Brevetoxin activation of voltage-gated sodium channels regulates Ca ²⁺ dynamics and ERK1/2 phosphorylation in murine neocortical neurons

Voltage-gated sodium channels (VGSC) are involved in the generation of action potentials in neurons. Brevetoxins (PbTx) are potent allosteric enhancers of VGSC function and are associated with the periodic 'red tide' blooms. Using PbTx-2 as a probe, we have characterized the effects of activation of VGSC on Ca²⁺ dynamics and extracellular signal-regulated kinases 1/2 (ERK1/2) signaling in neocortical neurons. Neocortical neurons exhibit synchronized spontaneous Ca²⁺ oscillations, which are mediated by glutamatergic signaling. PbTx-2 (100 nm) increased the amplitude and reduced the frequency of basal Ca²⁺ oscillations. This modulatory effect on Ca²⁺ oscillations produced a sustained rise in ERK1/2 activation. At 300 nm, PbTx-2 disrupted oscillatory activity leading to a sustained increase in intracellular Ca²⁺ ([Ca²⁺]_i) and induced a biphasic, activation followed by dephosphorylation, regulation of ERK1/2. PbTx-2-induced ERK1/2 activation was Ca²⁺ dependent and was mediated by Ca²⁺ entry through manifold routes. PbTx-2 treatment also increased cAMP responsive element binding protein (CREB) phosphorylation and increased gene expression of brain-derived neurotrophic factor (BDNF). These findings indicate that brevetoxins, by influencing the activation of key signaling proteins, can alter physiologic events involved in survival in neocortical neurons, as well as forms of synaptic plasticity associated with development and learning.