Pharmacological and Morphological Properties of Two Functionally Distinct Subpopulations of Striatal Neurons

Abstract

We have shown recently that two subpopulations of striatal neurons in the rat, Type I and Type II, can be distinguished on the basis of their electrophysiological responses to paired impulse stimulation of cortical afferents (Berger et al., 1987; Nisenbaum et al., 1988b). The experimental paradigm involves recording from single striatal neurons while stimulating the corticostriatal pathway with pairs of electrical impulses; intervals between the impulses of each pair vary systematically, e.g., from 10–1000 ms. The excitatory cortical volley evoked by the first impulse elicits a spike discharge from Type I and Type II neurons. As a consequence of this initial response, however, voltage-dependent conductances of target striatal neurons are activated, and activity is generated within local circuitry (e.g., axon collateral systems and interneurons) and within efferent projections to other brain systems with which the striatum is reciprocally connected (e.g., substantia nigra). These additional events provide the basis for feedforward/feedback modulation of striatal cell response to subsequent cortical input. The second impulse of each pair of impulses reveals the net effect of this modulation. Our analyses have shown that, for a wide range of interstimulus intervals (ISIs), the probabilities of Type I and Type II spike discharge to the second impulse are inversely related, indicating that the two populations are modulated by different feedforward and/or feedback mechanisms.