Effects of a sustained-release naloxone pellet on luteinizing hormone secretion in female rats

Studies were undertaken to develop a naloxone implant capable of chronically blocking opioid receptors for several weeks in an effort to evaluate the effect of this prolonged narcotic antagonism on luteinizing hormone (LH) secretion in female rats. Antagonism of opiate receptors was achieved with a tablet formulation which contained 75 mg naloxone free base and a high content of the insoluble binding material, Mg stearate. Subcutaneous placement of this implant prevented morphine-induced analgesia for 2 weeks and antagonized the LH suppressory effects of morphine (15 or 30 mg/kg) administration. Thus, this naloxone delivery system is capable of chronically occupying the opioid receptors which mediate morphine's effects on analgesia and LH secretion. Despite this, the naloxone pellet only moderately enhanced the initial rate of increase in LH secretion following ovariectomy (day 1) and was ineffective in further augmenting LH secretion at 3 and 7 days after implantation. In rats which were ovariectomized and implanted immediately with estradiol-containing Silastic capsules, the naloxone pellet was ineffective in altering LH secretion 1, 3 or 7 days later. Thus, while chronic exposure to naloxone persistently antagonizes the pharmacologic actions of morphine, the naloxone pellet only transiently blocked the tonic inhibitory effect of endogenous opioid peptides. The mechanism by which the LH secretory effects of naloxone are lost following chronic exposure to the antagonist are at present unknown, but may involve the activation of compensatory mechanisms which are inhibitory to LH secretion.