Recent meta-analyses of structural imaging studies in the clinical literature have detected volumetric changes in a number of brain regions in MDD relative to controls. The most robust finding in the literature is reduced hippocampal volume in patients (Campbell et al., 2004; Videbech, 2004; Kempton et al., 2011; Arnone et al., 2012; Schmaal et al., 2016), but some reports have highlighted a decrease in the volume of prefrontal, dorsomedialprefrontal, orbitofrontal and cingulate cortices, and striatum (Arnone et al., 2012; Bora et al., 2012; Sacher et al., 2012; Lai, 2013) as well as increased volume in the lateral ventricles (Kempton et al., 2011).

Although hippocampal volume loss might be dependent on patient age and disease duration (McKinnon et al., 2009; Schmaal et al., 2016), a meta-analysis of studies in first episode patients revealed significant hippocampal volume reductions, suggesting that smaller hippocampal volume may be a possible risk factor for depression, rather than a marker of disease progression (Cole et al., 2011). This is in agreement with a 3-year longitudinal study, which reported no significant reduction in hippocampal volume of patients, but did show that smaller baseline volume was associated with poorer clinical outcome (Frodl et al., 2008a). It should be noted that, following a whole brain voxel based morphometry analysis, a decrease in gray matter density in the hippocampus in patients with ongoing depression over a 3 year period was detected, thus highlighting a potential impact of the method of analysis on the findings (Frodl et al., 2008b).

Changes in hippocampal volume also appear to be sensitive to antidepressant treatment, showing an increase in volume following traditional antidepressant (Frodl et al., 2008a) and electroconvulsive (ECT; Abbott et al., 2014) therapies. Fu et al. (2013) suggested that lower right hippocampal volume could even be a predictor of poor antidepressant response. Reduced subgenual anterior cingulate cortex volume persists despite successful treatment with antidepressant drugs (Drevets et al., 1997) but chronic lithium treatment, which exerts robust neurotrophic effects in animal models, has been associated with an increase in gray mater volume in treatment responders in this and other prefrontal areas (Drevets et al., 2008; Moore et al., 2009).

Interestingly, smaller hippocampi in depressed patients have been found to be associated with increased markers of glucocorticoid receptor activation in peripheral plasma, suggesting that gray matter loss in this structure might be related to changes in HPA axis function (Frodl et al., 2012).

Translational neuroimaging studies have reported structural alterations in the rodent brain in animal models of depression. The Wistar-kyoto (WKY) rat is an inbred rat strain with an inherent anxiety- and depressive-like behavioral phenotype. In vivo MRI has shown increased lateral ventricular volume and decreased hippocampal volume in adult WKY rats compared to the Wistar comparison strain with no change in total cortical volume (Gormley et al., 2016). Alterations in T1 and T2 relaxation times can provide information about tissue characteristics and increased T2 relaxation times were evident in the hippocampus of WKY rats compared to Wistar controls (Gormley et al., 2016), a finding that may be linked to oedema or changes in vascular permeability. Work in our group has also investigated MRI changes in the olfactory bulbectomized rat (OB) and showed opposite changes with reductions in T2 relaxation times in the hippocampus, visual cortex and left retrosplenial cortex (Gigliucci et al., 2014). This highlights the heterogeneity of animal models and may indicate alterations in relaxation times could be linked to specific phenotypes. Furthermore, this emphasizes the importance of selecting an appropriate animal model as the OB rat may be more related to anxiety rather than depressive-like behaviors. Structural analysis revealed a trend toward increased ventricular volume in OB rats however no changes in hippocampal volume were observed (Gigliucci et al., 2014). Reductions in T2 relaxation times have been linked to microglial activation and also increased cell packing density in the brain parenchyma (Ding et al., 2008; Blau et al., 2012). Given the lack of overt inflammation, it is most likely that changes in cell density or morphology may account for the decreased T2 relaxation times reported in the OB rats.

