Evidence for nonsynaptic neuronal interaction

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Abstract
Evidence is presented that synaptic interactions within and between the statocyst and visual pathways of Hermissenda are eliminated after 0.5-4 min exposure to 20-40 mM Co2+. Synaptic blockade was produced by perfusion with low Ca2+ (5 mM) plus 10-20 mM Co2+. Depolarization of hair cells by impulses of type A photoreceptors remains after the same exposure to Co2+, or low Co2+ plus Co2+. Increased resistance previously observed during this depolarization of hair cells cannot be observed after exposure to Co2+. The depolarization which remains after exposure to Co2+ did not change with different levels of membrane potential from -20 mV below to +10 mV above the resting level. The time course of K+ accumulation, monitored by the amplitude of the type A impulse afterpotential, closely followed the time course of hair cell depolarization and of changes in the amplitude of the hair cell afterpotential. The depolarization of hair cells by type A impulses decreased with increased extracellular K+, but was only slightly reduced by lowered extracellular K+. The amount of K+ accumulation following a type A impulse train could be estimated from the effects of changes in extracellular K+ in the perfusate on the type A impulse afterpotential. From this estimated increase of extracellular K+ it was possible to predict the observed hair cell depolarization with some accuracy. Although type A cells are not electrically coupled to ipsilateral hair cells, firing of these hair cells slightly depolarized the type A photoreceptor which excites them. Strophanthidin (10-4 M) did not block the depolarization of hair cells by type A impulses. The data are evidence for nonsynaptic excitation of hair cells by type A photoreceptor impulses. The data are consistent with the interpretation that the excitation arises from K+ accumulation around the type A and hair cell axonal membranes.