Reduction of zif268 messenger RNA expression during prolonged withdrawal following "binge" cocaine self-administration in rats

The Periaqueductal Gray and Its Extended Participation in Drug Addiction Phenomena

The periaqueductal gray (PAG) is a complex mesencephalic structure involved in the integration and execution of active and passive self-protective behaviors against imminent threats, such as immobility or flight from a predator. PAG activity is also associated with the integration of responses against physical discomfort (e.g., anxiety, fear, pain, and disgust) which occurs prior an imminent attack, but also during withdrawal from drugs such as morphine and cocaine. The PAG sends and receives projections to and from other well-documented nuclei linked to the phenomenon of drug addiction including: (i) the ventral tegmental area; (ii) extended amygdala; (iii) medial prefrontal cortex; (iv) pontine nucleus; (v) bed nucleus of the stria terminalis; and (vi) hypothalamus. Preclinical models have suggested that the PAG contributes to the modulation of anxiety, fear, and nociception (all of which may produce physical discomfort) linked with chronic exposure to drugs of abuse. Withdrawal produced by the major pharmacological classes of drugs of abuse is mediated through actions that include participation of the PAG. In support of this, there is evidence of functional, pharmacological, molecular. And/or genetic alterations in the PAG during the impulsive/compulsive intake or withdrawal from a drug. Due to its small size, it is difficult to assess the anatomical participation of the PAG when using classical neuroimaging techniques, so its physiopathology in drug addiction has been underestimated and poorly documented. In this theoretical review, we discuss the involvement of the PAG in drug addiction mainly via its role as an integrator of responses to the physical discomfort associated with drug withdrawal.

Resting state hypothalamic and dorsomedial prefrontal cortical connectivity of the periaqueductal gray in cocaine addiction

Cocaine-dependent (CD) individuals demonstrate significant anxiety and dysphoria during withdrawal, a negative emotional state that may perpetuate drug seeking and consumption. An extensive body of work has focused on characterizing reward circuit dysfunction, but relatively little is known about the pain circuit during cocaine withdrawal. In an earlier study, we highlighted how cue-elicited functional connectivity between the periaqueductal gray (PAG), a subcortical hub of the pain circuit, and ventromedial prefrontal cortex supports tonic craving in recently abstinent CD. The functional organization of the brain can be characterized by intrinsic connectivities, and it is highly likely that the resting state functional connectivity (rsFC) of the PAG may also be altered in association with cocaine use variables. Here, we examined this issue in 52 CD and 52 healthy control (HC) participants. Imaging data were processed with published routines, and the findings were evaluated with a corrected threshold. In a covariance analysis, CD as compared with HC showed higher PAG rsFC with the hypothalamus, dorsomedial prefrontal, and inferior parietal cortices. Further, these connectivities were correlated negatively with tonic cocaine craving and recent cocaine use, respectively. Higher hypothalamic and frontoparietal rsFC with the PAG may reflect a compensatory process to regulate craving and compulsive drug use. The findings provide additional evidence in humans implicating the PAG circuit and may help research of the role of negative reinforcement in sustaining habitual drug use in cocaine addiction.

Rostrocaudal subregions of the ventral tegmental area are differentially impacted by chronic stress

RATIONALE

The ventral tegmental area (VTA) is implicated in the pathophysiology of depression and addictive disorders and is subject to the detrimental effects of stress. Chronic stress may differentially alter the activity pattern of its different subregions along the rostrocaudal and dorsoventral axes, which may relate to the variable behavioral sensitivity to stress mediated by these subregions.

OBJECTIVES

Here, chronic stress-exposed rats were tested for depressive-like reactivity. In situ hybridization for zif268 as a marker of neuronal activation was combined with in vivo single-unit recording of dopaminergic neurons to assess modifications in the activity of the rostral VTA (rVTA) and caudal VTA (cVTA). Changes in the expression of stress-responsive glucocorticoid receptors (GR) and brain-derived neurotrophic factor (BDNF) were also assessed.

RESULTS

Stress-induced anhedonia-like, hyper-anxious, and passive-like responding were associated with reductions in dopaminergic burst activity in the cVTA and an increase in local GABAergic activity, particularly in GABA_A receptor sensitivity. On the other hand, stress increased single-spiking activity, burst activity, and zif268 mRNA levels in the rVTA, which were associated with increased glutamatergic tonus and enhanced GR and AMPA receptor (AMPAR) expression. rVTA and cVTA activity differentially correlated with sucrose preference and passivity measures.

CONCLUSIONS

These data demonstrate that the rVTA and cVTA respond differently to stress and suggest that while cVTA activity may be related to passivity-like states, the activity of both subregions appears to be related to anhedonia and the processing of incentive value. These region-dependent abnormalities indicate the multi-modular composition of the VTA, which could provide multiple substrates for different symptom features.

