Effects of chronic exposure to cocaine are regulated by the neuronal protein Cdk5

Epigenetic codes in cognition and behaviour

The epigenetic marking of chromatin provides a ubiquitous means for cells to shape and maintain their identity, and to react to environmental stimuli via specific remodeling. Such an epigenetic code of the core components of chromatin, DNA and histone proteins, can thus be stable but is also highly dynamic. In the nervous system, epigenetic codes are critical for basic cellular processes such as synaptic plasticity, and for complex behaviours such as learning and memory. At the same time, epigenetic marks can be stably transmitted through mitosis and meiosis, and thereby underlie non-genomic transgenerational inheritance of behavioural traits. In this review, we describe recent findings on the role and mechanisms of epigenetic codes in the brain, and discuss their implication in synaptic plasticity, cognitive functions and psychiatric disorders. We provide examples of transgenerational inheritance of epigenetic marks that affect simple morphological traits or complex processes such as disease susceptibility, and point to the potential implication of epigenetic codes in medicine and evolution.

Cdk5 modulates cocaine reward, motivation, and striatal neuron excitability

Cyclin-dependent kinase 5 (Cdk5) regulates dopamine neurotransmission and has been suggested to serve as a homeostatic target of chronic psychostimulant exposure. To study the role of Cdk5 in the modulation of the cellular and behavioral effects of psychoactive drugs of abuse, we developed Cre/loxP conditional knock-out systems that allow temporal and spatial control of Cdk5 expression in the adult brain. Here, we report the generation of Cdk5 conditional knock-out (cKO) mice using the alphaCaMKII promoter-driven Cre transgenic line (CaMKII-Cre). In this model system, loss of Cdk5 in the adult forebrain increased the psychomotor-activating effects of cocaine. Additionally, these CaMKII-Cre Cdk5 cKO mice show enhanced incentive motivation for food as assessed by instrumental responding on a progressive ratio schedule of reinforcement. Behavioral changes were accompanied by increased excitability of medium spiny neurons in the nucleus accumbens (NAc) in Cdk5 cKO mice. To study NAc-specific effects of Cdk5, another model system was used in which recombinant adeno-associated viruses expressing Cre recombinase caused restricted loss of Cdk5 in NAc neurons. Targeted knock-out of Cdk5 in the NAc facilitated cocaine-induced locomotor sensitization and conditioned place preference for cocaine. These results suggest that Cdk5 acts as a negative regulator of neuronal excitability in the NAc and that Cdk5 may govern the behavioral effects of cocaine and motivation for reinforcement.

Adenosine A2A receptors and basal ganglia physiology

Adenosine A2A receptors are highly enriched in the basal ganglia system. They are predominantly expressed in enkephalin-expressing GABAergic striatopallidal neurons and therefore are highly relevant to the function of the indirect efferent pathway of the basal ganglia system. In these GABAergic enkephalinergic neurons, the A2A receptor tightly interacts structurally and functionally with the dopamine D2 receptor. Both by forming receptor heteromers and by targeting common intracellular signaling cascades, A2A and D2 receptors exhibit reciprocal antagonistic interactions that are central to the function of the indirect pathway and hence to basal ganglia control of movement, motor learning, motivation and reward. Consequently, this A2A/D2 receptors antagonistic interaction is also central to basal ganglia dysfunction in Parkinson's disease. However, recent evidence demonstrates that, in addition to this post-synaptic site of action, striatal A2A receptors are also expressed and have physiological relevance on pre-synaptic glutamatergic terminals of the cortico-limbic-striatal and thalamo-striatal pathways, where they form heteromeric receptor complexes with adenosine A1 receptors. Therefore, A2A receptors play an important fine-tuning role, boosting the efficiency of glutamatergic information flow in the indirect pathway by exerting control, either pre- and/or post-synaptically, over other key modulators of glutamatergic synapses, including D2 receptors, group I metabotropic mGlu5 glutamate receptors and cannabinoid CB1 receptors, and by triggering the cAMP-protein kinase A signaling cascade.

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.

