Elizabeth N. Holly

Social stress and CRF-dopamine interactions in the VTA: role in long-term escalation of cocaine self-administration.

The nature of neuroadaptations in the genesis of escalated cocaine taking remains a topic of considerable interest. Intermittent social defeat stress induces both locomotor and dopaminergic cross-sensitization to cocaine, as well as escalated cocaine self-administration. The current study examines the role of corticotropin releasing factor receptor subtypes 1 and 2 (CRFR1, CRFR2) within the ventral tegmental area (VTA) during social defeat stress. This study investigated whether injecting either a CRFR1 or CRFR2 antagonist directly into the VTA before each social defeat would prevent the development of later (1) locomotor sensitization, (2) dopaminergic sensitization, and (3) escalated cocaine self-administration in rats. CRFR1 antagonist CP376395 (50 or 500 ng/side), CRFR2 antagonist Astressin2-B (100 or 1000 ng/side), or vehicle (aCSF) was microinjected into the VTA 20 min before social defeat stress (or handling) on days 1, 4, 7, and 10. Ten days later, rats were injected with cocaine (10 mg/kg, i.p.) and assessed for either locomotor sensitization, measured by walking activity, or dopaminergic sensitization, measured by extracellular dopamine (DA) in the nucleus accumbens shell (NAcSh) through in vivo microdialysis. Locomotor sensitization testing was followed by intravenous cocaine self-administration. Intra-VTA antagonism of CRFR1, but not CRFR2, inhibited the induction of locomotor cross-sensitization to cocaine, whereas both prevented dopaminergic cross-sensitization and escalated cocaine self-administration during a 24 h “binge.” This may suggest dissociation between locomotor sensitization and cocaine taking. These data also suggest that interactions between CRF and VTA DA neurons projecting to the NAcSh are essential for the development of dopaminergic cross-sensitization to cocaine.

Episodic Social Stress-Escalated Cocaine Self-Administration: Role of Phasic and Tonic Corticotropin Releasing Factor in the Anterior and Posterior Ventral Tegmental Area

Intermittent social defeat stress escalates later cocaine self-administration. Reward and stress both activate ventral tegmental area (VTA) dopamine neurons, increasing downstream extracellular dopamine concentration in the medial prefrontal cortex and nucleus accumbens. The stress neuropeptide corticotropin releasing factor (CRF) and its receptors (CRF-R1, CRF-R2) are located in the VTA and influence dopaminergic activity. These experiments explore how CRF release and the activation of its receptors within the VTA both during and after stress influence later cocaine self-administration in rats. In vivo microdialysis of CRF in the VTA demonstrated that CRF is phasically released in the posterior VTA (pVTA) during acute defeat, but, with repeated defeat, CRF is recruited into the anterior VTA (aVTA) and CRF tone is increased in both subregions. Intra-VTA antagonism of CRF-R1 in the pVTA and CRF-R2 in the aVTA during each social defeat prevented escalated cocaine self-administration in a 24 h “binge.” VTA CRF continues to influence cocaine seeking in stressed animals long after social defeat exposure. Unlike nonstressed controls, previously stressed rats show significant cocaine seeking after 15 d of forced abstinence. Previously stressed rats continue to express elevated CRF tone within the VTA and antagonism of pVTA CRF-R1 or aVTA CRF-R2 reverses cocaine seeking. In conclusion, these experiments demonstrate neuroadaptive changes in tonic and phasic CRF with repeated stress, that CRF release during stress may contribute to later escalated cocaine taking, and that persistently elevated CRF tone in the VTA may drive later cocaine seeking through increased activation of pVTA CRF-R1 and aVTA CRF-R2. SIGNIFICANCE STATEMENT Corticotropin releasing factor (CRF) within the ventral tegmental area (VTA) has emerged as a likely candidate molecule underlying the fundamental link between stress history and escalated drug self-administration. However, the nature of CRF release in the VTA during acute and repeated stress, as well as its role in enduring neuroadaptations driving later drug taking and seeking, are poorly understood. These experiments explore how CRF is released and interacts with its receptors in specific regions of the VTA both during and after stress to fuel later escalated cocaine taking and seeking behavior. Understanding these acute and persistent changes to the VTA CRF system may lead to better therapeutic interventions for addiction.

