Kisspeptin Cells in the Ovine Arcuate Nucleus Express Prolactin Receptor but not Melatonin Receptor

Abstract

Melatonin is secreted at night by the pineal gland and governs the reproductive system in seasonal breeders, such as sheep. The mechanism by which melatonin regulates reproduction is not known. The circannual rhythmicity of other factors, including prolactin, is also regulated by photoperiod via changes in melatonin secretion. In sheep, plasma prolactin levels are higher in the nonbreeding season than the breeding season. Kisspeptin, synthesised by neurones in the ovine arcuate nucleus (ARC) and preoptic area, is a key regulator of reproduction through stimulation of gonadotrophin-releasing hormone secretion and its expression in the ARC is reduced during the nonbreeding season. We hypothesised that kisspeptin expression is directly, or indirectly, regulated by melatonin and/or prolactin. We first examined the expression of melatonin receptor (MTNR1A) in kisspeptin (Kiss1 mRNA) neurones in the ARC of ovariectomised (OVX) sheep using double-label in situ hybridisation. MTNR1A mRNA was not expressed by kisspeptin neurones, whereas strong expression was detected in the pars tuberalis. We then examined the expression of the long-form prolactin receptor (PRLR-L) in ARC kisspeptin neurones. In OVX ewes, approximately 60% of kisspeptin neurones expressed PRLR-L mRNA at similar levels in the breeding and nonbreeding seasons. We then aimed to determine whether prolactin treatment during the breeding season regulates kisspeptin expression in the ARC. Continuous central infusion of prolactin (20 μg/h for 7 days) in oestradiol-treated OVX sheep did not alter Kiss1 mRNA expression or luteinising hormone secretion, although it induced substantial phosphorylated signal transducer and activator of transcription 5-immunoreactive nuclei staining in the mediobasal hypothalamus. We conclude that the seasonal change in kisspeptin neurones cannot be regulated directly by melatonin, although it may be a result of changes in prolactin levels. Despite this, kisspeptin expression was unchanged after exogenous prolactin treatment in breeding season ewes.

Rapid modulation of hypothalamic Kiss1 levels by the suckling stimulus in the lactating rat

“…Changes in serum PRL levels via modulation of lactating conditions were largely consistent with the idea that high serum concentrations of PRL are required for continued milk production (Mattheij et al 1984). PRL receptor expression in kisspeptin neurons (Kokay et al 2011, Li et al 2011, and pSTAT5 expression in kisspeptin neurons after i.c.v. administration of PRL (Araujo-Lopes et al 2014) indicates that kisspeptin neurons are responsive to PRL.…”

Evidence that Neurokinin B Controls Basal Gonadotropin-Releasing Hormone Secretion but Is Not Critical for Estrogen-Positive Feedback in Sheep

“…Double-label in situ hybridization was performed as described previously [46,47,48], using 3 sections from the ARC of each ewe, representing the rostral, middle, and caudal regions (fig. 1a).…”

Interactions between prolactin and kisspeptin to control reproduction

“…The function of PRLRs in TIDA neurons is to allow these cells to sense circulating prolactin levels and consequently regulate pituitary prolactin secretion through negative feedback mechanisms. Several other cell populations also express PRLRs, including different brain regions, as well as the bone, adipose tissue, gut, skin, immune system and reproductive tract (2,(4)(5)(6)(7)(8)(9).…”