Danielle L. Graham

Molecular adaptations underlying susceptibility and resistance to social defeat in brain reward regions.

While stressful life events are an important cause of psychopathology, most individuals exposed to adversity maintain normal psychological functioning. The molecular mechanisms underlying such resilience are poorly understood. Here, we demonstrate that an inbred population of mice subjected to social defeat can be separated into susceptible and unsusceptible subpopulations that differ along several behavioral and physiological domains. By a combination of molecular and electrophysiological techniques, we identify signature adaptations within the mesolimbic dopamine circuit that are uniquely associated with vulnerability or insusceptibility. We show that molecular recapitulations of three prototypical adaptations associated with the unsusceptible phenotype are each sufficient to promote resistant behavior. Our results validate a multidisciplinary approach to examine the neurobiological mechanisms of variations in stress resistance, and illustrate the importance of plasticity within the brains reward circuits in actively maintaining an emotional homeostasis.

Dynamic BDNF activity in nucleus accumbens with cocaine use increases self-administration and relapse

A single exposure to cocaine rapidly induces the brief activation of several immediate early genes, but the role of such short-term regulation in the enduring consequences of cocaine use is poorly understood. We found that 4 h of intravenous cocaine self-administration in rats induced a transient increase in brain-derived neurotrophic factor (BDNF) and activation of TrkB-mediated signaling in the nucleus accumbens (NAc). Augmenting this dynamic regulation with five daily NAc BDNF infusions caused enduring increases in cocaine self-administration, and facilitated relapse to cocaine seeking in withdrawal. In contrast, neutralizing endogenous BDNF regulation with intra-NAc infusions of antibody to BDNF subsequently reduced cocaine self-administration and attenuated relapse. Using localized inducible BDNF knockout in mice, we found that BDNF originating from NAc neurons was necessary for maintaining increased cocaine self-administration. These findings suggest that dynamic induction and release of BDNF from NAc neurons during cocaine use promotes the development and persistence of addictive behavior.

CREB regulation of nucleus accumbens excitability mediates social isolation-induced behavioral deficits

Here, we characterized behavioral abnormalities induced by prolonged social isolation in adult rodents. Social isolation induced both anxiety- and anhedonia-like symptoms and decreased cAMP response element–binding protein (CREB) activity in the nucleus accumbens shell (NAcSh). All of these abnormalities were reversed by chronic, but not acute, antidepressant treatment. However, although the anxiety phenotype and its reversal by antidepressant treatment were CREB-dependent, the anhedonia-like symptoms were not mediated by CREB in NAcSh. We found that decreased CREB activity in NAcSh correlated with increased expression of certain K+ channels and reduced electrical excitability of NAcSh neurons, which was sufficient to induce anxiety-like behaviors and was reversed by chronic antidepressant treatment. Together, our results describe a model that distinguishes anxiety- and depression-like behavioral phenotypes, establish a selective role of decreased CREB activity in NAcSh in anxiety-like behavior, and provide a mechanism by which antidepressant treatment alleviates anxiety symptoms after social isolation.

Environmental Enrichment Produces a Behavioral Phenotype Mediated by Low Cyclic Adenosine Monophosphate Response Element Binding (CREB) Activity in the Nucleus Accumbens

BACKGROUND Previous research has shown that rats reared in an enriched condition (EC) are more sensitive to the acute effects of amphetamine than rats reared in an isolated condition (IC); yet, EC rats self-administer less amphetamine than IC rats. The present study used cocaine to further explore this environmental enrichment behavioral phenotype, as well as the underlying molecular mechanisms involved. METHODS Enriched condition and IC rats were studied in a broad battery of behavioral tests, including cocaine conditioned place preference (CPP) and self-administration and several measures of anxiety- and depression-related behavior. The involvement of the transcription factor, cyclic adenosine monophosphate (cAMP) response element binding protein (CREB), in mediating EC versus IC differences was investigated. RESULTS Enriched condition rats exhibited less cocaine self-administration, despite showing enhanced cocaine CPP. Enriched condition rats also displayed less depression-like behavior but higher levels of anxiety-like behavior. This behavioral phenotype is consistent with low CREB activity in the nucleus accumbens, a key brain reward region. Indeed, EC rats have less phospho-CREB (the transcriptionally active form of the protein) in the nucleus accumbens than IC rats, and a selective knockdown of CREB in this brain region of normally reared rats, by use of a novel viral vector expressing a short hairpin RNA (shRNA) directed against CREB, reproduced the EC behavioral phenotype. CONCLUSIONS These studies identify a potential molecular mechanism for how rearing environment-a nonpharmacological, nonsurgical manipulation-can modify a wide range of complex emotional behaviors.

