GABAB receptor mRNA in the raphe nuclei:: co-expression with serotonin transporter and glutamic acid decarboxylase

We have used double-label in situ hybridization techniques to examine the cellular localization of GABA(B) receptor mRNA in relation to serotonin transporter mRNA and glutamic acid decarboxylase mRNA in the rat dorsal raphe, median raphe and raphe magnus nuclei. The degree of cellular co-localization of these markers notably varied among the different nuclei. In the dorsal raphe, cell bodies showing GABA(B) receptor mRNA were very abundant, the 85% being also labelled for serotonin transporter mRNA, and a low proportion (5%) showing glutamic acid decarboxylase mRNA. In the median raphe, the level of co-expression of GABA(B) receptor mRNA with serotonin transporter mRNA was significantly lower. Some cells were also identified that contained GABA(B) receptor mRNA in the absence of either one of the other mRNA species studied. Our results support the presence of GABA(B) receptors in serotonergic as well as GABAergic neurones in the dorsal and median raphe, providing the anatomical basis for the reported dual inhibitory/disinhibitory effect of the GABA(B) agonist baclofen on serotonergic function.