Cholinergic and non-cholinergic neurons in the rat pedunculopontine tegmental nucleus

Choline acetyltransferase immunhistochemistry was employed at light and electron microscopic levels in order to determine the distribution of cholinergic neurons in two subdivisions of the rat pedunculopontine tegmental nucleus that were previously defined on cytoarchitectonic grounds, and to compare the synaptic inputs to cholinergic and non-cholinergic somata in the subnucleus dissipatus, which receives major input from the substantia nigra. Large cholinergic neurons were found in both the pars compacta and the pars dissipata of the pedunculopontine nucleus. However, they were intermingled with non-cholinergic neurons and did not respect the cytoarchitectural boundaries of the nucleus. Ultrastructural study showed that all cholinergic neurons in the subnucleus dissipatus exhibited similar features. The majority had large somata (largest diameter ≥20 μm) containing abundant cytoplasmic organelles and nuclei displaying a few shallow invaginations. Synaptic terminals on the cholinergic cell bodies were scarce and unlabeled boutons containing spherical synaptic vesicles and establishing asymmetric synaptic junctions were the dominant type. In contrast, the non-cholinergic neurons presented prominent differences in the size of their somata as well as in the distribution of axosomatic synapses. Two almost equally represented classes of non-cholinergic neurons which are referred to as large (largest diameter ≥20 μm) and small (largest diameter <20 μm) were recognized. Large non-cholinergic cell bodies were ultrastructurally similar to the cholinergic ones, but they received rich synaptic input by unlabeled nerve terminals which contained pleomorphic vesicles and were engaged in symmetric synaptic junctions. Small non-cholinergic cell bodies were characterized by deeply invaginated nuclei surrounded by a narrow rim of cytoplasm, and were often found near or in direct apposition to the cholinergic somata. Their major input consisted of axosomatic boutons containing round synaptic vesicles. These results demonstrate that cells in the pedunculopontine tegmental nucleus are differentiated with regard to their axosomatic synaptic inputs which may influence their firing properties. Furthermore, they support previous suggestions that nigral afferents may be preferentially distributed to a subpopulation of the pedunculopontine neurons.