Inhibition by melatonin of dopamine release from rat hypothalamus: regulation of calcium entry

The [Ca²⁺] dependency of dopamine release evoked by electrical field stimulation of hypothalamic tissue from female rats at the estrous stage was assessed in the presence of melatonin. At concentrations of 10^-9-10^-6 M, melatonin inhibited the [Ca²⁺]-dependent dopamine release. Melatonin reduced tissue uptake of ⁴⁵Ca²⁺ during stimulation either by an electrical field or by elevated K⁺ concentration. About 90% of this inhibition by melatonin of ⁴⁵Ca²⁺ uptake, as well as of dopamine release, was not observed in the presence of the calcium ionophore A23187. Hence, the inhibitory effect on dopamine release by melatonin may stem from the reduction of calcium entry into the presynaptic nerve endings.