Cholinergic limbic projections and behavioral role of basal forebrain nuclei in the rat.

The purposes of the present study were to identify cholinergic non-neocortical projections of the basal forebrain and to determine the role of this region in the regulation of estrogen-dependent reproductive behaviors in the rat. Bilateral electrolytic lesions were placed in an area encompassing the horizontal limb of the diagonal band, as well as portions of the substantia innominata and magnocellular preoptic nucleus, and choline acetyltransferase (CAT) activity was assayed in microdissected brain areas seven days after lesion. Compared to sham surgery, lesions of this region significantly reduced CAT activity in the basal amygdala (34%), dorsal hippocampus (14%), cingulate cortex (25%), piriform cortex (36%), and entorhinal cortex (34%). Other limbic and midbrain structures do not appear to receive significant cholinergic innervation from this locus since no reductions in CAT were detected after bilateral lesions. These included the anterior hypothalamus, ventromedial hypothalamus, mammillary nucleus, habenula, subiculum, ventral hippocampus, insular cortex, central gray, and interpeduncular nucleus. Behaviorally, female rats with bilateral lesions of the basal forebrain displayed an unusually high incidence of rejection behavior in response to attempted mounts by stimulus male rats in sexual behavior tests. There was no effect of basal forebrain lesions on the incidence of lordosis exhibited by these females. The dissociation of rejection and lordosis suggests that distinct neural pathways mediate the occurrence of these reproductive behaviors and that rejection behavior may be regulated by basal forebrain pathways.