Spinal dorsal horn neurons involved in the alleviating effects of cannabinoid receptor agonists on neuropathic allodynia-like behaviors in rats

Mechanical allodynia, the pain caused by innocuous tactile stimuli, is a hallmark symptom of neuropathic pain that is often resistant to currently available treatments. Cannabinoids are widely used for pain management; however, their therapeutic mechanisms for neuropathic mechanical allodynia remain unclear. Using transgenic rats that enable to optogenetically stimulate touch-sensing Aβ fibers in the skin, we found that the intrathecal administration of the synthetic cannabinoid, WIN 55,212-2, alleviated the Aβ fiber-derived neuropathic allodynia. Furthermore, we injected adeno-associated virus vectors incorporating the rat cannabinoid receptor 1 (CB₁ receptor) (encoded by Cnr1) promoter and tdTomato or short hairpin RNA targeting the CB₁ receptor into the spinal dorsal horn (SDH) and demonstrated that the conditional knockdown of CB₁ receptors in Cnr1⁺ SDH neurons attenuates the anti-allodynic effects of intrathecally administered WIN 55,212-2. Electrophysiological analysis revealed that Cnr1⁺ SDH neurons received excitatory synaptic inputs from the primary afferent Aβ fibers. Collectively, our results suggest that the CB₁ receptors in Cnr1⁺ SDH neurons are molecular and cellular targets of intrathecal WIN 55,212-2 to alleviate neuropathic allodynia.