The modulation of neuronal activity by melatonin: In vitro studies on mouse hippocampal slices

The influence of melatonin on evoked potentials recorded from the CAI field of mouse hippocampal slices was investigated. Melatonin (0.1-2.0 mM) and i ts analog, 6-chloromelatonin (0.1-0.5 mM) depressed evoked potentials (EPSP and the population spike) in a concentration-dependent manner. The melaton in-induced depression was followed by a slow recovery phase. Since the fibe r potential was not affected, it was concluded that melatonin influenced sy naptic efficiency and-or cell excitability. Luzindole, an antagonist of MT2 melatonin receptors, although slightly depressing evoked potentials when a pplied by itself (100 muM), blocked any further inhibition by melatonin whe n added afterwards. We concluded that melatonin reduced synaptic efficiency and:or excitability of hippocampal neurons most likely through interaction with MT2 melatonin receptors, but other possible mechanisms of melatonin a ction are also considered.