Neurohypophysial hormone functions in marsupials

Abstract

Eutherian mammals secrete the nonapeptides oxytocin (OT) andnarginine vasopressin (AVP) as their neurohypophysial hormones. OT has wellndocumented roles in lactation and parturition whereas AVP is involved in fluidnhomeostasis and blood pressure regulation. However the functions of thesenpeptides are much wider than these and include roles during various types ofnstress. Marsupials secrete a variety of neurohypophysial hormones in additionnto or instead of OT and AVP. There is very little known about the roles of thesendifferent OT- and AVP-like peptides in marsupials. This thesis has primarilyninvolved characterization and localization of these peptides leading to somenpreliminary investigations into the possible functions of these peptides innmarsupials. The brushtail possum (Trichosurus vulpecula) has been used asnthe primary model with some studies also on the northern brown bandicootn(Isoodon macrourus). The brushtail possum secretes the typically reptiliannmesotocin (MT) together with AVP as its neurohypophysial hormones,nwhereas the bandicoot secretes OT and MT together with AVP and lysine-nvasopressin (LVP). The brain distribution and plasma release of both MT andnOT were studied in the bandicoot and the brain distribution and plasmanrelease of AVP was studied in the possum. The regulation of the plasmanrelease of MT and AVP by various factors including different types of stressnwas also studied in the possum. Neurohypophysial hormones are also foundnin a number of peripheral tissues in eutherian mammals and other vertebrates.nThere is also some evidence for tissue specific expression ofnneurohypophysial hormones. Therefore peripheral tissues of the possum andnbandicoot were examined for the presence of neurohypophysial hormones. Asnthe functions of peptides are mediated via receptors this thesis has alsonexamined OT and AVP receptors in the possum, OT and AVP receptors In thenpossum were compared to OT and AVP receptors in the rat. As OT receptorsnhave not previously been characterized in the testis of eutherian mammals thenrat testis OT receptor was characterized together with the testis AVP receptornfor comparison to OT and AVP receptors in the possum testis.nMT and OT were measured In the brain and plasma of the northernnbrown bandicoot (Isoodon macrourus) using reverse-phase high-performancenliquid chromatography (HPLC) and specific radloimmunoassays (RIA) for bothnOT and MT. Both peptides were found in the pituitary (1.25 pn0.10 mg/MT;n0.725 pn0.077 mg/OT) and hypothalamus (38.37 pn6.46 ng/MT; 19.1 pn4.61nng/OT) (mean pnSEM). Concentrations in the cerebral cortex were very lownand consequently only two animals had detectable levels (311 and 154npg/MT; 360 and 92 ng/OT). Basal plasma concentrations ranged from 1.5 - 8.1npg/ml for both peptides (n=14) and were elevated by the combined stress ofnsurgery, anaesthesia and hypovolemia. n