Clinical implications of the pharmacology of serotonin reuptake inhibitors

The selective serotonin reuptake inhibitors (SSRIs) are a tribute to the ingenuity of pharmacologists and designers of molecules. Not only do these drugs have remarkable selectivity for the reuptake of serotonin compared with other monoamines, but also they have a commendable lack of affinity for receptors including the serotonin receptor. In contrast, the classical tricyclic antidepressants (TCAs) are less specific in their pharmacological action. In addition to inhibiting the reuptake of serotonin, TCAs inhibit the uptake of noradrenaline, dopamine and tyramine, and antagonize cholinergic (muscarinic), adrenergic and histaminergic receptors. Moreover, TCAs have quinidine-like anti-arrhythmic activity and lower the seizure threshold. Clinical investigations have shown that the SSRIs have equivalent therapeutic efficacy compared with the TCAs in the treatment of depression. However, the pharmacological specificity of the SSRIs is a clinical advantage since they lack the propensity to cause dry mouth, blurred vision, urinary hesitancy, constipation, hypotension and arrhythmia. Furthermore, the SSRIs are relatively safe in overdosage. The similarities between the SSRIs are more obvious than their differences: all are highly potent and selective inhibitors of serotonin reuptake with efficacy in the treatment of depression. Nevertheless, each has a distinctive pharmacological profile. In this review the characteristics desired in an "ideal" antidepressant are examined, and the ways in which the TCAs and SSRIs fit this ideal are compared.