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Research Paper Article de recherche Return to September 5, 2000 Table of Contents Effects of sustained (±) administration on neurotransmission in rats

Nasser Haddjeri, PhD; Pierre Blier, MD, PhD

Haddjeri — Centre de recherche en sciences neurologiques, Université de Montréal, Montreal, Que.; Blier — Department of Psychiatry, Brain Institute, University of Florida, Gainesville, Fla.

β Objective: Given reports that (±)pindolol, a -–5-HT1A/1B antagonist, accelerates the onset of the therapeutic effect of certain in major depression, the aim of this study was to assess the effect of sustained (±)pindolol administration on the sensitivity of pre- and postsynaptic

5-HT1A receptors, terminal 5-HT1B autoreceptors and on overall 5-HT neurotransmission. Design: Prospective animal study. Animals: Sprague-Dawley rats. Outcome measures: Modifications of the sen-

sitivity of somatodendritic and postsynaptic 5-HT1A receptors using in vivo electrophysiological paradigms in animals treated with vehicle or (±)pindolol (20 mg/kg/day, subcutaneously) through osmotic minipumps for 2 weeks. Results: (±)Pindolol attenuated the suppressant effect of the 5-HT autoreceptor lysergic acid diethylamide (LSD) on the firing activity of 5-HT neurons, suggesting that (±)pindolol antago-

nized somatodendritic 5-HT1A autoreceptors in the dorsal raphe nucleus. However, following a 2-day washout period, the suppressant effect of LSD was still attenuated, indicating rather a desensitization of 5-

HT1A autoreceptors had occurred. In the CA3 region of the dorsal hippocampus, (±)pindolol treatment did

not modify the responsiveness of postsynaptic 5-HT1A receptors to microiontophoretic applications of 5- HT. Moreover, such a treatment modified neither the effectiveness of the electrical stimulation of 5-HT

fibers nor the function of terminal 5-HT autoreceptors. Finally, the administration of the selective 5-HT1A WAY 100635 (100 µg/kg, intravenously) did not increase the firing activity of dorsal

hippocampus CA3 pyramidal neurons in rats treated with (±)pindolol, thus failing to reveal the enhanced

tonic activation of postsynaptic 5-HT1A receptors associated with major classes of antidepressant treat- ments. Conclusion: Prolonged administration of (±)pindolol by itself is not sufficient to enhance overall 5-HT neurotransmission; pindolol should therefore not be endowed with intrinsic antidepressant activity.

Although pindolol is capable of antagonizing the 5-HT1A autoreceptor upon the initiation of a 5-HT reup-

take-blocker treatment, it also induces a desensitization of this 5-HT1A autoreceptor, which could explain why patients do not relapse upon its discontinuation when they continue taking a 5-HT reuptake blocker.

Objectif : Des études cliniques ont récemment démontré que le (±)pindolol, un antagoniste des récep- β teurs -adrenergic/5-HT1A/1B, accélérait la réponse thérapeutique de certains médicaments antidépresseurs chez des patients atteints de dépression majeure. Le but de la présente étude était d’évaluer les effets d’une

administration prolongée de (±)pindolol sur la sensibilité des récepteurs pré et postsynaptiques 5-HT1A,

Correspondence to: Dr. Pierre Blier, Department of Psychiatry, Brain Institute, University of Florida, PO Box 100256, Gainesville FL 32610; fax 352 392-2579 Medical subject headings: antidepressant agents; depression; lysergic acid diethylamide; pindolol; rats, Sprague-Dawley; receptors, serotonin

J Psychiatry Neurosci 2000;25(4):378-88.

Submitted July 19, 1999 Accepted Apr. 5, 2000 © 2000 Canadian Medical Association

378 Revue de psychiatrie et de neuroscience Vol. 25, no 4, 2000 (±)Pindolol and 5-HT neurotransmission

celle des autorécepteurs 5-HT1B terminaux ainsi que sur la neurotransmission serotoninèrgique du cerveau de rat. Conception : Étude prospective sur des animaux. Animaux : Rats males Sprague-Dawley. Mesures des résul-

tats : Modifications de la sensibilité des récepteurs 5-HT1A somatodendritiques et postsynaptiques en utilisant des méthodes électrophysiologiques in vivo chez des rats contrôles et traités par le (±)pindolol pour une période de 2 semaines à une dose de 20 mg/kg/jour (s.c.) en utilisant des mini-pompes osmotiques. Résultats : Un traite- ment à long-terme avec du (±)pindolol a atténué l’effet inhibiteurs du LSD, un agoniste des autorecepteurs 5-HT, sur la fréquence de décharge des neurones 5-HT du noyau du raphé dorsal, ce qui suggère un antagonisme des

autorécepteurs somatodendritiques 5-HT1A. Toutefois, après une période d’écimination de 2 jours, l’effet inhibi-

