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192 The APA Publishing Textbook of Mood Disorders, Second Edition
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dependent (BOLD) measures and oxygen perfusion using arterial spin labeling (ASL). BOLD imaging essentially measures the amount of oxygenated hemoglobin in the brain as a positive signal; the level of oxygenation increases in regions of activation because the brain is designed to oversupply blood to brain areas that are in use. ASL works by using magnetic pulses to “label” a volume of blood that can then be followed as it travels through the brain. MRI can also measure neurotransmitters and energy sources that allow the brain to function using magnetic resonance spectroscopy (MRS). MRS capitalizes on the magnetic differences of neurochemicals to assess their concentrations. PET and SPECT can be used to measure blood flow, neurotransmitter concentrations, and glucose metabolism by measuring the effects of radioactive me tabolites (radiotracers) injected into the bloodstream, although these approaches are less commonly used since MRI technology has rapidly advanced. However, PET and SPECT use labeled isotopes to study specific brain activities (e.g., the number of dopa mine receptors or the metabolism of glucose) that cannot yet be studied with MRI. Detailed descriptions of these techniques exceed the scope of this chapter; interested readers are referred to the reviews of Gonul et al. (2009) and Videbech (2000).
These neuroimaging modalities have been used to identify biological markers of mood disorders that may be used for early identification and diagnosis and to de velop more effective treatments. At present, however, no neuroimaging techniques have a direct clinical application in the care of people with mood disorders.
In the sections that follow, we introduce the neural networks that are believed to underlie key clinical features of mood disorders—the emotion processing and regu lation, reward processing, and resting state networks (Figure 11–1)—and discuss ab­normal activity and abnormal functional connectivity within these networks. Most of the findings presented are from research utilizing functional MRI (fMRI), given that these studies dominate the current literature. We then address gray and white matter structural abnormalities associated with mood disorders. Findings from studies us­ing PET and MRS that examined regional neurotransmitter concentration and neu­roreceptor density are discussed in relation to larger-scale neural network models of mood disorders. Together, these results contribute to emerging functional neuroana tomical models of mood disorders that may eventually increase diagnostic precision, identify markers of risk for future mood disorders, and lay the foundation for person alized treatments.
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Abnormal Activity and Functional Connectivity in Neural Networks Underlying Clinical Features of Mood Disorders
Emotion Processing and Regulation Neurocircuitry
Emotion processing and regulation arise from ventral prefrontal-striatal-thalamic networks and their interactions with the amygdala and other brain regions. More spe­cifically, emotion processing involves a network that includes the ventrolateral and ventromedial, dorsal, and medial prefrontal cortices; anterior cingulate cortex; amyg­dala; and hippocampus (see Figure 11–1A). Studies of people with major depressive
FIGURE 11–1. Key nodes in emotion processing and regulation and in reward processing neural circuitries in healthy individuals.
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(A) Key nodes in emotion processing and emotion regulation neural circuitries in healthy individuals. (B) Key nodes in reward processing neural circuitry in healthy individuals. ACC=anterior cingulate cortex; OFC=orbitofrontal cortex; PFC= prefrontal cortex.
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disorder typically find abnormally reduced activity in these networks during expo­sure to positive emotional stimuli, and increased activation during exposure to nega­tive emotional stimuli. Specifically, findings include abnormally decreased activity in the lateral, orbital, and medial portions of the prefrontal cortex and elevated activity in the amygdala in depressed individuals during exposure to negative emotional stimuli (Groenewold et al. 2013; Rive et al. 2013). In contrast, activity during exposure to positive emotional stimuli is less predictable (Groenewold et al. 2013; Rive et al.
2013). Overrecruitment of the anterior cingulate cortex in response to both positive and negative emotional stimuli suggests that depressed individuals have difficulty recruiting prefrontal cortical regions to regulate the overactive limbic regions, partic­ularly during exposure to negative emotional stimuli. Functional connectivity stud­ies, which describe patterns of coactivation between two or more neural regions, also support this deficit in amygdala-prefrontal functional connectivity in response to negative emotional stimuli in depressed individuals relative to healthy individuals (Anand et al. 2005).
