Changes of [³H]naloxone binding in oestrogen stimulated rat uterus

Effects of a single dose of oestradiol (Oe) on [³H]naloxone (Nal) binding in ovariectomized rat uterus were studied. Specific [³H]Nal binding was assessed by saturation analysis in 800 g supernatants and pellets of uterine homogenates. Two binding sites with higher (K_d ∼ 1nM) and lower affinity (K_d ∼ 15nM) for Nal were observed, their binding capacities and affinities have changed after Oe treatment in a time-dependent manner. The high affinity binding sites, detected only in the cytoplasmic fraction, disappeared after 1 h and only became detectable again at 24 h after hormone treatment, the lower affinity binding sites, after an initial drop, slowly increased, peaking at the 9th hour of hormone injection. The competition experiments indicate the involvement of different opiate receptor subpopulations in Oe induced changes. In the nuclear fraction, the B_max values started to increase at 15 h, reaching the highest level at 18 h. The K_d values of lower affinity sites, in both studied compartments, were increased, i.e. the affinity decreased in the second half of the examined period.