Neuromedin B (NMB) is a highly conserved bombesin-related peptide found in mammals. The mammalian bombesin family of receptors consists of three closely related G protein coupled receptors, BB1, BB2 and BB3. The BB1 receptor subtype has the highest affinity for NMB. NMB mRNA is detected in the CNS and is expressed at relatively high levels in the rat hypothalamus, in particular the medial preoptic area and the arcuate nucleus.
NMB has well documented roles in the regulation of the thyroid axis and the stress axis in rats. However, there is little available data regarding the role of NMB in the regulation of the hypothalamo-pituitarygonadal (HPG) axis. It is known that the NMB receptor is expressed in immortalised GnRH-releasing GT1-7 cells, and that anterior pituitary NMB-immunoreactivity is altered by changes in the sex steroid environment. The objective of these studies was thus to further investigate the effects of NMB on the HPG axis.
I.c.v. administration of NMB (10 nmol) to adult male rats significantly increased plasma LH levels 30 min after injection (plasma LH ng/ml; saline 0.7±0.1, 10 nmol NMB 1.3±0.2, P<0.01). In vitro, NMB stimulated GnRH release from hypothalamic explants from male rats and from GT1-7 cells. NMB had no significant effect on LH release from anterior pituitary explants from male rats, or from LβT2 cells in vitro.
These results suggest a previously unreported role for NMB in the stimulation of the HPG axis via hypothalamic GnRH. Further work is now required to determine the receptor mediating the effects of NMB on the reproductive axis and the physiological role of NMB in reproduction.