Role of calcium on the modulation of spontaneous acetylcholine efflux by the D 2 dopamine receptor subtype in rat striatal synaptosomes

Abstract

The role of calcium in the modulation of spontaneous [ 3 H ]acetylcholine ([ 3 H ]ACh) efflux through presynaptic D 2 dopamine hetero-receptors was investigated in rat striatal synaptosomes. The kinetic studies of [ 3 H ]ACh efflux in the presence or absence of Ca 2+ were carried out in nonstimulating conditions. When Ca 2+ was omitted from the superfusion medium, a notable and significant ( P<0.001) decrease of tritium efflux (39%) was obtained. While [ 3 H ]ACh efflux was insensitive to tetrodotoxin (TTX) 1 μM, cadmium (10 μM), a nonselective antagonist of calcium channels, significantly reduced the tritium efflux by 24% ( P<0.001), while the l-type calcium antagonist, nifedipine, (30 μM) inhibited the tritium efflux by only 10% ( P<0.02). 2-(4-Fenylpiperidine)cyclohexanol (vesamicol), an inhibitor of the vesicular [ 3 H ]ACh carrier, significantly depressed the spontaneous tritium efflux in the presence of Ca 2+ (60%; P<0.001) and in a low-calcium medium (20%; P<0.001). Although 1 μM of 7-hydroxy- N, N-di- n-propyl-2-aminotetraline (7-OH-DPAT) inhibited spontaneous [ 3 H ]ACh efflux in the presence of calcium, this dopaminergic agonist did not modify the neurotransmitter release in either the low-Ca 2+ medium or in the presence of vesamicol. These results suggest that the spontaneous [ 3 H ]ACh efflux is a process involving a Ca 2+-dependent component (39%), sensitive to calcium channel-blockers and vesamicol, in rat striatal synaptosomes. In addition, activation of the D 2 dopamine hetero-receptor only modulates the calcium-dependent component of spontaneous [ 3 H ]ACh efflux.