teur du LSD était toujours atténué, ce qui indique plutôt une désensibilisation des autorécepteurs 5-HT1A. Dans

la région CA3 de l’hippocampe dorsal, des applications microiontophorétiques de 5-HT ont révelé que le traite-

ment au (±)pindolol ne modifiait pas les réponses des récepteurs postsynaptiques 5-HT1A. De plus, un tel traite- ment ne changeait ni l’efficacité des stimulations électriques des fibres 5-HT ni la fonction autorecepteurs 5-HT terminaux. Finalement, l’administration intraveineuse du WAY 100635 (100 µg/kg), un antagoniste sélectif des

récepteurs 5-HT1A, n’augmentait pas la fréquence de décharge des neurones des neurones pyramidaux CA3 de l’hippocampe dorsal chez des traités par le (±)pindolol, ne dévoilant pas une augmentation de l’activation tonique

des récepteurs postsynaptiques 5-HT1A, comme c’est le cas pour les traitements antidépresseurs. Conclusion : Ces résultats indiquent qu’une administration prolongée de (±)pindolol en soi n’est pas suffisante pour augmenter la neurotransmission 5-HT et que cet agent serait dépourvu d’activité antidépressive. Bien que le pindolol soit

capable d’antagoniser les autorécepteurs 5-HT1A dès le début de l’administration soutenue d’un bloqueur de la recapture de 5-HT, il produit également leur désensibilisation. Ceci expliquerait le fait que les patients ne

Introduction SSRIs on extracellular 5-HT concentration by prevent-

ing the activation of somatodendritic 5-HT1A autore- Although the pathophysiology of major depression has ceptors.14,15 Paradoxically, however, the active enan- not been elucidated, there is a growing body of evi- tiomer, (–)pindolol, possesses some 16 dence that supports the involvement of the activity at human 5-HT1A receptors, and it has been (5-HT) system in the therapeutic effect of antidepres- shown that its intravenous administration can sant treatments.1,2 Various classes of antidepressant decrease the firing activity of dorsal raphe 5-HT neu- treatments enhance 5-HT neurotransmission with a rons in freely-moving cats and anesthetised rats.17–19 time course that is consistent with their delayed thera- These results appear incompatible with a potentiation peutic effect. This would be mediated via different of extracellular 5-HT levels in postsynaptic structures mechanisms such as postsynaptic sensitization to 5-HT, resulting from 5-HT1A autoreceptor blockade by pin- desensitization of the somatodendritic or terminal 5-HT dolol when given in combination with SSRIs.14 α autoreceptors, or both, or a desensitization of 2-adren- Nevertheless, a biphasic effect of pindolol on the firing ergic heteroreceptors located on 5-HT terminals.1,3 activity of 5-HT neurons has been reported recently. A novel strategy to accelerate the antidepressant At low doses, (±)pindolol acts as a somatodendritic 5- response of acting on 5-HT neurons is to HT1A autoreceptor antagonist, but at a higher dose, it combine the arylalkylamine (±)pindolol, a β-adrener- decreases the firing rate of 5-HT neurons, in part by gic–5-HT1A/1B receptor antagonist, with a selective sero- attenuating the tonic excitatory input from noradren- tonin (SSRI), a monoamine oxidase ergic neurons to 5-HT neurons.19 inhibitor (MAOI) or a 5-HT1A receptor agonist from the To mimic as much as possible the clinical condition in beginning of a treatment to obtain a more rapid onset which pindolol has been given 3 times daily in double- of antidepressant action.4 Although negative results blind trials, we treated rats with (±)pindolol for 2 weeks have been reported by one group,5,6 there are now 7 and used in vivo electrophysiological paradigms in the placebo-controlled studies showing that this approach rat dorsal raphe and dorsal hippocampus to assess the can result in a rapid onset of action and, in some cases, effect of sustained treatment with a constant infusion of a greater efficacy in treating major depression.7–13 (±)pindolol on the sensitivity of pre- and postsynaptic

Accordingly, preclinical studies have shown that 5-HT1A receptors and terminal 5-HT1B autoreceptors and (±)pindolol can potentiate the enhancing effect of on overall 5-HT neurotransmission.

