miR-7a alleviates the maintenance of neuropathic pain through regulation of neuronal excitability

Neuronal damage in the somatosensory system causes intractable chronic neuropathic pain. Plastic changes in sensory neuron excitability are considered the cellular basis of persistent pain. Non-coding microRNAs modulate specific gene translation to impact on diverse cellular functions and their dysregulation causes various diseases. However, their significance in adult neuronal functions and disorders is still poorly understood. Here, we show that miR-7a is a key functional RNA sustaining the late phase of neuropathic pain through regulation of neuronal excitability in rats. In the late phase of neuropathic pain, microarray analysis identified miR-7a as the most robustly decreased microRNA in the injured dorsal root ganglion. Moreover, local induction of miR-7a, using an adeno-associated virus vector, in sensory neurons of injured dorsal root ganglion, suppressed established neuropathic pain. In contrast, miR-7a overexpression had no effect on acute physiological or inflammatory pain. Furthermore, miR-7a downregulation was sufficient to cause pain-related behaviours in intact rats. miR-7a targeted the beta 2 subunit of the voltage-gated sodium channel, and decreased miR-7a associated with neuropathic pain caused increased beta 2 subunit protein expression, independent of messenger RNA levels. Consistently, miR-7a overexpression in primary sensory neurons of injured dorsal root ganglion suppressed increased beta 2 subunit expression and normalized long-lasting hyperexcitability of nociceptive neurons. These findings demonstrate miR-7a downregulation is causally involved in maintenance of neuropathic pain through regulation of neuronal excitability, and miR-7a replenishment offers a novel therapeutic strategy specific for chronic neuropathic pain.