Designing a new generation of antidepressant drugs.

Although longer-term adaptive changes in receptor sensitivity may better explain the delayed onset of action of antidepressants, the mechanism based on acutely elevated noradrenaline (NA) and serotonin (5-HT) synaptic levels remains the basis for new drug design. The dual action concept, which postulates that effects on both NA and 5-HT are more advantageous than a selective action on serotonin reuptake (SSRI), has been used to design new antidepressants such as venlafaxine and mirtazapine. Both drugs enhance NA and 5-HT neurotransmission with little affinity for receptors mediating tricyclic-like side effects. Mirtazapine, the prototype noradrenergic and specific serotonergic antidepressant (NaSSA), specifically enhances 5-HT1 neurotransmission and blocks 5-HT2 and 5-HT3 receptors, and in contrast to venlafaxine lacks SSRI-like and adverse cardiovascular side effects. The unique pharmacological action of mirtazapine is a result of implementation of two concepts: dual action as a basis of efficacy combined with receptor-specific action as a basis of tolerability.