The effects of mirtazapine on central noradrenergic and serotonergic neurotransmission

Mirtazapine is a new antidepressant with a unique mode of action: it preferentially blocks the noradrenergic alpha2-auto- and heteroreceptors held responsible for controlling noradrenaline and serotonin release. In addition, mirtazapine has a low affinity for serotonin (5-HT)1A receptors but potently blocks 5-HT2 and 5-HT3 receptors. It increases serotonergic cell-firing in the dorsal raphe and 5-HT release in the hippocampus as measured by microdialysis. These effects are explained by noradrenergic enhancement of 5-HT cell-firing and blockade of noradrenaline-mediated inhibition of hippocampal 5-HT release. Because mirtazapine blocks 5-HT2 and 5-HT3 receptors, only 5-HT1-mediated transmission is enhanced. The noradrenergic activation and the consequent indirect enhancement of serotonergic transmission most probably underlie the marked therapeutic activity of mirtazapine. The blockade of 5-HT2 and 5-HT3 receptors prevents development of the side effects associated with non-selective 5-HT activation and may contribute to the anxiolytic and sleep-improving properties of this new compound. Therefore mirtazapine can be described as a noradrenergic and specific serotonergic antidepressant (NaSSA).