Dual transmission at morphologically mixed synapses: Evidence from postsynaptic cobalt injections

Two experimental approaches have been utilized to test the possibility that morphologically mixed synaptic terminals of the eighth nerve fibers mediate both electrotonic and chemical excitation of the goldfish Mauthner cell. First, the spatial distributions of electrotonic and chemical postsynaptic potentials, evoked by stimulation of the eighth nerve, have been determined with intracellular recordings from the Mauthner cell soma and several locations along the lateral dendrite. In some instances, both synaptic components were maximal at distal dendritic recording sites. In that region, it appears that the only presynaptic terminals with morphological characteristics consistent with excitatory chemical transmission are the large myelinated club endings, which actually establish mixed synapses with the lateral dendrite. Second, we have analyzed the effects of postsynaptic Co²⁺ injections on these synaptic responses. With high iontophoretic currents, there was a rapid uncoupling of the electrotonic component. However, with smaller current intensities, uncoupling is accompanied, or preceded, by a transient reduction in the later chemically mediated postsynaptic potentials. This latter effect on chemical transmission is only observed if the postsynaptic potentials are associated with electrotonic synaptic inputs. We speculate that Co²⁺ diffuses across the gap junctions and into the presynaptic terminals, acting there to reduce evoked transmitter release.The results of these two experimental approaches support the hypothesis that mixed synapses on the lateral dendrite of the Mauthner cell do actually mediate transmission by both chemical and electrical modes.