FEW CHOLINERGIC NEURONS IN THE RAT BASAL FOREBRAIN COEXPRESS GALANIN MESSENGER-RNA

The concept that galanin (GAL) is cosecreted with acetylcholine (ACh) into the ventral hippocampus is a major component of the current model delineating GAL regulation of the cholinergic memory pathways in the rat. Although GAL-immunoreactivity coexists in 50-70% of cholinergic n eurons in the basal forebrain (BF) of colchicine-treated rats, the act ual coexistence of these neurotransmitters in the basal state may be l ower, because colchicine treatment was recently shown to both induce G AL gene expression and inhibit choline acetyltransferase (ChAT) gene e xpression in this brain region. We have used single and double in situ hybridization histochemistry to examine the distribution and coexiste nce of GAL and ChAT mRNAs in the BF of male and female rats. Compared with other forebrain regions, few GAL mRNA-expressing neurons are pres ent within the cholinergic fields of the BF. The greatest number of GA L mRNA-expressing cells in this region are located within the nucleus of the horizontal limb of the diagonal band; but, even in this region, they represent only a small percentage (<20%) of ChAT mRNA-expressing cells. Our results indicate that few cholinergic neurons in the rat B F coexpress GAL mRNA and suggest that, in the basal state, GAL is not widely cosecreted with ACh into hippocampal memory centers. (C) 1998 W iley-Liss, Inc.