Potential Role of the 5-HT6 Receptor in Depression and Anxiety: an Overview of Preclinical Data

Review

Potential role of the 5-HT6 receptor in depression and anxiety: an overview of preclinical data

Anna Weso³owska

Department of Clinical Pharmacy, Jagiellonian University Medical College, Medyczna 9, PL 30-688 Kraków, Poland

Correspondence: Anna Weso³owska, e-mail: [email protected]

Abstract: Mental disorders, such as depression and anxiety, pose both medical and social challenges. The clinical efficacy of current antidepressant/anxiolytic therapies is unsatisfactory; both antidepressant and anxiolytic drugs induce a variety of unwanted effects and have delayed onsets of action. Thus, a search for better and safer agents is continuously in progress. Preclinical results published so far have brought new insights into the possible role of recently discovered serotonin 5-HT6 receptors in these disorders. This review surveys the current state of knowledge regarding potential antidepressant and anxiolytic activities of selective 5-HT6 receptor ligands, namely, full agonists and antagonists, in animal models commonly used to predict such activity. Evidence indicates that both 5-HT6 agonists and antagonists may evoke identical responses in animal models of depression and anxiety; however, the possible mechanisms of these effects seem to be diverse and are not clearly understood. Especially interesting are the augmented effects achieved by combining antidepressants or diazepam with a selective 5-HT6 receptor antagonist.

Key words: serotonin, 5-HT$ receptor, 5-HT$ agonist, 5-HT$ antagonist, depression, anxiety, hippocampus, mice, rats

Introduction The 5-HT6 receptor was first identified and se- quenced in 1993 by separate groups [68, 78, 87]. It is Medicinal therapies for mood disorders, such as de- one of three serotonin receptors positively coupled to pression and anxiety, neither fully serve the efficacy the Gs-sensitive AC5 isoform [2], which induces 3’5’- demands of patients, nor are they free of side effects. cyclic adenosine monophosphate (cAMP) production Despite advances in pharmacotherapy, there continue [68, 78, 87]. The human and mouse 5-HT6 receptors to be many unmet clinical needs, ranging from effi- are glycoproteins consisting of 440 amino acids; in rats, cacy in treatment-resistant patients to improved onset the protein have 438 amino acids. All known homo- of action and to reductions in unwanted effects. Numer- logues have 7 transmembrane domains that form 3 cy- ous combination therapies and novel targets have been toplasmic and 3 extracellular loops [51, 52, 68]. identified that may demonstrate improvements in one In adult animals, in situ hybridization histochemis- or more of these desired areas. One of these ap- try assays analyzing 5-HT6 receptor mRNA expres- proaches is directly targeting monoamine receptors, sion, as well as immunohistochemical and autoradio- such as the recently discovered serotonin receptor graphic studies, have exhibited high concordance and 5-HT6. have demonstrated the highest 5-HT6 receptor expres-

564 Pharmacological Reports, 2010, 62, 564577 Role of 5-HT$ receptors in depression and anxiety Anna Weso³owska

sion in the striatum, nucleus accumbens, olfactory tu- ceptor is located outside serotonin neurons and does bercle, and cortex, with moderate density in the not function as an autoreceptor. amygdala, hippocampus, hypothalamus, thalamus, During the last several years, a large amount of in- and cerebellum [8, 13, 25, 28, 32, 33, 35, 41, 42, 52, formation has been collected about the 5-HT6 recep- 68, 84, 87, 106, 118]. There appears to be a negligible tor. A physiological role for this receptor within the density of 5-HT6 receptors outside the central nervous central nervous system has been clearly established in system; faint expression is also detectable in the rat studies focusing on learning and memory [36, 62, 67, stomach, spleen, thymus, and peripheral blood lym- 85, 113]. Strong evidence also supports the involvement phocytes [87, 100] as well as sympathetic ganglia of the 5-HT6 receptor in centrally-regulated feeding [77]. The postsynaptic localization of the 5-HT6 re- behavior [16, 37]. Currently, 5-HT6 receptor antago- ceptor is supported by the following: (I) electron mi- nists are being developed in clinical trials for treat- croscopic analysis of 5-HT6 immunohistochemistry ment of Alzheimer’s type dementia, cognitive impair- revealed receptor staining primarily on dendritic and ment associated with schizophrenia, and obesity [47, cilia processes, with little expression on cell bodies 48, 55]. [32, 35] and high expression of 5-HT6 receptor Preclinical efforts to evaluate a possible link be- mRNA in serotonin projection fields [106]; (II) the tween the 5-HT6 receptor and affective disorders have presence of 5-HT6 receptors is confined to the den- generally been inconclusive. These studies have dritic compartment [35]; and (III) no depletion of se- mainly used pharmacological tools, such as 5-HT6 re- rotoninergic innervation via 5,7-dihydroxytryptamine ceptor antisense oligonucleotides and/or mice lacking on 5-HT6 receptor expression in the rat nucleus ac- functional 5-HT6 receptors, in animal behavioral cumbens, striatum and hippocampus has been ob- models designed to mimic, at least in part, human dis- served [33]. All these data suggest that the 5-HT6 re- eases. Very recently, selective antagonists and ago-

Tab. 1. The binding profile of selective 5-HT$ receptor antagonists Tab. 2. The binding profile of selective 5-HT$ receptor agonists = [15, 41, 42, 44, some data concerning the affinity of SB-399885 were ( % inhibition at 1 µM drug concentration or KE (nM) determination at received from GlaxoSmithKline company] respective receptor according to Schechter et al. [93] and Cole et al. [18]), > data taken from [34]

Receptor pKi Receptor WAY-181187a WAY-208466a 11qa EMDTb SB-271046 SB-399885 SB-258585 Ki (nM) 5-HT 2.2 4.8 2.0 16 5-HT6 8.9 9.1 8.6 6 5-HT 0% 0% 170 5-HT1A 6.4 5.7 6.2 1A 5-HT 36% 30% 36% 5-HT1B 6.6 5.4 6.4 1B 5-HT 21% 16% 21% 290 5-HT1D 6.6 5.8 6.4 1D 5-HT 520 5-HT1E < 5.0 < 5.0 5.6 1E 5-HT 40% 22% 40% 5-HT1F < 6.0 < 5.1 6.2 1F 5-HT 25% 351 nM 25% > 10000 5-HT2A < 5.6 6.0 6.0 2A

5-HT2B < 5.4 6.4 5.5 5-HT2B 459 nM 313 nM 458 nM

5-HT2C 5.7 6.7 5.9 5-HT2C (Ant) 51% 217 nM 51% 1810

5-HT4 5.4 5.0 < 5.0 5-HT2C (Ago) 124 nM 644 nM 124 nM

5-HT7 5.4 < 5.1 5.5 5-HT7 1579 nM 4764 nM

D2 5.6 5.3 5.4 D2 1% 0%

D3 6.3 5.9 6.1 D3 1% 0%

D4 < 5.0 D4 1% 0% a a 1B 5.7 5.6 5.5 1 1334 nM 0% 300

Pharmacological Reports, 2010, 62, 564577 565 nists have become useful tools for studying the func- ceptor ligands in animal models of depression and tion of the 5-HT6 receptor in mood control. Among anxiety are summarized in Tables 3 and 4, respec- them, SB-271046 (5-chloro-N-[4-methoxy-3-(1-pipe- tively. The experiments, listed in tables together with razinyl)phenyl]-3-methyl-benzothiophene-2-sulfona- additional supporting and sometimes contradicting mide), SB-399885 (N-(3,5-dichloro-2-methoxyphenyl)- evidence, are discussed below. 4-methoxy-3-(1-piperazinyl)benzenesulfonamide), and SB-258585 (4-iodo-N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]benzenesulfonamide) are the most widely used, selective, and potent 5-HT6 receptor antagonists (Tab. 1) [15, 41, 42, 44]. Three other compounds, i.e., Possible role of the 5-HT6 receptor WAY-181187 (2-{1-[(6-chloroimidazo[2,1-b][1,3]thiazol- in depression 5-yl)sulfonyl]-1H-indol-3-yl}-N,N-dimethylethanamine), WAY-208466 [93], and LY-586713 [30], have recently been described as novel, selective, and full 5-HT6 re- Monsma et al. [68], Kohen et al. [51], and Sebben et ceptor agonists (Tab. 2); however, some chemical al. [94] showed that several tricyclic antidepressant structures and receptor affinities for these compounds drugs, such as amitriptyline, and atypical antidepres- 11q N are not available. Another compound, ( 1-(6- sants, such as mianserin, display high affinity and an- b chloroimidazo[2,1- ][1,3]-thiazole-5-sulfonyl)trypta- tagonistic activity at the 5-HT6 receptor. This finding mine), was shown to belong to a group of high affin- together with the localization of the receptor in limbic ity, potent, and full 5-HT6 receptor agonists (Tab. 2) and cortical brain areas [7, 8] has led to the implica- N N [18]. EMDT (2-ethyl-5-methoxy- , -dimethyltryptamine) tion of a role for the 5-HT6 receptor in the pathogene- is a successive 5-HT6 receptor agonist (Tab. 2) [34] sis and/or treatment of affective disorders. Further- used in animal studies to examine the role of 5-HT6 more, 5-HT6 receptor expression appears to be regu- receptor in depression [101], but there is some uncer- lated by glucocorticoids. Both adrenalectomy and tainty about the selectivity of EMDT [34]. The most blockade of glucocorticoid synthesis by metyrapone relevant findings concerning the effects of 5-HT6 re- or aminoglutethimidine treatment selectively up-

