Basal forebrain innervation of rodent neocortex: studies using acetylcholinesterase histochemistry, Golgi and lesion strategies.

Acetylcholinesterase (AChE)-rich projections from basal forebrain to neocortex cerebri were characterized in the present study. The purpose was to investigate 3 aspects of these projections in rats and mice that have been incompletely described in previous work: intracortical organization of the fibers, subcortical pathways and axonal branching patterns of individual basal forebrain neurons. AChE histochemistry, lesions and Golgi impregnations were the principal strategies employed in this light microscopic study. The moderately dense, AChE-stained innervation of neocortex can be altered by intracortical lesions. The results depended on the region involved and the orientation of the lesion. Sagittal knife cuts had barely detectable effects, regardless of sites. Coronal knife cut lesions in medial cortex resulted in substantial loss of staining in cingulate and medial occipital fields. In contrast, coronal lesions of lateral or anterior cortex produce only small zonal reductions in staining. The interpretation of the latter findings that we favor is that AChE-rich basal forebrain fibers enter lateral/anterior cortex and branch densely there, but in tangentially limited and overlapping terminal domains. Observations on the topography and targets of AChE-rich basal forebrain cortical afferents revealed that the fibers could be grouped based on certain characteristics. Three sets of fibers were distinguishable: anterior pathway innervating cortex of the frontal pole. These fibers were traceable to the region of the substantia innominata/nucleus basalis. They crossed the neostriatum and external capsule in the sagittal plane, forming in 3 dimensions an orderly sheet-like array of fibers bridging the anteroventral surface of the neostriatum with nearby polar cortex medial pathway innervating cingulate and medial occipital cortex. Emerging predominantly from the region of the diagonal band, the fibers run caudally as a triangular bundle in deep layer VI of cingulate cortex. lateral pathway innervating most of remaining lateral neocortex. The fibers radiate out from substantia innominata/nucleus basalis with a complex 3-dimensional organization. In all pathways, fibers enter and initially run within layer VI before ascending pialward, although the intracortical course in layer VI differs between pathways. These fibers primarily terminate in layer V with a secondary concentration in layer I. However, the latter appears to receive substantial AChE-stained inputs from other sources, possibly intracortical, as well. The pathways overlap at their respective boundary zones. This system is comparably organized in rats and mice.(ABSTRACT TRUNCATED AT 400 WORDS)