Localization of serotonin5A receptors in discrete regions of the circadian timing system in the Syrian hamster

Endogenous serotonin and serotonergic drugs influence many aspects of circadian rhythms, including phase shifts, onset of locomotor activity, and period length and integrity of rhythms during exposure to constant light. The receptor subtype(s) mediating all of these circadian effects of serotonin has (have) not been identified. Immunoreactivity for the serotonin(5A) (5-HT5A) receptor has recently been identified in the rat suprachiasmatic nucleus (SCN). In this study, we investigated the distribution of the 5-HT5A receptors in four neural components of the circadian timing system (the SCN, the intergeniculate leaflet, and the median and dorsal raphe nuclei), in the Syrian hamster. Single and dual immunohistochemistry were conducted using an affinity-purified rabbit antibody generated against a peptide sequence unique to the 5-HT5A receptor, guinea pig anti-5-HT antisera and guinea pig anti-GABA antisera. For single labeling, immunoreactivity was visualized using DAB-nickel as the chromagen. All four regions showed strong, yet distinct, immunoreactivity for the 5-HT5A receptor. No specific labeling was present in the absorption or omission controls. For double labeling, immunoreactivity was visualized using immunofluorescence with Cy5- and FITC-labeled second antibodies followed by confocal microscopy. In the raphe nuclei, 5-HT-immunoreactivity and 5-HT5A-immunoreactivity were co-localized in cell bodies and axons. GABA-immunoreactive fibers surrounded some of the 5-HT5A receptor-immunoreactive cell bodies in the raphe nuclei. In conclusion, the 5-HT5A receptors are localized within several important neuroanatomical substrates of the circadian timekeeping system, and within the raphe nuclei, appear to be present on serotonin neurons. These findings suggest that some of the circadian effects of 5-HT may be mediated by the 5-HT5A receptor, which may function as a presynaptic autoreceptor. (C) 2000 Elsevier Science B.V. All rights reserved.