EFFECT OF NALTREXONE ON THE SECRETION PAT TERN OF THE LUTEINIZING-HORMONE IN PREPUBERTAL EWES

Secretion of LH is inhibited by opioid peptides during the prepubertal period in ewe lambs. The chronic blockade of opioid receptors with a long acting opioid receptor antagonist might accelerate the increase o f LH secretion observed before the initiation of puberty. The objectiv e of the present study was to explore the effect of naltrexone-a long acting opioid receptor antagonist-on the 24 h pattern of LH secretion and on the pulsatile LH secretion after 3 days of treatment. Ewe lambs (4 months old, 21 +/- 1 kg, n=5) were given 1.5 mg naltrexone/kg body weight every 12 h for 3 days with a total of 7 doses. Pattern of LH s ecretion was assessed in plasma samples obtained at hourly intervals f or 24 h which were analyzed with the COSINOR program; and with plasma samples obtained at 15 min intervals for six hours, which were analyze d with the PULSAR program before naltrexone treatment and before the t reatment ended. Naltrexone did not modify the 24 h rhythm of LH secret ion. On the other hand, the LH pulse frequency diminished from 7.8 +/- 0.5 to 4.8 +/- 0.4 pulses/6 h (p < 0.05). The mean pulse amplitude de creased from 0.83 +/- 0.1 to 0.44 +/- 0.07 (p < 0.05). Results suggest that naltrexone may be acting as an opioid agonist instead of antagon ist in immature ewes.