Distinct regulation pattern of Egr-1, BDNF and Arc during morphine-withdrawal conditioned place aversion paradigm: Role of glucocorticoids

Negative affective aspects of opiate abstinence contribute to the persistence of substance abuse. Importantly, interconnected brain areas involved in aversive motivational processes, such as the ventral tegmental area (VTA) and medial prefrontal cortex (mPFC), become activated when animals are confined to withdrawal-paired environments. In the present study, place aversion was elicited in sham and adrenalectomized (ADX) animals by conditioned naloxone-precipitated drug withdrawal following exposure to chronic morphine. qPCR was employed to detect the expression of brain derived neurotrophic factor (Bdnf) and the immediate early genes (IEG) early growth response 1 (Egr-1) and activity-regulated cytoskeletal-associated protein (Arc) mRNAs in the VTA and mPFC at different time points of the conditioned place aversion (CPA) paradigm: after the conditioning phase and after the test phase. Sham + morphine rats exhibited robust CPA, which was impaired in ADX + morphine animals. Egr-1 and Arc were induced in the VTA and mPFC after morphine-withdrawal conditioning phase. Furthermore, Bdnf expression was enhanced in the VTA during the test phase. Bdnf induction seemed to be glucocorticoid-dependent, given that was correlated with HPA axis function and was not observed in morphine-dependent ADX animals. In addition, BDNF regulation and function was opposite in the VTA and mPFC during aversive-withdrawal memory retrieval. Our results suggest that IEGs and BDNF in these brain regions may play key roles in mediating the negative motivational component of opiate withdrawal.

Viral-mediated Zif268 expression in the prefrontal cortex protects against gonadectomy-induced working memory, long-term memory, and social interaction deficits in male rats

In humans, some males experience reductions in testosterone levels, as a natural consequence of aging or in the clinical condition termed hypogonadism, which are associated with impaired cognitive performance and mood disorder(s). Some of these behavioral deficits can be reversed by testosterone treatment. Our previous work in rats reported that sex differences in the expression of the transcription factor Zif268, a downstream target of testosterone, within the medial prefrontal cortex (mPFC) mediates sex differences in social interaction. In the present study, we aimed to examine the effects of gonadectomy (GNX) in male rats on mPFC Zif268 expression, mood and cognitive behaviors. We also examined whether reinstitution of Zif268 in GNX rats will correct some of the behavioral deficits observed following GNX. Our results show that GNX induced a downregulation of Zif268 protein in the mPFC, which was concomitant with impaired memory in the y-maze and spontaneous object recognition test, reduced social interaction time, and depression-like behaviors in the forced swim test. Reinstitution of mPFC Zif268, using a novel adeno-associated-viral (AAV) construct, abrogated GNX-induced working memory and long-term memory impairments, and reductions in social interaction time, but not GNX-induced depression-like behaviors. These findings suggest that mPFC Zif268 exerts beneficial effects on memory and social interaction, and could be a potential target for novel treatments for behavioral impairments observed in hypogonadal and aged men with declining levels of gonadal hormones.

Ultrasonic Vocalizations as a Measure of Affect in Preclinical Models of Drug Abuse: A Review of Current Findings

The present review describes ways in which ultrasonic vocalizations (USVs) have been used in studies of substance abuse. Accordingly, studies are reviewed which demonstrate roles for affective processing in response to the presentation of drug-related cues, experimenter- and self-administered drug, drug withdrawal, and during tests of relapse/reinstatement. The review focuses on data collected from studies using cocaine and amphetamine, where a large body of evidence has been collected. Data suggest that USVs capture animals’ initial positive reactions to psychostimulant administration and are capable of identifying individual differences in affective responding. Moreover, USVs have been used to demonstrate that positive affect becomes sensitized to psychostimulants over acute exposure before eventually exhibiting signs of tolerance. In the drug-dependent animal, a mixture of USVs suggesting positive and negative affect is observed, illustrating mixed responses to psychostimulants. This mixture is predominantly characterized by an initial bout of positive affect followed by an opponent negative emotional state, mirroring affective responses observed in human addicts. During drug withdrawal, USVs demonstrate the presence of negative affective withdrawal symptoms. Finally, it has been shown that drug-paired cues produce a learned, positive anticipatory response during training, and that presentation of drug-paired cues following abstinence produces both positive affect and reinstatement behavior. Thus, USVs are a useful tool for obtaining an objective measurement of affective states in animal models of substance abuse and can increase the information extracted from drug administration studies. USVs enable detection of subtle differences in a behavioral response that might otherwise be missed using traditional measures.

Life-long consequences of juvenile exposure to psychotropic drugs on brain and behavior

Psychostimulants such as methylphenidate (MPH) and antidepressants such as fluoxetine (FLX) are widely used in the treatment of various mental disorders or as cognitive enhancers. These medications are often combined, for example, to treat comorbid disorders. There is a considerable body of evidence from animal models indicating that individually these psychotropic medications can have detrimental effects on the brain and behavior, especially when given during sensitive periods of brain development. However, almost no studies investigate possible interactions between these drugs. This is surprising given that their combined neurochemical effects (enhanced dopamine and serotonin neurotransmission) mimic some effects of illicit drugs such as cocaine and amphetamine. Here, we summarize recent studies in juvenile rats on the molecular effects in the mid- and forebrain and associated behavioral changes, after such combination treatments. Our findings indicate that these combined MPH+FLX treatments can produce similar molecular changes as seen after cocaine exposure while inducing behavioral changes indicative of dysregulated mood and motivation, effects that often endure into adulthood.

