Purinergic signalling: Sensitisation of recombinant P2X receptors

ATP acts as a fast neurotransmitter for a family of ligand-gated ion channels (P2X receptors) in the central and peripheral nervous systems. The potential for H+, Zn2+ and diadenosine polyphosphates to modify purinergic signalling was investigated on ATP-evoked inward currents at recombinant P2X1-4 receptors expressed in Xenopus oocytes, under voltage-clamp conditions. These test substances are contained in synaptic vesicles and, accordingly, are co-released with ATP. The effects of H+ differed at P2X1-4 receptors. An increasingly acidic superfusate (pH 7.5-5.5) increased ATP-potency at the P2X2 receptor whereas agonist potency was reduced at P2X1,3,4 receptors. Potentiation of ATP-responses at the P2X2 receptor by various neurotransmitters and related substances was, in many cases, an effect entirely due to a change in pH of the bathing medium (i.e. drugs were weak acids). The effect of a few substances (substance P, CGRP and arachidonic acid) was pH-independent. The effects of Zn2+ were also different for P2X1-4 receptors. Zn2+ inhibited ATP responses at the P2X1 receptor in a time-dependent but pH-independent manner. Zn2+ potentiated ATP-responses at P2X2,3,4 receptors. The effects of Zn2+ on ATP- responses were similar at P2X2 and P2X3 receptors - a time-dependent potentiation which, with prolonged incubation periods, was replaced by an inhibitory action at high concentrations. The effects of Zn2+ were pH-dependent at the P2X2 receptor but not the P2X3 receptor. Zn2+ potentiation of ATP responses at the P2X4 receptor was pH- dependent but time-independent. Diadenosine polyphosphates (ApnA, n=2-6) were either agonists, potentiators or inactive at P2X1-4 receptors. Ap6A alone was a full agonist at the P2X1 receptor. Ap4A alone was a full agonist at the P2X2 receptor. Ap4A, Ap5A and Ap6A were all full agonists at the P2X3 receptor. None of the dinucleotides were foil agonists at the P2X4 receptor. Potentiators included: Ap5A (P2X2); Ap2A (P2X3); Ap2A and Ap3A (P2X4). In conclusion, P2X1-4 receptors were found to be differentially sensitive to H+, Zn2+ and adenine dinucleotides which, separately or together, failed to elicit a common effect. It is evident from these data that H+, Zn2+ and adenine dinucleotides do have the potential to modulate purinergic signalling in a variety of ways which will be dependent on the subunit assembly of native P2X receptors. The results of this work may offer a means of identifying P2X subunits present in native P2X receptors