2 TYPES OF CHOLINERGIC NEURONS IN THE RAT TEGMENTAL PEDUNCULOPONTINE NUCLEUS - ELECTROPHYSIOLOGICAL AND MORPHOLOGICAL CHARACTERIZATION

Two types of tegmental pedunculopontine nucleus neurons have been repo rted previously based on their electrophysiological characteristics: t ype I neurons were characterized by low-threshold Ca spikes and type I I neurons displayed a transient outward current. This report describes the membrane properties, synaptic inputs, morphologies and axonal pro jections of two subgroups of type II neurons examined in an in vitro s lice preparation. Type II neurons were divided into two groups based o n their spike durations: short-duration neurons with an action potenti al duration of 0.7-1.5 ms and long-duration neurons with an action pot ential duration of 1.6-2.9 ms. Choline acetyltransferase immunohistoch emistry combined with biocytin labeling indicated that 56% of short-du ration neurons and 61% of long-duration neurons were immunopositive fo r choline acetyltransferase. Short-duration neurons had a high input r esistance and the capacity to discharge with high frequency. By contra st, long-duration neurons had a low input resistance and low firing fr equency and upon current injection displayed an accommodation (spike-f requency adaptation) before reaching a steady firing frequency. Micros timulation of the substantia nigra pars compacta evoked antidromic res ponses in both short-duration neurons (n=5/14, 36%) and long-duration neurons (n=20/39, 51%). Stimulations of the subthalamic nucleus and th e substantia nigra pars reticulata induced in these neurons excitatory and inhibitory postsynaptic potentials, respectively. Short-duration neurons were dispersed equally throughout the extent of the tegmental pedunculopontine nucleus area, while long-duration neurons were locate d more in the rostral tegmental pedunculopontine nucleus. Short-durati on neurons were small with two to four thin primary dendrites. Long-du ration neurons were medium to large with three to six thick primary de ndrites. Cell size was positively correlated with spike duration and a xonal conduction velocity, but negatively with input resistance and sp ontaneous firing frequency. Both groups of neurons had ascending (towa rd thalamus, prerectal areas and tectum) and descending (toward pontom edullary reticular formation) axons in addition to nigropetal axons. A scending axons were observed in 75% (6/8) of short-duration neurons an d in 45% (15/33) of long-duration neurons, while nigropetal axons were observed in 50% (4/8) of short-duration neurons and in 76% (25/33) of Long-duration neurons. These results suggest that the tegmental pedun culopontine nucleus cholinergic projection system is composed of heter ogeneous populations of neurons in terms of electrophysiological and m orphological characteristics as well as their distribution patterns in the nucleus. (C) 1997 IBRO. Published by Elsevier Science Ltd.