TREATMENT OF LACTATING SOWS WITH THE DOPAMINE AGONIST CABERGOLINE - EFFECTS ON LH AND PROLACTIN SECRETION AND RESPONSES TO CHALLENGES WITH NALOXONE AND MORPHINE

This study was conducted to examine the effects of chronic administrat ion of a long-acting dopamine agonist, Cabergoline, on LH and prolacti n secretion during lactation in the sow. The effect of the administrat ion of the opioid antagonist naloxone and the agonist morphine in Cabe rgoline treated animals was also evaluated. In Part I of the experimen t, 16 sows were treated as either CONT sows (n = 4; control, no treatm ent); CAB sows (n = 4; treated with Cabergoline from days 10 to 26 of lactation); CAB + NAL sows (n = 4; received Cabergoline treatment and naloxone challenges); CAB + MORP sows (n = 4; treated with Cabergoline and morphine challenges). Plasma LH and prolactin concentrations were measured in blood samples taken from all sows during 6-h periods at d ays 12, 19 and 26 of lactation. To extend the results at the most crit ical response period at day 26, another 11 sows were allocated in Part II to either Control (n = 3), Cabergoline (n = 4) or Cabergoline and morphine (n = 4) treatments as for Part I, but the effect of treatment s were only confirmed in a single period of sampling at day 26 of lact ation. Cabergoline treatment alone increased (P < 0.001) mean plasma L H concentrations at day 26 but not at days 12 and 16 of lactation. In contrast, naloxone challenges given in the presence of Cabergoline tre atment increased (P < 0.05) mean LH at days 12 and 19 of lactation but not at day 26. Morphine challenges in the presence of Cabergoline tre atment decreased(P < 0.05) mean LH concentrations only at day 26 of la ctation, but did not completely reverse the effect of Cabergoline. No treatment differences in plasma oestradiol-17 beta were detected at an y time. Plasma prolactin decreased (P < 0.001) in response to treatmen t with Cabergoline but there were no additional effects of naloxone or morphine. These data provide evidence for the existence of dopaminerg ic and opioidergic regulation of LH secretion in lactation in the sow and the relative influence of these systems changes as lactation progr esses. Furthermore, the data suggest that the stimulatory effect of Ca bergoline treatment on LII secretion in late lactation may be mediated by its effects on an inhibitory opioidergic mechanism. Finally, the d ata provide conclusive proof that prolactin does not directly influenc e LPI secretion or estrogenic activity of the ovary during lactation i n the sow. (C) 1998 Elsevier Science B.V. All rights reserved.