The milk ejection reflex in the pig

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Abstract
The milk ejection reflex in response to suckling was studied in conscious sows by continuous recording of intrammary pressure, radioimmunoassay of plasma concentrations of neurohypophyseal hormones, and observation of the behavior of the sows and piglets. Neurohypophyseal hormone assays performed on serial blood samples showed an increase in plasma concentration of oxytocin up to 30 s before milk ejection. The concentration of lysine-vasopressin did not rise above basal levels. In 21.4% of the suckling periods, no rise in intrammamary pressure was observed. For 3 of these periods, hormone assay showed no increase in oxytocin or vasopresin concentrations in blood. Oxytocin given i.v. produced variations in intrammamary pressure which depended on the dose and the rate of injection. Trains of electrical pulses were applied to the posterior pituitary of 4 anesthetized sows. At frequencies of stimulation above 10 Hz, a rise in intrammary pressure and an increase in plasma oxytocin and vasopressin concentrations were observed. At frequencies of stimulation of 30-50 Hz, the response of the mammary gland and the time course of the variations in oxytocin plasma concentrations were similar to those observed during natural reflex milk ejection. Reflex milk ejections during suckling in the pig apparently are caused by the intermittent and spurt-like release of oxytocin, without concomitant vasopressin release. The release of oxytocin is probably precipitated by a brief and massive activation of oxytocin-secreting neurons in the hypothalamus. Central mechanisms controlling the intermittent release of oxytocin are discussed.