Puberty presents remarkable individual differences in timing reaching over 5 years in humans. We put emphasis on the two edges of the age distribution of pubertal signs in humans and point to an extended distribution towards earliness for initial pubertal stages and towards lateness for final pubertal stages. Such distortion of distribution is a recent phenomenon. This suggests changing environmental influences including the possible role of nutrition, stress and endocrine disruptors. Our ability to assess neuroendocrine effects and mechanisms is very limited in humans. Using the rodent as a model, we examine the impact of environmental factors on the individual variations in pubertal timing and the possible underlying mechanisms. The capacity of environmental factors to shape functioning of the neuroendocrine system is thought to be maximal during fetal and early postnatal life and possibly less important when approaching the time of onset of puberty.

In order to evaluate the involvement of gonadotropin-releasing hormone (GnRH) in the effects of neuroexcitatory amino acids on luteinizing hormone (LH) secretion, N-methyl-D,L-aspartate (NMDA; 30 mg/kg s.c.) was administered to 50-day-old male rats. The in vitro release of GnRH from the hypothalamus showed a maximal increase (4.6-fold) in animals sacrificed 7.5 min after NMDA injection, while serum LH levels rose concomitantly. Incubation of rat hypothalami in vitro with kainate or NMDA concentrations > 0.1 mM resulted in a dose-related release of GnRH, NMDA being twofold more potent than kainate. Quisqualate (10 mM) did not affect the release of GnRH. On a molar basis, quinolinate (50 mM), a possible endogenous ligand for NMDA receptors, was the most effective in inducing GhRH secretion (34.9 ± 4.9 pg/7.5 min, mean increment ± SEM, n= 10). The effects of kainate and NMDA were mediated through different types of receptors, since GnRH response to kainate was unchanged in the absence of glycine or in the presence of increased concentrations of Mg²⁺ (2 mM) or Ca²⁺ (5.8 mM). In contrast, the GnRH response to NMDA was reduced by Ca²⁺ (5.8 mM) and abolished in the absence of glycine or in the presence of Mg²⁺ (2 mM). In addition, DL-amino-5-phosphonopentanoic acid (AP5), a competitive antagonist of NMDA receptors, prevented the NMDA-induced release of GnRH. The permissive effect of glycine on GnRH response to NMDA was 2.7-fold more important using glycine concentrations of 0.01 µM than when concentrations >100 µM were used. Intermittent incubation with NMDA in vitro (every other 7.5-min period) did not affect the amplitude of GnRH response, while continuous exposure to NMDA resulted in an initial and transient increase in GnRH release followed by a prolonged desensitization period. It is concluded that different neuroexcitatory amino acids acting through distinct receptor types may be involved in the hypothalamic control of LH release by regulating the secretion of GnRH.

An increase in the frequency of pulsatile gonadotropin-releasing hormone (GnRH) secretion in vitro and a reduction in LH response to GnRH in vivo characterize hypothalamic-pituitary maturation before puberty in the female rat. In girls migrating for international adoption, sexual precocity is frequent and could implicate former exposure to the insecticide dichlorodiphenyltrichloroethane (DDT), since a long-lasting DDT derivative has been detected in the serum of such children. We aimed at studying the effects of early transient exposure to estradiol (E(2)) or DDT in vitro and in vivo in the infantile female rat. Using a static incubation system of hypothalamic explants from 15-day-old female rats, a concentration- and time-dependent reduction in GnRH interpulse interval (IPI) was seen during incubation with E(2) and DDT isomers. These effects were prevented by antagonists of alpha-amino-3-hydroxy-5-methylisoxazole-4 propionic acid (AMPA)/kainate receptors and estrogen receptor. Also, o,p'-DDT effects were prevented by an antagonist of the aryl hydrocarbon orphan dioxin receptor (AHR). After subcutaneous injections of E(2) or o,p'-DDT between Postnatal Days (PNDs) 6 and 10, a decreased GnRH IPI was observed on PND 15 as an ex vivo effect. After DDT administration, serum LH levels in response to GnRH were not different from controls on PND 15, whereas they tended to be lower on PND 22. Subsequently, early vaginal opening (VO) and first estrus were observed together with a premature age-related decrease in LH response to GnRH. After prolonged exposure to E(2) between PNDs 6 and 40, VO occurred at an earlier age, but first estrus was delayed. We conclude that a transient exposure to E(2) or o,p'-DDT in early postnatal life is followed by early maturation of pulsatile GnRH secretion and, subsequently, early developmental reduction of LH response to GnRH that are possible mechanisms of the subsequent sexual precocity. The early maturation of pulsatile GnRH secretion could involve effects mediated through estrogen receptor and/or AHR as well as AMPA/kainate subtype of glutamate receptors.

In the male rat the timing of puberty can be estimated by the rapid increase in testicular weight occurring between 25-50 days of age. We found that elongated spermatids, the most mature germ cells identified using flow cytometry, were first seen at 25 days (4% of the testicular cells), while an adult proportion (63%) was attained by 45 days of age. We have shown previously that hypothalamic explants could release GnRH in a pulsatile fashion at a frequency increasing around the age of 25 days, thus consistent with the time of onset of puberty. Since pulsatile GnRH secretion could be suppressed by MK-801, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptor activation, we postulated that an increased activation of those receptors could be involved in the neuroendocrine mechanism that activates pulsatile GnRH secretion at the onset of puberty. Such a concept was supported by the NMDA-induced release of GnRH, which was observed using 1 mM NMDA at 25 days, while a dose of 20-50 mM was required at 15 or 50 days of age. MK-801 could provide an index of NMDA receptor activation, since the antagonistic effect of MK-801 is use dependent. This particular property was confirmed by the inability of MK-801 (5 pM) to block the depolarization (veratridine)-induced release of GnRH in the presence of 0.001 mM NMDA, while partial or complete suppression was obtained in the presence of 0.1 and 10 mM NMDA, respectively. Using explants obtained at 5, 10, 15, 20, 25, 30, 35, and 50 days of age, the lowest concentrations of MK-801 that blocked the veratridine-induced release of GnRH were, respectively, 10(7), 10(7), 10(7), 10(3), 10, 10(2), 10(4), and 10(8) pM. In contrast, there was no age-related difference in sensitivity to the inhibitory effect of Mg2+, a noncompetitive NMDA receptor antagonist which is not use dependent. The pulsatile secretion of GnRH occurred at a similar frequency at 25 and 50 days of age (4.7 and 5.4 pulses/3.5 h, respectively) but it was suppressed by a lower MK-801 concentration at 25 days (10(4) pM) than at 50 days (10(8) pM). These data indicate that the NMDA receptors involved in the control of pulsatile GnRH secretion are markedly and transiently activated around the time of onset of puberty in the male rat.