Recent functional, autoradiographic, and molecular investigations have shown that the pineal secretory product melatonin reduces the forskolin-stimulated insulin secretion from isolated pancreatic islets of neonate rats. Autoradiographic and binding studies as well as reverse transcriptase-polymerase chain reaction (RT-PCR) experiments proved that these effects are mediated through specific, high-affinity pertussis-toxin-sensitive Gi-protein-coupled MT(1) receptors and subsequent inhibition of the adenylyl cyclase/cyclic adenosine monophosphate (cAMP) system. This hypothesis was proved by blocking the intracellular signal transduction pathway using the non-hydrolyzable guanosine triphosphate analog guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) or the competitive melatonin receptor antagonist luzindole. Both GTPgammaS and luzindole diminished the melatonin effect. We have published these prior results elsewhere. So far, however, no information is available on both whether the MT1 receptors are located on the beta-cells and whether the consecutive functional reactions are based on a direct influence of melatonin on the insulin producing beta-cells. In order to examine this question, we used a glucose responsive insulin producing insulinoma cell line INS-1 isolated from rats. Comparable with the results of islets the competitive receptor antagonist luzindole diminished the insulin-decreasing effect of melatonin. In addition, our RT-PCR experiments, using specific primers for the rat melatonin receptor MT(1) showed that this melatonin receptor mRNA is also expressed in the INS-1 cells. Furthermore we radioimmunologically analyzed the forskolin-stimulated cAMP concentration in the superfusate. Similar to insulin secretion, the cAMP concentration was significantly reduced by melatonin. Following the hypothesis that cAMP is actively secreted from INS-1 cells by an energy-dependent mechanism based on either a OAT1/ROAT1 like anion exchanger or MDR-like transport systems, we used probenecid (p-[dipropylsulfamoyl] benzoic acid), a known inhibitor of cAMP extrusion. Probenecid blocks the export of cAMP by acting on transport mechanisms which are as yet not completely understood. Consistently, insulin secretion was increased and cAMP concentration diminished. The application of the phosphodiesterase inhibitor IBMX (3-isobutyl-1-methylxanthine) caused a marked rise of insulin secretion as well as cAMP concentration in the perifusate. From these data we conclude that the MT1 receptor is located on the INS-1 cell and therefore in general on pancreatic beta-cells.