Vol. 25, no 4, 2000 Journal of Psychiatry & Neuroscience 379 Haddjeri and Blier

Methods mean of firing rate of neurons from about 1 to 2 minutes before the intravenous administration of the drugs and Male Sprague-Dawley rats (Charles River Laboratories, a mean after, until a plateau was reached. A percentage St-Constant, Que.) weighing 250 to 300 g were kept under change was then calculated. standard laboratory conditions (12-hour light–dark cycle with free access to food and water). Groups of 12 to 22 rats Recordings from CA3 dorsal hippocampus were treated for 2 weeks with (±)pindolol (20 pyramidal neurons mg/kg/day) or vehicle (saline), delivered by osmotic minipumps (ALZA, Palo Alto, Calif.) inserted subcuta- Recording and microiontophoresis were performed neously. The rats were tested with the minipumps in with 5-barrelled glass micropipettes broken back to place, and some were also tested after a 2-day washout 8–12 µm under microscopic control (ASI Instruments, period to allow for elimination. Animals were anes- Warren, Mich.). The central barrel, used for extracellu- thetized with chloral hydrate (400 mg/kg, given lar unitary recordings, was filled with a 2 M NaCl solu- intraperitoneally) for recordings, and supplemental doses tion. The side barrels contained the following solutions: were given to maintain constant anaesthesia and to pre- 5-HT creatinine sulphate (2 mM in 200 mM NaCl, pH vent any nociceptive reaction to a tail pinch. 4), quisqualate (1.5 mM in 200 mM NaCl, pH 8) and 2 M NaCl used for automatic current balancing. Rats were Drugs mounted in a stereotaxic apparatus, and the microelec-

trodes were lowered into the CA3 region of the dorsal (±)Pindolol, WAY 100635 ((N-{2-[4(2-methoxyphenyl)- hippocampus (4.2 mm lateral and 4.2 mm anterior to 1-piperazinyl]ethyl}-N-(2-pyridinyl)cyclohexanecar- lambda). Pyramidal neurons were identified by their boxamide trihydroxychloride), 8-OH-DPAT, quisqualic large amplitude (0.5–1.2 mV) and long-duration (0.8–1.2 acid and (see Fig. 1) were purchased from ms) simple spikes that alternated with complex spike Research Biochemicals (Natick, Mass.), methiothepin discharges.21 Since most hippocampal pyramidal neu- maleate from Hoffmann-La Roche Ltd (Etobicoke, Ont.) rons are not spontaneously active under chloral hydrate and lysergic acid diethylamide (LSD) from Health anaesthesia, a leak or a small ejection current of Canada (Ottawa, Ont.). The concentrations and doses quisqualate (+1 nA to –4 nA) was used to activate them used were chosen on the basis of previous successful within their physiological firing range (10–15 Hz).22 experiments carried out in our and other laboratories. Neuronal responsiveness to the microiontophoretic application of 5-HT was assessed by determining the Extracellular recordings of dorsal raphe 5-HT neurons number of spikes suppressed per nanoampere of ejec- tion current. The duration of the microiontophoretic Extracellular recordings were performed with single- application of the agonist was 50 seconds, and the same barrelled glass micropipettes preloaded with fibreglass ejection current was used before and after the intra- filaments to facilitate filling. The tip was broken back to venous injection of the selective 5-HT1A receptor antag- 2–4 µm and filled with a 2 M NaCl solution. Rats were onist WAY 100635 (100 µg/kg). Finally, to assess the placed in a stereotaxic frame and a burr hole was drilled degree to which postsynaptic 5-HT1A receptor activation on the midline 1 mm anterior to lambda. Dorsal raphe was inhibiting CA3 pyramidal neuron firing activity, the 5-HT neurons were encountered over a distance of 1 ejection current of quisqualate was reduced to about 5 mm, starting immediately below the ventral border of Hz to lower the firing rate, and WAY 100635 (100 the Sylvius aqueduct. These neurons were identified µg/kg) was injected intravenously. Disinhibition using the criteria of Aghajanian20 (i.e., slow [0.5–2.5 Hz] would best be determined if the neuron was not already and regular firing rate and long-duration [0.8–1.2 ms] firing at high rate. positive action potentials). For the controls and 2-week treated rats, the responsiveness of 5-HT neurons was Electrical activation of afferent 5-HT fibers assessed using an intravenous injection of LSD (10 to the hippocampus µg/kg) and of 8-OH-DPAT (5 µg/kg). In both the dor- sal raphe nucleus and the dorsal hippocampus, the A bipolar electrode (NE-110; David Kopf, Tujunga, change of firing activity was assessed by calculating the Calif.) was implanted on the midline with a 10° back-

380 Revue de psychiatrie et de neuroscience Vol. 25, no 4, 2000 (±)Pindolol and 5-HT neurotransmission

ward angle in the ventromedial tegmentum to stimulate (100 µg/kg, intravenously). A 2-week treatment with the raphe 5-HT fibres (1 mm anterior to lambda and 8.3 (±)pindolol (20 mg/kg/day, given subcutaneously) sig- mm below the cortical surface). A stimulator (S8800; Grass Instruments, Quincy, Mass.) delivered 200 square pulses of 0.5 ms at a frequency of 1 Hz and an intensity of 300 µA. The stimulation pulses and the firing activity of the hippocampal neuron recorded were fed to an IBM- PC computer equipped with a Tecmar interface, and peristimulus time histograms were generated to deter- mine the duration of suppression of firing activity of the