Reduced hippocampal volume has also been linked to anxiety-like behavior, with mice bred for high anxiety showing reduced hippocampal volume compared to those bred for low anxiety traits (Kalisch et al., 2006). The structural changes may also be linked to the depressive-like behavioral phenotype observed in these animals, however the authors show that in control animals, hippocampal volume positively correlated with anxiety-like but not depressive-like behavior and therefore suggest the differences in hippocampal volume are primarily linked to anxiety-like traits.

Chronic stress paradigms have also been shown to produce changes in rodent brain morphology. Structural MRI of post-mortem brains of Wistar rats that were subjected to a 10 day immobilization stress, have shown an increase in lateral ventricular volume without any change in hippocampal volume or morphology when compared to non-stressed controls (Henckens et al., 2015). Delgado y Palacios et al. (2011) also report no change in hippocampal volume in rats exposed to 8 weeks of CMS, although subtle changes in hippocampal morphology were evident. In contrast, Lee et al. (2009) report that 21 days of immobilization stress in rats results in a 3% reduction in baseline hippocampal volume, assessed by longitudinal MRI, a change which was not observed in control animals. There was no change observed in anterior cingulate or retrosplenial cortical volumes reported (Lee et al., 2009). The contradictory reports relating to hippocampal volume may be related to stressor severity and/or the ex vivo acquisition of structural data. Ex vivo MRI on post-mortem brains following 10 days of social defeat in mice shows that the volume of the ventral tegmental area (VTA), bed nucleus of the stria terminalis (BNST) and the dorsal raphe nucleus (DRN) positively correlate with social avoidance scores in animals, indicating that these areas may be linked to stress vulnerability (Anacker et al., 2016). Social avoidance scores also negatively correlated with size of both the cingulate cortex and nucleus accumbens suggesting decreased volume in stress-sensitive mice. Interestingly DTI metrics largely correlated in opposite directions to volumetric changes indicating increased volume is accompanied by reduced diffusion in the brain regions investigated.

Volumetric alterations have also been associated with antidepressant effects in offspring following administration of a ketogenic (high ketone) diet (KD) during pregnancy. Whilst small structural changes were observed at weaning (PND 21.5), KD offspring showed increased frontal cortical and cerebellar volume, but interestingly, decreased volume of the hippocampus, striatum, and some cortical areas in adulthood (Sussman et al., 2015). The observed decreased hippocampal volume is somewhat counterintuitive to an antidepressant effect given that reductions in hippocampal volume are associated with depressive symptoms. The authors suggest this may be due to reduced dendritic branching or abnormal neuronal differentiation resulting from low protein consumption however as this is a single study examining MRI related KD effects, further work is required to comprehensively assess the long-term effects of KD diet on behavior and brain structure.

Together these data suggest that increased lateral ventricular volume and decreased hippocampal volume are relatively consistent hallmarks in both patients with MDD and animal models, adding to their face validity. Although contrasting findings may be due to study design, analysis method or method of acquisition they could also reflect subtypes of stress/depression and the diversity of patient populations that may be modeled in animals.

Diffusion tensor imaging (DTI) allows for an assessment of in vivo white matter microstructural integrity through measurement of the restriction in diffusion of water molecules in brain tissue (Moser et al., 2009). Several DTI studies in patients with unipolar depression have reported decreased cortical fractional anisotropy (FA), a measure of white matter integrity and fiber directionality in the frontal and temporal lobes (for review see Sexton et al., 2009) as well as in the amygdala (Arnold et al., 2012). This seems to be evident in both late life depression (Bae et al., 2006; Nobuhara et al., 2006) and in young adults (Li et al., 2007). Data suggests that altered FA values may be a marker of pre-disposition to depression or may occur early in the course of disease, as patients with first onset depression have been shown to exhibit decreased FA values in frontal cortical regions such as the anterior cingulate cortex (Zhu et al., 2011) and the frontal gyrus (Ma et al., 2007).

Currently, tractography studies show that first episode depressed patients exhibit increased cortico-limbic structural connectivity (Fang et al., 2012) and that depression is associated with a disruption in the pathways linking subcortical regions, including the hippocampus, to the prefrontal cortex (Liao et al., 2013) as well as alterations in the superior longitudinal and fronto-occipital fascicule (Murphy and Frodl, 2011). Finally, preliminary results from the largest study to date have also highlighted alterations in the corpus callosum of MDD subjects (Kelly et al., 2016).

