The effect of low doses of naloxone on the preovulatory surge of LH and on the onset and duration of oestrus in the ewe with induced oestrus during the non-breeding season

The objective of this work was to study the effect of the endogenous opiate peptide (EOP) antagonist, naloxone, on the preovulatory LH surge and on the time of onset and duration of oestrus in the ewe with induced oestrus during the non-breeding season. Forty Suffolk X Hampshire ewes 2–3-years-old and 50±4 kg were studied, ewes were divided at random in two groups of 20, housed in open paddocks under natural photoperiod (19° latitude N); were fed with hay and commercial pellets, and provided water ad libitum. Group one received an intravaginal sponge with 45 mg of medroxiprogesterone acetate for 14 days, and upon sponge withdrawal 250 IU of eCG was administered i.m. Group two received the same treatment as group 1 but in addition they received two i.m. injections of 0.5 mg of naloxone, one given on sponge withdrawal and the second 24 h later (total dose 1.0 mg). Oestrus in naloxone-treated ewes was present 32±2 h and in control ewes in 35±3 h after sponge withdrawal. Duration of oestrus in control ewes was shorter (27±2.5 h), than naloxone-treated ewes (39±6 h); (P<0.0001). The LH surge in naloxone-treated ewes was initiated 5 h after onset of oestrus, and 8 h after onset of oestrus in control ewes, and the difference was significative (P<0.0006). It was concluded that EOP are important modulators of reproductive function in the ewe.