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.