The transcription factor Zif268/Egr1, brain plasticity, and memory

The capacity to remember our past experiences and organize our future draws on a number of cognitive processes that allow our brain to form and store neural representations that can be recalled and updated at will. In the brain, these processes require mechanisms of neural plasticity in the activated circuits, brought about by cellular and molecular changes within the neurons activated during learning. At the cellular level, a wealth of experimental data accumulated in recent years provides evidence that signaling from synapses to nucleus and the rapid regulation of the expression of immediate early genes encoding inducible, regulatory transcription factors is a key step in the mechanisms underlying synaptic plasticity and the modification of neural networks required for the laying down of memories. In the activated neurons, these transcriptional events are thought to mediate the activation of selective gene programs and subsequent synthesis of proteins, leading to stable functional and structural remodeling of the activated networks, so that the memory can later be reactivated upon recall. Over the past few decades, novel insights have been gained in identifying key transcriptional regulators that can control the genomic response of synaptically activated neurons. Here, as an example of this approach, we focus on one such activity-dependent transcription factor, Zif268, known to be implicated in neuronal plasticity and memory formation. We summarize current knowledge about the regulation and function of Zif268 in different types of brain plasticity and memory processes.

Juvenile and adult rats differ in cocaine reward and expression of zif268 in the forebrain

Adolescents are more likely to experiment with and become addicted to drugs of abuse. A number of studies indicate that the developmental forebrain may be responsible for making adolescents vulnerable to the addictive properties of such drugs. The aim of this study was to first compare behavioral responses to novelty and cocaine between juvenile and adult rats and then compare levels of the immediate-early gene zif268 activation in several forebrain areas via in situ hybridization. We found that juveniles demonstrated higher locomotion scores and required a higher dose of cocaine than adults to establish a conditioned place preference. Additionally, at this higher dose, juvenile rats exhibited higher levels of zif268 mRNA in the prefrontal cortex compared with adults. A developmental effect for increased zif268 mRNA was also observed in the striatum and nucleus accumbens, but there was no interaction with the cocaine dose. These findings hold interesting implications for the study of the molecular mechanisms underlying juvenile drug addiction.

Juvenile administration of concomitant methylphenidate and fluoxetine alters behavioral reactivity to reward- and mood-related stimuli and disrupts ventral tegmental area gene expression in adulthood

There is a rise in the concurrent use of methylphenidate (MPH) and fluoxetine (FLX) in pediatric populations. However, the long-term neurobiological consequences of combined MPH and FLX treatment (MPH + FLX) during juvenile periods are unknown. We administered saline (VEH), MPH, FLX, or MPH + FLX to juvenile Sprague Dawley male rats from postnatal day 20 to 34, and assessed their reactivity to reward- and mood-related stimuli 24 h or 2 months after drug exposure. We also assessed mRNA and protein levels within the ventral tegmental area (VTA) to determine the effect of MPH, FLX, or MPH + FLX on the extracellular signal-regulated protein kinase-1/2 (ERK) pathway--a signaling cascade implicated in motivation and mood regulation. MPH + FLX enhanced sensitivity to drug (i.e., cocaine) and sucrose rewards, as well as anxiety (i.e., elevated plus maze)- and stress (i.e., forced swimming)-eliciting situations when compared with VEH-treated rats. MPH + FLX exposure also increased mRNA of ERK2 and its downstream targets cAMP response element-binding protein (CREB), BDNF, c-Fos, early growth response protein-1 (Zif268), and mammalian target of rapamycin (mTOR), and also increased protein phosphorylation of ERK2, CREB, and mTOR 2 months after drug exposure when compared with VEH-treated rats. Using herpes simplex virus-mediated gene transfer to block ERK2 activity within the VTA, we rescued the MPH and FLX-induced behavioral deficits seen in the forced-swimming task 2 months after drug treatment. These results indicate that concurrent MPH + FLX exposure during preadolescence increases sensitivity to reward-related stimuli while simultaneously enhancing susceptibility to stressful situations, at least in part, due to long-lasting disruptions in ERK signaling within the VTA.

Role of BDNF and GDNF in drug reward and relapse: a review

Brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) are neurotrophic factors that are critical for the growth, survival, and differentiation of developing neurons. These neurotrophic factors also play important roles in the survival and function of adult neurons, learning and memory, and synaptic plasticity. Since the mid-1990s, investigators have studied the role of BDNF and GDNF in the behavioral effects of abused drugs and in the neuroadaptations induced by repeated exposure to drugs in the mesocorticolimbic dopamine system. Here, we review rodent studies on the role of BDNF and GDNF in drug reward, as assessed in the drug self-administration and the conditioned place preference procedures, and in drug relapse, as assessed in extinction and reinstatement procedures. Our main conclusion is that whether BDNF or GDNF would facilitate or inhibit drug-taking behaviors depends on the drug type, the brain site, the addiction phase (initiation, maintenance, or abstinence/relapse), and the time interval between site-specific BDNF or GDNF injections and the reward- and relapse-related behavioral assessments.

