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Effect of Morphine on Serum Gonadotropin Concentrations in Postpartum Beef Cows^1,2,3,

The University of Georgia, Athens 30602

Abstract

Morphine (M), an opioid agonist, was administered to postpartum (PP) Angus cows to investigate opioid modulation of gonadotropin secretion. In Exp. 1, eight PP cows (36.9 ± 2.3 d) received either M (1 mg/kg; n = 4) or saline solution (S) (n = 4) via i.v. injection 36 h after calf removal. Morphine decreased (P < .01) the number of serum LH pulses (3.0 ± 1.1 pre- vs .3 ± .3 post-pulses/h) and, compared with pretreatment values (3.3 ng/ml), decreased (P < .05) mean LH at 105 min (2.1 ng/ml) through 270 min (1.9 ng/ml ± .4). Serum prolactin (PRL) increased (P < .01) following M from 16.4 ng/ml to a peak of 59.3 ng/ml (± 3.9). Serum FSH concentrations were unaffected. In Exp. 2, M (.31 mg/kg i.v. injection followed by .15 mg/(kg · h) infusion; n = 6) or S (n = 6) treatments were given for 7 h beginning 36 h after calf removal. Serum LH was similar between groups during the pretreatment and the first 6 h of infusion, but M decreased (P < .001) the number of serum LH pulses (.44 ± .09 vs .06 ± .04 pulses/h). Morphine increased (P < .05) serum PRL. It is concluded that M differentially modulated gonadotropin secretion in the cow such that PRL increased, LH decreased and FSH was unchanged.

¹ This research was supported by funds from the Georgia Vet. Exp. Sta., Georgia Agric. Exp. Sta. and USDA Grant #85-CRCR-1-1854. Published as manuscript no. 2659 from the Georgia Vet. Exp. Sta.

² The authors appreciate the use of facilities and equipment in the Anim. Physiol. Unit of Russell Res. Center, USDA-ARS, Athens, GA.

³ The authors acknowledge the skilled technical assistance of C. K. Smith and K. P. Wood.

⁴ Dept. of Physiol.-Pharmacol., College of Vet. Med.

⁵ Dept. of Anim. and Dairy Sci., College of Agric.