Plasma oxytocin and oxytocin neurone activity during delivery in rabbits

The extracellular electrical activity of magnocellular neurones was recorded from unanaesthetized, unrestrained rabbits in birth and the post partum suckling period. The activity of oxytocin neurones was differentiated from that of vasopressin cells on the basis of their stereotyped activity in suckling. Oxytocin neurones showed five to fourteen discrete bursts of accelerated discharge in parturition. Each burst lasted 2-22 s and represented a 3-100-fold increase in the rate of firing, compared with pre-partum values, and was followed 10-34 s later by delivery. After parturition, the spontaneous activity of these neurones returned to pre-partum rates of firing. Vasopressin neurones did not show any bursts of discharge in delivery. There was a significant fall in the discharge frequency compared with pre-partum levels (P less than 0.05: Student's t test) and a significant (P less than 0.01) lengthening of the modal interspike interval. Serial blood samples were obtained during parturition in ten rabbits. Simultaneous recordings of magnocellular neurones activity and plasma oxytocin measurements were made in four of these experiments. Plasma oxytocin profiles were related to the observed events of parturition. Frequent blood samples (0.2-0.3 ml every 10-15 s) were taken throughout delivery and plasma oxytocin measured by sensitive radioimmunoassay in unextracted plasma (lower limit sensitivity of assay 5 pg ml-1). Before birth, plasma oxytocin was 53 +/- 12 pg ml-1 (mean +/- S.E. of mean) and rose to 2846 +/- 326 pg ml-1 within 40-120 s of the onset of the expulsive phase of delivery. Peak concentrations of oxytocin were coincident with delivery of the first or second fetus. No sign of pulsatile release of oxytocin was demonstrated in the profiles but significantly (P less than 0.001) greater variance in oxytocin titres was found during birth compared with pre-partum values which is suggestive of pulsatile release. There was a straight line relationship between peak oxytocin concentrations in the plasma and the speed of delivery, implying that oxytocin facilitates as well as maintains labour in the rabbit.