Two decades of measuring GnRH secretion.

It is almost 20 years since it was first demonstrated that serial hypophysial portal blood samples could be obtained from conscious sheep and, subsequently, from cattle, goats and pigs. Accumulated data have defined the patterns of secretion of GnRH in the normal physiological state and the effects of gonadal steroids have also been largely defined. In particular, changes across the oestrous cycle of females have been studied in detail. These data show that there is an increase in the frequency of pulsatile discharges of GnRH in the transition from the luteal to the follicular phase of the oestrous cycle, with an inverse change in the amplitude of GnRH pulses; such changes are reflected in the patterns of LH secretion from the pituitary gland. A surge in GnRH secretion is observed in a number of species, and is the primary stimulus for the preovulatory LH surge. Patterns of GnRH secretion and regulation by gonadal steroids have also been described in males. With respect to FSH, secretion per se is not dependent on pulsatile GnRH input to the pituitary gland and regulation of secretion is dependent on other factors, such as inhibin. With the bank of knowledge that has now accumulated on the secretion of GnRH in sheep under normal conditions and with respect to steroidal feedback regulation, we are now in a commanding position to define and decipher the neural systems that are involved in the regulation of its secretion.