Neuroadaptations of total levels of adenylate cyclase, protein kinase A, tyrosine hydroxylase, cdk5 and neurofilaments in the nucleus accumbens and ventral tegmental area do not correlate with expression of sensitized or tolerant locomotor responses to cocaine

Protein kinases in natural versus drug reward

Natural and drug rewards act on the same neural pathway, the mesolimbic dopaminergic system. In brain regions such as the nucleus accumbens and ventral tegmental area, drugs of abuse-induced stimulation of signaling pathways can lead to synaptic reshaping within this system. This is believed to be underlying the maladaptive alterations in behaviors associated with addiction. In this review, we discuss animal studies disclosing the implication of several protein kinases, namely protein kinase A (PKA), extracellular signal regulated kinase (ERK) mitogen-activated protein kinases (MAPK), p38 MAPK, and calcium/calmodulin-dependent kinase II (CaMKII), in reward-related brain regions in drug and natural reward. Furthermore, we refer to studies that helped pave the way toward a better understanding of the neurobiology underlying non-drug and drug reward through genetic deletion or brain region-specific pharmacological inhibition of these kinases. Whereas the role of kinases in drug reward has been extensively studied, their implication in natural reward, such as positive social interaction, is less investigated. Discovering molecular candidates, recruited specifically by drug versus natural rewards, can promote the identification of novel targets for the pharmacological treatment of addiction with less off-target effects and being effective when used combined with behavioral-based therapies.

Involvement of cAMP-Dependent Protein Kinase in the Nucleus Accumbens in Cocaine Versus Social Interaction Reward

Evidence suggests that PKA activity in the nucleus accumbens (NAc) plays an essential role in reward-related learning. In this study, we investigated whether PKA is differentially involved in the expression of learning produced by either natural reinforcers or psychostimulants. For that purpose, we inhibited PKA through a bilateral infusion of Rp-cAMPS, a specific PKA inhibitor, directly into the NAc. The effects of PKA inhibition in the NAc on the expression of concurrent conditioned place preference (CPP) for cocaine (drug) and social interaction (natural reward) in rats were evaluated. We found that PKA inhibition increased the expression of cocaine preference. This effect was not due to altered stress levels or decreased social reward. PKA inhibition did not affect the expression of natural reward as intra-NAc Rp-cAMPS infusion did not affect expression of social preference. When rats were trained to express cocaine or social interaction CPP and tested for eventual persisting preference 7 and 14 days after CPP expression, cocaine preference was persistent, but social preference was abolished after the first test. These results suggest that PKA in the NAc is involved in drug reward learning that might lead to addiction and that only drug, but not natural, reward is persistent.

Interruption of continuous opioid exposure exacerbates drug-evoked adaptations in the mesolimbic dopamine system

Drug-evoked adaptations in the mesolimbic dopamine system are postulated to drive opioid abuse and addiction. These adaptations vary in magnitude and direction following different patterns of opioid exposure, but few studies have systematically manipulated the pattern of opioid administration while measuring neurobiological and behavioral impact. We exposed male and female mice to morphine for one week, with administration patterns that were either intermittent (daily injections) or continuous (osmotic minipump infusion). We then interrupted continuous morphine exposure with either naloxone-precipitated or spontaneous withdrawal. Continuous morphine exposure caused tolerance to the psychomotor-activating effects of morphine, whereas both intermittent and interrupted morphine exposure caused long-lasting psychomotor sensitization. Given links between locomotor sensitization and mesolimbic dopamine signaling, we used fiber photometry and a genetically encoded dopamine sensor to conduct longitudinal measurements of dopamine dynamics in the nucleus accumbens. Locomotor sensitization caused by interrupted morphine exposure was accompanied by enhanced dopamine signaling in the nucleus accumbens. To further assess downstream consequences on striatal gene expression, we used next-generation RNA sequencing to perform genome-wide transcriptional profiling in the nucleus accumbens and dorsal striatum. The interruption of continuous morphine exposure exacerbated drug-evoked transcriptional changes in both nucleus accumbens and dorsal striatum, dramatically increasing differential gene expression and engaging unique signaling pathways. Our study indicates that opioid-evoked adaptations in brain function and behavior are critically dependent on the pattern of drug administration, and exacerbated by interruption of continuous exposure. Maintaining continuity of chronic opioid administration may, therefore, represent a strategy to minimize iatrogenic effects on brain reward circuits.

Systemic Administration of the Cyclin-Dependent Kinase Inhibitor (S)-CR8 Selectively Reduces Escalated Ethanol Intake in Dependent Rats

BACKGROUND

Chronic exposure to ethanol (EtOH) and other drugs of abuse can alter the expression and activity of cyclin-dependent kinase 5 (CDK5) and its cofactor p35, but the functional implication of CDK5 signaling in the regulation of EtOH-related behaviors remains unknown. In the present study, we sought to determine whether CDK5 activity plays a role in the escalation of EtOH self-administration triggered by dependence.

METHODS

We tested the effect of systemically administered (S)-CR8, a nonselective CDK inhibitor, on operant responding for EtOH or saccharin, a highly palatable reinforcer, in adult male Wistar rats. Half of the rats were made EtOH-dependent via chronic intermittent EtOH inhalation (CIE). We then sought to identify a possible neuroanatomical locus for the behavioral effect of (S)-CR8 by quantifying protein levels of CDK5 and p35 in subregions of the extended amygdala and prefrontal cortex from EtOH-naïve, nondependent, and dependent rats at the expected time of EtOH self-administration. We also analyzed the phosphorylation of 4 CDK5 substrates and of the CDK substrate consensus motif.

