Expression and function of P2X purinoceptors in rat histaminergic neurons

1 The pharmacology of ATP responses and the expression pattern of seven known subunits of the P2X receptor were investigated in individual histaminergic neurons of the tuberomamillary nucleus (TM). 2 ATP (3-1000 muM) evoked fast non-desensitizing inward currents in TM neurons. 2-methylthioATP (2MeSATP) displayed the same efficacy but a lower potency, EC(50)s 84 muM versus 48 pm, when compared with ATP. Adenosine-diphosphate (ADP), uridine-triphosphate (UTP) and alpha beta methylene-ATP (alphabeta-meATP) were inactive. 3 ATP-mediated whole cell currents were potentiated by acidification of the recording solution (pH 7.5 and 6.6 were compared). 4 Single-cell RT-PCR (scRT-PCR) analysis revealed that the P2X(2) receptor is expressed in all PCR-positive neurons. Each of the P2X(1), P2X(3), P2X(4), P2X(5) and P2X(6) mRNAs were detected in less than 35% of the cells. 5 Suramin antagonized ATP responses with an IC50 of 4.2 muM and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 1 mum) reduced ATP responses to 43% of control, when antagonists were pre-applied 90s before the agonist. Cibacron blue (3 muM) given together with ATP potentiated control responses by 67%, but inhibited it to 10% after pre-application. 6 2',3'-O-(2,4,6-Trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) antagonized ATP responses with an IC50 of 7 muM. 7 Pharmacological properties of ATP responses together with scRT-PCR data suggest that P2X2 is the major purinoceptor on the soma of TM neurons, however the presence of heteromeric P2X2/5 receptors in some neurons cannot be excluded. British Journal of Pharmacology (2003).