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Selective serotonin reuptake inhibitor

Encyclopedia : S : SE : SEL : Selective serotonin reuptake inhibitor

 

"SSRI" redirects here; for other uses, see SSRI (disambiguation).
Selective serotonin reuptake inhibitors (SSRIs) are a class of antidepressants for treating depression, anxiety disorders and some personality disorders. These drugs are designed to allow the available neurotransmitter serotonin to be utilized more efficiently. A low level of utilization of serotonin is currently seen as one among several neurochemical symptoms of depression. Low levels of serotonin in turn can be caused by an anxiety disorder, because serotonin is needed to metabolize stress hormones [[Citing sources citation needed]].

These medications evolve their effects at the serotonin transporter. They increase the extracellular level of the neurotransmitter serotonin by inhibiting its reuptake into the presynaptic cell. They have no or only weak effects on other monoamine transporters, thus having little direct influence on the level of other neurotransmitters. That distinguishes them from the older tricyclic antidepressants (TCAs), thus they are named selective. SSRIs are considered to be considerably safer than TCAs, since the toxic dose is much higher and they are said to have fewer and weaker side effects and drug interactions.

Contents

List of SSRIs

Many drugs in this class are familiar in the USA through advertising, including the following:
(Trade names in parentheses)

Escitalopram is simply a variant of citalopram (racemate), of which it is the active enantiomer. It has been introduced to the market after the patent protection for citalopram had expired. The advantages are marginal.

Note that trazodone is not a typical member of the SSRIs - while it is a serotonin reuptake inhibitor, it is believed that its anti-depressant properties may be due to some of its other pharmokinetic properties rather than its effect on serotonin. That said, it does still share many properties of the typical SSRIs, especially the possibility of the so-called 'discontinuation syndrome' (see the section on this below).

Medical indications

The main indication for SSRIs is clinical depression. Apart from this, SSRIs are frequently prescribed for anxiety disorders, panic disorders, obsessive-compulsive disorder (OCD), and eating disorders. Though not specifically indicated by the manufacturers, they are also sometimes prescribed to treat irritable bowel syndrome (IBS). Additionally, SSRIs have been found to be effective in treating premature ejaculation in up to 60% of men.

Mode of action

Basic understanding

In the brain, messages are passed between two neurons (nerve cells) via a synapse, a small gap between the cells. The neuron that sends the information releases neurotransmitters (with serotonin among them) into that gap. The neurotransmitters are then recognized by receptors on the surface of the recipient 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 in the sending cell (a process called reuptake).

Depression has been linked to a lack of stimulation of the recipient neuron at a synapse. 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 has the chance to be recognized again (and again) by the receptors of the recipient cell, which can finally be stimulated fully.

SSRIs inhibit the reuptake of the neurotransmitter serotonin (5-hydroxytryptamine or 5-HT) in the synaptic cell, increasing levels of 5-HT within the synaptic cleft.

Usually, several weeks of continuous SSRI use are necessary for the antidepressant effects to become fully manifested. Pharmacologically, this delay is due to a side-effect of the initially high levels of serotonin within the synaptic gap: high serotonin levels will not only activate the postsynaptic receptors, but also flood the autoreceptors of the presynaptic cell, triggering 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 is therefore not able to counterbalance it.

Consequently, during SSRI therapy, the body must first adapt to the high levels of serotonin within the synaptic gap by downregulating the sensitivity of the autoreceptors, which can take up to 3 weeks. To expedite the onset of the antidepressant effect, bifunctional SSRIs are currently under development, which will additionally occupy the autoreceptors, and thus deactivate the serotonin production throttling mechanism.

Interaction with carbohydrate metabolism

Serotonin is also involved in regulation of carbohydrate metabolism. Few analyses of the role of SSRI's in treating depression cover the effects on carbohydrate metabolism from intervening in serotonin handling by the body.

Neuroprotection

Studies have suggested that SSRIs may promote the growth of new neural pathways.Malberg JE et al. (2000): "Chronic antidepressant treatment increases neurogenesis in adult rat hippocampus" [J. Neurosci. 20 (24), 9104-10] Also, SSRIs may protect against neurotoxicity caused by other compounds (for instance MDMA and Fenfluramine) as well as from depression itself.

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. Because of this, SSRIs lack some of the side effects of the more general drugs.

