Voltage-gated ion currents in embryogenesis.

Excitability is not an exclusive property of differentiated cells, as egg cells already express a surprising variety of species-specific combinations of voltage-gated ion channels. After fertilization, ion currents are generally down-regulated and reappear at about the time of the last mitosis of neuronal precursor cells. When ganglia have formed, Na+ and Ca2+ currents are generally present. The first voltage-gated ion currents in differentiating neurons are unambiguously identified by kinetic and pharmacological criteria according to the channel classifications defined for ion currents in more mature cells. Specialization of different neuronal subtypes with respect to the expression of characteristic patterns of ion channels occurs before neurites grow out and contact their targets. During maturation of committed neurons, the densities of ion currents change. Whereas low voltage-activated Ca2+ current are only transiently expressed in several types of cells Na+, high voltage activated Ca2+, and K+ currents are usually up-regulated until stable final values are reached in adult cells. The most challenging questions to be answered by future research concern the molecular mechanisms regulating the expression of the specific patterns of ion channels characteristic of different subtypes of neurons.