DTI, based on Gaussian distribution of water molecules, and more recently diffusion kurtosis imaging (DKI) which attempts to account for biological variation in water diffusion and provide a more accurate model of diffusion, have both identified alterations in microstructural integrity in animal models of depression. Common metrics reported in DTI and DKI studies are mean diffusion or kurtosis across all directions (MD or MK), radial diffusion or kurtosis (RD or RK; movement in the transverse direction) and axial diffusion or kurtosis (AD or AK; movement along the main axis of diffusion). Although, interpretation of diffusion imaging has not been fully explored, microstructural organization of the brain, including cellular membranes, myelinated axons, intracellular organelles, and dendritic architecture, affects the orientation of water diffusion.

DTI and DKI studies have identified alterations in the microstructural integrity of gray matter regions in rats exposed to CMS. Increased mean diffusion values in the bilateral frontal cortex, hippocampus, hypothalamus, caudate putamen and corpus callosum were reported in rats exposed to CMS compared to control animals, which may relate to reduced membrane density or tissue degeneration (Hemanth Kumar et al., 2014). This was accompanied by increased radial and axial diffusion in the frontal cortex and increased AD in the hypothalamus. It is suggested that increased RD may be due to demyelination while the AD alterations may be attributed to axonal loss. Conversely, decreased mean and radial kurtosis (magnitude of restrictions perpendicular to the principle direction of diffusion) has been reported in the hippocampus of animals exposed to CMS (Delgado y Palacios et al., 2011) irrespective of sensitivity to the stress protocol. Further work by the same group expanded the neuroimaging markers relating to stress sensitivity and resilience. Specifically, increased axial diffusion in the caudate putamen (CPu) demonstrates microstructural alterations within the CPu (Delgado y Palacios et al., 2014). Furthermore, increases in radial diffusion were observed in the amygdala (Delgado y Palacios et al., 2011) as well as frontal cortex and hypothalamus (Hemanth Kumar et al., 2014) of CMS animals. This may reflect dendritic atrophy, augmented arborization or demyelination as alterations in hippocampal kurtosis parameters were accompanied by decreased staining of microtubule associated protein 2, which is involved in stabilization of neuronal cytoarchitecture (Delgado y Palacios et al., 2011). In addition, by subdividing the CMS group into those sensitive or resilient to CMS Delgado et al. showed decreased mean kurtosis values in the caudate putamen of animals exhibiting stress-induced behavioral anhedonia compared to stress-resilient counterparts. Such alterations in diffusion metrics may be attributed to differences in dendritic arborization or potential alterations in microglial morphology due to activation states. Although, further studies are required to elucidate the molecular and cellular mechanisms underlying these microstructural changes, these data highlight the CPu as a region altered by stress and potentially valuable in identifying individuals vulnerable to stress. Furthermore, this highlights the importance of analyzing cohorts based on their behavioral responses to stressors as failure to identify possible “resilient” subgroups may result in misleading conclusions.

The effect of stress on tissue architecture in both control and stress sensitive WKY rats has also been investigated. WKY rats show decreased FA in the corpus callosum and anterior commissures and increased mean diffusion in both the fornix and corpus callosum compared to the Wistar comparator strain (Zalsman et al., 2016). Furthermore, exposing WKY rats to early-life stress produced significantly higher mean diffusion values compared to control in the amygdala, cingulate cortex and external capsule. Conversely, early stress decreased mean diffusion in the cingulate cortex, amygdala, and external capsule in Wistar rats (Zalsman et al., 2015). This suggests that stress alters the structural integrity of brain areas linked to emotional regulation differently in animals with genetic vulnerability.

DTI studies in depressed patients have shown decreased fractional anisotropy in the frontal cortex, cingulate and corpus callosum, changes which are mirrored in the preclinical research supporting the translational relevance of these measures. Interestingly, diffusion kurtosis imaging is not widely used in clinical neuroimaging, however preclinical results from CMS studies suggest kurtosis parameters, particularly in the caudate putamen, may be useful in identifying susceptible from resilient phenotypes and highlights the importance of animal models.

