Effect of SNRI
As described above and as can be seen from the name, serotonin noradrenalin reuptake inhibitors (SNRI) inhibit the reuptake of serotonin and noradrenalin into the nerve cells. In order to understand this mechanism, one should consider the structure of a synapse, i.e. an interconnection point between two nerve cells. A synapse consists of the presynaptic end of one nerve cell and the postsynaptic end of another nerve cell.
In order to transmit certain information, the first nerve cell releases messenger substances (transmitters) into the gap between the two nerve cells. These messengers move towards the cell membrane of the second nerve cell, are absorbed into it and can transmit the information in various ways. Synapses in which serotonin or noradrenaline take over the role of transmitter are thus preferentially targeted by the serotonin noradrenaline reuptake inhibitors.SNRIs inhibit the transporters that channel some of the serotonin or norepinephrine molecules released from the first nerve cell back into the first nerve cell – these transporters are thus a kind of brake.
If this return transport is now inhibited by the SNRI, more serotonin or norepinephrine molecules reach the second nerve cell and can develop their effect there. In this way, the serotonin-noradrenaline reuptake inhibitors counteract the underlying lack of serotonin and noradrenaline between the two nerve cells. In Germany, several active ingredients are approved for use in the treatment of depression; they differ mainly in their effect on norepinephrine levels. The names of the active ingredients are venlafaxine, duloxetine and milnacipran.
Side effects of SNRI
An increase in serotonin and especially norepinephrine levels in the synaptic cleft leads to an increase in sympathetic nervous system activity. This is a system of nerve cells that controls basic bodily functions and – from an evolutionary biological point of view – has the task of adjusting the body to fight, flight or similar stressful situations. Therefore, in addition to an increase in heart rate and blood pressure, the effects of increased sympathetic nervous system activity include increased perspiration, sleep disorders and restlessness.
Other possible side effects based on an increase in sympathetic nervous system activity include dry mouth, nausea or difficulty urinating, and possibly sexual dysfunction. Especially due to the increase of serotonin activity in the synaptic cleft, many patients complain of nausea and vomiting at the beginning of treatment with serotonin noradrenalin reuptake inhibitors. However, in most cases these complaints disappear quite soon and can be temporarily treated with anti-nausea drugs, the so-called antiemetics.
Care should be taken when serotonin noradrenaline reuptake inhibitors are combined with other drugs, as this can lead to interactions in certain cases. First and foremost, other psychotropic drugs should be mentioned here, i.e. drugs that are used for mental illnesses such as depression. For this reason, monotherapy, i.e.
therapy with only one drug (e.g. SNRI), is generally recommended for the treatment of depression. Especially the combination with the so-called MAO-inhibitors, another group of antidepressants, or with triptans (migraine therapy) should be avoided under all circumstances, as here the effects of both drugs on serotonin activity add up and can lead to the dangerous picture of the serotonin syndrome with confusion, seizures or even coma. If the serotonin noradrenaline reuptake inhibitors are discontinued too abruptly at the end of the therapy, withdrawal symptoms such as circulatory problems, sleep or digestive disorders and the like may occur.
Due to their effect on the central nervous system, active participation in road traffic should be temporarily avoided under treatment with serotonin noradrenalin reuptake inhibitors.
- The side effects of antidepressants
- The Serotonin Syndrome
Due to the increasing effect of serotonin noradrenalin reuptake inhibitors on sympathetic nervous system activity, many patients under treatment with SNRI tend to lose weight. This is particularly remarkable as weight gain is one of the most common side effects of another large group of antidepressants, tricyclic antidepressants (e.g. : amitriptyline).
This difference should therefore be taken into account if the patient whose depression is to be treated is overweight. In rare cases, however, patients also react to the use of SNRIs by gaining weight – in this case, it may be worth considering accepting the weight gain for the limited period of SNRI therapy. Due to the rather favorable effect of serotonin noradrenaline reuptake inhibitors on the patient’s weight, SNRIs are therefore not only suitable as first-choice drugs for depression, but can also serve as an alternative in patients who were initially treated with tricyclic antidepressants or mirtazapine and have developed weight gain under this therapy.
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