COMPARATIVE LOCALIZATION OF SEROTONIN, (1A), (1C) AND (2) RECEPTOR SUBTYPE MESSENGER-RNAS IN RAT-BRAIN

Serotonin (5-HT) mediates its effects on neurons in the central nervou s system through a number of different receptor types. To gain better insight as to the localization of 5-HT responsive cells, the distribut ion of cells expressing mRNAs encoding the three 5-HT receptor subtype s (1A, 1C,) and (2) was examined in rat brain with in situ hybridizati on using cRNA probes. 5-HT1A receptor mRNA labeling was most pronounce d in the olfactory bulb, anterior hippocampal rudiment, septum, hippoc ampus (dentate gyrus and layers CA1-3), entorhinal cortex, interpedunc ular nucleus, and medullary raphe nuclei. 5-HT1C receptor mRNA labelin g was the most abundant and widespread of the three 5-HT receptor subt ypes examined. Hybridization signal was densest in the choroid plexus, anterior olfactory nucleus, olfactory tubercle, piriform cortex, sept um, subiculum, entorhinal cortex, claustrum, accumbens nucleus, striat um, lateral amygdala, paratenial and paracentral thalamic nuclei, subt halamic nucleus, substantia nigra, and reticular cell groups. 5-HT2 re ceptor mRNA was localized to the olfactory bulb, anterior hippocampal rudiment, frontal cortex, piriform cortex, entorhinal cortex, claustru m, pontine nuclei, and cranial nerve motor nuclei including the oculom otor, trigeminal motor, facial, dorsal motor nucleus of the vagus, and hypoglossal nuclei. The distributions of mRNAs for the three differen t 5-HT receptor subtypes overlap with regions that bind various 5-HT r eceptor-selective ligands and are present in nearly all areas known to receive serotonergic innervation. The results of this study demonstra te that neurons which express these 5-HT receptor subtypes are very wi despread in the central nervous system, yet possess unique distributio ns within the rat brain. Moreover, previously unreported regions of 5- HT receptor subtype expression were observed, particularly with the 5- HT2 receptor riboprobe in the brainstem. Finally, several brain areas contain multiple 5-HT receptor subtype mRNAs, which leads to the possi bility that individual cells may express more than one 5-HT receptor s ubtype. (C) 1995 Wiley-Liss, Inc.