Monoaminergic-cholinergic interactions in the primate basal forebrain

Anatomical studies in the rat have shown that the cholinergic cells of the nucleus basalis receive;e synapses from monoamine axons, but similar eviden ce is lacking in primates. We used single- and double-labeling immunocytoch emistry to visualize monoamine axons and their relationship with the cholin ergic cells of the basal forebrain of the monkey. Norepinephrine axons, lab eled with dopamine-P-hydroxylase antibodies, formed a bed of fine varicose axons that co-distributed with the cholinergic cells. Tyrosine hydroxylase- immunoreactive axons, presumed to be mainly dopaminergic, were 10-20 times more abundant than dopamine-P-hydroxylase axons throughout the basal forebr ain, except in the medial septal area, where their density was lower. Serot onin-immunoreactive axons formed a dense axon plexus throughout the basal f orebrain. Double-labeling light microscopy demonstrated that each of the th ree types of monoamine axons formed frequent direct contacts with the choli nergic,aic cells. Electron microscopy showed that the noradrenergic and the putative dopaminergic axone synapsed on the cholinergic cells. In the huma n brain, immunolabeling with antibodies to dopamine-P-hydroxylase, tyrosine hydroxylase and tryptophan hydroxylase (for serotonin axons) showed axon d ensities in the nucleus basalis comparable to those of the monkey brain. Th e data demonstrate that all three of these monoamine systems innervate the cholinergic and possibly also the non-cholinergic cells of the nucleus basa lis, and therefore affect the release of acetylcholine in the cerebral cort ex. (C) 1999 IBRO. Published by Elsevier Science Ltd.