Beef cows were used to determine if suckling influences release of LH via endogenous opioids at 28 ± 4 d after parturition. Cows of similar weight and body condition (6.8 ± .1, 1=emaciated, 9=obese) were assigned randomly to five groups (n=6 to 7): 1) control-suckled/saline (suckled 15 min every 6 hr for 48 hr); 2) control-suckled/ naloxone; 3) calf-removal/saline (calf removal for 52 hr); 4) calf-removal/naloxone; and 5) control-suckled/GnRH (Gonadotropin-Releasing Hormone). At 0 hr, saline was administered to all cows. This treatment was continued at 6 hr intervals for 24 hr. Either naloxone (0.5 mg/kg), GnRH (40 ng/kg) or saline was administered to cows in their respective groups every 6 hr during the ensuing 24-hr period in calf-removal groups, or immediately preceding each suckling episode in the control-suckled groups. Blood samples for analysis of luteinizing hormone (LH) were collected at 15-min intervals for 1 hr prior to and 3 hr after treatment at 0, 24, 36 and 48 hr. Cows were observed for estrus twice daily. All cows in the control-suckled/GnRH group released LH (P<.05) in response to exogenous GnRH, indicating the presence of releasable quantities of the gonadotropin. Mean concentrations of LH were not effected (P>.05) by the control-suckled regime. However, calf-removal alone, or in combination with naloxone, increased (P<.05) mean concentrations of LH by 48 hr. Though mean concentrations of LH in the 5 control-suckled cows after treatment with naloxone for 24 hr (48-hr treatment period) were 116, 145, 761, 159 and 140 percent of concentrations at 0 hr, values were intermediate between those concentrations observed in control-suckled and in calf-removal cows. Because treatment with naloxone prior to suckling simulated some of the effects of calf removal on the release of LH, it was concluded that suckling, at least partially, suppresses the release of LH through endogenous opioid mechanisms. © 1989.
Myers, T. R., Myers, D. A., Gregg, D. W., & Moss, G. E. (1989). Endogenous opioid suppression of release of luteinizing hormone during suckling in postpartum anestrous beef cows. Domestic Animal Endocrinology, 6(3), 183–190. https://doi.org/10.1016/0739-7240(89)90012-X