The effects of fluoxetine (Prozac), a widely used antidepressant drug, and its major metabolite, norfluoxetine, were examined using whole-cell configuration of patch-clamp techniques in primary cultured hippocampal neuron. In whole-cell configuration, low concentration of fluoxetine (3 μM) induced depolarization of resting membrane potential and reduced frequency of action potential. High concentration of fluoxetine (10 μM) eliminated the generation of action potentials. The resting membrane potential was repolarized when fluoxetine was washed out.
Fluoxetine and norfluoxetine accelerated the decay rate of inactivation of whole-cell K+ currents and thus decreased the current amplitude at the end of the pulse in a concentration-dependent manner. Delayed-rectifier K+ currents (KDR) and transient outward currents (KA) were isolated from whole-cell K+ currents using pulse protocol. Fluoxetine decreased KDR and KA currents in a concentration-dependent manner with an IC50 of 4.37 and 5.54 μM, respectively. Norfluoxetine also inhibited KDR and KA currents in a concentration-dependent manner with an IC50 of 0.70 and 0.90 μM, respectively. Fluoxetine and norfluoxetine inhibited the late currents (indicating KDR currents) of whole-cell K+ currents and the areas of KA currents at whole range of tested potential with a voltage dependence (bigger increase at more positive potentials). From the onset of fractional block of KA currents by fluoxetine during the initial 40 msec of a clamp step, we calculated k1 = 34.72 μM-1s-1 for the association rate constant, and k2 = 116.39 s-1 for the dissociation rate constant. The resulting apparent KD was 3.35 μM and is similar to the IC50 value. For norfluoxetine, the k1, k2 and KD value are 53.26 μM-1s-1, 70.24 s-1 and 1.32 μM, respectively.
Our results indicate that fluoxetine and norfluoxetine at therapeutic levels produced a concentration- and voltage-dependent inhibition of KA and KDR currents on primary cultured hippocampal neuron. These could lead to an increase in neuronal excitability and these effects may contribute to the therapeutic antidepressant action of fluoxetine.
Source: Korean Journal of Physiology & Pharmacology.2000 Oct;4(Suppl):S9-S9
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