Differential Effects of β-Endorphin and Met- and Leu-Enkephalin on Steroid Hormone-Induced Lordosis in Ovariectomized Female Rats

The effect of intrathirdventricular (ITV) injections of β-endorphin, anti-β-endorphin antiserum, Met-enkephalin, Leu-enkephalin, and naloxone on the initial activation and final development of steroid hormone-mediated induction of female sexual receptivity was studied in ovariectomized female rats. The lordosis response to male mounts in ovariectomized rats after subcutaneous (SC) estradiol benzoate (EB) and progesterone (Prog) priming was facilitated by β-endorphin, and Met-enkephalin (10 μg·5 μl −1), but inhibited by Leu-enkephalin, when the peptides were injected into the third ventricle at the time of SC EB priming. A lower dose Met-enkephalin had no effects. Lordosis behavior in steroid hormone-primed rats was significantly facilitated when ITV injections of Met-enkephalin were given 1 h prior to behavioral testing (47 h after EB priming). At 1 h prior to behavioral testing (47 h after EB priming), ITV injection of β-endorphin significantly inhibited lordosis behavior, especially at the higher dose of β-endorphin (10 μg·5 μl −1). Under those conditions, Leu-enkephalin had no effect. Lordosis behavior of ovariectomized female rats receiving SC steroid hormones and ITV injection of anti-β-endorphin antiserum was significantly inhibited when anti-β-endorphin antiserum was injected at the time of EB priming. However, lordosis was significantly facilitated when anti-β-endorphin antiserum was injected 1 h prior to the behavior testing (47 h after EB priming). In contrast, ITV injection of the opioid antagonist naloxone given either at the time of EB priming or 1 h prior to behavioral testing (47 h after EB priming) decreased lordosis behavior. The present results suggest that 1) β-endorphin, Met-enkephalin, and Leu-enkephalin have differential effects in the control of lordosis behavior; 2) the opioidergic systems may modulate initial-stage and final-stage estrogen-induced lordosis behavior; and 3) the opioidergic systems could be divided into the endorphinergic modulation-type and enkephalinergic modulation-type, based on their effects on lordosis behavior.