Localization of serotonin(5A) receptors in discrete regions of the circadian timing system in the Syrian hamster

Endogenous serotonin and serotonergic drugs influence many aspects of circa dian rhythms, including phase shifts, onset of locomotor activity, and peri od length and integrity of rhythms during exposure to constant light. The r eceptor subtype(s) mediating all of these circadian effects of serotonin ha s (have) not been identified. Immunoreactivity for the serotonin(5A) (5-HT5 A) receptor has recently been identified in the rat suprachiasmatic nucleus (SCN). In this study, we investigated the distribution of the 5-HT5A recep tors in four neural components of the circadian timing system (the SCN, the intergeniculate leaflet, and the median and dorsal raphe nuclei), in the S yrian hamster. Single and dual immunohistochemistry were conducted using an affinity-purified rabbit antibody generated against a peptide sequence uni que to the 5-HT5A receptor, guinea pig anti-5-HT antisera and guinea pig an ti-GABA antisera. For single labeling, immunoreactivity was visualized usin g DAB-nickel as the chromagen. All four regions showed strong, yet distinct , immunoreactivity for the 5-HT5A receptor. No specific labeling was presen t in the absorption or omission controls. For double labeling, immunoreacti vity was visualized using immunofluorescence with Cy5- and FITC-labeled sec ond antibodies followed by confocal microscopy. In the raphe nuclei, 5-HT-i mmunoreactivity and 5-HT5A-immunoreactivity were co-localized in cell bodie s and axons. GABA-immunoreactive fibers surrounded some of the 5-HT5A recep tor-immunoreactive cell bodies in the raphe nuclei. In conclusion, the 5-HT 5A receptors are localized within several important neuroanatomical substra tes of the circadian timekeeping system, and within the raphe nuclei, appea r to be present on serotonin neurons. These findings suggest that some of t he 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.