PATTERN OF EXPRESSION OF THE SEROTONIN(2C) RECEPTOR MESSENGER-RNA IN THE BASAL GANGLIA OF ADULT RATS

The distribution of the serotonin (5-HT) receptor 5-HT2C mRNA was exam ined at the single-cell level with in situ hybridization histochemistr y and emulsion autoradiography in the basal ganglia and mesolimbic sys tem of adult rats, with focus on the pallidum and the substantia nigra , which receive striatal inputs and play a critical role in basal gang lia function. 5-HT2C receptor mRNA expression was always restricted to a subpopulation of neurons in the regions examined. In the neostriatu m, labeled neurons were more numerous in the rostral nucleus accumbens than in the caudal nucleus accumbens and were more numerous in the ve ntral and ventrolateral caudate-putamen than in the dorsal caudate-put amen, where labeled neurons were restricted to isolated clusters. In s triatal target areas, dense labeling in the entopeduncular nucleus (in ternal pallidum, direct striatal output pathway) contrasted with an ab sence of labeling in the globus pallidus (external pallidum, indirect striatal output pathway). Double-label in situ hybridization in the su bstantia nigra revealed coexpression of 5-HT2C receptor mRNA with glut amic acid decarboxylase but not with tyrosine hydroxylase mRNA, indica ting that it was restricted to gamma-aminobutyric acid (GABA)ergic neu rons. In this region, dense labeling for 5-HT2C mRNA was found in half of the neurons at middle and caudal levels of both the pars compacta and the pars reticulata, with little labeling rostrally. The data sugg est that drugs acting on the 5-HT2C receptor could selectively affect discrete neuronal populations in the basal ganglia and mesolimbic syst ems and indicate a new level of neurochemical heterogeneity among GABA ergic neurons of the substantia nigra. (C) 1997 Wiley-Liss, Inc.