RESULTS

(S)-CR8 dose-dependently reduced EtOH self-administration in dependent rats. It had no effect on water or saccharin self-administration, nor in nondependent rats. The abundance of CDK5 or p35 was not altered in any of the brain regions analyzed. In the bed nucleus of the stria terminalis, CDK5 abundance was negatively correlated with intoxication levels during EtOH vapor exposure but there was no effect of dependence on the phosphorylation ratio of CDK5 substrates. In contrast, EtOH dependence increased the phosphorylation of low-molecular-weight CDK substrates in the basolateral amygdala (BLA).

CONCLUSIONS

The selective effect of (S)-CR8 on excessive EtOH intake has potential therapeutic value for the treatment of alcohol use disorders. Our data do not support the hypothesis that this effect would be mediated by the inhibition of up-regulated CDK5 activity in the extended amygdala nor prefrontal cortex. However, increased activity of CDKs other than CDK5 in the BLA may contribute to excessive EtOH consumption in alcohol dependence. Other (S)-CR8 targets may also be implicated.

Molecular Mechanism: ERK Signaling, Drug Addiction, and Behavioral Effects

Addiction to psychostimulants has been considered as a chronic psychiatric disorder characterized by craving and compulsive drug seeking and use. Over the past two decades, accumulating evidence has demonstrated that repeated drug exposure causes long-lasting neurochemical and cellular changes that result in enduring neuroadaptation in brain circuitry and underlie compulsive drug consumption and relapse. Through intercellular signaling cascades, drugs of abuse induce remodeling in the rewarding circuitry that contributes to the neuroplasticity of learning and memory associated with addiction. Here, we review the role of the extracellular signal-regulated kinase (ERK), a member of the mitogen-activated protein kinase, and its related intracellular signaling pathways in drug-induced neuroadaptive changes that are associated with drug-mediated psychomotor activity, rewarding properties and relapse of drug seeking behaviors. We also discuss the neurobiological and behavioral effects of pharmacological and genetic interferences with ERK-associated molecular cascades in response to abused substances. Understanding the dynamic modulation of ERK signaling in response to drugs may provide novel molecular targets for therapeutic strategies to drug addiction.

Transcriptional regulation of L-type calcium channel subtypes Cav1.2 and Cav1.3 by nicotine and their potential role in nicotine sensitization

INTRODUCTION

L-type calcium channel (LTCC) activity in the brain is mediated by 2 subtypes, Ca(v)1.2 and Ca(v)1.3. The individual contributions of these LTCC subtypes to the long-term pharmacological and behavioral effects of nicotine are unknown.

METHODS

Using quantitative in situ hybridization, we examined expression levels of Ca(v)1.2 and Ca(v)1.3 in forebrain regions of mice treated with nicotine (0.175 mg/kg) or saline for 1 or 14 days and sacrificed 24 hr or 7 days following the last injection. Additionally, we treated mice with nicotine for 14 days and then administered the nonspecific LTCC antagonist nifedipine twice daily during a 7-day abstinence period prior to testing for nicotine sensitization to determine the effect of LTCC blockade on sensitization.

RESULTS

Ca(v)1.2 mRNA was unaffected 24 hr following a single nicotine exposure, whereas Ca(v)1.3 mRNA was upregulated in several brain regions. Following 14 days of nicotine treatment and 24 hr of abstinence, Ca(v)1.2 mRNA was downregulated throughout the areas examined, whereas Ca(v)1.3 mRNA had mostly returned to control values. Following 7 days of abstinence, a strong upregulation of Ca(v)1.2 transcripts was observed, whereas Ca(v)1.3 mRNA was largely unaffected. In our sensitization study, nifedipine administered during nicotine abstinence impaired subsequent nicotine sensitization.

CONCLUSIONS

Our data suggest a differential involvement of Ca(v)1.2 and Ca(v)1.3 in nicotine-related processes. Ca(v)1.3 seems to be involved primarily during early exposure to nicotine. Ca(v)1.2 appears to play a role in the long-term molecular and behavioral changes that occur following chronic nicotine and abstinence. Nifedipine may counteract those nicotine-induced alterations in LTCC activity to impair nicotine sensitization.

Relapse to cocaine-seeking after abstinence is regulated by cAMP-dependent protein kinase A in the prefrontal cortex

Abstinence from cocaine self-administration (SA) is associated with neuroadaptations in the prefrontal cortex (PFC) and nucleus accumbens (NAc) that are implicated in cocaine-induced neuronal plasticity and relapse to drug-seeking. Alterations in cAMP-dependent protein kinase A (PKA) signaling are prominent in medium spiny neurons in the NAc after repeated cocaine exposure but it is unknown whether similar changes occur in the PFC. Because cocaine SA induces disturbances in glutamatergic transmission in the PFC-NAc pathway, we examined whether dysregulation of PKA-mediated molecular targets in PFC-NAc neurons occurs during abstinence and, if so, whether it contributes to cocaine-seeking. We measured the phosphorylation of cAMP response element binding protein (Ser133) and GluA1 (Ser845) in the dorsomedial (dm) PFC and the presynaptic marker, synapsin I (Ser9, Ser62/67, Ser603), in the NAc after 7 days of abstinence from cocaine SA with or without cue-induced cocaine-seeking. We also evaluated whether infusion of the PKA inhibitor, 8-bromo-Rp-cyclic adenosine 3', 5'-monophosphorothioate (Rp-cAMPs), into the dmPFC after abstinence would affect cue-induced cocaine-seeking and PKA-regulated phosphoprotein levels. Seven days of forced abstinence increased the phosphorylation of cAMP response element binding protein and GluA1 in the dmPFC and synapsin I (Ser9) in the NAc. Induction of these phosphoproteins was reversed by a cue-induced relapse test of cocaine-seeking. Bilateral intra-dmPFC Rp-cAMPs rescued abstinence-elevated PKA-mediated phosphoprotein levels in the dmPFC and NAc and suppressed cue-induced relapse. Thus, by inhibiting abstinence-induced PKA molecular targets, relapse reverses abstinence-induced neuroadaptations in the dmPFC that are responsible, in part, for the expression of cue-induced cocaine-seeking.

