Cholinergic neurons in the telencephalon of the reptile Caiman crocodilus.

Choline acetyltransferase (ChAT) immunohistochemistry was used to characterize the distribution of cholinergic neurons in the telencephalon of the reptile, Caiman crocodilus. ChAT-positive cell bodies were observed within the small-celled portion of the ventrolateral area of the telencephalon (VLA s.c.), a region previously considered comparable to the mammalian caudate nucleus and putamen. A large field of cholinergic neurons was observed within the ventral paleostriatum (VP), a region comparable to the substantia innominata and ventral pallidum of mammals. Cholinergic neurons were also observed within cranial motor nuclei of the brainstem, within the isthmic nucleus and within portions of the lateral reticular formation of the pons and medulla. A rich plexus of cholinergic fibers was observed within the intermediate and deep layers of the optic tectum. The results of this study indicate that many aspects of cholinergic system organization are similar in caiman and mammals, and suggest a common derivation of these systems from ancestral forms.