IDENTIFICATION AND CHARACTERIZATION OF STRIATAL CELL SUBTYPES USING IN-VIVO INTRACELLULAR-RECORDING IN RATS .1. BASIC PHYSIOLOGY AND RESPONSE TO CORTICOSTRIATAL FIBER STIMULATION

The electrophysiological characteristics of two subtypes of striatal neurons, identified by their distinct patterns of response to paired impulse stimulation of corticostriatal afferents, were compared using in vivo intracellular recordings in rats. As observed in previous extracellular recording studies, the majority of neurons (73%) were found to be of the Type II class, with the remaining cells exhibiting the Type I response patern. For all cells, cortical stimulation elicited 5-30 mV EPSPs at latencies ranging from 2.0-5.3 msec. Increasing the stimulating current intensity caused a progressive increase in the amplitude of the evoked EPSPs without altering their latencies, suggesting that the EPSPs are monosynaptically mediated. Both the average amplitude and duration of the evoked EPSPs at spike threshold in Type I neurons (9.8 +/- 1.7 mV, 11.8 +/- 2.8 msec; mean +/- SEM) were significantly smaller than those of Type II cells (20.3 +/- 1.4 mV, 22.7 +/- 2.1 msec). Although the average latency to the onset of the EPSP was similar for both cell classes (Type I cells: 2.3 +/- 0.3 msec; Type II cells: 2.2 +/- 0.2 msec), the EPSPs in Type I cells reached peak amplitude and the spikes were triggered at significantly longer latencies than in the Type II cells (peak I: 11.2 +/- 2.5 msec vs. II: 7.6 +/- 0.7 msec; spike I: 8.0 +/- 1.2 msec vs. II: 5.7 +/- 0.4 msec). Striatal neurons had a comparatively hyperpolarized resting membrane potential (- 70.2 +/- 2.1 mV) and had an average input resistance of 35.4 +/- 7.6 M Omega. Overall, striatal neurons exhibited low levels of spontaneous activity (0.6 +/- 0.7 Hz) with 50% of the neurons being quiescent. Type I cells exhibited significantly higher firing rates (3.2 +/- 0.8 Hz) than Type II cells (0.8 +/- 0.3 Hz), although their resting membrane potentials were not significantly different. Spontaneously occurring spikes had an average amplitude of 72.7 +/- 3.4 mV and spike thresholds of - 50.1 +/- 1.5 mV. Irregularly occurring depolarizing plateau potentials, which typically gave rise to spike discharge, were frequently observed in both spontaneously firing and quiescent neurons, A small proportion of the cells recorded (3/55) exhibited a Type I response pattern but had unique physiological characteristics that were similar to those described by others as arising from large, aspiny striatal neurons. The present study shows that these two physiologically distinct neuron types appear to be similar in terms of their passive membrane properties (e.g., resting membrane potentials, input resistance, etc.) and firing characteristics, despite their unique patterns of response to corticostriatal stimulation. Therefore, the source of the distinct paired impulse response profiles of these neurons is more likely to arise from differences in their afferent drive than from a heterogeneity in their membrane properties. (C) 1994 Wiley-Liss, Inc.