Adaptations in AMPA receptor transmission in the nucleus accumbens contributing to incubation of cocaine craving

Cue-induced cocaine craving in rodents intensifies or "incubates" during the first months of withdrawal from long access cocaine self-administration. This incubation phenomenon is relevant to human users who achieve abstinence but exhibit persistent vulnerability to cue-induced relapse. It is well established that incubation of cocaine craving involves complex neuronal circuits. Here we will focus on neuroadaptations in the nucleus accumbens (NAc), a region of convergence for pathways that control cocaine seeking. A key adaptation is a delayed (~3-4 weeks) accumulation of Ca(2+)-permeable AMPAR receptors (CP-AMPARs) in synapses on medium spiny neurons (MSN) of the NAc. These CP-AMPARs mediate the expression of incubation after prolonged withdrawal, although different mechanisms must be responsible during the first weeks of withdrawal, prior to CP-AMPAR accumulation. The cascade of events leading to CP-AMPAR accumulation is still unclear. However, several candidate mechanisms have been identified. First, mGluR1 has been shown to negatively regulate CP-AMPAR levels in NAc synapses, and it is possible that a withdrawal-dependent decrease in this effect may help explain CP-AMPAR accumulation during incubation. Second, an increase in phosphorylation of GluA1 subunits (at the protein kinase A site) within extrasynaptic homomeric GluA1 receptors (CP-AMPARs) may promote their synaptic insertion and oppose their removal. Finally, elevation of brain-derived neurotrophic factor (BDNF) levels in the NAc may contribute to maintenance of incubation after months of withdrawal, although incubation-related increases in BDNF accumulation do not account for CP-AMPAR accumulation. Receptors and pathways that negatively regulate incubation, such as mGluR1, are promising targets for the development of therapeutic strategies to help recovering addicts maintain abstinence. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Repeated cocaine exposure decreases dopamine D₂-like receptor modulation of Ca(2+) homeostasis in rat nucleus accumbens neurons

The nucleus accumbens (NAc) is a limbic structure in the forebrain that plays a critical role in cognitive function and addiction. Dopamine modulates activity of medium spiny neurons (MSNs) in the NAc. Both dopamine D₁-like and D₂-like receptors (including D1R or D(1,5)R and D2R or D(2,3,4)R, respectively) are thought to play critical roles in cocaine addiction. Our previous studies demonstrated that repeated cocaine exposure (which alters dopamine transmission) decreases excitability of NAc MSNs in cocaine-sensitized, withdrawn rats. This decrease is characterized by a reduction in voltage-sensitive Na(+) currents and high voltage-activated Ca(2+) currents, along with increased voltage-gated K(+) currents. These changes are associated with enhanced activity in the D1R/cAMP/PKA/protein phosphatase 1 pathway and diminished calcineurin function. Although D1R-mediated signaling is enhanced by repeated cocaine exposure, little is known whether and how the D2R is implicated in the cocaine-induced NAc dysfunction. Here, we performed a combined electrophysiological, biochemical, and neuroimaging study that reveals the cocaine-induced dysregulation of Ca(2+) homeostasis with involvement of D2R. Our novel findings reveal that D2R stimulation reduced Ca(2+) influx preferentially via the L-type Ca(2+) channels and evoked intracellular Ca(2+) release, likely via inhibiting the cAMP/PKA cascade, in the NAc MSNs of drug-free rats. However, repeated cocaine exposure abolished the D₂R effects on modulating Ca(2+) homeostasis with enhanced PKA activity and led to a decrease in whole-cell Ca(2+) influx. These adaptations, which persisted for 21 days during cocaine abstinence, may contribute to the mechanism of cocaine withdrawal.

Episodic withdrawal promotes psychomotor sensitization to morphine

The relative intermittency or continuity of drug delivery is a major determinant of addictive liability, and also influences the impact of drug exposure on brain function and behavior. Events that occur during the offset of drug action (ie, acute withdrawal) may have an important role in the consequences of intermittent drug exposure. We assessed whether recurrent episodes of acute withdrawal contribute to the development of psychomotor sensitization in rodents during daily morphine exposure. The acoustic startle reflex--a measure of anxiety induced by opiate withdrawal-was used to resolve and quantify discrete withdrawal episodes, and pharmacological interventions were used to manipulate withdrawal severity. Startle potentiation was observed during spontaneous withdrawal from a single morphine exposure, and individual differences in initial withdrawal severity positively predicted the subsequent development of sensitization. Manipulations that reduce or exacerbate withdrawal severity also produced parallel changes in the degree of sensitization. These results demonstrate that the episodic experience of withdrawal during daily drug exposure has a novel role in promoting the development of psychomotor sensitization--a prominent model of drug-induced neurobehavioral plasticity. Episodic withdrawal may have a pervasive role in many effects of intermittent drug exposure and contribute to the development of addiction.

Plasticity of L-type Ca2+ channels after cocaine withdrawal

Increased reactivity of certain frontal cortical brain regions to cocaine re-exposure or drug-associated cues in cocaine-abstinent human addicts is linked to drug craving. Similarly, in rats tested after withdrawal from repeated cocaine exposure, cocaine or other strong excitatory stimuli produce greater activation of pyramidal neurons in the medial prefrontal cortex (mPFC). Our recent findings indicate that the increased mPFC neuronal activation depends primarily upon enhanced voltage-sensitive Ca(2+) influx, most likely through high-voltage activated (HVA) L-type Ca(2+) channels, but the mechanism underlying the enhanced Ca(2+) currents is unknown. In this study, we used a protein crosslinking assay to show that repeated cocaine injections, resulting in behavioral sensitization, increased total protein levels and cell surface expression of HVA-Ca(v)1.2 L-type channels in pyramidal neurons in deep layers of the mPFC. These changes in Ca(v)1.2 L-channels were time dependent and subtype specific (i.e., differed from those observed for Ca(v)1.3 L-channels). Furthermore, we found enhanced PKA activity in the mPFC of cocaine-sensitized rats that persisted for 21 days after withdrawal. PKA phosphorylation of L-channels increases their activity, so Ca(2+) currents after cocaine withdrawal could be enhanced as a result of both increased activity and number of HVA-Ca(v)1.2 L-channels on the cell surface. By increasing the suprafiring threshold excitability of mPFC pyramidal neurons, excessive upregulation of HVA L-channel activity and number may contribute to the cortical hyper-responsiveness that enhances vulnerability to cocaine craving and relapse. More generally, our results are the first to demonstrate that repeated cocaine exposure alters the membrane trafficking of a voltage-sensitive ion channel.

