Melatonin Potentiates Glycine Currents Through A Distinct PC-PLC/PKC Signaling Pathway In Rat Retinal Ganglion Cells

Melatonin (MEL), a significant hormone, is widely distributed in nervous system. In CNS, MEL is secreted by the pineal gland, whereas it is synthesized and released by photoreceptors in the retina. By binding with specific G protein coupled receptors, in additions to its role as hormone, MEL exerts its profound influences on retinal functions as a neuromodulator.Previous studies in our lab have demanstrated that MEL modulates the activity of neurons in the outer retina of Carassius carassius. Most recently, we reported that MEL potentiated glycine currents of rat retinal ganglion cells. In this work we investigated the mechanisms underlying MEL-induced potentiation of glycine currents in rat retinal ganglion cells (RGCs).Immunofluorescence double labeling showed that rat RGCs were solely immunoreactive to MEL MT2 receptors. MEL potentiated glycine currents of RGCs, which was reversed by the MT2 receptor antagonist 4-P-PDOT. The MEL effect was blocked by intracellular dialysis of GDP-P-S.Preincubation with pertussis toxin (PTX) eliminated the MEL-induced potentiation of glycine currents. All these results suggest that PTX-sensitive Gi/o proteins mediated the MEL effect.It is suggested that three signaling pathways may be involved in MEL induced effects following activation of its receptors:cAMP-PKA, cGMP-PKG and PLC-PKC. To identify which of them mediated the effect reported in this work, we conducted the following experiments. Application of the phosphatidylcholine (PC)-specific phospholipase C (PLC) inhibitor D609, but not the phosphatidylinositol (PI)-PLC inhibitor U73122, blocked the MEL effect. The protein kinase C (PKC) activator PMA potentiated the glycine currents and in the presence of PMA MEL failed to cause further potentiation of the currents, whereas application of the PKC inhibitor bisindolylmaleimide IV (Bis IV) abolished the MEL-induced potentiation.The MEL effect persisted when intracellular Ca2+ was chelated by BAPTA, and MEL induced no increase in intracellular Ca2+. Neither cAMP-PKA nor cGMP-PKG signaling pathways seemed to be involved because 8-Br-cAMP or 8-Br-cGMP failed to cause potentiation of the glycine currents and both the PKA inhibitor H-89 and the PKG inhibitor KT5823 did not block the MEL-induced potentiation in consequence.We conclude that the distinct PC-PLC/PKC signaling pathway, following the activation of Gi/o-coupled MT2 receptors, is most likely responsible for the MEL-induced potentiation of glycine currents of rat RGCs. Furthermore, in rat retinal slices MEL potentiated light-evoked glycine receptor-mediated inhibitory postsynaptic currents in RGCs, which was reversed by 4-P-PDOT. These results suggest that MEL, being at higher levels at night, may help animals to detect positive or negative contrast in night vision by modulating inhibitory signals largely mediated by glycinergic amacrine cells in the inner retina.