Pregnancy Related Changes of Opiate Receptors Identified in Rat U terine Membranes by 3H -N aloxone Binding

Pregnancy rclat ed chung es ofopiat e /'('u;plOrs identified in rat nt erine membrane s bv 3H-n%xone binding . PEPTIDES 6(5) 97\-974, \985.~'H-Naloxone was used to demonstrate the presence of specific opiate binding sites in uterine membrane prepara- tions of rats. "H-Naloxone binding (0.4\-27 nM) was found to be rapid, saturable and reversible showing two populations of binding sites with the characteristic ofhigh (K" 2.2 nM; Bm", 46.6 fmollmg prot.) and low (Kil \8.1 nM; Bm", 143.7 fmol/mg prot.) affinity. The number and affinity ofthe binding sites labelled by "l-l-naloxone in the uterus were measured in the rat at mid (\4 days), late (2\ days) pregnancy and at parturition. The high and low affinity recognition sites labelled by 3H_ naloxone showed a consistent reduction during pregnancy and at parturition without changes in the affinity constant. We concluded that pregnancy and parturition are associated with significant changes in the number of the opiate receptors bound in the uierus by 'H-naloxone. This phenomenon which seems to be linked with the 'several pregnancy-related changes in the levels of endogenous peptides and hormones could be relevant IO further explain the pregnancy related changes in pain perception and maternal behavior.