Signaling pathway adaptations and novel protein kinase A substrates related to behavioral sensitization to cocaine

Behavioral sensitization is an animal model for aspects of cocaine addiction. Cocaine-sensitized rats exhibit increased AMPA receptor (AMPAR) surface expression in the nucleus accumbens (NAc) which may in turn enhance drug seeking. To identify signaling pathways contributing to AMPAR up-regulation, we measured AMPAR surface expression and signaling pathway activation in the NAc of cocaine-sensitized rats, cocaine-exposed rats that failed to sensitize and saline controls on withdrawal days (WD) 1, 7, and 21. We focused on calcium/calmodulin-dependent protein kinase II (CaMKII), extracellular signal-regulated protein kinase (ERK), and protein kinase A (PKA). In sensitized rats, AMPAR surface expression was elevated on WD7 and WD21 but not WD1. ERK2 activation followed a parallel time-course, suggesting a role in AMPAR up-regulation. Both sensitized and non-sensitized rats exhibited CaMKII activation on WD7, suggesting that CaMKII activation is not sufficient for AMPAR up-regulation. PKA phosphorylation, measured using an antibody recognizing phosphorylated PKA substrates, increased gradually over withdrawal in sensitized rats, from below control levels on WD1 to significantly greater than controls on WD21. Using proteomics, novel sensitization-related PKA substrates were identified, including two structural proteins (CRMP-2 and alpha-tubulin) that we speculate may link PKA signaling to previously reported dendritic remodeling in NAc neurons of cocaine-sensitized rats.

The neuronal nitric oxide synthase (nNOS) gene contributes to the regulation of tyrosine hydroxylase (TH) by cocaine

Recently, we demonstrated that intact nitric oxide (NO) signaling is essential for the development of cocaine behavioral sensitization in adulthood [M.A. Balda, K.L. Anderson, Y. Itzhak, Differential role of the nNOS gene in the development of behavioral sensitization to cocaine in adolescent and adult B6;129S mice, Psychopharmacology (Berl) 200 (2008) 509-519]. Given the requirement of dopamine (DA) transmission in cocaine-induced behavioral sensitization and the interactions between NO and DA systems, the present study investigated the role of the neuronal nitric oxide synthase (nNOS) gene and the effect of cocaine on the expression of tyrosine hydroxylase (TH)-immunoreactive (-ir) neurons. Adult (postnatal day 80) wild type (WT) and nNOS knockout (KO) mice received saline or a sensitizing regimen of cocaine (20mg/kg) for 5 days. After 24h, TH immunoreactivity was assessed in the ventral tegmental area (VTA) and the dorsal striatum (dST) using stereology and Western blotting, respectively. We report that (a) nNOS KO mice express lower levels of TH-ir neurons in the VTA compared to WT counterparts, (b) cocaine administration to WT mice significantly increased striatal TH expression, and (c) the same cocaine administration to nNOS KO mice significantly decreased striatal TH expression. Thus, the nitrergic system may contribute to cocaine-induced behavioral sensitization by regulating dopaminergic neurotransmission.

Sex differences in basal and cocaine-induced alterations in PKA and CREB proteins in the nucleus accumbens

INTRODUCTION

Alterations in protein kinase (PKA) protein levels have been implicated in the regulation of responses to and development of cocaine addiction. However, the contribution of differences in PKA intracellular cascade to the known sex differences in responses to cocaine is not well understood. This study examined whether there are intrinsic or cocaine-induced alterations in PKA-mediated responses, such as phosphorylation of cyclic AMP response element binding protein, in male and female rats.

MATERIALS AND METHODS

To this end, protein levels of PKA and phosphorylated CREB (pCREB) in the caudate putamen (CPu) and nucleus accumbens (NAc) of male and female rats were measured basally or after acute (one 30-mg/kg intraperitoneal injection) or chronic (twice-daily 15-mg/kg injections for 14 days) cocaine administration. Behavioral responses to both cocaine administration paradigms were also studied.

RESULTS

Similar to previous findings, ambulatory, rearing, and stereotypic activities were higher in female rats after acute cocaine administration. Sex differences in cocaine-induced responses were also observed after chronic cocaine administration: While males developed a robust sensitization in ambulatory activities to cocaine, females developed tolerance in cocaine-induced rearing and stereotypic activities. In the basal group, females had significantly higher PKA protein levels in the NAc. Regardless of the cocaine administration paradigm, PKA protein levels in the NAc were higher overall in females than in males. Furthermore, after cocaine administration, while pCREB protein levels in male rats were induced for a longer amount of time than in female rats, the magnitude of change on pCREB levels were higher in female than male rats. However, in the CPu, no sex differences in PKA or pCREB protein levels were observed either in the basal group or after acute or chronic cocaine administration.

DISCUSSION

Taken together, these findings suggest that sex differences in basal and cocaine-induced alterations in the PKA signaling regulation in the NAc may contribute to sex differences in the psychomotor responses to cocaine.

