GABA(B)-RI receptors in serotonergic neurons: effects of baclofen on 5-HT output in rat brain

The activation of GABA(B) receptors hyperpolarizes 5-HT neurons and reduces cell firing. In situ hybridization showed the presence of the GABA(B)-RI r eceptor transcript in virtually all 5-HT neurons of the dorsal and median r aphe nuclei (DR and MnR, respectively) whereas the GAD transcript was prese nt mainly outside these nuclei. The systemic administration of baclofen inc reased the in vivo 5-HT release in the DR, MnR and several projection areas . As shown previously in the DR, the application of baclofen in the MnR inc reased the local 5-HT output. Thus, although 5-HT neurons contain inhibitor y GABA(B)-RI receptors, baclofen increased 5-HT release in some brain areas , likely by a preferential action on terminal GABA(B) autoreceptors in inhi bitory inputs to 5-HT neurons. The scarcity of GAD-expressing cells in the DR and MnR suggests that these inputs originate mainly outside these nuclei . NeuroReport 11:941-945 (C) 2000 Lippincott Williams & Wilkins.