Anxiolytic-like effect of mirtazapine mediates its effect in the median raphe nucleus

Abstract

Mirtazapine, a noradrenergic and specific serotonergic antidepressant (NaSSA), blocks the α₂-adrenergic autoreceptors and heteroreceptors, which are responsible for controlling noradrenaline and 5-hydroxytryptamine (5-HT) release. Though preclinical and clinical studies have shown that mirtazapine exerts an anxiolytic action, its precise brain target sites remain unclear. In the present study, we investigated the brain area(s) in which mirtazapine exerts its anxiolytic-like effects on the expression of contextual conditioned freezing in rats. Mirtazapine (3 μg/site) was directly injected into three brain structures, the median raphe nucleus (MRN), hippocampus and amygdala. Freezing behavior tests were carried out 10 min after injections. Our results showed that the intra-MRN injection of mirtazapine reduced freezing significantly, whereas injections into the hippocampus or the amygdala did not. In addition, the intra-MRN injection of mirtazapine did not affect locomotor activity. These results suggest that the anxiolytic-like effect of mirtazapine might be mediated by its action on the MRN.

Introduction

Mirtazapine, an antagonist of central α₂-adrenergic autoreceptors and heteroreceptors, and postsynaptic 5-hydroxytryptamine₂ (5-HT₂) and 5-HT₃ receptors, achieves its unique therapeutic effect by enhancing noradrenaline and serotonin release (Millan, 2006, Yamauchi et al., 2012). Several clinical trials demonstrated the effectiveness of mirtazapine in the treatment of not only depression but also anxiety disorders, including post-traumatic stress disorder (Davidson et al., 2003), generalized anxiety disorder (Gambi et al., 2005), panic disorder (Ribeiro et al., 2001) and social anxiety disorder (Van Veen et al., 2002). Although its beneficial effects in relieving anxiety symptoms and curing anxiety disorders have been reported consistently, the mechanism by which mirtazapine exerts its anxiolytic effect has not been fully clarified.

Evidence from animal experiments also supports the anxiolytic-like effect of mirtazapine. The systemic administration of mirtazapine decreases conditioned freezing behavior in the contextual fear conditioning model (Kakui et al., 2009). A number of studies have demonstrated the reliability of the fear conditioning test as a behavioral paradigm with which to clarify the mechanisms involved in fear and anxiety and to evaluate the efficacy of therapeutic agents such as SSRIs and benzodiazepines in the treatment of anxiety disorders (Inoue et al., 2011). Studies using an in vivo microdialysis technique have found that the acute systemic administration of mirtazapine increases extracellular serotonin concentrations in the hippocampus but not the prefrontal cortex of rats (Yamauchi et al., 2012), suggesting that the anxiolytic effect of mirtazapine might be mediated by the facilitation of central 5-HT neurotransmission in the hippocampus.

In contextual fear conditioning, the amygdala and hippocampus are two main brain structures implicated in the acquisition, expression and retrieval of fear conditioning (LeDoux, 2000, Maren, 2008). In addition, the median raphe nucleus (MRN) also has an important role in the acquisition and expression of conditioned fear (Almada et al., 2009, Avanzi et al., 2003, Borelli et al., 2005). Lesions or inactivation of the MRN were shown to reduce the expression of contextual fear conditioning (Borelli et al., 2005, Silva et al., 2004). The MRN includes a great density of α₁-adrenoceptors and α₂-adrenoceptors in addition to the high density of 5-HT receptors (Day et al., 1997, Kia et al., 1996, Rosin et al., 1993, Talley et al., 1996). Administration of the α₂-adrenergic agonist, clonidine, into the MRN decreases the level of 5-HT in this nucleus, whereas the administration of an α₂-adrenergic antagonist into the MRN enhances 5-HT release (Adell and Artigas, 1999). These results suggest that α₁- and α₂-adrenoceptors in the MRN may affect contextual conditioned fear by increasing 5-HT release. However, little is known about the sites of anxiolytic action of mirtazapine in the brain.

The aim of this study is to clarify the brain regions where mirtazapine acts as an anxiolytic in contextual conditioned fear. In this study, we examined the effects of intracranial injections of mirtazapine administered directly into the amygdala, hippocampus or MRN on the expression of contextual conditioned fear by using freezing as an index of fear.