In the current study, the onset of sucking by the young rats was associated with a gradual rise in basal circulating levels of oxytocin. It is unlikely that the sampling technique used caused the release of hormone, as preliminary experiments established that 12-15 samples (total volume 2-2.5 ml) did not evoke the release of oxytocin in rats per se. Onset of nursing in rats is accompanied by a slight, but statistically significant, increase in the background firing rate of oxytocin neurones in conscious rats, which might explain the rise in oxytocin. As oxytocin is known to have a positive effect on its own release, it is possible that the initial rise in circulating oxytocin is critical to crossing a threshold that permits reflex milk ejection to start. The latency from the onset of sucking to the start of reflex milk ejection, about 20 min, may reflect the time for oxytocin concentrations to reach and cross this threshold. However, the similarity between the latency to the first milk ejection in anesthetized rats and the time for plasma oxytocin concentrations to rise may be a coincidence. Certainly, the latency to the first milk ejection in conscious rats in usually much shorter, although there are no data on the relationship between the onset of sucking and increases in plasma oxytocin concentrations in conscious rats.
Rapid blood sampling throughout the period of reflex milk ejection in the present study confirmed the pulsatile release of oxytocin at milk ejection in anesthetized rats. There was a consistent latency (5-10 sec) from the onset in the rise in plasma oxytocin and the rise in intramammary pressure. flovent inhaler
The rapid but short-term release of oxytocin at reflex milk ejection represents a 10-fold increase in plasma concentrations compared with basal concentrations between milk ejections and probably accounts for the pulse of oxytocin associated with reflex milk ejection. In our study, the high concentrations of oxytocin observed decreased rapidly (within 10 sec). It is possible that these high concentrations represent the bolus of hormone release from the pituitary in the blood draining into the jugular. This pulse of hormone is mixed with systemic blood and is distributed throughout the body. Ten seconds after the peak, oxytocin concentrations were approximately 65 pM and thereafter declined at a rate that reflects the half-life of the hormone. Higuchi et al. reported that plasma oxytocin levels were approximately 65 pM in anesthetized lactating rats. However, these workers took blood samples effectively at 30-sec intervals from anesthetized rats, so the frequency of sampling may not have been sufficient to demonstrate the initial pulses of oxytocin observed in our experiments.