Reciprocal functional inhibition between P2X and GABA(A/C) receptors represents a novel mechanism fine-tuning neuronal excitability. However, the participating receptors and underlying mechanisms are not fully understood. P2X(4) receptor is widely found in neurons that express GABA(C) rho 1 receptor. Thus, we co-expressed P2X(4) and rho 1 receptors in HEK293 cells and, using patch-clamp recording, examined whether they have Mutual functional inhibition. Currents evoked by simultaneous application of ATP and GABA (IATP+GABA) were significantly smaller compared to the addition of I-ATP and I-GABA. Furthermore, I-ATP were strongly suppressed during rho 1 receptor activation. Similarly, I-GABA were greatly attenuated during P2X(4) receptor activation. Such mutual inhibition was absent in cells only expressing P2X(4) or rho 1 receptor. Taken together, these functional data Support negative cross-talk between P2X(4) and rho 1 receptors. (C) 2008 Elsevier Inc. All rights reserved.