Although melatonin is necessary for circadian regulation of sleep, the mechanisms underlying this effect of melatonin are still unclear. In the present study, we showed that melatonin suppressed the activity of GABAergic neurons in the lateral hypothalamus, which has been reported to play a crucial role in maintaining wakefulness.

The inhibitory effect of the melatonin was mediated by activation of melatonin 1 receptors and depended on the inhibition of hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels. At behavioral levels, infusion of melatonin into the lateral hypothalamus significantly decreased the locomotor and exploratory activities and increased the time of immobility in open filed. Additionally, using electroencephalogram (EEG) and electromyogram (EMG) recordings, we found that infusion of melatonin into the lateral hypothalamus decreased the time spent in wakefulness and increased the amount of sleep. Overall, these results suggest that melatonin inhibits GABAergic neurons in the lateral hypothalamus via melatonin 1 receptor-dependent inhibition of the HCN channels, which is consistent with a decrease in wakefulness. These findings provide a new mechanism underlying the hypnotic effect of the melatonin.