Persistent proteomic alterations in the medial prefrontal cortex with abstinence from cocaine self-administration

Neuroproteomic studies of drug abuse offer the potential for a systems-level understanding of addiction. Understanding cocaine-responsive alterations in brain protein expression that persist even with extended abstinence may provide insight into relapse liability. In the current study, protein changes in the medial prefrontal cortex of cocaine self-administering rats following 1 and 100 days of enforced abstinence were quantified by 2D-DIGE. We have previously reported increased drug-seeking and drug-taking, as well as mRNA and epigenetic changes in this model even after 100 days of enforced abstinence. A number of statistically-significant changes in proteins relating to synapse function and neuronal remodeling were evident, including neurofilament medium and heat shock protein 73 (Hsp73) which increased at 1 day of abstinence, but returned to normal levels following 100 days of abstinence. -1 and synaptosome-associated protein 25 kDa (SNAP-25) were unchanged at 1 day of abstinence, but were significantly decreased after 100 days. These data demonstrate that while some protein changes return to normal levels following enforced cocaine abstinence, a number remain or become altered after long periods, up to 100 days, of cocaine abstinence. Those protein expression changes that do not reset to pre-cocaine exposure levels may contribute to the persistent relapse potential that occurs in response to cocaine abstinence.

WFS1 gene as a putative biomarker for development of post-traumatic syndrome in an animal model

Post-traumatic stress disorder (PTSD) is an anxiety disorder that may develop after the experiencing or witnessing of a life-threatening event. PTSD is defined by the coexistence of three clusters of symptoms: re-experiencing, avoidance and hyperarousal, which persist for at least 1 month in survivors of the event (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition). Using an established model of PTSD, we addressed the well-accepted clinical finding that only a minority (about 20%) of the individuals exposed to a traumatic event develop PTSD. Moreover, we followed individual rat behavior for up to a month, and then treated the PTSD-like animals with citalopram. Our data demonstrate high face (20% of rats exposed to a reminder of the stressor develop symptoms characteristic of PTSD) and predictive (response to citalopram) validities. Based on these validities we identified alterations in the Wolframin gene in the CA1 and amygdala regions, specifically in exposed PTSD-like rats, which were normalized after treatment with citalopram. We suggest the Wolframin gene as a putative biomarker for PTSD. Since Wolframin gene undergoes alternative splicing and has polymorphism in the population, it may serve a future marker for identification of the vulnerable population exposed to a traumatic event.

Acute and chronic dopamine receptor stimulation modulates AMPA receptor trafficking in nucleus accumbens neurons cocultured with prefrontal cortex neurons

Postsynaptic interactions between dopamine (DA) and glutamate receptors in the nucleus accumbens (NAc) are critical for addiction. To determine the effect of acute and repeated DA receptor stimulation on AMPA receptor (AMPAR) synaptic targeting in medium spiny NAc neurons, we developed a model system consisting of rat NAc neurons cocultured with prefrontal cortex neurons from enhanced green fluorescent protein-expressing mice. Cortical neurons restore excitatory input onto NAc neurons but are distinguishable based on fluorescence. First, we showed that brief D1-like agonist exposure increased AMPAR insertion onto extrasynaptic regions of NAc neuronal processes through a mechanism requiring protein kinase A. This facilitated the Ca2+/calmodulin dependent protein kinase II (CaMKII)-dependent synaptic incorporation of AMPARs in response to subsequent NMDA receptor (NMDAR) stimulation. Through this mechanism, DA may promote reward- and drug-related plasticity in the NAc. Then, to model effects of repeated in vivo cocaine exposure, we treated cocultures with DA (1 microm, 30 min) on days 7, 9, and 11 in culture. On day 15, NAc neurons exhibited increased synaptic AMPAR levels. This was associated with CaMKII activation and was blocked by the CaMKII inhibitor KN-93 (N-[2-[N-(4-chlorocinnamyl)-N-methylaminomethyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulfonamide phosphate salt). Furthermore, D1-like agonist exposure on day 15 no longer increased AMPAR surface expression. This refractoriness was associated with decreased D1 receptor surface expression. NMDAR surface expression was not altered by acute or repeated DA receptor stimulation. These results suggest that (1) after repeated DA treatment, NAc neurons are more responsive to glutamate inputs but D(1)-like receptor regulation of plasticity is impaired, and (2) NAc/prefrontal cortex cocultures are useful for studying dopamine-induced neuroadaptations.

Morphine-induced analgesic tolerance, locomotor sensitization and physical dependence do not require modification of mu opioid receptor, cdk5 and adenylate cyclase activity

Acute morphine administration produces analgesia and reward, but prolonged use may lead to analgesic tolerance in patients chronically treated for pain and to compulsive intake in opioid addicts. Moreover, long-term exposure may induce physical dependence, manifested as somatic withdrawal symptoms in the absence of the drug. We set up three behavioral paradigms to model these adaptations in mice, using distinct regimens of repeated morphine injections to induce either analgesic tolerance, locomotor sensitization or physical dependence. Interestingly, mice tolerant to analgesia were not sensitized to hyperlocomotion, whereas sensitized mice displayed some analgesic tolerance. We then examined candidate molecular modifications that could underlie the development of each behavioral adaptation. First, analgesic tolerance was not accompanied by mu opioid receptor desensitization in the periaqueductal gray. Second, cdk5 and p35 protein levels were unchanged in caudate-putamen, nucleus accumbens and prefrontal cortex of mice displaying locomotor sensitization. Finally, naloxone-precipitated morphine withdrawal did not enhance basal or forskolin-stimulated adenylate cyclase activity in nucleus accumbens, prefrontal cortex, amygdala, bed nucleus of stria terminalis or periaqueductal gray. Therefore, the expression of behavioral adaptations to chronic morphine treatment was not associated with the regulation of micro opioid receptor, cdk5 or adenylate cyclase activity in relevant brain areas. Although we cannot exclude that these modifications were not detected under our experimental conditions, another hypothesis is that alternative molecular mechanisms, yet to be discovered, underlie analgesic tolerance, locomotor sensitization and physical dependence induced by chronic morphine administration.

