Cholinergic projections from the basal forebrain to the frontal cortex: a combined fluorescent tracer and immunohistochemical analysis in the rat.

Cholinergic projections from the basal forebrain to some regions of the frontal cortex were studied by infusing propidium iodide (PI), a fluorescent tracer, into areas 6 and 10 and microscopically assessing the cellular co-localization of PI and immunohistochemically demonstrated choline-O-acetyltransferase (ChAT). The same brain sections were additionally processed for acetylcholinesterase (AChE, pharmacohistochemical regimen) and Nissl material (cresyl violet stain). Basal forebrain neurons projecting to the frontal cortex were found primarily in nucleus basalis, but others were located in association with the substantia innominata/lateral preoptic area, magnocellular preoptic area, and ansa lenticularis. These projection neurons were large (greater than 25 micrometers in maximum soma extent), demonstrated ChAT-like immunoreactivity, stained intensely for AChE following systemic administration of bis-(1-methylethyl)phosphorofluoridate, and were highly chromophilic.