Spontaneous neural activity of a mechanoreceptive system is undiminished by replacement of external calcium with equimolar magnesium in the presence of EGTA.

Abstract

Primary afferent neurons of the lateral-line mechanosensory organs, which are believed to be closely related to the auditory and vestibular organs, exhibit "spontaneous" action potentials in the absence of mechanical stimulation of the receptor cells (hair cells). Sinusoidal mechanical stimulation of the hair cells enhances the impulse rate of the afferent neurons. The spontaneous activity is found to be a decreasing function of increasing concentration of either external magnesium or calcium, when each cation is varied in the absence of the other and bath-applied to the synaptic side of the lateral-line mechanoreceptors. One mM to 6 mM magnesium with 5 mM EGTA (the latter for chelation of remaining traces of calcium) permits undiminished spontaneous afferent activity of lateral-line neurons for as long as 3 to 4 hours. With bath-applied calcium, mechanical stimulation results in evoked incremental activity--defined as total activity with stimulation minus spontaneous activity--which significantly increases with increasing calcium concentration. However, with magnesium and EGTA in the bath, mechanical stimulation produces no increase in the neural firing rate above spontaneous rate for any magnesium concentration tested. Taken together, these results suggest that spontaneous activity, in contrast to evoked incremental activity, does not require external calcium in the bath, and production of spontaneous neural action potentials may proceed via mechanisms that are modifications of those of classical stimulus-secretion coupling.