Antidepressants are commonly supposed to enhance serotonergic and/or noradrenergic neurotransmission by inhibition of neurotransmitter reuptake through binding to the respective neurotransmitter transporters or through inhibition of the monoamine oxidase. Using the concentration-clamp technique and measurements of intracellular Ca 2 þ , we demonstrate that different classes of antidepressants act as functional antagonists at the human 5-HT 3A receptor stably expressed in HEK 293 cells and at endogenous 5-HT 3 receptors of rat hippocampal neurons and N1E-115 neuroblastoma cells. The tricyclic antidepressants desipramine, imipramine, and trimipramine, the serotonin reuptake inhibitor fluoxetine, the norepinephrine reuptake inhibitor reboxetine, and the noradrenergic and specific serotonergic antidepressant mirtazapine effectively reduced the serotonin-induced Na þ -and Ca 2 þ -currents in a dose-dependent fashion. This effect was voltage-independent and, with the exception of mirtazapine, noncompetitive. Desipramine, imipramine, trimipramine, and fluoxetine also accelerated receptor desensitization. Moclobemide and carbamazepine had no effect on the serotonin-induced cation current. By analyzing analogues of desipramine and carbamazepine, we found that a basic propylamine side chain increases the antagonistic potency of tricyclic compounds, whereas it is abolished by an uncharged carboxamide group. The antagonistic effects of antidepressants at the 5-HT 3 receptor did not correlate with their effects on membrane fluidity. In conclusion, structurally different types of antidepressants modulate the function of this ligand-gated ion channel. This may represent a yet unrecognized pharmacological principle of antidepressants.

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The serotonin type 3 (5-HT 3 ) receptor is the only ligand-gated ion channel receptor for serotonin (5-HT). 5-HT 3 receptors play an important role in modulating the inhibitory action of dopamine in mesocorticolimbic brain regions. Neuroleptic drugs are commonly thought to exert their psychopharmacological action mainly through dopamine and serotonin type 2 (5-HT 2 ) receptors. Except for clozapine, a direct pharmacological interaction of neuroleptics with 5-HT 3 receptors has not yet been described. Using the concentration-clamp technique, we investigated the effects of flupentixol, various phenothiazines, haloperidol, clozapine and risperidone on Na þ -inward currents through 5-HT 3 receptors stably expressed in human embryonic kidney 293 cells, and through endogenous 5-HT 3 receptors of murine N1E-115 neuroblastoma cells. In addition, we studied their effects on Ca 2 þ influx, measured as a change in intracellular Ca 2 þ concentrations ([Ca 2 þ ] i ). All neuroleptic drugs, but not risperidone, antagonized Na þ -and Ca 2 þ -inward currents evoked by 5-HT (10 lM for 2 s and 1 lM, respectively) in a voltage-independent manner. Only clozapine was a competitive antagonist, while all other compounds turned out to be noncompetitive. Fluphenazine and haloperidol affected membrane anisotropy at concentrations below their IC 50 values, indicating that a change in membrane anisotropy might contribute to their antagonistic effect at the 5-HT 3 receptor. Only structure analogues of flupentixol and fluphenazine with a lipophilic side chain were potent antagonists against 5-HT-evoked Na þ and Ca 2 þ currents. Since 5-HT 3 receptors modulate mesolimbic and mesocortical dopaminergic activity, the functional antagonism of neuroleptics at 5-HT 3 receptors may contribute to their antipsychotic efficacy and may constitute a not yet recognized pharmacological principle of these drugs.