INTERACTION OF GONADAL STEROIDS AND ENDOGENOUS OPIOIDS IN THE INHIBITION OF LH RELEASE (NALOXONE, ENDORPHINS)

WILLIAM EVERETT TROUT, Purdue University

Abstract

Opioid-mediated inhibition of luteinizing hormone (LH) was investigated in sheep and cattle. Naloxone (NAL) injection abruptly stimulated LH release (i.e. disinhibited the opioid-mediated suppression) in intact ewes with corpora lutea or those receiving progesterone (P4). In ovariectomized (OVEX) ewes, P4 suppressed serum LH and NAL injection in these ewes disinhibited the release of LH. After treatment with P4 for 30 days, serum LH was no longer suppressed and NAL injection no longer disinhibited the release of LH. A single injection of estradiol-17B (E2) in these ewes reinstated the suppression of LH secretion and the ability of NAL to disinhibit LH release. In other OVEX ewes, continuous administration of E2 or E2 and P4 suppressed LH release, but NAL failed to disinhibit consistently the release of LH in these ewes. Procedures were developed and validated for the in vitro quantification of specific binding of ('3)H-NAL to homogenates of brain tissue of sheep and cattle. In steroid-treated OVEX ewes, brain tissue was removed for assay of ('3)H-NAL binding several hours after the serum LH response to exogenous NAL had been determined. Administration of E2 appeared to antagonize the opioid-mediated suppression of LH, and ewes were divided retrospectively into NAL-sensitive or insensitive groups based upon their in vivo LH response to NAL. Binding of ('3)H-NAL to homogenates of POA or HYP did not differ between the two groups of ewes, but the NAL-sensitive ewes had a higher concentration of binding sites in the ventral SEP area when compared with the NAL-insensitive ewes. Brain tissue was obtained from suckled beef cows at several stages of the postpartum period. Specific binding of ('3)H-NAL was demonstrated in homogenates of HYP, POA, and ventral SEP with the highest concentration of binding sites in the ventral SEP as also observed in sheep. The escape of cows from the inhibitory effects of suckling and the occurrence of estrous cycles were associated with a decrease in the concentration of binding sites for ('3)H-NAL in homogenates of POA and ventral SEP. (Abstract shortened with permission of author.)

Degree

Ph.D.

Subject Area

Livestock

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