In individuals with bipolar disorder, this emotion processing neurocircuitry shows heightened emotional salience even during exposure to nonemotional content. More specifically, individuals with bipolar disorder show abnormally decreased ventrolat­eral prefrontal cortical activity during tasks involving emotion processing and emo­tion regulation, and response inhibition (Chen et al. 2011). These studies report abnormally decreased inferior frontal cortical activity, especially in the ventrolateral prefrontal cortex, and abnormally decreased ventrolateral prefrontal cortex–amyg­dala functional connectivity during different positive and negative emotion process­ing and emotion regulation tasks in adults with bipolar disorder across different mood states (Strakowski et al. 2011).
Furthermore, individuals with bipolar disorder also show abnormally increased ac­tivity in the amygdala, ventral striatum, and medial prefrontal cortex and decreased functional connectivity between the amygdala and prefrontal cortex, during exposure to positive emotional stimuli (Phillips and Swartz 2014). Here, the studies in bipolar disorder show abnormally increased amygdala and medial prefrontal cortex activity (Almeida et al. 2009; Chen et al. 2011) and abnormally decreased positive bilateral or bitofrontal cortex–amygdala effective connectivity in response to positive emotional stimuli, especially happy faces (Almeida et al. 2009), suggesting a dysregulated emo tional network response to these stimuli. These findings may reflect an underlying at­tentional bias to positive emotional stimuli in bipolar disorder, predisposing to or associated with risk for mania. However, others found that the prefrontal-amygdala­striatal circuit distinguished depressed from manic mood states during processing of negatively valenced information (Grotegerd et al. 2014; Man et al. 2019). In summary, in both (unipolar) major depressive disorder and bipolar disorder, dysregulation of emotion processing and emotion regulation networks has been consistently demon­strated through a variety of neuroimaging approaches and tasks.
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Reward Processing Neurocircuitry
Reward-related neurocircuitry is centered on medial and lateral prefrontal cortical modulation of the ventral striatum (Figure 11–1B), primarily through dopaminergic systems. Reward processes are related to mood state, so it is not surprising that there
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is overlap in the neural networks involved in reward and emotion processing. Re­ward processing includes periods of motivation, anticipation, and satiety. Consump­tion of rewards induces feelings of pleasure, leading to reinforcement of reward cues and behaviors (Schultz 2006). Reward-seeking and learning tend to decrease during depressive episodes and are heightened during manic periods. Neuroimaging stud ies assess the neurocircuitry of reward using a variety of paradigms, including mon­etary, social, and appetitive rewards. These human studies (Beckmann et al. 2009; Kumar et al. 2008; Ongür and Price 2000) have confirmed the presence of a reward network previously identified in animal studies (Haber 2016), consisting of the ven tral striatum with connections to the ventral tegmental area and the ventral pallidum, which then connect (through the thalamus and anterior cingulate) to the orbitofrontal cortex. In animal studies, reward processing tends to localize within the striatum to the nucleus accumbens; however, in humans, this nucleus is less well delineated, so activation occurs at the ventral striatal–putamen junction. This activation is blunted or unsustained in depressed individuals (Zhang et al. 2013), and cortical regions that regulate the ventral striatum, such as the anterior cingulate and middle frontal cortex, show abnormal activation in response to rewarding stimuli and reward anticipation (Zhang et al. 2013). Studies in bipolar disorder have reported abnormally increased left ventrolateral prefrontal cortex and ventral striatal activity during reward antici­pation in adults with bipolar disorder in different mood states (Bermpohl et al. 2010; Nusslock et al. 2012; Phillips and Swartz 2014). However, others found no differences in activity in the ventral striatum in response to reward receipt versus omission in manic adults with bipolar disorder (Abler et al. 2008). Collectively, data suggest that depression and mania may be associated with dysfunction of these neural networks that manage rewards and reward learning, but further study is required.
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Resting State Neurocircuitry
Studies of resting state functional connectivity examine measures of context­independent neural circuits and may represent fundamental abnormalities in the functional neuroanatomy of mood disorders. Measuring resting state functional con­nectivity is based on the discovery that low-frequency (<~0.1 Hz) BOLD fluctuations in functionally related gray matter regions show strong temporal correlations at rest (i.e., they vary together over time). Resting state studies are useful in clinical popula­tions because of the ease of image acquisition; however, this technique is vulnerable to individual variations in behavior, arousal, and head motion in the scanner.
