Effect of ovariectomy on the binding characteristics of hypothalamic mu opioid receptors in the rat

The present experiments have been performed to analyze whether ovariectomy, performed in adult female rats, might influence the binding characteristics of hypothalamic mu binding sites. To this purpose, different groups of ovariectomized females have been killed by decapitation 1, 3, 7, 15 and 21 days after surgery, and the binding characteristics (Bmax and Kd) of the mu receptors have been determined in their hypothalami. Dihydromorphine was used as a specific mu ligand. The data obtained have been compared with those derived from the hypothalami of adult female rats in the morning of proestrus, when the density of hypothalamic mu opioid receptors reaches its highest concentration. Blood from trunk vessels was also collected for the evaluation of serum LH levels by radioimmunoassay. The results obtained have shown that one day after ovariectomy, the number of hypothalamic mu opioid binding sites is as high as that found in proestrus animals. The number of these binding sites tends to decrease on days 3, 7 and 15 after surgery. On day 21 after gonadectomy, the number of mu binding sites results to be significantly lower than that observed in the hypothalami of intact female rats in the morning of proestrus and in the hypothalami of female animals castrated since one day. The value of the Kd did not show any variation on days 1, 3, 15 and 21; however, a significant increase of this parameter (which indicates a decrease of the affinity of the ligand for its receptors) was observed on day 7 after ovariectomy.(ABSTRACT TRUNCATED AT 250 WORDS)