I would like to highlight an exciting new study referencing how knocking down Kv9.1 Potassium Channel Subunit in vivo mediates neuropathic pain: Christoforos Tsantoulas, Lan Zhu, Yasin Shaifta, John Grist, Jeremy P. T. Ward, Ramin Raouf, Gregory J. Michael, and Stephen B. McMahon. Sensory Neuron Downregulation of the Kv9.1 Potassium Channel Subunit Mediates Neuropathic Pain following Nerve Injury. The Journal of Neuroscience, 28 November 2012, 32(48): 17502-17513; doi: 10.1523/JNEUROSCI.3561-12.2012.
Highlights: Here, we report that the potassium channel subunit Kv9.1 is expressed in myelinated sensory neurons, but is absent from small unmyelinated neurons. Kv9.1 expression was strongly and rapidly downregulated following axotomy, with a time course that matches the development of spontaneous activity and pain hypersensitivity in animal models. Interestingly, siRNA-mediated knock-down of Kv9.1 in naive rats led to neuropathic pain behaviors. Diminished Kv9.1 function also augmented myelinated sensory neuron excitability, manifested as spontaneous firing, hyper-responsiveness to stimulation, and persistent after-discharge. Intracellular recordings from ex vivo dorsal root ganglion preparations revealed that Kv9.1 knock-down was linked to lowered firing thresholds and increased firing rates under physiologically relevant conditions of extracellular potassium accumulation during prolonged activity. Similar neurophysiological changes were detected in animals subjected to traumatic nerve injury and provide an explanation for neuropathic pain symptoms, including poorly understood conditions such as hyperpathia and paresthesias. In summary, our results demonstrate that Kv9.1 dysfunction leads to spontaneous and evoked neuronal hyperexcitability in myelinated fibers, coupled with development of neuropathic pain behaviors.
n vivo RNA interference: Anesthetized rats were subjected to a thoracic laminectomy and a silastic tube was inserted subdurally to lie just rostral to L3 DRG and externalized to deliver bolus injections (one injection per day for 4 consecutive days). Animals were allowed to recover for 5 d before treatment commenced. On the day of injection, siRNA was mixed with i-Fect (Neuromics) to a final concentration of 0.2 μg μl−1, according to published protocols (Luo et al., 2005). For each treatment, 10–20 μl of Kv9.1 siRNA or scrambled control mixture was injected, followed by a 10 μl saline flush. Twenty-four hours after the fourth injection animals were killed and L5 DRGs fresh dissected for qRT-PCR analysis. A separate set of animals were PFA perfused and DRGs retrieved for IHC. Passenger strand sequences for Kv9.1 and scrambled control siRNAs were cuuggaaucuguaggauca and gaggcctaatcgatatgtt, respectively (Dharmacon; “in vivo processing” option).
Kv9.1 knock-down triggers ectopic activity and a form of peripheral wind-up in response to stimulation. A, Schematic illustrating the positions of stimulating and recording electrodes. B, Example recordings from centrally disconnected L4/L5 strands demonstrating SA in Kv9.1 siRNA-treated or nerve-injured rats, but not in control (scrambled siRNA) animals. C, Frequency-dependent SEA (denoted by double arrowheads) in Kv9.1 siRNA-treated (middle) and injured (right), but not control (left) animals. This activity is not locked in time and can be seen in between stimulation events (vertical arrows on top of 5 Hz stimulation traces, only first 5 shown). Also note the prolonged after-discharge (AD) observed in siRNA-treated and injured animals. D, Percentage of units showing SA and SEA in control (n = 269), Kv9.1 siRNA-treated (n = 369) and injured (n = 176) animals (*p < 0.05, **p < 0.01, ***p < 0.001 vs control, χ2 test). E, Firing rate of SEA units at different stimulation frequencies (mean ± SEM; control, n = 4; siRNA, n = 22; injured, n = 17; *p < 0.05 vs control, two-way ANOVA with Tukey's). F, Quantification of AD rate per SEA unit (mean ± SEM; *p < 0.05 vs control, Mann–Whitney test).
Results propose that Kv9.1 downregulation after nerve injury may be the molecular switch controlling myelinated sensory neuron hyperexcitability. Intriguingly, a recent wide-genome association screen in humans identified a Kv9.1 polymorphism associated with susceptibility to develop chronic neuropathic pain after back surgery or leg amputation (Costigan et al., 2010), suggesting that the mechanisms described in our studies will be of direct clinical relevance to human pain. Future efforts to elucidate the precise pathways involved, combined with approaches aiming to compensate loss of Kv9.1 function, may create novel therapeutic opportunities for neuropathic pain management.