Do neurons have a reserve of sodium channels for the generation of action potentials? A study on acutely isolated CA1 neurons from the guinea-pig hippocampus

The density of voltage-gated sodium channels is high in several regions of the neuronal membrane. It is unclear if this density of channels represents a reserve for the neuron, or if it fulfils a special role in action potent ial firing. This problem was addressed by studying sodium currents and acti on potentials in acutely isolated hippocampal CA1 neurons whose number of a ctive sodium channels was acutely changed by applying the sodium channel bl ocker tetrodotoxin (TTX) at different concentrations. The results show that more than a third of the sodium channels can fail without affecting the si ngle action potential. Thus, the neurons have a remarkable surplus of sodiu m channels. The surplus, however, is necessary for repetitive action potent ial firing, as every decrease in the fraction of sodium channels reduces th e maximal frequency of action potentials that can be generated by the neuro n.