Experimental and Modeling Studies of Desensitization of P2X sub(3) Receptors

The function of ATP-activated P2X sub(3) receptors involved in pain sensation is modulated by desensitization, a phenomenon poorly understood. The present study used patch-clamp recording from cultured rat or mouse sensory neurons and kinetic modeling to clarify the properties of P2X sub(3) receptor desensitization. Two types of desensitization were observed, a fast process (t sub(1/2) = 50 ms; 10 mu M ATP) following the inward current evoked by micromolar agonist concentrations, and a slow process (t sub(1/2) = 35 s; 10 nM ATP) that inhibited receptors without activating them. We termed the latter high-affinity desensitization (HAD). Recovery from fast desensitization or HAD was slow and agonist-dependent. When comparing several agonists, there was analogous ranking order for agonist potency, rate of desensitization and HAD effectiveness, with 2-methylthioadenosine triphosphate the strongest and beta , gamma -methylene-ATP the weakest. HAD was less developed with recombinant (ATP IC sub(50) = 390 nM) than native P2X sub(3) receptors (IC sub(50) = 2.3 nM). HAD could also be induced by nanomolar ATP when receptors seemed to be nondesensitized, indicating that resting receptors could express high-affinity binding sites. Desensitization properties were well accounted for by a cyclic model in which receptors could be desensitized from either open or closed states. Recovery was assumed to be a multistate process with distinct kinetics dependent on the agonist-dependent dissociation rate from desensitized receptors. Thus, the combination of agonist-specific mechanisms such as desensitization onset, HAD, and resensitization could shape responsiveness of sensory neurons to P2X sub(3) receptor agonists. By using subthreshold concentrations of an HAD-potent agonist, it might be possible to generate sustained inhibition of P2X sub(3) receptors for controlling chronic pain.