The central oxytocin pulse generator: a pacemaker for the ovarian cycle
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During luteolysis in sheep, episodic pulses of oxitocin (OT), contributed by the neurophysis and the corpus luteum (CL), stimulate uterine luteolytic pulses of prostaglandin (PG) F2alpha via endometrial OT receptors.To distinquish relative contributions of neurophysical and luteal OT, overoctomized sheep were given estradiol-17 beta (E) and progeterone (P) to simulate levelas during the cycle.In intact sheep, luteectomy was performed to exclude the CL as a source of OT and initiate P withdrawal.IN overiectomized sheep, E (mug/h for 12 to 36h) superimposed on basal E (0.05 mug/h) caused a series of 4 to 6 episodes of high frequency pulses of OT, each episode lasting 1 to 2 h at intervals of 3 h, and commencing at 24 h.Withdrawal of P (500 mug/h), superimposed on basal E in ovariectomized sheep, oe luteectomy in intact sheep, evoked similar episodes of high frequency pulses of OT begining at 24h.We conclude that (1) an increase in E levels, or the return of E actin following P withdrawal, causes intermittent increases in tyhe frequency of the central OT pulse generator, (2) high frequency pulses of OT initiate subluteolytic levels of uterine PGF2 alpha which trigger a supplemental release of luteal OT; (3) luteal OT amplifies the secretion of uterine PGF2 alpha which initiates luteolysis and causes more luteal OT to be secreted; and (4) in addition to the established hypothalmic-antrior pituitarty-gonadal axis for initiating the ovarian cycle (via the gonadotrophins), there is now evidence for a hypothalmic-posterior pituitary-gonadal axis for terminating the ovarian cycle (via OT).
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