Involvement of the Central Noradrenergic System in Opioid Modulation of Luteinizing Hormone and Prolactin Secretion in the Pig

Abstract

The role of norepinephrine (NE) in the progesterone (P)-dependent endogenous opioid peptide (EOP) suppression of LH and prolactin (PRL) secretion in the pig was studied. Fifteen ovariectomized gilts treated with P (0.85 mg/kg b.w. i.m.) twice daily for 10 days were assigned, 5 gilts each, to the following treatment groups: 1) control, 2) AIMAX (N-methyl-N¹[1-methyl-2-propenyl]1,2 hydrazinedicarbothioamide), and 3) anti-GnRH serum. On day 4 of P treatment, AIMAX (125 mg per 1.8 kg feed/day), previously shown to be an NE synthesis inhibitor, was fed daily for 7 days to group 2. On the last day of P treatment, group 3 gilts received an i.v. injection of anti-GnRH serum at 0800 h while Group 1 animals received an i.v. injection of serum collected from hypophysectomized gilts. Jugular blood samples were collected every 15 min for 9 h starting at 1000 h. All gilts received two injections of naloxone (NAL; 1 mg/kg b.w., i.v.) at 1400 h and 1600 h and an injection of a GnRH analogue (D-[Ala⁶-desGly-NH¹⁰]-ethylamide; 5 µg/pig, i.v.) at 1800 h. Pretreatment serum LH concentrations were lower (p < 0.05) in AIMAX-treated gilts (0.19 ± 0.09 ng/ml) than in controls (0.53 ± 0.09 ng/ml), but were similar to those in GnRH antiserum-treated gilts (0.32 ± 0.09 ng/ml). NAL increased (p < 0.001) serum LH concentrations in control animals and averaged 1.6 ± 0.2 ng/ml the first h. AIMAX and anti-GnRH serum blocked the NAL-induced LH secretion. Serum PRL concentrations averaged 6.0 ± 0.1 ng/ml for all groups. NAL increased (p < 0.05) serum PRL secretion in all gilts during the first h after treatment. Magnitude of PRL response to NAL was similar for all groups and averaged 9.8, 8.4, and 8.1 ng/ml (SEM = 0.7 ng/ml) for controls, AIMAX- , and GnRH antiserum-treated gilts, respectively. Serum LH concentrations increased (p < 0.0005) in all gilts after GnRH and averaged 10 ± 0.2, 17.6 ± 0.2, and 8.7 ± 0.2 ng/ml for control, AIMAX-, and GnRH antiserum-treated animals, respectively. In conclusion, NAL acts primarily at the hypothalamic level, possibly via activation of the NE system to induce LH secretion in Ptreated gilts, whereas EOP modulation of PRL secretion is independent of the noradrenergic system.