Behavioral expression of cocaine sensitization in rats is accompanied by a distinct pattern of modifications in the PKA/DARPP-32 signaling pathway

Repeated cocaine administration induces behavioral sensitization and modifications in the phosphorylation pattern of dopamine and cAMP-regulated phosphoprotein of Mr 32,000 (DARPP-32), characterized by a tonic increase in the Thr75 phosphorylated form, and a decrease in the Thr34 phosphorylated form. This study further investigated the correlations between cocaine sensitization and modifications in the DARPP-32 phosphorylation pattern, cAMP-dependent protein kinase (PKA) activity, and mGluR5 tone in the medial prefrontal cortex and nucleus accumbens. Behavioral sensitization and modifications in these neurochemical markers followed a similar temporal pattern. Moreover, in sensitized rats acute cocaine administration modified phosphorylation levels of Thr75- and Thr34-DARPP-32, GluR1, and NR1 subunits in the nucleus accumbens only at a dose double the efficacious dose in control rats. These results suggest that the high levels of phospho-Thr75 DARPP-32 maintain PKA in a prevalent inhibited state. Furthermore, in sensitized rats the acute administration of 6-methyl-2-(phenylethynyl)-pyridine, a mGluR5 antagonist, reinstated the phosphorylation levels of Thr75- and Thr34-DARPP-32, GluR1, and NR1 to control values, and a subsequent cocaine challenge did not elicit a sensitized response. These data suggest that a tonic increase in mGluR5 transmission in cocaine-sensitized rats sustains both the increase in phospho-Thr75 DARPP-32 levels and the expression of behavioral sensitization.

Cocaine-induced sensitization is associated with altered dynamics of transcriptional responses of the dopamine transporter, tyrosine hydroxylase, and dopamine D2 receptors in C57Bl/6J mice

RATIONALE

Behavioural sensitization is a long lasting phenomenon that has been proposed to be involved in drug addiction. Although the expression of cocaine-induced sensitization has been associated with the activity of the mesencephalic dopaminergic neurons, little is known about the transcriptional adaptations of these neurons to a new challenge with cocaine long after cessation of repeated exposure to the drug.

OBJECTIVES

We studied the time course of the mRNA levels of three main regulatory elements of dopaminergic transmission after a challenge with cocaine (15 mg/kg) that followed 21 days of withdrawal from a cocaine pretreatment (20 mg/kg, ip, every 2 days for 21 days) in C57Bl/6J mice.

MATERIALS AND METHODS

Mice were placed 45 min in activity chambers and were killed 45 min, 2 h or 24 h after the challenge injection. Dopamine transporter (DAT), D2 auto-receptor (D2) and tyrosine hydroxylase (TH) mRNA levels were assessed by in situ hybridization in the ventral tegmental area and the substantia nigra compacta.

RESULTS

As compared to vehicle challenge, cocaine challenge in vehicle pretreated mice induced a rapid increase (+208%) in DAT mRNA (45 min) followed by a delayed decrease (-70%) (24 h), while TH and D2 mRNA were both increased (+45%) 24 h after the challenge. In cocaine pretreated mice, cocaine-induced short-term increase and long-term decrease in DAT mRNA levels were amplified (+328%) and reduced (-40%), respectively.

CONCLUSIONS

Repeated exposure to cocaine alters the transcriptional response of DA neurons to a new cocaine challenge long after cessation of repeated exposure to the drug. They point to the DAT mRNA as a major responsive element to a new presentation of cocaine.

Effect of cocaine self-administration on striatal PKA-regulated signaling in male and female rats

RATIONALE

Chronic cocaine produces changes in the dopamine (DA)/D1/cAMP/protein kinase A (PKA)-regulated signaling pathway that may underlie the development of addiction.

OBJECTIVE

Given sex differences in the progression to cocaine addiction, we examined the possibility that the PKA pathway is differentially activated by cocaine in male and female rats.

MATERIALS AND METHODS

Rats were given 24-h access to cocaine (1.5 mg/kg) or saline for 7 days under a discrete trial procedure (four trials per hour). Rats were then retested on responding for cocaine under a progressive-ratio schedule after either 0 (no-delay retest) or 10 (10-day-delay retest) days of abstinence. Markers of PKA-regulated signaling in the striatum and nucleus accumbens were evaluated by Western blotting, including phosphorylation of DA and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) at Thr 34 and glutamate receptor 1 (GluR1) at Ser 845.

RESULTS

Compared to males, females had higher levels of DARPP-32 phosphorylated at the PKA site in the striatum. Increased phosphorylation of DARPP-32 at the PKA site was also seen in the nucleus accumbens of females compared to males, particularly among controls and rats tested after a 10-day abstinence period. DARPP-32 phosphorylation was also increased as a consequence of cocaine when tested after a 0-day abstinence period in male rats but not female rats.

CONCLUSION

These findings indicate sex differences in PKA-regulated signaling in drug-naïve controls. Furthermore, these data suggest that regulation of PKA signaling by cocaine is differentially influenced in male and female rats as a consequence of cocaine exposure and cocaine abstinence period.

Long-term upregulation of protein kinase A and adenylate cyclase levels in human smokers

Repeated injections of cocaine and morphine in laboratory rats cause a variety of molecular neuroadaptations in the cAMP signaling pathway in nucleus accumbens and ventral tegmental area. Here we report similar neuroadaptations in postmortem tissue from the brains of human smokers and former smokers. Activity levels of two major components of cAMP signaling, cAMP-dependent protein kinase A (PKA) and adenylate cyclase, were abnormally elevated in nucleus accumbens of smokers and in ventral midbrain dopaminergic region of both smokers and former smokers. Protein levels of the catalytic subunit of PKA were correspondingly higher in the ventral midbrain dopaminergic region of both smokers and former smokers. Protein levels of other candidate neuroadaptations, including glutamate receptor subunits, tyrosine hydroxylase, and other protein kinases, were within normal range. These findings extend our understanding of addiction-related neuroadaptations of cAMP signaling to tobacco smoking in human subjects and suggest that smoking-induced brain neuroadaptations can persist for significant periods in former smokers.

