Plasma oxytocin and oxytocin neurone activity during delivery in rabbits.

Abstract

SUMMARY1. 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.2. 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.3. 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 < 0'05: Student's t test) and a significant (P < 0-01) lengthening of the modal interspike interval.4. 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.5. Frequent blood samples (0-2-043 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-').6. Before birth, plasma oxytocin was 53 + 12 pg ml-1 (mean + S.E. of mean) and rose to 2846 + 326 pg ml-' 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.7. No sign of pulsatile release of oxytocin was demonstrated in the profiles but significantly (P < 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.

Aspects of the neuronal and endocrine components of reflex milk ejection in conscious rabbits

“…These might include speed of sampling with regard to the half-life of the hormone (0-8-8-5 min) (Ginsburg, 1968;Fuchs & Dawood, 1980), sensitivity of the assay, or simply that pulses of oxytocin are released into the circulation too quickly to appear as discrete pulses of hormone. In birth, where oxytocin neurones provide much longer bursts of neurosecre¬ tory activity, it has been shown that bursts of discharge may be related to pulses of hormone in the circulation (O'Byrne, Ring & Summerlee, 1985). After the doe terminated nursing by moving away from the litter oxytocin levels either declined at an average of 1 -8 pmol/1 per s, in cases where the doe did not groom her nipples, or fell much more slowly (0-5 pmol/1 per s) in animals where the doe did groom her nipples.…”

Sex, parturition and motherhood without oxytocin?

“…There is clear evidence that the pulsatile pattern of oxytocin secretion is important in driving parturition. The pulsatile secretion derives from the synchronised intense bursting activity of oxytocin neurones in the hypothalamus in the rat and rabbit (Summerlee 1981, O'Byrne et al 1986, activity similar to that in the rat during reflex milk ejection in response to suckling. The increased electrical activity is accompanied by induction in these neurones of the immediate-early gene c-fos, an indicator of trans-synaptic activation of the neurones, and consequent prominent expression of the protein product Fos in the nuclei of oxytocin cells (Luckman et al 1993).…”

Pulsatile secretion of oxytocin during parturition in the pig: temporal relationship with fetal expulsion.

“…It is possible that this bolus release is what was observed in rats by Summerlee (1981) and in rabbits by O'Byrne et al (1986) as the 'birth-burst' phenomenon (a high frequency discharge of spikes from putative oxytocin cells coincident with delivery) and the more sporadic cell firing in between birth bursts receives its stimulus from a separate source. It remains a possibility that a more constant low grade stimulation of the cervix occurs to account for the high circulating levels of oxytocin between Ferguson reflexes, but there is no evidence for this hypothesis from our experiments.…”

“…During late pregnancy in the rat the peptide's stores increase in the neural lobe of the pituitary (Fuchs & Saito, 1971;Kumeresan, Subramanian, Anandarangam & Kumaresan, 1979), but on the day of parturition significant increases in plasma concentrations do not occur until the birth of the first pup (Higuchi, Honda, Fukuoka, Negoro & Wakabayashi, 1985;Higuchi, Tadokoro, Honda & Negoro, 1986a). In the rabbit, a rise in circulating oxytocin is measurable during the expulsive phase about 10 min before birth (Fuchs & Dawood, 1980; O'Byrne, Ring & Summerlee, 1986), whilst in larger species such as the miniature pig (Forsling, Taverne, Parvizzi, Elsaesser, Smidt & Ellendorf, 1979;Taverne et al 1979) or man (Vasicka, Kumaresan, Han & Kumaresan, 1978;Fuchs & Fuchs, 1984) the interval between the first appearance of high plasma oxytocin concentrations and delivery may be longer.…”

“…During normal parturition, electrophysiological recordings from single units in the supraoptic nucleus show that firing rate is high, with additional high frequency 'birth bursts' of action potentials occurring in synchrony with fetal expulsion (Summerlee, 1981;O'Byrne et al 1986). In the periphery, Higuchi et al (1986a) have demonstrated that plasma oxytocin concentrations become raised during second-stage labour, with a suggestion of further elevations following fetal ejection.…”