Synaptic potentials in afferent fibers innervating hair cells of the lateral line organ in Xenopus laevis.

Spontaneous synaptic activity in afferent fibers innervating hair cells of the lateral line organ was monitored with micropipettes at visually identified locations near the peripheral terminations of the fibers. Spontaneous action potentials as well as spontaneous excitatory post-synaptic potentials (EPSPs) were observed. Some action potentials were preceded by EPSPs. Most EPSPs did not reach threshold to generate action potentials. The frequency of occurrence of spontaneous EPSPs was related to the number of neuromasts innervated by a fiber. Single EPSPs were most readily observed in fibres innervating only a few neuromasts. Perfusion of the synapse with solutions containing cobalt reversibly diminished discharge rate by reducing the occurrence of EPSPs. Manganese produced similar effects on discharge rate. The blockage of spontaneous EPSPs by cobalt, an agent that can block voltage-dependent transmitter release, supports the hypothesis that spontaneous discharge in afferent fibers of the lateral line organ is due to voltage-dependent release of transmitter from the hair cells.