Serotonin1A autoreceptor activation by S 15535 enhances circadian activity rhythms in hamsters: evaluation of potential interactions with serotonin2A and serotonin2C receptors.

Mammalian circadian activity rhythms are generated by pacemaker cells in the suprachiasmatic nucleus (SCN). As revealed by the actions of diverse agonists, serotonergic input from raphe nuclei generally inhibits photic signaling in the suprachiasmatic nucleus. In contrast, the serotonin (5HT)1A partial agonist, 4-(benzodioxan-5-yl)1-(indan2-yl)piperazine (S 15535), was found to enhance the phase-shifting influence of light on hamster circadian rhythms [Gannon, Neuroscience 119 (2003) 567]. Herein, we extend this observation in showing that S 15535 (5.0 mg/kg, i.p.) markedly (275%) enhanced the light-induced phase shift in circadian activity rhythms: further, this action was dose-dependently abolished by the highly-selective 5HT1A receptor antagonist, WAY 100,635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]N-2-pyridinyl-cyclohexane-carboxamide maleate) (0.1-0.5 mg/kg, i.p.). WAY 100,635, which was inactive alone, shares the antagonist actions of S 15535 at postsynaptic 5HT1A sites, yet blocks its effects at their presynaptic counterparts. Thus, 5HT1A autoreceptor activation must be involved in this effect of S 15535 which contrasts with the opposite, inhibitory influence upon phase shifts of the "full" agonist, 8-OH-DPAT, which acts by stimulation of postsynaptic 5HT1A receptors [Rea et al., J Neurosci 14 (1994) 3635]. Despite the occurrence of 5HT2A and 5HT2C receptors in the (rat) suprachiasmatic nucleus, their influence on circadian rhythms is unknown since actions of selective ligands have never been evaluated. This issue was investigated with the most selective agents currently available. However, the 5HT2A agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.25 and 0.5 mg/kg), and the 5HT2C agonist, alphaS-6-chloro-5-fluoro-a-methyl-1H-indole-1-ethanamine fumarate (Ro-60-0175) (1.0 and 5.0 mg/kg), failed to affect light-induced phase shifts in hamsters. Moreover, even over broad dose-ranges, the 5HT2A antagonist, (+)-(2,3-dimethoxy-phenyl)-[1-[2-(4-fluoro-phenyl)-ethyl]-piperidin-4-yl]methanol (MDL 100,907) (0.1-1.0 mg/kg), and the 5HT2C antagonist, 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-yl carbamoyl]indoline (SB 242,084) (1.0-10.0 mg/kg), were likewise inactive. In view of evidence that 5HT2A and 5HT2C sites functionally interact with 5HT1A receptors, we also examined the influence of these agents upon the actions of S 15535, but no significant alteration was seen in its enhancement of rhythms. In conclusion, S 15535 elicits a striking enhancement of light-induced phase shifts in circadian rhythms by specifically recruiting 5HT1A autoreceptors, which leads to suppression of serotonergic input to the suprachiasmatic nucleus. Surprisingly, no evidence for a role of 5HT2A or 5HT2C sites was found, though comparable functional studies remain to be undertaken in rats. Indeed, the present work underlines the importance of comparative studies of circadian rhythms in various species, as well as the need for further study of potential interactions among 5HT receptor subtypes in their control.