CA3 pyramidal neuron, measured as absolute silence value (SIL) in milliseconds. This value was obtained by dividing the total number of events suppressed follow- ing the stimulation by the mean frequency of firing of the 23 recorded neuron. The CA3 region of the hippocampus receives extensive innervation from 5-HT neurons of the dorsal and median raphe nuclei.24 This brief suppressant effect (about 50 ms with a stimulation of 300 µA), result- ing from the electrical stimulation of the ascending 5-HT pathway, is due to the release of 5-HT for each impulse applied to the 5-HT axons and is mediated by postsy- 23 naptic 5-HT1A receptors. Thus, for the CA3 pyramidal neurons tested in control rats, the effect of the stimulation of the ascending 5-HT pathway was first determined before and after an intraperitoneal injection of (±)pin- dolol (20 mg/kg) and then after an intravenous injection of the 5-HT autoreceptor antagonist methiothepin (1 mg/kg). To determine the possible changes of the responsiveness of the terminal 5-HT autoreceptors, in each group, 2 series of stimulations (1 Hz and 5 Hz) were carried out while recording the same neuron, since it has previously been demonstrated in vitro and in vivo that the activation of terminal 5-HT autoreceptors decreases the release of 5-HT, and this reduction is enhanced by increasing the frequency of the stimulation.23,25,26 Fig. 1: Integrated firing rate histograms of dorsal raphe 5- Results HT neurons showing their response to lysergic acid diethylamide (LSD; 10 µg/kg), WAY 100635 and prazosin Effect of sustained administration of (±)pindolol on in control rats (A), rats treated with (±)pindolol (20 firing activity of dorsal raphe 5-HT neuron mg/kg subcutaneously) for 14 days (B), and rats treated rat (±)pindolol for 14 days followed by a 2-day washout period (C). The α -adrenoceptor antagonist prazosin was It has been demonstrated that 5-HT neuron firing activ- 1 used to suppress 5-HT neuron firing activity through the ity is dependent on the activation of somatodendritic 5- blockade of this excitatory noradrenergic receptor. The 27 HT1A autoreceptors. Accordingly, and as illustrated in degree of suppression of the firing of 5-HT neurons by Fig. 1A, the intravenous administration of the 5-HT LSD (% and standard errors of the means [SEM]) in con- µ autoreceptor agonist LSD (10 g/kg, given intra- trols and rats treated with (±)pindolol is summarized in venously) suppressed the firing activity of dorsal raphe D. The numbers within the histograms indicate the num- 5-HT neurons, and this suppressant effect was reversed ber of neurons or rats tested for each treatment (*p < by the selective 5-HT1A receptor antagonist WAY 100635 0.05; unpaired Student’s t-test).

Vol. 25, no 4, 2000 Journal of Psychiatry & Neuroscience 381 Haddjeri and Blier

nificantly attenuated (by 70%) the suppressant effect of ronal firing, and this was reversed by the subsequent LSD (Figs. 1B and 1D). Morever, in rats treated for 14 intravenous administration of 100 µg/kg of WAY days with (±)pindolol followed by a 2-day washout 100635 (data not shown). period, the suppressant effect of a dose of LSD (10 µg/kg, intravenously, n = 7) was still attenuated by 80% Effect of sustained administration of (±)pindolol on

(Figs. 1C and 1D), indicating a desensitization of soma- CA3 dorsal hippocampus pyramidal neuron respon- todendritic 5-HT1A autoreceptors. In contrast, the 2- siveness to 5-HT week treatment with (±)pindolol did not modify the suppressant effect of the more selective 5-HT1A receptor The microiontophoretic application of 5-HT onto rat agonist 8-OH-DPAT, as was previously observed with dorsal hippocampus pyramidal neurons suppresses fir- its acute administration.19 Both in control (n = 6) and ing activity, and this effect is mediated by postsynaptic 27–30 treated rats (n = 5), the intravenous administration of 5 5-HT1A receptors. For all CA3 pyramidal neurons test- µg/kg of 8-OH-PAT completely suppressed 5-HT neu- ed in this study, 5-HT (10 nA) markedly reduced firing

Fig. 2: Integrated firing rate histograms of dorsal hippocampus CA3 pyramidal neurons showing their responsiveness to the microiontophoretic application of 5-HT before and after the intravenous administration of WAY 100635 (100 µg/kg intravenously) in a control (A) and a rat treated for 14 days with (±)pindolol (B). These neurons were activated with quisqualate ejection current. Horizontal bars indicate the duration of the applications for which the current was given in nanoampers. In C, the responsiveness to 5-HT is expressed as the mean number (and SEM) of spikes suppressed per nanoampere. Note the significant difference in the effectiveness of 5-HT to suppress firing activity after WAY 100635 administration in each group. (*p < 0.05, unpaired Student’s t-test). In D, the responsiveness to WAY 100635 is expressed as percent change (and SEM) of the firing of CA3 pyramidal neurons (no significant differences were observed). The num- bers within the histograms in C and D indicate the number of neurons tested.

