Subregion-Specific Modulation of Excitatory Input and Dopaminergic Output in the Striatum by Tonically Activated Glycine and GABA(A) Receptors

Abstract

The flow of cortical information through the basal ganglia is a complex spatiotemporal pattern of increased and decreased firing. The striatum is the biggest input nucleus to the basal ganglia and the aim of this study was to assess the role of inhibitory GABA(A) and glycine receptors in regulating synaptic activity in the dorsolateral striatum (DLS) and ventral striatum (nucleus accumbens, nAc). Local field potential recordings from coronal brain slices of juvenile and adult Wistar rats showed that GABA(A) receptors and strychnine-sensitive glycine receptors are tonically activated and inhibit excitatory input to the DLS and to the nAc. Strychnine-induced disinhibition of glutamatergic transmission was insensitive to the muscarinic receptor inhibitor scopolamine (10 μM), inhibited by the nicotinic acetylcholine receptor antagonist mecamylamine (10 μM) and blocked by GABA(A) receptor inhibitors, suggesting that tonically activated glycine receptors depress excitatory input to the striatum through modulation of cholinergic and GABAergic neurotransmission. As an end-product example of striatal GABAergic output in vivo we measured dopamine release in the DLS and nAc by microdialysis in the awake and freely moving rat. Reversed dialysis of bicuculline (50 μM in perfusate) only increased extrasynaptic dopamine levels in the nAc, while strychnine administered locally (200 μM in perfusate) decreased dopamine output by 60% in both the DLS and nAc. Our data suggest that GABA(A) and glycine receptors are tonically activated and modulate striatal transmission in a partially subregion-specific manner.

Keywords: basal ganglia; dorsolateral striatum; nucleus accumbens; rat.

Figure 1

Excitatory input to the striatum is inhibited by GABAergic neurotransmission. (A) Schematic picture showing the location of stimulating electrodes (black) and recording electrodes (gray) in the dorsolateral striatum (DLS) and in the nAc core. (B,C) Treatment with 20 μM, but not 2 μM, of the GABA_A receptor antagonist bicuculline enhanced PS amplitude in both the DLS and the nAc. (D) Example figure showing absolute PS amplitude over time in the DLS and nAc during bicuculline-treatment. (E,F) The increase in PS amplitude induced by 20 μM bicuculline sustained in slices from adult Wistar rats. (G) Treatment with the GABA_A receptor blocker picrotoxin (100 μM) increased PS amplitude in the DLS, the dorsomedial striatum (DMS), and in the nAc. Data show mean PS amplitudes compared to baseline with SEM.

Figure 2

Glycine receptors are tonically activated and depress excitatory input. (A) Inhibition of glycine receptors with 0.1 μM strychnine induced a small increase in PS amplitude in the nAc, and there was a trend for a similar effect in the DLS. Example traces show evoked PSs at baseline (gray), and after 15–20 min exposure to 0.1 μM strychnine (black) in the DLS. (B) Treatment with a higher concentration of strychnine enhanced PS amplitude in both the DLS and in the nAc. Example traces show evoked PSs at baseline (gray), and after 15–20 min exposure to 1 μM strychnine (black) in the DLS. (C) The enhancing effect displayed by strychnine was not depressed over time. Example traces show evoked PSs at baseline (gray), and after 50–55 min exposure to 1 μM strychnine (black) in the nAc. (D,E) Strychnine enhanced PS amplitude to a similar extent in slices from adult Wistar rats. (F) PS amplitudes in the DLS and in the nAc were also enhanced by the glycine receptor antagonist PMBA (50 μM). Data show mean PS amplitudes compared to baseline with SEM. Calibration: 0.2 mV, 2 ms.

Figure 3

Strychnine-induced enhancement of PS amplitude involves nicotinic but not muscarinic acetylcholine receptors. (A) Treatment with the nicotinic acetylcholine receptor inhibitor mecamylamine (10 μM) did not enhance PS amplitude in the DLS or in the nAc core. Example traces show evoked population spikes in the DLS at baseline (gray) and after 15–20 min of mecamylamine-treatment (black). (B) The strychnine-induced enhancement of PS amplitudes recorded in the nAc core remained in slices treated with the muscarinic acetylcholine receptor antagonist scopolamine (10 μM), but was significantly depressed in mecamylamine-treated slices. Example traces show evoked population spikes at baseline (gray) and after 15–20 min of strychnine administration in a scopolamine-treated slice (black). Data show mean PS amplitudes compared to baseline with SEM. Calibration: 0.2 mV, 2 ms.

Figure 4

Tonically activated glycine receptors depress excitatory input to the striatum via GABA_A receptors. (A,B) The strychnine- induced enhancement of PS amplitude was significantly depressed in slices treated with the GABA_A receptor blockers picrotoxin (100 μM) or bicuculline (20 μM). Example traces show evoked population spikes at baseline (gray) and after 15–20 min of strychnine-treatment (black) in a control slice (A) and in a bicuculline-treated slice (B). (C) The increase in PS amplitude induced by the glycine receptor antagonist PMBA (50 μM) was also prevented in slices treated with bicuculline (20 μM). Example traces show evoked population spikes at baseline (black) and after 15–20 min of PMBA-treatment (gray). (D) Slices treated with a lower concentration of bicuculline (2 μM) also inhibited strychnine-induced enhancement of PS amplitudes. Example traces show evoked population spikes in the nAc at baseline (black) and after 15–20 min of strychnine perfusion in a bicuculline-treated slice (gray). Data show mean PS amplitudes compared to baseline with SEM. Calibration: 0.2 mV, 2 ms.

Figure 5

Location of the microdialysis probes in the DLS and nAc. The black lines indicate the track of individual microdialysis probes in the DLS (A) and the nAc (B). Distance from bregma (in mm) is shown to the right in each figure.

Figure 6

The microdialysate concentration of dopamine is regulated by GABA_A and glycine receptors. (A) The microdialysate concentration of dopamine was significantly enhanced by 100 μM but not by 50 μM bicuculline in the DLS. (B) Bicuculline (50 μM) applied by reversed microdialysis in the nAc enhanced the extracellular level of dopamine. (C) The increase in dopamine output induced by bicuculline administration in the nAc was not significantly modulated by mecamylamine co-perfused in the VTA. (D,E) The microdialysate concentration of dopamine was not depressed by 20 μM strychnine in either brain region. (F) A higher concentration of strychnine (200 μM) significantly depressed dopamine levels in both the DLS and in the nAc.