The pubertal process with its multifaceted neuroendocrine control provides an excellent model for the study of the regulation of FSH heterogeneity. We tested the hypothesis that during the pubertal transition in the female lamb 1) an increase in both pituitary and circulating bioactive FSH concentrations occur and 2) that the increase in bioactivity is associated with a change in the distribution pattern of both pituitary and circulating FSH isoforms. Pituitary and serum immunoreactive (I), and bioactive (B, Sertoli cell bioassay) FSH concentrations were measured in six prepubertal lambs (18 +/- 1 weeks, 29.9 +/- 2.8 kg body weight; mean +/- SE) and compared to those of six others (24.2 +/- 2.2 weeks of age, 41.4 +/- 2.5 kg body weight) during the pubertal transition period. Puberty was synchronized by pulsatile iv administration of GnRH (2 ng/kg every 2 h for 24 h and then at hourly intervals for the next 12 h) in a manner mimicking the I-LH pulse patterns observed during the natural transition to adulthood. Blood samples were collected at 12-min intervals for 4 h from both groups of lambs; for the pubertal group this included the final 32-36 h of GnRH administration. At the end of the study, a 25 ml volume of peripheral blood was collected from both prepubertal and pubertal females for the determination of serum FSH distribution patterns; the lambs were then euthanised, and pituitaries were removed for determination of pituitary hormone content and FSH isoform distribution patterns. In addition, the distribution pattern of I-FSH isoforms in the pituitary and serum from both groups of lambs were compared. The pubertal stages of all lambs were verified by measuring the size of follicles, the circulating concentrations of estradiol (E2) and inhibin, and the I-LH pulse patterns. Prepubertal lambs had low frequency I-LH pulses, small (2-3 mm) size ovarian follicles and low circulating concentrations of E2 (4.1 +/- 0.4 pg/ml) and inhibin (38.0 +/- 2.9 U/ml WHO). By contrast, all the pubertal lambs had hourly I-LH pulse frequency (induced with exogenous GnRH), a large (5-6 mm) follicle (in one lamb a 4-mm follicle), follicular phase levels of E2 (7.1 +/- 0.8 pg/ml), and higher concentrations of inhibin (53.2 +/- 3.1 U/ml).(ABSTRACT TRUNCATED AT 400 WORDS)

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.