Fast Ripples: What Do New Data about Gap Junctions and Disrupted Spike Firing Reveal about Underlying Mechanisms?

Gap junctions have been postulated to exist between the axons of excitatory cortical neurons based on electrophysiological, modeling, and dye‐coupling data. Here, we provide ultrastructural evidence for axoaxonic gap junctions in dentate granule cells. Using combined confocal laser scanning microscopy, thin‐section transmission electron microscopy, and grid‐mapped freeze–fracture replica immunogold labeling, 10 close appositions revealing axoaxonic gap junctions (30–70 nm in diameter) were found between pairs of mossy fiber axons (100–200 nm in diameter) in the stratum lucidum of the CA3b field of the rat ventral hippocampus, and one axonal gap junction (100 connexons) was found on a mossy fiber axon in the CA3c field of the rat dorsal hippocampus. Immunogold labeling with two sizes of gold beads revealed that connexin 36 was present in that axonal gap junction. These ultrastructural data support computer modeling and in vitro electrophysiological data suggesting that axoaxonic gap junctions play an important role in the generation of very fast (>70 Hz) network oscillations and in the hypersynchronous electrical activity of epilepsy. Ripples are sharp‐wave‐associated field oscillations (100–300 Hz) recorded in the hippocampus during behavioral immobility and slow‐wave sleep. In epileptic rats and humans, a different and faster oscillation (200–600 Hz), termed fast ripples, has been described. However, the basic mechanisms are unknown. Here, we propose that fast ripples emerge from a disorganized ripple pattern caused by unreliable firing in the epileptic hippocampus. Enhanced synaptic activity is responsible for the irregular bursting of CA3 pyramidal cells due to large membrane potential fluctuations. Lower field interactions and a reduced spike‐timing reliability concur with decreased spatial synchronization and the emergence of fast ripples. Reducing synaptically driven membrane potential fluctuations improves both spike‐timing reliability and spatial synchronization and restores ripples in the epileptic hippocampus. Conversely, a lower spike‐timing reliability, with reduced potassium currents, is associated with ripple shuffling in normal hippocampus. Therefore, fast ripples may reflect a pathological desynchronization of the normal ripple pattern.