Anales RANF

S1-02 THE ASIC3/P2X3 COGNATE RECEPTOR SENSES ACIDIC AND PURINERGIC PAIN IN A NOVEL MANNER P. Rubini 1 , G. Stephan 1 , H. Lumei 2 , Z. Ying 2 , E. Fabbretti 3 , Y. Tang 2 , P. Illes 1 1 University of Leipzig, Leipzig, Germany; 2 TCM University of Chengdu, Chengdu, China; 3 International School for Advanced Studies, Trieste, Italy. Two subclasses of acid-sensing ion channels (ASIC3) and of ATP-sensitive P2X receptors (P2X3Rs) show a partially overlapping expression in sensory neurons. Although the amino-acid composition of the two receptor-channel types is different, their trimeric structure and ion conductive pathways are similar. Our aim was to clarify whether the ASIC3 and P2X3 subunits can compose a heteromeric channel or whether they build up a multiprotein complex responding to stimulation by protons and ATP in a novel manner. For this purpose we transfected COS cells with the respective plasmids to generate recombinant receptors and investigated dorsal root ganglion neurons in cultures. Patch-clamp measurements indicated that ASIC3 and P2X3Rs interact with each other in that the two receptor-channels have a lower ionic permeability than each channel alone. Further, ASIC3 stimulation strongly inhibited the P2X3R current partly by a Ca 2+ -dependent mechanism. By contrast, the stimulation of P2X3Rs failed to modify the ASIC3 current. The proton-binding site was critical for this effect and the two receptor channels appeared to switch their ionic permeabilities during activation. Co-immunoprecipitation proved the close association of the two protein structures. The injection of either acidic medium or α , β -methylene ATP into the rat paw caused thermal hypersensitivity. Selective ASIC3 antagonists reversed both the acidic and purinergically-induced pain, while selective antagonists for P2X3Rs reversed only this latter pain condition. We conclude that the receptor subunits do not appear to form a heteromeric channel, but tightly associate with each other to form a protein complex, mediating unidirectional inhibition.