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Quantification of Naloxone Binding Sites in Brains from Suckled Beef Cows during Postpartum Anestrus and Resumption of Estrous Cycles^1,2

Purdue University, West Lafayette, IN 47907

Abstract

Thirty beef cows, approximately 3 yr of age, were randomly assigned to be slaughtered on d 7, 14, 28, 42 or 56 postpartum. Each cow suckled one calf until slaughter. Data from cows slaughtered on d 42 and 56 were pooled and further classified as anestrous or cyclic based on the presence of a corpus luteum and elevated serum concentrations of progesterone at slaughter. Specific binding of [³H]naloxone (3H-NAL) to homogenates of tissue from hypothalamus (HYP), preoptic area (POA) and basal forebrain (BF) was assessed using multiple-point Scatchard analyses. Nonspecific binding was estimated in the presence of 10^-6 M naloxone. Separation of bound from free 3H-NAL was achieved by centrifugation at 20,000 x g. Concentration (fmol/mg original tissue wet wt) of 3H-NAL binding sites in POA tissue was higher (P < .05) on d 28 postpartum in anestrous cows than in cyclic cows on d 42 + 56 postpartum (2.58 ± .32 vs 1.58 ± .10). When all anestrous cows were compared with cyclic cows, concentrations of 3H-NAL binding sites in POA tissues and in BF tissue were higher (P < .05) in anestrous cows (anestrous POA, 2.12 ± .17, cyclic POA, 1.58 ± .10; anestrous BF, 2.94 ± .41, cyclic BF, 2.19 ± .16). Compared across brain regions for all cows, the concentration of specific binding sites for 3H-NAL was greater (P < .01) in BF (2.5 ± .2) than in POA (1.9 ± .1) and greater (P < .01) in POA than in HYP (1.5 ± .1). In summary, tissue concentration of 3H-NAL binding sites was higher in POA and BF tissues of suckled cows that were anestrous than in suckled cows that were cyclic. In addition, binding sites for 3H-NAL were more concentrated in neural tissue of the BF than in tissues of POA and HYP.

¹ Published as Journal Paper No. 11,272, Purdue Univ. Agric. Exp. Sta. The authors acknowledge the generosity of E.M. Convey, L. C. Spicer and K. Leung, Michigan State Univ., who supplied the tissues used in this study.

² Portions of these results have been reported previously in preliminary form (J. Anim. Sci. 65 [Suppl. 1]:396, [Abstr.], 1987).

³ Present address: U.S. Meat Anim. Res. Center, Clay Center, NE 68933.

⁴ Dept. of Anim. Sci.