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Serotonin-norepinephrine reuptake inhibitors (SNRIs) are a class of antidepressant used in the treatment of clinical depression and other affective disorders. They are also sometimes used to treat anxiety disorders, obsessive-compulsive disorder, attention deficit hyperactivity disorder (ADHD) and chronic neuropathic pain. They act upon two neurotransmitters in the brain that are known to play an important part in mood, namely, serotonin and norepinephrine. This can be contrasted with the more widely-used selective serotonin reuptake inhibitors (SSRIs), which act only on serotonin.
The abbreviation "SNRI" should not be used for "'selective' norepinephrine reuptake inhibitors".
Mode of action
Depression is thought to be caused by a lack of information flow between neurons in certain parts of the brain. Neurons pass information to each other by means of chemicals known as neurotransmitters, which shoot across the tiny synapses between the cells. After firing, most of the neurotransmitter is reabsorbed by the presynaptic cell in a process called reuptake.
Antidepressants work by increasing the amount of neurotransmitters active in the synapse, thereby enhancing neuronal activity and increasing the responsiveness of mood. Modern antidepressants usually achieve this effect by blocking the transporter proteins that reabsorb certain neurotranmitters, hence the name "reuptake inhibitors".
SNRIs were developed more recently than SSRIs, and there are relatively few of them. Their efficacy as well as their tolerability appears to be somewhat better than the SSRIs, owing to their compound effect.
As with the SSRIs, abrupt discontinuation of SNRI-medication usually leads to a discontinuation syndrome which could include states of anxiety and further symptoms. It is therefore recommended to slowly taper down the dose under the supervision of a psychopharmacologist when discontinuing SNRIs.
SNRIs currently available
- venlafaxine (tradenames Effexor®, Efexor®) is the first and most commonly used SNRI. Although it also works on dopamine somewhat at high dosages, the majority of its effect is on serotonin and norepinephrine.
- nefazodone (tradename Serzone®) is an antidepressant with efficacy similar to SSRIs, but without the sexual side effects. In fact, Serzone at times may act similar to Wellbutrin in its neutral, or at times positive effect on function. It has been discontinued in several countries due to rare cases of liver failure. The tradename "Serzone®" has been discontinued, however generic nefazodone is currently available (May 06). However, the liver failure is rare, and a simple blood test every 6 months to assess liver enzyme levels is sufficient. Nefazodone has an active metabolite which at higher doses (> 250mg/day) can increase anxiety.
- milnacipran (tradename Dalcipran®/ Portugal; Ixel®/ France) has shown to be significantly effective in the treatment of depression and Fibromyalgia syndrome (FMS). Although it has not yet been approved by the Food and Drug Administration (FDA) for use in the United States, it has been commercially available in Europe and Asia for several years.
- desipramine (tradenames Norpramine®, Pertofraneis®) is technically a tricyclic antidepressant, and is usually categorized as such. It works, however, on both serotonin and norepinephrine, so it can also be considered an SNRI.
- duloxetine (tradename Cymbalta®) by Eli Lilly and Company, also inhibits serotonin and has been approved for the treatment of depression and neuropathic pain in August of 2004.
Effexor is the "newer" version of Serzone, while Cymbalta is the "newer" version of Effexor. These drugs have been considered a second tier drug for treating less severe adult ADHD, because they combine the serotonin reuptake inhibition (successful in treating some anxiety/depression and obsessive-compulsive disorder), and/or the "ruminating" done in ADHD patients with norepinephrine's stimulant effect.
Please note that some of the above medications may not be considered "true" SNRIs; refer to specific peer-reviewed scientific journals for more in-depth coverage on classifications and pharmaco-kinetics.
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