Tab. 3. Effects of selective 5-HT$ receptor antagonists and agonists in animal behavioral models of depression

Compound Animal model Effect (active doses) Tested doses Reference Antagonists: SB-399885 Tail suspension test in mice Antidepressant-like (10–30 mg/kg) 3–30 mg/kg [109] Forced swim test in mice Antidepressant-like (20 and 30 mg/kg) 10–30 mg/kg [109] Forced swim test in rats Antidepressant-like (1 ´ 10 mg/kg) 3–20 mg/kg [107, 109] Antidepressant-like (3 ´ 3 or 10 mg/kg) 3 ´ 0.3–10 mg/kg [39] SB-399885 + Forced swim test in rats Antidepressant-like 3 mg/kg + antidepressant [110] antidepressantsa SB-399885 + Forced swim test in rats No change 3 mg/kg + 20 mg/kg [110] citalopramb SB-271046 Tail suspension test in mice No change 1–10 mg/kg [101] Forced swim test in rats Antidepressant-like (3 ´ 10 or 30 mg/kg) 3 ´ 3–30 mg/kg [39] SB-258585c Forced swim test in rats Antidepressant like (3 µg) 1–10 µg [111] Agonist: EMDT Tail suspension test in mice Antidepressant like (2.5–15 mg/kg) 1–15 mg/kg [101]

= Synergistic enhancement when using a combination of inactive doses of SB-399885 together with imipramine, desipramine, bupropion, or moclobemide. > Combination of non-active doses of SB-399885 and citalopram. ? SB-258585 administered into the CA1 region of the hippocampus

566 Pharmacological Reports, 2010, 62, 564577 Role of 5-HT$ receptors in depression and anxiety Anna Weso³owska

Tab. 4. Effects of selective 5-HT$ receptor antagonists and agonists in animal behavioral models of anxiety

Compound Animal model Effect (active doses) Tested doses Reference Antagonists: SB-399885 Four plate test in mice Anxiolytic-like (3–20 mg/kg) 1–20 mg/kg [109] Elevated plus maze in rats Anxiolytic-like (0.3–3 mg/kg) 0.1–3 mg/kg [109] Conflict drinking test in rats Anxiolytic-like (1 and 3 mg/kg) 0.3–3 mg/kg [108, 109] SB-399885 + diazepama Conflict drinking test in rats Anxiolytic-like 0.3 mg/kg + 2.5 mg/kg [108] SB-258585c Conflict drinking test in rats Anxiolytic-like (1 µg) 0.3–3 µg [111] Agonists: WAY-181187 Schedule-induced polydipsia test in rats Anxiolytic-like (178 mg/kg) 56–178 mg/kg [93] 11q Schedule-induced polydipsia test in rats Anxiolytic-like (178 mg/kg) 56–178 mg/kg [18]

= Synergistic enhancement when using a combination of inactive doses of SB-399885 together with diazepam. > SB-258585 administered into the CA1 region of the hippocampus

regulated the 5-HT6 receptor mRNA in the CA1 and 5-HT6 receptor antagonist used was weaker than the CA3 pyramidal cells of the rat hippocampus [116]. As effect observed after treatment with imipramine and metyrapone and aminoglutethimidine treatments have was not dose-dependent [109]. been used in the clinic to treat resistant depression It is well established that acute administration of [54, 69], the authors speculated that this effect might antidepressants reduces immobility in behavioral tests have involved the 5-HT6 receptor. Moreover, prelimi- of despair (i.e., the forced swim and the tail suspen- nary genetic studies revealed that bipolar affective sion tests) and increases expression of the immediate disorder may be associated with variation in the early gene c-fos mRNA in the brain [4, 20, 46]. Com- 5-HT6 gene [103]. pound SB-271046, another 5-HT6 receptor antagonist, did not change immobility behavior of mice in the tail suspension test as fluoxetine did. In agreement with behavioral data, treatment with SB-271046 alone had no effect on c-fos mRNA expression in certain Selective 5-HT6 receptor antagonists limbic regions of the frontal cerebral cortex, including the cingulated cortex and the endopiriform cortex [101]. The reason for the discrepancies in the action of These results have urged scientists to conduct experi- two selective 5-HT6 receptor antagonists and chemical ments with selective 5-HT6 receptor ligands, adminis- analogues in the mouse tail suspension test is unclear. tered acutely in animal models commonly used for It should be noted that both experiments were per- evaluating the antidepressant potential of a drug. It formed on C57BL/6J mice. Although the breeding in- has been shown that one of the antagonists, com- stitutions providing male mice were different [101], pound SB-399885, exerted an antidepressant-like ef- the behavioral outcome should have been similar, fect in the forced swim and tail suspension tests in even though mice are not an ideal species in which to mice. These effects seem to be specific because conduct behavioral or physiological experiments per- SB-399885, administered at antidepressant doses, did taining to 5-HT6 receptor function. As described by not stimulate locomotor activity in mice [109]. In the Hirst et al. [40], the mouse is unique compared with forced swim test in mice, an anti-immobility effect of the rat, pig, and human, all of which express relatively SB-399885 was dose-dependent and comparable to high levels of 5-HT6 receptors. Hence, Svenningsson that of imipramine, which was used as a reference an- et al. [101] confirmed in their study that SB-271046 tidepressant in that study, whereas in the tail suspen- has a high affinity for 5-HT6 receptors in the mouse sion test, the potential antidepressant activity of the forebrain by showing that it displaced specific [125I]-

Pharmacological Reports, 2010, 62, 564577 567 SB-258585 binding at nanomolar concentrations 5-HT6 receptor antagonistic properties because both (EC50 = 4 nM). Nevertheless, it is noteworthy that doses compounds are selective ligands and blockers of used for both compounds and experimental protocols 5-HT6 sites [15, 44, 86]. Hence, direct involvement of were inconsistent. SB-399885 produced a weak, but other receptors in their effect ought to be excluded. similar, anti-immobility effect at all doses used (i.e., The shortening of immobility time, induced by an- 10, 20 and 30 mg/kg) [109], whereas SB-271046 was tidepressants in behavioral tests of despair, depends administered up to a dose of 10 mg/kg only; higher on the enhancement of the central serotonin and cate- doses were not tested [101]. Hirano et al. [39] and cholamine neurotransmission [10, 12, 20, 80]. Unfor- Hirst et al. [44] demonstrated in ex vivo binding as- tunately, no information is available on the effect of says that administration of SB-399885 significantly SB-399885 on the basal levels of serotonin. Yet, a mi- inhibited specific [125I]-SB258585 binding in the rat crodialysis study has shown that SB-271046 had no striatum with lower doses than those required for SB- influence on the basal release of serotonin in the rat prefrontal cortex, frontal cortex, striatum and hippo- 271046 to significantly occupy brain 5-HT6 receptors. The above results prove the higher efficacy of SB- campus [21, 22, 56, 63]. Moreover, 5-HT6 receptor 399885 in comparison to that of SB-271046, at least antagonists, SB-399885 and/or SB-271046 signifi- in rats. The improved potency of SB-399885 seems to cantly increase cortical and hippocampal extracellular be due to an increase in brain penetration when com- concentrations of dopamine, norepinephrine and ace- pared to SB-271046, where it shows a 3-fold im- tylcholine in freely moving rats [43, 44, 56, 59, 63]. provement in the brain: plasma ratio of 5% to 15% They also can potentiate the behavioral effects of amphetamine [23, 31, 82]. Furthermore, high levels of [44, 86]. Moreover, the duration of the tail suspension 5-HT receptor-like immunoreactivity have been tests also varied: 6 min for SB-399885 [109] and 5 6 found in dopaminergic areas [32]. The microdialysis min for SB-271046 [101]. findings have been extended by a study described by In the forced swim test in rats, the findings for both Weso³owska et al. [107], which demonstrated that ad- 5-HT receptor antagonists are consistent. Treatment 6 ministration of p-chloramphetamine under their labo- with an acute dose of 10 mg/kg of SB-399885 pro- ratory conditions reduced cortical and hippocampal duced specific antidepressant-like activity by shorten- concentrations of serotonin and its metabolite and did ing the immobility time without a stimulatory effect not modify the antidepressant-like effect of SB- on exploratory activity observed in the open field 399885 in the forced swim test in rats. However, its [107, 109]. The absence of a potential antidepressant anti-immobility action in that test was abolished by action after administration of a higher dose of SB- the preferential D1- and D2-like receptor antagonists 399885 (i.e., 20 mg/kg) may be due to a sedative ef- SCH-23390 and sulpiride, respectively, and by the fect, as was evident in the open field test in rats [109]. a 2-adrenoceptor antagonist idazoxan, but not by pra- In line with that study, Hirano et al. [39] demonstrated a zosin, a blocker of 1-adrenoceptors [107]. On the ba- antidepressant-like activity for both SB-399885 and sis of microdialysis and behavioral studies, it could be SB-271046 administered three times in rats. In agree- proposed that the anti-immobility effect of 5-HT6 re- ment with brain 5-HT6 receptor occupancy (62% and ceptor antagonists does not actually require any integ- 96% for 3 and 10 mg/kg of SB-399885, respectively; rity of serotonin neurons and seems to be connected 56% and 84% for 10 and 30 mg/kg of SB-271046, re- with activation of dopamine and norepinephrine sys- spectively) [39] and better brain penetration (15% for via a tems D1- and D2-like receptors and 2-adrenoce- SB-399885 vs. 5% for SB-271046) [44], SB-399885 ptors. was approximately 3 times more potent than SB-271046 It is well documented that 5-HT6 receptor antago- in the forced swim test in rats. Both SB-399885 and nists, including SB-399885, increase extracellular SB-271046 significantly suppressed immobility be- levels of acetylcholine in the hippocampus and pre- havior of rats with doses of 3 and 10 mg/kg, respec- frontal cortex in freely moving rats [44, 83]. Acetyl- tively; their effects were slightly weaker than those of choline is of great importance in cognitive function imipramine [39]. and diseases, including Alzheimer’s disease [91]; The antidepressant-like activity of SB-399885 in however, its influence on emotional regulation is less mice and rats and such activity of SB-271046 in rats known. Janowsky et al. [49] and Shytle et al. [99] are most probably connected with these compounds’ postulated that there is relative or actual cholinergic

