Endocrine control of ovarian function in dogs and other carnivores
Concannon, P. WCastracane, V. DTemple, MMontanez, A
Ovarian function in dogs is minimally but successfully evolved and adapted for fertility, and represents a basic model for examining the more complex evolution of ovarian activity in other carnivores and mammals in general. Canids are monoestrous, polytocous, spontaneous ovulators with a spontaneous luteal function producing progesterone for the duration of a normal 2-month pregnancy and unaffected by hysterectomy. They have no acute luteolytic mechanism in the absence of pregnancy although PGF is luteolytic and participates in prepartum luteolysis. The cellular mechanisms of luteal and follicular tissues appear unlikely to differ meaningfully from those described in other species, with the spontaneously prolonged luteal function being similar to, and in some instances shorter than, the luteal lifespan of hysterectomized polyestrous species. All or nearly all female caniform carnivore species have photo-entrained annual life-cycles and annual or biennial reproduction. However, the domestic dog, a subspecies of the grey wolf, is an exception and non-seasonal; but, as an exception to the exception, the basenji dog like the dingo, another wolf subspecies, is seasonal, having its cycle in the autumn. The canine obligate anestrus lasts 2-10 months and is terminated by increased GnRH and LH pulsatility. The timing is under multiple regulatory inputs. These include recovery from progesterone effects at variable times after progesterone declines to nadir values; increased dopaminergic and/or decreased opioidergic tones and/or sensitivities, presumably under the influence of an endogenous circannual cycle assumed to persists despite the lack of photoresponsiveness; and, stimulatory pheromonal input from other females (as well as photoperiod in the case of Basenji). The only clear adaptations or unique attributes seen in dogs that are likely beyond what occurred in a more primitive ancestor are two. One, there is a pregnancy specific increase in prolactin that as a potent luteotrophin (as in rodents) acts to enhance progesterone production during pregnancy, which appears likely to be the case in all carnivores. And, two, the bitch has a fertile-mating window as wide as 11 days, and up to 8 days after ovulation. The latter involves the delayed post-ovulatory maturation of oocytes (also seen in foxes), prolonged post-maturation oocyte viability, and a uterine environment hospitable to sperm survival for up to 7 days during estrus. This relative simplicity contrasts to more complicated adaptive strategies like (1) delayed implantation seen in many caniform carnivores (including many mustelids, ursids, and phocid and otarid seals); (2) reflex, induced ovulation (as seen in many feliform carnivores); and (3) prolongation of post-implantation gestation via placental secretion of progesterone (some feliform, some artiodactyls, primates) or gonadotrophin (primates, equids). Also considered in the review are the endocrine mechanisms triggering the LH surge and estrus behavior in dogs, and factors involved in termination of obligate anestrus.(AU)
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