Calbindin D-28k and choline acetyltransferase are expressed by different neuronal populations in pedunculopontine nucleus but not in nucleus basalis in squirrel monkeys.

Single- and double-immunostaining procedures were used to study the distribution of the acetylcholine synthesizing enzyme choline acetyltransferase (ChAT) and the calcium binding protein calbindin D-28k in the nucleus basalis of Meynert (nbM) and in the pedunculopontine nucleus (PPN) of the squirrel monkey (Saimiri sciureus). As expected from previous studies in other primates, including humans, the nbM in the squirrel monkey is enriched with large ChAT-immunoreactive neurons that form clusters in the substantia innominata. Some ChAT-positive neurons are also scattered more dorsally within the internal and external medullary laminae of the pallidal complex. A smaller number of calbindin-immunoreactive cells occur in the same locations and their mean cross-sectional somatic area (424 microns 2) is not significantly different from that of the ChAT-immunoreactive cells (450 microns 2). Furthermore, 60% of the ChAT-immunopositive cells in the nbM display calbindin immunoreactivity. Most of these double-immunoreactive neurons occur in the typical clusters of the nbM, whereas the large neurons scattered in between the clusters display ChAT immunoreactivity only. In the PPN, ChAT-positive neurons are scattered around and partly within the superior cerebellar peduncle and also form a dense cluster in the lateral portion of the mesopontine tegmentum. Calbindin-immunoreactive cells also abound around the superior cerebellar peduncle, but they are more sparsely distributed and cover a larger sector of the tegmentum than the ChAT-positive neurons. These calbindin-immunoreactive cells are significantly smaller (200 microns 2) than the ChAT-immunoreactive cells (471 microns 2) and no double-immunostained neurons are present in the PPN.(ABSTRACT TRUNCATED AT 250 WORDS)