568 Pharmacological Reports, 2010, 62, 564577 Role of 5-HT$ receptors in depression and anxiety Anna Weso³owska

hyperactivity in depression. Moreover, in animal changes [104]. An involvement of the hippocampus models anticholinergic drugs enhance the potential in the action of antidepressant drugs has also been antidepressant activity of imipramine [79] and sup- demonstrated in animal studies. For example, imi- press immobility behavior of mice in the forced swim pramine [81] or desipramine [53] injected into the rat and the tail suspension tests [11, 20]. Thus, these find- hippocampus was able to reduce immobility in the ings indirectly suggest that it is unlikely that the forced swim test. Moreover, electrolytic lesion of that antidepressant-like effect of 5-HT6 receptor antago- structure completely abolished the effect of peripher- nists observed in animal models develops as a conse- ally administered desipramine [53]. In addition, Sher- quence of enhanced acetylcholine release. man and Allers [95] found a strong correlation be- The behavioral evidence indicates that the selective tween the imipramine concentration in the rat hippocampus and the positive effect of the drug in the blockade of 5-HT6 receptors evoked by SB-399885 may facilitate the anti-immobility effects of antide- behavioral learned helplessness test, an animal model pressants in which the mechanism of action is con- predictive of antidepressant potential. Finally, the nected with the inhibition of norepinephrine/dopa- finding that antidepressants increase hippocampal mine uptake and monoamine oxidase-A [110]. Com- brain-derived neurotrophic factor (BDNF) expression bining a sub-efficacious dose of SB-399885 with inef- seems to support the idea that hippocampal neuro- fective doses of imipramine, desipramine, bupropion genesis is involved in antidepressant action [64]. or moclobemide results in a pronounced decrease of As has been mentioned above, 5-HT6 receptors are immobility time in the forced swim test in rats [110]. present in the hippocampus, including its CA1 region Only citalopram injected in a non-active dose jointly [32, 33, 68, 106, 118], and the hippocampus has been proposed as one of the neuroanatomical structures in- with SB-399885 did not induce any effects characteris- volved in antidepressant-like activity of selective tic of antidepressants in that test. However, no data are 5-HT receptor antagonists [111]. In fact, recent be- available so far on the effect of SB-399885 on antide- 6 havioral results demonstrated that SB-258585, a se- pressants’ blood-brain barrier penetration and pharma- lective 5-HT receptor blocker, when injected into the cokinetics parameters; thus, pharmacokinetic interac- 6 CA1 region of the rat hippocampus, produced an tion between a selective 5-HT antagonist and the anti- 6 antidepressant-like effect in the forced swim test. Its depressants studied cannot be ruled out. All in all, the potential antidepressant activity cannot be attributed above cited behavioral and neurochemical results seem to changes in general activity because this drug, given to point to an involvement of enhanced monoaminer- at a dose producing an anti-immobility effect, did not gic, namely, dopaminergic and/or noradrenergic, neu- change exploratory locomotor activity measured in rotransmission in the antidepressant-like activity of the open field test in rats. It is noteworthy that the a 5-HT6 antagonist [110]. antidepressant-like effect of SB-258585 was compa- The hippocampus is a limbic region that plays an rable with the effect of imipramine [111]. important role in emotional states [38]. In recent years, clinical and laboratory experiments have strengthened the evidence for the role of the hippocampus in the pathophysiology of depression. Multi- ple imaging studies have revealed hippocampal vol- Selective 5-HT6 receptor agonists ume reductions in depressed patients, which are changes that correlate well with disease duration and 5-HT6 receptor agonists may also play a role in de- executive dysfunction [57, 71, 72, 92, 102, 112]. Fur- pression. Svenningsson et al. [101] showed that thermore, it seems that the hippocampus may play an fluoxetine exerts a stimulatory action on cortical c-fos important role in the action of antidepressants as well. mRNA and phospho-Ser845-GluR1 and reduces the It has been demonstrated that treatment patients with immobility time of mice in the tail suspension test. major depressive disorder using these drugs signifi- All its actions were partially blocked by a selective cantly improved memory and depressive symptoms 5-HT6 receptor antagonist, SB-271046, administered without altering hippocampal volume, which could at a dose that by itself produced no effect in those as- suggest that antidepressants improve hippocampal says. Because fluoxetine has only low-to-moderate af- function in the absence of detectable structural finity for 5-HT6 receptors [68], the authors concluded

