D1-like dopamine receptors selectively block P/Q-type calcium channels to reduce glutamate release onto cholinergic basal forebrain neurones of immature rats

Whole-cell patch-clamp recordings of non-NMDA glutamatergic EPSCs were made from identified cholinergic neurones in slices of basal forebrain (BF) of young rats (P13–P18), to investigate the subtypes of calcium channels involved in dopamine D 1 -like receptor-mediated presynaptic inhibition of the EPSCs. The BF cholinergic neurones were pre-labelled by intracerebroventricular injection of a fluorescent marker, Cy3-192IgG. A D 1 -like receptor agonist, SKF 81297 (30 μ m ) suppressed the EPSCs reversibly by about 30%, and this inhibition was reproducible. Calcium channel subtypes involved in the glutamatergic transmission were elucidated using selective Ca 2+ channel blockers. The N-type Ca 2+ channel blocker ω-conotoxin (ω-CgTX, 3 μ m ) suppressed the EPSCs by 57.5%, whereas the P/Q-type channel selective blocker ω-agatoxin-TK (ω-Aga-TK, 200 n m ) suppressed the EPSCs by 68.9%. Simultaneous application of both blockers suppressed the EPSCs by 96.1%. The R-type Ca 2+ channel blocker SNX-482 (300 n m ) suppressed the EPSCs by 18.4%, whereas nifedipine, the L-type Ca 2+ channel blocker (10 μ m ), had little effect. In the presence of ω-Aga-TK, SKF 81297, a dopamine D 1 -like receptor agonist, had no effect on the EPSCs. On the other hand, SKF 81297 could still inhibit the EPSCs in the presence of either ω-CgTX, SNX-482 or nifedipine. SKF 81297 had no further effect on the EPSCs when external Ca 2+ concentration was raised to 7.2 m m in the presence of ω-Aga-TK, but could still inhibit the EPSCs in high Ca 2+ solution after ω-CgTX application. Forskolin (FK, 10 μ m ), an activator of adenylyl cyclase pathway, suppressed the EPSCs, and the FK-induced effect was mostly blocked in the presence of ω-Aga-TK but not that of ω-CgTX. These results suggest that D 1 -like receptor activation selectively blocks P/Q-type calcium channels to reduce glutamate release onto BF cholinergic neurones.