Disappearance of Opioidergic Tone on LH Secretion in Underfed Prepubertal Sheep

Summary

A study was conducted to determine whether an opioid tonus inhibitory of LH secretion is present in underfed prepubertal sheep. Ten Suffolk ewe lambs were subjected to food restriction during 60 days. During this period they were allowed to pasture only 2 hours per day while control ewe lambs were allowed for 10 hours. Body weight and plasma blood levels of glucose, urea and total proteins were measured weekly. At the end of this period, an intravenous injection of Naloxone (NAL, 1.5 mg/kg BW) was given to control and underfed animals followed 60 min later by an intravenous injection of LHRH to test the pituitary responsiveness. Underfed animals did not show an increase in plasma LH while control animals presented a rise from 0.28±0.08 to 2.02±0.6 ng/ml after the NAL stimulus (P < 0.05). The response to LHRH was similar in both group of animals. Basal plasma levels of insulin were lower in underfed ewe lambs than in control animals (P < 0.05). Underfed animals were placed on plain feeding with a schedule similar to control lambs for 30 days and the same experiment was repeated. During this occasion, NAL increased plasma LH concentration in both group of lambs. Levels of plasma insulin were not different in both groups. The lack of effect of NAL on LH secretion in food restricted ewe lambs suggests that the opioid modulation of LH secretion is absent by underfeeding in female prepubertal sheep. The underlying neuroendocrine mechanism(s) to be accounted for this divergent manifestation of response to opioidergic blockade may be located at the hypothalamic levels since the pituitary response to LHRH was comparable in both groups. The opioidergic activity could be restored by placing the underfed ewe lamb on regular feeding. Thus, the endogenous inhibitory opioid activity appears to critically depend on the internal status of prepubertal ewe lambs.

Key words

LH - Naloxone - LHRH - Prepuberty - Sheep - Underfeeding