Local Release of GABAergic Inhibition in the Motor Cortex Induces Immediate-Early Gene Expression in Indirect Pathway Neurons of the Striatum

Abstract

The neocortex is thought to exert a powerful influence over the functions of the basal ganglia via its projection to the striatum. It is not known, however, whether corticostriatal effects are similar across different types of striatal projection neurons and interneurons or are unique for cells having different functions within striatal networks. To examine this question, we developed a method for focal synchronous activation of the primary motor cortex (MI) of freely moving rats by local release of GABAergic inhibition. With this method, we monitored cortically evoked activation of two immediate-early gene protein products, c-Fos and JunB, in phenotypically identified striatal neurons. We further studied the influence of glutamate receptor antagonists on the stimulated expression of c-Fos, JunB, FosB, and NGFI-A.Local disinhibition of MI elicited remarkably selective induction of c-Fos and JunB in enkephalinergic projection neurons. These indirect pathway neurons, through their projections to the globus pallidus, can inhibit thalamocortical motor circuits. The dynorphincontaining projection neurons of the direct pathway, with opposite effects on the thalamocortical circuits, showed very little induction of c-Fos or JunB. The gene response of striatal interneurons was also highly selective, affecting principally parvalbumin-and NADPH diaphorase-expressing interneurons. The glutamate NMDA receptor antagonist MK-801 strongly reduced the cortically evoked striatal gene expression in all cell types for each gene examined. Because the gene induction that we found followed known corticostriatal somatotopy, was dose-dependent, and was selectively sensitive to glutamate receptor antagonists, we suggest that the differential activation patterns reflect functional specialization of cortical inputs to the direct and indirect pathways of the basal ganglia and functional plasticity within these circuits.Key words: immediate-early genes; neural plasticity; basal ganglia; striatum; motor cortex; corticostriatal; coherent activation; picrotoxin; GABA-A; enkephalin; dynorphin; rat A broad range of experimental evidence implicates the basal ganglia in functions related to procedural learning, context-dependent motor control, and reward-related behavior (Apicella et al., 1992;Robbins and Everitt, 1992;Graybiel et al., 1994;Graybiel, 1995;Hikosaka et al., 1995;Houk et al., 1995;Schultz, 1995;Knowlton et al., 1996). These functions share the property of requiring both rapid and long-term plasticity of neural connections and synaptic efficacies. Electrophysiological and biochemical findings support the view that the glutamatergic cortico-basal ganglia pathways exhibit such plasticity. Both long-term depression (LTD) and long-term potentiation (LTP) have been demonstrated in the striatum after cortical stimulation (for review, see Calabresi et al., 1996). Immediate-early genes, considered potential indicators of neuronal activity capable of producing long-term alterations in neuronal properties, also have been shown to be induced in t...

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“…The bulk of evidence in favor of a synapse-to-nucleus signal in vertebrate neurons lies in the observation that N-methyl-Daspartate (NMDA) receptor blockade inhibits immediate early gene expression in most cases (45,46; but see ref. 47).…”