Voltage dependent sodium currents in cultured rat cerebellar granule cells

Activation of voltage-dependent sodium channels in cultured cerebellar poffule cells induces neurotoxicity that is not mediated by glutamate release

Exposure of rat cerebellar granule cell cultures to neurotoxins that specifically enhance the open state probability of voltage-dependent Na+ channels, resulted in neuronal death as estimated by a cell viability assay based on fluorescent staining and 51Cr-uptake. Toxicity was detected within 1 h after addition of 100 microM veratridine and was complete within 10-18 h; it was dose-dependent and was found to be completely abolished by tetrodotoxin, an Na+ channel blocker. When veratridine was replaced by an alpha-scorpion toxin, similar observations were done. In contrast, when cultured neurons prepared ffom the cerebral hemisphere of fetal rat brain were exposed to either veratridine or alpha-scorpion toxin for 18 h or even for a longer time of incubation, no neuronal death was observed. DNA fragmentation analysis showed that the toxicity was not mediated by apoptosis. Neuronal death was neither prevented by glutamate receptor antagonists, nor by depletion of endogenous glutamate, nor by voltage sensitive calcium channel antagonists such as omega-Conotoxin-GVIA (N-type channels), omega-Agatoxin-IVA (P-type channels), nimodipine and nitrendipine (L-type channels). Our study indicates that prolonged opening of Na+ channels induced neuronal death of cerebellar granule cells which is not mediated by glutamate and reveals novel neurotoxic mechanism in addition to the well-established excitatory amino acid receptor pathway.

Sodium, calcium and late potassium currents are reduced in cerebellar granule cells cultured in the presence of a protein complex conferring resistance to excitatory amino acids

Whole-cell, patch-clamp recordings were used to study voltage-gated currents generated by cerebellar granule cells that were cultured in medium containing either 10% fetal calf serum (hereafter termed S + granules) or neurite outgrowth and adhesion complex (NOAC, hereafter called NOAC granules). NOAC is a protein complex found in rabbit serum that renders granules resistant to the excitotoxic action of excitatory amino acids. During depolarizing commands both S+ and NOAC granules generated Na+ and Ca2+ inward currents and an early and a late K+ outward currents. However, Na+ and Ca2+ inward currents and late outward K+ currents recorded in NOAC granules were smaller than those seen in S+ granules. Furthermore, although of similar amplitude, early K+ currents displayed different kinetics in the two types of neurons. Thus, these data demonstrate that the electrophysiological properties of cerebellar granules, and probably of other neuronal populations, depend upon serum components and raise the possibility that an analogous modulation might be operative in vivo, and play a role in development, synaptic plasticity or neuropathological processes.

Voltage dependent calcium channels in cerebellar granule cell primary cultures

Voltage activated calcium channels were studied in rat cerebellar granule cells in primary culture. Macroscopic currents, carried by 20mM Ba2+, were measured in the whole-cell configuration. Slowly inactivating macroscopic currents, with a maximum value at a membrane potential around 5 mV, were recorded between the 1st and the 4th day in culture. These currents were completely blocked by 5mM Co2+, partially blocked by 10 microM nifedipine, and increased by 2 to 5 microM BAY K-8644. Two types of channels, in the presence of 80 mM Ba2+, were identified by single channel recording in cell-attached patches. The first type, which was dihydropyridine agonist sensitive, had a conductance of 18 pS, a half activation potential of more than 10 mV and did not inactivate. This type of channel was the only type found during the first four days in culture, although it was also present up to the 11th day. The second type of channel was dihydropyridine insensitive, had a conductance of 10 pS, a half activation potential less than -15 mV, and displayed voltage dependent inactivation. This second type of channel was found in cells for more than four days in culture.

Competitive blockage of the sodium channel by intracellular magnesium ions in central mammalian neurones

The aim of this study was to determine from macroscopic current analysis how intracellular magnesium ions, Mgi2+, interfere with sodium channels of mammalian neurones. It is reported here that permeation across the sodium channel is voltage- and concentration-dependently reduced by Mgi2+. This results in a general reduction of sodium membrane conductance and an outward sodium peak current at large positive potentials. 30 mM Mgi2+ leads ot a negative shift of voltage dependence of sodium channel gating parameters, probably due to the surface potential change of the membrane. This shift alone is, however, insufficient to explain the reduction of outward sodium currents. The blockage by Mgi2+ is decreased upon increasing intracellular or extracellular Na+ concentration, which suggests that Mgi2+ interferes with sodium permeation by competitively occupying sodium channels. Using a kinetic model to describe the sodium permeation, the dissociation constant (at zero membrane potential) of Mgi2+ for the sodium channel has been calculated to be 8.65 +/- 1.51 mM, with its binding site located at 0.26 +/- 0.05 electrical distance from the inner membrane. This dissociation constant is smaller than that of Nai+, which is 83.76 +/- 7.60 mM with its binding site located at 0.75 +/- 0.23. The low dissociation constant of Mgi2+ reflects its high affinity for the sodium channel.