Purinergic receptor antagonism or ATP degradation prevents sensitization and CHS. (A) The P2X₇ receptor antagonist KN-62 was injected into the pinna of the left ear of WT mice 4 h before sensitization. Mice were sensitized with 3% TNCB in acetone on the left ear and were challenged 5 d later by application of 1% TNCB on the right ear. Data represent the mean ear swelling of groups of five mice ± SD. (B) Mice received apyrase in PBS or PBS as a vehicle control into the left ear pinna at the time of sensitization. Mice were sensitized with 3% TNCB or acetone as a vehicle control on the left ear for sensitization. Challenge was performed with 1% TNCB applied on the right ear 5 d later. The increase of ear thickness was determined 24 h after challenge. Data represent the mean increase in ear thickness ± SD of groups of five mice. (A and B) One representative of three independent experiments is shown (**, P < 0.005; ***, P < 0.001).