Melatonin is involved in a variety of physiological functions through activating specific receptors coupled to GTP-binding protein.Melatonin and its receptors are abundant in the retina. Here we showfor the first time that melatoninmodulates glutamatergic synaptic transmission fromcones to horizontal cells (HCs) in carp retina. Immunocytochemical data revealed the expression of the MT1 receptoroncarpHCs.Whole-cell recordings further showedthat melatonin ofphysiological concentrationspotentiatedglutamate-inducedcurrents fromisolatedcone-drivenHCs(H1cells) in a dose-dependent manner, by increasing the efficacy and apparent affinity of the glutamate receptor. The effects of melatonin were reversed by luzindole, but not by K 185, indicating the involvement of the MT1 receptor. Like melatonin, methylene blue (MB), a guanylate cyclase inhibitor, also potentiated the glutamate currents, but internal infusion of cGMP suppressed them. The effects of melatonin were not observed in cGMP-filled and MB-incubated HCs. These results suggest that the melatonin effects may be mediated by decreasing the intracellular concentration of cGMP. Consistent with these observations, melatonin depolarized the membrane potential of H1 cells and reduced their light responses, which could also be blocked by luzindole. These effects of melatonin persisted in the presence of the antagonists of receptors for dopamine, GABA and glycine, indicating a direct action of melatonin on H1 cells. Such modulation by melatonin of glutamatergic transmission from cones to HCs is thought to be in part responsible for circadian changes in light responsiveness of cone HCs in teleost retina.

Glycinergic inhibitory postsynaptic currents (IPSCs) focally elicited at the dendrites and axon terminals were recorded from bipolar cells in the bullfrog retinal slice, using the whole-cell clamp technique. IPSCs driven by input from interplexiform cells at bipolar cell dendrites (ipc-IPSCs) had a much slower decay time constant (25.29

pots, which was reversed by co-application of 10μM strychnine. It was further found that illumination of the receptive field surround did not affect the depolarizing center response of RBCs. The above result therefore suggests that glycine modulates the center response of RBCs. Focal application of glycine to either dendrites or axon terminals of RBCs failed to induce any currents in both isolated cell and retinal slice preparations. On the other hand, glycine of 4mM increased the amplitude of the scotopic electroretinographic PIIIcomponent, which reflects the activity of rod photoreceptors. It seems likely that modulation by glycine of the RBC center response may be in part ascribed to a consequence of the potentiation of rod responses by glycine.

Natriuretic peptides (NPs) may work as neuromodulators through their associated receptors [NP receptors (NPRs)]. By immunocytochemistry, we showed that NPR-A and NPR-B were expressed abundantly on both ON-type and OFF-type bipolar cells (BCs) in rat retina, including the dendrites, somata, and axon terminals. Whole-cell recordings made from isolated ON-type BCs further showed that brain natriuretic peptide (BNP) suppressedGABAA receptor-, but notGABAC receptor-, mediated currents of the BCs, which was blocked by the NPR-A antagonist anantin. The NPR-C agonist c-ANF [des(Gln18, Ser19, Gln20, Leu21, Gly22)ANF4 -23-NH2] did not suppress GABAA currents. The BNP effect on GABAA currents was abolished with preincubation with the pGC-A/B antagonist HS-142-1 but mimicked by application of 8-bromoguanosine-3',5'-cyclomonophosphate. These results suggest that elevated levels of intracellular cGMP caused by activation of NPR-A may mediate the BNP effect. Internal infusion of the cGMP-dependent protein kinase G (PKG) inhibitor KT5823 essentially blocked the BNP-induced reduction of GABAA currents. Moreover, calcium imaging showed that BNP caused a significant elevation of intracellular calcium that could be caused by increased calcium release from intracellular stores by PKG. The BNP effect was blocked by the ryanodine receptor modulators caffeine, ryanodine, and ruthenium red but not by the IP3 receptor antagonists heparin and xestospongin-C. Furthermore, the BNP effect was abolished after application of the blocker of endoplasmic reticulum Ca2+-ATPase thapsigargin and greatly reduced by the calmodulin inhibitorsW-7and calmidazolium.Wetherefore conclude that the increased calcium release from ryanodine-sensitive calcium stores byBNPmaybe responsible for the BNP-causedGABAA response suppression in ON-type BCs through stimulating calmodulin.

Subcellular localization and complements of GABAA and GABAC receptors on bullfrog retinal bipolar cells. J Neurophysiol 84: 666-676, 2000. γ-Aminobutyric acid (GABA) receptors on retinal bipolar cells (BCs) are highly relevant to spatial and temporal integration of visual signals in the outer and inner retina. In the present work, subcellular localization and complements of GABAA and GABAC receptors on BCs were investigated by whole cell recordings and local drug application via multi-barreled puff pipettes in the bullfrog retinal slice preparation. Four types of the BCs (types 1-4) were identified morphologically by injection of Lucifer yellow. According to the ramification levels of the axon terminals and the responses of these cells to glutamate (or kainate) applied at their dendrites, types 1 and 2 of BCs were supposed to be OFF type, whereas types 3 and 4 of BCs might be ON type. Bicuculline (BIC), a GABAA receptor antagonist, and imidazole-4-acetic acid (I4AA), a GABAC receptor antagonist, were used to distinguish GABA receptor-mediated responses. In all BCs tested, not only the axon terminals but also the dendrites showed high GABA sensitivity mediated by both GABAA and GABAC receptors. Subcellular localization and complements of GABAA and GABAC receptors at the dendrites and axon terminals were highly related to the dichotomy of OFF and ON BCs. In the case of OFF BCs, GABAA receptors were rather evenly distributed at the dendrites and axon terminals, but GABAC receptors were predominantly expressed at the axon terminals. Moreover, the relative contribution of GABAC receptors to the axon terminals was prevalent over that of GABAA receptors, while the situation was reversed at the dendrites. In the case of ON BCs, GABAA and GABAC receptors both preferred to be expressed at the axon terminals; relative contributions of these two GABA receptor subtypes to both the sites were comparable, while GABAC receptors were much less expressed than GABAA receptors. GABAA, but not GABAC receptors, were expressed clusteringly at axons of a population of BCs. In a minority of BCs, I4AA suppressed the GABAC responses at the dendrites, but not at the axon terminal, implying that the GABAC receptors at these two sites may be heterogeneous. Taken together, these results suggest that GABAA and GABAC receptors may play different roles in the outer and inner retina and the differential complements of the two receptors on OFF and ON BCs may be closely related to physiological functions of these cells.