The regulation of pulsatile GnRH release is highly complex. A number of upstream neuronal populations— along with their associated neurotransmitters and neuropeptides— substantially impact GnRH neuron activity and largely mediate ovarian feedback— largely via circulating sex steroid concentrations— in addition to mediating the influence of energy availability and stress. Relevant signalling compounds include dopamine, norepinephrine, glutamate, γ- aminobutyric acid (GABA), nitric oxide, kisspeptin, neurokinin B, and endogenous opiates.
Kisspeptin is a critical requirement for normal GnRH secretion. The human kisspeptin gene (KISS1) encodes a 154- amino acid precursor protein (kisspeptin 1– 145); and proteolytic modification yields a number of bioactive kisspeptin fragments: kisspeptin- 54, - 14, - 13, and - 10. Functional native kisspeptin species contain the carboxy- terminal amino acids 112 to 121, which are important for receptor binding and function. Kisspeptin neurons form synaptic contacts with GnRH neurons, including at GnRH neuron terminals in the median eminence. Kisspeptin stimulates GnRH neurons via the kisspeptin receptor (KISSR; also known as the G- protein coupled receptor 54, or GPR54)— a seven transmembrane domain, G- protein- coupled receptor expressed by a majority of GnRH neurons. Persons with inactivating KISS1R or KISS1 mutations exhibit pubertal failure and hypogonadotropic hypogonadism. Kisspeptin- related stimulation of LH release is blocked by GnRH antagonists, and pulsatile GnRH administration can fully restore re productive function in patients with KISS1R mutations. These findings suggest that the effects of kisspeptin on LH release is mediated by GnRH. Of interest, the effects of kisspeptin on LH release appear to be influenced by several factors including menstrual cycle phase and hormonal milieu.
Rodents exhibit two distinct populations of kisspeptin neurons. Kisspeptin neurons in the arcuate nucleus (mediobasal hypothalamus) mediate the negative feedback effects of oestrogen on GnRH release; while kisspeptin neurons in the anteroventral periventricular nucleus (AVPV) of the preoptic area mediate the positive feedback effects of oestrogen on GnRH release (i.e. GnRH surge generation). Kisspeptin neurons are primarily located in the in fundibular (arcuate) nucleus in humans and monkeys. It remains uncertain whether unique populations of kisspeptin neurons play similar respective roles in primates.
Neurokinin B (NKB) is a decapeptide encoded by the tachykinin 3 gene (TAC3). NKB preferentially binds to and acts via the neurokinin 3 receptor (NK3R), encoded by the TACR3 gene. The role of NKB in central reproductive function is complex. Homozygous loss- of- function TAC3 or TACR3 mutations cause pubertal failure and hypogonadotropic hypogonadism. Senktide, a selective NK3R agonist, can provoke LH release, and this effect is abolished by GnRH receptor antagonism. Of interest, a number of studies suggest that NKB primarily influences GnRH release via its effects on kisspeptin release. For example, kisspeptin neurons ex press NK3R; senktide increases kisspeptin neuronal activity; LH responses to senktide are absent or impaired in Kiss1R knockout mice, with Kiss1R antagonism, or after Kiss1R desensitization; and continuous kisspeptin infusion enhances LH secretion in patients with loss- of- function TAC3 or TACR3 mutations. NKB analogues may have therapeutic use in the future.
Endogenous opioid peptides (EOP)— including endorphins, enkephalins, and dynorphins— participate in numerous neuroendocrine functions. Endorphins (e.g. β- endorphin) are derived from proopiomelanocortin (POMC) and act primarily via μ (micro)- opioid receptors; while dynorphins are products of the precursor prodynorphin and act chiefly at κ (kappa)- opioid receptors (KOR). Although derived from different precursor proteins, active endorphins, enkephalins, and dynorphins contain a common sequence at the amino- terminal (Tyr- Gly- Gly- Phe- [Met or Leu]). Ample evidence suggests that sex steroid negative feedback on GnRH release is at least partly mediated by hypothalamic opiates. Hypophyseal blood concentrations of β- endorphin increase during the luteal phase in monkeys; opiate receptor antagonists increase LH pulse frequency when administered to women in the luteal phase, and exogenous opiates (e.g. morphine) can cause hypogonadotropic hypogonadism. Recent animal studies suggest that dynorphin is an important mediator of GnRH pulse frequency suppression by progesterone. For example, progesterone receptors colocalize with arcuate nucleus dynorphin neurons; dynorphin- containing varicosities associate with GnRH neurons; progesterone treatment increases dynorphin A concentrations in the third ventricle cere brospinal fluid; and delivery of κ- opioid receptor antagonists to the mediobasal hypothalamus reverses progesterone inhibition of LH pulse secretion.
Animal studies suggest that kisspeptin, NKB, and dynorphin are often coexpressed in single neurons. Such KNDy (‘candy’) neurons (Kisspeptin, Neurokinin B, Dynorphin) are extensively interconnected and project to GnRH neurons, including GnRH neuron fibres within the median eminence. Although corresponding data in humans are limited, autopsy studies generally support the notion that kisspeptin and NKB colocalize in a portion of neurons in the infundibular nucleus. Regardless, kisspeptin, NKB, and dynorphin— released from neurons that do or do not colocalize with the other peptides— substantially influence GnRH neuronal function in humans, and such neurons are likely a funda mental component of the GnRH pulse generator (vide infra).
RFamide- related peptides (RFRPs)— the mammalian orthologues of gonadotropin- inhibitory hormone (GnIH)— also appear to exert an important influence on central reproductive function [19]. RFRP- immunoreactive cells have been identified in human hypo thalami, and RFRP- immunoreactive fibres approximate GnRH neurons. Animal studies suggest that RFRP- 3 reduces GnRH neuronal firing rates, but an understanding of the roles of RFRPs in hu mans requires further study.
