CONTROL OF LUTEINIZING-HORMONE SECRETION IN EWES BY ENDOGENOUS OPIOIDS AND THE DOPAMINERGIC SYSTEM DURING SHORT SEASONAL ANESTRUS - ROLE OFPLANE OF NUTRITION

The role of endogenous opioids and the dopaminergic system on the inhi bition of luteinizing hormone (LH) secretion during early and late ano estrus, together with its modulation by the plane of nutrition were in vestigated in ewes with a short anoestrous season. In early anoestrus (22 March; day 0), two groups of ovariectomized, oestradiol-treated ad ult Rasa Aragonesa ewes, maintained tinder natural photoperiod at 41 d egrees N, were given enough food to provide 1.4 x (high; H; no. = 6) o r 0.5 x (low; L; no. = 6) energy requirements for maintenance. The eff ects of administration of the opiate receptor antagonist naloxone (I m g/kg at four I-h intervals) (day 15) and of the dopaminergic(2) recept or antagonist pimozide (0.08 mg/kg) (day 21) on LH secretion were asse ssed. A second experiment ions carried out in late anoestrus (21 June) using the same protocol. A significant increase in LH pulse frequency after naloxone treatment for both H and L groups was detected in late anoestrus. Number of LH pulses after naloxone injections in early ano estrus also increased in H (P < 0.05) and L ewes (P = 0.08). The effec t of pimozide injection on mean LH pulse frequency was greater in earl y than in late anoestrus, especially in ewes receiving a high plane of nutrition (P < 0.05 and P = 0.07 for H and L ewes, respectively in Ap ril and P = 0.07 for H ewes in July). A significant increase of LH pul se amplitude was also detected in early anoestrus in H ewes (P < 0.01) . These results provide evidence that endogenous opioid mechanisms are involved in the inhibition of LH pulsatile release both in early and late anoestrus in ewes with a short seasonal anoestrus. The ability of pimozide to increase LH pulse frequency in early anoestrus could be e nhanced by a high plane of nutrition in the breed studied.