Neural signals that regulate GnRH neurones directly during the oestrous cycle

GnRH, produced by a loose network of neurones in the basal forebrain, is th e primary brain signal responsible for the release of LH and FSH from the a nterior pituitary gland. The ovarian steroid hormone oestradiol feeds back at both the central nervous system and the anterior pituitary to regulate t he patterns of release of GnRH and the gonadotrophins. Although recent evid ence indicates that oestradiol may act directly on some GnRH neurones throu gh classical genomic mechanisms, data from published studies have demonstra ted that neurotransmission of afferent neuronal systems that are receptive to oestradiol is necessary to drive reproductive cyclicity. Many classical neurotransmitters and neuropeptides alter GnRH neuronal activity, through d irect and sometimes indirect actions. This review focuses on the neurotrans mitters that regulate GnRH neurones by binding to and activating specific m embrane receptors that are expressed in GnRH neurones. These include the ca techolamines, gamma -aminobutyric acid, glutamate, neuropeptide Y, neuroten sin, beta -endorphin and vasoactive intestinal polypeptide. On the basis of recent molecular and neuroanatomical evidence, it is proposed that oestrad iol influences the activity of these neurotransmitter and neuropeptide syst ems within the GnRH network to drive reproductive cyclicity.