8-bromocyclic-gmp and 4-phenyl-2-propionamidotetraline

In vertebrate retina, melatonin regulates various physiological functions. In this work we investigated the mechanisms underlying melatonin-induced potentiation of glycine currents in rat retinal ganglion cells (RGCs). Immunofluorescence double labelling showed that rat RGCs were solely immunoreactive to melatonin MT(2) receptors. Melatonin potentiated glycine currents of RGCs, which was reversed by the MT(2) receptor antagonist 4-P-PDOT. The melatonin effect was blocked by intracellular dialysis of GDP-beta-S. Either preincubation with pertussis toxin or application of the phosphatidylcholine (PC)-specific phospholipase C (PLC) inhibitor D609, but not the phosphatidylinositol (PI)-PLC inhibitor U73122, blocked the melatonin effect. The protein kinase C (PKC) activator PMA potentiated the glycine currents and in the presence of PMA melatonin failed to cause further potentiation of the currents, whereas application of the PKC inhibitor bisindolylmaleimide IV abolished the melatonin-induced potentiation. The melatonin effect persisted when [Ca(2+)](i) was chelated by BAPTA, and melatonin induced no increase in [Ca(2+)](i). Neither cAMP-PKA nor cGMP-PKG signalling 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 melatonin-induced potentiation. In consequence, a distinct PC-PLC/PKC signalling pathway, following the activation of G(i/o)-coupled MT(2) receptors, is most likely responsible for the melatonin-induced potentiation of glycine currents of rat RGCs. Furthermore, in rat retinal slices melatonin potentiated light-evoked glycine receptor-mediated inhibitory postsynaptic currents in RGCs. These results suggest that melatonin, 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.

Topics: 8-Bromo Cyclic Adenosine Monophosphate; Animals; Bridged-Ring Compounds; Calcium; Carbazoles; Cyclic GMP; Cyclic GMP-Dependent Protein Kinases; Egtazic Acid; Estrenes; Glycine; Guanosine Diphosphate; Indoles; Isoquinolines; Male; Maleimides; Melatonin; Norbornanes; Pertussis Toxin; Protein Kinase C; Pyrrolidinones; Rats; Rats, Sprague-Dawley; Receptor, Melatonin, MT2; Retinal Ganglion Cells; Signal Transduction; Sulfonamides; Tetradecanoylphorbol Acetate; Tetrahydronaphthalenes; Thiocarbamates; Thiones; Type C Phospholipases

Recent investigations have demonstrated an influence of melatonin on insulin secretion in pancreatic beta-cells. The effects are receptor-mediated via two parallel signaling pathways. The aim of this study was to examine the relevance of a second melatonin receptor (MT2) as well as the involvement of a third signaling cascade in mediating melatonin effects, i.e. the cyclic guanosine monophosphate (cGMP) pathway. Our results demonstrate that the insulin-inhibiting effect of melatonin could be partly reversed by preincubation with the unspecific melatonin receptor antagonist luzindole as well as by the MT2-receptor-specific antagonist 4P-PDOT (4-phenyl-2-propionamidotetraline). As melatonin is known to modulate cGMP concentration via the MT2 receptor, these data indicate transmission of the melatonin effects via the cGMP transduction cascade. Molecular investigations established the presence of different types of guanylate cyclases, cGMP-specific phosphodiesterases and cyclic nucleotide-gated channels in rat insulinoma beta-cells (INS1). Moreover, variations in mRNA expression were found when comparing day and night values as well as different states of glucose metabolism. Incubation experiments provided evidence that 3-isobutyl-1-methylxanthine (IBMX)-stimulated cGMP concentrations were significantly decreased in INS1 cells exposed to melatonin for 1 hr in a dose- and time-dependent manner. This effect could also be reversed by application of luzindole and 4P-PDOT. Stimulation with 8-Br-cGMP resulted in significantly increased insulin production. In conclusion, it could be demonstrated that the melatonin receptor subtype MT2 as well as the cGMP signaling pathway are involved in mediating the insulin-inhibiting effect of melatonin.

Topics: 1-Methyl-3-isobutylxanthine; Animals; Brain; Cell Line, Tumor; Colforsin; Cyclic GMP; Cyclic Nucleotide Phosphodiesterases, Type 2; Cyclic Nucleotide-Gated Cation Channels; Diabetes Mellitus, Type 2; Dose-Response Relationship, Drug; Glucose; Guanylate Cyclase; Insulin; Insulin Secretion; Insulin-Secreting Cells; Insulinoma; Melatonin; Pineal Gland; Rats; Rats, Wistar; Receptor, Melatonin, MT2; Signal Transduction; Tetrahydronaphthalenes; Tryptamines