382 Revue de psychiatrie et de neuroscience Vol. 25, no 4, 2000 (±)Pindolol and 5-HT neurotransmission

activity (Figs. 2A and 2B). This suppressant effect did not alter the shape of the action potentials. As illustrat- ed in Fig. , long-term treatment with (±)pindolol did not modify the suppressant effect of microiontophoret- ically applied 5-HT (number of spikes suppressed per nanoampere of 5-HT: controls, 114 (standard error of the mean [SEM] 6), n = 15; (±)pindolol-treated rats, 104 [SEM 5], n = 14). WAY 100635 (100 µg/kg, intravenous- ly) had an antagonistic effect on the responsiveness of postsynaptic 5-HT1A receptors, and the significant reduction of the suppressant effect of 5-HT was similar for both controls and rats treated with (±)pindolol (by 73% and 72% respectively; Fig. 2C). Finally, WAY 100635 (100 µg/kg, intravenously) did not modify the quisqualate-activated firing activity of CA3 pyramidal neurons in control rats or in (±)pindolol-treated rats (t = 1.07, p = 0.3; Fig. 2D).

Electrical stimulation of afferent 5-HT fibers to the hippocampus

To determine whether treatment with (±)pindolol could modulate the in vivo release of 5-HT (per electrical impulse reaching 5-HT terminals), its capacity to modi- fy the duration of the firing suppression produced by the electrical activation of the ascending 5-HT pathway was examined. As illustrated in Fig. 3B, long-term (±)pindolol treatment (20 mg/kg, subcutaneously for 14 days) did not modify the effectiveness of the stimula- tion of the 5-HT pathway (SIL value in controls, 41 [SEM 3] ms and in (±)pindolol-treated rats, 43 [SEM 4] ms; t = 0.37, p > 0.7). The responsiveness of terminal 5- HT autoreceptors was evaluated by increasing the fre- quency of the stimulation from 1 Hz to 5 Hz. In control rats, 5-Hz stimulations were 37% less effective than 1- Hz stimulations (Figs. 3A and 3B). The decremental Fig. 3: (A) Peristimulus time histograms illustrating the effect obtained by increasing the frequency from 1 Hz to effect of the electrical stimulation of the ascending 5-HT 5 Hz was similar in rats treated with (±)pindolol and pathway at the level of the ventromedial tegmentum on that observed in the controls (41%), indicating the func- the firing activity of dorsal hippocampus CA3 pyramidal tion of terminal 5-HT autoreceptors was unaltered by neurons at 1 Hz (upper) and 5 Hz (lower) in a control rat. pindolol treatment (Fig. 3B). Note the reduction of the absolute silence value (SIL) in milliseconds, which represents the duration of suppres- To ensure that the 5-HT antagonistic property of 1B sion of firing, by increasing frequency. (B) Effect of (±)pin- (±)pindolol was detectable in the present paradigm, the dolol treatment on the efficacy of stimulating the ascend- daily amount of the drug infused over a 24-hour period ing 5-HT pathway and of an increase in frequency from 1 was injected intraperitoneally in a single bolus. The SIL to 5 Hz (*p < 0.05, paired Student’s t-test). The respon- value after (±)pindolol administration was increased by siveness of terminal 5-HT autoreceptors in rats treated 16% (t = 3.56, p < 0.05, Figs. 4B and 4D), and the decre- with (±)pindolol was not significantly different than that mental effect of the 5-HT pathway stimulation was also of control rats. The numbers within the histograms indi- no longer present (Fig. 4D), confirming a blockade of cate the number of neurons tested.

Vol. 25, no 4, 2000 Journal of Psychiatry & Neuroscience 383 Haddjeri and Blier

the terminal 5-HT1B autoreceptor by (±)pindolol when administered acutely. However, the subsequent intra- venous injection of the 5-HT autoreceptor antagonist methiothepin (1 mg/kg) further increased, by 36%, the duration of firing suppression induced by the electrical stimulation of the ascending 5-HT pathway at 1 Hz (t = 2.9, p < 0.05, Figs. 4C and 4D).

Discussion

The sustained administration of (±)pindolol for 2 weeks prevented the suppressant effect of the 5-HT autorecep- tor agonist LSD, suggesting that (±)pindolol antago-

nizes somatodendritic 5-HT1A autoreceptors in the dor- sal raphe nucleus. However, following a 2-day washout period, the suppressant effect of LSD was still attenuat- ed, indicating rather a desensitization of somatoden-

dritic 5-HT1A autoreceptors. Accordingly, there is some evidence suggesting that (±)pindolol may act as a par- 18 tial 5-HT1A autoreceptor agonist.

