Activation of ASIC3 in nociceptors induces itch without overt pain in a novel mouse model of acid-evoked itch.
OBJECTIVE: Acid-sensing ion channel 3 (ASIC3), a proton-gated cation channel predominantly expressed in primary afferent nociceptors, is an acidosis-related pain generator. Previous experiments suggested that ASIC3 is also involved in the generation of itch. However, mechanistic links between ASIC3 and itch, including the expression of ASIC3 in itch-mediating primary sensory neurons, remain unclear. We examined ASIC3 expression in these sensory neurons and then investigated whether mild acid stimulation could induce ASIC3-dependent itch without overt pain in mice. METHODS: Immunohistochemical analyses were performed using ASIC3-FLAG-enhanced green fluorescent protein-FLAG (FEF) expressing mice. Citric acid was applied with a brush to shaved skin of the nape of the neck or cheek in wild-type and ASIC3 knockout (ASIC3 -/- ) mice. Hindlimb scratching and, in the cheek model, forelimb facial wiping were recorded. RESULTS: ASIC3-expressing neurons and plexin C1-positive/tachykinin 1-negative itch-mediating neurons essentially belonged to distinct subpopulations in dorsal root and trigeminal ganglia. Application of 0.2 M citric acid to the nape induced hindlimb scratching directed toward the citric acid-applied area in wild-type mice, and this response was significantly attenuated in ASIC3 -/- mice. Application of 0.5 M citric acid to the cheek induced ASIC3-dependent itch behavior (hindlimb scratching), accompanied by minimal or no pain behavior (forelimb wiping). CONCLUSION: Given the absence of ASIC3 in typical itch-mediating primary sensory neurons, citric acid-induced ASIC3 activation in nociceptive skin afferents primarily involved in pain likely underlies the observed itch behavior. As 0.5 M citric acid likely represents a weak noxious stimulus, weak activation of these pain-mediating afferents can evoke itch, supporting the intensity theory of itch.