Effect of naloxone and luteinizing hormone releasing hormone pulses on plasma luteinizing hormone concentration in ewe lambs.
A study was conducted to determine the effect of pulses of naloxone on plasma LH concentration in prepubertal ewes and to see if this treatment affects the pituitary response to subsequent LHRH pulses. Prepubertal ewes (n = 5) received three intracarotidal pulses of naloxone (NAL, 1 mg/kg BW) and four pulses of Luteinizing Hormone Releasing Hormone (LHRH, 1 micrograms/pulse) at hourly intervals. Control ewes (n = 5) received saline instead of NAL followed by the LHRH pulses at the same frequency. Blood samples were collected from one hour before the first pulse of saline or NAL at 15 min intervals for four hours and at 30 min intervals for another four hours. Plasma LH concentration was measured by radioimmunoassay. The first pulse of NAL increased plasma LH levels compared to the basal levels and compared to control animals (P less than 0.01). The succeeding pulses were ineffective. LHRH provoked an increase in plasma LH concentration in control as well as in treated animals but the amplitude of the net peak height increased progressively up to the third pulse in NAL treated animals, while there was no increase in pituitary responsiveness to LHRH in control prepubertal ewes. Also, the area under the LHRH response curve was greater (P less than 0.05) in animals pretreated with NAL than in lambs pretreated with saline. The results suggest that there is an inhibitory opioid tone over LH secretion in female lambs. NAL increases the responsiveness to exogenous LHRH pulses, probably as a result of endogenous LHRH release.
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Mora
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D000818:Animals; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007261:Infusions, Intra-Arterial / Q000662:veterinary; D007986:Luteinizing Hormone / Q000097:blood*; D009270:Naloxone / Q000494:pharmacology*; D011673:Pulsatile Flow; D012756:Sheep / Q000097:blood*
D007987:Gonadotropin-Releasing Hormone; D009270:Naloxone; D007986:Luteinizing Hormone