Cholinergic and Nadph‐δ neurons in the pedunculopontine and laterodorsal tegmental nuclei of human and nonhuman primates

The brainstem pedunculopontine (PPN) and laterodorsal tegmental (LDTg) nuclei are involved in multifarious activities, including motor control. Yet, their exact cytoarchitectural boundaries are still uncertain. We therefore initiated a comparative study of the topographical and neurochemical organization of the PPN and LDTg in cynomolgus monkeys (Macaca fascicularis) and humans. The distribution and morphological characteristics of neurons expressing choline acetyltransferase (ChAT) and/or nicotinamide adenine dinucleotide phosphate diaphorase (Nadph‐δ) were documented. The number and density of the labeled neurons were obtained by stringent stereological methods, whereas their topographical distribution was reported upon corresponding magnetic resonance imaging (MRI) planes. In both human and nonhuman primates, the PPN and LDTg are populated by three neurochemically distinct types of neurons (ChAT‐/Nadph‐δ+, ChAT+/Nadph‐δ‐, and ChAT+/Nadph‐δ+), which are distributed according to a complex spatial interplay. Three‐dimensional reconstructions reveal that ChAT+ neurons in the PPN and LDTg form a continuum with some overlaps with pigmented neurons of the locus coeruleus, dorsally, and of the substantia nigra (SN) complex, ventrally. The ChAT+ neurons in the PPN and LDTg are —two to three times more numerous in humans than in monkeys but their density is —three to five times higher in monkeys than in humans. Neurons expressing both ChAT and Nadph‐δ have a larger cell body and a longer primary dendritic arbor than singly labeled neurons. Stereological quantification reveals that 25.6% of ChAT+ neurons in the monkey PPN are devoid of Nadph‐δ staining, a finding that questions the reliability of Nadph‐δ as a marker for cholinergic neurons in primate brainstem. Cholinergic neurons in the brainstem are 2‐3 times more numerous in humans than in monkeys but their density is 3‐5 times higher in monkeys. More than 25% of cholinergic neurons in the pedunculopontine nucleus of the monkey are devoid of Nadph‐δ staining, a finding that questions the reliability of Nadph δ as a marker for cholinergic neurons in primate brainstem.