[Involvement of actin in electrotonic conductivity in the mixed synapses of goldfish Mauthner neurons]

The effect of highly specific and selective actin-polymerizing and labelling agent, phalloidin, on electrotonic conductivity and structure of the mixed synapses of goldfish Mauthner neurons (MN) was studied. It was shown that the paired subthreshold electrostimulation of afferent input against a background of phalloidin application resulted in the average 80% increase of the amplitude of MN response to the second stimulus. In control group it increased by only 10% and was observed only after suprathreshold stimulation, while subthreshold stimuli were ineffective. We interpret these data as the manifestation of increased conductivity of the mixed synapses, induced by actin polymerization. At the ultrastructural level, phalloidin application at MN and their mixed synapses increased the size and number of actin-containing desmosome-like junctions, as well as the number of fibrillar bridges crossing their cleft. Using the phalloidin-colloid gold marker, the actin nature of these bridges was demonstrated. Interdependent morpho-functional changes found in the mixed synapses, provide the indication of actin involvement in the conduction of electrotonic signal through the mixed synapse. The bridges crossing the cleft of desmosome-like junction could be the structural substrate of this process.