Relationships among Concentrations of Four Opioid Neuropeptides and Luteinizing Hormone-Releasing Hormone in Neural Tissues of Beef Cows Following Early Weaning2

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Abstract
Acute changes associated with removal of the inhibition of estrus caused by suckling were examined in beef cows. Calves were weaned during the fifth week after parturition and cows were slaughtered at 0 (n=8), 36 (n=8) or 72 h (n=8) after calf removal. Tissues of preoptic area (POA), hypothalamus (HYP), pituitary stalk-median eminence (SME) and pituitary neurointermediate lobe (NIL) were obtained for analyses of luteinizing hormone-releasing hormone (LHRH) and four opioid neuropeptides. In addition, one-half of each SME was superfused in vitro for measurement of basal and potassium-induced release of LHRH. The following opioid neuropeptides were quantified: methionine-enkephalin (Met-Enk), β-endorphin (β-EP), dynorphin-A, 1–17 (DYN-17) and dynorphin-A, 1–8 (DYN-8). All four opioid neuropeptides were most concentrated in the pituitary NIL. Luteinizing hoimone-releasing hormone was most concentrated in the SME tissue, which also contained substantial concentrations of Met-Enk and (β-EP, but very little DYN-17 or DYN-8. In addition, weaning increased the weight of NIL between 0 and 36 h (P<.05), and the concentrations of LHRH, Met-Enk, and DYN-17 in the combined POA + HYP (P<.05) tissue between 36 and 72 h. No differences occurred among groups in SME content of LHRH or in vitro release of LHRH from the superfused SME. Although they were not affected by weaning, within-cow correlations among parameters revealed that: 1) concentrations of DYN-17 and DYN-8 were always positively correlated (P<.05); 2) concentrations of LHRH were positively correlated with Met-Enk (P<.01), β-EP (P<.05) and DYN-17 (Pβ-EP (P<.05), but not of LHRH or DYN-17 and 4) in vitro release of LHRH from the pituitary SME was correlated with concentrations of DYN-8 in various tissues including the SME (P<.01). In summary, bovine neural tissues differ widely in concentrations of the four opioid neuropeptides with NIL tissue having the greatest concentrations. Weaning calves at 36 and 72 h before slaughter caused parallel changes in LHRH, Met-Enk and DYN-17 in preoptic and hypothalamic tissues. We suggest that suckling suppresses estrous cycles by affecting the release and(or) synthesis of LHRH and, because of the concurrent changes observed, endogenous opioid peptides may be involved in the regulation of this process. Copyright © 1986. American Society of Animal Science . Copyright 1986 by American Society of Animal Science.

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