Peptides derived from Cdk5 activator p35, specifically inhibit deregulated activity of Cdk5

Normal Cdk5 activity, conferred mainly by association with its primary activator p35, is critical for normal function of the cell and must be tightly regulated. During neurotoxicity, p35 is cleaved to form p25, which becomes a potent and mislocalized hyperactivator of Cdk5, resulting in a deregulation of Cdk5 activity. p25 levels have been found to be elevated in Alzheimer's disease (AD) brain and overexpression of p25 in a transgenic mouse results in the formation of phosphorylated tau, neurofibrillary tangles and cognitive deficits that are pathological hallmarks of AD. p25/Cdk5 also hyperphosphorylates neurofilament proteins that constitute pathological hallmarks found in Parkinson's disease and amyotrophic lateral sclerosis. The selective targeting of p25/Cdk5 activity without affecting p35/Cdk5 activity has been unsuccessful. In this review we detail our recent studies of selective p25/Cdk5 inhibition without affecting p35/Cdk5 or mitotic Cdk activities. We found that a further truncation of p25 to yield a Cdk5 inhibitory peptide (CIP) can specifically inhibit p25/Cdk5 activity in transfected HEK cells and primary cortical neurons. CIP was able to reduce tau hyperphosphorylation and neuronal death induced caused by p25/Cdk5 and further studies with CIP may develop a specific Cdk5 inhibition strategy in the treatment of neurodegeneration.

Activation of the cAMP/PKA/DARPP-32 signaling pathway is required for morphine psychomotor stimulation but not for morphine reward

Activation of the cAMP/PKA pathway in the dopaminoceptive neurons of the striatum has been proposed to mediate the actions of various classes of drugs of abuse. Here, we show that, in the mouse nucleus accumbens and dorsal striatum, acute administration of morphine resulted in an increase in the state of phosphorylation of the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) at Thr34, without affecting phosphorylation at Thr75. The ability of morphine to stimulate Thr34 phosphorylation was prevented by blockade of dopamine D1 receptors. DARPP-32 knockout mice and T34A DARPP-32 mutant mice displayed a lower hyperlocomotor response to a single injection of morphine than wild-type controls. In contrast, in T75A DARPP-32 mutant mice, morphine-induced psychomotor activation was indistinguishable from that of wild-type littermates. In spite of their reduced response to the acute hyperlocomotor effect of morphine, DARPP-32 knockout mice and T34A DARPP-32 mutant mice were able to develop behavioral sensitization to morphine comparable to that of wild-type controls and to display morphine conditioned place preference. These results demonstrate that dopamine D1 receptor-mediated activation of the cAMP/DARPP-32 cascade in striatal medium spiny neurons is involved in the psychomotor action, but not in the rewarding properties, of morphine.

Signaling in the basal ganglia: Postsynaptic and presynaptic mechanisms

The selection and execution of appropriate motor behavior result in large part from the ability of the basal ganglia to collect, integrate and feedback information coming from the cerebral cortex. The GABAergic medium spiny neurons (MSNs) of the striatum represent the main receiving station of the basal ganglia. These cells are innervated by excitatory glutamatergic fibers from cortex and thalamus, and modulatory dopaminergic fibers from the midbrain. MSNs comprise two populations of projection neurons, which give rise to the direct, striatonigral pathway, and indirect, striatopallidal pathway. Changes in transmission at the level MSNs affect the activity of thalamocortical projection neurons, thereby influencing motor behavior. For instance, the cardinal symptoms of Parkinson's disease, such as tremor, rigidity and bradykinesia, are caused by the selective degeneration of dopaminergic neurons originating in the substantia nigra pars compacta, which modulate the activity of MSNs in the dorsal striatum. The therapy for Parkinson's disease relies on the use of levodopa, but is hampered by neuroadaptive changes affecting dopaminergic and glutamatergic transmission in striatonigral neurons. MSNs are also the target of many psychoactive drugs. For example, caffeine affects motor activity by blocking adenosine receptors in the basal ganglia, thereby affecting neurotransmission in striatopallidal neurons. The present review focuses on studies performed in our laboratory, which provide a molecular framework to understand the effects on motor activity of adenosine and caffeine.

Structure-activity relationships of 3,4-dihydro-1H-quinazolin-2-one derivatives as potential CDK5 inhibitors

Cyclin-dependent kinase 5 (CDK5) is a serine/threonine kinase that plays a critical role in the early development of the nervous system. Deregulation of CDK5 is believed to contribute to the abnormal phosphorylation of various cellular substrates associated with neurodegenerative disorders such as Alzheimer's disease, amyotrophic lateral sclerosis, and ischemic stroke. Acyclic urea 3 was identified as a potent CDK5 inhibitor and co-crystallographic data of urea 3/CDK2 enzyme were used to design a novel series of 3,4-dihydroquinazolin-2(1H)-ones as CDK5 inhibitors. In this investigation we present our synthetic studies toward this series of compounds and discuss their biological relevance as CDK5 inhibitors.

