Mesotocin and luteal function in macropodid marsupials

Pituitary glands and corpora lutea collected at various stages of the reproductive cycle of the tammar wallaby (Macropus eugenii), were extracted and fractionated by high-performance liquid chromatography, and specific radioimmunoassays were used to measure mesotocin ([Ile8]-oxytocin) and oxytocin. Mesotocin, but not oxytocin, was identified in extracts of pituitary; the mean concentration of mesotocin in this tissue was 0.75 nmol/g wet weight. Neither mesotocin nor oxytocin was detected in extracts of corpus luteum. In female Bennett's wallabies passively immunized against mesotocin during seasonal reproductive quiescence, there was no significant effect on peripheral progesterone concentrations and there were no births, matings or changes in vaginal smears in the 2 months following treatment. Thus mesotocin is unlikely to act as a systemic luteostatic agent during seasonal quiescence.