Ambivalent Effects on P2X3 Receptors in Rat Sensory Neurons in the Presence of Opioid Receptor Antagonists

In cultured neurons of the dorsal root ganglia (DRGs) of rats, it was found that incubation of these neurons in the presence of 50 nM naloxone (an opioid receptor antagonist, OR antagonist) significantly increased the efficacy of blocking of P2X3-mediated transmembrane currents induced by application of an endogenous opioid, Leu-enkephalin (LEK); the above currents were evoked by application of α,β-Me-ATP. Under conditions of preliminary 2- to 4-min-long incubation of neurons with naloxone, 10 nM LEK completely blocked P2H3 currents (blocking developed within 2 min). Under control conditions (without incubation), the respective effect was observed only at much higher LEK concentrations (1 μM). Thus, the IC50 of inhibitory LEK action was reduced in this case from 10 nM to 1 nM. Naloxone in high concentrations (1 μM), when applied after the development of the LEK inhibitory effect, exerted a dissimilar influence on the amplitude of P2X3 currents. For a time (within 2-4 min), this amplitude exceeded the control values; later on, it returned to the initial value. Such ambivalent effect of the opioid peptide (LEK) is related to the fact that G-proteins, when binding to the ORs, can form complexes in two conformations, inhibitory (Gi/o) and stimulating (Gq/s). It is known that naloxone can increase the analgesic effect of morphine due to direct competitive antagonism with the Gq/s-bound conformation. Thus, ambivalent effects of opioids on P2X3 receptors are explained by an antagonist-induced shift of the equilibrium between conformations of G-protein complexes.