Involvement of the endogenous opioid peptides in suckling-induced inhibition of LH release

Abstract

Two studies were conducted to further understand the involvement of the endogenous opioid peptides (EOPs) in suckling-induced inhibition of LH release. Experiment A in the first study was designed to evaluate the effect of naloxone (NAL) infusion (1.0 mg/kg bw/h) on serum LH and hypothalamic LHRH content in 1 week ovariectomized suckled (8 pups/animal) rats after 18 h pup removal, and after 18 h resuckling. Pup removal resulted in an increase (p<0.05) in serum LH and a decrease (p<0.10) in hypothalamic LHRH content. On the other hand, subsequent resuckling decreased (p<0.05) serum LH and increased (p<0.10) hypothalamic LRH content. NAL did not influence (p>0.10) serum LH in suckled or nonsuckled rats. Hypothalamic LHRH content in NAL-treated rats was not changed upon pup removal or subsequent resuckling. Experiment B was designed to evaluate the effect of NAL on serum LH and hypothalamic HRH content in suckled rats. Serum LH increased (p<0.10 and p<0.005, respectively) in response to 18- and 36-h NAL infusion. Hypothalamic LHRH content decreased (p<0.025) after 18 h of NAL infusion, but increased to control concentrations after 36 h of NAL infusion Experiment C was designed to evaluate the effect of pup removal during NAL infusion on serum LH and hypothalamic LHRH content. Pup removal during the latter 18 h of a 36 h NAL infusion did not further influence (p>0.10) LH release nor did it influence (p>0.10) hypothalamic LHRH content. The second study was designed to evaluate the effect of NAL infusion (1.0 mg/kg bw/h for 7 days) in ovariectomized suckled rats, and ovariectomy on LHRH neuron morphology using immunocytochemistry. NAL did not influence LHRH immunoreactivity in the infundibulum (INF) nor was it able to influence neuronal morphology. In contrast, rats ovariectomized for 3 weeks had a greater proportion (48% vs 25%, respectively) of smooth LHRH neurons compared to diestrous rats...