Lisa A. Briand

Persistent Alterations in Cognitive Function and Prefrontal Dopamine D2 Receptors Following Extended, but Not Limited, Access to Self-Administered Cocaine

Drug addicts have deficits in frontocortical function and cognition even long after the discontinuation of drug use. It is not clear, however, whether the cognitive deficits are a consequence of drug use, or are present prior to drug use, and thus are a potential predisposing factor for addiction. To determine if self-administration of cocaine is capable of producing long-lasting alterations in cognition, rats were allowed access to cocaine for either 1 h/day (short access, ShA) or 6 h/day (long access, LgA) for 3 weeks. Between 1 and 30 days after the last self-administration session, we examined performance on a cognitively demanding test of sustained attention that requires an intact medial prefrontal cortex. The expression levels of dopamine D1 and D2 receptor mRNA and D2 protein in the prefrontal cortex were also examined. Early after discontinuation of drug use, LgA (but not ShA) animals were markedly impaired on the sustained attention task. Although the LgA animals improved over time, they continued to show a persistent pattern of performance deficits indicative of a disruption of cognitive flexibility up to 30 days after the discontinuation of drug use. This was accompanied by a significant decrease in DA D2 (but not D1) mRNA in the medial and orbital prefrontal cortex, and D2 receptor protein in the medial prefrontal cortex of LgA (but not ShA) animals. These findings establish that repeated cocaine use is capable of producing persistent alterations in the prefrontal cortex and in cognitive function, and illustrate the usefulness of extended access self-administration procedures for studying the neurobiology of addiction.

Molecular and genetic substrates linking stress and addiction.

Drug addiction is one of the top three health concerns in the United States in terms of economic and health care costs. Despite this, there are very few effective treatment options available. Therefore, understanding the causes and molecular mechanisms underlying the transition from casual drug use to compulsive drug addiction could aid in the development of treatment options. Studies in humans and animal models indicate that stress can lead to both vulnerability to develop addiction, and increased drug taking and relapse in addicted individuals. Exposure to stress or drugs of abuse results in long-term adaptations in the brain that are likely to involve persistent alterations in gene expression or activation of transcription factors, such as the cAMP Response Element Binding (CREB) protein. The signaling pathways controlled by CREB have been strongly implicated in drug addiction and stress. Many potential CREB target genes have been identified based on the presence of a CRE element in promoter DNA sequences. These include, but are not limited to CRF, BDNF, and dynorphin. These genes have been associated with initiation or reinstatement of drug reward and are altered in one direction or the other following stress. While many reviews have examined the interactions between stress and addiction, the goal of this review was to focus on specific molecules that play key roles in both stress and addiction and are therefore posed to mediate the interaction between the two. Focus on these molecules could provide us with new targets for pharmacological treatments for addiction.

Stress-Induced Potentiation of Cocaine Reward: A Role for CRF R1 and CREB

Both clinical and preclinical research have shown that stress can potentiate drug use; however, the underlying mechanisms of this interaction are unknown. Previously, we have shown that a single exposure to forced swim (FS) reinstates extinguished conditioned place preference (CPP) to cocaine and that cAMP response element binding protein (CREB) is necessary for this response. CREB can be activated by corticotropin releasing factor (CRF) receptor type 1 (CRFR1) binding, which mediates neuroendocrine and behavioral responses to stress as well as to drugs of abuse. The present experiments investigate whether changes in cocaine reward elicited by previous exposure to stress are mediated by CREB and/or CRFR1. Chronic exposure to FS in advance of conditioning enhances cocaine CPP in wild-type mice, but this is blocked in CREB-deficient mice. In addition, pretreatment with the CRFR1 antagonist, antalarmin, before FS exposure blocks this stress-induced enhancement of cocaine CPP. Furthermore, FS-induced increase in phosphorylated CREB (pCREB), specifically in the lateral septum (LS) and nucleus accumbens (NAc) is also blocked by antalarmin. Taken together, these studies suggest that both CREB and CRFR1 activation are necessary for stress-induced potentiation of drug reward.

