Identification of alpha1B adrenergic receptor protein in gonadotropin releasing hormone neurones of the female rat.

Noradrenaline is an important neurotransmitter which regulates GnRH release from the median eminence in the female rat during both basal GnRH secretion and the preovulatory or steroid hormone-induced GnRH-mediated LH surge. However, it is not clear at which sites in the brain this predominantly stimulatory influence is exerted nor is it known which adrenergic receptor subtype(s) mediate(s) the effects of noradrenaline. In order to determine if the GnRH neurones in the septum-diagonal band-preoptic area and/or their axon terminals in the median eminence are direct targets for noradrenaline, immunohistochemical triple-labelling studies were conducted to localize simultaneously GnRH peptide, dopamine-beta-hydroxylase and alpha1B adrenergic receptor protein. The results show that about 80% of all GnRH neurones examined contained patches of immunoreactive alpha1B adrenergic receptor protein at or near the plasma membrane and that some of these alpha1B adrenergic receptors were adjacent to dopamine-beta-hydroxylase containing axons. The GnRH neurones which did not contain alpha1B adrenergic receptors were preferentially located in the rostral portion of the septum and diagonal band while all GnRH neurones in the caudal septum, diagonal band and in the preoptic area expressed alpha1B adrenergic receptors. In the median eminence, a few alpha1B adrenergic receptor patches were seen in the external layer and these receptors were only rarely observed to be associated with GnRH containing axon terminals. The results suggest that the effects of noradrenaline on GnRH release are, at least in part, mediated by the activation of alpha1B adrenergic receptors which are located on most GnRH perikarya while the median eminence is not a likely site at which GnRH release is regulated by alpha1B adrenergic receptors.