OPIOIDERGIC AND NUTRITIONAL INVOLVEMENT IN THE CONTROL OF LUTEINIZING-HORMONE SECRETION OF POSTPARTUM RASA-ARAGONESA EWES LAMBING IN THE MID-BREEDING SEASON

The role of endogenous opioids and nutrition on the inhibition of lute inizing hormone (LH) secretion during the postpartum period was invest igated in a Spanish breed of sheep lambing in the mid-late breeding se ason. Two groups of adult Rasa Aragonesa ewes housed in individual pen s and lambing on 30 December were fed during the suckling period to pr ovide maintenance requirements and the production of 1.1 (M; n = 8) or 0.55 (L; n = 8) kg of milk per day. On days 10, 20 and 30 after lambi ng, the effect of a treatment with the opiate receptor antagonist nalo xone (1 mg/kg at four hourly intervals) on LH secretion was assessed i n half of the ewes of each group, the remaining females receiving four saline injections. After weaning, animals were fed to provide require ments for maintenance of liveweight. Blood samples were collected twic e a week from day 20 postpartum until the end of March, and assayed fo r progesterone and prolactin. Although underfed ewes showed significan tly lower mean plasma concentrations during the control period on day 20 postpartum, nutrition did not seem to modify LH secretion before na loxone or saline injections. Moreover, no differences between nutritio nal groups in the response to naloxone injections on pattern of LH sec retion were found. In fact, naloxone treatment induced an increase of mean LH concentrations on days 10, 20 and 30 postpartum (at least, P < 0.05), of LH pulse frequency on days 20 and 30 (P < 0.05), and of LH pulse amplitude on days 10 and 20 (P < 0.05). Underfed ewes during the postpartum period showed a slower decline in plasma prolactin levels, with significant differences on days 29, 36 and 39 after lambing(P < 0.05), Only 3 M ewes ovulated before the onset of the seasonal anoestr us period. It is concluded that endogenous opioids are involved in the inhibition of LH secretion during the early suckling period of a redu ced seasonality breed of sheep without any influence of nutrition on t he response to naloxone treatment; however, ewes underfed before weani ng failed to reactivate their cyclicity prior to the onset of the seas onal anoestrus. (C) 1998 Elsevier Science B.V. All rights reserved.