Although melatonin receptors are widely expressed in the mammalian central nervous system and peripheral tissues, there are limited data regarding the functions of melatonin in cerebellar Purkinje cells. Here, we identified a novel functional role of melatonin in modulating P‐type Ca2+ channels and action‐potential firing in rat Purkinje neurons. Melatonin at 0.1 μm reversibly decreased peak currents (IBa) by 32.9%. This effect was melatonin receptor 1 (MTR1) dependent and was associated with a hyperpolarizing shift in the voltage dependence of inactivation. Pertussis toxin pretreatment, intracellular application of QEHA peptide, and a selective antibody raised against the Gβ subunit prevented the inhibit... More
Although melatonin receptors are widely expressed in the mammalian central nervous system and peripheral tissues, there are limited data regarding the functions of melatonin in cerebellar Purkinje cells. Here, we identified a novel functional role of melatonin in modulating P‐type Ca2+ channels and action‐potential firing in rat Purkinje neurons. Melatonin at 0.1 μm reversibly decreased peak currents (IBa) by 32.9%. This effect was melatonin receptor 1 (MTR1) dependent and was associated with a hyperpolarizing shift in the voltage dependence of inactivation. Pertussis toxin pretreatment, intracellular application of QEHA peptide, and a selective antibody raised against the Gβ subunit prevented the inhibitory effects of melatonin. Pretreatment with phosphatidylinositol 3‐kinase (PI3K) inhibitors abolished the melatonin‐induced decrease in IBa. Surprisingly, melatonin responses were not regulated by Akt, a common downstream target of PI3K. Melatonin treatment significantly increased protein kinase C (PKC) activity 2.1‐fold. Antagonists of PKC, but not of protein kinase A, abolished the melatonin‐induced decrease in IBa. Melatonin application increased the membrane abundance of PKCδ, and PKCδ inhibition (either pharmacologically or genetically) abolished the melatonin‐induced IBa response. Functionally, melatonin increased spontaneous action‐potential firing by 53.0%; knockdown of MTR1 and blockade of P‐type channels abolished this effect. Thus, our results suggest that melatonin inhibits P‐type channels through MTR1 activation, which is coupled sequentially to the βγ subunits of Gi/o‐protein and to downstream PI3K‐dependent PKCδ signaling. This likely contributes to its physiological functions, including spontaneous firing of cerebellar Purkinje neurons.