Cyclin-dependent kinase 5 governs learning and synaptic plasticity via control of NMDAR degradation

Learning is accompanied by modulation of postsynaptic signal transduction pathways in neurons. Although the neuronal protein kinase cyclin-dependent kinase 5 (Cdk5) has been implicated in cognitive disorders, its role in learning has been obscured by the perinatal lethality of constitutive knockout mice. Here we report that conditional knockout of Cdk5 in the adult mouse brain improved performance in spatial learning tasks and enhanced hippocampal long-term potentiation and NMDA receptor (NMDAR)-mediated excitatory postsynaptic currents. Enhanced synaptic plasticity in Cdk5 knockout mice was attributed to reduced NR2B degradation, which caused elevations in total, surface and synaptic NR2B subunit levels and current through NR2B-containing NMDARs. Cdk5 facilitated the degradation of NR2B by directly interacting with both it and its protease, calpain. These findings reveal a previously unknown mechanism by which Cdk5 facilitates calpain-mediated proteolysis of NR2B and may control synaptic plasticity and learning.

Epigenetic regulation in psychiatric disorders

Many neurological and most psychiatric disorders are not due to mutations in a single gene; rather, they involve molecular disturbances entailing multiple genes and signals that control their expression. Recent research has demonstrated that complex 'epigenetic' mechanisms, which regulate gene activity without altering the DNA code, have long-lasting effects within mature neurons. This review summarizes recent evidence for the existence of sustained epigenetic mechanisms of gene regulation in neurons that have been implicated in the regulation of complex behaviour, including abnormalities in several psychiatric disorders such as depression, drug addiction and schizophrenia.

Cyclin dependent kinase 5 (Cdk5) mediated inhibition of the MAP kinase pathway results in CREB down regulation and apoptosis in PC12 cells

Cyclin dependent kinase 5 (cdk5) is active mainly in postmitotic cells like neurons and regulates important cellular functions by phosphorylating a wide variety of targets. Nerve growth factor stimulates the MEK-ERK MAP kinase pathway and causes neuronal differentiation and survival. It was reported previously that Cdk5 inhibits the MAP kinase pathway by phosphorylating Map kinase kinase-1 (MEK1) [1]. We have delineated the functional consequence of this cross talk and found that the cdk5 mediated inhibition of MEK1 results in apoptosis. We also demonstrate that the activity of transcription factor CREB, which is known to play pro-survival roles in neuronal cells, is attenuated as a result of this cross-talk.

Accumbal neurons that are activated during cocaine self-administration are spared from inhibitory effects of repeated cocaine self-administration

Hypoactivity of the accumbens is induced by repeated cocaine exposure and is hypothesized to play a role in cocaine addiction. However, it is difficult to understand how a general hypoactivity of the accumbens, which facilitates multiple types of motivated behaviors, could contribute to the selective increase in drug-directed behavior that defines addiction. Electrophysiological recordings, made during sessions in which rats self-administer cocaine, show that most accumbal neurons that encode events related to drug-directed behavior achieve and maintain higher firing rates during the period of cocaine exposure (Task-Activated neurons) than do other accumbal neurons (Task-Non-Activated neurons). We have hypothesized that this difference in activity makes the neurons that facilitate drug-directed behavior less susceptible than other neurons to the chronic inhibitory effects of cocaine. A sparing of neurons that facilitate drug-directed behavior from chronic hypoactivity might lead to a relative increase in the transmission of neuronal signals that facilitate drug-directed behavior through accumbal circuits and thereby contribute to changes in behavior that characterize addiction (ie differential inhibition hypothesis). A prediction of the hypothesis is that neurons that are activated in relation to task events during cocaine self-administration sessions will show less of a decrease in firing across repeated self-administration sessions than will other neurons. To test this prediction, rats were exposed to 30 daily (6 h/day) cocaine self-administration sessions. Chronic extracellular recordings of single accumbal neurons were made during the second to third session and the 30th session. Between-session comparisons showed that decreases in firing were exhibited by Task-Non-Activated, but not by Task-Activated, neurons. During the day 30 session, the magnitude of the difference in firing rate between the two groups of neurons was positively related to the propensity of animals to seek and take cocaine. The findings of the present study are consistent with a basic prediction of the differential inhibition hypothesis and may be relevant to understanding cocaine addiction.

