Electrophysiological Biomarkers for Translational Research: A Study on Addiction
Substance abuse disorder (SAD) is characterized by the inability to inhibit the compulsive use of psychoactive substances in face of negative emotional, relational and physiological consequences. Anxiety and mood disorders are the most common psychopathologies diagnosed in the United Sates each year and share over 40% comorbidity. The rate of mood and anxiety disorder diagnoses are expected to increase, in part, as a function of environmental stressors, illustrated by the prevalence of these disorders in urban areas. Stress increases the likelihood of anxiety and mood disorders, which highly correlates with increased frequency of SAD4, 5. Relatedly, rewarding substances of abuse modify these same regions 6, 7
The neurological pathways of stress and reward overlap and each modulates the other. This overlap occurs in a distributed network and provides the target regions for creating anxiety, mood and SAD therapies. Stress hormones and psychoactive substances of abuse can increase the activity of the ventral tegmental area (VTA), resulting in the subsequent release of dopamine (DA) output to its target sites. These target sites include the nucleus accumbens (NAc), the amygdala (AMG), the prefrontal cortex (PFC) and the HPC, which together form the larger circuitry of stress and reward processing. Stressful events cause the release of the stress hormone corticotropin releasing factor (CRF), which activates the hypothalamic-pituitary-adrenal (HPA) axis. HPA activation by CRF increases the release of glucocorticoids (CORT) and binds to two glucocorticoid receptors subtypes, GR1 and GR2. In addition to activating the HPA axis, CRF binds to two receptor subtypes, CRFR1 and CRFR2, expressed in the brain regions stated above. CRFRs and GRs modulate VTA activity and the release of DA at VTA targets sites. This functional overlap between stress and reward pathways indicates a role, CRFRs, GRs and DA receptors in the modification of neuronal function from a healthy to pathogenic state.
Therapeutic targets can be discovered by studying the mechanisms of stress hormone and DA interplay in the stress and reward circuit and will require multidimensional experimental strategies. Although the observation of interplay between the concentration of stress hormones and DA has been well outlined, very little is known about the mechanisms of this interaction. Moreover, the mechanisms of interaction in the stress and reward circuit will differ given the anatomical, genetic and physiological differences in each region. As a result, it is difficult to predict the affects of stress hormones and DA on this distributed network. Functional connectivity (e.g. in vivo local field synchrony) between brain regions offers a way to overcome this problem and has been instrumental in understanding the pathogenesis and pathophysiology of neurological disorders such as obsessive compulsive disorder and rodent models of anxiety and schizophrenia. This highlights the utility of electrophysiology as a potential biomarker for translational research. Consequently, understanding the functional connectivity between each subcircuit during behavioral assessment, as it relates to stress and reward, in conjunction with pharmacological intervention can be used to understand the aetiology of anxiety, mood and SAD. The goal is to mitigate aberrant functional connectivity using pharmacological tools in order to ameliorate dysfunctional behavior. I will propose two experimental studies that focus on behavioral, electrophysiological and pharmacological experiments with the goal of determining specific therapeutic targets for pharmacological intervention to treat anxiety, mood and SAD
Study 1: Characterization of Aberrant Functional Connectivity Between the PFC, HPC, AMG and NAc in Drug Dependent Animals
Individuals with SAD exhibit an imbalance between cognitive control and stress-reward based motivation. In this regard, the PFC regulates impulse control and the HPC negatively controls the HPA axis while the AMG and NAc underlie stress-reward based motivation. The PFC and HPC project to the AMG, NAc (PFC-AMG/NAc and HPC-AMG/NAc, respectively) and cognitively modulate stress-reward mediated behavior. Chronic stress results in physiological dysfunction of the PFC and HPC, alters NAc excitability and increases AMG activity. This may parallel the loss of top down cognitive control and increased affective response to stressors observed in individuals with anxiety and mood disorders. Imbalanced functional connectivity between the PFC-AMG/NAc and HPC-AMG/NAc may reinforce stress-reward behavior, leading to compulsive use of substances. Accordingly, the extinction of drug use behavior (i.e. abstinence) may require restoring functional connectivity between the these structures.
Restoring PFC control over the AMG and NAc requires functional characterization of the PFC-AMG/NAc circuit during abstinence. Early abstinence requires the metabolism and detoxification of psychoactive substances. During this stage, substance withdrawal initiates CRF and CORT release, increasing AMG excitability. The AMG projects to the NAc and regulates reward-seeking behavior. Given that the PFC is critical in behavioral extinction, increased PFC top down modulation of AMG activity may relieve drug-seeking behavior. In addition, individuals with anxiety disorders show enhanced functional connectivity between the PFC and NAc. Deep brain stimulation of the NAc decouples the PFC-NAc interaction and relieves aberrant behavior. This leads to Hypothesis I: Initial abstinence exhibits dysfunctional connectivity between the PFC-AMG/NAc and switches to a functional state via enhanced PFC control of AMG activity. Protracted abstinence requires PFC mediated impulse inhibition to substance seeking and use. Impulsive inhibition can rapidly deteriorate in response to acute stressors, lending support for Hypothesis II: During acute stress induced relapse, the AMG drives NAc mediated reward-seeking behavior and rapidly switches PFC-AMG/NAc connectivity from a functional to dysfunctional state, independent of abstinence duration.
