Fluoxetine Modulation Of Inhibitory System In Rat Hippocampus

Fluoxetine is a selective serotonin reuptake inhibitor widely used for treating depression.However,its underlying mechanism remains largely unknown.Altered GABAergic function is evident in depressed patients and animal models of depression,which could be normalized by treatment of antidepressants including fluoxetine.Also,it is well-known that GABA agonists have antidepressant effects. Although fluoxetine was found to normalize the GABA deficit during depression,the GABA concentration measured in these studies did not reflect the synaptic GABA concentration.The effects of fluoxetine on GABAergic synaptic neurotransmission remain poorly investigated.Beside its benefit effects,accumulating evidence has noticed that fluoxetine treatment can lead to some adverse-effects,including seizures.Seizures associated with fluoxetine therapy generally occur at higher doses.Less commonly,seizures can also occur at therapeutic doses.Unfortunately,the underlying mechanism remains largely unknown.Fluoxetine has been reported to inhibit the NMDA receptorand enhance GABA type-A receptor activity.However,both of these interactions fail to explain the fluoxetine-associated seizures.The glycine receptor(GlyR),one of the major inhibitory receptors in the adult mammalian central nervous system,has never been investigated as a potential target during fluoxetine therapy.Thus,whole-cell patch clamp recordings from cultured rat hippocampal neurons were conducted to investigate the effects of fluoxetine on GABAergic neurotransmission and GlyR activity.1.Fluoxetine potentiates GABAergic IPSC in rat hippocampal neuronsIn this study,we investigate the effects of fluoxetine on GABAergic neurotransmission.The spontaneous inhibitory postsynaptic current(sIPSC) was completely blocked by 10μM bicuculline and reversibly potentiated by 30μM fluoxetine.The fluoxetine potentiation on either amplitude or frequency of sIPSCs was dose-dependent,with the EC₅₀ values of 10.96 and 14.26μM,respectively.This potentiation was also TTX-insensitive,suggesting independence of presynaptic action potentials.The ritanserin(5μM),a selective 5-HT₂ receptor antagonist,did not alter the fluoxetine potentiation on miniature inhibitory postsynaptic currents.Taken together,our data suggest that fluoxetine can potentiate GABAergic neurotransmission without depending on presynaptic firing of action potentials and its elevating of 5-HT₂ receptor activities.This potentiation by fluoxetine may normalize the hippocampal GABA deficit during depression and in part exert its antidepressant activity.2.Fluoxetine inhibition of GlyR activity in rat hippocampal neuronsIn this study,we examined the effects of fluoxetine on GlyR activity.We found that fluoxetine and its metabolite norfluoxetine inhibited glycine-induced currents in cultured rat hippocampal neurons.This inhibition was dose-dependent,and voltage-independent.Fluoxetine shifted the glycine concentration-response curve to the right without altering the maximal current.Both Lineweaver-Burk and Schild plots suggest competitive inhibition.The amount of fluoxetine inhibition significantly increased when homomeric GlyRs were selectively inhibited with picrotoxin. Moreover,fluoxetine inhibited the current mediated by heteromericα2β-but not homomericα2-GlyRs transiently expressed in HEK293T cells.These results suggest that fluoxetine is a competitive and subtype-selective GlyR inhibitor,which may explain its capacity to induce seizures.