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.

AKT and CDK5/p35 Mediate Brain-derived Neurotrophic Factor Induction of DARPP-32 in Medium Size Spiny Neurons in Vitro

Mature striatal medium size spiny neurons express the dopamine and cyclic AMP-regulated phosphoprotein, 32 kDa (DARPP-32), but little is known about the mechanisms regulating its levels or the specification of fully differentiated neuronal subtypes. Cell extrinsic molecules that increase DARPP-32 mRNA and/or protein levels include brain-derived neurotrophic factor (BDNF), retinoic acid, and estrogen. DARPP-32 induction by BDNF in vitro requires phosphatidylinositide 3-kinase (PI3K), but inhibition of phosphorylation of protein kinase B/Akt does not entirely abolish expression of DARPP-32. Moreover, the requirement for Akt has not been established. Using pharmacologic inhibitors of PI3K, Akt, and cyclin-dependent kinase 5 (cdk5) and constitutively active and dominant negative PI3K, Akt, cdk5, and p35 viruses in cultured striatal neurons, we measured BDNF-induced levels of DARPP-32 protein and/or mRNA. We demonstrated that both the PI3K/Akt/mammalian target of rapamycin and the cdk5/p35 signal transduction pathways contribute to the induction of DARPP-32 protein levels by BDNF and that the effects are on both the transcriptional and translational levels. It also appears that PI3K is upstream of cdk5/p35, and its activation can lead to an increase in p35 protein levels. These data support the presence of multiple signal transduction pathways mediating expression of DARPP-32 in vitro, including a novel, important pathway via by which PI3K regulates the contribution of cdk5/p35.

Inhibition of Cdk5 in the nucleus accumbens enhances the locomotor-activating and incentive-motivational effects of cocaine

Neuronal adaptations in striatal dopamine signaling have been implicated in enhanced responses to addictive drugs. Cyclin-dependent kinase 5 (Cdk5) regulates striatal dopamine signaling and is a downstream target gene of the transcription factor DeltaFosB, which accumulates in striatal neurons after chronic cocaine exposure. Here we investigated the role of Cdk5 activity in the nucleus accumbens (NAc) on cocaine-induced locomotor sensitization, responding for reward-associated stimuli (conditioned reinforcement), and cocaine self-administration under a progressive ratio schedule. Repeated infusions of the Cdk5 inhibitor roscovitine into the NAc before cocaine injections augmented both the development and expression of cocaine sensitization without having any intrinsic stimulant actions of its own. Additionally, repeated intra-NAc infusions of roscovitine to saline-injected rats enhanced locomotor responses to a subsequent cocaine challenge. Similar effects were found after infusions of another Cdk5 inhibitor, olomoucine, but not its inactive congener, iso-olomoucine. Repeated inhibition of Cdk5 within the NAc also robustly enhanced the incentive-motivational effects of cocaine, similar to the effect of prior repeated cocaine exposure. The enhanced responding with conditioned reinforcement induced by cocaine persisted at least 2 weeks after the final roscovitine infusion. NAc infusions of olomoucine also produced acute and enduring increases in "breakpoints" achieved on a progressive ratio schedule for cocaine reinforcement. These results demonstrate profound and persistent effects of NAc Cdk5 inhibition on locomotor sensitization and incentive-motivational processes and provide direct evidence for a role for striatal Cdk5-induced alterations in the brain's long-term adaptations to cocaine.

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.

GABA(B) receptor-positive modulation decreases selective molecular and behavioral effects of cocaine

