D2‐dopamine receptor‐mediated inhibition of intracellular Ca2+ mobilization and release of acetylcholine from guinea‐pig neostriatal slices

1 The effect of dopamine receptor activation on electrically‐or high K+ (30 mm)‐evoked neurotransmitter release and rise in intracellular Ca2+ concentration was investigated using slices of guinea‐pig neostriatum. 2 A specific D2‐dopamine receptor agonist, LY‐171555 (a laevorotatory enantiomer of LY‐141865: N‐propyl tricyclic pyrazole) at 10−6 m inhibited electrical stimulation‐and high K+‐evoked release of [3H]‐acetylcholine ([3H]‐ACh) to 47.7 ± 6.0% and 54.1 ± 5.0% of control, respectively. The maximal inhibition by LY‐171555 at 10−5 m was 54.8 ± 5.1% reduction of the control. The half‐maximal effective concentration (EC50) of LY‐171555 for the inhibition of [3H]‐ACh release was 2.3 × 10−7 m. A specific D2‐dopamine receptor antagonist, (−)‐sulpiride (10−7 m) reversed the inhibition of [3H]‐ACh release induced by LY‐171555. A specific D1‐dopamine receptor agonist, SK&F 38393 (2,3,4,5‐tetrahydro‐7,8‐dihydroxy‐1‐phenyl‐1H‐benzazepine) (10−5 m) had no effect on the release of [3H]‐ACh. LY‐171555 (10−6 m) also inhibited the high K+‐evoked endogenous glutamate release, by 47% of control. This inhibitory effect was reversed by (−)‐sulpiride (10−7 m). 3 We used a fluorescent, highly selective Ca2+ indicator, ‘quin 2’ to measure intracellular free Ca2+ concentrations ([Ca2+]i). Electrical stimulation of slices preloaded with quin 2 led to an elevation of relative fluorescence intensity and this response was reduced by the removal of Ca2+ from the bathing medium. These results indicate that the enhanced elevation in fluorescence intensity in the quin 2‐loaded slices reflects the increase of intracellular free Ca2+ concentration, [Ca2+]i. 4 The mixed D1‐and D2‐receptor agonist, apomorphine and LY‐171555 inhibited the increase of [Ca2+]i induced by electrical stimulation or high K+ medium, in a concentration‐dependent manner, while SK&F 38393 did not affect the increase of [Ca2+]i. The maximal inhibitory effect of LY‐171555 at 3 × 10−5 m was 35 ± 3% reduction in control values. The inhibitory effect of LY‐171555 was antagonized by (−)‐sulpiride (10−7 m). 5 There was a high correlation (r = 0.997, P < 0.05) between the D2‐receptor‐mediated inhibition of the stimulated rise of [Ca2+]i and [3H]‐ACh release. 6 When the slices were superfused with the Ca2+‐free medium containing EGTA (10−4 m) for 5 min, the rise in [Ca2+]i was markedly suppressed to 18.0% of control by LY‐171555 (10−6 m). 7 These data indicate that activation of the D2‐dopamine receptor suppresses the elevation of [Ca2+]i induced by depolarizing stimuli. This may be due to inhibition of mobilization of Ca2+ from the intracellullar store. We propose that the D2‐receptor‐mediated inhibition of transmitter release is probably due to a reduction in intracellular Ca2+ mobilization.