Abstract
A regulatory mechanism for neuronal excitability consists in controlling sodium channel density at the plasma membrane. In cultured fetal neurons, activation of sodium channels by neurotoxins, e.g., veratridine and alpha-scorpion toxin (alpha-ScTx) that enhance the channel open state probability induced a rapid down-regulation of surface channels. Evidence that the initial step of activity-induced sodium channel down-regulation is mediated by internalization was provided by using 125I-alpha-ScTx as both a channel probe and activator. After its binding to surface channels, the distribution of 125I-alpha-ScTx into five subcellular compartments was quantitatively analyzed by EM autoradiography. 125I-alpha-ScTx was found to accumulate in tubulovesicular endosomes and disappear from the cell surface in a time-dependent manner. This specific distribution was prevented by addition of tetrodotoxin (TTX), a channel blocker. By using a photoreactive derivative to covalently label sodium channels at the surface of cultured neurons, we further demonstrated that they are degraded after veratridine-induced internalization. A time-dependent decrease in the amount of labeled sodium channel alpha subunit was observed after veratridine treatment. After 120 min of incubation, half of the alpha subunits were cleaved. This degradation was prevented totally by TTX addition and was accompanied by the appearance of an increasing amount of a 90-kD major proteolytic fragment that was already detected after 45-60 min of veratridine treatment. Exposure of the photoaffinity-labeled cells to amphotericin B, a sodium ionophore, gave similar results. In this case, degradation was prevented when Na+ ions were substituted by choline ions and not blocked by TTX. After veratridine- or amphotericin B-induced internalization of sodium channels, breakdown of the labeled alpha subunit was inhibited by leupeptin, while internalization was almost unaffected. Thus, cultured fetal neurons are capable of adjusting sodium channel density by an activity-dependent endocytotic process that is triggered by Na+ influx.