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 dynorphin-containing 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.

Fig. 5.

Top. Stimulation of MI with picrotoxin elicits selective gene expression in striatal projection neurons and interneurons. Cortically driven c-Fos induction in the striatum was found very rarely in dynorphin-positive projection neurons (A) but was intense in projection neurons expressing enkephalin (B) and in parvalbumin-containing interneurons (C). JunB was found even more rarely in dynorphin-expressing projection neurons (D) but was strongly expressed in many enkephalin-positive projection neurons (E). JunB was almost never induced in parvalbumin-containing interneurons (F). Arrows indicate examples of doubly labeled neurons; arrowheads indicate examples of singly labeled nuclei. Double-immunolabeling for c-Fos or JunB (black) and dynorphin or parvalbumin (brown) was obtained by combining two different chromogens. In B and E, c-Fos or JunB was labeled with a gold-conjugated antibody followed by silver intensification; enkephalin was labeled with DAB (brown) in B and with Vector VIP (purple) in E. The inset shows an example of triple immunolabeling for c-Fos nuclei (black dot-likestaining) expressed in enkephalin-positive neurons (purple) and in a parvalbumin-positive neuron (brown). Arrows indicate examples of double-labeling. Scale bar (shown in C): 10 μm for all panels.

Fig. 1.

Focal epidural application of picrotoxin onto the MI through a chronically implanted, well induced c-Fos expression and increased metabolic activity in the underlying cortex and in the dorsolateral caudoputamen. A, An example of the microstimulation maps of MI on which stereotaxic coordinates for the implanted wells were based. The area in gray represents the size and the position of the well. F, Foot;V, vibrissae; E, elbow; W, wrist; T, trunk; N, neck;H, hand; D5F, digit 5; X, no response. Coordinates relative to bregma. B, Schematic representation (anterior = 9.7; Paxinos and Watson, 1986) of the most intense c-Fos induction observed in the cortex after picrotoxin application (gray) superimposed on points at which microstimulation elicited movements of the elbow (E) and of the elbow and vibrissae (E/V) at the same position in a different experiment. C, Serial transverse sections through the caudoputamen illustrating the longitudinally extended dorsolateral induction of c-Fos in the striatum after picrotoxin application to the motor cortex. Scale bar, 500 μm.

Fig. 2.

Focal epidural application of picrotoxin induces c-Fos expression in a concentration-dependent manner. A,B, Photomicrographs of sections immunostained for c-Fos from experiments in which 75 μm (A) or 300 μm (B) picrotoxin was applied to MI. A wedge-shaped area of intense c-Fos induction, corresponding to the site of picrotoxin application, is detectable. In both cases, c-Fos induction in the striatum is restricted to the dorsolateral caudoputamen. The inset (a) shows bracketed region at higher magnification. Markers above the overlying cortex indicate the approximate location of the well.C, Relationship between the number of immunodetectable c-Fos-positive nuclei in the ipsilateral caudoputamen and the concentration of picrotoxin applied to MI (mean ± SEM;N = 18; n = 3/group). Scale bar, 1 mm.

Fig. 4.

Picrotoxin-induced excitation of MI elicits different patterns of c-Fos, JunB, FosB, and NGFI-A induction in the cerebral cortex (A) and striatum (B). Immediate-early gene induction in the striatum of saline-treated controls (C) was found only along the medial edge of the caudoputamen. Scale bar, 500 μm.

Fig. 3.

Autoradiograms of two parasagittal sections demonstrating the effects of picrotoxin application to MI on the uptake of [¹⁴C] 2-DG. In A, thearrowhead indicates a wedge-shaped region of high uptake approximating the site of picrotoxin application (lateral = 2.4;Paxinos and Watson, 1986). In B, thearrowhead indicates the corresponding region of heightened labeling in the dorsolateral caudoputamen (lateral = 4.6; Paxinos and Watson, 1986). Scale bar, 1 mm.

Fig. 7.

Effects of glutamate receptor antagonists on cortically driven gene expression in the striatum. A, The glutamate NMDA receptor antagonist MK-801 injected systemically before picrotoxin application (n = 10) significantly reduced picrotoxin-stimulated c-Fos, JunB, NGFI-A, and FosB induction in the caudoputamen relative to the levels of induction found in control rats that received vehicle injection before picrotoxin application (n = 10; p ≤ 0.025). B, The glutamate AMPA receptor antagonist GYKI 52466 increased JunB induction in the caudoputamen but did not have detectable effects on c-Fos, NGFI-A, or FosB induction relative to control levels in rats treated with vehicle before picrotoxin (n = 10; p ≤ 0.025).Asterisks indicate values significant by Kruskal–Wallis testing followed by comparisons of treatments versus control;circled asterisks indicate differences by ANOVA testing followed by Scheffé’s post hoc test.

Fig. 8.

Top. Model of MI activation of the sensorimotor striatum. Local application of picrotoxin to MI relieves projection neurons of inhibitory control by intrinsic GABAergic interneurons. Corticostriatal neurons (Ctx;red) become activated as a consequence and in turn stimulate a restricted population of striatal neurons (CPu; red) to express the immediate-early gene products c-Fos and JunB. Most of the activated striatal neurons are enkephalinergic (e) and thus are part of the indirect basal ganglia pathway projecting to the external pallidal segment (GPe). Dynorphin-positive projection neurons (d), part of the direct pathway projecting to the internal pallidum (EN/SN), show very little cortically driven induction of c-Fos and JunB after MI stimulation. Differential cortical effects on these two striatal projection neuron subtypes potentially amplify the specificity with which MI controls the indirect and direct pathways of the basal ganglia. GPe, External pallidum; EN, entopeduncular nucleus;SN, substantia nigra; CPu, caudoputamen;Ctx, motor cortex.