Cholinergic neurons in the rat substantia nigra

The distribution of choline acetyltransferase (ChAT)-like immunoreactivity (LI), demonstrated according to an avidin-biotin procedure, was evaluated in the substantia nigra (SN) of rats. Although ChAT positivity was considerably less in the nigral neuropil than in the interpeduncular nucleus, it was also higher, particularly in caudal pars reticulata, than background as exampled by ChAT staining in the mesencephalic tegmentum, medial lemniscus and cerebral peduncle. Unexpectedly, a few ChAT-containing cells (approximately 1-5 per brain section) were also observed within the borders of pars reticulata of the substantia nigra but only at caudal levels. These latter cells were large (20-30 μm in maximum soma extent) and were morphologically similar to ChAT-positive neurons in, for example, the laterodorsal tegmental nucleus and substantia innominata. Although the origins of ChAT-LI in the nigral neuropil are unknown, possible sources include the processes of intranigral ChAT-containing cells, as well as extranigral cholinergic neurons. It is suggested that the ChAT-positive cells in the SN may be ectopically located neurons of the pontomesencephalotegmental cholinergic system.