Effect of cocaine on the 5-HT3 receptor-mediated ion current in Xenopus oocytes

The cloned 5-HT3 receptor from NCB-20 neuroblastoma cells was expressed in Xenopus oocytes. In these oocytes, 5-HT, the selective 5-HT3 receptor agonists, 2-methyl-5-HT and m-chlorophenylbiguanide activated an inward current which was sensitive to the specific 5-HT3 receptor antagonist LY278584. Cocaine (0.1 to 10 microM) reversibly inhibited the current activated by 1 microM 5-HT in a concentration-dependent manner. The IC50 value is 0.7 microM and the apparent Hill coefficient is 1.55. This effect of cocaine was not dependent on membrane potential. Cocaine also produced a parallel shift of the 5-HT concentration-response curve to the right and did not reduce the maximal current induced by 5-HT. In the presence of 3 microM cocaine, the EC50 value of 5-HT was increased from 3.08 microM to 6.1 microM. Other local anesthetics such as tricaine and lidocaine also inhibited the current induced by 5-HT. These results suggest that the 5-HT3 receptors expressed in Xenopus oocytes exhibit properties similar to those in sensory neurons and neuroblastoma cells and were blocked by cocaine in a competitive manner.