Long-term potentiation of electrotonic coupling at mixed synapses. Long-term potentiation of chemical synapses is closely related to memory and learning. Studies of this process have concentrated on chemically mediated excitatory synapses. By contrast, activity-dependent modification of gap junctions, which also widely exist in higher structures such as hippocampus and neocortex, has not been described. Here we report that at mixed synapses between sensory afferents and an identified reticulospinal neuron, the electrotonic coupling potential can be potentiated, as well as the chemically mediated excitatory postsynaptic potential, for a prolonged time period using a stimulation paradigm like that which produces long-term potentiation in hippocampus. The effect on coupling is due to an increase in gap-junctional conductance. Our data indicate that the potentiation of both synaptic components requires an increase in intracellular calcium, involves activation of NMDA (N-methyl-D-aspartate) receptors, and is specific to the tetanized pathway. 