Gonadotroph-lactotroph associations and expression of prolactin receptors in the equine pituitary gland throughout the seasonal reproductive cycle

Abstract

An interaction between gonadotroph and lactotroph cells of the pituitary gland has long been recognized in several species. The current study was conducted to investigate whether an association between gonadotrophs and lactotrophs occurs in mares and whether prolactin receptors are expressed within the pituitary gland of this species. The effects of both reproductive state and season on these variables were examined in pituitary glands obtained from sexually active mares in July (breeding season), sexually active mares in November (non-breeding season) and anoestrous mares in November. Pituitaries were dissected out immediately after death and immunofluorescent staining was carried out on 6 micrometer sections using specific antibodies to the LHbeta subunit, FSHbeta subunit, prolactin and prolactin receptor. Gonadotrophs were observed in both the pars distalis and pars tuberalis; although they appeared mostly as isolated cells, small groups of gonadotrophs were also identified in the pars distalis. In contrast, lactotrophs were observed only as clusters of cells exclusively in the pars distalis of sexually active and anoestrous mares in November and in most of the sexually active mares in July. A specific gonadotroph-lactotroph association was identified only between large isolated gonadotrophs and lactotroph clusters. Double immunofluorescent staining for FSHbeta and prolactin revealed a similar gonadotroph-lactotroph association to the one detected for LH gonadotrophs. No statistical difference in the gonadotroph:lactotroph ratio was observed as a result of changes in reproductive status or season. However, a tendency for a simultaneous decrease in the number of gonadotrophs and an increase in the number of lactotrophs was detected in anoestrous animals. Prolactin receptor immunoreactivity was found in the pars distalis, but not in the pars tuberalis, of sexually active (July and November) and anoestrous animals for both long and short forms of the receptor. No prolactin receptor co-localization for either form of the receptor was observed in LH or FSH gonadotrophs in either of the reproductive states examined during both summer and winter seasons. Furthermore, no significant difference was apparent in the proportion of cells expressing prolactin receptors between mares of different reproductive state or season. The specific anatomical association between gonadotroph and lactotroph cells and the expression of prolactin receptors in the equine pituitary gland indicate a potential role of prolactin in the regulation of gonadotrophin secretion. However, the absence of evidence for co-localization of prolactin receptors in LH or FSH cells does not support the hypothesis of a direct effect of prolactin on the gonadotroph as reported in a short day breeder. The results raise the possibility that, in horses, an intermediate regulatory cell may mediate the action of prolactin on gonadotroph function.

Ovarian and PGF2α responses to stimulation of endogenous PRL pulses during the estrous cycle in mares

“…The presence of PRL receptors in the pituitary [23] indicates a potential role of PRL on pituitary function. Secretion of PRL has been reported to inhibit LH secretion in several species [24], especially in rats and humans [25,26].…”

Molecular characterization, modeling, in silico analysis of equine pituitary gonadotropin alpha subunit and docking interaction studies with ganirelix

“…Distinctive cell types in the anterior pituitary gland including somatotrophs and gonadotrophs, secrete polypeptide hormones such as prolactin, growth hormone and the gonadotropins viz. luteinizing (LH) and follicle stimulating (FSH) hormones (Kalra et al 1997;Gregory et al 2000). Pregnant mare serum gonadotropin (PMSG), also known as equine chorionic gonadotropin (eCG) is responsible for maintenance of pregnancy in mares during early gestation (Christakos and Bahl 1979).…”

Dinâmica hormonal durante o processo luteolítico nas espécies equina e bovina; com ênfase sobre o papel da prolactina

“…Em equinos, a maioria dos estudos envolvendo a PRL está relacionada à prenhez (AURICH et al, 1995;IRELAND et al, 1991), parto (WORTHY et al, 1986) e sazonalidade reprodutiva (KING et al, 2010b;GREGORY et al, 2000;HOFFMAN Jr;DEPEW, 1997). Os níveis da concentração de PRL são elevados no verão e baixos no inverno (THOMPSON et al, 1986).…”

“…Os níveis da concentração de PRL são elevados no verão e baixos no inverno (THOMPSON et al, 1986). Durante o período transicional entre a estação anovulatória para a ovulatória, a estimulação da secreção de PRL acelera o crescimento folicular e antecipa a primeira ovulação da estação ovulatória (BESOGNET;HANSEN; NEQUIN et al, 1993;THOMPSON et al, 1986), agindo como um potencial regulador da secreção de gonadotrofinas (GREGORY et al, 2000). Na estação ovulatória, os níveis de PRL são mais elevados no estro do que no diestro (WORTHY et al, 1987;SHAND et al, 2000), sendo observado um efeito positivo da concentração de estradiol sobre a PRL no início da fase luteal (ROSER et al, 1987).…”

“…In horses, the majority of the published studies on PRL are related to pregnancy (AURICH et al, 1995;IRELAND et al, 1991) parturition (WORTHY et al, 1986), and seasonality of reproduction (KING et al, 2010b;JOHNSON, 1986;GREGORY et al, 2000;EVANS et al, 1991;THOMPSON et al, 1986;HOFFMAN Jr;DEPEW, 1997). There is a rise in serum and pituitary PRL concentration in the summer, whereas concentrations are low in winter (THOMPSON et al, 1986).…”