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.Anderson IM (2000): "Selective serotonin reuptake inhibitors versus tricyclic antidepressants: a meta-analysis of efficacy and tolerability", [J. Affect. Disord. 58(1), 19-3] 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 were initially claimed to have fewer and milder side effects.

SSRIs versus 5-HT-

Serotonin cannot be administered directly because when ingested orally, it will not cross the blood-brain barrier, and therefore won't have an 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.

Biosynthetically serotonin is made from tryptophan, an amino acid. If depression is caused by lack of serotonin, rather than insensitivity to it, SSRIs alone will not work well, whereas supplementing with tryptophan will. 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. Pharmaceutical grade L-tryptophan is currently available by prescription in the U.S. However the supplement 5-HTP can be bought over the counter and is a direct precursor to serotonin.

Adverse effects

General side effects

General side effects are mostly present during the first 1-4 weeks while the body adapts to the drug. Almost all SSRIs are known to cause either one or more of these symptoms:

It is not recommended to quit the medication because of the side effects, as they usually disappear after the adaptation phase and at the same time the antidepressive effects begin to show. However, despite being called general, the side effects and their duration is highly individual and drug-specific, so usually the treatment is begun with a small dose to see how the patient's body reacts to the drug. After that either the dose can be increased or the drug can be changed to some other if the side effects won't disappear or the patient feels they are too uncomfortable.

Sexual side effects

It is well known that the selective serotonin reuptake inhibitors (SSRIs) can cause various types of sexual dysfunction such as anorgasmia, erectile dysfunction, and diminished libido. Initial studies found that such side effects occur in less than 10% of patients, but those studies relied on unprompted reporting, so the frequency of such problems was underestimated. In more recent studies, doctors have specifically asked about sexual difficulties, and found that they are present in between 41% and 83% of patients. This dysfunction occasionally disappears spontaneously without stopping the SSRI, and in most cases resolves after discontinuance. In some cases, however, it does not; this is known as PSSD.

It is believed that sexual dysfunction is caused by an SSRI induced reduction in dopamine. Stimulation of postsynaptic 5-ht2 and 5-ht3 receptors decreases dopamine release from the Substantia Nigra. Sexual dysfunction caused by SSRI's has been shown to be mitigated by several different drugs. These include bupropion, buspirone, methylphendiate, mirtazapine, amphetamine, pramipexole and ropinirole.

Because of these sexual side effects, the SSRI fluoxetine (Prozac) was recently classified as a reproductive and developmental toxin by the Center for the Evaluation of Risks to Human Reproduction (CERHR), an expert panel at the National Institute of Environmental Health Sciences at the National Institutes of Health. The panel concluded "that there is sufficient evidence in humans that fluoxetine can produce reproductive toxicity in men and women as manifested by reversible, impaired sexual function, specifically orgasm."

Discontinuation syndrome

SSRIs are not addictive in the conventional medical use of the word (i.e. animals given free access to the drug do not actively seek it out and do not seek to increase the dose), but suddenly discontinuing their use is known to produce both somatic and psychological withdrawal symptoms, a phenomenon known as "SSRI discontinuation syndrome". Compared to the withdrawal symptoms of such drugs as opiates, alcohol, or cocaine, these reactions are quite different and frequently less significant, although the prescribing labels acknowledge the possibility of "intolerable" discontinuation reactions and some patients are never able to completely withdraw from SSRI drugs. In Europe, SSRI manufacturers are not permitted to promote their products as "non-habit forming", in the U.S., this statement is used to promote SSRIs: "SSRIs meet the World Health Organization definition of 'addictive'." Many physicians do not get informed consent at the time of initial prescription that covers the difficulties of later withdrawal from the drug, so this syndrome can be an unexpected barrier to patients, especially those who tried the drug in response to a specific crisis, who expected an easy withdrawal once their emotional situation stabilized. In addition, warnings to patients not to stop taking the drug without doctor's approval, while indicated, may lead to a reluctance to discontinue SSRI therapy.

PANES

Persistent Adverse Neurological Effects following SSRI discontinuation (PANES) are serious side-effects induced by an SSRI that remain or worsen after both the medicine has been completely eliminated from the body, and after SSRI discontinuation syndrome (if it occurs) ends.

