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.