Cocaine self-administration produces a persistent increase in dopamine D2High receptors

Cocaine addicts are reported to have decreased numbers of striatal dopamine D2 receptors. However, in rodents, repeated cocaine administration consistently produces hypersensitivity to the psychomotor activating effects of both indirect dopamine agonists, such as cocaine itself, and importantly, to direct-acting D2 receptor agonists. The current study reports a possible resolution to this long-standing paradox. The dopamine D2 receptor exists in both a low and a high-affinity state, and dopamine exerts its effects via the more functionally relevant high-affinity D2 receptor (D2 High). We report here that cocaine self-administration experience produces a large (approximately 150%) increase in the proportion of D2 High receptors in the striatum with no change in the total number of D2 receptors, and this effect is evident both 3 and 30 days after the discontinuation of cocaine self-administration. Changes in D2 High receptors would not be evident with the probes used in human (and non-human primate) imaging studies. We suggest, therefore, that cocaine addicts and animals previously treated with cocaine may be hyper-responsive to dopaminergic drugs in part because an increase in D2 High receptors results in dopamine supersensitivity. This may also help explain why stimuli that increase dopamine neurotransmission, including drugs themselves, are so effective in producing relapse in individuals with a history of exposure to cocaine.

Ventral Tegmental Afferents in Stress-Induced Reinstatement: The Role of cAMP Response Element-Binding Protein

The transcription factor cAMP response element-binding protein (CREB) is required for stress- but not drug-induced reinstatement of cocaine conditioned place preference. To reveal the neural circuitry associated with this CREB dependence, we injected a retrograde tracer into the ventral tegmental area (VTA) and identified afferents that were activated after stress or cocaine exposure in both naive and cocaine-conditioned mice. Neuronal activation, as assessed by Fos expression, was greatly reduced in the dorsal and ventral bed nucleus of the stria terminalis (BNST), lateral septum, and nucleus accumbens shell in mice lacking CREB (CREBαΔ mice) after a 6 min swim stress but not after cocaine exposure (20 mg/kg). Additionally, activation of VTA afferent neurons in the ventral BNST and the infralimbic cortex in CREBαΔ mice was blunted in response to stress. This pattern of neuronal activation persisted in mice that were conditioned to a cocaine place preference procedure before stress exposure. Furthermore, lidocaine inactivation (0.4 μl, 4%) studies demonstrated the necessity of BNST activation for swim-stress-induced reinstatement of cocaine-conditioned reward. Together, the present studies demonstrate that CREB is required for the activation of a unique circuit that converges on the dopamine reward pathway to elicit reinstatement of drug reward and points to the BNST as a key intersection between stress and reward circuits.

Cocaine-related behaviors in mice with deficient gliotransmission

RationaleAstrocytes play an integral role in modulating synaptic transmission and plasticity, both key mechanisms underlying addiction. However, while astrocytes are capable of releasing chemical transmitters that can modulate neuronal function, the role of these gliotransmitters in mediating behaviors associated with drugs of abuse has been largely unexplored.ObjectivesThe objective of the present study was to utilize mice with astrocytes that lack the ability to release chemical transmitters to evaluate the behavioral consequence of impaired gliotransmission on cocaine-related behaviors. These mice have previously been used to examine the role of gliotransmission in sleep homeostasis; however, no studies to date have utilized them in the study of addictive behaviors.MethodsMice expressing a dominant-negative SNARE protein selectively in astrocytes (dnSNARE mice) were tested in a variety of behavioral paradigms examining cocaine-induced behavioral plasticity. These paradigms include locomotor sensitization, conditioned place preference followed by cocaine-induced reinstatement of CPP, and cocaine self-administration followed by cue-induced reinstatement of cocaine-seeking behavior.ResultsWild-type and dnSNARE mice demonstrated no significant differences in the development or maintenance of locomotor sensitization. While there were non-significant trends for reduced CPP following a low dose of cocaine, drug-induced reinstatement of CPP is completely blocked in dnSNARE mice. Similarly, while dnSNARE mice demonstrated a non-significant trend toward reduced cocaine self-administration compared with wild-type mice, dnSNARE mice do not demonstrate cue-induced reinstatement in this paradigm.ConclusionsGliotransmission is necessary for reinstatement of drug-seeking behaviors by cocaine or associated cues.

IMPAIRED OBJECT RECOGNITION FOLLOWING PROLONGED WITHDRAWAL FROM EXTENDED-ACCESS COCAINE SELF-ADMINISTRATION

Cocaine addicts have a number of cognitive deficits that persist following prolonged abstinence. These include impairments in executive functions dependent on the prefrontal cortex, as well as deficits on learning and memory tasks sensitive to hippocampal function. Recent preclinical studies using non-human animals have demonstrated that cocaine treatment can produce persistent deficits in executive functions, but there is relatively little evidence that treatment with cocaine produces persistent deficits in performance on hippocampal-dependent tasks. We recently demonstrated that extended (but not limited) access to self-administered cocaine is especially effective in producing persistent deficits on a test of cognitive vigilance, and therefore, we used this procedure to examine the effects of limited or extended access to cocaine self-administration on recognition memory performance, which is sensitive to hippocampal function. We found that extended access to cocaine produced deficits in recognition memory in rats that persisted for at least 2 weeks after the cessation of drug use. We conclude that the deficits in learning and memory observed in cocaine addicts may be at least in part due to repeated drug use, rather than just due to a pre-existing condition, and that in studying the neural basis of such deficits procedures involving extended access to self-administered cocaine may be especially useful.

