Is oxytocin an ovarian hormone?

Abstract

Oxytocin is a nonapeptide hormone produced by the hypothalamus and released from the posterior pituitary. It is cleaved from a large molecular weight precursor which is synthesized in cell bodies of hypothalamic magnocellular neurones and then packaged into membrane-bound granules¹. During axonal transport the precursor is cleaved to produce oxytocin and an oxytocin-related neurophysin. Apart from its established roles in lactation and labour, oxytocin is thought to be an important regulator of the oestrous cycle; it causes luteolysis in cattle² and immunization against oxytocin increases the length of the ovine oestrous cycle³, McCracken⁴ has proposed that oxytocin acts by stimulating prostaglandin F_2α (PGF_2α) release from the uterine endometrium and that steroid hormones regulate PGF_2α secretion by altering the availability of endometrial oxytocin receptors^4,5. Steriods may also have a direct influence on oxytocin release from the posterior pituitary, oestradiol-17β increasing and progesterone inhibiting oxytocin secretion in response to vaginal distension in the goat^6,7. However, recent radioimmunoassay (RIA) measurements of peripheral oxytocin levels during the ovine oestrous cycle^8–10 consistently show that oxytocin and progesterone concentrations increase and decrease in synchrony during the luteal phase, reaching basal levels after luteolysis at a time when oestradiol-17β titres are known to rise¹¹. Therefore, there may exist different controls of reflex release and basal secretion of oxytocin, and our discovery of large amounts of oxytocin within the ovine corpus luteum reported here may provide an explanation.