The effect of polarizing current on action potential and transmitter release in crayfish motor nerve terminals

Summary Potential changes were recorded from the terminal regions of crayfish motor nerve fibres by means of extracellular microelectrodes. Near to the recording site, a nerve branch was cut and sucked into a close fitting glass capillary electrode. Nerve action potentials were recorded through this electrode; in addition, current could be passed which de- or hyperpolarized the nerve and the terminal.Hyperpolarization of the terminal led to an increased average amplitude of the EPSP (up to 400% of control), which was due to an increased probability of release of quanta of transmitter. The average amplitude of the EPSP rose steeply with increasing current strength up to –6 A, larger hyperpolarizing current had little more effect. Part of this facilitatory effect of hyperpolarization was instantaneous, the full effect, however, developed only within 10 min after hyperpolarization. Similarly, the EPSP returned to the control amplitude only 10 min after the hyperpolarizing current was switched off. During hyperpolarization, the action potentials recorded from the nerve and from the terminal were prolonged by about 100% and increased in amplitude. The frequency of spontaneous synaptic potentials was reduced (down to 12% of control) during hyperpolarization.Depolarization of the terminal had little effect on the EPSP and on terminal potential changes, probably because small current strengths led to block of conduction proximal to the terminal. The frequency of spontaneous synaptic potentials was increased during depolarization of the terminal.The effects of polarizing current on the crayfish motor nerve terminal are very similar to the respective effects on cholinergic vertebrate terminals indicating a very similar mechanism of release of transmitter. Part of the facilitatory effect of hyperpolarization should be due to the prolongation of the action potential, in addition a mobilization of transmitter must be assumed. Possible relations between synaptic facilitation due to repetitive stimulation and hyperpolarization of the terminal are discussed.