Hypothalamic versus Pituitary Stimulation of Luteinizing Hormone Secretion in the Prepubertal Female Lamb

Abstract

Glutamate and aspartate have been hypothesized to function as neurotransmitters in the regulation of the gonadotropin-releasing hormone (GnRH) neurosecretory system. We, therefore, determined if hypothalamic stimulation of luteinizing hormone (LH) secretion in the intact prepubertal female lamb could be achieved by intravenous injection of N-methyl-D,L-aspartate (NMA), a glutamate agonist. A pilot study determined a dose of NMA that would induce physiologic pulses of LH (GnRH). Subsequently, we compared the ability of NMA with exogenous GnRH to induce ovulation in the prepubertal lamb when administered chronically. Eighteen prepubertal lambs (21 weeks of age, 34.2 ± 1.5 kg body weight) were treated intravenously with either NMA (2 mg/kg, n = 6) or GnRH (68 ng/injection or approximately 2 ng/kg, n = 6) for 3 days, every 2 h on day 1 and every 1 h on days 2 and 3, or received no treatment (controls, n = 6). Gonadotropin surges were detected only in GnRH-treated lambs (5/6 lambs, onset = 54.0 ± 4.5 h from the start of study, mean ± SE). Compared to 83% of GnRH injections inducing LH pulses, only 47% of NMA injections induced LH pulses. Because each injection of NMA did not induce a pulse of LH, a second experiment was performed in an attempt to optimize the LH respone to NMA. Ten prepubertal lambs (25 weeks of age) were injected every 2 h for 24 h with higher doses of NMA, either 4 mg/kg (n = 5) or 16 mg/kg (n = 5). At the outset (first and second injections), the lower dose of NMA did not consistently induce LH pulses in all lambs; by 22 and 24 h, however, a more consistent LH response was observed. By contrast, in the high dose group, the first two and last two injections of NMA all induced LH pulses. Similar amplitudes of LH pulses were induced by the last two injections of either dose of NMA (6.3 ± 1.0 vs. 8.8 ± 2.3 ng/ml, 4 vs. 16 mg/kg, respectively); similar LH pulse amplitudes were also induced by NMA (4 and 16 mg/kg doses combined) and GnRH (2 ng/kg dose) (7.6 ± 1.2 vs. 5.7 ± 0.8 ng/ml). These results suggest that acute administration of NMA can be used to induce pulses of LH in the prepubertal lamb, and that the response appears to be ‘all-or-none’. In addition, although we were unable to sustain pulsatile LH secretion sufficiently to induce an LH surge and ovulation using a low dose of NMA (Experiment 1), a higher dose could be effective (Experiment 2, 4 mg/kg). The highest dose of NMA (16 mg/kg) was the only dose consistently able to induce LH pulses, however, adverse behavioral responses were also observed at this dose. In conclusion, glutamate/aspartate neurotransmitters may regulate the GnRH neurosecretory system in the prepubertal lamb by stimulating both inhibitory and stimulatory pathways. The dominant pathway which is activated would determine the level of reproductive activity of the lamb, with stimulatory pathways more likely to be activated during the onset of puberty.

Regulation of Luteinizing Hormone-Releasing Hormone (LHRH) Secretion by Melatonin in the Ewe. II. Changes in N-Methyl-D,L-aspartic acid-induced LHRH Release during the Stimulation of Luteinizing Hormone Secretion by Melatonin1

“…A complementary hypothesis is that the increased LHRH secretion during the induction of breeding season also implicates stimulatory mechanisms. In support of this hypothesis, administration of N-methyl-D,L-aspartic acid (NMDA), an agonist of neuroexcitatory amino acid receptors, acutely stimulates LH secretion in seasonal species such as the hamster and the sheep [10][11][12][13][14][15]. However, this stimulatory effect is larger during periods of photoinhibited LH secretion than during photostimulated periods, suggesting that excitatory amino acids are involved in the photoperiodic regulation of LH secretion.…”

Role of GABA in the Mechanism of the Onset of Puberty in Non‐Human Primates

“…NMDA stimulates release of LH and LHRH in adult rats and monkeys in vivo (Price et al, 1978, Olney et al, 1976;Brann and Mahesh, 1997;van den Pol et al, 1994;Bourguignon et al, 1995), and glutamate, NMDA, and kainate all stimulate LHRH/ LH release in sexually immature monkeys (Gay and Plant, 1987;Medhamurthy et al, 1990), rats (Bourguignon et al, 1989;Brann and Mahesh, 1992;Cicero et al, 1988), sheep (I'Anson et al, 1993), and fetal sheep (Bettendorf et al, 1999) in vitro and in vivo. Moreover, stimulation of NMDA receptors results in precocious puberty in rats and monkeys (Plant et al, 1989;Urbanski and Ojeda, 1990), whereas administration of the NMDA receptor blockers, MK-801 or 2-amino-5-phosphonovaleric acid (AP-5), delays the timing of puberty in rats (MeijiRoelofs et al, 1991;Urbanski and Ojeda, 1990;Wu et al, 1990;MacDonald and Wilkinson, 1990).…”

Dietary Effects on Ruminant Livestock Reproduction with Particular Reference to Protein

“…fed lambs which suggested that the effect of feed restriction was on central mechanisms controlling LHRH release rather than on LHRH synthesis. It is further evident that N-methyl-D-aspartate activates the release of accumulated GnRH from the hypothalamus which is inhibited as a result of undernutrition (Ebling et al 1990;I'Anson et al 1990), suggesting that the central nervous system was responsible for the release of GnRH (Dunn & Moss, 1992). Since dietary protein:energy ratios influence intermediary metabolism leading to changes in the circulatory concentrations of insulin, certain amino acids and non-esterified fatty acids (Brockman & Laarveld, 1986;Howard et al 1987;Patil, 1993), it is proposed that these metabolites may act as nutritional signals influencing LH secretion.…”