Functional melatonin receptors and metabolic coupling in cultured chick astrocytes

Melatonin is an important hormone regulating circadian clocks in birds, but the specific cellular sites of action are not completely known. The present study was designed to determine whether astrocytes derived from chick brain contained functional melatonin receptors. Primary cell cultures of diencephalon astrocytes that express glial fibrillary acidic protein (GFAP), but not neuron-specific enolase (NSE) immunoreactivity, were employed to determine the cellular distribution and physiological role for the three known receptor subtypes. Saturation and Scatchard analysis of 2-[(125)I]iodomelatonin binding demonstrated melatonin receptor binding with a high affinity and a pharmacological profile similar to that obtained from brain. In situ hybridization for receptor subtypes revealed Mel(1A) and Mel(1C) receptor mRNA, but not Mel(1B). Administration of pharmacological levels of melatonin acutely inhibited forskolin-stimulated 2-deoxyglucose (2DG) uptake, while rhythmic administration of physiological levels of melatonin gradually imposed a rhythm in 2DG uptake and of the release of both lactate and pyruvate into the medium. These results indicate that (1) there are functional Mel(1A) and Mel(1C) melatonin receptors in astrocyte-rich cultures, and (2) rhythmic administration of melatonin plays an important role in the regulation of astrocytic metabolic activity. Together, the data suggest that the circadian secretion of melatonin probably plays a role in the global metabolic economy of the avian brain through rhythmic regulation of metabolism in astrocytes.