Excitatory effects of oxytocin and cholecystokinin on oxytocin neurones: differences between virgin, pregnant and lactating rats.

Extracellular recordings were made from supraoptic oxytocin (OT) neurones in rat brain slices to investigate the effect of reproductive state on their excitation by OT and cholecystokinin (CCK). Stable background activity was induced by local glutamate application, and OT neurones were identified by their continuous firing and opioid inhibition. The excitatory effect of OT (indicated by percentage neurones excited, peak increment in firing, and response magnitude) was maximal in lactating rats, intermediate in virgin, and lowest in pregnant animals. Excitation by CCK was high in both lactating and virgin rats but, as with OT tests, was minimal in the pregnant group. These adaptations in the responses to OT and CCK may help to ensure that the firing of OT neurones is appropriately regulated to meet changing demands for OT release.