IN-SITU HYBRIDIZATION EVIDENCE FOR THE SYNTHESIS OF 5-HT1B RECEPTOR IN SEROTONINERGIC NEURONS OF ANTERIOR RAPHE NUCLEI IN THE RAT-BRAIN

The regional distribution of the mRNA encoding the serotonin 5-HT1B re ceptor was studied in the central nervous system of the rat by in situ hybridization histochemistry and Northern blot analysis. A 180 base p air probe, corresponding to a highly selective portion of the third in tracellular loop of the rat 5-HT1B receptor, was used. In most regions , a single 5 kb message was found by Northern blot analysis. However, two additional bands (2.5 and 4 kb) were detected in the striatum. The rank order of 5-HT1B mRNA abundance was striatum >> septum = ventral tegmentum greater than or equal to colliculi = hypothalamus = hippocam pus > brain stem greater than or equal to cerebellum greater than or e qual to dorsal horn of the spinal cord > cerebral cortex greater than or equal to ventral horn of the spinal cord > olfactory tubercle. This distribution was confirmed by in situ hybridization, which further re vealed that the 5-HT1B mRNA was present in dorsal root ganglia, the la yer IV of the cerebral cortex, the Purkinje cell layer of the cerebell um, the pyramidal neurons in the CA1 area of the hippocampus, and the dorsal and median raphe nuclei. In situ hybridization was also perform ed in nomifensine (10 mg/kg/i.p.)-pretreated rats whose serotoninergic neurons were extensively and selectively lesioned by microinjection o f 5,7-dihydrox-ytryptamine (8 mu g/1 mu l) directly into the anteroven tral vicinity of anterior raphe nuclei 3 weeks before sacrifice. In le sioned rats, 5-HT1B mRNA was present in the same areas and at the same levels as in control rats, except in the dorsal and median raphe nucl ei, where a marked decrease (-75%) in its local concentration was obse rved. These data provide the first demonstration of the synthesis of 5 -HT1B receptor within serotoninergic neurons, as expected of their pre synaptic autoreceptor function at the level of serotoninergic terminal s. (C) 1995 Wiley-Liss, Inc.