Studies during rest have identified distributed regions—including the default mode, salience, attentional, and emotion regulation networks—that are abnormally activated during rest in participants with MDD (Kaiser et al. 2015). The default mode network, which comprises coactivation among the hippocampus, medial prefrontal cortex, posterior cingulate cortex, and temporal cortex (Drevets et al. 2008), is norma tively activated during rest. The salience network consists of coactivation between the anterior cingulate cortex and frontoinsular regions (Chiong et al. 2013). The attentional
network consists of dorsal and ventral networks that work in concert to orient atten­tion and consists of visual areas, inferior frontal cortex, and parietal and temporal cor­tices (Vossel et al. 2014). The emotion regulation network includes regulatory regions of the anterior cingulate cortex and the lateral and medial prefrontal cortex and subcor-
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tical regions of the amygdala, insula, and caudate (Etkin et al. 2015). Findings suggest that interference of other networks during rest may be related to mood disorder pa thology. Specifically, in individuals with MDD during rest, abnormally low connec­tivity within the attentional and emotion regulation networks (Kaiser et al. 2015) and abnormally high connectivity within the default mode and salience networks (Kaiser et al. 2015; Sundermann et al. 2014) have been shown, and these abnormalities may become more pronounced with repeated depressive episodes (Meng et al. 2014).
In individuals with bipolar disorder, Brady et al. (2017) found two patterns of al­tered functional connectivity that differentiated mood states in bipolar disorder as well as distinguishing bipolar euthymia from a healthy population: 1) dorsal atten tional network—the bipolar euthymia group demonstrated hypoconnectivity and the mania group demonstrated hyperconnectivity relative to a healthy control population;
2) default mode network—the bipolar euthymic state demonstrated hypoconnectivity, while bipolar mania demonstrated connectivity comparable to that of a healthy popu lation between the dorsal frontal nodes and the rest of the network. Moreover, Martino et al. (2020) found increased functional connectivity between the sensorimotor net work and the thalamus during bipolar mania that was absent during bipolar depres­sion. In summary, mood disorders may be associated with a specific functional neuro­anatomic architecture, although the details of this architecture are still incompletely defined.
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Abnormal Cerebral Blood Flow
Cerebral blood flow during emotion processing, reward processing, and rest shows hypoperfusion in bipolar participants during both depressed and manic mood epi sodes (Toma et al. 2018). Both bipolar and unipolar depressed participants showed hypoactive cerebral blood flow during rest, especially in regions involved in emotion processing and cognitive control, such as the anterior cingulate, frontoparietal, and striatal regions (Toma et al. 2018; Vasic et al. 2015).
Neural Structural Abnormalities
Abnormal Gray Matter Structure
In the brain, gray matter measurements are thought to reflect the density of dendrites, short-range axons, and glial cell bodies (Zatorre et al. 2012). Individuals with mood disorders, relative to healthy subjects, showed decreased gray matter in cortical (ante­rior cingulate cortex and across the prefrontal cortex) and subcortical regions (caudate, putamen, hippocampus, and insula), especially after recurrent affective episodes (Ar none et al. 2012; Bore et al. 2012; Phillips and Swartz 2014). In bipolar disorder, gray matter volume may also be used as a proxy measure of neurotrophic exposure to lith­ium medication, because several studies and a meta-analysis suggest that individuals with bipolar disorder who are being treated with lithium have a greater global and
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regional (e.g., anterior cingulate gyrus) gray matter volume than individuals with bi­polar disorder who are lithium-free (Sun et al. 2018), although not all investigators have replicated these findings (see, e.g., Eker et al. 2014). These findings provide evi dence of structural abnormalities that parallel the functional abnormalities associated with emotion processing, emotion regulation, and reward processing neurocircuitries in mood disorders.
Abnormal White Matter Structure Measured With Diffusion Imaging
Diffusion imaging allows measurement of the architecture of the white matter fiber tracts by approximating the movement of water along the axon. Greater longitudinal diffusivity reflects high cohesion along the length of the axis, perhaps reflecting greater myelination and higher tract organization. By contrast, greater radial diffusiv- ity reflects greater movement of water molecules perpendicular to the tract length and suggests a less linear structure and may reflect damaged axonal membranes and/or myelin sheaths. Fractional anisotropy is the ratio of longitudinal diffusivity to radial diffusivity, with high fractional anisotropy characterizing densely packed collinear white matter fibers, along with few noncollinear fibers.
