Electrical activity of septal neurones during suckling and the milk ejection reflex in the lactating rat

Extracellular recordings from neurones in the lateral septum were performed in urethane anaesthetised lactating rats to study the eventual role of the septum in the control of suckling-induced oxytocin release. The connections of these neurones with the supraoptic nucleus, which contains cells secreting oxytocin, were assessed electrophysiologically by single pulse stimulation of the ipsilateral supraoptic nucleus. The neurones were thus classified into four categories: antidromically activated, orthodromically activated or inhibited, and unresponsive neurones. One hundred septal neurones were recorded in animals not exposed to suckling. A second group of 40 cells were analysed during suckling and one or more reflex milk ejections. The mean firing rates of each category of septal neurone did not differ significantly during suckling from the values observed in the absence of suckling. During suckling, almost all the recorded septal cells showed no significant alteration in their level of firing in relation to milk ejections. Two neurones presented an activation in the period between two milk ejections that seemed related to arousal. One neurone was clearly inhibited at the time of milk ejection. Our observations suggest that the septum does not represent an essential component of the pathways necessary for the milk ejection reflex induced by suckling, although it could exert an inhibitory action modulating either the intervals between two successive milk ejections, or the amount of oxytocin released.