Intrapituitary mechanisms underlying the control of fertility: key players in seasonal breeding

Abstract

Recent studies have shown that, in conjunction with dynamic changes in the secretion of GnRH from the hypothalamus, paracrine interactions within the pituitary gland play an important role in the regulation of fertility during the annual reproductive cycle. Morphological studies have provided evidence for close associations between gonadotropes and lactotropes and gap junction coupling between these cells in a variety of species. The physiological significance of this cellular interaction was supported by subsequent studies revealing the expression of prolactin receptors in both the pars distalis and pars tuberalis regions of the pituitary. This cellular interaction is critical for adequate gonadotropin output because, in the presence of dopamine, prolactin can negatively regulate the LH response to GnRH. Receptor signaling studies showed that signal convergence at the level of protein kinase C and phospholipase C within the gonadotrope underlies the resulting inhibition of LH secretion. Although this is a conserved mechanism present in all species studied so far, in seasonal breeders such as the sheep and the horse, this mechanism is regulated by photoperiod, as it is only apparent during the long days of spring and summer. At this time of year, the nonbreeding season of the sheep coincides with the breeding season of the horse, indicating that this inhibitory system plays different roles in short- and long-day breeders. Although in the sheep, it contributes to the complete suppression of the reproductive axis, in the horse, it is likely to participate in the fine-tuning of gonadotropin output by preventing gonadotrope desensitization. The photoperiodic regulation of this inhibitory mechanism appears to rely on alterations in the folliculostellate cell population. Indeed, electron microscopic studies have recently shown increased folliculostellate cell area together with upregulation of their adherens junctions during the spring and summer. The association between gonadotropes and lactotropes could also underlie an interaction between the gonadotropic and prolactin axes in the opposite direction. In support of this alternative, a series of studies have demonstrated that GnRH stimulates prolactin secretion in sheep through a mechanism that does not involve the mediatory actions of LH or FSH and that this stimulatory effect of GnRH on the prolactin axis is seasonally regulated. Collectively, these findings highlight the importance of intercellular communications within the pituitary in the control of gonadotropin and prolactin secretion during the annual reproductive cycle in seasonal breeders.

Profile of gonadotropic hormone secretion in sheep with disturbed rhythm of seasonality

“…In mature, sexually active sheep, melatonin is a hormone that modulates their sexual activity. The period of increased concentration of melatonin in the bloodstream depends on the length of the night and is subject to seasonal changes; hence, the highest concentration of melatonin in the bloodstream of sheep is found during the autumn-winter season in the northern hemisphere (Malpaux et al 1993;Tortonese 2016).…”

“…These results suggest that pineal hormone can demonstrate the synchronization effect on the daily rhythm of LH release, which is observed during the sexual activity period. The characteristic rhythm of LH is that it occurs in sheep in the breeding season and that the rise in LH concentration in the bloodstream occurs usually in the evening or during night hours (Misztal et al 2002;Tortonese 2016). It may be presumed that additional introduction of melatonin to the body, especially in the afternoon hours, induces some physiological processes, which are characteristic for the period of darkness.…”

A New Perspective on Regulation of Pituitary Plasticity: The Network of SOX2-Positive Cells May Coordinate Responses to Challenge

“…In contrast, the increased requirement for ACTH and thyrotropin principally occurs in response to environmental challenges, such as stress ( 6 ) and low dietary iodine ( 7 ), respectively, and are unpredictable, occurring with a varying frequency and duration. These features of pituitary plasticity can also be species dependent, with prolactin and the gonadotrophins varying dramatically with a circannual cycle in seasonal breeding animals ( 8 ).…”

Impact of an increased exposure with manganese and nickel on pituitary hormones in women with ovarian polycystosis

“…У високих концентраціях пролактин здатний у 3-4 рази збільшувати рівень ЛГ. Припускають, що ефекти пролактину в гонадотропних клітинах опосередковуються через дофамін [6], який у свою чергу виконує роль інгібітора секреції гонадотропінів [7]. Крім того, пролактин підвищує чутливість наднирників до адренокортикотропного гормону (АКТГ), що приводить до збільшення секреції кортизолу навіть за низьких показників АКТГ.…”