Ivermectin (IVM) applied extracellularly increases the sensitivity of P2X(4) receptor (P2X(4)R) to ATP, enhances the maximum current amplitudes, and greatly prolongs the deactivation kinetics. In this manuscript, we focused on identification of receptor-specific residues responsible for IVM effects on channel gating using the wild-type rat homomeric P2X(4)R, several chimeric P2X(2)/P2X(4) receptors, and single-point P2X(4)R-specific mutants in the ectodomain and two transmembrane domains. Experiments with chimeric receptors revealed that the Val(49)-Val(61) but not the Val(64)-Tyr(315) ectodomain sequence is important for the effects of IVM on channel deactivation. Receptor-specific mutations placed in the Gly(29)-Val(61) and Asp(338)-Leu(358) regions showed the importance of Trp(50), Val(60), and Val(357) residues in IVM regulation of the rate of channel deactivation, but not on the maximum current amplitude. These results suggest that the transmembrane domains and the nearby ectodomain region contribute to the effects of IVM on channel deactivation. (c) 2006 Elsevier Inc. All rights reserved.