Ja. Mccracken et al., THE CENTRAL OXYTOCIN PULSE-GENERATOR - A PACEMAKER FOR THE OVARIAN CYCLE, Acta Neurobiologiae Experimentalis, 56(3), 1996, pp. 819-832
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).