Mechanism of block by fluoxetine of 5-hydroxytryptamine3 (5-HT3)-mediated currents in NCB-20 neuroblastoma cells

The effect of fluoxetine (Prozac) on 5-hydroxytryptamine(3) (5-HT3)-mediated currents in NCB-20 neuroblastoma cells was examined using the whole-cell patch-clamp technique. Fluoxetine produced a significant reduction of peak amplitude without altering the activation time course of 5-HT3-mediated currents. These effects were concentration-dependent, with an IC50 value of 4.15 muM. No voltage dependence was evident in fluoxetine's block of 5-HT3-mediated currents over the entire voltage range tested. The extent of block by pre-application of fluoxetine was significantly greater than that by co-application. Fluoxetine also increased the apparent rate of current desensitization to 5-HT application. Using a first-order kinetics analysis, the open-channel blocking rate constants were 0.06 muM(-1) s(-1) (k(+1), association rate constant) and 0.05 s(-1) (k(-1), dissociation rate constant), with an apparent K-d (=k(-1)/k(+1)) of 0.83 muM. This value is close to an IC50 of 1.11 muM obtained from the reduction in tau, the time constant of desensitization. Intracellular application of fluoxetine for long durations had no effect on the amplitude or kinetics of 5-HT3-mediated currents. Similarly, norfluoxetine, the major metabolite of fluoxetine, reduced the peak current, and enhanced the rate of current desensitization in a concentration-dependent manner with an IC50 of 2.66 muM, indicating that norfluoxetine is more potent than fluoxetine in blocking 5-HT3-mediated currents. These results indicate that, at clinically relevant concentrations, fluoxetine and its metabolite, norfluoxetine, block 5-HT3-mediated currents in NCB-20 neuroblastoma cells. (C) 2003 Elsevier Inc. All rights reserved.