Contraindications

SSRIs are contraindicated with concomitant use of MAOIs (monoamine oxidase inhibitors). This can lead to increase serotonin levels which could cause serotonin syndrome. People taking SSRIs should also avoid taking pimozide (a diphenylbutylpiperidine derivative). The non-opioid analgesic tramadol hydrochloride (or Ultram, Ultracet) can, in rare cases, produce seizures when taken in conjunction with an SSRI or tricyclic antidepressant.

Criticism of SSRIs

SSRIs have been the focus of controversy. Some feel that SSRIs are prescribed by overzealous doctors or psychiatrists in cases where their use is only marginally indicated. According to this argument, societal pressures have created a precedent for the pursuit of "normal" mental or emotional functioning by chemical means versus a more holistic approach (diet, exercise, sleep, stress reduction, etc). Furthermore, in late 2004 much media attention was given to a proposed link between SSRI use and juvenile suicide. For this reason, the use of SSRIs in pediatric cases of depression is now recognized by the FDA as warranting a cautionary statement to the parents of children who may be prescribed SSRIs by a family doctor. The FDA's currently required packaging insert for SSRIs includes a warning (known as a "black box warning") that a pooled analysis of placebo controlled trials of 9 antidepressant drugs (including multiple SSRIs) resulted in a risk of suicidality that was twice that of placebo. Other studies have shown no increase in rates of suicide but a small increase of non-fatal self-harm and even of reduced incidence of suicide. [UCLA (2005)] ["Medpagetoday" (2006)]

Critics have also alleged that the widely disseminated television and print advertising of SSRI drugs promotes an inaccurate message, oversimplifying what these medications actually do and perhaps misinforming the public, contributing to the problems listed above. Much of the criticism stems from questions about the validity of claims that such drugs work by correcting chemical imbalances.

Mechanism not well understood

Some say that the biological causes of depression, which SSRIs were designed to treat, have never in fact been scientifically established. They claim that there is no scientific evidence for the existence of the disorders that SSRIs are designed to treat, or that they are based on a chemical imbalance of the brain, or that SSRIs effectively handle this chemical imbalance. [[Citing sources citation needed]]

Certain depressed patients describe complex social situations as the root cause of their depression. Critics seize on this to point out that it is an unproven assumption that depression results from "hardware" errors in the brain, whereas exogenous factors like jobs, love, money, family frustrations, are clearly in the realm of "social software". Changing the "hardware" chemistry, they claim, is an unproven methodology and on a different level from the one where the adjustments may need to be made.

The mode of action of antidepressant drugs on their direct target, the serotonin transport protein, and possible regulatory mechanisms with respect to long-term alleviation of depression, although having been thoroughly investigated both neurobiologically and clinically over decades, are not yet fully understood. A comprehensive theory of how and why a slower fading of signals on the serotonin pathway will affect depression and not impair other brain functions does not yet exist, despite the vast body of empirical evidence.

Other medications to treat depression

The majority of medications most recently approved to treat depression work on multiple neurotransmitters. Venlafaxine and duloxetine (Cymbalta) are both members of the SNRI class of antidepressant medication. SNRIs (serotonin-norepinephrine reuptake inhibitors) work on the norepinephrine and serotonin neurotransmitters. Mirtazapine also increases levels of norepinephrine and serotonin, but it is a tetracyclic antidepressant, not a SSRI or SNRI. The arrival of these new drugs suggest that future antidepressants will not work on serotonin exclusively. Since the expiration of Eli Lilly's Prozac patent, Lilly has been promoting their new SNRI, duloxetine. Natural healing professionals often recommend 5-HTP supplements instead of standard SSRI/MAOI prescriptions as 5-HTP allegedly accomplishes the same goal without resorting to disturbing the brain's natural metabolic procedures, although this has not been scientifically proven.

External links

References


Monoamine oxidase inhibitors (MAOI):

Reversible Inhibitor of Monoamine oxidase A (RIMA):

Dopamine Reuptake Inhibitor (DARI):

Norepinephrine-dopamine reuptake inhibitors:

Norepinephrine Reuptake Inhibitor (NRI) or (NARI):

Serotonin-Norepinephrine Reuptake Inhibitor (SNRI):

Selective Serotonin Reuptake Inhibitor (SSRI):

Selective Serotonin Reuptake Enhancer (SSRE):

Tricyclic antidepressants (TCA):

Tetracyclic antidepressants:

Noradrenergic and Specific Serotonergic Antidepressant (NaSSA):

 

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