Increased dopamine receptor expression and anti-depressant response following deep brain stimulation of the medial forebrain bundle

Highlights

• Animals receiving MFB-DBS showed antidepressant-like response in the FST.

• Locomotor activity was unaltered at the same intensity of brain stimulation.

• MFB-DBS led to a striking increase in protein levels of only dopamine D2 receptors in the PFC.

• No significant change was seen in the expression of dopamine receptors (D1 through D5), DAT and TH.

Abstract

Background

Among several potential neuroanatomical targets pursued for deep brain stimulation (DBS) for treating those with treatment-resistant depression (TRD), the superolateral-branch of the medial forebrain bundle (MFB) is emerging as a privileged location. We investigated the antidepressant-like phenotypic and chemical changes associated with reward-processing dopaminergic systems in rat brains after MFB-DBS.

Methods

Male Wistar rats were divided into three groups: sham-operated, DBS-Off, and DBS-On. For DBS, a concentric bipolar electrode was stereotactically implanted into the right MFB. Exploratory activity and depression-like behavior were evaluated using the open-field and forced-swimming test (FST), respectively. MFB-DBS effects on the dopaminergic system were evaluated using immunoblotting for tyrosine hydroxylase (TH), dopamine transporter (DAT), and dopamine receptors (D1-D5), and high-performance liquid chromatography for quantifying dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) in brain homogenates of prefrontal cortex (PFC), hippocampus, amygdala, and nucleus accumbens (NAc).

Results

Animals receiving MFB-DBS showed a significant increase in swimming time without alterations in locomotor activity, relative to the DBS-Off (p<0.039) and sham-operated groups (p<0.014), indicating an antidepressant-like response. MFB-DBS led to a striking increase in protein levels of dopamine D2 receptors and DAT in the PFC and hippocampus, respectively. However, we did not observe appreciable differences in the expression of other dopamine receptors, TH, or in the concentrations of dopamine, DOPAC, and HVA in PFC, hippocampus, amygdala, and NAc.

Limitations

This study was not performed on an animal model of TRD.

Conclusion

MFB-DBS rescues the depression-like phenotypes and selectively activates expression of dopamine receptors in brain regions distant from the target area of stimulation.

Introduction

Major depressive disorder (MDD) is a chronic and disabling mental disorder with high amount of morbidity and shortened lifespan (De Hert et al., 2011; Ferrari et al., 2013; Whiteford et al., 2013). MDD patients display depressive mood, anhedonia, suicidal behavior, and other cognitive and somatic symptoms (Bostwick and Pankrazt, 2000; Leung et al., 2012; Charlson et al., 2013; Zarate et al., 2013). Despite the availability of several conventional antidepressant medications, the number of treatment refractory patients is increasing (Rush et al., 2006, Nierenberg et al., 2010, Romera et al., 2013). Thus, alternative therapeutic approaches are in high demand. In recent clinical trials, deep brain stimulation (DBS) ensued promising neuromodulatory effects in drug-resistant neuropsychiatric disorders like obsessive-compulsive disorder (OCD), Tourette syndrome, addiction, and MDD (Krack et al., 2010). Moreover, the substantial benefits of DBS for patients with severe neurological diseases like Parkinson's disease (PD), essential tremor, and motor symptoms of dystonia and dyskinesia are well recognized, as is its safety, and its use is approved by the US FDA (Kleiner-Fisman et al., 2006, Andrews, 2010; Mentzel al, 2012; Creed et al., 2013; Hariz et al., 2013; Kalia et al., 2013; Vidailhet et al., 2013).

