Effects of Antidepressants on 5-HT7 Receptor Regulation in the Rat Hypothalamus U. Lena Mullins, Ph.D., G. Gianutsos, Ph.D., and A. S. Eison, Ph.D., Recent evidence suggests that a novel serotonin receptor agents produces a neuroadaptive downregulation of the 5-HT7 localized in the hypothalamus downregulates in 5-HT7 receptor in the hypothalamus. The current studies response to treatment with the antidepressant fluoxetine extend the previous observations to include several (Sleight et al. 1995). This receptor has also been implicated pharmacologically distinct antidepressants. In addition, in the regulation of circadian rhythms (Lovenberg et al. these studies provide further evidence to support the role of 1993). Here, we show that several agents administered in a the 5-HT7 receptor in the mechanism of antidepressant profile consistent with activity at the 5-HT7 receptor produce action and in the regulation of circadian rhythms controlled significant functional Fos immunoreactivity in the by the SCN. [Neuropsychopharmacology 21:352–367, suprachiasmatic nucleus (SCN), an effect reduced upon 1999] © 1999 American College of chronic exposure. Furthermore, binding studies Neuropsychopharmacology. Published by Elsevier Science Inc. demonstrate that chronic administration of Fos-inducing KEY WORDS: Serotonin; 5-HT7; Antidepressants; SCN; panded to include disturbances in biological rhythm Circadian rhythm regulation. Impairment of the efficiency of rhythm maintenance or rhythm desynchronization has been Depression may involve many possible abnormalities suggested by many to lead to mental fatigue and de- corresponding to multiple biological correlates of dys- pression (Goodwin et al. 1982; Hallonquist et al. 1986; function or dysregulation in either one or several brain Healy 1987; Partonen 1994; Schwartz 1993; Wirz-Justice neurotransmitter systems (Cooper et al. 1996; Nair and and Campbell 1982; Wirz-Justice et al. 1995). Clinically, Sharma 1989), a property that may contribute to the ap- it has been extensively documented that the timing and parent disparity in symptomology and response to anti- structure of rhythms in physiological, behavioral, and depressant therapy. The strict classical notions of neu- endocrinological functions seem to be abnormal in de- rotransmitter dysregulation hypotheses that associate pression (Coiro et al. 1993; Duncan 1996; File 1990; depression with a deficiency of a vailable neurotrans- Kupfer 1995; Nair and Sharma 1989; Wehr et al. 1979). mitter or subresponsivity of mainly noradrenergic and/ Furthermore, studies investigating the patterns of circa- or serotonergic receptor systems are recently being ex- dian rhythms of patients diagnosed with depression have been undertaken with the premise of disturbed rhythmicity as a central theme underlying the etiology From the Bristol-Myers Squibb Company (ULM, ASE), Neuro- of some affective disorders (Duncan 1996; Goodwin et science Drug Discovery, Wallingford, Connecticut; and University al. 1982; Healy 1987; Siever and Davis, 1985; Souetre et of Connecticut, Department of Pharmaceutical Sciences (GG), al. 1988). Storrs, Connecticut, USA. Address correspondence to: U. Lena Mullins, Ph.D., Bristol- Although melatonin is generally thought to be a pri- Myers Squibb Company, Neuroscience Drug Discovery, Depart- mary modulator of circadian function through the su- ment 408, 5 Research Parkway, Wallingford, Connecticut 06492, prachiasmatic nucleus (SCN) of the hypothalamus USA. Received January 1, 1999; revised March 23, 1999; accepted March (Armstrong and Redman 1993; Binkley 1993; Cassone et 26, 1999. al. 1993; Dubocovich 1991; Ibata et al. 1997; Reiter 1993; NEUROPSYCHOPHARMACOLOGY 1999–VOL. 21, NO. 3 © 1999 American College of Neuropsychopharmacology Published by Elsevier Science Inc. 0893-133X/99/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0893-133X(99)00041-X NEUROPSYCHOPHARMACOLOGY 1999–VOL. 21, NO. 3 Effects of Antidepressants 353 Stankov et al. 1993), the serotonergic system also plays 5-HT1A antagonist. Other studies showed that phos- a critical role in circadian modulation. The SCN re- phodiesterase inhibitors and nonhydrolyzable cAMP ceives dense projections from the raphe serotonergic analogs were able to mimic the effects of exogenously neurons originating in the brainstem (Jacobs and Azmi- applied 5-HT and produce an equivalent phase shift re- tia 1992; Meyer-Bornstein and Morin 1996; Van De Kar sponse in the SCN slice (Prosser and Gillette 1989). Still and Lorens 1979). Lesions of the median raphe seroto- other investigations demonstrated that H8 and Rp- nergic system using the neurotoxic agent 5,7-dihydrox- cAMP (a stereoisomer of cAMP) blocked quipazine- ytryptamine (5,7-DHT) produce a severe disruption in induced phase