The antidromic compound action potential of the hypothalamo-neurohypophysial tract, a tool for assessing posterior pituitary activity in vivo.

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 fibre tract in response to electrical stimulation of the neurohypophysis. It was estimated that all fibres in the tract were electrically activated and contributed to the compound potential. It was shown that a decrease of peak amplitude could be 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, haemorrhage and plasma hyperosmolality, and it correlated with neurohypophysial hormone release. This method may usefully complement hormone assays and single cell analysis.