Noradrenergic modulation of central serotonergic neurotransmission: acute and long-term actions of mirtazapine.

Mirtazapine (Org 3770, Remeron) is a new alpha2-adrenoceptor antagonist which is an effective antidepressant drug. An in vivo electrophysiological paradigm in the rat was used to assess the effects of acute and long-term treatment with mirtazapine on pre- and postsynaptic alpha2-adrenoceptors and to determine whether this drug can modulate serotonin (5-HT) neurotransmission. The acute administration of mirtazapine produced a rapid increase in both noradrenaline and 5-HT neurotransmission by blocking both alpha2-adrenergic auto- and heteroreceptors, resulting in an enhanced tonic activation of postsynaptic 5-HT receptors. Long-term administration showed that this tonic activation of postsynaptic 5-HT receptors was most likely enhanced after such a treatment, as a result of a sustained increase in 5-HT neuronal activity in the presence of and due to inactivated alpha2-adrenergic heteroreceptors.