Pharmacological Reports, 2010, 62, 564577 569 that the inhibitory action of SB-271046 on fluoxetine- matched controls. The preclinical studies and prelimi- mediated effects involves blocking 5-HT6 receptor ac- nary clinical trials with ketamine, an NMDA receptor tivation that had been elicited by the fluoxetine- antagonist [74, 117, 119], demonstrated its potential induced elevations of extracellular levels of serotonin antidepressant activity, which supports the notion that at- and does not involve the direct competition of either tenuating glutamatergic neurotransmission can be benefi- compound at 5-HT6 sites [101]. Additionally, they as- cial. Thus, improvement of GABAergic neurotransmission sessed the ability of the 5-HT6 receptor agonist in connection with dampening stimulated glutamatergic EMDT to mimic some of the antidepressant-like bio- transmission could be proposed as a mechanism of the chemical and behavioral effects of fluoxetine [101]. antidepressant-like activity of 5-HT6 receptor agonists. EMDT, like fluoxetine, increased the phosphorylation The above findings may suggest a difference in the 34 state of Thr -DARPP-32 both in brain slices and in mechanisms of the anti-immobility action of 5-HT6 the intact brain. It also increased phospho-Ser845- receptor agonists and antagonists, although each of GluR1 and the expression of c-fos mRNA throughout them seems to be primarily connected with stimulat- the striatum and cerebral cortex. Moreover, in a simi- ing or blocking, respectively, 5-HT6 receptors, given lar range of doses, EMDT significantly reduced the that the studied compounds are selective 5-HT6 recep- immobility time of mice in the tail suspension test. tor ligands [15, 34, 43, 44, 86]. The limited available SB-271046 completely blocked the biochemical and data do not, however, permit us to provide a definite behavioral antidepressant-like effects produced by explanation. EMDT [101]. Several independent studies have demonstrated It is well established that the shortening of immo- that repeated, but not acute, administration of antide- bility time, induced by antidepressant drugs and ob- pressants increases the expression of genes whose served in behavioral tests of despair, depends on the corresponding proteins regulate synaptic plasticity in enhancement of the central serotonin and catechola- the brain [19, 29, 70, 88]. As a result, BDNF and the mine neurotransmission [10, 12, 20, 80]. Unfortunately, effector immediate early gene activity-regulated cyto- no information is available on the effect of EMDT on skeletal associated protein (Arc) have been identified the levels of serotonin and catecholamines. A micro- as possible targets for antidepressant action [75, 96, dialysis study has only shown that a selective 5-HT6 97]. Experiments conducted by de Foubert et al. [30] receptor agonist, WAY-181187, decreased the basal provided the first evidence for the involvement of the Arc release of serotonin, dopamine and norepinephrine in 5-HT6 receptors in regulating BDNF and mRNA the frontal cortex, striatum and amygdala of freely expression. They showed that a selective 5-HT6 re- moving rats [93]. The latter effects were found to be ceptor agonist, compound LY-586713, when adminis- blocked by local infusion of a GABAA receptor an- tered acutely, caused marked increases of BDNF and tagonist, bicuculline, which confirms a relationship Arc mRNA levels in hippocampal and cortical re- between the 5-HT6 receptor and GABAergic system gions. These increases were attenuated by SB-271046 and is entirely consistent with a study showing dense in all regions of the hippocampus and the parietal cor- colocalization of the 5-HT6 receptor with the GABA- tex. In conclusion, the above results may suggest that synthesizing enzyme glutamic acid decarboxylase in direct 5-HT6 receptor activation results in a more the rat cortex, hippocampus and striatum [105, 114]. rapid rise in BDNF and Arc mRNA expression, which Additionally, acute administration of WAY-181187 in- does not require repeated administration, as is the creased the extracellular levels of GABA in the rat case for some antidepressant drugs [19, 29, 70, 88]. frontal cortex, dorsal hippocampus, striatum and Because LY-586713 is described as a selective 5-HT6 amygdala without altering basal levels of glutamate. receptor agonist, it is plausible that the 5-HT6 recep- However, in hippocampal slices, pretreatment with tor-mediated onset of BDNF gene expression follow- WAY-181187 significantly and dose-dependently at- ing a single dose of LY-586713 is directly linked to tenuated sodium azide-stimulated increases in gluta- the activation of 5-HT6 receptors. Subsequently, in- mate concentrations [93]. Impairment of GABAergic creases in cAMP levels lead to activation of CREB, neurotransmission has been described in patients with a known transcription factor for the BDNF gene [30]. a variety of depressive illness [58, 76, 90]. Moreover, From the above description, it is clear that 5-HT6 Sanacora et al. [89] observed higher glutamate levels receptors are involved in the antidepressant-like activ- in depressed patients compared with healthy age- ity observed in animal models. An acute and selective

570 Pharmacological Reports, 2010, 62, 564577 Role of 5-HT$ receptors in depression and anxiety Anna Weso³owska

blockade of 5-HT6 receptors may evoke an antide- dips [9]. A major concern with data arising from this pressant-like effect that seems to be predominantly approach is that different developmental compensa- due to the enhancement of brain noradrenergic and/or tions may mask the true function of the receptor de- dopaminergic neurotransmission, and the hippocam- leted, as is observed, for example, with 5-HT1A and pus seems to be one of neuroanatomical sites involved 5-HT1B receptor knockouts [1]. Additionally, the very in that effect. Moreover, the selective blockade of low expression of 5-HT6 receptors in the mouse brain 5-HT6 receptors induced by SB-399885 may facilitate [40] should call into question the value of using this the anti-immobility activity of antidepressants whose species to examine basic 5-HT6 receptor function. mechanism of action is connected with norepinephrine/dopamine uptake inhibition or with the inhibition of monoamine oxidase-A. However, in this case, one cannot exclude pharmacokinetic interaction yet. In Selective 5-HT6 receptor antagonists contrast, the stimulation of 5-HT6 receptors may also evoke antidepressant-like activity, and 5-HT6 receptors seem to play at least a partial role in the potential an- With the development of selective ligands, useful tidepressant effect of fluoxetine. The ability of a 5-HT6 tools for further understanding the functions of 5-HT6 receptor agonist to enhance extracellular GABA lev- receptors have been available. Recently, Weso³owska els and decrease stimulated glutamatergic neurotrans- and Nikiforuk [109] have observed that the potent and mission supports the hypothesis that 5-HT6 receptor selective 5-HT6 receptor antagonist SB-399885 pro- agonists may be effective agents for the treatment of duced specific anxiolytic-like activity in animal mod- depression, particularly when glutamate levels are en- els of anxiety. It dose-dependently and significantly hanced under pathologic circumstances. increased the number of shocks accepted in the conflict drinking (Vogel) test in rats, a model that is widely used and considered to be one of the most specific methods for the detection of potential anxiolytic Possible role of the 5-HT receptor in activity [66], without an effect on either the shock 6 threshold or unpunished water consumption. This anxiety finding is supported by results obtained in an elevated plus maze test in rats, a procedure based on rodents’ Few studies using 5-HT6 receptor antisense oligonu- natural aversion to heights and open space. In this cleotides have explored the involvement of 5-HT6 re- model, SB-399885 dose-dependently increased the ceptors in rodent models of anxiety, and the reported percentage of time spent in and the number of entries results have been inconsistent. In the Yoshioka et al. into the open arms of the maze, without stimulating [118] study, seven days of 5-HT6 receptor-directed the general exploratory activity of rats detected in the antisense oligonucleotide treatment caused a 30% re- open field test. Moreover, SB-399885 also had anti- duction in [3H]-LSD binding sites, accompanied by anxiety-like activity in the four-plate test in mice a reduction in conditioned fear stress-induced sero- [109]. Quantitatively, the potential anxiolytic effect of tonin release in the rat prefrontal cortex, which is sug- the 5-HT6 receptor antagonist tested in all three mod- gestive of an anxiolytic-like response. Conversely, els employed was approximately equivalent to that of Hamon et al. [35] and Otano et al. [73] demonstrated diazepam, particularly in rats. anxiogenic-like activity following chronic administra- The potential anxiolytic-like effect of SB-399885 tion of the same oligonucleotide sequence in two al- was not modified in rats whose serotonin neurons ternative rat models of anxiety: the elevated plus- were destroyed by prior administration of p-chlor- maze and the social interaction tests, respectively. amphetamine [108], which suggests that the above ac- 5-HT6 receptor-knockout mice, however, displayed tivity is not conditioned by the integrity of serotonin few phenotypic abnormalities, except for differences neurons. Such a concept is in line with neuroanatomi- in open field anxiety-related behaviors and in an ele- cal results that show that 5-HT6 receptors are located vated zero maze; i.e., there were no differences in to- outside serotonin neurons [32, 106]. Thus, the anti- tal open quadrant dwell time, number of transitions conflict activity of SB-399885 observed in the Vogel between open and sheltered maze quadrants or head test in rats does not depend on a serotonergic mecha-