The use of neuroproteomics in drug abuse research

The number of discovery proteomic studies of drug abuse has begun to increase in recent years, facilitated by the adoption of new techniques such as 2D-DIGE and iTRAQ. For these new tools to provide the greatest insight into the neurobiology of addiction, however, it is important that the addiction field has a clear understanding of the strengths, limitations, and drug abuse-specific research factors of neuroproteomic studies. This review outlines approaches for improving animal models, protein sample quality and stability, proteome fractionation, data analysis, and data sharing to maximize the insights gained from neuroproteomic studies of drug abuse. For both the behavioral researcher interested in what proteomic study results mean, and for biochemists joining the drug abuse research field, a careful consideration of these factors is needed. Similar to genomic, transcriptomic, and epigenetic methods, appropriate use of new proteomic technologies offers the potential to provide a novel and global view of the neurobiological changes underlying drug addiction. Proteomic tools may be an enabling technology to identify key proteins involved in drug abuse behaviors, with the ultimate goal of understanding the etiology of drug abuse and identifying targets for the development of therapeutic agents.

Role of ventral tegmental area glial cell line-derived neurotrophic factor in incubation of cocaine craving

BACKGROUND

Ventral tegmental area (VTA) brain-derived neurotrophic factor (BDNF) contributes to time-dependent increases in cue-induced cocaine seeking after withdrawal (incubation of cocaine craving). Here, we studied the role of glial cell line-derived neurotrophic factor (GDNF) in incubation of cocaine craving because, like BDNF, GDNF provides trophic support to midbrain dopamine neurons.

METHODS

We first trained rats to self-administer intravenous cocaine for 10 days (6 hours/d, cocaine injections were paired with a tone-light cue). We then manipulated VTA GDNF function and assessed cue-induced cocaine seeking in extinction tests after withdrawal from cocaine.

RESULTS

VTA injections of an adeno-associated virus (AAV) vector containing rat GDNF cDNA (5 x 10(8) viral genomes) on withdrawal Day 1 increased cue-induced cocaine seeking on withdrawal days 11 and 31; this effect was not observed after VTA injections of an AAV viral vector containing red fluorescent protein (RFP). Additionally, VTA, but not substantial nigra (SN), GDNF injections (1.25 microg or 12.5 microg/side) immediately after the last cocaine self-administration session increased cue-induced drug seeking on withdrawal days 3 and 10; this effect was reversed by VTA injections of U0126, which inhibits the activity of extracellular signal-regulated kinases (ERK). Finally, interfering with VTA GDNF function by chronic delivery of anti-GDNF monoclonal neutralizing antibodies via minipumps (600 ng/side/d) during withdrawal Days 1-14 prevented the time-dependent increases in cue-induced cocaine seeking on withdrawal days 11 and 31.

CONCLUSIONS

Our results indicate that during the first weeks of withdrawal from cocaine self-administration, GDNF-dependent neuroadaptations in midbrain VTA neurons play an important role in the development of incubation of cocaine craving.

Persistent alterations in mesolimbic gene expression with abstinence from cocaine self-administration

Cocaine-responsive gene expression changes have been described after either no drug abstinence or short periods of abstinence. Little data exist on the persistence of these changes after long-term abstinence. Previously, we reported that after discrete-trial cocaine self-administration and 10 days of forced abstinence, incubation of cocaine reinforcement was observable by a progressive ratio schedule. The present study used rat discrete-trial cocaine self-administration and long-term forced abstinence to examine extinction responding, mRNA abundance of known cocaine-responsive genes, and chromatin remodeling. At 30 and 100 days of abstinence, extinction responding increased compared to 3-day abstinent rats. Decreases in both medial prefrontal cortex (mPFC) and nucleus accumbens c-fos, Nr4a1, Arc, and EGR1 mRNA were observed, and in most cases persisted, for 100 days of abstinence. The signaling peptides CART and neuropeptide Y (NPY) transiently increased in the mPFC, but returned to baseline levels following 10 days of abstinence. To investigate a potential regulatory mechanism for these persistent mRNA changes, levels of histone H3 acetylation at promoters for genes with altered mRNA expression were examined. In the mPFC, histone H3 acetylation decreased after 1 and 10 days of abstinence at the promoter for EGR1. H3 acetylation increased for NPY after 1 day of abstinence and returned to control levels by 10 days of abstinence. Behaviorally, these results demonstrate incubation after discrete-trial cocaine self-administration and prolonged forced abstinence. This incubation is accompanied by changes in gene expression that persist long after cessation of drug administration and may be regulated by chromatin remodeling.

NMDA receptors in the rat VTA: a critical site for social stress to intensify cocaine taking

RATIONALE

Cocaine strengthens behaviors associated with its administration. The stress response by individuals that are defeated in a brief aggressive confrontation can also promote enduring behavioral consequences similar to those of stimulants.

