The neuroendocrine timing of puberty.

Puberty is the attainment of fertility, a process encompassing morphological, physiological and behavioural development. The increased hypothalamic secretion of the gonadotrophin-releasing hormone decapeptide (GnRH) is essential for the activation of the pituitary-gonadal axis at puberty. The GnRH secretory network initially develops and is temporarily active during species-specific periods of fetal/neonatal development, so puberty is the secondary reactivation of an existing system. From a neurobiological perspective, the timing of puberty is therefore a function of changes in the neural systems controlling GnRH release. The large variability between individuals in the onset and progression of puberty indicates that the timing of puberty is not simply a function of chronological age. Rather, the neurotransmitter and neuromodulatory systems that impact upon the GnRH secretory network convey information about metabolic fuels, energy stores and somatic development and, for many species, information about season and social environment. The clear links demonstrated between metabolic fuel availability and reproductive function in many animal models provides evidence that the earlier onset of pubertal development observed in girls in certain US study populations is likely to relate to the increasing prevalence of overweight and obesity in adolescents.