Noradrenergic regulation of cyclic GnRH secretion.

The GnRH cells represent the final output neurones of an integrated neuronal network used by the brain to generate pulsatile LH secretion from the pituitary gland. Changes in LH secretion profile throughout the ovarian cycle, including the preovulatory LH surge, result principally from alterations in the output of this GnRH network and it has been a key goal of many neurobiologists to elucidate the components and nature of this network. This review documents recent progress in understanding the role of noradrenaline within the GnRH network and highlights and explains its 'enabling' or permissive characteristics. Network behaviour analysis suggests that noradrenaline should be considered as a permissive agent promoting high output states of the GnRH network. On the basis of recent molecular and neuroanatomical data, it is proposed that oestrogen influences brainstem noradrenergic neurones specifically within the nucleus tractus solitarius to facilitate synaptic transmission within the GnRH network. In this manner, noradrenaline is likely to play a role in bringing about the increased GnRH messenger RNA expression and secretion necessary for ovulation.