ULTRASTRUCTURAL-STUDY OF CHOLINERGIC AND NONCHOLINERGIC NEURONS IN THE PARS COMPACTA OF THE RAT PEDUNCULOPONTINE TEGMENTAL NUCLEUS

A group of medium-to-large cholinergic neurons situated in the dorsola teral mesopontine tegmentum comprises the pedunculopontine tegmental n ucleus (PPT). The PPT pars compacta (PPT-pc), which occupies the later al part of the caudal two-thirds of the nucleus, contains a dense aggr egation of cholinergic neurons. In the present study, we have employed immunohistochemistry for choline acetyltransferase (ChAT) and electro n microscopy to investigate the ultrastructure and synaptic organizati on of neuronal elements in the PPT-pc. Our results demonstrate that: ( 1) ChAT-immunoreactive (i.e., cholinergic) PPT-pc neurons are characte rized by abundant cytoplasm and organelles, and have few axosomatic sy napses (both asymmetric and symmetric); (2) ChAT-immunoreactive dendri tes comprise 6-15% of total dendritic elements in the neuropil; the me an percentage of dendritic membrane covered by synaptic terminals is a pproximately 15%, and nearly all synapses with ChAT-immunoreactive den drites are asymmetric; (3) within the boundaries described by choliner gic PPT-pc, there are noncholinergic neurons which, in contrast, exhib it a lucent cytoplasm and a higher frequency of axosomatic synapses (1 0.5% versus 3.7% for cholinergic neurons); and (4) noncholinergic neur ons are morphologically heterogeneous with one subpopulation exhibitin g a mean diameter that approximates that of cholinergic cells (i.e., > 15 mu m and <20 mu m) and a very high frequency of axosomatic synapses (>20%). Only 0.2-0.7% of terminal elements in the neuropil were ChAT- immunoreactive and these were not observed to synapse with cholinergic dendrites or somata. This relative paucity of terminal labeling and l ack of cholinergic-cholinergic interactions seems inconsistent with th e recognized and prominent physiological actions of acetylcholine on c holinergic PPT-pc neurons, and suggests a methodological limitation an d/or a potential paracrine-like action of nonsynaptically released ace tylcholine in the PPT region. J. Comp. Neurol. 382:285-301, 1997. (C) 1997 Wiley-Liss, Inc.