NEURO-ENDOCRINE REGULATION OF THE ESTROUS-CYCLE AND SEASONAL BREEDING IN THE EWE

A generalized scheme for neuroendocrine regulation of the estrous cycle and seasonal breeding season, occurrence of each estrous cycle requires a cascade of causally related events culminating in ovulation (decreasing P, increasing LH, increasing E, LH surge and estrus). Interruption of a single step in the sequence (decreasing P → increasing LH) breaks the entire cycle and results in anestrus. Much emphasis has been placed on control of the tonic mode of LH secretion because this system appears to occupy a central position in regulating the estrous cycle and in determining the seasonal reproductive state. During the breeding season progesterone, with the help of estradiol and possibly other ovarian steroids, plays the dominant role in governing tonic LH secretion; its withdrawal initiates the sequence of events which leads to ovulation. In this sense, progesterone may be viewed as an organizer of the preovulatory events of the estrous cycle of the ewe. Estradiol, on the other hand, may be considered an organizer of the seasonal reproductive cycle. By virtue of seasonal changes in its effects on the system which governs tonic LH secretion, estradiol determines whether or not the normal progression of preovulatory events can occur. Thus, the sequence is permitted when the response to estradiol is low; it is actively inhibited when the response to estradiol is high. The actions of estradiol on the system governing tonic LH secretion, in turn, subserve the influences of environmental photoperiod on the ovarian cycle. Thus, by means of seasonal alternations between stimulatory (short day) and inhibitory (long day) photoperiods and the attendant changes in response to estradiol, the natural process of reversible fertility is achieved.