THE CENTRAL OXYTOCIN PULSE-GENERATOR - A PACEMAKER FOR THE OVARIAN CYCLE

During luteolysis in sheep, episodic pulses of oxytocin (OT), contribu ted by the neurohypophysis and the corpus luteum (CL), stimulate uteri ne luteolytic pulses of prostaglandin (PG) F-2 alpha via endometrial O T receptors. To distinguish relative contributions of neurohypophysial and luteal OT, ovariectomized sheep were given estradiol-17 beta (E) and progesterone (P) to simulate levels during the cycle. In intact sh eep, luteectomy was performed to exclude the CL as a source of OT and to initiate P withdrawal. In ovariectomized sheep, E(1 mu g/h for 12 t o 36 h) superimposed on basal E (0.05 mu g/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 mu g/h), superimposed on basal E in ovariectomized sheep, or luteectom y in intact sheep, evoked similar episodes of high frequency pulses of OT beginning at 24 h. We conclude that (1) an increase in E levels, o r the return of E action following P withdrawal, causes intermittent i ncreases in the frequency of the central OT pulse generator. (2) high frequency pulses of OT initiate subluteolytic levels of uterine PGF(2) alpha which trigger a supplemental release of luteal OT; (3) luteal O T amplifies the secretion of uterine PGF(2) alpha which initiates lute olysis and causes more luteal OT to be secreted; and (4) in addition t o the established hypothalamic-anterior pituitary-gonadal axis for ini tiating the ovarian cycle (via the gonadotrophins), there is now evide nce for a hypothalamic-posterior pituitary-gonadal axis for terminatin g the ovarian cycle (via OT).