The antidepressant fluoxetine but not citalopram suppresses synapse formation and synaptic transmission between Lymnaea neurons by perturbing presynaptic and postsynaptic machinery

Abstract

Depression is a debilitating mental disorder, and selective serotonin reuptake inhibitors (SSRIs) constitute the first-line antidepressant treatment choice for the clinical management of this illness; however, the mechanisms underlying their therapeutic actions and side effects remain poorly understood. Here, we compared the effects of two SSRIs, fluoxetine and citalopram, on synaptic connectivity and the efficacy of cholinergic synaptic transmission between identified presynaptic and postsynaptic neurons from the mollusc Lymnaea. The in vitro paired cells were exposed to clinically relevant concentrations of the two SSRIs under chronic and acute experimental conditions, and the incidence of synapse formation and the efficacy of synaptic transmission were tested electrophysiologically and with fluorescent Ca(2+) imaging. We demonstrate that chronic exposure to fluoxetine, but not to citalopram, inhibits synapse formation and reduces synaptic strength, and that these effects are reversible following prolonged drug washout. At the structural level, we demonstrate that fluoxetine, but not citalopram, prevents the expression and localization of the presynaptic protein synaptophysin. Acute exposure to fluoxetine substantially reduced synaptic transmission and synaptic plasticity (post-tetanic potentiation) in established synapses, whereas citalopram reduced synaptic transmission, but not short-term synaptic plasticity. We further demonstrate that fluoxetine, but not citalopram, directly inhibits voltage-gated Ca(2+) currents in the presynaptic neuron, as well as postsynaptic responsiveness to exogenously applied neurotransmitter. This study provides the first direct evidence that fluoxetine and citalopram exert characteristic, non-specific side effects that are unrelated to their function as SSRIs, and that fluoxetine is more detrimental to synaptic physiology and structure than citalopram.

Failure of delayed nonsynaptic neuronal plasticity underlies age-associated long-term associative memory impairment

“…It should be noted however that recent studies suggest that fluoxetine’s spectrum of pharmacological activities in the Lymnaea nervous system extends beyond the compound’s “classical SSRI activity. [26,49]. Therefore, further studies are required to examine the mechanisms through which fluoxetine ameliorates the effects of age on neural- and behavioural plasticity in this model system.…”

Antidepressant Pharmacotherapy – Do the Benefits Outweigh the Risks?

“…These trophic factor-mediated intracellular Ca 2+ oscillations required for excitatory synapse formation were inhibited in the presence of fluoxetine, indicating that fluoxetine prevents the expression of proteins involved in synaptogenesis (Xu et al, 2010). Attesting to this, we found that the expression and synaptic localization of synaptophysin, a synaptic vesicle-associated protein and presynaptic biomarker, were significantly reduced in synapses formed in the presence of fluoxetine, but not citalopram (Getz et al, 2011). Importantly, we also demonstrated that the inhibitory effect of fluoxetine on synapse formation is reversible following prolonged drug washout, and that this recovery is dependent on protein synthesis -that is, new synapses developed following the removal of fluoxetine.…”

“…Furthermore, ratiometric Ca 2+ imaging revealed that the influx of Ca 2+ during action potential burst firing was significantly reduced by both fluoxetine and citalopram, suggesting that a loss of depolarizing driving force (i.e. through the inhibition of sodium and/or potassium channels and the development of action potential clamping) and subsequent indirect reduction in Ca 2+ entry is most likely responsible for the citalopram-mediated reduction of synaptic transmission (Getz et al, 2011). In Lymnaea neurons, the mechanism underlying the short-term plasticity of PTP has been shown to involve presynaptic Ca 2+ entry through voltage-gated calcium channels and subsequent activation of CaMKII, a protein kinase that mediates the effects of intracellular signalling cascades (Luk et al, 2011).…”

“…These pharmacological characteristics lent to the formulation of our hypothesis that different SSRIs would exert characteristic non-specific neuronal side effects, and, because of its lesser selectivity profile, that fluoxetine would exert a more detrimental effect on synaptic physiology than citalopram. Using a combination of intracellular and patch-clamp electrophysiological recordings, calcium imaging, immunocytochemistry and various neuroimaging techniques, we have for the first time demonstrated that clinically-relevant concentrations of fluoxetine, but not citalopram (dosage range up to 5 μg/mL; Bolo et al, 2000), exhibit detrimental effects on neurite outgrowth, synapse formation, synaptic transmission and synaptic plasticity, and that the negative effects of fluoxetine on synaptic function involves a direct perturbation of both preand post-synaptic machinery (Xu et al, 2010;Getz et al, 2011).…”

Studies of Long-Term Use of Antidepressants

“…Most of the discussion regarding the symptoms seen after antidepressants are withdrawn, known as the antidepressant discontinuation syndrome, have focused on psychiatric and physical symptoms such as irritability, anxiety, agitation, sweating, tremor or trembling, insomnia, mouth movements, gastrointestinal symptoms and others. [60] Chronic fluoxetine has been reported to reduce the number of synapses in the brains of rats. However, this is a product of the preconception that a de-pressive syndrome would represent a relapse or recurrence, not withdrawal.…”