ATP-gated ionotropic receptors (P2X receptors) are distributed widely in the nervous system. For example, a hetero-oligomeric receptor containing both P2X2 and P2X3 subunits is involved in primary afferent sensation. Each subunit has two membrane-spanning domains. We have used disulfide bond formation between engineered cysteines to demonstrate close proximity between the outer ends of the first transmembrane domain of one subunit and the second transmembrane domain of another. After expression in HEK 293 cells of such modified P2X2 or P2X4 subunits, the disulfide bond formation is evident because an ATP-evoked channel opening requires previous reduction with dithiothreitol. In the hetero-oligomeric P2X2/3 receptor the coexpression of doubly substituted subunits with wild-type partners allows us to deduce that the hetero-oligomeric channel contains adjacent P2X3 subunits but does not contain adjacent P2X2 subunits. The results suggest a "head-to-tail" subunit arrangement in the quaternary structure of P2X receptors and show that a trimeric P2X2/3 receptor would have the composition P2X2(P2X3)2.

We have cloned P2X4, a member of the P2-purinoceptor family, which has a new pharmacological profile. Rat P2X 4 is distantly related to P2XI, P2X2 and P2X 3 and is expressed in brain, spinal cord, lung, thymus, bladder, adrenal, testis and vas deferens. This ligand gated ion channel is activated by ATP and analogs with the potency order ofATP > ATP3,S > 2-methylthio ATP > ADP = ai~-methylene ATP. However, none of the currently used P2X purinoceptor antagonists suramin, reactive blue 2 and PPADS blocked ATP evoked currents; in contrast their application resulted in potentiation of the agonist response. Due to lack of any known antagonist for P2X 4 it is unlikely that native P2X 4 has previously been recognized as a P2X purinoceptor.

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We constructed a full-length human P2X 5 purinoceptor cDNA by incorporating a sequence corresponding to exon 10, which is missing in cDNAs cloned previously from human tissues. We studied the functional properties by patch-clamp recording and fluorescence imaging after expression in human embryonic kidney 293 cells. ATP (1-100 M; half-maximal current at 4 M) elicited inward currents at Ϫ60 mV; these persisted during brief (2 s) applications but declined during longer applications. The peak current was dependent on the holding potential and showed little rectification; however, both the desensitization during the application and the decline in the current when ATP was washed out were slower at ϩ30 mV than at Ϫ60 mV. 2Ј,3Ј-O-(4-Benzoyl)-benzoyl-ATP and ␣␤-methylene-ATP mimicked the action of ATP (half-maximal concentrations 6 and 161 M, respectively). The currents were inhibited by suramin, pyridoxal-5-phosphate-6-azo-2Ј,4Ј-disulfonic acid and Brilliant Blue G, with half-maximal inhibition at 3, 0.2, and 0.5 M, respectively; 2Ј,3Ј-O-(2Ј,4Ј,6Ј-trinitrophenol)-ATP (1 M) was ineffective. Removing divalent cations did not significantly alter ATP concentration-response curves. Reversal potential measurements showed that the human P2X 5 receptor was permeable to calcium (P Ca /P Na ϭ 1.5) and N-methyl-D-glucamine (NMDG) (P NMDG /P Na ϭ 0.4); it was also permeable to chloride (P Cl /P Na ϭ 0.5) but not gluconate (P gluc /P Na ϭ 0.01) ions. The permeability to NMDG developed as quickly as the channel opened, in contrast to the P2X 7 receptor where the NMDG permeability develops over several seconds. Cells expressing human P2X 5 receptors also rapidly accumulated the propidium dye YO-PRO-1 in response to ATP.A P2X 5 subunit cDNA was first isolated from libraries of rat sympathetic ganglia (Collo et al., 1996) and heart (Garcia-Guzman et al., 1996). The RNA is expressed predominantly in heart (Garcia-Guzman et al., 1996) but there is also expression in brain, spinal cord, and adrenal gland (GarciaGuzman et al., 1996). Immunoreactivity for P2X 5 subunits has also been described in developing skeletal muscle of the rat (Meyer et al., 1999;Ryten et al., 2001); recently, the receptor has been implicated in the differentiation of satellite cells into mature multinucleated muscle fibers (Ryten et al., 2002). Mammalian skin and endocrine (Glass and Burnstock, 2001) cells also show P2X 5 immunoreactivity.When expressed in HEK293 cells, the rat P2X 5 subunit cDNAs gave rise to currents activated by ATP, but these were very small and their properties not extensively studied. The current was elicited by ATP [half-maximal concentration (EC 50 ) about 10 M] but not by ␣␤meATP, and it was readily blocked by suramin and PPADS in the 1 to 10 M concentration range (Collo et al., 1996;Garcia-Guzman et al., 1996). A mouse cDNA has also been cloned and expressed; again, the currents recorded from HEK293 cells were only a few tens of picoamperes and they were not studied in detail . On the other hand, although these two mammalian recep...