Daily changes in melatonin binding sites and the effect of castration

The effect of castration and/or neonatal administration of testosterone propionate (TP) on 125I-melatonin binding and its daily changes was studied in rat anterior pituitary (AP) and in pars tuberalis/median eminence (PT/ME). In animals kept on a light/dark cycle of 12:12 h there was a marked increase in binding site density (Bmax) in the evening as compared to the morning, while there were no differences in the affinity (Kd). On a light/dark cycle of 8:16 h the daily rhythm in Bmax was abolished and the values were intermediate. Neonatal TP administration which increases the sensitivity of the reproductive axis to melatonin and photoperiodic regulations had no effect on the binding parameters in AP and in PT/ME. Castration, however, increased binding site density in AP by 85% while it had no effect on the affinity of the binding site (Kd).