Nuclear [3H]naloxone binding in rat hypothalamus.

High affinity (Kd approximately 1 nM) and low capacity [3H]naloxone binding sites were detected in the nuclear fraction of rat hypothalamus. The profile of this binding changed with age. In immature rats from 11 days of age until vaginal opening (approximately 33 day) the Scatchard plots of saturation data were linear and the Hill coefficient was 1, while just after vaginal opening (< 6 h) Scatchard analysis of [3H]naloxone binding gave a curvilinear component, with the Hill coefficient approximately 2, followed by an increase in binding sites and a decrease in Kd. In adults, after ovariectomy, the pattern of [3H]naloxone binding was similar to that observed in immature rats before vaginal opening. Following oestradiol treatment, binding sites with linear Scatchard plots disappeared and returned at least 24 h after hormone administration.