Distribution of central cholinergic neurons in the baboon ( papio papio ). II. A topographic atlas correlated with catecholamine neurons

The topographic distribution of central cholinergic and catecholaminergic neurons has been investigated in the baboon ( Papio papio ). The perikarya were mapped on an atlas through the brain and spinal cord employing sections processed for acetylcholinesterase (AChE) pharmacohistochemistry coupled with choline acetyltransferase (ChAT) immunohistochemistry or aqueous catecholamine‐fluorescence histochemistry. Compared with subprimates, there is a remarkable increase in the volume occupied by and the number of cholinergic cells contained in the nucleus basalis and nucleus tegmenti pedunculopontinus (subnucleus compacta). The elaboration of these parts of the cholinergic system is accompanied by a large extension of catecholaminergic cell groups in the midbrain (groups A8–A10), particularly the substantia nigra (pars compacta), and in the dorsolateral pontine tegmentum (A5–A7 complex). Although cholinergic and catecholaminergic soma generally occupy distinctly different regions of the brain, a close apposition of cholinergic and noradrenergic neurons occurs in the dorsolateral pontine tegmentum. In the peripeduncular region ChAT‐positive cells and green fluorescent neurons of the A6–A7 complex form parallel lines and do not intermingle as has previously been demonstrated in the cat. Two distribution patterns, aggregated or disseminated, are another common feature of central cholinergic and catecholaminergic perikarya. The cholinergic neurons in the nucleus tegmenti pedunculopontinus and the catecholaminergic neurons in A6–A7 complex display both patterns. This comparative study of three transmitter systems in the baboon suggests that the cholinergic as well as the catecholaminergic neurons that give rise to ascending telencephalic and dorsal diencephalic projections undergo phylogenetic development in terms of cell number and nuclear volume.