GALR1 GALANIN RECEPTOR MESSENGER-RNA IS COEXPRESSED BY GALANIN NEURONS BUT NOT CHOLINERGIC NEURONS IN THE RAT BASAL FOREBRAIN

The neuropeptide galanin (GAL) has been proposed to be an inhibitory m odulator of cholinergic transmission in the hippocampus and may impair memory by directly affecting the activity of basal forebrain (BF) cho linergic neurons. Alternatively, GAL may act indirectly and modulate t he activity of other neurotransmitter systems which, in turn, influenc e cholinergic transmission. We have used double in situ hybridization histochemistry to evaluate the co-expression of the GAL receptor subty pe, GALR1, within cholinergic neurons in the medial septum/diagonal ba nd of adult male rats. In alternate brain sections, we assessed the co -expression of GALR1 mRNA within another forebrain cell group implicat ed in memory functions, the neurons of the bed nucleus of the stria te rminalis (BNST) and medial amygdala (AMe) which co-express vasopressin (VP) and GAL and project to septo-hippocampus. Despite the abundance of GALR1 mRNA-expressing neurons in the cholinergic BF, we found no ev idence for the co-expression of this receptor subtype within cholinerg ic neurons in the medial septum/diagonal band. In contrast, we detecte d an extensive co-expression (95%) of GALR1 mRNA within extrahypothala mic VP/GAL neurons. These results do not support the idea that GAL, ac ting via the GALR1 receptor, directly impairs BF cholinergic neurons b ut suggest, instead, that non-cholinergic neurons in the BF may play a role in mediating the inhibitory actions of GAL on cholinergic functi on. However, our findings provide anatomical evidence that GAL could d irectly modulate the activity and/or secretion pattern of extrahypotha lmic VP/GAL neurons into septo-hippocampal regions. (C) 1997 Elsevier Science B.V.