Modulation By Melatonin Of Signal Processing Of Outer Retinal Neurons

Melatonin, the major hormone of the pineal gland, is involved in a variety ofphysiological functions through activating specific receptors coupled to GTP-bindingprotein. Melatonin and its receptors are abundant in the retina. In the present work,modulation by melatonin of activities of carp retinal second-order neurons, horizontalcells and bipolar cells, is studied using multidisciplinary approaches.Immunocytochemical data revealed MT1 receptor was diffusely distributed inthe retina, with the labeling being rather strong in the outer plexiform layer and thedistal part of the inner nuclear layer. Double labeling with GAD showed that H1 cellswere strongly MT1-immunoreactive. By the patch clamp technique, melatonin wasfurther shown to potentiate glutamate-induced currents, which was mediatedexclusively through AMPA receptor, from isolated H1 cells in a dose-dependentmanner, without affecting the kinetics parameters of rising time and desensitization.The effects of melatonin were reversed by co-application of luzindole, but not by K185, indicating the involvement of the MT1 receptor. Like melatonin, methylene blue(MB) and ODQ, the guanylate cyclase inhibitors, also potentiated the glutamatecurrents, but internal infusion of cGMP suppressed them. The potentiation of theglutamate response by melatonin was no longer observed in the presence of cGMP orMB. Furthermore, additive effects were found when the concentrations of melatoninand MB were lower. These results suggest that the melatonin effects may be mediatedby decreasing the intracellular concentration of cGMP in the horizontal cells.Consistent with the above observations, melatonin of physiological concentrationsdepolarized the membrane potential of H1 cells and reduced their light responses,intracellularly recorded in the isolated superfused carp retina. The melatonin effectspersisted in the presence of spiperone, SCH 23390, picrotoxin and strychnine,suggesting that the effects must be produced, at least in part, by a direct action ofmelatonin on the H1 cells. Such modulation by melatonin of glutamatergictransmission from cones to horizontal cells may be in part responsible for circadianchanges in light responsiveness of cone horizontal cells, which have been shown inteleost retina.We further tested effects of melatonin on bipolar cells. Intracellular recordingsshowed that melatonin depolarized rod-dominant ON bipolar cells (Rod-ON-BCs)and decreased their light responses, but did not affect cone-dominant ON bipolar cells(Cone-ON-BCs). The effects of melatonin on Rod-ON-BCs were reversed by co-application of luzindole or 4-P-PDOT, indicating the involvement of the MT2receptor. Consistent with intracellular recordings, melatonin suppressed the scotopicelectroretinographic (ERG) b-wave, which was also reversed by 4-P-PDOT, but waswithout effects on the photopic ERG b-wave. Moreover, melatonin did not affect thescotopic ERG PIII component, suggesting that the effects of melatonin on the Rod-ON-BC were not due to possible modulation by melatonin of rods.Immunocytochemical assay revealed that melatonin increased the intracellular cGMPof Rod-ON-BCs, and whole-cell patch clamp recordings further showed thatmelatonin activated in these cells a current with a reversal potential of near 0 mV,which was mediated by a cGMP-activated cation channel. Like melatonin, 8-Br-cGMP also depolarized the membrane potential and decreased the light responseswhile MB had opposite effects. These results suggest that modulation by melatonin ofRod-ON-BCs may be mediated by an incread intracellular cGMP induced byactivation of MT2 receptors.In sum, our results provide further evidence that melatonin may function as animportant endogenous neuromodulator by differentially modulating transmission ofrod and cone signals through activating different receptor subtypes.