HYPOTHALAMIC VERSUS PITUITARY STIMULATION OF LUTEINIZING-HORMONE SECRETION IN THE PREPUBERTAL FEMALE LAMB

Glutamate and aspartate have been hypothesized to function as neurotra nsmitters in the regulation of the gonadotropin-releasing hormone (GnR H) neurosecretory system. We, therefore, determined if hypothalamic st imulation of luteinizing hormone (LH) secretion in the intact prepuber tal female lamb could be achieved by intravenous injection of N-methyl -D,L-aspartate L-aspartate (NMA), a glutamate agonist. A pilot study d etermined a dose of NMA that would induce physiologic pulses of LH (Gn RH). Subsequently, we compared the ability of NMA with exogenous GnRH to induce ovulation in the prepubertal lamb when administered chronica lly. 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) o r GnRH (68 ng/injection or approximately 2 ng/kg, n = 6) for 3 days, e very 2 h on day 1 and every 1 h on days 2 and 3, or received no treatm ent (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 stu dy, 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 performe d in an attempt to optimize the LH respone to NMA. Ten prepubertal lam bs (25 weeks of age) were injected every 2 h for 24 h with higher dose s of NMA, either 4 mg/kg (n = 5) or 16 mg/kg (n = 5). At the outset (f irst and second injections), the lower dose of NMA did not consistentl y induce LH pulses in all lambs; by 22 and 24 h, however, a more consi stent LH response was observed. By contrast, in the high dose group, t he first two and last two injections of NMA all induced LH pulses. Sim ilar amplitudes of LH pulses were induced by the last two injections o f either dose of NMA (6.3 +/- 1.0 vs. 8.8 +/- 2.3 ng/ml, 4 vs. 16 mg/k g, 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 v s. 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, an d that the response appears to be 'all-or-none'. In addition, although we were unable to sustain pulsatile LH secretion sufficiently to indu ce 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 d ose of NMA (16 mg/kg) was the only dose consistently able to induce LH pulses, however, adverse behavioral responses were also observed at t his dose. In conclusion, glutamate/aspartate neurotransmitters may reg ulate the GnRH neurosecretory system in the prepubertal lamb by stimul ating both inhibitory and stimulatory pathways. The dominant pathway w hich is activated would determine the level of reproductive activity o f the lamb, with stimulatory pathways more likely to be activated duri ng the onset of puberty.