Key white matter tracts that connect the prefrontal cortical and subcortical regions involved in emotion regulation and reward processing neural circuits include the su perior longitudinal fasciculus, uncinate fasciculus, corpus callosum, and cingulum (Versace et al. 2014). Meta-analyses of studies in depressed patients have shown re ductions in fractional anisotropy in the corpus callosum extending to the body of the corpus callosum and the left anterior limb of the internal capsule (Chen et al. 2016; Jiang et al. 2017). Moreover, decreased uncinate fasciculus fractional anisotropy is co nsistent ly f ound in ad ults with dep res sio n (Bracht et a l. 20 15). Findin gs of abnormal fractional anisotropy in the medial forebrain bundle and cingulum bundle have also been shown and may be associated with severity of depressive symptoms (Bracht et al. 2015). Abnormally reduced fractional anisotropy in tracts such as the uncinate fas ciculus, which connects the prefrontal cortical and subcortical regions, may underlie some of the functional abnormalities in emotion regulation and reward processing observed in depressed individuals. In bipolar disorder, several meta-analyses of white matter volumes converged to identify reductions in the posterior corpus callo­sum (Arnone et al. 2008; Ganzola and Duchesne 2017; Kempton et al. 2008; Pezzoli et al. 2018), and in one study, reduced volumes extended to the adjacent posterior cin­gulate cortex (Pezzoli et al. 2018). The observed white matter decrease seems to be a trait marker of disease, because it is not typically correlated with other clinical vari ables. The white matter region lies adjacent to the posterior cingulate cortex, which is a core region of the default mode network (Greicius et al. 2003). Finally, increased white matter hyperintensities continue to be the most replicated finding in bipolar disorder. However, this finding lacks specificity, given that it is also seen in other psy­chiatric disorders, including major depressive disorder, as well as in normal aging and in other medical illnesses that impact the brain.
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Magnetic Resonance Spectroscopy Studies of Neurotransmitters Implicated in Mood Disorders
Noninvasive detection of metabolites, such as choline, N-acetylaspartate, and lactate, can be measured using MRS. Utilizing the chemically specific radiofrequency signals within metabolites and varying the magnetic fields, MRS can estimate neurotransmit ter concentrations in neural regions of interest. We describe here two neurotransmit­ters implicated in mood disorders because of their roles in inhibition and excitation: GABA and glutamate.
GABA is the principal mediator of inhibition in the CNS. GABA concentration deficits have been demonstrated in depressed individuals in occipital, medial pre frontal, and anterior cingulate cortices. These deficits appeared to resolve with remis­sion from depression but to remain low in individuals with recurrent or treatment­resistant depression (Schür et al. 2016). GABA concentrations were also negatively as sociated with anhedonia severity (Gabbay et al. 2012). These findings suggest that high concentrations of GABA in the cortex are needed to successfully regulate nega tive symptoms of anhedonia.
Glutamate may also have a relationship with mood disorders. It is the principal ex­citatory neurotransmitter in the CNS. In depressed individuals, glutamate concentra­tions have been reported to be increased in the occipital cortex and decreased across the prefrontal cortex and anterior cingulate cortex (Lener et al. 2017). Changes in gluta mate levels after treatment for depression were not consistently associated with symp­tom improvement (Lener et al. 2017). Additional research is needed to understand the relationship between glutamate concentrations in the cortex and depression.
Individuals with bipolar I disorder who were being treated with long-term phar­macotherapy showed increased glutamate and reduced N-acetylaspartate concentra­tions, which, combined, suggested neuronal metabolic dysfunction (Bustillo et al.
2019). Moreover, glutamate and N-acetylaspartate concentrations were also relatively lower in the anterior cingulate cortex of bipolar individuals who had experienced multiple episodes compared with those who were experiencing their first episode (Borgelt et al. 2019).
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Newer Areas of Neuroimaging Research in Mood Disorders
Neuroimaging studies have focused on specific subgroups of individuals with mood disorders. This research includes studies comparing individuals with major depres­sive disorder and individuals with bipolar depression, as well as studies of specific mood states, neural correlates, and predictors of treatment response in individuals with mood disorders, risk for the development of mood disorders, and pediatric-onset mood disorders. Although an in-depth discussion is beyond the scope of this chapter, briefly, this research supports findings from studies of adults with mood disorders and suggests that emotion processing and reward processing neurocircuitries are ab-
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normal in mood disorder pathology. It also identifies other neural regions, such as the insula and precuneus, that may be implicated in mood disorders. However, use of these findings as biomarkers of disease is not yet feasible. Machine learning is a prom ising approach for identifying individual subject–level patterns of neurocircuitry ab­normalities in mood disorders. These approaches can identify individuals with mood disorders with 70%–80% accuracy. Improvements in the use of machine learning tech nologies in conjunction with neuroimaging measures have the potential to improve the diagnosis of psychopathology as well as identification of optimal treatment choices.
