Aquaporin 4 is involved in chronic pain but not acute pain

Accumulating evidence has revealed that spinal glia plays an important role in the processing of pain, particularly chronic pain. Aquaporin 4 (AQP4), the predominant water channel exists in astrocytes, has been proved to modulate astrocytic function and thus participate in many diseases of the central nervous system. However, there is still controversy over whether AQP4 is involved in pain modulation. In the present study, we investigated the effects of AQP4 on pain by examining chronic inflammatory pain, neuropathic pain, and thermal, chemical, and mechanical stimuli-induced acute pain in AQP4 knockout mice. In Complete Freund's adjuvant-induced chronic inflammatory pain and spared nerve injury-induced neuropathic pain models, AQP4^-/- mice attenuated pain-related behavioral responses compared with AQP4^+/+ mice, demonstrating that AQP4 deficiency relieved chronic inflammatory pain and neuropathic pain. In the tail-flick and hot-plate tests, two acute pain models of thermal stimuli, no differences in pain-related behaviors were detected between AQP4^+/+ and AQP4^-/- mice. In the formalin and capsaicin tests, two models of chemical stimuli-induced acute pain, no differences in the durations of licking the injected hindpaw were found between AQP4^+/+ and AQP4^-/- mice. In the von Frey hair test, a model of mechanical stimuli-induced acute pain, no significant differences in withdrawal thresholds were found between these two genotypes mice as well. These results indicated that AQP4 deficiency did not affect acute pain induced by thermal, chemical, and mechanical stimuli. Taken together, our findings suggested that AQP4 contributes to chronic pain, but not acute pain.