Inhibition of ATP-activated currents by met-Enk in isolated DRG neurons of the rat

The present study aimed to explore modulation of ATP-activated currents (I_ATP) by met-Enk in rat DRG neurons. I_ATP were recorded using the whole-cell patch clamp technique. The majority of the neurons examined responded to ATP (90.0%, 45/50) with inward currents. In the 45 ATP sensitive neurons three kinds of responses to application of met-Enk were distinguished: (1) inward currents (29/45), (2) outward currents (9/45), and (3) no effect (7/45). Pretreatment with met-Enk (10^-9-10^-5 mol/L) suppressed I_ATP(10^-4 mol/L) in 29 neurons responding to met-Enk with inward currents. The inhibition by met-Enk of I_ATP could be blocked by naloxone (10^-7 mol/L) in a concentration-dependent manner. Met-Enk of 10^-9, 10^-8, 10^-7, 10^-6 and 10^-5 mol/L suppressed I_ATP by 13.2 ± 5.4% (n = 5); 39.2 ± 8.6% (n = 8),54.1 ± 8.6% (n = 8),43.3 ± 7.9% (n = 7) and 43.1 ± 7.9% (n = 7) (mean ± MSE), respectively. A comparison of concentration - response relations of ATP with and without preapplication of met-Enk indicated that after pretreatment with met-Enk (10^-7 mol/L) the curve shifted downward markedly with a decrease of 25% of the maxmum value of I_ATP and unchanged K_d value. The suppression of I_ATP by met-Enk was reversed as evidenced by intracellular dialysis of H-9 by using the repatch technique. Taken together, it is suggested that the inhibition by met-Enk of I_ATP is caused by activation of opiate receptor, which eventually results in phosphorylation of ATP receptor, mediated by modulation of G protein coupling and intracellular signal transduction.