Ca2+- and K+-dependent communication between central nervous system myelinated axons and oligodendrocytes revealed by voltage-sensitive dyes.

The interactions between myelinated axons and surrounding glia cells, in rat optic nerve, were investigated by optical recording with voltage-sensitive dyes. Electrical stimulation of the nerve evoked an optical signal revealing two clearly distinct components: a fast propagating component, corresponding to the compound action potential, and a prominent slow component. Several lines of evidence suggest that part of the slow component originated from depolarization of the oligodendrocytes by potassium accumulation in the paranodal or internodal region. In addition, the experiments suggest that in this preparation axons also have voltage-dependent Ca2+ channels, and a Ca2+-dependent K+ conductance involved in the depolarization of oligodendrocytes. Thus, axons and oligodendrocytes communicate in an intimate, ionically-mediated fashion, and oligodendrocytes may play an important functional role beyond that of providing the myelin sheath.