NE Receptor Antagonist

Mirtazapine. FDA approved for the treatment of depression in the summer of 1998, mirtazapine is unique among antidepressants by virtue of the fact that it does not inhibit the reuptake 5-HT, NE, or DA. Its primary mechanism of action relates to its potent antagonism of a2-adrenergic receptors and 5-HT2 receptors. It is also a potent antagonist of 5-HT3 and histamine H1 receptors, effects that influence its side effect profile. Mirtazapine has no effects on DA, cholinergic, or a1 -adrenergic receptors (De Boer, 1996). By blocking a2- but not the ai-adrenergic receptors, mirtazapine leads to an increase in firing rate and release of both NE and 5-HT. This is because a2-adrenergic receptors are localized on both NE and 5-HT neurons. On NE neurons, presynaptic a2 receptors function as autoreceptors, inhibiting the release of NE. Blocking these receptors leads to an increase firing rate and release of NE in most brain regions. NE released near the cell bodies of 5-HT neurons activate a1-adrenergic receptors located on 5-HT cell bodies, and since these receptors act in an excitatory fashion, the firing rate of 5-HT neurons is increased. 5-HT neurons also have a2-adrenergic receptors, but in this case, the receptors are localized on 5-HT terminals and function to inhibit the release of 5-HT. Blocking these a2-adrenergic receptors enhances the amount of 5-HT released each time the neurons fire.

Mirtazapine has been shown to be more effective than placebo in both hospitalized patients and outpatients, and patients with "severe" depression (17-item Hamilton Depression Scale score >25). It has comparable efficacy with amitriptyline (Bremner, 1995), doxepin (Marttila et al., 1995), and chlorimipramine (Richou et al., 1995) and has been shown to be more efficacious than trazodone (Van Moffaert et al., 1995) and fluoxetine (Wheatley et al., 1998) in severely ill depressed patients.

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