OBJECTIVES

The study intends to find whether intermittent episodes of defeat promote cocaine's reinforcing effects by triggering N-methyl-D: -aspartic acid (NMDA)-receptor-mediated plasticity in the ventral tegmental area (VTA).

MATERIALS AND METHODS

Long-Evans rats were investigated after four social defeats in three experiments. Two experiments examined systemic or intra-VTA antagonism of NMDA receptors during stress on the later expression of behavioral sensitization and cocaine self-administration during fixed and progressive ratio (PR) schedules of reinforcement (0.3 mg/kg/infusion), including a novel 24-h variable-dose continuous access binge (0.2, 0.4, and 0.8 mg/kg/infusion, delivered in an irregular sequence). Third, the expression of receptor proteins NR1 (NMDA) and GluR1 [alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)] were examined in VTA and nucleus accumbens.

RESULTS

Intermittent defeats augment locomotor responses to cocaine and increase cocaine taking. Rates of responding during binges are increased after defeat stress. These effects are prevented when NMDA or AMPA receptor antagonists are administered before defeats. VTA infusions of the NMDA antagonist AP-5 (5 nmol/side) before stress prevents locomotor sensitization to cocaine and intensified responding for cocaine during a PR schedule or binge. Episodic defeats increase GluR1 AMPA subunit protein expression in the VTA.

CONCLUSIONS

Social stress stimulates NMDA receptors in the VTA, and this neural action of defeat may be essential for prompting a later increase in cocaine intake during binges.

Functional internal complexity of amygdala: focus on gene activity mapping after behavioral training and drugs of abuse

The amygdala is a heterogeneous brain structure implicated in processing of emotions and storing the emotional aspects of memories. Gene activity markers such as c-Fos have been shown to reflect both neuronal activation and neuronal plasticity. Herein, we analyze the expression patterns of gene activity markers in the amygdala in response to either behavioral training or treatment with drugs of abuse and then we confront the results with data on other approaches to internal complexity of the amygdala. c-Fos has been the most often studied in the amygdala, showing specific expression patterns in response to various treatments, most probably reflecting functional specializations among amygdala subdivisions. In the basolateral amygdala, c-Fos expression appears to be consistent with the proposed role of this nucleus in a plasticity of the current stimulus-value associations. Within the medial part of the central amygdala, c-Fos correlates with acquisition of alimentary/gustatory behaviors. On the other hand, in the lateral subdivision of the central amygdala, c-Fos expression relates to attention and vigilance. In the medial amygdala, c-Fos appears to be evoked by emotional novelty of the experimental situation. The data on the other major subdivisions of the amygdala are scarce. In conclusion, the studies on the gene activity markers, confronted with other approaches involving neuroanatomy, physiology, and the lesion method, have revealed novel aspects of the amygdala, especially pointing to functional heterogeneity of this brain region that does not fit very well into contemporarily active debate on serial versus parallel information processing within the amygdala.

Multidisciplinary perspectives on impaired control over substance use

This article presents the proceedings of a symposium held at the June 2005 meeting of the Research Society on Alcoholism in Santa Barbara, California. Impaired control over substance use has long been considered a central feature of alcohol and drug dependence. However, much remains to be learned about the nature of impaired control, the mechanisms by which acute and chronic substance use can lead to impaired control, and how this construct is best assessed in the laboratory and the clinic. The goal of this symposium was to describe current perspectives on impaired control over alcohol and drug use from diverse research areas, to promote future multidisciplinary work in this area. Four speakers described their work on impaired control using human clinical samples (Dr. Chung), animal models (Dr. Miczek), experimental laboratory paradigms in humans (Dr. Fillmore), and neuroimaging studies (Dr. Easdon). Taken together, the talks highlighted the heterogeneous nature of constructs such as impaired inhibitory control, and patterns of impulsive and compulsive substance use. Future clinical and experimental research should attempt to carefully define and measure particular aspects of impaired control and to seek insights from other disciplines.

Previous experience of ethanol withdrawal increases withdrawal-induced c-fos expression in limbic areas, but not withdrawal-induced anxiety and prevents withdrawal-induced elevations in plasma corticosterone

RATIONALE

Increased anxiety is a characteristic of the acute ethanol withdrawal syndrome. Repeated exposure of rats to withdrawal from chronic ethanol increases sensitivity to seizures.

OBJECTIVES

We investigated whether repeated withdrawal experience increases withdrawal-induced anxiety and stress, and if it changes withdrawal-induced activation of related brain areas.

METHODS

Rats were chronically treated with an ethanol-containing liquid diet either for 24 days continuously (single withdrawal, SWD) or interspersed with 2x3-day withdrawal periods (repeated withdrawal, RWD), or with a control diet. Eight hours after ethanol withdrawal, anxiety-like behaviour was tested in the elevated plus-maze, blood corticosterone levels were measured, and expression level of markers of neuronal activity and plasticity, c-fos and zif268, was assessed.

RESULTS

Eight hours after ethanol withdrawal, SWD rats showed increased anxiety on the elevated plus-maze relative to control rats. Rats given previous withdrawal experiences did not show further increases in measures of anxiety. Corticosterone levels were elevated during withdrawal in SWD rats but not in RWD rats. RWD resulted in marked increases in c-fos expression in amygdala, hippocampus, nucleus accumbens and dorsolateral periaqueductal grey. In contrast, zif268 expression was not increased after RWD, and in central amygdala the marked increase in zif268 seen after SWD was absent after RWD.