Throughout the years, the investigation of the biochemical alterations associated with MDD has highlighted changes in neurotransmitter systems of depressed patients (Crow et al., 1984; Cheetham et al., 1989; Yildiz-Yesiloglu and Ankerst, 2006; Rajkowska and Stockmeier, 2013). In particular, proton MRS is a non-invasive technique that has been often used to detect differences in tissue concentration of various chemical compounds in vivo (Jansen et al., 2006). Even on routinely available 1.5 T scanners, this technique can identify N-acetylaspartate (NAA), choline (Cho), and creatine (Cr). At higher field strengths (3-7 T), glutamate (Glu), glutamine (Gln) and myoinositol (Ins), can also be individually resolved. Finally, γ-Aminobutyric acid (GABA) as well as glutathione (GSH) are optimally quantified at the highest field strengths with special editing and acquisition techniques (Bustillo, 2013; Rae, 2014), although GSH has been shown to be reliably measured at lower fields using fitting routines such as LCModel after acquisition (Terpstra et al., 2003).

MRS studies have often reported a reduction of glutamate, the most common excitatory neurotransmitter, in the prefrontal cortex of MDD compared to controls (Yildiz-Yesiloglu and Ankerst, 2006; Luykx et al., 2012; Miladinovic et al., 2015). Glutamine (Gln), an amino acid synthetized from glutamate in the glia (Martinez-Hernandez et al., 1977), and GABA have also been found to be decreased in MDD in this region (Walter et al., 2009; Pehrson and Sanchez, 2015). Glutathione, which is involved in synthesis and degradation of proteins and DNA (Maher, 2005), has recently been found to be reduced in the occipital cortex of depressed patients, suggesting reduced anti-oxidative capacity in this cohort (Godlewska et al., 2015). Choline, which is related to membrane metabolism and turnover, has been found to correlate with mood in healthy subjects (Jung et al., 2002) and to be increased in MDD patients compared to healthy controls in the basal ganglia (Yildiz-Yesiloglu and Ankerst, 2006). Most studies have not shown any difference in NAA levels in depressed patients (Yildiz-Yesiloglu and Ankerst, 2006) and inconsistent results for Cr levels (Venkatraman et al., 2009). Thus, the most consistent findings in MDD are decreased excitatory and inhibitory neurotransmitters as well as glutathione, and elevated Ch levels in some regions, indicative of increased membrane phospholipid turnover.

The low signal to noise ratio and large voxel size required has hindered the progression of preclinical MRS studies, however with greater sensitivity achieved with magnetic fields as high as 11.7T currently available, several recent studies have shown alterations in a range of metabolites in animal models of depression.

Exposing the offspring of dams subjected to CMS to an acute stressor in adolescence (a single forced swim test) decreased glutamate MR signal in the right hippocampus (Huang et al., 2016). Similarly, chronic exposure to the FST has been reported to decrease the glutamate signal in both the prefrontal cortex and hippocampus in rats (Li et al., 2008). Reductions in the combined glutamate and glutamine signal have also been reported in the prefrontal cortex of mice exposed to CMS (Hemanth Kumar et al., 2012). In contrast, Delgado y Palacios et al. (2011) report increased hippocampal glutamate levels in animals susceptible to CMS compared to both resilient and control groups. These conflicting changes in hippocampal glutamate may be due to the type of stressor used (acute compared to chronic) or regional differences within the hippocampus. Delgado et al. positioned a 2 mm³ voxel in the ventral part of the left hippocampus whereas Huang et al. (2016) positioned their 2.5 × 4 × 4 mm voxel in the dorsal area of the right hippocampus. Similar decreases in dorsal hippocampal glutamate have been reported in rats bred for learned helplessness (cLH) compared to Sprague-Dawley controls and also following CMS (Hemanth Kumar et al., 2012; Schulz et al., 2013).

