Elicitation of release of luteinizing hormone by N-methyl-d,l-aspartic acid during three paradigms of suppressed secretion of luteinizing hormone in the female pig

Abstract

Two experiments were conducted to determine the minimal effective dose during lactation and site of action of N-methyl-d,l-aspartic acid (NMA) for elicitation of release of luteinizing hormone (LH) in female pigs. In the first experiment, three doses of NMA were given to lactating primiparous sows in which endogenous LH was suppressed by suckling of litters. In the second experiment, ovariectomized gilts were pretreated with estradiol benzoate or porcine antisera against GnRH to suppress LH and then given NMA to determine if it elicited secretion of LH directly at the anterior pituitary or through release of GnRH. In experiment 1, 3 lactating sows (17 ± 1.5 d postpartum) were each given three doses of NMA (1.5, 3.0 and 5.0 mg/kg body weight [BW]; IV) on 3 consecutive days in a Latin Square design. Blood samples were collected every 10 min from −1 to 1 hr from injection of NMA. NMA at 1.5 and 3.0 mg/kg did not affect (p>.5) secretion of LH; however, 5 mg NMA/kg elicited a 114% increase (p<.001) in circulating levels of LH during 1 hr after treatment. In experiment 2, 8 ovariectomized gilts were given either estradiol benzoate (EB; 10 μg/kg BW; IM n = 4) to suppress release of GnRH or porcine antiserum against GnRH (GnRH-Ab; titer 1:8,000; 1 ml/kg BW; IV; n = 4) to neutralize endogenous GnRH. Gilts infused with GnRH-Ab were given a second dose of antiserum 24 hr after the first. Gilts were then given NMA (10 mg/kg BW; IV) 33 hr after EB or initial GnRH-Ab. Blood samples were drawn every 6 hr from −12 to 24 hr from EB or GnRH-Ab treatments, and every 10 min from −2 to 2 hr from NMA. Serum LH declined (p<.001) after EB (from 1.87 ± .2 ng/ml at 12 hr before EB to 0.46 ± .02 ng/ml during 24 hr after EB) and GnRH-Ab (from 1.97 ± .1 to 0.59 ± .02 ng/ml). In gilts treated with EB, the area under the curve (AUC) for the LH response (ng·ml −1·min) 1 hr after NMA (38.7 ± 3) was significantly greater (p<.01) than the 1 hr prior to NMA (21.3 ± 1.5). Treatment with NMA had no effect (p>.5) on secretion of LH in gilts infused with GnRH-Ab. These results demonstrate that as little as 5 mg NMA/kg evokes LH secretion in female pigs and that NMA elicits LH release via stimulation of release of GnRH.