Inhibition of 5-HT(3) receptors-activated currents by cannabinoids in rat trigeminal ganglion neurons.

This study investigated the modulatory effect of synthetic cannabinoids WIN55,212-2 on 5-HT(3) receptor-activated currents (I(5-HT3)) in cultured rat trigeminal ganglion (TG) neurons using whole-cell patch clamp technique. The results showed that: (1) The majority of examined neurons (78.70%) were sensitive to 5-HT (3-300 μmol/L). 5-HT induced inward currents in a concentration-dependent manner and the currents were blocked by ICS 205-930 (1 μmol/L), a selective antagonist of the 5-HT(3) receptor; (2) Pre-application of WIN55,212-2 (0.01-1 μmol/L) significantly inhibited I(5-HT3) reversibly in concentration-dependent and voltage-independent manners. The concentration-response curve of 5-HT(3) receptor was shifted downward by WIN55,212-2 without any change of the threshold value. The EC(50) values of two curves were very close (17.5±4.5) μmol/L vs. (15.2±4.5) μmol/L and WIN55,212-2 decreased the maximal amplitude of I(5-HT3) by (48.65±4.15)%; (3) Neither AM281, a selective CB1 receptor antagonist, nor AM630, a selective CB2 receptor antagonist reversed the inhibition of I(5-HT3) by WIN55,212-2; (4) When WIN55,212-2 was given from 15 to 120 s before 5-HT application, inhibitory effect was gradually increased and the maximal inhibition took place at 90 s, and the inhibition remained at the same level after 90 s. We are led to concluded that-WIN55,212-2 inhibited I(5-HT3) significantly and neither CB1 receptor antagonist nor CB2 receptor antagonist could reverse the inhibition of I(5-HT3) by WIN55,212-2. Moreover, WIN55,212-2 is not an open channel blocker (OCB) of 5-HT(3) receptor. WIN55,212-2 significantly inhibited 5-HT-activated currents in a non-competitive manner. The inhibition of I(5-HT3) by WIN55,212-2 is probably new one of peripheral analgesic mechanisms of WIN55,212-2, but the mechanism by which WIN55,212-2 inhibits I(5-HT3) warrants further investigation.