On the search for the electrical synapse: a glimpse at the future.

The traditional concept of electrotonic synapses suggests that they synchronize outputs from coupled neurons and provide rapid impulse propagation between pre- and postsynaptic elements. These properties have provided an evolutionary advantage in certain behavioral repertoires, for example, in the rapid impulse propagation between axonal segments in the crayfish and in electrotonic synapses on motoneurons. Recent theoretical and experimental evidence, in particular with regard to neuronal synchronization, ultrastructure and molecular biology, shows that this concept has new relevance. In particular, computer simulations demonstrated that neurons synchronize and alter their firing patterns depending on gap-junctional communication. The cloning of neuronal gap-junction proteins and the ablation of the neuronal connexin36 (Cx36) provided novel insights into the extent and functional significance of electrotonic coupling between paired interneurons. Furthermore, electrophysiological recording of gap-junctional communication supports its importance in network behavior. Hence, in addition to chemical transmission, direct coupling by electrotonic synapses is now accepted to provide a second major pathway contributing to normal and abnormal physiological rhythms.