Mirtazapine acts as an antagonist at central pre-synaptic alpha(_)-receptors, inhibiting negative feedback to the presynaptic nerve and causing an increase in NE release. Blockade of heteroreceptors, alpha(_)-receptors contained in serotenergic neurons, enhances the release of 5-HT, increasing the interactions between 5-HT and 5-HT_ receptors and contributing to the anxiolytic effects of mirtazapine. Mirtazapine also acts as a weak antagonist at 5-HT_ receptors and as a potent antagonist at 5-HT_ (particularly subtypes _A and _C) and 5-HT_ receptors. Blockade of these receptors may explain the lower incidence of adverse effects such as anxiety, insomnia, and nausea. Mirtazapine also exhibits significant antagonism at H_-receptors, resulting in sedation. Mirtazapine has no effects on the reuptake of either NE or 5-HT and has only minimal activity at dopaminergic and muscarinic receptors.
Mirtazapine, an antidepressant of the piperazinoazepine class, is a tetracyclic compound with an anxiolytic effect. Mirtazapine has fewer ADRs than tricyclic antidepressants and is better tolerated. Selective blockade of specific serotonin receptors by mirtazapine likey minimizes side effects typical of other antidepressants.
Mirtazapine is extensively metabolized by demethylation and hydroxylation followed by glucuronide conjugation. Cytochrome P450 2D6 and cytochrome P450 1A2 are involved in formation of the 8-hydroxy metabolite of mirtazapine, and cytochrome P450 3A4 is responsible for the formation of the N-desmethyl and N-oxide metabolites. Several metabolites possess pharmacological activity, but plasma levels are very low.
Symptoms of overdose include disorientation, drowsiness, impaired memory, and tachycardia. LD50 is 600-720mg/kg (oral, mice) and 320-490mg/kg (oral, rat) [PMID: 10333982]