Contraindications and drug interaction For SSRIs
One major contraindication of SSRIs is the concomitant use of MAOIs (monoamine oxidase inhibitors). This is likely to cause severe serotonin syndrome/toxidrome.
People taking SSRIs should also avoid taking pimozide (an antipsychotic diphenylbutylpiperidine derivative). Tramadol hydrochloride (or Ultram, Ultracet) can, in rare cases, produce seizures when taken in conjunction with an SSRI or tricyclic antidepressant. Liver impairment is another contraindication for medications of this type.
SSRIs may increase blood levels and risk of toxicities of certain medications:
highly protein-bound medications like warfarin (coumadin) and digoxin
antiarrhythmic agents like propafenone (Rythmol) or flecainide (Tambocor)
beta blockers like metoprolol (Toprol xl) or propranolol (Inderal)
Tricyclic antidepressants like amitriptyline (Elavil, Endep) etc.
triptans like sumatriptan (Imitrex, Imigran) etc.
benzodiazepines like alprazolam (Xanax) or diazepam (Valium)[citation needed]
carbamazepine (Tegretol)
cisapride (Propulsid)
clozapine (Clozaril)
ciclosporin (Neoral)
haloperidol (Haldol)
phenytoin (Dilantin)
pimozide (Orap)
theophylline (Theo-dur)
Certain drugs may increase toxicities of SSRIs:
alcohol and other CNS depressants
methylene blue dye
diuretics (water pills)
MAOIs โ possibly fatal serotonin syndrome/toxidrome
sympathomimetic drugs like pseudoephedrine (Sudafed)
lithium
sibutramine (Meridia)
MDMA (ecstasy)
zolpidem (ambien)[75]
dextromethorphan (cough suppressant) โ increased risk of serotonin syndrome/toxidrome
tramadol (synergistic serotoninergic effect said to increase risk of seizure or serotonin syndrome/toxidrome)
pethidine/meperidine โ increased risk of serotonin syndrome/toxidrome
herbal Saint John's wort or yohimbe โ increased risk of serotonin syndrome/toxidrome
SSRIs also directly interfere with ligands of 5-HT receptors, like the psychedelics and entactogens. SSRIs strongly attenuate the effects of tryptamine psychedelics like psilocybin and LSD, and almost completely eliminate the serotonergic effects of phenethylamine psychedelics like mescaline and MDMA. The exact mechanism that causes this interaction is still unclear.[citation needed]
[edit] Mechanism of action
SSRIs are believed to act by inhibiting the reuptake of serotonin after being released in synapses. How much an individual will respond to this, however, also depends on genetics. In addition, several other mechanisms are suggested for the desired effect, e.g. neuroprotection and anti-inflammatory and immunomodulatory factors. Taken together, SSRI has several advantages compared with tricyclic antidepressants (TCA)s and 5-HT-prodrugs. However, the latter might be required in addition to SSRIs in certain situations.
[edit] Basic understanding
Further information: Chemical synapse
In the brain, messages are passed between two nerve cells via a chemical synapse, a small gap between the cells. The (presynaptic) cell that sends the information releases neurotransmitters (including serotonin) into that gap. The neurotransmitters are then recognized by receptors on the surface of the recipient (postsynaptic) cell, which upon this stimulation, in turn, relays the signal. About 10% of the neurotransmitters are lost in this process; the other 90% are released from the receptors and taken up again by monoamine transporters into the sending (presynaptic) cell (a process called reuptake).
To stimulate the recipient cell, SSRIs inhibit the reuptake of serotonin. As a result, the serotonin stays in the synaptic gap longer than it normally would, and may repeatedly stimulate the receptors of the recipient cell. The current model of SSRIs (the Monoamine Hypothesis) assumes that a lower homeostatic level of serotonin is primarily responsible for depression. While this holds in cases of major depression, minor to moderate cases are not as clear cut, and may in fact be caused by excess serotonin in specific areas of the brain.
Some current research points to more than just a single type of chemical signaling - the classic synapse model - involving serotonin. Astrocytes are "helper cells" in the brain that do not participate directly in chemical signaling, but play a part in homeostasis for many chemical levels in the brain. Recent research[76] suggests that serotonin is one of the hormones regulated by astrocytes, and that astrocytes actually uptake, package, and resend serotonin in a way similar to neuronal axons, but do not have corresponding post-synaptic terminals, therefore appearing to function only to control the local levels of serotonin in the cerebrospinal fluid.
Still more research illustrates that the current model for the antidepressant activity of SSRIs may be misdirected, as a drug that works entirely opposite to SSRIs - Tianeptine, a selective serotonin reuptake enhancer - also exhibits antidepressant activity, especially in patients resistant to SSRI therapy. The effect of an SSRE in comparison to an SSRI requires that the nature of serotonin signaling in the areas of the brain related to mood and cognition needs to be further elucidated. If serotonin firing is regularly phasic (related to brain waves), or rapid and discrete, then SSRIs simply compress the signal potential at affected receptors (bringing down the maximum potential and bring up the minimum) by causing a constant leftover signal (serotonin left in the synaptic gap) coupled with weaker subsequent signals (due to the decrease in presynaptic serotonin available to send new signals). By this hypothetical model, SSREs increase the signal potential separation (min to max) at affected 5-HT sites by reducing the level of free cerebrospinal serotonin and increasing the amount uptaken into axons to send new signals.
[edit] Pharmacodynamics
SSRIs inhibit the reuptake of the neurotransmitter serotonin (5-hydroxytryptamine or 5-HT) into the presynaptic cell, increasing levels of 5-HT within the synaptic cleft.