The Influence of ΔFosB in the Nucleus Accumbens on Natural Reward-Related Behavior

The transcription factor deltaFosB (ΔFosB), induced in nucleus accumbens (NAc) by chronic exposure to drugs of abuse, has been shown to mediate sensitized responses to these drugs. However, less is known about a role for ΔFosB in regulating responses to natural rewards. Here, we demonstrate that two powerful natural reward behaviors, sucrose drinking and sexual behavior, increase levels of ΔFosB in the NAc. We then use viral-mediated gene transfer to study how such ΔFosB induction influences behavioral responses to these natural rewards. We demonstrate that overexpression of ΔFosB in the NAc increases sucrose intake and promotes aspects of sexual behavior. In addition, we show that animals with previous sexual experience, which exhibit increased ΔFosB levels, also show an increase in sucrose consumption. This work suggests that ΔFosB is not only induced in the NAc by drugs of abuse, but also by natural rewarding stimuli. Additionally, our findings show that chronic exposure to stimuli that induce ΔFosB in the NAc can increase consumption of other natural rewards.

Tropomyosin-Related Kinase B in the Mesolimbic Dopamine System: Region-Specific Effects on Cocaine Reward

BACKGROUND Previous studies found that brain-derived neurotrophic factor (BDNF) derived from nucleus accumbens (NAc) neurons can mediate persistent behavioral changes that contribute to cocaine addiction. METHODS To further investigate BDNF signaling in the mesolimbic dopamine system, we analyzed tropomyosin-related kinase B (TrkB) messenger RNA (mRNA) and protein changes in the NAc and ventral tegmental area (VTA) in rats following 3 weeks of cocaine self-administration. To study the role of BDNF-TrkB activity in the VTA and NAc in cocaine reward, we used localized viral-mediated Cre recombinase expression in floxed BDNF and floxed TrkB mice to knockdown BDNF or TrkB in the VTA and NAc in cocaine place conditioning tests and TrkB in the NAc in cocaine self-administration tests. RESULTS We found that 3 weeks of active cocaine self-administration significantly increased TrkB protein levels in the NAc shell, while yoked (passive) cocaine exposure produced a similar increase in the VTA. Localized BDNF knockdown in either region reduced cocaine reward in place conditioning, whereas only TrkB knockdown in the NAc reduced cocaine reward. In mice self-administering cocaine, TrkB knockdown in the NAc produced a downward shift in the cocaine self-administration dose-response curve but had no effect on the acquisition of cocaine or sucrose self-administration. CONCLUSIONS Together, these data suggest that BDNF synthesized in either VTA or NAc neurons is important for maintaining sensitivity to cocaine reward but only BDNF activation of TrkB receptors in the NAc mediates this effect. In addition, up-regulation of NAc TrkB with chronic cocaine use could promote the transition to more addicted biological states.

Overexpression of CREB in the nucleus accumbens shell increases cocaine reinforcement in self-administering rats

Chronic exposure to addictive drugs enhances cAMP response element binding protein (CREB)-regulated gene expression in nucleus accumbens (NAc), and these effects are thought to reduce the positive hedonic effects of passive cocaine administration. Here, we used viral-mediated gene transfer to produce short- and long-term regulation of CREB activity in NAc shell of rats engaging in volitional cocaine self-administration. Increasing CREB expression in NAc shell markedly enhanced cocaine reinforcement of self-administration behavior, as indicated by leftward (long-term) and upward (short-term) shifts in fixed ratio dose–response curves. CREB also increased the effort exerted by rats to obtain cocaine on more demanding progressive ratio schedules, an effect highly correlated with viral-induced modulation of BDNF protein in the NAc shell. CREB enhanced cocaine reinforcement when expressed either throughout acquisition of self-administration or when expression was limited to postacquisition tests, indicating a direct effect of CREB independent of reinforcement-related learning. Downregulating endogenous CREB in NAc shell by expressing a short hairpin RNA reduced cocaine reinforcement in similar tests, while overexpression of a dominant-negative CREBS133A mutant had no significant effect on cocaine self-administration. Finally, increasing CREB expression after withdrawal from self-administration enhanced cocaine-primed relapse, while reducing CREB levels facilitated extinction of cocaine seeking, but neither altered relapse induced by cocaine cues or footshock stress. Together, these findings indicate that CREB activity in NAc shell increases the motivation for cocaine during active self-administration or after withdrawal from cocaine. Our results also highlight that volitional and passive drug administration can lead to substantially different behavioral outcomes.

