CHOLINERGIC AND NONCHOLINERGIC TEGMENTAL PEDUNCULOPONTINE PROJECTION NEURONS IN RATS REVEALED BY INTRACELLULAR LABELING

Morphological features of rat pedunculopontine projection neurons were investigated in in vitro preparation by using intracellular labeling with biocytin combined with choline acetyltransferase (ChAT) immunohis tochemistry. These neurons were classified into two types (Type I and II), based on their electrical membrane properties: Type I had low-thr eshold Ca2+ spikes, and Type II had A-current. All Type I neurons (n = 17) were ChAT immunonegative (ChAT(-)). Type II neurons were either C hAT immunopositive (ChAT(+); n = 49) or ChAT(-) (n = 20). In terms of topography in the tegmental pedunculopontine nucleus (PPN), Type I neu rons were dispersed throughout the extent of the nucleus, whereas Type II neurons tended to be located more in the rostral and middle sectio ns. Both Type I and II neurons consisted of small (long axis <20 mu m) , medium (20-35 mu m), and large (>35 mu m) cells. The small cells wer e round or oval; medium cells were round, triangular, or fusiform; and the large cells were primarily fusiform in shape. In terms of the som a size, there was a difference in Type I (15-38 mu m) and Type II (11- 50 mu m) neurons, but no significant difference was found between Type II ChAT(+) and ChAT(-) cells. Both types of neurons had three to six primary dendrites, but the dendritic field was more prominent in Type II neurons. Most of the axons originated from one of the primary dendr ites, which gave off axon collaterals, some of which projected out of the nucleus. The intrinsic collaterals were thin and branched partly w ithin the dendritic field of the parent cell. The extrinsic collateral s were thicker and could be grouped into three categories: 1) collater als arborizing in the substantia nigra, 2) collaterals ascending mainl y toward the thalamus, pretectal, and tectal area; and 3) collaterals descending toward the mesencephalic and/or pontine reticular formation . It was noted that the collaterals of both ChAT(+) and ChAT neurons w ere traced into the substantia ni;ya. There was no significant differe nce in antidromic latencies between Type I (m = 1.47 msec) and Type II (m = 1.36 msec) neurons following electrical stimulation of the subst antia nigra. (C) 1996 Wiley-Liss, Inc.