ANTIDROMIC COMPOUND ACTION POTENTIAL OF THE HYPOTHALAMO-NEUROHYPOPHYSEAL TRACT, A TOOL FOR ASSESSING POSTERIOR PITUITARY ACTIVITY INVIVO

Two neurohypophysial hormones, vasopressin and oxytocin, are synthesized in the perikarya of supraoptic and paraventricular neurones, transported via the pituitary stalk to the axonal endings in the neurohypophysis and are released into the blood stream by a process depending on action potentials. A sensitive technique was developed for measuring continuously the summated neural activity of the rat hypothalamo-neurohypophysial tract. Large compound potentials of 4-9 mV peak amplitude were recorded from the fiber tract in response to electrical stimulation of the neurohypophysis. It was estimated that all fibers in the tract were electrically activated and contributed to the compound potential. A decrease of peak amplitude was used as a measure of changes in endogenous neural activity. The decrease in peak amplitude during an increase of endogenous neural activity was due to fatigue and temporal spread of the compound potential and also to collision of antidromic and orthodromic action potentials. Decrease of the compound potentials was related to the intensity of vaginal distension, hemorrhage and plasma hyperosmolality, and it correlated with neurohypophysial hormone release. This method may usefully complement hormone assays and single cell analysis.