Melatonin (Mel) promotes sleep through G protein-coupled receptors. However, the downstream molecular target(s) is unknown. We identified the Caenorhabditis elegans BK channel SLO-1 as a molecular target of the Mel receptor PCDR-1-. Knockout of pcdr-1, slo-1, or homt-1 (a gene required for Mel synthesis) causes substantially increased neurotransmitter release and shortened sleep duration, and these effects are nonadditive in double knockouts. Exogenous Mel inhibits neurotransmitter release and promotes sleep in wild-type (WT) but not pcdr-1 and slo-1 mutants. In a heterologous expression system, Mel activates the human BK channel (hSlo1) in a membrane-delimited manner in the presence of the Mel receptor MT1 but not MT2. A peptide acting to release free Gβγ also activates hSlo1 in a MT1-dependent and membrane-delimited manner, whereas a Gβλ inhibitor abolishes the stimulating effect of Mel. Our results suggest that Mel promotes sleep by activating the BK channel through a specific Mel receptor and Gβλ.
Melatonin (Mel) is generally known as a hormone of darkness or a sleep hormone because it plays a pivotal role in sleep. In the human brain, Mel is secreted by the pineal gland with low levels during the day and high levels at night. This rhythmic secretion of Mel is under the control of neurons in the suprachiasmatic nucleus (SCN), which is the site of a circadian master clock, and receives synaptic inputs from photosensitive neurons in the retina. Many people take Mel supplements as a sleep aid.
Mel is believed to produce its sleep effect through Mel receptors. Two Mel receptors exist in mammals: MT1 and MT2. Experiments with knockout mice indicate that MT1 and MT2 deficiencies compromise rapid eye movement (REM) sleep and non-REM sleep, respectively. MT1 signaling is also important to circadian rhythmic expression of several clock genes. However, it is unclear whether MT1 and MT2 play similar roles in humans, mainly due to a lack of sufficiently selective receptor agonists and antagonists. Although it is well established that MT1 and MT2 function through Gi-type G proteins, the downstream molecular targets leading to its sleep-promoting effect remain mysterious. Protein interactome mining indicates that MT1 but not MT2 is an integral component of a presynaptic protein complex, but the physiological functions of presynaptic MT1 are undetermined.