However, there is one counteracting effect: high serotonin levels will not only activate the postsynaptic receptors, but also flood presynaptic autoreceptors, which serve as a feedback sensor for the cell. Activation of the autoreceptors (by agonists like serotonin) triggers a throttling of serotonin production. The resulting serotonin deficiency persists for some time, as the transporter inhibition occurs downstream to the cause of the deficiency and therefore, is not able to counterbalance the serotonin deficiency. The body adapts gradually to this situation by lowering (downregulating) the sensitivity of the autoreceptors.[77]
Another adaptive process provoked by SSRIs is the downregulation of postsynaptic serotonin 5-HT2A receptors. After the use of an SSRI, since there is more serotonin available, the response is to decrease the number of postsynaptic receptors over time and in the long run, this modifies the serotonin/receptor ratio. This downregulation of 5-HT2A occurs when the antidepressant effects of SSRIs become apparent. Also, deceased suicidal and otherwise depressed patients have had more 5-HT2A receptors than normal patients. These considerations suggest that 5-HT2A overactivity is involved in the pathogenesis of depression.[78]
Most of the serotonin receptors on the surface of the cell are coupled to a G-protein inside it. These proteins activate or inhibit second messengers, which in turn affect transcription factors. Transcription factors are proteins that fit to the beginning of a gene and tell the cell to start using it.
These (slowly proceeding) neurophysiological adaptations of the brain tissue are the reason why usually several weeks of continuous SSRI use is necessary for the antidepressant effect to become fully manifested,[78] and why increased anxiety is a common side effect in the first few days or weeks of use.
[edit] Role in BDNF release
SSRIs act on signal pathways such as cAMP (Cyclic AMP) on the postsynaptic neuronal cell, which leads to the release of Brain Derived Neurotrophic Factor (BDNF). BDNF enhances the growth and survival of cortical neurons and synapses.[79]
[edit] Pharmacogenetics
Further information: Pharmacogenetics
Large bodies of research are devoted to using genetic markers to predict whether patients will respond to SSRIs or have side effects that will cause their discontinuation, although these tests are not yet ready for widespread clinical use.[80] Single-nucleotide polymorphisms of the 5-HT(2A) gene correlated with paroxetine discontinuation due to side effects in a group of elderly patients with major depression, but not mirtazapine (a non-SSRI antidepressant) discontinuation.[81]
[edit] Neuroprotection
Studies have suggested that SSRIs may promote the growth of new neural pathways or neurogenesis in rats.[82] Also, SSRIs may protect against neurotoxicity caused by other compounds (for instance fenfluramine) as well as from depression itself. SSRIs have been found to induce programmed cell death in Burkitt lymphoma and the brain tumors neuroblastoma and glioma with minimal effect on normal tissue.[83][84]
[edit] Anti-inflammatory and immunomodulation
Recent studies show pro-inflammatory cytokine processes take place during depression, mania and bipolar disorder, in addition to somatic disease (such as autoimmune hypersensitivity) and it is possible that symptoms manifest in these psychiatric illnesses are being attenuated by pharmacological effect of antidepressants on the immune system.[85][86][87][88][89]
SSRIs have been shown to be immunomodulatory and anti-inflammatory against pro-inflammatory cytokine processes, specifically on the regulation of Interferon-gamma (IFN-gamma) and Interleukin-10 (IL-10), as well as TNF-alpha and Interleukin-6 (IL-6). Antidepressants have also been shown to suppress TH1 upregulation.[90][91][92][93]
Future serotonergic antidepressants may be made to specifically target the immune system by either blocking the actions of pro-inflammatory cytokines or increasing the production of anti-inflammatory cytokines.[94]
[edit] SSRIs versus TCAs
SSRIs are described as 'selective' because they affect only the reuptake pumps responsible for serotonin, as opposed to earlier antidepressants, which affect other monoamine neurotransmitters as well, and as a result, SSRIs have fewer side effects.
There appears to be no significant difference in effectiveness between SSRIs and tricyclic antidepressants, which were the most commonly used class of antidepressants before the development of SSRIs.[95] However, SSRIs have the important advantage that their toxic dose is high, and, therefore, they are much more difficult to use as a means to commit suicide. Further, they have fewer and milder side effects. Tricyclic antidepressant also have a higher risk of serious cardiovascular side effects which SSRIs lack.
[edit] SSRIs versus 5-HT-Prodrugs
Further information: Prodrugs
Serotonin cannot be administered directly because when ingested orally, it will not cross the blood-brain barrier, and therefore would have no effect on brain functions. Also, serotonin would activate every synapse it reaches, whereas SSRIs only enhance a signal that is already present, but too weak to come through. In addition, hope for breaching the blood-brain barrier for causes such as severe depression is underway. The selectivity of the membrane can be reduced for a drug by injecting it in a concentrated sugar solution. The high osmotic pressure of the sugar solution causes the endothelial cells of the capillaries to shrink, which opens gaps between their tight junctions and makes the barrier more permeable. As a result the drug can enter the brain tissue.
[edit] SSRIs together with 5-HT-Prodrugs
Biosynthetic serotonin is made from tryptophan, an amino acid. In 1989, the Food and Drug Administration made tryptophan available by prescription only, in response to an outbreak of eosinophilia-myalgia syndrome caused by impure L-tryptophan supplements sold over-the-counter. With current standards, L-tryptophan is again available over the counter in the US as well as supplement 5-HTP which is a direct precursor to serotonin.
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