Pharmacological Reports, 2010, 62, 564577 571 nism. Weso³owska [108] has also demonstrated that The hippocampus seems to be an important site of the selective 5-HT6 receptor antagonist tested helped action of anxiolytic compounds with diverse mecha- to reveal the anticonflict action of diazepam; both nisms. For example, 5-HT1A receptor partial agonists, were administered in a non-active dose. The additive benzodiazepine receptor agonists, and ligands of effect of SB-399885 and diazepam may be regarded ionotropic or metabotropic glutamatergic receptors as a result of pharmacodynamic and/or pharmacoki- exhibit anxiolytic-like effects in various models of netic interaction. Because the levels of SB-399885 anxiety after administration into the hippocampus and diazepam administered alone or jointly have not [17, 66]. Similarly, the 5-HT6 receptor blocker SB- been analyzed, one cannot rule out a pharmacokinetic 258585, when injected into the CA1 region of the rat interaction at this stage of experimentation. Further- hippocampus, produced anxiolytic-like activity in the more, the anticonflict activity of SB-399885 was re- conflict drinking test [111]. Its effect seems to be spe- duced by the benzodiazepine receptor antagonist flu- cific because this agent, when administered at an anx- mazenil, which was used at a dose reported to antago- iolytic dose, affected neither the shock threshold nor nize diverse effects of diazepam, including its unpunished water consumption. However, its effect anxiolytic-like effect [6, 26, 61, 108]. Because SB- observed in the Vogel test in rats was weaker than that 399885 exhibits no affinity for GABA and benzodi- of diazepam. azepine receptors [44], its anti-anxiety-like effect stems from a functional interaction between 5-HT6 receptors and the GABA/benzodiazepine system. Such a conclusion is supported by neuroanatomical find- Selective 5-HT6 receptor agonists ings concerning the expression of 5-HT6 receptor mRNA on GABAergic neurons [33, 105]. Moreover, As suggested by Schechter and his co-investigators Benes et al. [5] demonstrated an increase in 5-HT re- 6 [93], selective 5-HT receptor agonists may play a po- ceptor mRNA in the rat hippocampus after GABAergic 6 tential therapeutic role in the treatment of some types transmission had been interrupted by a local infusion of of anxiety-related disorders. Thus, WAY-181187 and picrotoxin. It has also been shown that another 5-HT 6 WAY-208466 [93], as well as compound 11q [18], receptor antagonist, SB-357134, induced a concentra- when administered acutely, effectively decreased wa- tion-dependent increase in the K+-evoked GABA ef- ter intake by rats that had not been water-deprived in flux in rat striatal slices [65]. the schedule-induced polydipsia test, a model consid- Unfortunately, there is no information about the ac- ered to be predictive for efficacy in obsessive compul- tion of SB-399885 on the release of GABA in animal sive disorder [45]. Moreover, pharmacological studies brain areas. However, it has been established that have demonstrated that selective serotonin re-uptake 5-HT6 receptor antagonists, including SB-399885, in- inhibitors (SSRI) can also decrease adjunctive drink- crease the activity of the acetylcholine system [27, 44, ing in this test [115]; however, they have to be admin- 50, 60, 83, 98]. As has been presented in an excellent istered chronically to correspond with their clinical review written by Millan [66], cholinergic pathways efficacy. do not play a pivotal role in the control of anxiety; In addition to behavioral data, neurochemical re- however, some experimental evidence indicates that sults have revealed that selective 5-HT6 receptor ago- stimulation of cholinergic transmission is accompa- nists increased extracellular levels of GABA in sev- nied by anxiolytic-like activity in animals [3, 14, 24]. eral areas of the rat brain associated with affective Thus, involvement of the cholinergic system in the disorders [93]. Thus, an acute administration of WAY- anxiolytic-like activity of SB-399885 and in the syn- 181187 increased extracellular GABA concentrations ergistic effect of the 5-HT6 receptor antagonist tested in the rat frontal cortex, dorsal hippocampus, stria- and diazepam cannot be excluded. On the other hand, tum, and amygdala; all effects were blocked by a se- Marcos et al. [65] demonstrated that the GABAA re- lective 5-HT6 receptor antagonist SB-271046. In ad- ceptor antagonist bicuculline did not alter the acetyl- dition to the acute effects, a fourteen-day treatment choline release produced by SB-357134, another with another 5-HT6 receptor agonist, WAY-208466, 5-HT6 receptor antagonist. Further studies are neces- resulted in robust elevations in the extracellular levels sary to explain this interaction. of GABA in the rat dorsolateral frontal cortex, an ef-

572 Pharmacological Reports, 2010, 62, 564577 Role of 5-HT$ receptors in depression and anxiety Anna Weso³owska

fect similar in terms of magnitude and duration to that receptor synergistically potentiates the effect of clini- produced by WAY-181187 in the same brain region. cally used antidepressants and anxiolytics. Combining These findings highlight the fact that chronic activa- a lower dose of an antidepressant and/or anxiolytic tion of 5-HT6 receptors does not evoke desensitiza- drug with a 5-HT6 receptor antagonist might acceler- tion. Moreover, employing in vivo microdialysis tech- ate the onset of action and minimize the side effect niques, Schechter et al. [93] have also shown that profiles. The combination might also be very useful WAY-181187 did not alter basal glutamate levels, but for patients who either does not respond to classic an- in hippocampal slices, this 5-HT6 receptor agonist at- tidepressant/anxiolytic treatment or for whom monother- tenuated sodium azide-stimulated glutamate release. apy provides insufficient efficacy. Alternately, 5-HT6 re- In summary, the above-cited results support the ceptor agonists administered alone may have some contention that selective 5-HT6 receptor antagonists, advantages compared with SSRI, including acute on- when administered peripherally or into the hippocam- set of action in the treatment of some types of anxiety- pus, may evoke an anxiolytic-like effect that can pos- related disorders as well as depressive symptoms. sibly be explained by a functional interaction between This finding opens up numerous possibilities for new 5-HT6 receptors and the benzodiazepine system. The individual therapies targeting the 5-HT6 receptor or selective blockade of 5-HT6 receptors evoked by SB- combinations of new drugs and current antidepres- 399885 may also facilitate the anticonflict effect of sants/anxiolytics. But only clinical testing will deter- diazepam. Alternately, stimulation of 5-HT6 receptors mine the extent to which this approach shows distinct can also produce potential anxiolytic activity. The advantages over existing therapies and finally demon- ability of 5-HT6 receptor agonists to enhance extra- strate the true innovation associated with this novel cellular GABA levels and decrease stimulated gluta- potential mechanism. matergic neurotransmission seems to support the hypothesis that 5-HT6 receptor agonists may be effective agents for the treatment of anxiety. References:

1. Ase AR, Reader TA, Hen R, Riad M, Descarries L: Al- tered serotonin and dopamine metabolism in the CNS of Conclusions and prospective future for serotonin 5-HT1A or 5-HT1B receptor knockout mice. J Neurochem, 2000, 75, 2415–2426. 5-HT6 receptor ligands 2. Baker LP, Nielsen MD, Impey S, Metcalf MA, Poser SW, Chan G, Obrietan K et al.: Stimulation of type 1 and 2+ Over the last several years, a number of studies have type 8 Ca /calmodulin-sensitive adenylyl cyclases by attempted to evaluate the potential role of the 5-HT the Gs-coupled 5-hydroxytryptamine subtype 5-HT7A 6 receptor. J Biol Chem, 1998, 273, 17469–17476. receptor in affective disorders. From the data pro- 3. Balerio GN, Aso E, Maldonado R: Role of cannabinoid vided in this review, it is evident that the 5-HT6 recep- system in the effects induced by nicotine on anxiety-like tor has emerged as a very interesting molecular target behaviour in mice. Psychopharmacology, 2006, 184, that interacts with antidepressant/anxiolytic drugs. It 504–513. 4. is uncertain whether antagonists or agonists of this re- Beck CH: Acute treatment with antidepressant drugs selectively increases the expression of c-fos in the rat ceptor will best serve the potential therapeutic indica- brain. J Psychiatry Neurosci, 1995, 20, 25–23. tions, i.e., depression and/or anxiety. This uncertainty 5. Benes FM, Burke RE, Walsh J, Berrette S, Matzilevich arises from the observation that equivalent antidepres- D, Minns M, Konradi C: Acute amygdalar activation in- sant and anxiolytic potency and efficacy can be deliv- duces an upregulation of multiple monoamine G protein ered in animal models by both 5-HT receptor antago- coupled pathways in rat hippocampus. Mol Psychiatry, 6 2004, 9, 932–945. nists and agonists. However, it should be kept in mind 6. Boast CA, Bernard PS, Barbaz BS, Bergen KM: The that the most of presented results concern the effects neuropharmacology of various diazepam antagonists. of 5-HT receptor ligands after their acute administra- Neuropharmacology, 1983, 22, 1511–1521. 6 7. tion. Yet, such findings may underline further pre- Boess FG, Monsma FJ, Carolo C, Meyer V, Rudler A, Zwingelstein C, Sleight AJ: Functional and radioligand clinical studies performed after repeated administra- binding characterization of rat 5-HT6 receptors stably ex- tion of selective 5-HT6 receptor ligands. Of particular pressed in HEK293 cells. Neuropharmacology, 1997, 36, interest is the observation that inhibition of the 5-HT6 713–720.

