Biochemical and functional aspects of gonadotrophin-releasing hormone and gonadotrophins.

Reproductive function in mammals is governed by the hypothalamic-pituitary-gonadal axis, which conforms a functional unit. Sexual maturation and the subsequent development of reproductive competence depend on the precise and coordinated function of this axis. The components of the reproductive axis communicate each other through endocrine signals. The hypothalamus synthesizes gonadotrophin-releasing hormone or GnRH, which in turn stimulates synthesis and secretion of the pituitary gonadotrophins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The ovarian follicles and the interstitial and Sertoli cells of the testis are the targets for these pituitary signals. Under gonadotrophic stimulation, the gonads produce and secrete several steroid and non-steroid (polypeptide) factors, which in turn regulate in different ways the function of the hypothalamic-pituitary axis. An episodic and pulsatile mode of secretion of hormonal signals characterize (as in other endocrine systems) the function of the reproductive axis, particularly that of the hypothalamic-pituitary unit. The target cell response, and consequently the harmonic function of the corresponding gland, will depend on the adequate dynamics of this pulsatile secretion. The function of each component of the reproductive axis is strongly influenced by locally-produced signals acting either in a paracrine or autocrine manner; these particular signals represent fine-tuning regulation systems that eventually amplify or restrain the magnitude of response to a particular endocrine signal, providing additional mechanisms for tissue homeostasis and a better functional plasticity of the target gland. The design and rational use of novel therapeutic strategies for an optimal exogenously-controlled reproductive function largely depend on the detailed knowledge of the hypothalamic-pituitary-gonadal axis function and the structure and mechanism of action of those factors and signals involved in its regulation.