The modulatory roles of dopamine (DA) in inhibitory transmission onto striatal large cholinergic interneurones were investigated in rat brain slices using patch‐clamp recording. Pharmacologically isolated GABAA receptor‐mediated IPSCs were recorded by focal stimulation within the striatum. Bath application of DA reversibly suppressed the amplitude of evoked IPSCs in a concentration‐dependent manner (IC50, 10.0 μm). A D2‐like receptor agonist, quinpirole (3–30 μm), also suppressed the IPSCs, whereas a D1‐like receptor agonist, SKF 81297, did not affect IPSCs. Sulpiride, a D2‐like receptor antagonist, blocked the DA‐induced suppression of IPSCs (apparent dissociation constant (KB), 0.36 μm), while a D1‐like receptor antagonist, SCH 23390 (10 μm), had no effect. DA (30 μm) reduced the frequency of spontaneous miniature IPSCs (mIPSCs) without changing their amplitude distribution, suggesting that GABA release was inhibited, whereas the sensitivity of postsynaptic GABAA receptors was not affected. The effect of DA on the frequency of mIPSCs was diminished when extracellular Ca2+ was replaced by Mg2+ (5 mm), indicating that DA affected the Ca2+ entry into the presynaptic terminal. An N‐type Ca2+ channel selective blocker, ω‐conotoxin GVIA (ω‐CgTX, 3 μm), suppressed IPSCs by 65.4%, whereas a P/Q‐type Ca2+ channel selective blocker, ω‐agatoxin IVA (ω‐Aga‐IVA, 200 nm), suppressed IPSCs by 78.4%. Simultaneous application of both blockers suppressed IPSCs by 95.9%. Assuming a 3rd power relationship between Ca2+ concentration and transmitter release, the contribution of N‐, P/Q‐ and other types of Ca2+ channels to presynaptic Ca2+ entry is estimated to be, respectively, 29.8, 40.0 and 34.5% at this synapse. After the application of ω‐CgTX, DA (30 μm) no longer affected IPSCs. In contrast, ω‐Aga‐IVA did not alter the level of suppression by DA, suggesting that the action of DA was selective for N‐type Ca2+ channels. A G protein alkylating agent, N‐ethylmaleimide (NEM), significantly reduced the DA‐induced suppression of IPSCs. These results suggest that presynaptic D2‐like receptors are present on the terminals of GABAergic afferents to striatal cholinergic interneurones, and down‐regulate GABA release by selectively blocking N‐type Ca2+ channels through NEM‐sensitive G proteins.

The effects of dopamine (DA) on non‐NMDA glutamatergic transmission onto dopaminergic neurones in the ventral tegmental area (VTA) were examined in rat midbrain slices using the whole‐cell patch‐clamp technique. EPSCs in dopaminergic neurones evoked by focal stimulation within the VTA were reversibly blocked by 5 μM CNQX in the presence of bicuculline (20 μM), strychnine (0.5 μM) and D‐amino‐5‐phosphonopentanoic acid (D‐AP5, 25 μM). Bath application of DA reduced the amplitude of EPSCs up to 65.1 ± 9.52% in a concentration‐dependent manner between 0.3‐1000 μM (IC50, 16.0 μM) without affecting the holding current at −60 mV measured using a Cs+‐filled electrode. The effect of DA on evoked EPSCs was mimicked by the D2‐like receptor agonist quinpirole but not by the D1‐like receptor agonist SKF 81297, and was antagonized by the D2‐like receptor antagonist sulpiride (KB, 0.96 μM), but not by the D1‐like receptor antagonist SCH 23390 (KB, 228.6 μM). Dopamine (30 μM) reduced the mean frequency of spontaneous miniature EPSCs (mEPSCs) without affecting their mean amplitude, and the DA‐induced effect on the mEPSCs was dependent on the external Ca2+ concentration. These results suggest that afferent glutamatergic fibres which terminate on VTA dopaminergic neurones possess presynaptic D2‐like receptors, activation of which inhibits glutamate release by reducing Ca2+ influx.

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