Differential ability of D1 and D2 dopamine receptor agonists to induce and modulate expression and reinstatement of cocaine place preference in rats

RationaleD1-Like agonists are self-administered by drug-naive animals, whereas D2-like agonists reinstate cocaine-seeking behavior, but the rewarding and reinstating effects of D1- and D2-like agonists in pavlovian-based conditioned place preference are equivocal.ObjectiveTo compare the ability of D1 and D2 agonists to produce conditioned place preference with their modulation of expression and reinstatement of an established cocaine place preference.MethodsUsing an unbiased procedure, we measured the place preference induced by the D1 receptor agonist SKF 81297 and the D2/D3 receptor agonist quinpirole in drug-naive or cocaine-exposed rats. The rewarding effects of the D1 agonists SKF 82958, ABT-431, A-77636, and the D2/D3 receptor agonist 7-OH-DPAT were also tested. Additionally, we tested the ability of SKF 81297 and quinpirole to modulate expression and reinstatement of an established cocaine place preference.ResultsThe D1 receptor agonists SKF 81297, SKF 82958, and ABT-431 produced dose-dependent conditioned place preferences, whereas A-77636 produced only place aversion, and the D2/D3 agonists quinpirole and 7-OH-DPAT were without effect in drug naive rats. In cocaine-treated rats, SKF-81297-induced place preference was reduced, whereas quinpirole-induced place preference was revealed. Pretreatment using either D1 or D2/D3 agonists blocked expression of an established cocaine place preference, but only the D1 agonist SKF 81297 and cocaine dose-dependently reinstated an extinguished cocaine place preference, whereas the D2/D3 agonist quinpirole induced place aversion but failed to alter cocaine-induced reinstatement.ConclusionsD1, but not D2/D3, agonists mediate rewarding effects and reinstatement of cocaine place preference, but the reinstating effects differ markedly from self-administration paradigms.

Yohimbine Increases Impulsivity Through Activation of cAMP Response Element Binding in the Orbitofrontal Cortex

BACKGROUND Stress can increase impulsivity and has a negative impact on psychiatric outcome. Norepinephrine is heavily implicated in responses to stress, and the alpha(2) antagonist yohimbine is used clinically to study this aspect of the stress response. Yohimbine induces mild anxiety and increases impulsivity in healthy volunteers but has more detrimental effects in some psychiatric populations, triggering mania in bipolar patients and drug craving in substance-dependent individuals. Understanding the mechanism by which yohimbine affects brain function could provide insight into the heightened reaction to stress in these patients. METHODS Yohimbines effects were assessed in rats using the five-choice serial reaction time test of attention and impulse control. We then examined whether yohimbine altered activity of cyclic adenosine monophosphate response element binding (CREB) protein-a transcription factor implicated in the stress response-in brain areas that regulate impulsivity. The behavioral consequences of any changes in CREB activity were subsequently assessed using viral-mediated gene transfer to regionally overexpress CREB or the dominant negative antagonist mCREB. RESULTS Yohimbine increased impulsive responding in rats and selectively increased CREB phosphorylation within the orbitofrontal cortex but not medial prefrontal cortex or nucleus accumbens. Overexpressing mCREB within the orbitofrontal cortex blocked yohimbines effects on impulsivity, whereas overexpressing CREB in this region increased impulsive responding and potentiated the proimpulsive actions of yohimbine. DISCUSSION These data suggest a novel molecular mechanism contributing to impulsivity that may be sensitive to stress. Such findings may improve our understanding of the neurobiological pathways linking the response to stress and impulsivity in both healthy and psychiatric populations.

Region-specific tolerance to cocaine-regulated cAMP-dependent protein phosphorylation following chronic self-administration

Chronic cocaine self‐administration can produce either tolerance or sensitization to certain cocaine‐regulated behaviours, but whether differential alterations develop in the biochemical response to cocaine is less clear. We measured cocaine‐induced phosphorylation of multiple cAMP‐dependent and ‐independent protein substrates in mesolimbic dopamine terminal regions following chronic self‐administration. Changes in self‐administering rats were compared to changes produced by passive yoked injection to identify reinforcement‐related regulation, whereas acute and chronic yoked groups were compared to identify the development tolerance or sensitization in the biochemical response to cocaine. Microwave‐fixed brain tissue was collected immediately following 4 h of intravenous cocaine administration, and subjected to Western blot analysis of phosphorylated and total protein substrates. Chronic cocaine produced region‐ and substrate‐specific tolerance to cAMP‐dependent protein phosphorylation, including phosphorylation in striatal and amygdala subregions and phosphorylation in the CA1 subregion of the hippocampus. Tolerance also developed to cAMP‐independent phosphorylation in the prefrontal cortex. In contrast, sensitization to presynaptic regulation of synapsinS9 phosphorylation developed in the hippocampal CA3 subregion while cAMP‐dependent tyrosine hydroxylaseS40 phosphorylation decreased in striatal dopamine terminals. Cocaine‐induced ERK and CREBS133 phosphorylation were dissociated in many brain regions and failed to develop either tolerance or sensitization with chronic administration. Positive reinforcement‐related correlations between cocaine intake and protein phosphorylation were found only in self‐administering animals, while negative dose‐related correlations were found primarily with yoked administration. These regional‐ and substrate‐specific adaptations in cocaine‐induced protein phosphorylation are discussed in view of their potential impact on the development of cocaine addiction.