Adenosine 5′-tetraphosphate (Ap4), a new agonist on rat midbrain synaptic terminal P2 receptors

The aim of this study was to sec whether the compound adenosine 5'-tetraphosphate (Ap(4)) is active in the central nervous system, by examining its effect on isolated rat brain synaptic terminals, Ap(4) proved to be more resistant to ecto-enzymatic hydrolysis than adenosine triphosphate (ATP), showing only 2% hydrolysis after a 2-min incubation, compared to 75% for ATP. In addition, Ap(4) was able to produce concentration-dependent increases in intracellular Ca2+ when applied extracellularly. This action was dependent upon the presence of extracellular calcium. Ap(4) acts through ionotropic ATP receptors (P2X receptors) and not through diadenosine polyphosphate receptors, since ATP abolished the response elicited by Ap(4) whereas Ap(5)A did not. Ap(4), ATP and ATP-gamma-S were of similar potency (EC50 approximate to 20 mu M) while 2MeSATP, alpha,beta-meATP and ADP-beta-S possessed slightly lower potency (EC50 approximate to 50 mu M). The P2-purinoceptor antagonists suramin and PPADS blocked the Ap(4) effect. The IC50 values for these compounds were 35.5 and 7.8 mu M respectively. Diinosine polyphosphates and inosine tetraphosphate inhibited the response elicited by Ap(4) with IC50 values that varied between approximately 40 and 50 mu M. These results show that Ap(4) is as good an agonist as ATP on synaptosomal P2X receptors, being more resistant to extracellular hydrolysis by ecto-nucleotidases. (C) 2000 Elsevier Science Ltd. All rights reserved.