Schulz et al. also report reductions in choline in both the hippocampus and frontal cortex (encompassing the cingulated and motor cortex) of cLH rats (Schulz et al., 2013). Conversely, a recent study reported increased choline levels in the prefrontal cortex following chronic psychosocial stress (Grandjean et al., 2014), which may reflect differences in preclinical models used (genetic vs. stress) or regional differences between the frontal and prefrontal cortex. Alterations in choline levels have also been shown by Han et al. with increased levels in both the hippocampus and amygdala following a single prolonged restraint stress (Han et al., 2015). Choline has also been shown to be sensitive to antidepressant treatments as Sartorius et al. (2003) showed electroconvulsive stimulation (ECS), the preclinical model of ECT, increased choline levels in the hippocampus of normal animals. Similarly, ECS increased the absolute concentrations of choline in the hippocampus of cLH rats but in contrast to the study by Sartorius et al., there was no effect in wild type control animals (Biedermann et al., 2012). This may be due to normalization of the data or comparison to a baseline scan in Sprague-Dawley control rats. Interestingly, choline MRS signal was also increased in the prefrontal cortex of patients treated with the SSRI paroxetine (Zhang et al., 2015).

Previous studies have examined the effect of early life adversity in the form of maternal separation (MS) on metabolite concentrations, however no difference between MS animals and controls was observed however both environmental enrichment and escitalopram treatment increased NAA levels in the (Hui et al., 2010, 2011).

Myoinositol (mI) levels can be indicative of glial cell function or proliferation and it has been considered a proxy marker of inflammation (Kousi et al., 2013). Preclinical MRS studies have shown increased hippocampal myoinositol levels following both CMS (Hemanth Kumar et al., 2012) and the acute FST (Kim et al., 2010). Similarly, increased mI levels have been observed in the amygdala following chronic psychosocial stress (Grandjean et al., 2016). Together, these data suggest a possible link between glial function and depression pathology.

Increased levels of GABA have been reported in the hippocampus of CMS-offspring exposed to an acute stressor (Huang et al., 2016). Ex vivo ¹H-MRS studies support elevated GABA levels following CMS and restraint stress with increases observed in the anterior cingulate and prefrontal cortex, respectively (Perrine et al., 2014; Drouet et al., 2015) suggesting that altered inhibitory neurotransmission may be involved in the pathophysiology of CMS.

Alterations in the N-acetyl aspartic acid (NAA), a marker commonly used to indicate neuronal integrity have also been reported, with decreased hippocampal NAA levels following both mild and unpredictable chronic stress paradigms (Xi et al., 2011; Hemanth Kumar et al., 2012) as well as chronic exposure to the FST (Li et al., 2008). Xi et al. (2011) report that hippocampal NAA reductions were normalized by chronic escitalopram treatment (4 weeks). Conversely, Han et al. (2015) showed increased NAA in both the amygdala and hippocampus following a single prolonged stress. This may be due to single vs. repeated stress protocols used. In addition, MRS studies in three mouse lines bred for HPA axis stress reactivity have revealed decreased N-acetylaspartate (NAA) levels in the dorsal hippocampus and prefrontal cortex which is coupled with decreased levels of performance in hippocampal dependent spatial memory tasks when compared with mice with low HPA axis stress reactivity (Knapman et al., 2012).

It is worth noting that an advantage of animal models is that metabolite concentrations in can also be assessed through invasive or post-mortem methods such as in vivo microdialysis or ex vivo assessment of tissue concentrations. These techniques could be used to compliment or validate in vivo MRS findings.

Results from both preclinical and clinical MRS studies implicate altered glutamatergic signaling in depression with studies in both depressed patients and animal models reporting decreased glutamate and/or glutamine signal, particularly in the prefrontal cortex (Yildiz-Yesiloglu and Ankerst, 2006; Li et al., 2008; Walter et al., 2009; Hemanth Kumar et al., 2012). Clinical MRS results for other metabolites are inconsistent and depend upon subregion investigated, disease duration, depression subtype, and medication (Yildiz-Yesiloglu and Ankerst, 2006) however some studies report increased choline signal in the frontal cortex (Steingard et al., 2000; Farchione et al., 2002), which is also shown in animal models (Han et al., 2015; Grandjean et al., 2016). Interestingly, reductions in the hippocampal glutamate signal differentiated susceptible from resilient animals when exposed to CMS (Delgado y Palacios et al., 2011) and similar alterations have been shown in depressed patients (Block et al., 2009). Alterations in certain metabolite concentrations in preclinical models could be validated through in vivo microdialysis or ex vivo assessment of tissue concentrations.

