Central melatonin receptors in red deer (Cervus elaphus).

Red deer display characteristic seasonal changes in appetite, growth, and reproduction which are mediated by the pineal hormone, melatonin, which provides a direct neuroendocrine transduction of the ambient photoperiod. To identify potential central sites of action for this hormone, [2(-125)I]iodomelatonin binding sites were localized and characterized within the cervine brain and pituitary by in vitro autoradiography. Specific binding was distributed over a number of discrete regions of the brain, including the cortex, septum, putamen, hippocampus, cerebellum, and superior colliculus. The choroid plexus, the pars tuberalis, and the pars distalis of the pituitary were also specifically labeled. Specific binding was completely abolished in the presence of 10(-7) M melatonin and inhibited in the presence of 10(-4) M GTP gamma S (guanosine-5-O-(3-thiotriphosphate)), a nonhydrolysable analogue of GTP, in the pars tuberalis, pars distalis, choroid plexus, and all neuronal regions examined apart from the hippocampus and layer III of the cerebral cortex. Inhibition of [2(-125)I]iodomelatonin binding by GTP gamma S is indicative that binding is representative of a G-protein-coupled receptor. Characterization studies showed that [2(-125)I]iodomelatonin binding was time-dependent and saturable with a dissociation constant (Kd) in the low picomolar range (approximately 10 pM). Competition studies with iodomelationin, melatonin, and N-acetylserotonin gave IC50 values similar to those previously characterized for the melatonin receptor in the ovine pars tuberalis.