CONCLUSIONS

The data suggest increased ability of withdrawal to activate neuronal circuits but reduced plasticity after RWD. We suggest parallels between the consequences of repeated ethanol withdrawal and repeated exposure to stress, and discuss implications of withdrawal for brain plasticity.

Differential involvement of the prelimbic cortex and striatum in conditioned heroin and sucrose seeking following long-term extinction

Relapse to drug taking is triggered by stimuli previously associated with consumption of drugs of misuse (cues) and involves brain systems controlling motivated behaviour towards natural reinforcers. In this study, we aimed to identify and compare neuronal pathways in corticostriatal systems that control conditioned heroin or natural reward (sucrose) seeking. To that end, rats were trained to self-administer heroin or sucrose in association with an identical compound cue. After more than 3 weeks of abstinence during extinction training, cue exposure robustly reinstated heroin and sucrose seeking, but induced distinct and even opposing changes in the expression of the neuronal activation marker zif268 in the prelimbic cortex and striatal complex, respectively. Because in the prelimbic area zif268 expression was enhanced during cue-induced heroin seeking but unaffected during sucrose seeking, a pharmacological intervention was aimed at this prefrontal region. Injection of a GABA agonist mixture within the prelimbic area enhanced conditioned heroin seeking, but had no effect on conditioned sucrose seeking. Our findings suggest a differential role of the prelimbic area and the striatum in the persistence of heroin vs. sucrose seeking following long-term extinction.

Cholecystokinin-2 (CCK2) receptor-mediated anxiety-like behaviors in rats

Cholecystokinin (CCK) is a neurotransmitter in the brain closely related to anxiety. Of the two CCK receptor subtypes, CCK(2) receptors are most implicated in the control of anxiety-related behavior. CCK(2) receptor activation causes anxiogenic effects while the blockade of this receptor has anxiolytic effects. This review focuses on the molecular mechanisms of CCK(2) receptors underlying anxiety-related behaviors of PVG hooded and Spraque-Dawley (SD) rats in two anxiety models (elevated plus-maze [EPM] and cat exposure test). PVG hooded rats showed prolonged freezing behavior in the cat exposure test while SD rats showed very low levels of freezing. A CCK(2) receptor antagonist (LY225910) attenuated freezing behavior in PVG hooded rats while a CCK(2) receptor agonist (CCK-4) increased freezing behavior in SD rats. In contrast, the two strains behaved similarly on the EPM. CCK-4 caused a pronounced anxiogenic effect in PVG hooded rats but only a slight effect in SD rats. CCK(2) antagonists also showed more pronounced anxiolytic effects in PVG hooded rats than in SD rats. CCK(2) receptor expression was greater in PVG hooded than in SD rats in the cortex and hippocampus. Genetic studies also demonstrated four differences in the DNA sequence of the CCK(2) receptor gene between the two rat strains.

Brief social defeat stress: long lasting effects on cocaine taking during a binge and zif268 mRNA expression in the amygdala and prefrontal cortex

Social stress can engender behavioral and neural sensitization and this process appears to enhance the transition to compulsive drug abuse. Exposures to brief social defeat stress in rats have significant consequences on cocaine-reinforced behavior and on the level of functional activation within regions of the mesocorticolimbic dopamine system. The objectives of the current study were to examine the enduring consequences of brief episodes of social defeat stress on cocaine bingeing (during 24 h of continuous access) and on the emergence of neural adaptations as revealed by zif268 immediate early gene expression. Adult, male Long-Evans rats were subjected to four 25 min episodes of social defeat (once every 72 h). After 2 months, cocaine binges or zif268 mRNA gene expression were studied after confirming behavioral cross-sensitization to stimulant challenge. Sensitization to social defeat increased cocaine intake during a 24 h binge, effectively abolishing the typical circadian pattern of intake. Furthermore, 60 days after exposure to the sensitizing regimen of social defeat, levels of functional activation, measured by zif268 mRNA expression, in the central and medial amygdala were increased, while levels of activation in the medial prefrontal cortex were decreased. Persistent stress-induced levels of zif268 in the central and medial amygdala were attenuated by an injection of amphetamine (1.0 mg/kg). Divergent changes in zif268 within the amygdala and cortex 2 months after social defeat stress indicate the vulnerability of distinct cellular populations in networks that modulate the behavioral actions of psychomotor stimulants.

Psychostimulant withdrawal as an inducing condition in animal models of depression

A large body of evidence indicates that the withdrawal from high doses of psychostimulant drugs in humans induces a transient syndrome, with symptoms that appear isomorphic to those of major depressive disorder. Pharmacological treatment strategies for psychostimulant withdrawal in humans have focused mainly on compounds with antidepressant properties. Animal models of psychostimulant withdrawal have been shown to demonstrate a wide range of deficits, including changes in homeostatic, affective and cognitive behaviors, as well as numerous physiological changes. Many of these behavioral and physiological sequelae parallel specific symptoms of major depressive disorder, and have been reversed by treatment with antidepressant drugs. These combined findings provide strong support for the use of psychostimulant withdrawal as an inducing condition in animal models of depression. In the current review we propound that the psychostimulant withdrawal model displays high levels of predictive and construct validity. Recent progress and limitations in the development of this model, as well as future directions for research, are evaluated and discussed.