Functional MRI (fMRI) provides a tool to map regional changes in cerebral blood flow at resting state, i.e., in the absence of task execution. Depressed patients have been reported to have decreased dorsolateral prefrontal cortex activation (Alcaro et al., 2010) as well as increased baseline activity in the orbitofrontal and subgenual anterior cingulate cortices (for review see Hasler and Northoff, 2011) relative to controls. Increased resting state perfusion as assessed via arterial spin labeling (ASL) in the subgenual anterior cingulate cortex has also been shown specifically in treatment resistant depressed patients relative to controls (Duhameau et al., 2010). These alterations are sensitive to treatment: resting state perfusion as assessed via ASL in the anterior cingulate cortex, for example, was reduced (Clark et al., 2006) and dorsomedial prefrontal cortical activity was increased following chronic venlafaxine and fluoxetine treatment in depressed patients (Savitz and Drevets, 2009). Hence, unlike some of the structural alterations discussed above, brain functional alterations appear to be state dependent markers of MDD. This is potentially unsurprising as quite modest changes in plasticity may induce marked changes in blood flow or neuronal activity however alterations in brain structure at a resolution detectable would be harder to induce.

The default mode network is a set of brain regions which are more active at rest than during task execution, which include the orbital frontal cortex, the medial prefrontal/anterior cingulate cortex, the lateral temporal cortex, the inferior parietal lobe, the posterior cingulate and retrosplenial cortex, the hippocampus and parahippocampal cortex (Lu et al., 2012). Hyperactivity in the default mode network during task performance in depressed patients has been reported and interpreted as a possible MRI marker for rumination (for review see Whitfield-Gabrieli and Ford, 2012). Hyper connectivity between the subgenual anterior cingulate and default mode network at rest has also been observed in MDD (Greicius et al., 2007; Berman et al., 2011), and increased default mode network connectivity with the dorsolateral prefrontal cortex are thought to underlie some aspects of emotional dysregulation characteristic of the disorder (Sheline et al., 2010).

Preclinical studies have begun to employ fMRI to investigate regional activity and functional connectivity in vivo and extraction of the blood oxygen level dependent (BOLD) signal is commonly used as surrogate marker of neuronal activation. Recent studies have shown altered BOLD signal intensity in several brain regions involved in mood regulation following exposure to early-life adversity or stress. Hui et al. (2010) recently showed that animals subjected to the early life maternal separation protocol exhibited increased BOLD signal in the insular lobe, hypothalamus, limbic system, hippocampus, and frontal lobe in adulthood. Similarly, increased BOLD activation was observed in the hippocampus, limbic system and temporal lobe of rats exposed to CMS.

Recent advances in preclinical neuroimaging have allowed the identification of similar resting-state networks in rodents and humans (Lu et al., 2012; Sierakowiak et al., 2015; Zerbi et al., 2015; Gozzi and Schwarz, 2016) and the analysis of these networks in disease models.

