Biophysical characterization of sodium(v)1.7 and sodium(v)1.8 rat isoforms of the voltage gated sodium channels from dorsal root ganglions: Regulation by auxiliary beta-subunits and by protein kinase A and C

Introduction. Voltage gated Na+ channels are critical determinants of the electrophysiological properties especially in nociceptive neurons. Factors that regulate the Na+ channel function are of great interest from both the pathophysiological and therapeutic standpoint. To date ten different isoforms have been identified and at least six are expressed in the nervous system. Importantly a single type of neuron, such as the C-fibres, has been identified to express at least three different Na+ channel types: the tetrodotoxin-sensitive (TTX-S) Nav1.7 and TTX-resistant (TTX-R) Nav1.8 and Nav1.9 Na+ channels. The roles of the individual channels are still unknown. This present work therefore describes the modulation of two dorsal root ganglion specific Na+ channel isoforms; the tetrodotoxin-sensitive Nav1.7 and tetrodotoxin-resistant Na v1.8 by various auxiliary beta-subunits and protein kinases.; Methods. The biophysical properties of the channels and their modulation by the beta-subunits, PKA and PKC were studied by heterologous expression in Xenopus oocytes and the two-electrode voltage clamp method.; Results. The studies showed that Nav1.7 and Nav1.8 have different biophysical properties in the presence of the auxiliary beta1-subunit and show different sensitivity to frequency stimulations. The Nav1.7 channels are resistant to low frequency stimulations (0.5--20 Hz) suggest that these channels may play an important role pain related to low firing rates. In contrast Nav1.8 channels availability is higher frequencies of 25--100 Hz suggest that the resistance of Nav1.8 + beta1 channels to fully enter into the slow inactivated state during high frequency (>20 Hz) stimulation, coupled with the high threshold for activation (V0.5 = -3.3 mV), could maintain a minimal level of sodium channel activity in rapidly firing or chronically depolarized neurons during sustain noxious stimuli. The various beta-subunits differentially modulate the expression levels, gating and kinetics of Nav1.8; the beta1-subunit was the most potent regulator of the channel expression levels. Also PKA and PKC activation increases and decreases peak current of Nav1.8 respectively, but both kinases decreases the peak current of Nav1.7. Current reduction was due to the &egr;PKC and betaIIPKC isozymes for Nav1.7 and &egr;PKC for Nav1.8.; Conclusion. Thus the role of Nav1.7 and Na v1.8 in nociception will depend on the modulation by various auxiliary factor (such as the beta-subunits) and the phosphorylation states of the neurons.