Cyto- and chemoarchitecture of basal forebrain cholinergic neurons in the common marmoset (Callithrix jacchus)

The cyto- and chemoarchitecture of basal forebrain cholinergic neurons (BFC N) was investigated in the lower primate, the common marmoset (Callithrix j acchus). A large population of magnocellular, hyperchromic, and choline ace tyltransferase (ChAT)-positive neurons was detected in the marmoset basal f orebrain. The distribution of these neurons was similar to those in higher primates. Thus, ChAT-positive neurons were observed in the medial septum (C h2), the vertical (Ch2) and horizontal (Ch3) limbs of the diagonal band of Broca, and the nucleus basalis of Meynert (Ch4). The Ch4 complex was relati vely well differentiated and displayed distinct sectors. We detected anteri or (Ch4a, with a medial and a lateral subdivision), intermediate (Ch4i, wit h a dorsal and a ventral subdivision), and posterior (Ch4p) sectors in the marmoset Ch4. The Ch4i was relatively small while the Ch4p was large. Simil ar to the rodent, the marmoset Chi extended quite a distance posteriorly, a nd the Ch4p displayed a major interstitial component distributed within the globus pallidus, its medullary laminae, and the internal capsule. Virtuall y all of the marmoset BFCN displayed acetylcholinesterase activity, and low affinity (p75(NTR)) and high affinity (Trk) neurotrophin receptor immunore activity. A majority contained immunoreactivity for calbindin-D-28k and cal retinin. Many of the Ch4 neurons also displayed tyrosine hydroxylase immuno reactivity. The BFCN lacked galanin immunoreactivity, but were innervated b y galanin-positive fibers. None of the marmoset BFCN were NADPH-d-positive. Thus, the BFCN display major anatomical and biochemical differences in the marmoset when compared with higher primates. The marmoset BFCN also displa y many characteristics common to other primates. This fact, combined with t he relatively short life span of the marmoset, indicates that this species may be ideal for studies of age-related changes in the BFCN. (C) 2000 Acade mic Press.