Pindolol interacts with 5-HT1A receptors with

nanomolar affinity (Ki = 8 nM in recombinant human 5-

HT1A receptors using (–)pindolol and Ki = 13 nM in rat hippocampal membranes using (±)pindolol)16,31,32 and prevents some biological responses mediated via 5- 33–35 HT1A receptor activation. One peculiar characteristic found with this is its preferential activity at the

presynaptic 5-HT1A receptor. In vitro, it has been report- ed that 1 µM (±)pindolol antagonizes the inhibitory effect of on the firing activity of rat dorsal raphe 5-HT neurons.36 In vivo, it has been also shown that a 2-day treatment with (–)pindolol (15 mg/kg/day, subcutaneously) prevents the inhibitory effect of LSD on rat dorsal raphe 5-HT neuronal activity, as well as Fig. 4: Peristimulus time histograms illustrating the effect the reduction of firing activity observed after a 2-day of electrical stimulation of the ascending 5-HT pathway treatment with the SSRI , thus supporting its on the firing activity of dorsal hippocampus CA3 pyrami- antagonistic property for dorsal raphe somatodendritic 14 dal neurons in a control rat before (A) and after (B) the 5-HT1A autoreceptors. However, whether (–)pindolol intraperitoneal administration of (±)pindolol (20 mg/kg) was given using the latter regimen or administered and following the intravenous administration of methio- acutely, it did not antagonize postsynaptic 5-HT1A thepin (1 mg/kg) (C). Each histogram was constructed receptors mediating the suppression of extracellularly from 200 consecutive stimulations (300 µA, 0.5 ms, 1 Hz) recorded firing of dorsal hippocampus CA –CA pyra- with a bin width of 2 ms. The SIL value represents the 1 3 midal neurons,14,37,38 further suggesting the existence of duration of suppression of firing. The mean (and SEM) pharmacologically distinct subpopulations of 5-HT1A duration of suppression of the firing activity of CA3 hip- 29,39,40 pocampus pyramidal neurons for each treatment (at 1 receptors. Nevertheless, in vitro, antagonistic activ- and 5 Hz) is summarized in D (*p < 0.05, paired Student’s ity of (±)pindolol for postsynaptic 5-HT1A receptors on 41 t-test). Note that the decremental effect of the 5-HT CA1–CA3 pyramidal neurons has been reported, and in pathway stimulation was no longer present in rats receiv- vivo (±)pindolol (0.1–1.0 mg/kg, intravenously) fails to ing an acute dose of (±)pindolol. antagonize the suppressant effect of 8-OH-DPAT (10

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µg/kg, intravenously) on the firing activity of dorsal 100635 is the most potent and selective antagonist acting at 17 53 raphe 5-HT neurons in freely moving cats. both pre- and postsynaptic 5-HT1A receptors. In contrast In an attempt to untangle these discrepancies, both to (±)pindolol, WAY 100635 (100 µg/kg, intravenously) LSD and 8-OH-DPAT were used in an earlier study to significantly antagonized the suppressant effect of 5-HT determine the contribution of somatodendritic 5-HT1A microiontophoretically applied onto CA3 pyramidal neu- autoreceptors and of 5-HT1A receptors located on a long- rons, thus showing its capacity to block 5-HT1A receptors feedback loop originating from the forebrain in modify- on the cell body of CA3 pyramidal neurons. In the hip- ing 5-HT neuronal firing in the rat dorsal raphe nucleus. pocampus, however, it has been shown that 5-HT termi- The intravenous administration of 200 µg/kg (±)pin- nals are almost exclusively located in apposition to the dolol, a dose which does not modify 5-HT neuronal fir- dendritic tree of pyramidal neurons54,55 and that the ing rate, prevented only the suppressant effect of LSD on endogenous 5-HT, released by the electrical stimulation of the firing activity of 5-HT neurons.19 As initially pro- the ascending 5-HT pathway, activates these intrasynaptic 27 posed, the difference between the effects of systemic 5-HT1A receptors on dendrites of hippocampus pyramidal injection of LSD and 8-OH-DPAT on 5-HT neuron firing neurons.28,29 It is important to emphasize here that pindolol activity can be explained by the existence of a negative fails to prevent the activation of both the intra- and feedback loop — the intravenous administration of 8- extrasynaptic 5-HT1A receptors located on CA3 pyramidal OH-DPAT at low doses would activate postsynaptic 5- neurons, whereas WAY 10635 appears to antagonize only 30,37 HT1A receptors located on neurons in the medial pre- extrasynaptic 5-HT1A receptors. frontal cortex, which exert a negative feedback influence In contrast to results obtained with SSRIs,23,28 a 2-week on dorsal raphe 5-HT neurons.27,42–44 (±)Pindolol would treatment with (±)pindolol did not modify the effective- thus be unable to antagonize the effect of 8-OH-DPAT ness of electrical stimulation of 5-HT afferents to the on 5-HT neurons, most likely because (±)pindolol may dorsal hippocampus and did not modify the function of not block this postsynaptic 5-HT1A receptor as the 5-HT1A terminal 5-HT autoreceptors (as indicated by the lack of receptors present on pyramidal neurons in the dorsal effect in the 1 Hz–5 Hz stimulation paradigm; Fig. 3). hippocampus (Fig. 2). However, both (±)pindolol and This is surprising because (±)pindolol is endowed with 31,56 WAY 100635 antagonized the suppressant effect of the antagonistic activity at the 5-HT1B autoreceptor. autoreceptor agonist LSD on the firing activity of dorsal Nonetheless, when (±)pindolol was acutely adminis- raphe 5-HT neurons because both antagonists are effec- tered intraperitoneally at a dose of 20 mg/kg, it signifi- 19 tive on the somatodendritic 5-HT1A autoreceptor. cantly increased the effectiveness of the electrical stim-

