The role of the AV3V region in the control of magnocellular oxytocin neurons.

The AV3V region is important in the control of body fluid and Na+ regulation and projects to the supraoptic and paraventricular nuclei. Oxytocin from the neurohypophysis mediates milk ejection and is involved in parturition, but has also been recently implicated as a candidate natriuretic hormone. We have studied the role of the AV3V region in the control of magnocellular oxytocin neurons in rats. Electrical stimulation of the AV3V region increased the firing rate of supraoptic oxytocin neurons and evoked a concomitant release of oxytocin. Acute electrolytic AV3V lesions silenced supraoptic neurons and abolished their excitation by hyperosmotic stimulation. The lesions also abolished osmotically-induced release of oxytocin. Re-activation of supraoptic neurons by local glutamate restored their osmoresponsiveness to about 50% normal. Thus, while supraoptic neurons are directly osmosensitive, the AV3V region is essential for their normal osmoresponsiveness. Electrolytic AV3V lesions did not affect suckling-induced oxytocin secretion or, in conscious rats, the release of oxytocin secretion during parturition. Thus the AV3V region is not involved in the activation of oxytocin neurons during suckling or parturition.