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Abstract
The 5-HT3 receptor, a ligand-gated ion channel, has been proposed to be involved in brain mechanisms associated with drug abuse (Costall et al., Pharmac. Ther. 47: 181, 1990). In the present study, the effect of cocaine on the activation of 5-HT3 receptors expressed in Xenopus oocytes was investigated. Xenopus oocytes were injected with the mRNA transcripts from the cloned 5-HT3 receptor (Maricq et al., 254: 432, Science 1991) and voltage-clamped at −70 mV. 5-HT and the selective 5-HT3 receptor agonists, 2-methyl-5-HT and m-chloro-phenylbiguanide activated a fast inward current which was blocked by the specific 5-HT3 receptor antagonist LY278584. Cocaine (0.1 to 10 μM) inhibited the current activated by 1 μM 5-HT in a concentration-dependent manner. The IC50 value is 0.5 μM and the apparent Hill coefficient is 1.42. This action of cocaine could be overcome by increasing 5-HT concentration, suggesting a competitive inhibition of 5-HT by cocaine. The results are similar to previous observations suggesting that cocaine competitively antagonizes the action of 5-HT at neuronal 5-HT3 receptors (Fan et al., Soc. Neurosci. Abst. 18: 800, 1992)
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