Due to the reversible nature of DBS, it is an appealing surgical option for patients with treatment-resistant depression (TRD). In DBS, electrodes are stereotactically implanted in specific neuroanatomical targets where electrical stimulation is applied via a stimulator device placed subcutaneously (Tye et al., 2009). Multiple DBS targets for TRD have been pursued by various research consortiums with promising therapeutic effects seen in preclinical and clinical settings (Mayberg et al., 2005, Schlaepfer et al., 2008, Schlaepfer et al., 2013, Johansen-Berg et al., 2008, Malone et al., 2009, Bewernick et al., 2012). Among them, the superolateral branch of the medial forebrain bundle (MFB) is particularly intriguing, because this fiber bundle is connected with the reward-seeking and appetite motivation neurocircuitry at crucial distant sites including Brodmann area 25 of the subcallosal cingulate gyrus (SCG), nucleus accumbens (NAc), and anterior limb of the internal capsule (ALIC) (Coenen et al., 2011, Torres-Sanchez et al., 2016). The dopaminergic reward system has been implicated in the pathophysiology of MDD (Nestler et al., 2002, Nestler and Carlezon, 2006). Notably, in contrast to traditional antidepressant medications, MFB-DBS showed dramatic therapeutic response occurring within just days after initiation of stimulation (Schlaepfer et al., 2013, Fenoy et al., 2016). Galvez et al. (2015) extensively reviewed the vital structural and functional pathways originating from MFB that connect with different brain areas possibly involved in the pathogenesis of mood disorders. This neuroanatomical location has been designated as a privileged target for the beneficial neuromodulatory effect after DBS treatment for neuropsychiatric disorders, and suggests that dopamine is the substrate mediating the effect (Döbrössy et al., 2015). While neuromodulation through MFB is considered as a potential therapeutic strategy in the treatment of resistant MDD (Coenen et al., 2011, Schlaepfer et al., 2013, Döbrössy et al., 2015, Fenoy et al., 2016), further investigations concerning the network of depression using neuromodulation platforms in animal models might provide insight into the mechanism of treatment of MDD.

While MFB-DBS in rats is not a new therapeutic approach to study depression, the underlying neurobiological mechanisms of its effect are still not clear. In the present study, we sought to investigate the hypothesis that DBS of the MFB induces antidepressant-like effects in rats by activating the dopaminergic reward pathways in the rat brain. We delivered MFB-DBS and its effect on depression-like behavior was tested using the forced swimming test (FST), a main behavioral screening method that has proven predictive validity for DBS in rats (Temel et al., 2007, Hamani et al., 2010a, Hamani et al., 2010b, Hamani et al., 2014, Hamani and Nobrega, 2010, Hamani and Nobrega, 2012, Hamani and Temel, 2012, Schlaepfer et al., 2013). It has been reported that MFB-DBS is effective for treating MDD because it activates the ventral tegmental area (VTA) via recruitment of descending glutamatergic fibers of the MFB and this might in turn change the fate of dopamine in the remote nodes of the network (Schlaepfer et al., 2013, Schlaepfer et al., 2014). Although no changes in monoamine levels were detected by microdialysis in the NAc after MFB-DBS (Bregman et al., 2015), dopamine's role in relieving the depressive phenotype following MFB-DBS is still inconclusive (Bregman et al., 2015, Furlanetti et al., 2016). Recently, ontogenetic activation of rat VTA dopaminergic neurons led to increased blood oxygen level-dependent (BOLD) signal on functional magnetic resonance imaging (fMRI) in the PFC concomitant with improvement in anhedonia (Ferenczi et al., 2016), further identifying dopamine as central to the neurocircuitry of reward. Therefore, additional mechanistic studies are required to ascertain potential substrates involved in the acute antidepressant-like effects of MFB-DBS. Herein, we have estimated the levels of dopamine receptors (D1 through D5), dopamine transporter (DAT), and tyrosine hydroxylase (TH) proteins using western blot technique and changes in content of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) employing high-performance liquid chromatography coupled with electrochemical detection (HPLC-ECD) in the prefrontal cortex (PFC), hippocampus, NAc and amygdala regions of rat brain. We selected these brain regions because MFB-DBS induces antidepressant-like effects and selectively modulates the neuronal activity in cortical and subcortical structures, PFC and NAc, which are involved in the underlying circuitry of depression (Hamani et al., 2014, Bregman et al., 2015).