In the CA3 region of the dorsal hippocampus, a 2- ulation and prevented the decremental effect of the 5- week treatment with (±)pindolol (20 mg/kg/day) did HT pathway stimulation (Fig. 4B). The fact that the sub- not modify the responsiveness of postsynaptic 5-HT1A sequent administration of the terminal 5-HT autorecep- receptors to the microiontophoretic application of 5-HT, tor antagonist methiothepin further prolonged the indicating that the sensitivity of these receptors remains duration of firing suppression suggests that the termi- unchanged following such a treatment. Hence, this nal 5-HT1B autoreceptors were not completely antago- observation provides yet an additional line of evidence nized by the acute dose of (±)pindolol. Consequently, for differential properties of pre- and postsynaptic 5- (±)pindolol appears to be, at best, a weak 5-HT1B autore-

HT1A receptors. For instance, in contrast to their capaci- ceptor antagonist. Nevertheless, when considering the ty to desensitize the 5-HT1A autoreceptors in the dorsal net effect of pindolol on extracellular levels of 5-HT in raphe, it has been shown that the long-term treatment microdialysis studies, there are striking discrepancies. with 5-HT1A receptor or with SSRIs does not Indeed, it was reported that pindolol, administered by alter the responsiveness of 5-HT1A receptors located on itself, locally or systemically, either decreases, enhances 27,40,45–50 CA3 pyramidal neurons. With respect to radioli- or has no effect on 5-HT levels in several brain struc- gand binding studies examining the density of 5-HT1A tures (e.g., frontal cortex, hippocampus, raphe nuclei or 14,18,57–64 receptor sites, long-term treatments with the 5-HT1A striatum) of rats and cats. The difference between receptor agonists or gepirone do not the acute and chronic administration of (±)pindolol in change the density of hippocampal 5-HT1A sites but do the blockade of terminal 5-HT1B autoreceptor activation affect receptor density in the frontal cortex.51,52 (Fig. 3 and Fig. 4) highlights the crucial importance of

Among the 5-HT1A receptor antagonists available, WAY the treatment regimen. The critical role of the concen-

