Cortical projections arising from the basal forebrain: A study of cholinergic and noncholinergic components employing combined retrograde tracing and immunohistochemical localization of choline acetyltransferase
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Abstract
The neurochemical identity of ascending putative cholinergic pathways from the rat
basal forebrain was investigated employing a method for simultaneously visualizing
choline acetyltransferase immunoreactivity and retrogradely transported horseradish
peroxidase-conjugated wheatgerm agglutinin. This histochemical procedure revealed
three distinct populations of neurons: (1) cells which stained only for choline acetyltransferase
immunoreactivity; (2) cells which stained only for retrograde tracer and (3) cells
which stained simultaneously for choline acetyltransferase immunoreactivity and retrograde
tracer. The results demonstrated that this projection is topographically organized
and consists of both cholinergic and noncholinergic components. The relative contribution
of each component varied with the telencephalic target area as follows: the olfactory
bulb receives a projection from cells of the horizontal limb nucleus, 10-20% of which
are cholinergic (Ch3); the hippocampal formation receives afferents from cells of
the medial septal and vertical limb nuclei, 35-45% of which are cholinergic (Ch1 and
Ch2); and the cortical mantle receives afferents primarily from cells within the substantia
innominata-nucleus basalis complex, 80-90% of which are cholinergic (Ch4). The topographical
organization of Ch4 projections is not as completely differentiated as we have previously
observed in the primate.