Possible binding of amines at the acidic pocket of ASIC1a. A and B, X-ray structures show interactions of Phe174 with psalmotoxin (A, PDB code 4fz0) and amiloride (B, PDB code 4ntx). C and D, putative binding modes of histamine. Aromatic moiety binds to ASIC1a-specific His173, whereas aminogroup can reach Asp349 (C) or Glu242 (D).