Effects of suckling and of a long interval after ovariectomy on hypothalamo-hypophyseal responsiveness to naloxone, morphine and GnRH in beef cows.
The response of serum luteinizing hormone (LH) to morphine, naloxone and gonadotropin-releasing hormone (GnRH) in ovariectomized, suckled (n=4) and nonsuckled (n=3) cows was investigated. Six months after ovariectomy and calf removal, the cows were challenged with 1mg, i.v. naloxone/kg body weight and 1 mg i.v. morphine/kg body weight in a crossover design; blood was collected at 15-minute intervals for 7 hours over a 3-day period. To evaluate LH secretion and pituitary responsiveness, 5 microg of GnRH were administered at Hour 6 on Day 1. On Days 2 and 3, naloxone or morphine was administered at Hour 3, followed by GnRH (5 microg/animal) at Hour 6. Mean preinjection LH concentrations (3.6 +/- 0.2 and 4.7 +/- 0.2 ng/ml), LH pulse frequency (0.6 +/- 0.1 and 0.8 +/- 0.1 pulses/hour) and LH pulse amplitude (2.9 +/- 0.5 and 2.9 +/- 0.6 ng/ml) were similar for suckled and nonsuckled cows, respectively. Morphine decreased (P < 0.01) mean serum LH concentrations (pretreatment 4.2 +/- 0.2 vs post-treatment 2.2 +/- 0.2 ng/ml) in both suckled and nonsuckled cows; however, mean serum LH concentrations remained unchanged after naloxone. Nonsuckled cows had a greater (P < 0.001) LH response to GnRH than did suckled cows (area of response curve: 1004 +/- 92 vs 434 +/- 75 arbitrary units). We suggest that opioid receptors are functionally linked to the GnRH secretory system in suckled and nonsuckled cows that had been ovariectomized for a long period of time. However, gonadotropin secretion appears not to be regulated by opioid mechanisms, and suckling inhibits pituitary responsiveness to GnRH in this model.
Rund
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LA
Department of Animal and Dairy Science, University of Georgia, Athens, GA 30602, USA.
Leshin
L S
LS
Thompson
F N
FN
Kiser
T E
TE