Sleep: A prerequisite for reflex milk ejection in the rat

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Abstract
Electroencephalographic activity (EEG) was recorded from the frontal cortex of unanaesthetized and urethane-anaesthetized lactating rats and analysed in relation to the pattern of milk ejection evoked by the nursing pups. The EEG of the anaesthetized rat fluctuated without experimental intervention between three distinctive patterns defined as synchronized, desynchronized, and stage III activity, whilst reflex milk ejection recurred at intervals of about 6 min (range 2->20 min) throughout the 1–4 h period the pups were left attached to the nipples. For > 10 s before and for up to 60 s after each milk ejection, as judged from recordings of intramammary pressure and pup behaviour, the EEG was invariably synchronized throughout. Conversely, milk ejection (n>300) was never observed during long periods of desynchronized, or stage III EEG activity. The vigorous increase in the sucking of the pups at milk ejection failed to produce a desynchronization (arousal) of the EEG as observed with other forms of sensory stimulation. Indeed, the sucking of the pups appeared to produce a soporific change in the maternal EEG for spontaneous periods of desynchronization were not observed in the 30–60 min following the initial attachment of the pups to the nipples. Similar EEG patterns were seen in the unanaesthetized rat, though arousal from the synchronized state was more easily produced, e.g., by weak auditory signals. Milk ejection, as judged from the behaviour of the pups, recurred at intervals of 2 min or more during each 20–80 min period of nursing. The rat appeared somnolent for most of the nursing period and the EEG was always synchronized for > 10 s before each milk ejection (n>200), though her eyes usually remained open. Arousal and desynchronization of the EEG was invariably observed in association with the increased pup behaviour at milk ejection. From these observations and the knowledge that oxytocin release from the neurohypophysis occurs about 10 s before milk ejection, we conclude that a synchronized EEG pattern, akin to that associated with ‘slow-wave sleep’, is an essential prerequisite for the expression of the milk-ejection reflex in the rat.