Topographic analysis of the innervation of the rat neocortex and hippocampus by the basal forebrain cholinergic system

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Abstract
The basal forebrain‐cortex connections of the rat were topographically mapped by retrograde tracer methods; and their contribution to the cholin‐ergic innervation of the cortex was assessed by excitotoxin lesions placed in the rostral and caudal aspects of the complex. Discrete injections of tracer into frontal cortex labeled the prominent multipolar acetylcholinesterase (AchE)‐positive cells of the ventromedial globus pallidus. Injections of tracer into the parietal cortex labeled cells in the ventral globus pallidus, the underlying substantia innominata, and the lateral hypothalamus. Separate injections of Fast Blue and Nuclear Yellow in the frontal and in the parietal cortex resulted in double‐labeled cells in the ventral globus pallidus, which indicates that at least some of these cells may possess collateralizing axons. The cingulate cortex is innervated predominantly by neurons in the nucleus of the horizontal limb of the diagonal band. The occipital cortex was also shown to receive a projection primarily from the nucleus of the horizontal limb of the diagonal band. The hippocampal formation is innervated primarily by cells located in the vertical limb of the diagonal band and in the medial septum. Consistent with the results of the retrograde tracing studies, excito‐toxin lesions affecting the diagonal band and medial septum decreased cho‐line acetyltransferase (CAT) activity up to 40% in the occipital cortex and by 64% in the hippocampus, but did not affect CAT activity in the rostral neo‐cortex. In contrast, ibotenate lesions of the caudal ventral globus pallidus and substantia innominata caused decreases in CAT activity in the frontal cortex of up to 65% without affecting enzyme activity in the hippocampal formation. The results of the present study provide details on the topographic organization of the cortical projections originating in the basal fore‐brain complex and indicate that these neurons are the predominant source of cortical cholinergic innervation.