Increased accumbens Cdk5 expression in rats after short-access to self-administered cocaine, but not after long-access sessions

Upregulation of cyclin-dependent kinase 5 (Cdk5) after chronic cocaine administration has led to speculation that Cdk5 plays an important role in drug addiction. However, as Cdk5 involvement is implicated in a variety of neural events, including neuronal development, synaptic plasticity and learning, a specific role in drug abuse is yet to be determined. The present study utilized cocaine self-administration and food-reinforced operant procedures to assess possible relationships between cocaine intake, food-reinforced operant responding, behavioral activity, and Cdk5 levels in the nucleus accumbens (NAcc), ventral tegmental area (VTA), and prefrontal cortex (PFC) in rats. In Experiment 1, animals undergoing daily cocaine self-administration (1-h/30 days) or food-reinforced operant sessions (20-min/30 days) showed significant between-group differences in operant responding and behavioral activity, but no significant differences in NAcc, VTA or PFC Cdk5 levels compared to a Handled Control group. In Experiment 2, animals that had self-administered cocaine in 10 daily 1-h sessions (Short-Access Cocaine) showed significantly greater NAcc Cdk5 expression compared to an Unhandled Control group, and no evidence of cocaine-induced behavioral sensitization. Animals given 4-h daily access to cocaine over the same number of sessions (Long-Access Cocaine) showed significantly enhanced cocaine-reinforced responding and locomotor activation by the end of the sessions, but no significant differences in Cdk5 expression compared to Control animals. These findings suggest that overexpression of Cdk5 may be a transient adaptation to cocaine experience that subsides with increased cocaine exposure and does not correspond with measures of cocaine-induced behavioral sensitization.

Use of surface enhanced laser desorption/ionization-time of flight mass spectrometry (SELDI-TOF MS) to study protein expression in a rat model of cocaine withdrawal

Surface enhanced laser desorption/ionization-time of flight mass spectrometry (SELDI-TOF MS) is an analytical technology for proteomic analysis that combines chromatography and mass spectrometry. At present, this technology is most commonly being exploited for the simultaneous measurement of numerous proteins in serum, but has also been utilized in organ tissue, although rarely in the brain. We applied SELDI-TOF MS technology to study protein expression in the brain of rats withdrawn from repeated cocaine exposure. Our goals were to optimize sample preparation and ProteinChip Array protocols for brain tissue, to verify the reproducibility of SELDI-TOF mass spectra and to determine whether SELDI-TOF MS detects differentially expressed proteins in cocaine- versus saline-treated rats. Consequently, we have developed an optimal protocol and generated a reproducible spectral pattern with six dominant peaks in all test samples. We have detected two smaller peaks (m/z: 5179, 5030) that were significantly increased (p < 0.05) in cocaine-treated rats compared to saline-treated rats. In summary, the application of SELDI-TOF MS to the study of protein expression in a rat model of cocaine withdrawal is feasible and has the potential to generate new hypotheses.

Chronic cocaine administration modulates the expression of transcription factors involved in midbrain dopaminergic neuron function

Chronic cocaine use leads to pronounced alterations in neuronal functions in brain circuits associated with reward. In the present study, we examined in the rat midbrain the effects of acute, subchronic (5 days) and chronic cocaine treatments (14 days) on the gene expression of transcription factors involved in the development and maintenance of dopaminergic neurons. We show that chronic, but not acute or subchronic, cocaine administration downregulates Nurr1 and Pitx3 transcripts whereas En1 transcripts are upregulated. Conversely, Lmx1b and En2 transcripts are not affected by the drug treatment, indicating that the modulation of the midbrain transcription factors analyzed is highly selective. Interestingly, modification of the gene expression for these transcription factors persists in midbrain as long as two weeks after the last drug administration, suggesting that it may account for some of the enduring alterations in midbrain dopaminergic circuits associated with chronic cocaine use.

Cocaine withdrawal enhances long-term potentiation induced by corticotropin-releasing factor at central amygdala glutamatergic synapses via CRF1, NMDA receptors and PKA

Cocaine addiction is an enduring, relapsing, behavioural disorder in which stressors reinstate cocaine-seeking even after prolonged abstinence. Evidence suggests that the 'anxiety-like' behaviour and stress associated with protracted withdrawal may be mediated by increased corticotropin-releasing factor (CRF) in the central nucleus of the amygdala (CeA), a part of the limbic circuitry engaged in the coding and transmission of stimulus-reward associations. In the present study we describe a long-lasting potentiation of glutamatergic transmission induced at lateral amygdala (LA)-to-CeA synapses by rat/human CRF. After 2 weeks of withdrawal from repeated intermittent exposure to cocaine, CRF-induced long-term potentiation (LTP) was greatly enhanced compared to the respective saline control group while, after short-term withdrawal (24 h), there was no significant difference between the two treatment groups, indicating alterations in CRF systems during protracted withdrawal from chronic cocaine. After prolonged withdrawal, CRF-induced LTP was dependent on activation of CRF2, CaV2.3 (R-type) calcium channels and intracellular signalling through protein kinase C in both saline- and cocaine-treated groups. The enhanced CRF-induced LTP after 2 weeks of withdrawal was mediated through augmented CRF1 receptor function, associated with an increased signalling through protein kinase A, and required N-methyl-D-aspartate (NMDA) receptors. Accordingly, single-cell recordings revealed a significantly increased NMDA/AMPA ratio after prolonged withdrawal from the cocaine treatment. These results support a role for CRF1 receptor antagonists as plausible treatment options during withdrawal from chronic cocaine and suggest Ca(V)2.3 blockers as potential candidates for pharmaceutical modulation of CRF systems.