Grandjean et al. have shown that chronic psychosocial stress increased within-network functional connectivity in the cingulate cortex and sensory cortical networks, specifically the supplementary, barrel field 1 and 2, and visual cortices in mice (Grandjean et al., 2016). In addition there was also increased between-network functional connectivity between the prefrontal cortex and both the amygdala and piriform cortex; ventral hippocampus and the amygdala and between the cingulate cortex and both the piriform cortex and the amygdala following psychosocial stress. Rodents have also been shown to have a default mode network similar to that seen in humans (Lu et al., 2012) and 10 days of immobilization stress has been reported to increase resting state functional connectivity within this network in addition to the visual and somatosensory networks (Henckens et al., 2015). Altered functional connectivity has also been shown in the congenital learned helplessness rat with increased correlation between activity in the DRN and the somatosensory cortex, orbital cortex, frontal cortex, and caudate-putamen. Enhanced correlations were also observed between the ventral hippocampus and the retrosplenial cortex and caudate putamen and also between the retrosplenial cortex and area 1 of the cingulate cortex (Gass et al., 2014a). Interestingly hippocampal-cingulate connectivity has been positively correlated with depression severity in patients, drawing parallels with the increased hippocampal-retrosplenial connectivity reported in cLH rats. By contrast, there are reductions in inter-hemispheric connectivity, particularly in the sensory, motor, cingulated and infralimbic cortices, the nucleus accumbens and the raphe nucleus of cLH rats (Ben-Shimol et al., 2015). Similarly, the WKY rat has shown increased functional connectivity between the hippocampus and the left frontal association cortex/dorsolateral orbital cortex in a “more immobile” cohort compared to “less immobile” counterparts (Williams et al., 2014). In contrast, hypoconnectivity was observed with the hippocampus and the left somatosensory cortex, the left ventral striatum partially inclusive of the nucleus accumbens core, the bilateral cingulate cortex, the bilateral lateral septum and the left caudate.

A study investigating resting state fMRI in serotonin receptor 1A knockout mice showed reduced functional connectivity between cortical areas (including prefrontal, retrosplenial, and entorhinal cortices) and the hippocampus (mainly CA1 and dentate gyrus, DG). Similarly, reduced functional correlations were observed between the dorsomedial thalamus and both the cortex and hippocampus (Razoux et al., 2013).

Alterations in functional connectivity reported in animal models of depression are displayed in Figure 1. Studies in both depressed patients and animal models of depression consistently report hyperactivity within the default mode network, comprised of the orbital, prelimbic, auditory/temporal association, parietal, and retrosplenial cortices and the dorsal hippocampus in the rat. In addition, studies in depressed patients have also shown hyperactivity between the cingulate cortex and the amygdala, parts of the affective network, which is also reported in the rodent literature. Interestingly, sub-anaesthetic doses of ketamine increased functional connectivity between frontocortical regions and also the prefrontal cortex and the hippocampus in both rats and humans (Grimm et al., 2015; Becker et al., 2016) suggesting that resting state fMRI may be a valuable translational tool for investigating the actions of novel antidepressants.

Manganese-enhanced MRI (MEMRI) is an emerging neuroimaging technique that exploits the capability of manganese (Mn) ions to enter excitable cells through voltage-gated calcium channels. As Mn ions decrease water longitudinal relaxation time this uptake can be visualized by MRI and as such MEMRI is being increasingly used in preclinical studies to investigate alterations in functional networks. Daducci et al. (2014) showed that chronic administration of interferon alpha to rats, commonly used to model chemotherapy-induced depression, reduced the pituitary volume and also reduced activation in cortical areas, particularly the visual and sensory cortices. Reduced MEMRI signal following manganese injection into the raphe nucleus has been shown in rats exposed to CMS in the substantia nigra, hippocampus, entorhinal cortex and the insular cortex, while increased signal was observed in the medial septal nucleus compared to non-stressed control animals (Gordon and Goelman, 2016) suggesting impaired serotonergic connectivity between the raphe and the substantia nigra and hippocampal areas.

High resolution MRI has revealed discrete reductions in regional cerebral blood volume (rCBV) in the left habenula, particularly the lateral area, and increased rCBV in the bed nucleus of the stria terminalis (BNST) in the congenital learned helplessness (cLH) model of depression (Gass et al., 2014a). Altered cerebral blood perfusion in preclinical models of depression has also been measured by bolus-tracking arterial spin labeling (bt-ASL). This non-invasive method directly assesses blood perfusion and provides quantitative measures relating to blood flow. Decreased cerebral perfusion was observed in the striatum and pre-limbic cortex of WKY rats compared to Wistar controls and interestingly, cerebral perfusion correlated with GFAP-positive cell number such that increased GFAP expression corresponded with increased cerebral perfusion (Gormley et al., 2016). In contrast no alterations in regional cerebral perfusion were reported in the OB rat model of depression (Gigliucci et al., 2014), again highlighting that many MR markers associated with depressed phenotypes may be “model” or patient specific.

