ORGANIZATION OF THE BASAL FOREBRAIN IN THE CAT - LOCALIZATION OF L-ENKEPHALIN, SUBSTANCE-P, AND CHOLINE-ACETYLTRANSFERASE IMMUNOREACTIVITY

The present study uses immunocytochemical techniques to determine whet her cholinergic basal forebrain neurons in the cat are in a position t o receive a homogeneous pattern of inputs, or if specific immunocytoch emically defined afferent systems are localized to only selected regio ns of the basal forebrain. Monoclonal antibodies against choline acety ltransferase (ChAT) were used to identify the location of putative cho linergic neurons which are known to project to the cerebral cortex. In addition, polyclonal antibodies against substance P (SP) or enkephali n (Enk) were used on either adjacent or on the same histological secti ons reacted for ChAT to identify the neuropeptide plexuses that provid e input to the basal forebrain. ChAT-immunoreactive (ChAT-IR) perikary a were located throughout the vertical limb, genu and horizonal limb o f the diagonal band of Broca. ChAT-IR neurons also were located within the substantia innominata (SI), within the peripallidal zone around t he globus pallidus, and were intercalated within the internal capsule. Enk-IR and SP-IR were used to determine the distribution of putative peptidergic terminals within the basal forebrain. Extensive Enk-IR and SP-IR terminal label was localized within the globus pallidus and the surrounding peripallidal zones, as well as within the SI, whereas the components of the diagonal band of Broca demonstrated negligible Enk- IR and SP-IR label. These data predict that the subdivisions of the ch olinergic basal forebrain in the cat do not share a uniform afferent s ystem, and only selective portions of this cholinergic system are in a n anatomical position to receive a major direct input from the identif ied subcortical peptidergic afferents. The segregation of afferents ha s important consequences in the selective control of cortical function by the cholinergic basalocortical pathway.