Neural mechanisms controlling seasonal reproduction: Principles derived from the sheep model and its comparison with hamsters

Abstract

Seasonal reproduction is a common adaptive strategy among mammals that allows for breeding to occur at times of the year when it is most advantageous for the subsequent survival and growth of offspring. A major mechanism responsible for seasonal reproduction is a striking increase in the responsiveness of gonadotropin-releasing hormone (GnRH) neurons to the negative feedback effects of estradiol. The neural and neuroendocrine circuitry responsible for mammalian seasonal reproduction has been primarily studied in three animal models: the sheep, and two species of hamsters. In this review, we first describe the afferent signals, neural circuitry and transmitters/peptides responsible for seasonal reproductive transitions in sheep, and then compare these mechanisms with those derived from studies in hamsters. The results suggest common principles as well as differences in the role of specific brain nuclei and neuropeptides, including that of kisspeptin cells of the hypothalamic arcuate nucleus, in regulating seasonal reproduction among mammals.

New concepts of the central control of reproduction, integrating influence of stress, metabolic state, and season

“…Lesions in this region prevented the estrogen negative feedback effect seen in OVX ewes during anestrous, leading to considerable focus on the A15 dopaminergic cells. Pharmacological, electrophysiological, and histochemical data have consolidated a role for these A15 cells in the seasonal shift in negative feedback of estradiol on GnRH secretion, which has been reviewed elsewhere [53]. Interestingly, these cells do not express ERa or ERb [63,64] but do respond to systemic estradiol treatment of OVX ewes in the anestrous season.…”

“…The kisspeptin cells in the POA of the ovine brain are primarily involved in the positive feedback effect of estrogen that causes the preovulatory surge in GnRH and LH secretion [50], whereas the former are involved in negative feedback regulation and seasonal change. The role of kisspeptin and GnIH in the regulation of GnRH secretion in sheep has been reviewed extensively [3,5,[51][52][53] and will not be dealt with in detail herein; specific details as to the involvement of these 2 peptides in seasonality, stress, and metabolic effects will be discussed.…”

“…Interestingly, the A15 cells project caudally to the mediobasal hypothalamus (ARC and median eminence), but it is curious that there are no available data to indicate whether these projections are to kisspeptin cells or not [53,68]. One report [70] indicates that cells of the retrochiasmatic nucleus project to the kisspeptin-producing (KnDY) cells of the ARC, but whether these are dopaminergic neurons is not known.…”

“…The activity of these enzymes changes with season and is driven by TSH, secreted by thyrotrope-like cells in the pars tuberalis and reaching the basal hypothalamus via specialized vessels within the median eminence. Recent reviews [53,75] provide a detailed account of this system, which will be reviewed only briefly herein according to the summary of the circuitry shown in Figure 3. Essential features are that the DIO appear to be expressed in tanycytes and TSH receptor expression is localized to the same region [76].…”

Influencia de la Melatonina sobre la fisiología y la conducta de ungulados

“…Sin embargo, se ha observado que el patrón reproductivo en especies de cérvidos tropicales, ha evolucionado en respuesta a otras señales diferentes a la luz, como la disponibilidad de alimento, la competencia o la depredación (Asher, 2011). En ovinos ( Weems, Goodman, & Lehman, 2015;Manca et al, 2014;Bittman et al, 1983;Roche, Karsch, Foster, Takagi, & Dziuk, 1970) y en équidos (Grubaugh et al, 1982), se ha observado que la pinealectomia o extracción de la glándula pineal produce un retraso en la ovulación, modificando los ciclos reproductivos en los animales, sin embargo a largo plazo estos ciclos tienden a reanudarse, con un patrón similar. Hay autores que plantean una leve compensación hormonal en ausencia de la glándula pineal por parte de otros tejidos, como la retina y el intestino, evidenciado con bajas concentraciones de melatonina circulante en animales pinealectomizados ( B i n k l e y, H r y s h c h y s h y n , & R e i l l y, 1 9 7 9 ; Stanisiewski et al, 1988).Por otro lado, la melatonina materna en ovinos puede proporcionarle al feto información acerca de los ciclos diarios (Kennaway & Gilmore, 1984;Zemdegs, McMillen, Walker, Thorburn, & Nowak, 1988).…”