Diurnal changes in melatonin binding sites of hamster and rat brains. Correlations with neuroendocrine responsiveness to melatonin.

A diurnal variation in neuroendocrine sensitivity to melatonin is known to occur in hamsters and rats. The present experiments were carried out to examine the possibility that affinity and/or number of melatonin binding sites in brain could change accordingly at the two times when exogenous melatonin is known to be ineffective or effective to produce neuroendocrine changes, i.e., at 07 : 00 or 20 : 00 h (lights on from 07 : 00 to 21 : 00 h daily). The number of melatonin binding sites in hamster and rat brains was at 20 : 00 h 34--56% higher than at 07 : 00 h, without changing their affinity towards [3H]melatonin (hamster: Kd = 53 nM; rat: Kd = 73--77 nM). These alterations in melatonin receptor density may play a role in daily changes in sensitivity to the exogenous methoxyindole.