Electrical properties of axons and neurohypophysial nerve terminals and their relationship to secretion in the rat.

Isolated rat pituitary stalk-neurohypophysial complexes were electrically stimulated and the evoked compound action potentials were recorded at the level of both axons and nerve terminals. The latency of the nerve terminal response increased during continuous stimulation of the stalk at frequencies as low as 1 Hz. At similar frequencies continuous stimulation of the stalk produced an increase in the latency of the response of the nerve fibres and a decrease in the amplitude of the compound action potential. The increase in the latency of the response of both axons and nerve terminals was related to the frequency and number of stimuli. The time necessary for full recovery of the response of the axons and the nerve endings, following stimulation at frequencies above 5 Hz, was not linearly related to the frequency of stimulation. Stimulation of the stalk with a pulse pattern (bursts) imitating the electrical activity of vasopressin-containing magnocellular neurones showed that the latency of the compound action potential had increased by the end of the first burst. The latency of the response of axons and nerve endings was inversely proportional to the time interval between bursts. Prolonged stimulation of the isolated neural lobe with 'vasopressin'-like bursts induced the release of vasopressin. Twelve bursts, separated by 3 min intervals, released more hormone than fifty bursts given during the same period of time, but separated by a 21 s interval. Leu-enkephalin (10(-5) M) did not modify the latency or the amplitude of the action potentials evoked with low frequency of stimulation (0.5 Hz) or with 'vasopressin'-like bursts. In conclusion, it is suggested that the electrical properties of the nerve fibres and the nerve endings goes some way to explain the pattern of hormone release observed during sustained stimulation.