Serotonin–norepinephrine reuptake inhibitor

Serotonin–norepinephrine reuptake inhibitors (SNRIs) are a class of antidepressant drugs used in the treatment of major depression and other mood disorders. They are sometimes also used to treat anxiety disorders, obsessive-compulsive disorder (OCD), attention deficit hyperactivity disorder (ADHD), chronic neuropathic pain, fibromyalgia syndrome (FMS), and for the relief of menopausal symptoms.

SNRIs act upon and increase the levels of two neurotransmitters in the brain that are known to play an important part in mood, serotonin, and norepinephrine. This can be contrasted with the more widely-used selective serotonin reuptake inhibitors (SSRIs) that act only on serotonin.

Contents

Overview of SNRIs

Pharmacology

SNRIs work by inhibiting the reuptake of the neurotransmitters serotonin and norepinephrine. This results in an increase in the extracellular concentrations of serotonin and norepinephrine and, therefore, an increase in neurotransmission. Recent evidence suggests that the norepinephrine transporter also transports some dopamine as well, since dopamine is inactivated by norepinephrine reuptake in the prefrontal cortex, which largely lacks dopamine transporters, therefore SNRIs can increase dopamine neurotransmission in this part of the brain.[6]

Most SNRIs including venlafaxine, desvenlafaxine, and duloxetine, are several fold more selective for serotonin over norepinephrine, while milnacipran is three times more selective for norepinephrine than serotonin.

Elevation of norepinephrine levels is thought to be necessary for an antidepressant to be effective against neuropathic pain, a property shared with the older tricyclic antidepressants (TCAs), but not with the SSRIs.[7]

Comparison to SSRIs

The SNRIs were developed more recently than the SSRIs and as a result there are relatively few of them. However, the SNRIs are among the most widely used antidepressants today. In 2009, Cymbalta and Effexor were the 11th- and 12th-most-prescribed branded drugs in the United States. This translates to the 2nd- and 3rd-most-common antidepressants, behind Lexapro (#5), the SSRI escitalopram.[8] In some studies, SNRIs demonstrated slightly higher antidepressant efficacy than the SSRIs (response rates 63.6% versus 59.3%).[9] However, in one study escitalopram had a superior efficacy profile to venlafaxine.[10] It is not clear what the reasons were for this unexpected anomaly. The side-effects of SNRIs are reported to be slightly less severe in comparison to the SSRIs as well. One of the major complaints that many users of SSRIs have is the negative sexual side-effects that can be very difficult to treat.[2] Although SNRIs can have similar side-effects, many of them can have the opposite effect of increased libido. Wellbutrin has had official studies done,[3] and Strattera and Savella have commonly been reported as increasing libido in both men and women, even though studies have contradicted these reports.[4] However, the individuals who reported increased sexual functioning also tended to report increased anxiety, heart rate, blood pressure, and other negative effects associated with adrenaline increase.

Side-effects

Because the SNRIs and SSRIs both act similarly to elevate serotonin levels, they subsequently share many of the same side-effects, though to varying degrees. The most common include loss of appetite, weight, and sleep. There may also be drowsiness, dizziness, fatigue, headache, mydriasis, nausea/vomiting, sexual dysfunction, and urinary retention. There are two common sexual side-effects: diminished interest in sex (libido) and difficulty reaching climax (anorgasmia), which are usually somewhat milder with the SNRIs in comparison to the SSRIs. Nonetheless, sexual side-effects account for lack of compliance with both SSRIs and SNRIs.

While tricyclic antidepressants also produce similar sexual side-effects as SNRIs, discontinuation of TCAs is more often due to the other side-effects (like cardiovascular effects). Also Amitriptyline (a TCAs) is more commonly associated with orthostatic hypotension.

Elevation of norepinephrine levels can sometimes cause anxiety, mildly elevated pulse, and elevated blood pressure. People at risk for hypertension and heart disease should have their blood pressure monitored.

Discontinuation syndrome

As with SSRIs, the abrupt discontinuation of an SNRI usually leads to withdrawal, or "discontinuation syndrome", which could include states of anxiety and other symptoms. It is therefore recommended that users seeking to discontinue an SNRI slowly taper the dose under the supervision of a professional. Discontinuation syndrome has been reported to be markedly worse for venlafaxine when compared to other SNRIs. Accordingly, as tramadol is related to venlafaxine, the same conditions apply.[11] This is likely due to venlafaxine's relatively short half-life and therefore rapid clearance upon discontinuation.

Contraindications

Due to the effects of increased norepinephrine levels and therefore higher adrenergic activity, pre-existing hypertension should be controlled before treatment with SNRIs and blood pressure periodically monitored throughout treatment. Duloxetine has also been associated with cases of hepatic failure and should not be prescribed to patients with chronic alcohol use or liver disease.

SNRIs should be taken with caution when using St John's wort as the combination can lead to the potentially fatal serotonin syndrome.[12] There is also a significant risk when combining SNRIs with dextromethorphan, tramadol, cyclobenzaprine, meperidine/pethidine, and propoxyphene. They should never be taken within 14 days of any other antidepressant, especially the monoamine oxidase inhibitors (MAOIs).

See also

References

  1. ^ http://www.emedexpert.com/facts/venlafaxine-facts.shtml
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  6. ^ http://stahlonline.cambridge.org/prescribers_drug.jsf?page=0521683505c95_p539-544.html.therapeutics&name=Venlafaxine&title=Therapeutics
  7. ^ Sindrup SH, Otto M, Finnerup NB, Jensen TS (2005). "Antidepressants in the treatment of neuropathic pain.". Basic Clin Pharmacol Toxicol 96 (6): 399–409. doi:10.1111/j.1742-7843.2005.pto_96696601.x. PMID 15910402. 
  8. ^ "2009 Top 200 branded drugs by total prescriptions". SDI/Verispan, VONA, full year 2009. www.drugtopics.com. http://drugtopics.modernmedicine.com/drugtopics/data/articlestandard//drugtopics/252010/674969/article.pdf. Retrieved 6 April 2011. 
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  12. ^ Karch, Amy (2006). 2006 Lippincott's Nursing Drug Guide. Philadephia, Baltimore, New York, London, Buenos Aires, Hong Kong, Sydney, Tokyo: Lippincott Williams & Wilkins. ISBN 1-58255-436-6.