Running‐induced anxiety is dependent on increases in hippocampal neurogenesis

Exercise, specifically voluntary wheel running, is a potent stimulator of hippocampal neurogenesis in adult mice. In addition, exercise induces behavioral changes in numerous measures of anxiety in rodents. However, the physiological underpinnings of these changes are poorly understood. To investigate the role of neurogenesis in exercise‐mediated anxiety, we examined the cellular and behavioral effects of voluntary wheel running in mice with a reduction in hippocampal neurogenesis, achieved through conditional deletion of ataxia telangiectasia‐mutated and rad‐3‐related protein (ATR), a cell cycle checkpoint kinase necessary for normal levels of neurogenesis. Following hippocampal microinjection of an adeno‐associated virus expressing Cre recombinase to delete ATR, mice were exposed to 4 weeks of voluntary wheel running and subsequently evaluated for anxiety‐like behavior. Wheel running resulted in increased cell proliferation and neurogenesis, as measured by bromodeoxyuridine and doublecortin, respectively. Wheel running also resulted in heightened anxiety in the novelty‐induced hypophagia, open field and light–dark box tests. However, both the neurogenic and anxiogenic effects of wheel running were attenuated following hippocampal ATR deletion, suggesting that increased neurogenesis is an important mediator of exercise‐induced anxiety.

Increased Hippocampal Neurogenesis and Accelerated Response to Antidepressants in Mice with Specific Deletion of CREB in the Hippocampus: Role of cAMP Response-Element Modulator τ

The transcription factor cAMP response element-binding protein (CREB) has been implicated in the pathophysiology of depression as well as in the efficacy of antidepressant treatment. However, altering CREB levels appears to have differing effects on anxiety- and depression-related behaviors, depending on which brain region is examined. Furthermore, many manipulations of CREB lead to corresponding changes in other CREB family proteins, and the impact of these changes has been largely ignored. To further investigate the region-specific importance of CREB in depression-related behavior and antidepressant response, we used CrebloxP/loxP mice to localize CREB deletion to the hippocampus. In an assay sensitive to chronic antidepressant response, the novelty-induced hypophagia procedure, hippocampal CREB deletion, did not alter the response to chronic antidepressant treatment. In contrast, mice with hippocampal CREB deletion responded to acute antidepressant treatment in this task, and this accelerated response was accompanied by an increase in hippocampal neurogenesis. Upregulation of the CREB-family protein cAMP response-element modulator (CREM) was observed after CREB deletion. Viral overexpression of the activator isoform of CREM, CREMτ, in the hippocampus also resulted in an accelerated response to antidepressants as well as increased hippocampal neurogenesis. This is the first demonstration of CREMτ within the brain playing a role in behavior and specifically in behavioral outcomes following antidepressant treatment. The current results suggest that activation of CREMτ may provide a means to accelerate the therapeutic efficacy of current antidepressant treatment.

Disruption of glutamate receptor-interacting protein in nucleus accumbens enhances vulnerability to cocaine relapse.

Trafficking and stabilization of AMPA receptors at synapses in response to cocaine exposure is thought to be critical for expression of cocaine addiction and relapse. Glutamate receptor-interacting protein (GRIP) is a neuronal scaffolding protein that stabilizes GluA2 AMPARs at synapses but its role in cocaine addiction has not been examined. The current study demonstrates that conditional deletion of GRIP within the nucleus accumbens potentiates cue-induced reinstatement of cocaine seeking without affecting operant learning, locomotor activity, or reinstatement of natural reward seeking. This is the first study to demonstrate a role for accumbal GRIP in behavior. Electrophysiological recordings revealed increased rectification of AMPAR-mediated currents in the nucleus accumbens and increased AMPAR sensitivity to the GluA2-lacking AMPAR antagonist, 1-naphthylacetyl spermine, indicative of an increased contribution of GluA2-lacking calcium-permeable AMPARs. In addition, accumbal GRIP deletion was associated with blunted long-term depression, similar to what is seen following cocaine self-administration. Taken together, these results indicate that GRIP may modulate addictive phenotypes through its regulation of synaptic AMPARs by controlling their subunit composition and susceptibility to LTD. These effects are associated with changes in vulnerability to cocaine relapse and highlight GRIP as a novel target for the development of cocaine addiction therapeutics.