Different actions for acute and chronic administration of mirtazapine on serotonergic transmission associated with raphe nuclei and their innervation cortical regions

Abstract

The atypical antidepressant, mirtazapine enhances noradrenergic transmission, but its effects on serotonergic transmission remain to be clarified. The present study determined the effects of acute and chronic administration of mirtazapine on serotonergic transmissions in raphe nuclei and their innervation regions, frontal and entorhinal cortex, using multiple-probes microdialysis with real-time PCR and western blotting. Acute administration of mirtazapine did not affect extracellular serotonin level in raphe nuclei or cortex; however, chronic administration increased extracellular serotonin level in raphe nuclei without affecting that in cortex. Blockade of 5-HT1A receptor, but not that of the 5-HT2A/2C receptor, enhanced the effects of acute administration of mirtazapine on extracellular serotonin level in raphe nuclei. Chronic mirtazapine administration reduced the inhibitory function associated with somatodendritic 5-HT1A receptor in raphe nuclei, but enhanced postsynaptic 5-HT1A receptor in serotonergic innervated cortical regions. Chronic administration reduced the expression of mRNA and protein of serotonin transporter and 5-HT1A receptor in raphe nuclei, but not in the cortices. These results suggested that acute administration of mirtazapine probably activated serotonergic transmission, but its stimulatory action was abolished by activated inhibitory 5-HT1A receptor. Chronic administration of mirtazapine resulted in increased extracellular serotonin level via reduction of serotonin transporter with reduction of somatodendritic 5-HT1A autoreceptor function in raphe nuclei. These pharmacological actions of mirtazapine include its serotonergic profiles as noradrenergic and specific serotonergic antidepressant (NaSSA).

Introduction

The atypical antidepressant, mirtazapine (1,2,3,4,10,14b-hexa-hydro-2-methylpyrazino [2,1-a] pyrido[2,3-c]benzazepin) is a tetracyclic compound with antidepressant activity despite no monoamine transporter affinities (Croom et al., 2009, de Boer, 1995, de Boer, 1996). A recent meta-analysis study of multiple treatments has demonstrated that in terms of clinical response, mirtazapine is more efficacious than duloxetine, fluoxetine, fluvoxamine, paroxetine and reboxetine (Cipriani et al., 2009). Mirtazapine has attracted attention since its binding profile indicates an antagonistic action to α2 adrenoceptor, serotonin 5-HT2A, 5-HT2C and 5-HT3 receptors without affecting monoamine transporters (Millan et al., 2000, Tatsumi et al., 1997, Van der Mey et al., 2006). Therefore, mirtazapine has a unique binding profile different from that of the typical tricyclic antidepressants and selective serotonin reuptake inhibitor (SSRI), and could be described as a noradrenergic and specific serotonergic antidepressant, abbreviated as NaSSA (Croom et al., 2009, de Boer, 1995, de Boer, 1996). Furthermore, the inhibitory effects of mirtazapine on 5-HT2A/2C and 5-HT3 receptors reduce the likelihood of some serotonergic adverse effects associated with hyperactivated serotonin receptors induced by serotonin transporter inhibitors (e.g., restlessness, nausea and sexual dysfunction), and this appears to be supported by tolerability data from clinical trials (Croom et al., 2009).

With regard to the antidepressant mechanisms of mirtazapine, previous studies indicated that mirtazapine increases the extracellular norepinephrine level in raphe nuclei through inhibition of α2 adrenoceptor on the pre-synaptic terminal from locus coeruleus noradrenergic projection and indirectly enhances serotonergic transmission via activation of α1 adrenoceptor on the postsynaptic membrane of raphe nuclei (NaSSA hypothesis) (Croom et al., 2009, de Boer, 1995, de Boer, 1996). Systemic administration of mirtazapine increased extracellular levels of norepinephrine and dopamine in various brain regions, e.g., frontal cortex (FC), dorsal and ventral hippocampus (de Boer, 1996, de Boer et al., 1996, Devoto et al., 2004, Millan et al., 2000). Contrary to norepinephrine, the effects of mirtazapine on the extracellular serotonin level remain controversial, since a report demonstrated that systemic administration of mirtazapine increased extracellular serotonin level in ventral hippocampus (de Boer et al., 1996), whereas several other studies could not detect the stimulatory effects on extracellular serotonin level in FC, striatum, dorsal and ventral hippocampus (Bengtsson et al., 2000, Millan et al., 2000, Nakayama et al., 2004).

Both the median (MRN) and dorsal (DRN) raphe nuclei receive noradrenergic projections from the locus coeruleus (Adell et al., 2002), as well as serotonergic projections from each other raphe nuclei (Adell et al., 2002, Imai et al., 1986). The serotonergic neurons in DRN project to FC, ventral hippocampus and striatum, whereas the serotonergic neurons in MRN project to the entorhinal cortex (EC), dorsal hippocampus and nucleus accumbens (Adell et al., 2002, Imai et al., 1986). Based on the serotonergic networks, the present study was designed to determine the effects of mirtazapine on serotonergic transmission associated with raphe nuclei. For this purpose, the acute and chronic effects of mirtazapine on extracellular serotonin levels in raphe nuclei and their innervated cortical regions were analyzed in rats using multiple-probes microdialysis. Furthermore, the present study determined the expression of protein and mRNA of serotonin transporter (Slc6a4) and 5-HT1A receptor (Htr1a) in the same regions.