Pharmacological MRI is becoming a more widely used tool in both preclinical and clinical drug research (Jonckers et al., 2015). Insights gained from clinical MRI studies are frequently confounded by illness, chronicity and medication use when attempting to determine if antidepressant drug treatment may influence gray matter volumes or functional MR parameters. PharmacoMRI coupled with in vivo structural MRI in animal models represents an approach which allows for effects of acute and chronic drug treatment and subsequent withdrawal, with clinically relevant dosing to be determined on brain structure and function. Findings may then be further evaluated and confirmed in the post-mortem brain using stereological histochemistry. Vernon et al. (2012) reported that chronic lithium treatment in rats induced an increase in whole-brain volume and cortical gray matter without a significant effect on striatal volume. Increased total brain volumes were subsequently confirmed post-mortem. Comparisons with the antipsychotic haloperidol indicated that the distribution of changes was topographically distinct.

Reductions in brain activation are observed with phMRI following acute treatment with the serotonin (5-HT) reuptake inhibitor (SSRI) fluoxetine (10 mg/kg oral or 5 mg/kg i.v.) in rats (Bouet et al., 2012). Subsequently Klomp et al. (2012) employed pharmacoMRI to assess the effects of chronic treatment with fluoxetine (5 mg/kg orally for 3 weeks) in juvenile (post natal day 25) and adult rats (post natal day 65) following a 1 week washout, using an acute fluoxetine challenge (5 mg/kg i.v.) to trigger the serotonergic system. A significant age by treatment interaction was observed in several subcortical brain regions related to 5-HT neurotransmission, suggestive of differential neuronal effects of SSRI treatment in juveniles that may underlie emotional disturbances seen in adolescents treated with fluoxetine. Harris and Reynell (2016), using data derived from animal studies, consider the underlying mechanisms of antidepressant treatment-related changes in BOLD including alterations in neurovascular coupling and brain energetics and metabolism.

To aid interpretation of spatially distributed activation patterns in response to pharmacological stimuli, Bruns et al. (2015) have proposed a set of multivariate metrics termed domain gauges which are calibrated based on different classes of reference drugs including antidepressants. The profile provides quantitative activation patterns with high biological plausibility with the potential to be developed as a valuable analytical tool for interpretation and decision making in drug development.

A number of recent investigations have assessed the effects of sub-anaesthetic doses of ketamine on intrinsic BOLD connectivity within hippocampal-prefrontal circuits in the rat (Gass et al., 2014b; Becker et al., 2016). Hippocampal-prefrontal cortical connectivity is a key substrate hypothesized to be associated with cognitive and emotional state in central nervous system disorders. Ketamine, a NMDA receptor antagonist, is a psychomimetic agent and rapidly acting antidepressant in the clinic (Zarate et al., 2006) and thus understanding its acute modulatory effect on functional connectivity in the rat brain using rs-fMRI is of significant interest. Ketamine provokes a dose dependent increase in prefrontal connectivity (between the posterior hippocampus, retrosplenial cortex, and prefrontal regions) and these changes are highly concordant and directionally consistent with network reconfigurations observed in humans (Becker et al., 2016). A recent investigation of network reconfiguration induced by ketamine in anaesthetized monkeys assessed differences in functional networks 18 h after drug administration. Here, a down regulation most prominently in the orbital prefrontal cortex, the subgenual and posterior cingulate cortices and the nucleus accumbens were reported (Lv et al., 2016). The changes were reported to oppose the maladaptive alterations characteristic in the depressed brain and further suggested to reflect alterations in local synaptic plasticity triggered by blockade of NMDA receptors leading to network reconfiguration within cortico-limbic circuits. The technique may therefore serve as a novel approach for the identification of translational imaging biomarkers in drug development.