Na_v1.7 sodium channels can amplify weak stimuli in neurons and act as threshold channels for firing action potentials. Neurotrophic factors and pro-nociceptive cytokines that are released during development and under pathological conditions activate mitogen-activated protein kinases (MAPKs). Previous studies have shown that MAPKs can transduce developmental or pathological signals by regulating transcription factors that initiate a gene expression response, a long-term effect, and directly modulate neuronal ion channels including sodium channels, thus acutely regulating dorsal root ganglion (DRG) neuron excitability. For example, neurotrophic growth factor activates (phosphorylates) ERK1/2 MAPK (pERK1/2) in DRG neurons, an effect that has been implicated in injury-induced hyperalgesia. However, the acute effects of pERK1/2 on sodium channels are not known. We have shown previously that activated p38 MAPK (pp38) directly phosphorylates Na_v1.6 and Na_v1.8 sodium channels and regulates their current densities without altering their gating properties. We now report that acute inhibition of pERK1/2 regulates resting membrane potential and firing properties of DRG neurons. We also show that pERK1 phosphorylates specific residues within L1 of Na_v1.7, inhibition of pERK1/2 causes a depolarizing shift of activation and fast inactivation of Na_v1.7 without altering current density, and mutation of these L1 phosphoacceptor sites abrogates the effect of pERK1/2 on this channel. Together, these data are consistent with direct phosphorylation and modulation of Na_v1.7 by pERK1/2, which unlike the modulation of Na_v1.6 and Na_v1.8 by pp38, regulates gating properties of this channel but not its current density and contributes to the effects of MAPKs on DRG neuron excitability.

The sensory neuron-specific sodium channel Na_v1.8 and p38 mitogen-activated protein kinase are potential therapeutic targets within nociceptive dorsal root ganglion (DRG) neurons in inflammatory, and possibly neuropathic, pain. Na_v1.8 channels within nociceptive DRG neurons contribute most of the inward current underlying the depolarizing phase of action potentials. Nerve injury and inflammation of peripheral tissues cause p38 activation in DRG neurons, a process that may contribute to nociceptive neuron hyperexcitability, which is associated with pain. However, how substrates of activated p38 contribute to DRG neuron hyperexcitability is currently not well understood. We report here, for the first time, that Na_v1.8 and p38 are colocalized in DRG neurons, that Na_v1.8 within DRG neurons is a substrate for p38, and that direct phosphorylation of the Na_v1.8 channel by p38 regulates its function in these neurons. We show that direct phosphorylation of Na_v1.8 at two p38 phospho-acceptor serine residues on the L1 loop (S551 and S556) causes an increase in Na_v1.8 current density that is not accompanied by changes in gating properties of the channel. Our study suggests a mechanism by which activated p38 contributes to inflammatory, and possibly neuropathic, pain through a p38-mediated increase of Na_v1.8 current density.

Nociceptive dorsal root ganglion (DRG) neurons can be classified into nonpeptidergic IB₄⁺and peptidergic IB₄⁻subtypes, which terminate in different layers in dorsal horn and transmit pain along different ascending pathways, and display different firing properties. Voltage-gated, tetrodotoxin-resistant (TTX-R) Na_v1.8 channels are expressed in both IB₄⁺and IB₄⁻cells and produce most of the current underlying the depolarizing phase of action potential (AP). Slow inactivation of TTX-R channels has been shown to regulate repetitive DRG neuron firing behavior. We show in this study that use-dependent reduction of Na_v1.8 current in IB₄⁺neurons is significantly stronger than that in IB₄⁻neurons, although voltage dependency of activation and steady-state inactivation are not different. The time constant for entry of Na_v1.8 into slow inactivation in IB₄⁺neurons is significantly faster and more Na_v1.8 enter the slow inactivation state than in IB₄⁻neurons. In addition, recovery from slow inactivation of Na_v1.8 in IB₄⁺neurons is slower than that in IB₄⁻neurons. Using current-clamp recording, we demonstrate a significantly higher current threshold for generation of APs and a longer latency to onset of firing in IB₄⁺, compared with those of IB₄⁻neurons. In response to a ramp stimulus, IB₄⁺neurons produce fewer APs and display stronger adaptation, with a faster decline of AP peak than IB₄⁻neurons. Our data suggest that differential use-dependent reduction of Na_v1.8 current in these two DRG subpopulations, which results from their different rate of entry into and recovery from the slow inactivation state, contributes to functional differences between these two neuronal populations.