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Four studies were conducted to investigate the effect and site of action of exogenous and endogenous opioids on pituitary growth hormone (GH) and prolactin (PRL) secretion in Holstein calves. In the first study, the effect of the opioid agonist DAMME (D-Ala²⁺,N-Me-Phe⁴,Met(O)⁵-01 enkephalin) on plasma GH and PRL secretion was measured in Holstein calves in fall season. Plasma concentrations of both GH and PRL increased in response to DAM ME injection. Pretreatment with either the lipid soluble opioid antagonist naloxone (NAL), which readily penetrates the blood brain barrier (BBB), or the peripherally acting antagonist methyl levallorphan mesiltate (MLM), blocked the PRL response to DAMME. Naloxone, but not MLM, negated the GH response to DAMME. In spring, the experiment was repeated with similar results. In the second experiment, the opioid antagonists NAL and MLM were administered alone to detennine whether endogenous opioids mediate basal GH and PRL secretion, and the site of action of any of opioid-mediation of basal GH and PRL. In fall, NAL administration increased both plasma GH and PRL secretion. Methyl levallorphan mesilate did not affect PRL, but increased plasma GH concentrations. In spring, a second trial using 5 different doses of each antagonist was conducted. Naloxone did not affect GH levels at any dose in spring, but decreased plasma PRL at the same dose which increased plasma PRL in fall. Plasma PRL was again unaffected by MLM, but plasma GH was increased by 3 separate doses of MLM. The third experiment was designed to determine if the increases in plasma PRL seen after DAMME administration were mediated via dopaminergic mechanisms. Plasma PRL in calves again increased in response to DAMME injection alone. In calves pre-treated with the long-acting dopamine agonist 2-Br-&alp. ergocryptine (CB 154), plasma PRL was unresponsive to DAMME injection. The pituitaries of calves treated with CB 154 in this experiment were able to respond to thyrotropin-releasing hormone (TRH) injection with increased PRL secretion. In the final experiment, the role of growth hormone-releasing hormone (GRH) in facilitating GH release after DAMME injection was investigated, and whether endogenous opioidergic mechanisms play a role in mediating the effects of exogenous GRH on GH secretion. Plasma GH concentrations increased in calves receiving either DAMME or D-ala²⁺, fragment 1- 29 amide, a synthetic GRH. The immediate increase in plasma GH concentrations after GRH injection in calves pre-treated with DAMME was approximately 5 fold less than that in calves not pre- treated with DAMME. Calves receiving DAMME and GRH in combination also produced a GH response curve with greater area under it than either compound alone, indicating possible synergism between the synthetic GRH and a DAMME-sustained release of endogenous GRH. Naloxone administration concomitantly with synthetic GRH did not alter the ability of the synthetic GRH to increase GH secretion overall, compared to synthetic GRH alone. In conclusion, these studies are the first to indicate that dairy breeds are able to respond to exogenous opioids with increased secretion of pituitary GH and PRL, as is known to occur in other mammalian species. Also, they indicate that opioid receptors mediating pituitary GH secretion to exogenous opioids in Holstein calves are located somewhere within the BBB, and those mediating PRL secretion are at a site outside the BBB. It appears from these studies that endogenous opioids within the BBB play a role in regulating basal PRL secretion, and that this regulation differs in fall and spring. A role for endogenous opioids in the regulation of GH secretion in Holstein calves may exist also, at least in fall, but the results are less conclusive. The peripheral opioid antagonist MLM alone may facilitate increased GH secretion in Holstein calves via an agonistic, not antagonistic, mechanism. These studies indicate that the increased PRL secretion seen following opioid administration in Holstein calves is mediated through a dopaminergic mechanism. It appears that endogenous opioids do not mediate the pituitary response to exogenous GRH in Holstein calves, and that GH increases after DAMME injection are facilitated, at least in part, by increased release of endogenous GRH. / Ph. D.

