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

The effect of fluoxetine (Prozac) on 5-hydroxytryptamine₃ (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 4.15μM. 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 0.06μM^-1s^-1 (k₊₁, association rate constant) and 0.05s^-1 (k_-1, dissociation rate constant), with an apparent K_d (=k_-1/k₊₁) of 0.83μM. This value is close to an IC₅₀ of 1.11μM obtained from the reduction in τ, 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 IC ₅₀ of 2.66μM, 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.