Pharmacological Reports, 2010, 62, 564577 573 8. Boess FG, Riemer C, Bös M, Bentley J, Bourson A, 24. Degroot A, Treit D: Dorsal and ventral hippocampal cho- Sleight AJ: The 5-hydroxytryptamine6 receptor-selective linergic systems modulate anxiety in the plus-maze and radioligand [3H]Ro 63-0563 labels 5-hydroxytryptamine shock-probe tests. Brain Res, 2002, 949, 60–70. receptor binding sites in rat and porcine striatum. Mol 25. East SZ, Burnet PWJ, Leslie RA, Roberts JC, Harrison Pharmacol, 1998, 54, 577–583. PJ: 5-HT6 receptor binding sites in schizophrenia and 9. Bonasera SJ, Chu H-M, Brennan TJ, Tecott LH: A null following antipsychotic drug administration: autoradio- mutation of the serotonin 6 receptor alters acute re- graphic studies with [125I]SB-258585. Synapse, 2002, sponses to ethanol. Neuropsychopharmacology, 2006, 45, 191–199. 31, 1801–1813. 26. File SE, Pellow S, Wilks L: The sedative effects of CL 10. Borsini F: Role of the serotonergic system in the forced 218,872, like those of chlordiazepoxide, are reversed by swimming test. Neurosci Biobehav Rev, 1995, 19, 377–395. benzodiazepine antagonists. Psychopharmacology, 1985, 11. Borsini F, Meli A: Is the forced swimming test a suitable 85, 295–300. model for revealing antidepressant activity? Psychophar- 27. Foley AG, Murphy KJ, Hirst WD, Gallagher HC, Hagan macology, 1988, 94, 147–160. JJ, Upton N, Walsh FS, Regan CM: The 5-HT6 receptor 12. Borsini F, Meli A: The forced swimming test: its contri- antagonist SB-271046 reverses scopolamine-disrupted bution to the understanding of the mechanisms of action consolidation of a passive avoidance task and amelio- of antidepressants. Adv Biosci, 1990, 77, 63–76. rates spatial task deficits in aged rats. Neuropsychophar- 13. Brailov I, Bancila M,Brisorgueil M-J, Miquel M-C, Ha- macology, 2004, 29, 93–100. mon M, Verge D: Localization of 5-HT6 receptors at the 28. Fone KCF: The 5-ht6 receptor: potential CNS functions. plasma membrane of neural cilia in the rat brain. Brain Br J Pharmacol, 2000, 131, Suppl, 238P. Res, 2000, 872, 271–275. 29. de Foubert G, Carney SL, Robinson CS, Destexhe EJ, 14. Brioni JD, O’Neill AB, Kim DJ, Decker MW: Nicotinic Tomlinson R, Hicks CA, Murray TK et al.: Fluoxetine- receptor agonists exhibits anxiolytic-like effects on the induced change in rat brain expression of brain-derived elevated plus-maze test. Eur J Pharmacol, 1993, 238, 1–8. neurotrophic factor varies depending on length of treat- 15. Bromidge SM, Brown AM, Clarke SE, Dodgson K, ment. Neuroscience, 2004, 128, 597–604. Gager T, Grassam HL, Jeffrey PM et al.: 5-Chloro-N-(4- 30. de Foubert G, O’Neill MJ, Zetterström TSC: Acute onset methoxy-3-piperazin-1-yl-phenyl)-3-methyl-2- by 5-HT6-receptor activation on rat brain-derived neuro- benzothiophenesulfonamide (SB-271046): a potent, se- trophic factor and activity-regulated cytoskeletal- lective, and orally bioavailable 5-HT6 receptor antago- associated protein mRNA expression. Neuroscience, nist. J Med Chem, 1999, 42, 202–205. 2007, 147, 778–785. 16. Caldirola P: 5-HT6 receptor antagonism. A novel mecha- 31. Frantz KJ, Hansson KJ, Stouffer DG, Parsons LH: 5-HT6 nism for the management of obesity. SMI Inaugural Con- receptor antagonism potentiates the behavioral and neu- ference on Obesity and Related Disorders, 2003, London. rochemical effects of amphetamine but not cocaine. Neu- 17. Chojnacka-Wójcik E, K³odziñska A, Pilc A: Glutamate ropharmacology, 2002, 42, 170–180. receptor ligands as anxiolytics. Curr Opin Investig 32. Gerard C, Martres MP, Lefevre K, Miquel MC, Verge D, Drugs, 2001, 2, 1112–1119. Lanfumey L, Doucet E et al.: Immuno-localization of se- 18. Cole DC, Stock JR, Lennox WJ, Bernotas RC, Ellingboe rotonin 5-HT6 receptor-like material in the rat central JW, Boikess S, Coupet J et al.: Discovery of N1-(6- nervous system. Brain Res, 1997, 23, 207–219. chloroimidazo[2,1-b][1,3]-thiazole-5-sulfonyl)trypta- 33. Gerard C, el Mestikawy S, Lebrand C, Adrien J, Ruat M, mine as a potent, selective, and orally active 5-HT6 re- Traiffort E, Hamon M, Martres MP: Quantitative RT-PCR ceptor agonist. J Med Chem, 2007, 50, 5535–5538. distribution of serotonin 5-HT6 receptor mRNA in the 19. Coppell AL, Pei Q, Zetterström TSC: Bi-phasic change central nervous system of control or 5,7-dihydroxy- in BDNF gene expression following antidepressant drug tryptamine-treated rats. Synapse, 1996, 23, 164–173. treatment. Neuropharmacology, 2003, 44, 903–910. 34. Glennon RA, Lee M, Rangisetty JB, Dukat M, Roth BL, 20. Cryan JF, Mombereau C, Vassout A: The tail suspension Savage JE, McBride A et al.: 2-Substituted tryptamines: test as a model for assessing antidepressant activity: re- agents with selectivity for 5-HT6 serotonin receptors. view of pharmacological and genetic studies in mice. J Med Chem, 2000, 43, 1011–1018. Neurosci Biobehav Rev, 2005, 29, 571–625. 35. Hamon M, Doucet E, Lefevre K, Miquel M-C, Lan- 21. Dawson LA, Nguyen HQ, Li P: In vivo effects of the fumey L, Insausti R, Frechilla D et al.: Antibodies and 5-HT6 antagonist SB-271046 on striatal and frontal cor- antisense oligonucleotide for probing the distribution and tex extracellular concentrations of noradrenaline, dopa- putative functions of central 5-HT6 receptors. Neuropsy- mine, 5-HT, glutamate and aspartate. Br J Pharmacol, chopharmacology, 1999, 21, 68–76. 2000, 130, 23–26. 36. Hatcher PD, Brown VJ, Tait DS, Bate S, Overend P, Ha- 22. Dawson LA, Nguyen HQ, Li P: The 5-HT6 receptor an- gan JJ, Jones DN: 5-HT6-R antagonists improve per- tagonist SB-271046 selectively enhances excitatory neu- formance in an attentional set shifting tast in rats. Psy- rotransmission in the rat frontal cortex and hippocampus. chopharmacology, 2005, 181, 253–259. Neuropsychopharmacology, 2001, 25, 662–668. 37. Heal DJ, Smith SL, Fisas A, Codony X, Buschmann H: 23. Dawson LA, Nguyen HQ, Li P: Potentiation of Selective 5-HT6 receptor ligands: progress in the devel- amphetamine-induced changes in dopamine and 5-HT by opment of a novel pharmacological approach to the treat- a 5-HT6 receptor antagonist. Brain Res Bull, 2003, 59, ment of obesity and related metabolic disorders. Pharma- 513–521. col Ther, 2008, 117, 207–231.