Four experiments were conducted to investigate the effect of naloxone, an opioid receptor antagonist, on pituitary LH secretion in Holstein cows during two periods after parturition and two phases of the estrous cycle. In experiment 1, 24 cows (12 primiparous; 12 multiparous) received either saline (n = 12) or 1 mg/kg naloxone (n = 12) i. v. at 14 1 days postpartum. Blood samples were collected at 15-minute intervals for 2 hours before and 2.5 hours after naloxone or saline. Serum LH concentrations increased (P < .05) in response to naloxone injection in both primi- and multiparous cows. Saline injection did not affect LH concentrations. In experiment 2, 27 cows (13 primiparous; 14 multiparous) received either saline (n=14) or 1 mg/kg naloxone (n=13) i. v. at 28 ± 1 days postpartum. Blood samples were collected as in the previous experiment. Naloxone did not affect serum LH concentrations in either primi- or multi-parous cows at 28 days postpartum. In experiment 3, estrous cycles were synchronized via prostaglandin administration (25 mg) in 22 cows (10 primiparous; 12 multiparous). Cows received either saline (n=11) or 1 mg/kg naloxone (n=11) Lv. during the luteal phase of the estrous cycle. Blood samples were collected as in the previous experiments. Luteinizing hormone concentrations were not affected by naloxone in either primi- or multi-parous cows during the luteal phase of the estrous cycle. In experiment 4, the same cows used in experiment 3 received a second dose of prostaglandin (25 mg). Thirty-six hours later, during the follicular phase of the estrous cycle, the cows received either saline (n =9) or 1 mg/kg naloxone (n = 11) i. v. Naloxone increased (P < .05) serum LH concentrations in both primi- and multi-parous cows in the follicular phase. These results suggest that LH release in the early postpartum dairy cow is regulated, at least in part, by endogenous opioid pep tides , and the ability of naloxone to affect LH secretion may change as days postpartum increases, perhaps due to changes in degree of inhibition by endogenous opioid peptides, and (or) changes in serum progesterone concentration due to onset of ovarian activity during postpartum period. It appears that the modulation of LH secretion may be mediated via opioids during the follicular phase of the estrous cycle. However, an opioid-mediated mechanism for modulation of LH secretion was absent or overridden by progesterone feedback during the luteal phase of the estrous cycle. / Master of Science

The effects of selection on body weight (BW), dry matter intake (DMI), milk yield, fat percent, and on plasma concentration of insulin (INS), growth hormone (GH), and Insulin-like Growth Factor I (IGF-I) were studied in two groups of first lactation Holstein cows of differing genetic merit (selection vs control). Dry matter intake (DMI) was measured at 45, 90, 180, 225, 270, and 315 d postpartum. Serial blood samples were collected at 30 d intervals for a 5 hr period at 15 min intervals. Selection group cows were heavier (532 vs 514 kg, N.S.) than control cows with a group by days in milk (DIM) interaction (P<.01). Significant differences (P < .05) in energy intake occurred with an average of 25.6 Mcal/d for the control group and 28.6 Mcal/d for the selection group cows. Milk yield and mean milk fat were greater (P<.01) in the selection group cows. The mean estimated production efficiency (kg milk/Meal intake) was .84 and 1.02 (P < .05) for the control and selection group, respectively. Plasma GH was higher (P<.01) and IGF-I was lower (P<.1) in selection group cows compared to control cows. Mean plasma INS concentrations were 821 vs 763 pg/ml (N.S.) for control and selection group cows. A significant (P< .01) interaction occurred between group and month of lactation for GH. The mean IGF-I plasma levels were 170 ng/ml and 139 ng/ml (P < .1) for the control versus the selection cows respectively. The results indicate that selection for milk yield resulted in differences in DMI, milk fat and plasma concentrations of GH and IGF-I. Selection also resulted in increased estimates of production efficiency. In a follow up study the effects of selection on BW, DMI, milk yield, fat percent and response to Growth Hormone Releasing Factor (GRF) as well as glucose infusion were studied in early lactation control and selection group cows. Dry matter intake was measured at 45 and 90 DPP. Serial blood samples were collected at 15 d intervals for a period of either 6 or 15 hr at various intervals. Selection group cows were similar in body weight (494 vs 489, N.S.) than control cows, however energy intake tended to be greater for selection group cows than for control animals (23.3 vs 20.4 Mcal/d, P< .1). Milk yield was greater in selection group cows (P < .01 ). The mean estimated production efficiency was 1.31 vs 1.18 (kg milk/Meal intake) (N.S.) for the selection and control group, respectively. Mean plasma GH was higher on all test days (30, 60, 90, 120 DPP) for selection compared to control group cows (15.6 vs 23.5 ng/ml, P < .01). Mean plasma INS concentrations were 1017 vs 1032 pg/ml (N.S.) for the control and selection groups, respectively, following glucose infusion(.lmg/kg BW). Mean plasma IGF-I concentrations tended to be greater (P<.14) in control group cows compared to selection group cows. No increase in plasma IGF-I was observed in the thirteen hours following GRF (.2ug/kg BW) administration in either group of cows. The results indicate that selection for milk yield resulted in difference in DMI and plasma concentration of GH in response to GRF infusion but it did not affect plasma INS. / M.S.