Internalization of Voltage‐Dependent Sodium Channels in Fetal Rat Brain Neurons: A Study of the Regulation of Endocytosis

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Abstract
In fetal rat brain neurons, activation of voltage‐dependent Na+ channels induced their own internalization, probably triggered by an increase in intracellular Na+ level. To investigate the role of phosphorylation in internalization, neurons were exposed to either activators or inhibitors of cyclic AMP‐ and cyclic GMP‐dependent protein kinases, protein kinase C, and tyrosine kinase. None of the tested compounds mimicked or inhibited the effect of Na+ channel activation. An increase in intracellular Ca2+ concentration induced either by thapsigargin, a Ca2+‐ATPase blocker, or by A23187, a Ca2+ ionophore, was unable to provoke Na+ channel internalization. However, Ca2+ seems to be necessary because both neurotoxin‐ and amphotericin B‐induced Na+ channel internalizations were partially inhibited by BAPTA‐AM. The selective inhibitor of Ca2+/calmodulin‐dependent protein kinase II, KN‐62, caused a dose‐dependent inhibition of neurotoxin‐induced internalization due to a blockade of channel activity but did not prevent amphotericin B‐induced internalization. The rate of increase in Na+ channel density at the neuronal cell surface was similar before and after channel internalization, suggesting that recycling of internalized Na+ channels back to the cell surface was almost negligible. Pretreatment of the cells with an acidotropic agent such as chloroquine prevented Na+ channel internalization, indicating that an acidic endosomal/lysosomal compartment is involved in Na+ channel internalization in neurons.

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