574 Pharmacological Reports, 2010, 62, 564577 Role of 5-HT$ receptors in depression and anxiety Anna Weso³owska

38. Hensler JG: Serotonergic modulation of the limbic sys- 53. Kostowski W: Possible relationship of the locus tem. Neurosci Biobehav Rev, 2006, 30, 203–214. coeruleus-hippocampal noradrenergic neurons to depres- 39. Hirano K, Piers TM, Searle KL, Miller ND, Rutter R, sion and mode of action of antidepressant drugs. Pol Chapman PF: Procognitive 5-HT6 antagonists in the rat J Pharmacol Pharm, 1985, 37, 727–743. forced swimming test: Potential therapeutic utility in 54. Kramlinger KG, Petersen GC, Watson PK, Leonard LL: mood disorders associated with Alzheimer’s disease. Metyrapone for depression and delirium secondary to Life Sci, 2009, 84, 558–562. Cushing’s syndrome. Psychosomatics, 1985, 26, 67–71. 40. Hirst WD, Abrahamsen B, Blaney FE, Calver AR, Aloj 55. Kwon MO, Fischer F, Matthisson M, Herling P: List of L, Price GW, Medhurst AD: Differences in the central drugs in development for neurodegenerative diseases. nervous system distribution and pharmacology of the Update June 2004. Neurodegener Dis, 2004, 1, 113–154. mouse 5-hydroxytryptamine-6 receptor compared with 56. Lacroix LP, Dawson LA, Hagan JJ, Heidbreder CA: rat and human receptors investigated by radioligand 5-HT6 receptor antagpnist SB-271046 enhances extracel- binding, site-directed mutagenesis, and molecular model- lular levels of monoamines in the rat medial prefrontal ing. Mol Pharmacol, 2003, 64, 1295–1308. cortex. Synapse, 2004, 51, 158–164. 41. Hirst WD, Minton JAL, Bromidge SM, Moss SF, Latter 57. Lange C, Irle E: Enlarged amygdala volume and reduced AJ, Riley G, Routledge C et al.: Characterization of hippocampal volume in young women with major de- [125I]-SB-258585 binding to human recombinant and na- pression. Psychol Med, 2004, 324, 1059–1064. 58. tive 5-HT6 receptors in rat, pig and human brain tissue. Leung JWC, Xue H: GABAergic functions and depres- Br J Pharmacol, 2000, 130, 1597–1605. sion: from clinical therapies to herbal medicine. Curr 42. Hirst WD, Minton JAL, Bromidge SM, Routledge C, Drug Targets CNS Neurol Disord, 2003, 2, 363–373. 59. Middlemiss DN, Price GW: [125I]-SB-258585 – a selec- Li Z, Huang M, Prus AJ, Dai J, Meltzer HY: 5-HT6 re- tive antagonist radioligand for 5-HT6 receptors. ceptor antagonist SB-399885 potentiates haloperidol and Br J Pharmacol, 1999, 127, 23P. risperidone-induced dopamine efflux in the medial pre- 43. Hirst WD., Moss SF, Bromidge SM, Riley G, Stean TO, frontal cortex or hippocampus. Brain Res, 2007, 1134, Rogers DC, Sunter D et al.: Characterization of 70–78. 60. Lieben CK, Blokland A, Sik A, Sung E, van Nieuwen- SB-399885, a potent and selective 5-HT6 receptor antagonist. Soc Neurosci Meet, 2003, Abstr. 576.7. huizen P, Schreiber R: The selective 5-HT6 receptor antagonist Ro4368554 restores memory performance in 44. Hirst WD, Stean TO, Rogers DC, Sunter D, Pugh P, cholinergic and serotonergic models of memory defi- Moss SF, Bromidge SM et al.: SB-399885 is a potent, se- ciency in the rat. Neuropsychopharmacology, 2005, 30, lective 5-HT6 receptor antagonist with cognitive enhanc- 2169–2179. ing properties in aged rat water maze and novel object rec- 61. Liljequist S, Engel JA: The effects of GABA and benzo- ognition models. Eur J Pharmacol, 2006, 553, 109–119. diazepine receptor antagonists on the anti-conflict ac- 45. Hogg S, Dalvi A: Acceleration of onset of action in tions of diazepam or ethanol. Pharmacol Biochem Be- schedule-induced polydipsia: combinations of SSRI and hav, 1984, 21, 521–525. 5-HT1A and 5-HT1B receptor agonists. Pharmacol Bio- 62. Lindner MD, Hodges Jr DB, Hogan JB, Orie AF, Corsa chem Behav, 2004, 77, 69–75. JA, Barten DM, Polson C et al.: An assessment of the ef- 46. Horowitz JM, Goyal A, Ramdeen N, Hallas BH, Horow- fects of serotonin 6 (5-HT6) receptor antagonists in ro- itz AT, Torres G: Characterization of fluoxetine plus ol- dent models of learning. J Pharmacol Exp Ther, 2003, anzapine treatment in rats: a behavior, endocrine, and 307, 682–691. immediate-early gene expression analysis. Synapse, 63. Loiseau F, Dekeyne A, Millan MJ: Pro-cognitive effects 2003, 50, 353–364. of 5-HT6 receptor antagonists in the social recognition 47. Integrity Database, Prous Science, 2007. (http://www.in- procedure in rats: implication of the frontal cortex. Psy- tegrity.prous.com). chopharmacology, 2008, 196, 93–104. 48. Investigational Drugs Database IDdb3, Thomson Current 64. Malberg JE, Schechter LE: Increasing hippocampal neu- Drugs, 2007. (http://www.iddb3.com). rogenesis: a novel mechanism for antidepressant drugs. 49. Janowsky DS, el-Yousef MK, Davis JM, Sekerke HJ: Curr Pharm Des, 2005, 11, 145–155. A cholinergic-adrenergic hypothesis of mania and de- 65. Marcos B, Gil-Bea FJ, Hirst WD, Garcia-Alloza M, pression. Lancet, 1972, 2, 632–635. Ramirez MJ: Lack of localization of 5-HT6 receptors on 50. Jayarajan P, Vishwakarma SL, Pendharkar VV, Sharma cholinergic neurons: implication of multiple neurotrans- DC, Shirsath VS, Nirogi RVS: Effect of selective 5-HT6 mitter systems in 5-HT6 receptor-mediated acetylcholine receptor antagonists in rodent models of learning. Soc release. Eur J Neurosci, 2006, 24, 1299–1306. Neurosci Meeting, 2005, Abstr. 881.8. 66. Millan MJ: The neurobiology and control of anxious 51. Kohen R, Fashingbauer LA, Heidmann DE, Guthrie CR, states. Prog Neurobiol, 2003, 70, 83–244. Hamblin MW: Cloning of the mouse 5-HT6 serotonin re- 67. Mitchell ES, Neumaier JF: 5-HT6 receptors: a novel tar- ceptor and mutagenesis studies of the third cytoplasmatic get for cognitive enhancement. Pharmacol Ther, 2005, loop. Brain Res Mol Brain Res, 2001, 90, 110–117. 108, 320–333. 52. Kohen R, Metcalf MA, Khan N, Druck T, Huebner K, 68. Monsma FJ, Shen Y, Ward RP, Hamblin MW, Sibley Lachowicz JE, Meltzer HY et al.: Cloning, characteriza- DR: Cloning and expression of a novel serotonin recep- tion, and chromosomal localization of a human 5-HT6 tor with high affinity for tricyclic psychotropic drugs. serotonin receptor. J Neurochem, 1996, 66, 47–56. Mol Pharmacol, 1993, 43, 320–327.

Pharmacological Reports, 2010, 62, 564577 575 69. Murphy BE, Dhar V, Ghadirian AM, Chouinard G, Kel- tive 5-HT6 receptor antagonist. J Med Chem, 2003, 46, ler R: Response to steroid suppression in major depres- 1273–1276. sion resistant to antidepressant therapy. J Clin Psycho- 84. Roberts JC, Reavill C, East SZ, Harrison PJ, Patel S, pharmacol, 1991, 11, 121–126. Routledge C, Leslie RA: The distribution of 5-HT6 re- 70. Nestler EJ, Barrot M, DiLeone RJ, Eisch AJ, Gold SJ, ceptors in rat brain: an autoradiographic binding study Monteggia LM: Neurobiology of depression. Neuron, using the radiolabelled 5-HT6 receptor antagonist 2002, 34, 13–25. [125I]SB-258585. Brain Res, 2002, 934, 49–57. 71. Neumeister A, Wood S, Bonne O, Nugent AC, Lucken- 85. Rogers DC, Hagan JJ: 5-HT6 receptor antagonists en- baugh DA, Young T, Bain EE et al.: Reduced hippocam- hance retention of a water maze task in the rat. Psycho- pal volume in unmedicated, remitted patients with major pharmacology, 2001, 158, 114–119. depression versus control subjects. Biol Psychiatry, 86. Routledge C, Bromidge SM, Moss SF, Price GW, Hirst 2005, 57, 935–937. W, Newman H, Riley G et al.: Characterisation of 72. O’Brien JT, Lloyd A, McKeith I, Gholkar A, Ferrier N: SB-271046-A: a potent, selective and orally active A longitudinal study of hippocampal volume, cortisol 5-HT6 receptor antagonist. Br J Pharmacol, 2000, 130, levels, and cognition in older depressed subjects. Am 1606–1612. J Psychiatry, 2004, 161, 2081–2090. 87. Ruat M, Traiffort E, Arrang JM, Tardivel-Lacombe J, 73. Otano A, Frechilla D, Cobreros A, Cruz-Orive LM, In- Diaz J, Leurs R, Schwartz JC: A novel rat serotonin sausti A, Insausti R, Hamon M, Del Rio J: Anxiogenic- (5-HT6) receptor: molecular cloning, localization and like effects and reduced stereological counting of immu- stimulation of cAMP accumulation. Biochem Biophys nolabelled 5-hydroxytryptamine6 receptors in rat nucleus Res Commun, 1993, 193, 268–276. accumbens by antisense oligonucleotides. Neuroscience, 88. Russo-Neustadt AA, Chen MJ: Brain-derived neuro- 1999, 92, 1001–1009. trophic factor and antidepressant activity. Curr Pharm 74. Paul IA, Skolnick P: Glutamate and depression: Clinical Des, 2005, 11, 1495–1510. and preclinical studies. Ann NY Acad Sci, 2003, 1003, 89. Sanacora G, Gueorguieva R, Epperson CN, Wu YT, Ap- 250–272. pel M, Rothman DL, Krystal JH, Mason GF: Subtype- 75. Pei Q, Zetterström TSC, Sprakes ME, Tordera R, Sharp T: specific alterations of g-aminobutyric acid and glutamate Antidepressant drug treatment induces Arc gene expres- in patients with major depression. Arch Gen Psychiatry, sion in the rat brain. Neuroscience, 2003, 121, 975–982. 2004, 61, 705–713. 76. Petty F: GABA and mood disorders: a brief review and 90. Sanacora G, Mason GF, Krystal JH: Impairment of GA- hypothesis. J Affect Disord, 1995, 34, 275–281. BAergic function in depression: new insights from neu- 77. Pierce PA, Xie G-X, Levine JD, Peroutka SJ: roimaging. Crit Rev Neurobiol, 2000, 14, 23–45. 5-Hydroxytryptamine receptor subtype messenger RNAs 91. Sarter M, Parikh V: Choline transporters, cholinergic in rat peripheral sensory and sympathetic ganglia: a po- transmission and cognition. Nat Rev Neurosci, 2005, 6, lymerase chain reaction study. Neuroscience, 1996, 70, 48–56. 553–559. 92. Saylam C, Ucerler H, Kitis O, Ozand E, Gonul AS: Re- 78. Plassat JL, Amlaiky N, Hen R: Molecular cloning of duced hippocampal volume in drug-free depressed pa- a mammalin serotonin receptor that activates adenylate tients. Surg Radiol Anat, 2006, 28, 82–87. cyclase. Mol Pharmacol, 1993, 44, 229–236. 93. Schechter LE, Lin Q, Smith DL, Zhang G, Shan Q, Platt 79. Popik P, Kozela E, Krawczyk M: Nicotine and nicotinic B, Brandt MR et al.: Neuropharmacological profile of receptor antagonists potentiate the antidepressant-like ef- novel and selective 5-HT6 receptor agonists: fects of imipramine and citalopram. Br J Pharmacol, WAY-181187 and WAY-208466. Neuropsychopharma- 2003, 139, 1196–1202. cology, 2008, 33, 1323–1335. 80. Porsolt RD, Bertin A, Blavet N, Deniel M, Jalfre M: Im- 94. Sebben M, Ansanay H, Bockaert J, Dumuis A: 5-HT6 re- mobility induced by forced swimming in rats: effects of ceptors positively coupled to adenylyl cyclase in striatal agents which modify central catecholamine and sero- neurons in culture. NeuroReport, 1994, 5, 2553–2557. tonin activity. Eur J Pharmacol, 1979, 57, 201–210. 95. Sherman AD, Allers GL: Relationship between regional 81. Przegaliñski E, Tatarczyñska E, Dereñ-Weso³ek A, distribution of imipramine and its effect on learned help- Chojnacka-Wójcik E: Antidepressant-like effects of lessness in the rat. Neuropharmacology, 1980, 19, 159–162. a partial agonist at strychnine-insensitive glycine recep- 96. Shimizu E, Hashimoto K, Okamura N, Koike K, Ko- tors and a competitive NMDA receptor antagonist. Neu- matsu N, Kumakiri C, Nakazato M et al.: Alterations of ropharmacology, 1997, 36, 31–37. serum levels of brain-derived neurotrophic factor 82. Pullagurla M, Bondareva T, Young R, Glennon RA: (BDNF) in depressed patients with or without antide- Modulation of the stimulus effects of (+)amphetamine by pressants. Biol Psychiatry, 2003, 54, 70–75. the 5-HT6 antagonist MS-245. Pharmacol Biochem Be- 97. Shirayama Y, Chen AC, Nakagawa S, Russell DS, Du- hav, 2004, 78, 263–268. man RS: Brain-derived neurotrophic factor produces an- 83. Riemer C, Borroni E, Levet-Trafit B, Martin JR, Poli S, tidepressant effects in behavioural models of depression. Porter RH, Bos M: Influence of the 5-HT6 receptor on J Neurosci, 2002, 22, 3251–3261. acetylcholine release in the cortex: pharmacological 98. Shirazi-Southall S, Rodriguez DE, Nomikkos GG: Ef- characterization of 4-(2-bromo-6-pyrrolidin-1- fects of typical and atypical antipsychotics and receptor ylpyridine-4-sulfonyl)phenylamine, a potent and selec- selective compounds on acetylcholine efflux in the hip-

