Comparative aspects of oxytocin-like hormones in marsupials

Unlike eutherian mammals which secrete only oxytocin (OT), marsupials secrete the typically reptilian mesotocin (MT) and/or OT as their oxytocic peptides. Our laboratories have been conducting research on various aspects of the roles of OT-like peptides in three marsupials, the brushtail possum, the northern brown bandicoot and the tammar wallaby. By providing information on the functions of OT-like peptides in these species we hope to provide some clues as to the evolution of neurohypophysial hormones in marsupials. Brain and peripheral distributions of OT-like peptides have been studied in the possum and bandicoot. As in eutherian mammals, OT-like peptides are distributed throughout the brain and are present in the testis, corpus luteum, prostate and adrenal glands. Studied on the regulation of release of MT into plasma in the possum show that it is regulated by similar mechanisms to OT release in eutherian mammals. OT receptors have been characterized and localized in the possum and the tissue distributions and pharmacological characteristics of the receptor are similar to both the sheep and rat OT receptors. The marsupial OT receptor shows no pharmacological specificity for MT over OT which is reflected in the similar potency of these peptides in eliciting contractions of the uterus of the tammar wallaby in vitro. MT seems to play an important but not essential role in parturition in the tammar. MT concentrations are increased immediately after delivery in the tammar but infusion of an OT antagonist before expected birth delays but does not prevent parturition. The presence of OT receptors in the marsupial mammary gland and the sensitivity of the gland to exogenous OT and stimulation of mesotocinergic neurones demonstrates that these peptides are important for marsupial lactation. Our data suggest that the presence of MT with or without OT in marsupials is a result of a neutral mutation rather than functional evolution.