Attenuation of Stimulated Accumbal Dopamine Release by NMDA Is Mediated through Metabotropic Glutamate Receptors

Abstract

Electrically stimulated dopamine release from the nucleus accumbens is attenuated following application of N-methyl-d-aspartate (NMDA), which is likely to be mediated indirectly through intermediary neuronal mechanisms rather than by a direct action on dopamine terminals. On the basis of known modulatory processes in nucleus accumbens, the current experiments sought to test whether the effect of NMDA was mediated through cholinergic, GABA-ergic, or metabotropic glutamatergic intermediate mechanisms. Fast-scan cyclic voltammetry was used to measure electrically stimulated dopamine release in nucleus accumbens of rat brain slices in vitro. Stimulated dopamine release was attenuated by NMDA, confirming previous findings, but this attenuation was unaffected by either cholinergic or GABA-ergic antagonists. However, it was completely abolished by the nonselective group I/II/III metabotropic glutamate receptor antagonist α-methyl-4-carboxyphenylglycine (MCPG) and by the selective group II antagonist LY 341396. Therefore, group II metabotropic glutamate receptors, but not acetylcholine or GABA receptors, mediate the attenuation of stimulated dopamine release caused by NMDA, probably by presynaptic inhibition through receptors located extra-synaptically on dopamine terminals. This provides a plausible mechanism for the documented role of metabotropic glutamate receptor systems in restoring deficits induced by NMDA receptor antagonists, modeling schizophrenia, underlining the potential for drugs affecting these receptors as therapeutic agents in treating schizophrenia.

Keywords: Brain slices; N-methyl-d-aspartate (NMDA); dopamine; fast-scan cyclic voltammetry; metabotropic glutamate receptors; nucleus accumbens.

Figure 1

Effect of DHβE (1 μM) on attenuation of stimulated dopamine release caused by NMDA (30 μM). (a) Electrically stimulated dopamine release over repeated stimulations at 3 min intervals, presented as the mean ± SEM percentage of release during the baseline period (stimulations S1–S4). DHβE was applied in the superfusate for 12 min during stimulations S5–S12 (light gray panel, A), NMDA was applied for 12 min during stimulations S9–S14 (dark gray panel, B). *p < 0.05; **p < 0.01: significant difference from the no drug condition (post hoc Fisher’s LSD based on significant interaction in three-way ANOVA). ^†p < 0.05; ^††p < 0.01: significant difference from DHβE baseline immediately prior to NMDA + DHβE application (NMDA + DHβE condition (red line)); n = 8 per treatment condition. (b) Mean ± SEM responses during baseline (S4), drug A (S8), drug B (S12), and washout (S18) in the four treatment conditions. Stimulation application is indicated by the yellow arrow. Data are normalized to the maximum response during the baseline recording (S4): n = 8 per treatment condition.

Figure 2

Effect of scopolamine (1 μM) on attenuation of stimulated dopamine release caused by NMDA (30 μM). (a) Electrically stimulated dopamine release over repeated stimulations at 3 min intervals, presented as the mean ± SEM percentage of release during the baseline period (stimulations S1–S4). Drugs were applied in the superfusate, either alone or in combination, for 12 min during stimulations S5–S12 (gray panel). *p < 0.05; **p < 0.01: significant difference from the no drug condition (post hoc Fisher’s LSD based on significant interaction in three-way ANOVA); n = 8 per treatment condition. (b) Mean ± SEM responses during baseline (S4), drug (S8), and washout (S14) in the four treatment conditions (Scop = scopolamine). Stimulation application is indicated by the yellow arrow. Data are normalized to the maximum response during the baseline recording (S4): n = 8 per treatment condition.

Figure 3

Effect of CGP 54626 (1 μM) on attenuation of stimulated dopamine release caused by NMDA (30 μM). (a) Electrically stimulated dopamine release over repeated stimulations at 3 min intervals, presented as mean ± SEM percentage of release during the baseline period (stimulations S1–S4). CGP 54626 was applied in the superfusate for 12 min during stimulations S5–S12 (light gray panel, A), and NMDA was applied for 12 min during stimulations S9–S14 (dark gray panel, B). *p < 0.05; **p < 0.01: significant difference from the no drug condition (post hoc Fisher’s LSD based on significant interaction in three-way ANOVA). ^†p < 0.05; ^††p < 0.01: significant difference from CGP 54626 baseline immediately prior to NMDA + CGP 54626 application (NMDA + CGP 54626 condition (red line)); n = 8 per treatment condition. (b) Mean ± SEM responses during baseline (S4), drug A (S8), drug B (S12), and washout (S18) in the four treatment conditions (CGP = CGP 54626). Stimulation application is indicated by the yellow arrow. Data are normalized to the maximum response during the baseline recording (S4): n = 8 per treatment condition.

Figure 4

Effect of MCPG (100 μM) on attenuation of stimulated dopamine release caused by NMDA (30 μM). (a) Electrically stimulated dopamine release over repeated stimulations at 3 min intervals, presented as mean ± SEM percentage of release during the baseline period (stimulations S1–S4). Drugs were applied in the superfusate, either alone or in combination, for 12 min during stimulations S5–S12 (gray panel). *p < 0.05; **p < 0.01: significant difference from the no drug condition: ^††p < 0.01: significant difference between NMDA application in the absence or presence of MCPG (post hoc Fisher’s LSD based on significant interaction in three-way ANOVA): n = 8 per treatment condition. (b) Mean ± SEM responses during baseline (S4), drug (S8), and washout (S14) in the four treatment conditions. Stimulation application is indicated by the yellow arrow. Data are normalized to the maximum response during the baseline recording (S4): n = 8 per treatment condition.

Figure 5

Effect of LY 341495 (1 μM) on attenuation of stimulated dopamine release caused by NMDA (30 μM). (a) Electrically stimulated dopamine release over repeated stimulations at 3 min intervals, presented as mean ± SEM percentage of release during the baseline period (stimulations S1–S4). Drugs were applied in the superfusate, either alone or in combination, for 12 min during stimulations S5–S12 (gray panel). *p < 0.05; **p < 0.01: significant difference from the no drug condition; significant difference between NMDA application in the absence or presence of LY 341495 (post hoc Fisher’s LSD based on significant interaction in three-way ANOVA): n = 8 per treatment condition. (b) Mean ± SEM responses during baseline (S4), drug (S8), and washout (S14) in the four treatment conditions. Stimulation application is indicated by the yellow arrow. Data are normalized to the maximum response during the baseline recording (S4): n = 8 per treatment condition.

Figure 6

Summary of the effects of antagonists on the attenuation of electrically stimulated dopamine caused by NMDA: the effects of the mGluR antagonists, MCPG (100 μM) and LY 341495 (LY, 1 μM) (red bars); the cholinergic antagonists, DHβE (1 μM) and scopolamine (Scop, 1 μM) (blue bars); and the GABA-B antagonist, CGP 54626 (CGP, 1 μM) (green bars) alone (open bars) and of NMDA (30 μM) plus the respective antagonist (shaded bars). Noncolored bars depict the attenuation caused by NMDA with no antagonist present. All data are the percentage of the mean stimulated release in the three stimulations before the application of the respective drugs. *p < 0.05; **p < 0.01: significant difference between antagonist alone and agonist + antagonist (t test).