Familial Risk and Resilience
Risk for mood disorder development can be elevated by genetic risk (i.e., having a par­ent or sibling with a mood disorder) or symptomatic risk (i.e., having subthreshold symptoms or a clinically related disorder such as an anxiety disorder). Patients and high-risk relatives showed similar abnormalities in neural activation and functional connectivity during emotional tasks; however, resilient relatives had larger cerebral volumes, enhanced prefrontal connectivity during tasks, and heightened functional coactivation of the default mode network during rest (Frangou 2019). Unaffected rela tives also showed a constellation of differences, including greater gray matter volumes in the frontal, prefrontal, and temporal regions and hyperactivation in the amygdala, frontal cortex, and striatum (Cattarinussi et al. 2019), suggesting neural compensation in these individuals. In addition, higher anterior cingulate activity and greater func­tional connectivity with the amygdala has been highlighted as a potential marker of genetic risk for bipolar disorder (Acuff et al. 2018).
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Pediatric Mood Disorders
The emergence of mood disorders during childhood or adolescence often portends more severe symptoms and increased risk of suicide in comparison with mood dis order emergence in adulthood. Neural networks that are implicated in emotion and reward processing are also important in pediatric depression. In depressed youth, patterns of increased activity in the ventrolateral and dorsolateral prefrontal cortex and anterior cingulate cortex have been shown (Miller et al. 2015). In youth with bi­polar disorder, abnormal patterns of increased amygdala activity, decreased ventrolat­eral prefrontal cortical activity, and decreased prefrontal cortical–amygdala functional connectivity during emotion processing were reported (Frank et al. 2015). Corroborat­ing the activity and functional connectivity findings, decreased cortical and subcorti­cal gray and white matter measures have also been observed in youth with pediatric­onset bipolar disorder (Acuff et al. 2019).
Summary
Neuroimaging studies of mood disorders have identified specific patterns of neural network dysfunction and related structural abnormalities. Neural network abnor­malities include dysfunction in bilateral prefrontal cortical (especially ventrolateral prefrontal cortex and orbitofrontal cortex)–hippocampal–amygdala emotion process-
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ing and emotion regulation networks along with hyperactivity in predominantly left­sided ventrostriatal-ventrolateral prefrontal cortex reward processing neurocircuitry. Additionally, parallel abnormalities in resting-state functional connectivity and gray and white matter structure in these networks have been shown. Bioenergetic abnor­malities that may be related to mitochondrial dysfunction, abnormal neuronal glial interactions, and abnormal glucose metabolism in some of the same regions showing gray matter loss have been reported. The combination of these functional and struc tural abnormalities may underlie the behavioral abnormalities associated with mood disorders, such as emotional lability, emotional dysregulation, and reward processing dysregulation.
Future Directions in Neuroimaging Research in Mood Disorders
Neuroimaging techniques are helping us better understand the neurocircuitry of mood disorders and are making significant contributions to our understanding of mood disorder pathophysiology (Gotlib and Hamilton 2008; Zhang et al. 2016). Many consistencies across neuroimaging studies and methods are reported, although in­consistencies are also seen. Inconsistencies may be the result of imaging parameters or analytic differences but may also represent important differences in subcategories of mood disorders related to symptom dimensions and symptom development. Some approaches to improving current methods include the following:
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1. Using dimensional symptomatic approaches to examine pathological behaviors
that cut across conventionally defined diagnostic categories of mood disorders
2. Incorporating longitudinal designs to identify developmental trajectories of ab-
normal neurocircuitry associated with symptoms and with risk
3. Merging multimodal neuroimaging techniques and other biological system–level
approaches to examine the impact of genetic variation and molecular-level pro cesses on neurocircuitry development
4. Implementing pattern recognition techniques in combination with neuroimaging
methodologies to identify individual-level neurocircuitry markers that not only help classify individuals into present diagnostic groups but also help predict indi vidual-level clinical course
Collectively, these approaches will enhance our understanding of the complex neuropathophysiological processes underlying abnormal behaviors associated with mood disorders, and they may ultimately provide individual-level biological mark­ers that have clinical utility for diagnosis and future illness prediction, and for guid­ing personalized treatment choices.
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