VIP and Infertility: However the majority of metabolic peptidergic systems appear to play a redundant role, in that although pharmacological manipulation of such peptides affects gonadotropin secretion, the genetic ablation of most peptides or their cognate receptors implicated in metabolic or circadian control does not produce infertility e.g. NPY [52], MCH [53], orexin [54], melanocortin receptor 4 [55], pro-opiomelanocortin [56] and VIP [57].