A study on the neural mechanism of pulsatile secretion of oxzytocin during suckling in the rat

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Oxytocin is known to be released in a pulsatile fashion during suckling in the latating rat. However, not only the neural mechanism to produce the pulsatile release but also its location have not been made clear. We found neurons projecting to the supraoptic nucleus and showing bursts of action potentials before milk ejection (bursting neuron) in the dorsomedial nuclers and its vicinity. We tested the effect of suckling stimuli on the background activity of those neurons and found that suckling stimuli increased the firing rate of the neuron and the removal of pups from the mother decreased it. Since the background activity of oxytocin neuron is not influenced by suckling stimuli it is suggested the bursting neuron may participate in the gating mechanism for milk ejection bursts. In order to investigate the location of the neural mechanism responsible for the synchronization of milk ejection bursts of oxytocin neurones the activity of oxytocin neurons was recorded from the supraoptic nuclei (SON) bilaterally in the lactating rats with hemitransection of the midbrain. During bilateral suckling not only the oxytocin neurones recorded from the intact side but also those recorded from the lesioned side displayd milk-ejection bursts. When oxytocin neurons could be recorded on both sides and displayd milk-ejection bursts, they were always well synchronized. Recoding from bilateral SON was also made in the rat with hemitransection in the mid-hypothalamus. Milk ejection burst were recorded from the intact side but no bursts were recorded from the lesioned side. Thus it is very likely that the synchronization mechanism for milk ejection burst is located in the region between the midbrain and the midhypothalamus.