Cholinergic innervation of cerebral cortex in organotypic slice cultures: sustained basal forebrain and transient striatal cholinergic projections.

Slices of entire forebrain hemispheres were taken from early postnatal rat pups and maintained as organotypic slice cultures. Basal forebrain cholinergic neurons, identified by histochemical staining for acetylcholinesterase, develop axons that grow rapidly into cerebral cortex. Ingrowth occurs by two routes: some axons course laterally from the basal forebrain region to reach lateral neocortex; others course dorsally from the septum to reach medial cortex. By one to two weeks in vitro, acetylcholinesterase-positive axons have extended throughout most of the cortical territory. In addition to basal forebrain cholinergic axons, the normally local circuit cholinergic neurons of the striatum also send axons into cerebral cortex. These striatum-derived axons can be distinguished from basal forebrain axons by their distinct morphological characteristics and by their different response to excision of the striatum or basal forebrain. Further, acetylcholinesterase-positive axons in cortex that originate from striatum appear to retract or degenerate after about one week in culture, while those from basal forebrain remain present and apparently healthy beyond two weeks. These data document the basal forebrain cholinergic ingrowth into cerebral cortex using this whole hemisphere slice culture system and also demonstrate different degrees of maintenance of cortical afferents that are derived from different subcortical sources.