Evidence for a melatonin receptor within pancreatic islets of neonate rats: functional, autoradiographic, and molecular investigations

Abstract

In a recent perifusion investigation, we showed that the pineal secretory product melatonin reduces insulin secretion from isolated pancreatic islets of neonate rats stimulated with potassium chloride (KCl), glucose, and forskolin. This effect of melatonin was reproduced with doses ranging from 200 pmol/L to 5 μmol/L. Because it is generally accepted that melatonin exerts some of its biological effects through specific, high‐affinity pertussis‐toxin‐sensitive G‐protein‐coupled receptors, we blocked the putative melatonin receptor of pancreatic islets using both the non‐hydrolyzable guanosine triphosphate analog guanosine 5′‐O‐(3‐thiotriphosphate) (GTPΓS, 30 μmol/L) and the melatonin antagonist luzindole (10 μmol/L). Both GTPΓS and luzindole caused a near normalization of the melatonin‐induced inhibition of the forskolin‐stimulated insulin secretion. To localize putative melatonin receptors within the pancreatic islets autoradiographic studies were additionally carried out. These investigations showed specific binding of 2‐[125I]iodomelatonin, which were in exact correspondence with the localization of the islets. In addition, gray‐level analysis showed that unlabeled melatonin was able to reduce the binding of 2‐[125I]iodomelatonin in a dose‐dependent manner. Concentrations of unlabeled melatonin of 10−9 mol/L produced a 50% reduction in specific binding, whereas concentrations of 10−6 mol/L displaced the binding completely. Likewise, the results of molecular investigations showed that the rat pancreas contains a melatonin receptor, since reverse transcription polymerase chain reaction (RT‐PCR) experiments, using specific primers for the rat melatonin receptor Mel1a, showed that mRNA for this melatonin receptor type is expressed in pancreatic tissue of newborn rats. In summary, it may be said that our functional, autoradiographic, and molecular results indicate that the Mel1a receptor is located on the pancreatic islets, possibly in the beta cells.

Chronomedicine and type 2 diabetes: shining some light on melatonin

“…Insulin-stimulatory and -inhibitory effects Using rat INS-11 insulinoma cells it was determined that activation of inhibitory G-protein coupled (G i -coupled) MT1/MT2 receptors leads to significant reduction of cAMP synthesis, reduced activation of protein kinase A and, presumably, reduced insulin secretion [112]. Melatonin can also act through G i -coupled MT2 receptors in INS-1 cells to lower intracellular Ca 2+ levels [110,113], which is also predicted to reduce insulin secretion.…”

Melatonin receptors in pancreatic islets: good morning to a novel type 2 diabetes gene

“…This indeed appears to be the case, as inferred from studies using the non-hydrolysable GTP analogue guanosine 5′-O-(3-thiotriphosphate) and the melatonin antagonist luzindole [51], both of which block the effects of melatonin on insulin secretion from neonatal rat islets. MTNR1A mRNA was subsequently demonstrated in INS-1 cells [52,53] and in rat [54,55] and human islets [55].…”

The role of melatonin in diabetes: therapeutic implications

“…Due to melatonin there is reduction in cAMP production in pancreatic islets and the rat insulinoma β-cell line INS1, as well as forskolin-stimulated insulin secretion from isolated pancreatic islets of neonate rats (33,46,47). The insulin and cAMP levels were stimulated by forskolin, adenylyl cyclase activator (48).…”

“…The insulin and cAMP levels were stimulated by forskolin, adenylyl cyclase activator (48). Previously it was found that receptor antagonists like luzindole completely reversed the cAMP-and insulin diminishing effects of melatonin (33). The Giα-protein-inhibitor pertussis toxin (PTX) abolished the effect of melatonin on the levels of cAMP and insulin as well (48).…”