Psychoactive drugs and regulation of the cAMP/PKA/DARPP-32 cascade in striatal medium spiny neurons

Changes in activity of the medium spiny neurons (MSNs) of dorsal and ventral striatum result in alterations of motor performance, ranging from rapid increases or decreases in locomotor activity, to long-term modifications of motor behaviours. In the dorsal striatum, MSNs can be distinguished based on the organization of their connectivity to substantia nigra pars reticulata (SNpr) and internal segment of the globus pallidus (GPi), which, in turn, control thalamocortical neurons. Approximately half of the MSNs project directly to SNpr and GPi, their activation leading to disinhibition of thalamocortical neurons and increased motor activity. The other subpopulation of MSNs connects to SNpr and GPi indirectly and when activated promotes inhibition of thalamocortical neurons, thereby reducing motor activity. The dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) is a modulator of the cAMP signalling pathway, highly expressed in MSNs. This review discusses the regulation of DARPP-32 exerted by psychoactive substances in specific populations of striatal projection neurons and its involvement in short- and long-term motor responses.

Repeated amphetamine administration decreases D1 dopamine receptor-mediated inhibition of voltage-gated sodium currents in the prefrontal cortex

Adaptations in dopamine (DA) transmission in the prefrontal cortex (PFC) are thought to be critical to the development and persistence of drug addiction. Our previous findings showed that medial PFC (mPFC) neurons in rats treated repeatedly with amphetamine exhibit a decreased inhibitory response to iontophoretically applied DA, demonstrating altered DA receptor transmission. To determine the role postsynaptic DA D1 receptors play in this effect, we used whole-cell patch-clamp recordings of acutely dissociated pyramidal mPFC neurons and inhibition of transient voltage-sensitive sodium current (INaT) as a measure of D1 receptor function. After 3 d of withdrawal, neurons recorded from amphetamine-treated rats (5 mg/kg for 5 d) demonstrated a significant decrease in whole-cell INaT density and in the ability of D1 receptor stimulation to inhibit INaT. Application of a protein kinase A (PKA) inhibitor blocked the ability of D1 receptor activation to inhibit INaT and increased the current density of both groups to similar values. These results suggest that repeated amphetamine exposure results in subsensitivity of the INaT to D1 receptor-mediated inhibition because of a possible increase in basal PKA activity. This adaptation may contribute to perseverative behaviors in animals that self-administer psychostimulants as well as compromised PFC-dependent behaviors in human addicts.

The effect of ‘binge’ cocaine administration on the expression of cyclin-dependent kinase 5 and its activator p35 in various regions of rat brain

The present study was aimed at determining whether the administration of cocaine in 'binge' pattern regimen that evoked tolerance to the locomotor stimulant effects of cocaine also influenced the expression of cyclin-dependent kinase 5 (Cdk5) and its activator p35 in the amygdala, medial prefrontal cortex, nucleus accumbens septi and caudate-putamen. Western blot techniques revealed that acute and repeated 'binge' cocaine decreased expression of the Cdk5 protein in the amygdala. In the medial prefrontal cortex, only exposure to repeated 'binge' cocaine decreased the content of the Cdk5 protein. 'Binge' cocaine administration also altered the expression of Cdk5 activator p35 protein. In the amygdala, only repeated 'binge' cocaine decreased the expression of p35, while in the medial prefrontal cortex, a decrease was observed after acute and repeated 'binge' cocaine exposure. In neither the nucleus accumbens septi nor the caudate-putamen acute or repeated 'binge' cocaine modified the expression of Cdk5 and p35. The above data indicate that in contrast to sensitizing doses of cocaine, a single and repeated binge of cocaine, which evoked tolerance to its locomotor stimulant effects, decreases expression of Cdk5 and p35 and possibly decreases the efficacy of neurotransmission or induces brain plastic changes regulated by Cdk5 and its activator p35.

Cocaine-induced CREB phosphorylation in nucleus accumbens of cocaine-sensitized rats is enabled by enhanced activation of extracellular signal-related kinase, but not protein kinase A

Repeated cocaine administration to rats outside their home cages sensitizes the behavioral effects of the drug, and enhances induction of the immediate early gene product Fos in nucleus accumbens. We hypothesized that the same treatment regimen would also enhance cocaine-induced activation of intracellular signaling kinases that phosphorylate cyclic AMP-regulated element-binding protein (CREB), an important mediator of c-fos transcription. Phosphorylation levels of extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK), calcium/calmodulin kinases (CaMKs) II and IV, and CREB were used to assess endogenous functional activity of these signaling molecules in rats behaviorally sensitized outside their home cages. Protein kinase A (PKA)-specific phosphorylation of Ser845 in the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit GluR1 was used to assess endogenous functional activity of PKA. Using western blots and immunohistochemistry, we detected cocaine-induced CREB phosphorylation after repeated cocaine administration, but not after repeated saline administration. Using western blots and MAPK activity assays, we found that cocaine-induced phosphorylation and activation of ERK, but not of CaMKs II or IV or GluR1, was augmented in nucleus accumbens of cocaine-sensitized rats. Unilateral infusions of the MAPK kinase inhibitor U0126 into nucleus accumbens attenuated cocaine-induced ERK and CREB phosphorylation in cocaine-sensitized rats. In contrast, unilateral infusions of the PKA inhibitor Rp-isomer of adenosine-3',5'-cyclicmonophosphorothioate (Rp-cAMPs) did not affect cocaine-induced CREB phosphorylation. Therefore, enhanced activation of ERK, but not PKA, enables and mediates cocaine-induced CREB phosphorylation in nucleus accumbens of rats that are sensitized by repeated cocaine administration outside their home cages.