Exposure to cocaine induces selective behavioral and molecular adaptations. In rodents, acute cocaine induces increased locomotor activity, whereas prolonged drug exposure results in behavioral locomotor sensitization, which is thought to be a consequence of drug-induced neuroadaptive changes. Recent attention has been given to compounds activating GABA(B) receptors as potential antiaddictive therapies. In particular, the principle of allosteric positive GABA(B) receptor modulators is very promising in this respect, as positive modulators lack the sedative and muscle relaxant properties of full GABA(B) receptor agonists such as baclofen. Here, we investigated the effects of systemic application of the GABA(B) receptor-positive modulator GS39783 (N,N'-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4, 6-diamine) in animals treated with acute and chronic cocaine administration. Both GS39783 and baclofen dose dependently attenuated acute cocaine-induced hyperlocomotion. Furthermore, both compounds also efficiently blocked cocaine-induced Fos induction in the striatal complex. In chronic studies, GS39783 induced a modest attenuation of cocaine-induced locomotor sensitization. Chronic cocaine induces the accumulation of the transcription factor deltaFosB and upregulates cAMP-response-element-binding protein (CREB) and dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32). GS39783 blocked the induction/activation of DARPP-32 and CREB in the nucleus accumbens and dorsal striatum and partially inhibited deltaFosB accumulation in the dorsal striatum. In summary, our data provide evidence that GS39783 attenuates the acute behavioral effects of cocaine exposure in rodents and in addition prevents the induction of selective long-term adaptive changes in dopaminergic signaling pathways. Further investigation of GABA(B) receptor-positive modulation as a novel therapeutic strategy for the treatment of cocaine dependence and possibly other drugs of abuse is therefore warranted.

c-Fos facilitates the acquisition and extinction of cocaine-induced persistent changes

Development of drug addiction involves persistent neurobiological changes. The dopamine D1 receptor is involved in mediating cocaine-induced neuroadaptation, yet the underlying intracellular mechanisms remain unclear. We examined a potential role of the immediate early gene Fos, which is robustly and rapidly induced by cocaine via D1 receptors, in mediating cocaine-induced persistent neurobiological changes by creating and analyzing a mouse in which Fos is primarily disrupted in D1 receptor-expressing neurons in the brain. We show that the expression levels of several transcription factors, neurotransmitter receptors, and intracellular signaling molecules induced by repeated cocaine administration are altered in Fos-deficient brains. Dendritic remodeling of medium spiny neurons induced by repeated exposure to cocaine is blunted in the mutant mice. The mutant mice exhibit attenuated behavioral sensitization after repeated exposure to cocaine and more persistent memory of cocaine-induced conditioned place preference. Our findings indicate that c-Fos produced in D1 receptor-expressing neurons integrates mechanisms to facilitate both the acquisition and extinction of cocaine-induced persistent changes.

Dual alteration of limbic dopamine D1receptor-mediated signalling and the Akt/GSK3 pathway in dopamine D3receptor mutants during the development of methamphetamine sensitization

The central dopamine system plays significant roles in motor activity and drug-induced behavioural sensitization. Our goal was to determine the significance of dopamine D(3) receptors in the development of behavioural sensitization to methamphetamine, assessed with D(3) receptor mutant mice. The absence of D(3) receptors significantly increased the behavioural responses to acute methamphetamine and evoked a faster rate of behavioural sensitization to chronic methamphetamine. In addition, both D(3) receptor protein and mRNA levels in the limbic forebrain decreased in sensitized wild-type mice. Further analyses indicated that D(1)-dependent behavioural sensitization and the number of limbic D(1) receptors increased in sensitized D(3) mutants as compared with sensitized wild-type mice. Consistent with this finding, we observed higher levels of D(1) receptor-evoked cAMP accumulation and basal phosphoDARPP-32/Thr34 in the limbic forebrain of D(3) mutants than wild-type mice and the difference was more pronounced after chronic methamphetamine treatment. We also observed an increase in phospho-extracellular signal-regulated kinase 2 but a decrease in phosphoAkt/Ser473 and phosphoglycogen synthase kinase 3 (GSK3)-alpha/beta in the limbic forebrain of D(3) mutants compared with wild-type mice after methamphetamine treatment. The convergent results implicate D(3) receptors as a negative regulator of the development of methamphetamine sensitization. A compensatory up-regulation of D(1) receptor-mediated signals, in addition to an altered Akt/GSK3 pathway, could contribute to the accelerated development of behavioural sensitization.