HPC regulation of the HPA axis requires functional characterization of the HPC-AMG/NAc circuit during abstinence. Chronic stress impedes HPC function resulting in reduced control of the HPA axis and impaired contextual memory. This results in increased AMG activation of the NAc and HPA axis. This suggests Hypothesis III: HPC control of the HPA axis will reduce AMG activity, restoring functional connectivity between the HPC-AMG/NAc circuit. The HPC utilizes context-reward associative information to predict aversive and rewarding events, by sending this information to the AMG and NAc. Individuals with compulsive disorders show deficits in reward prediction, which may underlie contextual cue induced drug seeking and relapse in SAD. This supports Hypothesis IV: Functional connectivity of the HPC-AMG/NAc enters a dysfunctional state when reward predictive cues occur with stressful events. Accordingly, SAD may exhibit functional disparity between regions of the stress-reward circuitry and successful abstinence could require restoration of this functional connectivity, emphasizing the potential for in vivo electrophysiology as a biomarker in translational research.
In vivo electrophysiological recordings of the local field potential (LFP) will be utilized to characterize the functional connectivity between the PFC-AMG/NAc and HPC-AMG/NAc. Neuronal population activity is tightly correlated at distinct LFP frequency bands. The ventral (v)HPC theta band (4-12Hz) is an LFP that synchronizes to the theta LFP of the medial (m)PFC, AMG and NAc24, 25
Study 2: Pharmacological Reduction of the Withdrawal Induced Stress Response – An Approach to Enhance and Maintain Restored Functional Connectivity for Relapse Prevention
Chronic stress contributes to the aetiology of generalized anxiety and mood disorders, in part, through unregulated CRF activation of the HPA axis and subsequent CORT release. mPFC and NAc GR activation modulates dopaminergic tone in these regions10, 26, while in the HPC high levels of CRF down regulates DAR expression. In addition, GR and CRFR2 activation sensitizes VTA activity via glutamatergic receptor alteration. Sensitization of the VTA is also induced by chronic drug use. As a result, stress and drug use modulate local DA efflux of the mPFC, vHPC, AMG and NAc and by sensitization of the VTA.
CRFR, GR and DAR antagonists will be used to modulate DA tone to restore functional connectivity and ameliorate stress induced relapse. The PFC, HPC and VTA exhibit synchronized LFP activity when higher cognitive functions are engaged. Elevated PFC DA enhances PFC-HPC synchrony and correlates with successful rule learning and reward-predictive behavior. Similarly, stressful events increase theta synchrony between the PFC-HPC and elevates DA release in the PFC. Recent evidence suggests that CRFR1 may play a role in altering the theta LFP during the stress response. Stress hormone or DA concentration that is too high or too low results in impaired cognitive and behavioral performance32, 33Hypothesis V: CRFR and GR antagonists will reduce stress induced hyperdopaminergic output to the PFC and AMG. In contrast, these antagonists will increase DA output to the HPC, increasing control over the HPA axis and restore theta activity. This will result in reduced AMG activity, stabilize NAc excitability and decouple dysfunctional connectivity between the PFC-NAc and PFC-HPC. In addition, DA5 receptor (D5R) agonists may help to restore HPC activity. D5Rs expression is the greatest in the HPC and D5R activation reduces GABAAR currents . D5R activation may increase information transfer through the hippocampal trisynaptic pathway (see above Research Background). Hypothesis V will be tested with the APA renewal protocol described in Study 1. Study 2 provides a novel target for intervention to study SAD through pharmacologically mediated stress reduction. This study utilizes an innovative approach (i.e. LFP synchrony as a biomarker for pathology) to study the neurobiological and dysfunctional connectivity of brain regions underlying addiction.
The understanding of SAD through anxiety and mood disorders has only recently become a topic of neuroscientific investigation. In focusing on the stress response, there will be greater understanding of DA circuit modulation, anxiety, mood and SADs. This will allow for the use of similar pharmacological agents to treat and perhaps prevent several related psychopathologies. Additionally, there are no current clinical trials (searched via clinicaltrials.gov) that focus on CRFR, GR or D5Rs in the treatment of drug addiction. The studies I propose offer a new approach to understand the aetiology, pathogenesis, and pathophysiology of anxiety, mood, SAD and points to the high potential for developing electrophysiological biomarkers for translational research.
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