Prolactin in the Horse: Historical Perspective, Actions and Reactions, and Its Role in Reproduction

A xenotransplantation mouse model to study physiology of the mammary gland from large mammals

Although highly conserved in structure and function, many (patho)physiological processes of the mammary gland vary drastically between mammals, with mechanisms regulating these differences not well understood. Large mammals display variable lactation strategies and mammary cancer incidence, however, research into these variations is often limited to in vitro analysis due to logistical limitations. Validating a model with functional mammary xenografts from cryopreserved tissue fragments would allow for in vivo comparative analysis of mammary glands from large and/or rare mammals and would improve our understanding of postnatal development, lactation, and premalignancy across mammals. To this end, we generated functional mammary xenografts using mammary tissue fragments containing mammary stroma and parenchyma isolated via an antibody-independent approach from healthy, nulliparous equine and canine donor tissues to study these species in vivo. Cryopreserved mammary tissue fragments were xenotransplanted into de-epithelialized fat pads of immunodeficient mice and resulting xenografts were structurally and functionally assessed. Preimplantation of mammary stromal fibroblasts was performed to promote ductal morphogenesis. Xenografts recapitulated mammary lobule architecture and contained donor-derived stromal components. Mammatropic hormone stimulation resulted in (i) upregulation of lactation-associated genes, (ii) altered proliferation index, and (iii) morphological changes, indicating functionality. Preimplantation of mammary stromal fibroblasts did not promote ductal morphogenesis. This model presents the opportunity to study novel mechanisms regulating unique lactation strategies and mammary cancer induction in vivo. Due to the universal applicability of this approach, this model serves as proof-of-concept for developing mammary xenografts for in vivo analysis of virtually any mammals, including large and rare mammals.

Seasonality of prolactin in birds and mammals

In most animals, annual rhythms in environmental cues and internal programs regulate seasonal physiology and behavior. Prolactin, an evolutionarily ancient hormone, serves as a molecular correlate of seasonal timing in most species. Prolactin is highly pleiotropic with a wide variety of well-documented physiological effects; in a seasonal context prolactin is known to regulate annual changes in pelage and molt. While short-term homeostatic variation of prolactin secretion is under the control of the hypothalamus, long-term seasonal rhythms of prolactin are programmed by endogenous timers that reside in the pituitary gland. The molecular basis of these rhythms is generally understood to be melatonin dependent in mammals. Prolactin rhythmicity persists for several years in many species, in the absence of hypothalamic signaling. Such evidence in mammals has supported the hypothesis that seasonal rhythms in prolactin derive from an endogenous timer within the pituitary gland that is entrained by external photoperiod. In this review, we describe the conserved nature of prolactin signaling in birds and mammals and highlight its role in regulating multiple diverse physiological systems. The review will cover the current understanding of the molecular control of prolactin seasonality and propose a mechanism by which long-term rhythms may be generated in amniotes.

Direct contact may affect the efficacy of stallion exposure in hastening the onset of cyclicity in anestrous mares

Introducing males to seasonally anestrous females has been shown to increase LH concentrations and hasten the first ovulation in ewes and goat does. Investigation of this sociosexual phenomenon in mares has not been well studied. The objective of this study was to determine if direct or indirect exposure of anestrous mares to a stallion would result in a similar increase in LH and hasten the first ovulation. In late February, mares were stratified by parity and age and assigned randomly to three treatments (n = 7/treatment): (1) DXP (direct exposure; mares housed adjacent to stallion and allowed direct contact for 2 h for 3 d/wk by placing the stallion into a cage in the center of a tease pen); (2) IXP (indirect exposure; mares housed in an adjacent pen to the stallion but not allowed direct contact); and (3) CON (control; mares housed away from stallion and not allowed direct or indirect exposure). During the first, third, and fifth treatment days, blood was collected hourly for 8 h beginning just before treatment initiation for measurement of LH and prolactin (PRL) concentrations. Hormone concentrations were analyzed as repeated measures and time from the first day of treatment to ovulation was analyzed using one-way ANOVA. Interval from treatment initiation to first ovulation did not differ (P > 0.05) among treatments: 36.3 ± 6.7, 42.8 ± 4.2, and 48.3 ± 4.2 d for DXP, IXP, and CON, respectively. When comparing the mares based on social status within treatment and level of direct interaction with the stallion, socially dominant DXP mares ovulated sooner (P < 0.05) than socially subordinate DXP mares (23.8 ± 3.2 and 53.0 ± 6.8 d, respectively). Although LH concentrations did not differ (P > 0.05) among treatments, a significant (P < 0.05) day effect was detected with concentrations being greater on the fifth treatment day compared with the first and third days. Concentrations of PRL were greater (P < 0.05) in IXP compared with DXP and CON, regardless of treatment day. Regardless of treatment, PRL concentrations were greater (P < 0.05) on the third treatment day, compared with the first and fifth days. We conclude that exposure of anestrous mares to a stallion, whether direct or indirect, to anestrous mares failed to elicit the same effects that have been observed in short-day breeders. The effect of stallion exposure on the onset of cyclicity in anestrous mares may be dependent on the duration of direct contact.

In silico prediction of prolactin molecules as a tool for equine genomics reproduction

The prolactin hormone is involved in several biological functions, although its main role resides on reproduction. As it interferes on fertility changes, studies focused on human health have established a linkage of this hormone to fertility losses. Regarding animal research, there is still a lack of information about the structure of prolactin. In case of horse breeding, prolactin has a particular influence; once there is an individualization of these animals and equines are known for presenting several reproductive disorders. As there is no molecular structure available for the prolactin hormone and receptor, we performed several bioinformatics analyses through prediction and refinement softwares, as well as manual modifications. Aiming to elucidate the first computational structure of both molecules and analyse structural and functional aspects related to these proteins, here we provide the first known equine model for prolactin and prolactin receptor, which obtained high global quality scores in diverse software's for quality assessment. QMEAN overall score obtained for ePrl was (- 4.09) and QMEANbrane for ePrlr was (- 8.45), which proves the structures' reliability. This study will implement another tool in equine genomics in order to give light to interactions of these molecules, structural and functional alterations and therefore help diagnosing fertility problems, contributing in the selection of a high genetic herd.