Peripheral nociceptive neurons encode and convey injury-inducing stimuli toward the central nervous system. In normal conditions, tight control of nociceptive resting potential prevents their spontaneous activation. However, in many pathological conditions the control of membrane potential is disrupted, leading to ectopic, stimulus-unrelated firing of nociceptive neurons, which is correlated to spontaneous pain. We have investigated the role of KV 7/M channels in stabilizing membrane potential and impeding spontaneous firing of nociceptive neurons. These channels generate low voltage-activating, noninactivating M-type K+ currents (M-current, IM), which control neuronal excitability. Using perforated-patch recordings fromcultured, rat nociceptor-like dorsal root ganglion neurons, we showthat inhibition of M-current leads to depolarization of nociceptive neurons and generation of repetitive firing. To assess to what extent the M-current, acting at the nociceptive terminals, is able to stabilize terminals’ membrane potential, thus preventing their ectopic activation, in normal and pathological conditions, we built a multi-compartment computational model of a pseudo-unipolar unmyelinated nociceptive neuron with a realistic terminal tree. The modeled terminal tree was based on the in vivo structure of nociceptive peripheral terminal, which we assessed by in vivo multiphoton imaging of GFP-expressing nociceptive neuronal terminals innervating mice hind paw. By modifying the conductance of the KV 7/Mchannels at the modeled terminal tree (terminal gKV 7/M) we have found that 40% of the terminal gKV 7/M conductance is sufficient to prevent spontaneous firing, while ∼75% of terminal gKV 7/M is sufficient to inhibit stimulus induced activation of nociceptive neurons. Moreover, we showed that terminal M-current reduces susceptibility of nociceptive neurons to a small fluctuations of membrane potentials. Furthermore, we simulated how the interaction between terminal persistent sodium current and M-current affects the excitability of the neurons. We demonstrated that terminal M-current in nociceptive neurons impeded spontaneous firing even when terminal Na(V) 1.9 channels conductance was substantially increased. On the other hand, when terminal gKV 7/M was decreased, nociceptive neurons fire spontaneously after slight increase in terminal Na(V) 1.9 conductance. Our results emphasize the pivotal role of M-current in stabilizing membrane potential and hereby in controlling nociceptive spontaneous firing, in normal and pathological conditions.
Bibliographical noteFunding Information:
Support is gratefully acknowledged from the Deutsch-Israelische Projectkooperation program of the Deutsche Forschungsgemeinschaft (DIP) grant agreement BI 1665/1-1ZI1172/12-1 (OB, RG, YK, BK, SL, AB); European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n◦ 260914 (OB, RG, BK, SL, AB) and the Hoffman Leadership program (RHG). We would like to thank Dr. Maya Groysman, the manager of ELSC viral core facility for designing and providing viruses used in this study.
© 2017 Barkai, Goldstein, Caspi, Katz, Lev and Binshtok.
- Ectopic activity
- Nociceptive terminals
- Spontaneous firing