Sensitivities to dopamine, 5-hydroxytryptamine, Zn2+ and Cd2+ were studied in P2X1, P2X2, P2X3 and P2X4 purinoceptors and mutants of P2X2 purinoceptors expressed in Xenopus oocytes. Dopamine (10 and 100 microM) and 5-hydroxytryptamine (1 to 100 microM) enhanced the inward current activated by extracellular ATP through P2X2 and P2X4 purinoceptors. Zn2+ (1 to 100 microM) and Cd2+ (10 microM to 1 microM) enhanced the current through P2X2 purinoceptors. As for P2X4 purinoceptors, the ATP-activated current was, however, enhanced after the washout of Zn2+ (100 microM) or Cd2+ (1 mM). Three mutants of P2X2 purinoceptors were constructed by substituting negatively charged amino-acid residues. The magnitude of the enhancement by Zn2+, Cd2+ and dopamine was attenuated when Asp221 was replaced by histidine. The results suggest that dopamine, Zn2+ and Cd2+ require some common motif for the current enhancement.