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.

Since binding sites for melatonin have been found in the hippocampus of several mammals, it has been suggested that the pineal hormone melatonin is able to modulate neuronal functions of hippocampal cells. In order to get more insight into the role of melatonin for the functions of hippocampal cells, the following experiments were performed: male rats, maintained under a 12/12-h light-dark cycle, were sacrificed by decapitation at zeitgeber times (h) ZT2, ZT8, and ZT15 (ZT0 = lights on); for experiment 1, gene expression for melatonin receptors was detected in the hippocampus and in hippocampal subfields by means of the RT-PCR technique; for experiment 2, electrophysiological and pharmacological properties of melatonin receptors heterologously expressed in Xenopus oocytes after injection of mRNA from the hippocampus were analyzed by means of voltage clamp technique; and for experiment 3, effects of melatonin on the spontaneous firing rate of action potentials in the CA1 regions of hippocampal slices were analyzed by means of extracellular recordings. The RT-PCR data revealed that transcripts for both the MT1 and MT2 melatonin receptors are present in the dentate gyrus, CA3, and CA1 regions, and the subiculum of the hippocampus. Injection of mRNA from rat hippocampus into the Xenopus oocytes led to the functional reconstitution of melatonin-sensitive receptors, which activates calcium-dependent chloride inward currents. The melatonin responses were abolished by simultaneous administration of the antagonists 2-phenylmelatonin and luzindole, and were unaffected by the MT2 antagonist 4-phenyl-2-propionamidotetralin. Bath-applied melatonin (1 micromol/l) enhances the firing rate of neurons in the CA1 region. The effect was small in experiments performed at ZT8 (<2 times the initial level) and large in experiments performed at ZT15 (>6 times). The changes of neuronal firing rate induced by melatonin were completely suppressed with simultaneous administration of the melatonin receptor antagonist luzindole (10 micromol/l). The results indicate that melatonin may play an important role in modulating neuronal excitability in the hippocampus.

At a single evening dose of 5–10 mg, melatonin (MLT), the pineal gland hormone, can exert a positive effect on the frequency of epileptic attacks in children with sleep disturbances of various etiologies. We have shown that the sleep behavior can be normalized and an existing epilepsy can be favorably influenced. Pretherapeutic MLT secretion profiles can provide new information concerning the origin and treatment of these disturbances. In vitro experiments suggest that this effect might be the result of the interaction between MLT and MLT-specific receptors in the neocortex. Due to its favorable safety profile, MLT can be liberally administered in the specified doses and be considered as a useful antiepileptic drug.

Hypophyseal pars tuberalis (PT)-specific cells are known to exhibit remarkable seasonal changes in morphology especially in photoperiodic animals like the Djungarian hamster Phodopus sungorus. Their high density of melatonin-receptors leads to the supposition that fluctuations in circulating melatonin levels are a crucial factor for the morphological alterations induced by photoperiodic signals. To prove this hypothesis the nocturnal elevation of melatonin in long photoperiods was prolonged by late afternoon administration of melatonin. We investigated whether this treatment induces cytological changes usually observable under short photoperiod. Electron microscopy revealed that in contrast to hamsters maintained in long photoperiods PT-specific cells of hamsters injected with melatonin or those kept in short photoperiods appear inactive, containing a relatively high number of secretory granules, sparse endoplasmatic reticulum, irregularly outlined and invaginated cell nuclei and a high amount of glycogen. Furthermore immunoreactivity for the common alpha-chain of glycoprotein hormones and beta-TSH was significantly weaker in hamsters kept in short photoperiods or daily injected with melatonin than untreated or vehicle injected controls in long photoperiod. These results demonstrate that an exogenous prolongation of the elevated nocturnal melatonin levels causes a similar morphological appearance of PT-specific cells as observed in short photoperiods. It is tempting to speculate that the melatonin signal is a direct 'Zeitgeber' for the transduction of photoperiodic information to the secretory activity in this cell type.

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