Molecular basis for catecholaminergic neuron diversity

Catecholaminergic neurons control diverse cognitive, motor, and endocrine functions and are associated with multiple psychiatric and neurodegenerative disorders. We present global gene-expression profiles that define the four major classes of dopaminergic (DA) and noradrenergic neurons in the brain. Hypothalamic DA neurons and noradrenergic neurons in the locus coeruleus display distinct group-specific signatures of transporters, channels, transcription, plasticity, axon-guidance, and survival factors. In contrast, the transcriptomes of midbrain DA neurons of the substantia nigra and the ventral tegmental area are closely related with <1% of differentially expressed genes. Transcripts implicated in neural plasticity and survival are enriched in ventral tegmental area neurons, consistent with their role in schizophrenia and addiction and their decreased vulnerability in Parkinson's disease. The molecular profiles presented provide a basis for understanding the common and population-specific properties of catecholaminergic neurons and will facilitate the development of selective drugs.

Neuroadaptive effects of active versus passive drug administration in addiction research

Increasing knowledge of the genome sequences of several organisms and the development of genome-wide, high-throughput screening techniques for gene expression are likely to generate a vast amount of data aimed at elucidating the molecular mechanisms of addiction. These findings are likely to have potential for future addiction pharmacotherapies. However, it is important to employ animal models that dissociate the molecular and cellular consequences of the direct pharmacological effects of addictive drugs from those that result from the cognitive processes associated with self-administration of these drugs. In this article, we suggest that the short-term and long-term neuroadaptive effects of addictive drugs in the brain depend crucially on the drug-exposure paradigm used [i.e. passive (non-contingent) drug exposure and active (contingent) self-administration]. This has important ramifications for future molecular and cellular studies of drug addiction.

The transition from controlled to compulsive drug use is associated with a loss of sensitization

Rats provided limited daily access to cocaine (1 h) maintain stable levels of drug self-administration over time while those switched to longer access (6 h or more) exhibit escalating patterns of drug intake. These results are reminiscent of human recreational and compulsive drug-taking behavior, respectively. We found that the brains of cocaine-self-administering rats were also qualitatively different in subjects having experienced 6-h (Coc6h) daily access compared to 1-h (Coc1h) access. Fourteen days after an eight-day protocol of cocaine self-administration, all subjects received one infusion of cocaine. Coc1h animals showed enhanced c-Fos reactivity in dopaminergic mesocorticolimbic brain regions and a sensitized locomotor response to IV cocaine. In contrast, both the neural and behavioral sensitization to cocaine was diminished in Coc6h animals. These data suggest that the transition to escalating patterns of drug use is associated with neuroadaptive changes that counteract those initially associated with controlled stable patterns of drug use.

Induction of the learning and plasticity-associated gene Zif268 following exposure to a discrete cocaine-associated stimulus

We investigated whether the expression of the plasticity-associated gene, zif268, was associated with memories retrieved by exposure to a discrete stimulus that had been associated with cocaine, either self-administered or administered noncontingently. In the absence of drug, passive presentation of a cocaine-associated light stimulus induced changes in the expression of zif268 measured by in situ hybridization within a limbic cortical-ventral striatal circuit that was not only regionally selective but related to whether the rats had originally received response-contingent or noncontingent stimulus-drug pairings. In rats that had self-administered drug, the cocaine-conditioned stimulus (CS) increased zif268 expression in neurons of the ventral tegmental area, nucleus accumbens core and shell, and basal nucleus of the amygdala but not hippocampus, prelimbic area of the medial prefrontal cortex or amygdala central nucleus. The same CS that had been associated with cocaine administered noncontingently additionally increased zif268 mRNA levels in area Cg1 of the anterior cingulate cortex, ventral and lateral regions of the orbitofrontal cortex and lateral nucleus of the amygdala. Zif268 induction was related to the predictive relationship between the stimulus and cocaine as no changes were seen in cocaine-experienced rats that had received unpaired light and drug presentations during training. Thus, zif268 expression is increased by appetitively (drug) conditioned stimuli after Pavlovian learning. Zif268 may participate in the molecular mechanisms underlying the reconsolidation or re-encoding of Pavlovian stimulus-drug associations across a distributed limbic cortical-ventral striatal neural network and that may contribute to the basis of the enduring drug-seeking behaviour produced by environmental cues.