Vol. 25, no 4, 2000 Journal of Psychiatry & Neuroscience 385 Haddjeri and Blier

tration of (±)pindolol is also emphasized by recent stud- References ies using different models in humans (e.g., body tem- perature and cortisol levels) showing that an oral dose 1. Blier P, de Montigny C. Current advances and trends in the treatment of depression. Trends Pharmacol Sci 1994;15:220-6. of 30 mg of (±)pindolol acts as a weak 5-HT1A receptor 2. Maes M, Meltzer HY. The serotonin hypothesis of major agonist but also prevents the full effect of subsequent depression. In: Bloom FE, Kupfer DJ, editors. 35,65,66 Psychopharmacology: the fourth generation of progress New York: administration of potent 5-HT1A agonists. In this study, we did not observe any disinhibition of Raven Press; 1995. p. 933-44. 3. Mongeau R, Blier P, de Montigny C. The serotonergic and the firing activity of CA3 pyramidal neurons in rats treat- noradrenergic systems of the hippocampus: their interactions ed with (±)pindolol for 14 days. These negative results and the effects of antidepressant treatments. Brain Res Rev stand in contrast with previous data showing that repeat- 1997;23:145-95. 4. Blier P, Bergeron R. The use of pindolol to potentiate antide- ed electroconvulsive shock or chronic treatment with pressant . J Clin Psychiatry 1998;59:16-23. antidepressant drugs, such as the antidepressant 5. Berman RM, Darnell AM, Miller HL, Anand A, Charney DS. , the SSRI paroxetine, the selective and Effect of pindolol in hastening response to in the α treatment of major depression: a doubled-blind, placebo-con- reversible MAO-A inhibitor befloxatone, the 2-adrener- trolled trial. Am J Psychiatry 1997;154:37-43. gic antagonist , the 5-HT1A receptor agonist 6. Berman RM, Anand A, Cappiello A, Miller HL, Hu XS, Horen gepirone or the dual 5-HT–noradrenergic reuptake DA, Charney DS. The use of pindolol with fluoxetine in the blocker , enhanced the tonic activation of treatment of major depression: Final results from a double- blind, placebo-controlled trial. 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Int Clin also of inducing its desensitization. The former effect Psychopharmacol 1997;12:81-9. would thus contribute to accelerate the antidepressant 12. Zanardi R, Artigas F, Franchini L, Sforzini L, Gasperini M, Smeraldi E, Perez J. How long should pindolol be associated response to SSRIs by preventing the initial decrease in with paroxetine to improve the antidepressant response? J Clin 5-HT neuron firing activity and thereby increasing 5- Psychopharmacol 1997;17:446-50. HT neurotransmission more rapidly. The latter effect 13. Zanardi R, Franchini L, Gasperini M, Lucca A, Smeraldi E, Perez J. Faster onset of in combination with pin- could account for the lack of relapse of the depressive dolol in the treatment of delusional depression. J Clin syndrome when patients discontinue (±)pindolol and Psychopharmacol 1998;18:441-6. continue with their SSRI treatment regimen. Finally, the 14. Romero L, Bel N, Artigas F, de Montigny C, Blier P. Effect of pindolol on the function of pre- and postsynaptic 5-HT1A recep- observation that (±)pindolol did not alter overall 5-HT tors: In vivo microdialysis and electrophysiological studies in neurotransmission implies that this drug may not be the rat brain. Neuropsychopharmacology 1996;15:349-60. endowed with intrinsic antidepressant activity. 15. Hjorth S, Auerbach SB. 5-HT1A autoreceptors and the mode of action of selective serotonin reuptake inhibitor (SSRI). Behav Brain Res 1996;73:281-3. Acknowledgements 16. Newman-Tancredi A, Chaput C, Gavaudan S, Verriele L, Millan M. Agonist and antagonist actions of (–)pindolol at recombinant, human serotonin (5-HT ) receptors. This work was supported by the Medical Research 1A 1A Neuropsychopharmacology 1998;18:395-8. Council of Canada (MRC) grant (MT11014). P.B. was a 17. Fornal CA, Martin FJ, Meltzler CW, Jacobs BL. Pindolol sup- recipient of a Scientist award from the MRC. presses serotonergic neuronal activity (by a WAY 100635-sen-

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CANADIAN COLLEGE OF NEUROPSYCHOPHARMACOLOGY COLLÈGE CANADIEN DE NEUROPSYCHOPHARMACOLOGIE CCNP Innovations in Neuropsychopharmacology Research Award For Outstanding Innovations in Neuropsychopharmacology by an Individual or Group The Innovations in Neuropsychopharmacology Research Award is designed to recognize innovative research in neuropsychopharmacology. The Award consists of $5000 and a suitably engraved plaque. The Innovations in Neuropsychopharmacology Award shall be presented annually for work done primarily in Canada by Canadian scientists, unless there is, in view of the Awards Committee, no qualified nominee. The decision of the Awards Committee will be based solely on the outstanding innovative nature of the work by an individual or a group as demonstrated by a single piece of research contributed over a number of years. The Awardee(s) is/are expected to give a lecture based on his/her/their research at the Annual Meeting of the CCNP, and to contribute a manuscript, based on the lecture, to the CCNP official journal, the Journal of Psychiatry & Neuroscience, not later than six months after the lecture.

Nomination for 2001 Innovations in Neuropsychopharmacology Research Award The names of nominees should be received by Dr. Andrew Greenshaw by November 30th, 2000. Supporting documentation must be received by December 31st, 2000. For each award, this documentation shall consist of: 1. Six copies of a two-page summary prepared by the sponsor describing the nominee’s work and its importance in furthering the field of neuropsychopharmacology. 2. Six copies of manuscripts and reprints considered by the sponsor to be pertinent to the nomination and which highlight the nominee’s work. 3. Six copies of the nominee’s curriculum vitae and list of publications. 4. Six copies of a brief biographical sketch of the candidate prepared by the sponsor. Formal presentation of the Award will be made to the recipient during the Annual Meeting of the College. Note: The Innovations in Neuropsychopharmacology Research Award may be given for basic research or clinical research. Please send the name of the nominee and a short supporting letter to: Dr. Andrew J. Greenshaw President - CCNP University of Alberta, Department of Psychiatry 1E1.01, 8440-112 St. Edmonton AB T6G 2B7 Deadline for receipt of initial nomination and short supporting letter is November 30, 2000.

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