Opioid peptides involvement in endotoxin-induced suppression of LH secretion in ovariectomized Holstein heifers

Three groups of ovariectomized Holstein heifers were used in this study. Group I heifers (n = 6) were pretreated with saline (SAL), followed 3 h later by three injections of naloxone hydrochloride (NLX) given 1 h apart. In the same sequence, group II heifers (n = 3) received lipopolysaccharide (LPS), followed by the opioid antagonist NLX. Group III heifers (n = 3) received LPS followed by SAL. Concentrations of cortisol and progesterone increased (P < 0.05) following LPS injections in both groups II and III, whereas luteinizing hormone (LH) concentrations were suppressed (P < 0.05). Administration of NLX to heifers pretreated with LPS elicited significant increases (P < 0.05) in LH concentrations, whereas SAL infusion had no effect. These results indicate that the inhibitory actions of opiate mu-delta receptors, and possibly other POMC gene products, were at least partially involved in LPS-induced suppression of the gonadotropic hormone. These results are discussed, including the possibility that feedback suppression by LPS-triggered adrenal gland steroids may have interfered with complete restoration of LH secretion by the opioid antagonist.