Inhibition of milk ejection by a visual stimulus in lactating rats: implication of the pineal gland.

The effect of visual and auditory stimuli on milk ejection during suckling was studied in normal and pinealectomized lactating rats. The photic and auditory stimuli were applied to each mother for 10 s every 20 s during the 30 min suckling period. Both stimuli inhibited milk ejection without altering the nursing behavior. In mothers kept in complete darkness or in which the visual stimulus shone continuously during the suckling period, milk ejection was not affected. The inhibition of milk ejection is therefore produced by the light on-off sequence. In lactating rats exposed to the stimulus during 3 consecutive days, a significant inhibition of milk ejection was obtained each day. A normal milk-ejection response occurred in both non-stimulated pinealectomized and in stimulated pinealectomized lactating rats. Pinealectomy did not prevent the inhibitory effect of the sound stimulus. Treatment with methysergide prevented the inhibition of milk ejection induced by the visual stimulus but did not prevent the inhibitory effect of the auditory stimulus. It seems that the pineal gland mediates an inhibitory visual reflex acting on oxytocin release and milk ejection.