Endocrine Abstracts

Oxytocin drives parturition by contracting the uterus, but transgenic mice with no oxytocin appear to give birth normally. We have previously shown that oxytocin antagonist administration delays births showing that oxytocin plays a role in normal mouse parturition. Here, we tested the hypothesis that oxytocin neurones are active during parturition, using double immunocytochemistry for Fos and oxytocin to show neuronal activation and radioimmunoassay of blood samples to seek increased oxytocin secretion. Outbred mice, housed in a reverse light cycle, were mated and on the day of expected parturition were observed in red light for the onset of birth. Ninety min after the birth of pup 2 the mice were anaesthetised and perfused-fixed transcardially and their brains removed. Controls were virgin, pregnant and post partum mice. Free-floating brain sections containing SON were processed immunocytochemically; Fos+oxytocin-positive cells were counted. Double-labelled neurones in the SON were sparse in all groups except in the parturient mice (1 way ANOVA, p<0.0001); indicating that oxytocin neurones become activated during birth. Other virgin, pregnant and parturient mice (at the birth of pup 2) were decapitated and trunk blood was collected. Plasma oxytocin concentration was assayed and increased at the birth of pup 2 compared to non-parturient mice (1 way ANOVA, p<0.05) showing that oxytocin secretion increased during birth. Some of the parturient mice were stressed in glass jar at pup 2 for 15 min and then blood collected as above: stress halted further births and was associated with a lower plasma oxytocin concentration compared to mice at the birth of pup 2 (p<0.05). Thus, oxytocin neurones are active during parturition in the mouse and secrete oxytocin to promote birth. In conclusion, oxytocin plays an important role during birth in mice.

SW supported by a Physiological Society Vacation Studentship.