Neuroendocrine mechanisms controlling the onset of female puberty: the rat as a model.

The advent of female puberty represents the culmination of a diversity of developmental processes which affect all components of the reproductive axis. Development of neuroendocrine reproductive functions proceeds in a harmonious and interrelated manner. No unique 'trigger' of puberty can be discerned, but rather puberty represents the climax of a cascade of events, finely interconnected throughout the continuum of sexual maturation. A resetting of the hypothalamic 'gonadostat' to steroid negative feedback appears to be a phenomenon associated with puberty, but not its cause. Although the central nervous system plays a pivotal role in the development of the ovary, it is the acquisition of ovarian ovulatory capacity which finally determines the timing of the first preovulatory surge of gonadotropins. In contrast to primates, development of the central component of estradiol positive feedback is an early event in the female rat. However, in most species--including the rat--amplification (or initiation) of a particular, synchronous pattern of LHRH release appears essential for the initiation of puberty. The mechanisms underlying this functional change of the LHRH secreting system are not clearly understood. In the rat, ovarian development proceeds under the influence of gonadotropins, and the somatomammotropins PRL and GH. More intriguingly, evidence is now emerging that the central nervous system may convey direct information to the ovary via the ovarian nerves, thus providing a hormone-independent fine tuning for its control. Upon reaching adequate development, the ovary through its secretory products, acts on an already competent hypothalamic-pituitary axis to activate the central component of estradiol positive feedback.