Regulation of DeltaFosB transcriptional activity by Ser27 phosphorylation

The transcription factor, DeltaFosB, is an important mediator of the long-term plasticity induced in brain by chronic exposure to drugs of abuse, stress, or several other psychoactive stimuli. We have previously demonstrated that the casein kinase 2 (CK2)-mediated phosphorylation of a highly conserved N-terminal serine (Ser27) plays a critical role in regulating DeltaFosB's unusual stability, while it does not affect that of the full-length FosB protein. In the present study, we analysed whether CK2 and Ser27 phosphorylation also play a role in regulating DeltaFosB's transcriptional activity. Our findings indicate that CK2 activation increases DeltaFosB's transactivation potential, while CK2 inhibition decreases it. Further, we show that preventing Ser27 phosphorylation by mutating the site to Ala results in a significant decrease in DeltaFosB transactivation, without affecting DeltaFosB's subcellular localization or DNA-binding affinity. In contrast, Ser27 does not seem to play a role in the transactivation potential of full-length FosB. These findings constitute the first evidence of a role for phosphorylation in DeltaFosB's transcriptional activity.

Alterations of CREB and DARPP-32 phosphorylation following cocaine and monoaminergic uptake inhibitors

The cAMP response element-binding protein (CREB) is a transcription factor that can contribute to drug-induced changes in gene expression. It is well known that the dopamine and cAMP-regulated phosphoprotein (DARPP-32), via activation, is converted into a potent inhibitor of protein phosphatase-1 (PP-1), which regulates the activity of CREB. We previously reported that the continuous infusion of cocaine for 7 days produced a significant increase in prodynorphin mRNA in the rat caudate putamen and we also studied the role of the different monoamines in these cocaine effects. Since multiple cAMP response element (CRE) sequences are present on the prodynorphin gene promoter, the aim of our study was to investigate the effects of cocaine and monoaminergic uptake inhibitors on CREB and DARPP-32 phosphorylation and moreover the possible correlation with the changes already observed on prodynorphin gene expression. Here we investigated the alterations on phospho-Ser133 CREB, phospho-Thr34 DARPP-32 and phospho-Thr75 DARPP-32 induced by continuous infusions of cocaine, GBR12909, fluoxetine and nisoxetine. A significant decrease in both phospho-CREB at Ser133 and phospho-DARPP-32 at Thr34 in the rat caudate putamen was produced by cocaine, GBR 12909, fluoxetine or nisoxetine. No alterations were observed on phospho-Thr75 DARPP-32 levels. We hypothesize that the decrease in phospho-Thr34 DARPP-32 could evoke an increase in PP-1 activity which is responsible for the reduction of CREB activation. These effects could in turn elicit the reduction in the transcriptional cascade of the prodynorphin gene in the caudate putamen, observed following chronic fluoxetine and nisoxetine. On the other hand, these mechanisms do not seem to be involved in cocaine- or GBR 12909-induced effects.

Corticostriatal up-regulation of activity-regulated cytoskeletal-associated protein expression after repeated exposure to cocaine

We provide evidence that cocaine evokes short- and long-lasting increases in activity-regulated cytoskeletal-associated protein (Arc) expression after a finely tuned, time-dependent and regional-selective expression profile. Acute experiments revealed that cocaine up-regulates Arc expression primarily in striatum and prefrontal cortex through a dopamine D1-dependent mechanism and a combination of D1- and D2-dependent mechanisms, respectively. Aside from cocaine-dependent Arc elevation, we show for the first time that D1 and D2 receptors tonically regulate basal Arc expression following a regional-selective profile. As opposed to the effects of a single cocaine injection on Arc expression, which dissipate within 24 h, subchronic (five daily injections) or chronic (14 daily injections) cocaine administration, with animals sacrificed hours or days after the last treatment, demonstrated that Arc expression is still up-regulated long after treatment cessation, suggesting that adaptive changes have been set in motion by the prolonged administration of the psychostimulant. In summary, our findings are the first to demonstrate that repeated exposure to cocaine leads to long-lasting dysregulation of Arc expression in the corticostriatal network, thus establishing a molecular basis to explain, at least partially, the impaired synaptic transmission caused by cocaine abuse at this level. Furthermore, given the role exerted by Arc in cytoarchitectural rearrangements, it is conceivable to speculate that it mediates changes in synaptic connectivity brought about by cocaine. Our findings thus pinpoint this molecule as a neuropathological underpinning and molecular bridge that connects short- and long-term neuronal modifications associated with cocaine abuse.