shifts (Lovenberg et al. 1993). Accord- rodent circadian locomotor (wheel running) activity, re- ingly, in vitro evidence strongly suggests that treat- sulting in a decrease in amplitude and a 20% longer ac- ments that increase intracellular cAMP levels modulate tivity phase (Morin and Blanchard 1991; Smale et al. endogenous SCN firing rhythms. This is an effect in- 1990). Furthermore, it has been demonstrated that ani- consistent with activity at the 5-HT1A receptor, because mals lacking intact serotonin innervation exhibit “hy- this receptor is known to couple negatively to adenylate pernormal” rhythm responses to light (Meijer and cyclase through an inhibitory G-protein; Gi (Cooper et Groos 1988). These data suggests that the serotonin sys- al. 1996; Saxena 1995). tem may provide inhibitory modulation of the circadian Subsequent cloning efforts identified a novel seroto- system to activation of the SCN by light (Van Den Pol nin receptor, type 7, which exhibits less than 40% ho- and Dudek 1993; Moore 1995; Morin 1994). mology with other known serotonin receptors (Bard et Serotonin is further implicated in circadian function al. 1993). Using [3H]-5-CT autoradiography and in situ by experiments that show that serotonergic agents af- hybridization (Waeber and Moskowitz 1995), the 5-HT7 fect behavioral rhythms by re-establishing disrupted receptor and its mRNA were detected in relatively high rhythmicity to external cycles in both mammals and hu- densities in limbic areas, particularly the hypothala- mans (Dawson and Armstrong 1996; DeMet and Chicz- mus, hippocampus, amygdala, mammillary nuclei, and DeMet 1987; Duncan 1996). Studies involving circadian raphe nuclei, and also diffusely within the cortex, a lo- locomotor (wheel-running) activity in rats and ham- calization that suggests that this receptor may be in- sters (Cutrera et al. 1994) and body temperature rhythms volved in the pathophysiology of affective disease in humans (Brown and Seggie 1988) have shown that (Gutafson et al. 1996; Kupferman 1991; Plassat et al. several classes of such mechanistically distinct antide- 1993; Ranson 1934). Functionally, the 5-HT7 receptor is pressant agents as imipramine and desipramine (tricy- a G-protein (Gs) linked receptor positively coupled to clics), clorgyline and phenelzine (monoamine oxidase adenylate cyclase and, thereby, increases cAMP. Phar- inhibitors), fluvoxamine, fluoxetine (selective serotonin macologically, in the context of a lack of selective reuptake inhibitors), and quipazine (a nonspecific sero- ligands, the 5-HT7 receptor is currently defined as ago- tonin agonist) shift established rhythmicity and possess nist activity by the endogenous ligand serotonin in the 2 the ability to restore experimentally phase-shifted circa- presence of the 5-HT1A antagonist ( )-pindolol and the dian activity patterns (Edgar et al. 1994; Golda 1993; 5-HT1D agonist sumatriptan with sensitivity to the an- Nagayama and Lu 1996; Wirz-Justice et al. 1995; tagonists ritanserin, clozapine, and mesulergine. The . Wollnick 1992). rank order of potency of 5-HT7 agonists is: 5-CT 5-HT Several serotonergic receptor subtypes have been 8-OH-DPAT (Lovenberg et al. 1993; Roth et al. 1994; identified in the SCN and other areas implicated in cir- Ruat et al. 1993; Shen et al. 1993; To et al. 1995; Tsou et cadian function including type-1A, 1B, 1D, 2A, and 7 re- al. 1994). Given this, the preponderance of the in vitro ceptors (Roca et al. 1993; Sumner et al. 1992; To et al. data suggest that the intrinsic circadian pattern of elec- 1995; Van Den Pol and Dudek 1993). The putative trical activity rhythm of the excised SCN responds to 5-HT1A agonists 5-CT (5-carboxamidotryptamine), 8-OH- serotonergics in a manner consistent with the pharma- DPAT, and 5-HT (5-hydroxytryptamine) in in vitro ex- cological profile of the 5HT7 receptor subtype (Loven- cised SCN experiments produced a shift in the timing berg et al. 1993). pattern of the endogenous SCN firing rate. Moreover, The AP-1 family of transcription factors, examples of DOB, a 5-HT2A/2C agonist; CGS-12066B, a 5-HT1B ago- which are Fos, Jun, and CREB, are induced in response nist, and ICS205930, a 5-HT3/4 receptor agonist all had to a variety of stimuli. The acute response protein, Fos, no effect on established SCN firing rates (Edgar et al. and chronic Fos-related antigens (cFRAs) are thought to 1993; Lovenberg et al. 1993; Medanic and Gillette 1992; play important modulatory roles in the transcriptional Prosser and Gillette 1989; Prosser et al. 1990, 1993; Shi- regulation of regionally specific brain responses to bata et al. 1992; Ying and Rusak 1994). However, the chronic such perturbations as long-term exposure to 5-HT1A agonist-induced responses were blocked by the psychotropics,