Vocalizations during withdrawal from opiates and cocaine: possible expressions of affective distress

Intense anxiety has been postulated to trigger relapse to abuse of opiates and psychomotor stimulants. Preclinical research methodologies need to be developed to adequately characterize the affective or emotional component of withdrawal. Classically, withdrawal from psychomotor stimulants and opiates focuses on somatic and autonomic indices, foremost based on observational assessments and, additionally, on measures of disrupted conditioned behavior. These measures depict the intensity and time course of withdrawal from specific doses of opiates and psychomotor stimulants, but require large numbers of subjects due to single use of each individual. Behavioral disruptions have been attributed to anhedonia, a core symptom of drug withdrawal, as well as major depressive and psychotic disorders. In spite of some pharmacological validation, inferences about anxiety-like disturbances, based on observed somatic and autonomic signs or on changes in conditioned responses, have to remain tentative. High-pitched vocalizations may communicate affective expressions and, in rodents, different kinds of ultrasonic vocalizations communicate maternal separation distress in infants, accompany the intensely arousing phases of agonistic confrontations, signal submission and distress in defensive responses to threats and painful events, and are part of the excitatory and inhibitory phases of sexual behavior. While acute treatment with opiates, psychomotor stimulants, alcohol and benzodiazepines suppresses ultrasonic vocalizations in the 22-25-kHz range, rats emit high rates of ultrasonic vocalizations upon withdrawal from prolonged exposure to these drugs, particularly if they have been startled. Peak rates of ultrasonic distress calls occur ca. 1-3 days after cessation of cocaine or opiate treatment and decline within 5-7 days. Ultrasonic vocalizations during withdrawal from cocaine, alcohol or benzodiazepines can be attenuated by renewed access to the drug. It will be informative to learn how the neural circuit mediating vocalizations interacts with the ones subserving self-administration of alcohol, opiates and psychomotor stimulants.

cDNA microarray analysis of gene expression in anxious PVG and SD rats after cat-freezing test

To identify genes involved in the development of anxiety or fear, we analyzed the gene expression profiles of the cortex of anxious hooded PVG and Sprague-Dawley (SD) rats after exposure to the cat-freezing test apparatus. These two rat strains showed a marked difference in the extent of anxious behavior on the cat-freezing test; the hooded PVG rats showed highly anxious behavior while a low anxiety state was observed in SD rats. A cDNA microarray consisting of 5,931 genes was employed to investigate the global mRNA expression profiles of anxiety-related genes. According to the assumption that an abundance ratio of > or =1.5 is indicative of a change in gene expression, we detected 16 upregulated and 38 downregulated genes in PVG hooded and SD rats. Some of these genes have not yet been associated with anxiety (e.g. FGF), while other genes were recently found to be expressed in an anxious state (e.g., rat nerve growth factor-induced gene, NGFI-A). Our study also focused on the expression of some neurotransmitter receptors that have already been proven to be relevant to anxiety or fear, e.g., gamma-aminobutyric acid (GABA), cholecystokinin (CCK) and 5-HT(3) receptors. To further confirm the microarray data, the mRNA expressions of three genes: rat activity-regulated cytoskeleton-associated gene (Arc), rat NGFI-A gene and rat 5-HT(3) receptor (5-HT(3)R) mRNA, were studied by reverse transcription-polymerase chain reaction (RT-PCR). The results of RT-PCR were basically consistent with those from cDNA microarray. Our study therefore demonstrated that the microarray technique is an efficient tool for analyzing global expression profiles of anxiety-related genes, which may also provide further insight into the molecular mechanisms underlying the states of anxiety and fear.

Functional striatal hypodopaminergic activity in mice lacking adenosine A(2A) receptors

Adenosine and caffeine modulate locomotor activity and striatal gene expression, partially through the activation and blockade of striatal A(2A) receptors, respectively. The elucidation of the roles of these receptors benefits from the construction of A(2A) receptor-deficient mice (A(2A)-R(-/-)). These mice presented alterations in locomotor behaviour and striatal expression of genes studied so far, which are unexpected regarding the specific expression of A(2A) receptor by striatopallidal neurones. To clarify the functions of A(2A) receptors in the striatum and to identify the mechanisms leading to these unexpected modifications, we studied the basal expression of immediate early and constitutive genes as well as dopamine and glutamate neurotransmission in the striatum. Basal zif268 and arc mRNAs expression was reduced in mutant mice by 60-80%, not only in the striatum but also widespread in the cerebral cortex and hippocampus. Striatal expression of substance P and enkephalin mRNAs was reduced by about 50% and 30%, respectively, whereas the expression of GAD67 and GAD65 mRNAs was slightly increased and unaltered, respectively. In vivo microdialysis in the striatum revealed a 45% decrease in the extracellular dopamine concentration and three-fold increase in extracellular glutamate concentration. This was associated with an up-regulation of D(1) and D(2) dopamine receptors expression but not with changes in ionotropic glutamate receptors. The levels of tyrosine hydroxylase and of striatal and cortical glial glutamate transporters as well as adenosine A(1) receptors expression were indistinguishable between A(2A)-R(-/-) and wild-type mice. Altogether these results pointed out that the lack of A(2A) receptors leads to a functional hypodopaminergic state and demonstrated that A(2A) receptors are necessary to maintain a basal level in immediate early and constitutive genes expression in the striatum and cerebral cortex, possibly via their control of dopamine pathways.