Basal forebrain lesions differentially alter galanin levels and acetylcholinergic receptors in the hippocampus and neocortex.

The basal forebrain contains two subpopulations of cholinergic cells: the medial septal area (MSA) has projections to the hippocampus, while the nucleus basalis magnocellularis (NBM) has projections to the entire neocortex. In the rat, galanin-like immunoreactivity (GAL-LI) may coexist with acetylcholine (ACh) in MSA neurons but not in NBM neurons. In the monkey, GAL-LI may coexist with ACh in neurons throughout the basal forebrain. The present study investigated the differential distribution of GAL-LI within these regions by placing discrete lesions in the MSA and NBM of rats. Endogenous levels of GAL-LI were decreased in the hippocampus but not in the cortex. This differential decrease is consistent with the coexistence of GAL-LI with ACh in neurons within the MSA but not within the NBM. Markers for nicotinic and muscarinic cholinergic receptors, i.e. binding for [3H]nicotine and [3H]pirenzepine, were unchanged in the cortex and hippocampus following these lesions. This suggests that these cholinergic receptor sites do not exist upon projections originating in the NBM or MSA. These results provide new information about the similarities and differences of these two subpopulations of basal forebrain cells, which in turn may have functional ramifications.