A STUDY ON THE MECHANISM UNDERLYING MILK EJECTION BURSTS OF OXYTOCIN CELLS.

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In order to study the neural mechanisms to generate burst activities of oxytocin neurones during suckling in lactating rats, the following three studies were made :(1) the effect of midbrain-hemitransection on amplitudes and time courses of milk ejection bursts of oxytocin neurones in lactating rats.Extracellular recordings of the electrical activity of oxytocin neurones were made from the supraoptic nuclei of lactating rats, and the milk-ejection bursts were investigated in the midbrain-hemitransected rats. Of the burst recorded from oxytocin neurones on the lesioned side, the amplitude was smaller, the onset time was later and the burst span was shorter than those recorded on the non-lesion-ed side. These results indicated that the amplitude and time course of the burst of oxytocin neurones mainly depend on suckling information from the nipples on their opposite side.(2) the effects of unilateral suckling on milk ejection bursts of oxytocin neurones in intact rats.When application of pups was limited to the nipples on either the same side or the side opposite to the oxytocine neurone recorded, the burst amplitude was significantly lower than during bilateral suckling. Furthermore, the burst amplitude was significantly lower during ipsilateral suckling than during contralateral suckling. The finding indicates that the burst amplitude mainly depends on the amount of afferent suckling signals arising from the nipplies on the side opposite to the reocording side.(3) the effect of interhemispheric sections of the hypothalamus on milk-ejection bursts of supraoptic oxytocin neurones.Extensive interhemispheric sectioning did not abolish the burst of oxytocin neurones during bilateral and contralateral suckling but reduced the number of oxytocine neurones that showed the burst during ipsilateral suckling. The result suggest that there are two separate gate and summation mechanisms on each side of the hypothalamus.