MILK EJECTION BURSTS OF SUPRAOPTIC OXYTOCIN NEURONS DURING BILATERAL AND UNILATERAL SUCKLING IN THE RAT

Extracellular recordings of the electrical activity of oxytocin neuron es were made from the supraoptic nuclei (SON) of lactating rats, and t he milk-ejection bursts and the background activity of oxytocin neuron es were investigated during unilateral and bilateral suckling. When ap plication of pups was limited to the nipples on either the same side ( ipsilateral suckling) or the side opposite (contralateral suckling) to the oxytocin neurone recorded, the burst amplitude and background fir ing rate were significantly (P<0.05) lower and the inter-burst interva l was significantly (P<0.05) longer than during bilateral suckling. Fu rthermore, the burst amplitude was significantly (P<0.05) lower during ipsilateral suckling than during contralateral suckling. The majority of the oxytocin neurones showed a gradual increase in the burst ampli tude during bilateral (88.9%) and contralateral (77.3%) suckling, but during ipsilateral suckling only 40% of the neurones did. The inter-bu rst interval became shorter with the progress of the milk ejection ref lex during any mode of suckling. Three pairs of oxytocin neurones reco rded simultaneously from both SON were successfully tested for the eff ect of bilateral and unilateral suckling on the electrical activity, a nd the results showed the same direction of change in the burst amplit ude, background activity and burst interval as shown in single side re cordings. These findings indicate that the burst amplitude mainly depe nds on the amount of afferent suckling signals arising from the nipple s on the side opposite to the recording side, and that there may exist bilateral summation centres coordinating with the synchronization mec hanism of milk-ejection bursts of oxytocin neurones.