Increased mesocorticolimbic dopamine during acute and repeated social defeat stress: modulation by corticotropin releasing factor receptors in the ventral tegmental area

RationaleStress activates a subset of dopamine neurons in the ventral tegmental area (VTA), increasing extracellular dopamine in the medial prefrontal cortex (mPFC) and nucleus accumbens shell (NAcSh). The stress neuropeptide corticotropin releasing factor (CRF) and its receptors (CRF-R1 and CRF-R2) are located within the VTA and directly and indirectly influence dopaminergic activity. However, it has yet to be shown in vivo whether VTA CRF receptor activation is necessary for acute and repeated stress-induced dopamine efflux.ObjectiveWith intra-VTA CRF-R1 and CRF-R2 antagonism during social defeat, we assessed whether blockade of these receptors could prevent stress-induced dopamine increases in the mPFC and NAcSh using in vivo microdialysis.MethodsRats were microinjected with a CRF-R1 or CRF-R2 antagonist into the VTA prior to social defeat stress on days 1, 4, 7, and 10. In vivo microdialysis for dopamine in the mPFC and NAcSh was performed during stress on days 1 and 10.ResultsDuring the first social defeat, extracellular dopamine was significantly elevated in both the mPFC and NAcSh, and this increase in the NAcSh was blocked by intra-VTA CRF-R2, but not CRF-R1, antagonism. During the final social defeat, the dopaminergic increase was neither sensitized nor habituated in the mPFC and NAcSh, and intra-VTA CRF-R2, but not CRF-R1, antagonism prevented the dopamine increase in both brain regions.ConclusionThese findings show that CRF-R2 in the VTA is necessary for acute and repeated stress-induced dopamine efflux in the NAcSh, but is only recruited into mPFC-projecting dopamine neurons with repeated stress exposure.

Social stress-escalated intermittent alcohol drinking: modulation by CRF-R1 in the ventral tegmental area and accumbal dopamine in mice

RationaleExcessive alcohol (EtOH) drinking is difficult to model in animals despite the extensive human literature demonstrating that stress increases EtOH consumption.ObjectiveThe current experiments show escalations in voluntary EtOH drinking caused by a history of social defeat stress and intermittent access to EtOH in C57BL/6J mice compared to non-stressed mice given intermittent EtOH or continuous EtOH. To explore a mechanistic link between stress and drinking, we studied the role of corticotropin-releasing factor type-1 receptors (CRF-R1) in the dopamine-rich ventral tegmental area (VTA).ResultsIntra-VTA infusions of a CRF-R1 antagonist, CP376395, infused into the VTA dose-dependently and selectively reduced intermittent EtOH intake in stressed and non-stressed mice, but not in mice given continuous EtOH. In contrast, intra-VTA infusions of the CRF-R2 antagonist astressin2B non-specifically suppressed both EtOH and H2O drinking in the stressed group without effects in the non-stressed mice. Using in vivo microdialysis in the nucleus accumbens (NAc) shell, we observed that stressed mice drinking EtOH intermittently had elevated levels of tonic dopamine concentrations compared to non-stressed drinking mice. Also, VTA CP376395 potentiated dopamine output to the NAc only in the stressed group causing further elevations of dopamine post-infusion.ConclusionsThese findings illustrate a role for extrahypothalamic CRF-R1 as especially important for stress-escalated EtOH drinking beyond schedule-escalated EtOH drinking. CRF-R1 may be a mechanism for balancing the dysregulation of stress and reward in alcohol use disorders.

Capturing Individual Differences: Challenges in Animal Models of Posttraumatic Stress Disorder and Drug Abuse.

Many psychiatric disorders are comorbid with substance use disorder (SUD). Individuals with any mood or anxiety disorder are twice as likely to develop SUD compared to healthy individuals (1). Posttraumatic stress disorder (PTSD) is no exception, with PTSD patients four times more likely to develop SUD than those without PTSD (2). However, despite numerous clinical reports of increased drug use in PTSD and other mood disorders, preclinical studies have struggled to replicate these effects in rodents.

Integrated anatomical and physiological mapping of striatal afferent projections

The dorsomedial striatum, a key site of reward‐sensitive motor output, receives extensive afferent input from cortex, thalamus and midbrain. These projections are integrated by striatal microcircuits containing both spiny projection neurons and local circuit interneurons. To explore target cell specificity of these projections, we compared inputs onto D1‐dopamine receptor‐positive spiny neurons, parvalbumin‐positive fast‐spiking interneurons and somatostatin‐positive low‐threshold‐spiking interneurons, using cell type‐specific rabies virus tracing and optogenetic‐mediated projection neuron recruitment in mice. While the relative proportion of retrogradely labelled projection neurons was similar between target cell types, the convergence of inputs was systematically higher for projections onto fast‐spiking interneurons. Rabies virus is frequently used to assess cell‐specific anatomical connectivity but it is unclear how this correlates to synaptic connectivity and efficacy. To test this, we compared tracing data with target cell‐specific measures of synaptic efficacy for anterior cingulate cortex and parafascicular thalamic projections using novel quantitative optogenetic measures. We found that target‐specific patterns of convergence were extensively modified according to region of projection neuron origin and postsynaptic cell type. Furthermore, we observed significant divergence between cell type‐specific anatomical connectivity and measures of excitatory synaptic strength, particularly for low‐threshold‐spiking interneurons. Taken together, this suggests a basic uniform connectivity map for striatal afferent inputs upon which presynaptic–postsynaptic interactions impose substantial diversity of physiological connectivity.