576 Pharmacological Reports, 2010, 62, 564577 Role of 5-HT$ receptors in depression and anxiety Anna Weso³owska

pocampus of the rat. Neuropsychopharmacology, 2002, 110. Weso³owska A, Nikiforuk A: The selective 5-HT6 recep- 26, 583–594. tor antagonist SB-399885 enhances anti-immobility ac- 99. Shytle RD, Silver AA, Lukas RJ, Newman MB, Sheehan tion of antidepressants in rats. Eur J Pharmacol, 2008, DV, Sanberg PR: Nicotinic acetylcholine receptors as tar- 582, 88–93. gets for antidepressants. Mol Psychiatry, 2002, 7, 525–535. 111. Weso³owska A, Nikiforuk A, Stachowicz K: Anxiolytic- 100. Stefujl J, Jernej B, Cicin-Sain L, Rinner I, Schauenstein like and antidepressant-like effects produced by the se- K: mRNA expression of serotonin receptors incells of lective 5-HT6 receptor antagonist SB-258585 after intra- the immune tissues of the rat. Brain Behav Immun, 2000, hippocampal administration to rats. Behav Pharmacol, 14, 219–224. 2007, 18, 439–446. 101. Svenningsson P, Tzavara ET, Qi H, Carruthers R, Wit- 112. Winter H, Irle E: Hippocampal volume in adult burn pa- kins JM, Nomikos GG, Greengard P: Biochemical and tients with and without posttraumatic stress disorder. Am behavioral evidence for antidepressant-like effects of 5-HT6 J Psychiatry, 2004, 16, 2194–2200. receptor stimulation. J Neurosci, 2007, 27, 4201–4209. 113. Woolley ML, Bentley JC, Sleight AJ, Marsden CA, Fone 102. Videbech P, Ravnkilde B: Hippocampal volume and de- KC: A role for 5-ht6 receptors in retention of spatial pression: a meta-analysis of MRI studies. Am J Psychia- learning in the Morris water maze. Neuropharmacology, try, 2004, 161, 1957–1966. 2001, 41, 210–219. 103. Vogt IR, Shimron-Abarbanell D, Neidt H, Erdmann J, Ci- 114. Woolley ML, Marsden CA, Fone KCF: 5-HT6 receptors. chon S, Schulze TG: Investigation of the human serotonin Curr Drug Targets CNS Neurol Disord, 2004, 3, 59–79. 6 [5-HT6] receptor gene in bipolar affective disorder and 115. Woods A, Smith C, Szewczak M, Dunn RW, Cornfeldt schizophrenia. Am J Med Genet, 2000, 96, 217–221. M, Corbett R: Selective serotonin re-uptake inhibitors 104. Vythilingam M, Vermetten E, Anderson GM, Lucken- decrease schedule-induced polydipsia in rats: a potential baugh D, Anderson ER, Snow J, Staib LH et al.: Hippo- model for obsessive compulsive disorder. Psychophar- campal volume, memory, and cortisol status in major de- macology, 1993, 112, 195–198. pressive disorder: effects of treatment. Biol Psychiatry, 116. Yau JLW, Noble J, Widdowson J, Seckl JR: Impact of 2004, 56, 101–112. adrenalectomy on 5-HT6 and 5-HT7 receptor gene ex- 105. Ward RP, Dorsa DM: Colocalization of serotonin recep- pression in the rat hippocampus. Mol Brain Res, 1997, tor subtypes 5-HT2A, 5-HT2C, and 5-HT6 with neuropep- 45, 182–186. tides in rat striatum. J Comp Neurol, 1996, 370, 405–414. 117. Yilmaz A, Schulz D, Aksoy A, Canbeyli R: Prolonged 106. Ward RP, Hamblin MW, Lachowicz JE, Hoffman BJ, Si- effect of an anesthetic dose of ketamine on behavioral bley DR, Dorsa DM: Localization of serotonin subtype 6 despair. Pharmacol Biochem Behav, 2002, 71, 341–344. receptor messenger RNA in the rat brain by in situ hybridi- 118. zation histochemistry. Neuroscience, 1995, 64, 1105–1111. Yoshioka M, Matsumoto M, Togashi H, Mori K, Saito H: 107. Weso³owska A: Study into a possible mechanism respon- Central distribution and function of 5-HT6 receptor sub- sible for the antidepressant-like activity of the selective type in the rat brain. Life Sci, 1998, 62, 1473–1477. 119. 5-HT6 receptor antagonist SB-399885 in rats. Pharmacol Zarate CA, Singh JB, Carlson PJ, Brutsche NE, Ameli R, Rep, 2007, 59, 664–671. Luckenbaugh MA, Charney DS, Manji HK: A random- 108. Weso³owska A: The anxiolytic-like effect of the selective ized trial of an N-methyl-D-aspartate antagonist in 5-HT6 receptor antagonist SB-399885: the impact of treatment-resistant major depression. Arch Gen Psychia- benzodiazepine receptors. Eur J Pharmacol, 2008, 580, try, 2006, 63, 856–864. 355–260. 109. Weso³owska A, Nikiforuk A: Effects of the brain- penetrant and selective 5-HT6 receptor antagonist SB-399885 in animal models of anxiety and depression. Received: Neuropharmacology, 2007, 52, 1274–1283. October 19, 2009; in revised form: December 4, 2009.

Pharmacological Reports, 2010, 62, 564577 577