The effect of 9-tetrahydrocannabinol on 5-HT3 receptors depends on the current density

The effects of 9-tetrahydrocannabinol (THC), the psychoactive component of cannabis, on the function of 5-HT type 3 (5-HT3) receptors were investigated using a two-electrode voltage clamp technique in Xenopus oocytes, and a whole-cell patch clamp technique in rat nodose ganglion neurons. In oocytes injected with 3 ng cRNA of 5-HT3A receptor, THC reversibly inhibited currents evoked with 5-HT (1 mu M) in a concentration-dependent manner (IC50=1.2 mu M). The extent of THC inhibition was inversely correlated with the amount of cRNA injected and the mean 5-HT3A receptor current densities. Pretreatment with actinomycin D, which inhibits transcription, decreased the mean 5-HT3 receptor current density and increased the extent of THC inhibition on 5-HT3 receptor-mediated currents. The IC50 values for THC increased from 285 nM to 1.2 mu M in oocytes injected with 1 and 3 ng of 5-HT3A cRNA, respectively. In radioligand binding studies on membrane preparations of oocytes expressing 5-HT3A receptors, THC did not alter the specific binding of a 5-HT3A receptor antagonist, [3H]GR65630. In the presence of 1 mu M THC, the maximum 5-HT-induced response was also inhibited without a significant change in 5-HT potency, indicating that THC acts as a noncompetitive antagonist on 5-HT3 receptors. In adult rat nodose ganglion neurons, application of 1 mu M THC caused a significant inhibition of 5-HT3 receptors, extent of which correlated with the density of 5-HT-induced currents, indicating that the observed THC effects occur in mammalian neurons. The inhibition of 5-HT3 receptors by THC may contribute to its pharmacological actions in nociception and emesis.