Genetics of dopamine and its contribution to cocaine addiction

Cocaine addiction is a major health and social problem for which there are presently no effective pharmacotherapies. Many of the most promising medications target dopamine based on the large literature that supports its role in addiction. Recent studies show that genetic factors are also important. Rodent models and gene knock-out technology have helped elucidate the involvement of specific genes in the function of the dopamine reward system and intracellular cascades that lead to neuronal changes in this system. Human epidemiological, linkage, and association studies have identified allelic variants (polymorphisms) that give rise to altered metabolism of dopamine and its functional consequences. Individuals with these polymorphisms respond differently to psychostimulants and possibly to pharmacotherapies. Here we review the literature on genetic variations that affect dopamine neurotransmission, responses to psychostimulants and potential treatments for cocaine addiction. Behavioral responses to psychostimulants in animals with different or modified genetics in dopamine signaling are discussed. We also review polymorphisms in humans that affect dopaminergic neurotransmission and alter the subjective effects of psychostimulants. Pharmacotherapies may have increased efficacy when targeted to individuals possessing specific genetic polymophisms in dopamine's metabolic and intracellular messenger systems.

Identification of pyruvate kinase as an antigen associated with Tourette syndrome

Immune responses to beta-hemolytic streptococcal infections are hypothesized to trigger tic disorders and early-onset obsessive-compulsive disorder (OCD) in some pediatric populations. Here we identify the M1 isoform of the glycolytic enzyme, pyruvate kinase (PK) as an autoimmune target in Tourette syndrome and associated disorders. Antibodies to PK reacted strongly with surface antigens of infectious strains of streptococcus, and antibodies to streptococcal M proteins reacted with PK. Moreover, immunoreactivity to PK in patients with exacerbated symptoms who had recently acquired a streptococcal infection was 7-fold higher compared to patients with exacerbated symptoms and no evidence of a streptococcal infection. These data suggest that PK can function as an autoimmune target and that this immunoreactivity may be associated with Tourette syndrome, OCD, and associated disorders.

Neuroadaptations of Cdk5 in cholinergic interneurons of the nucleus accumbens and prefrontal cortex of inbred alcohol-preferring rats following voluntary alcohol drinking

BACKGROUND

Neurobiological studies have identified brain areas and related molecular mechanisms involved in alcohol abuse and dependence. Specific cell types in these brain areas and their role in alcohol-related behaviors, however, have not yet been identified. This study examined the involvement of cholinergic cells in inbred alcohol-preferring rats following 1 month of alcohol drinking. Cyclin-dependent kinase 5 (Cdk5) immunoreactivity (IR), a marker of neuronal plasticity, was examined in cholinergic neurons of the nucleus accumbens (NuAcc) and prefrontal cortex (PFC) and other brain areas implicated in alcohol drinking, using dual immunocytochemical (ICC) procedures. Single Cdk5 IR was also examined in several brain areas implicated in alcohol drinking.

METHODS

The experimental group self-administered alcohol using a 2-bottle-choice test paradigm with unlimited access to 10% (v/v) alcohol and water for 23 h/d for 1 month. An average of 6 g/kg alcohol was consumed daily. Control animals received identical treatment, except that both bottles contained water. Rats were perfused and brain sections were processed for ICC procedures.

RESULTS

Alcohol drinking resulted in a 51% increase in Cdk5 IR cholinergic interneurons in the shell NuAcc, while in the PFC there was a 51% decrease in the percent of Cdk5 IR cholinergic interneurons in the infralimbic region and a 46% decrease in Cdk5 IR cholinergic interneurons in the prelimbic region. Additionally, single Cdk5 IR revealed a 42% increase in the central nucleus of the amygdala (CNA).

CONCLUSIONS

This study identified Cdk5 neuroadaptation in cholinergic interneurons of the NuAcc and PFC and in other neurons of the CNA following 1 month of alcohol drinking. These findings contribute to our understanding of the cellular and molecular basis of alcohol drinking and toward the development of improved region and cell-specific pharmacotherapeutic and behavioral treatment programs for alcohol abuse and alcoholism.

DeltaFosB in the nucleus accumbens regulates food-reinforced instrumental behavior and motivation

Alterations in motivation have been implicated in the pathophysiology of several psychiatric disorders, including substance abuse and depression. Repeated exposure to drugs of abuse or stress is known to persistently induce the transcription factor deltaFosB in the nucleus accumbens (NAc) and dorsal striatum, effects hypothesized to contribute to neuroadaptations in dopamine-regulated signaling. Little is known, however, about the specific involvement of deltaFosB in dysregulation of appetitively motivated behaviors. We show here that inducible overexpression of deltaFosB in NAc and dorsal striatum of bitransgenic mice, or specifically in the NAc core of rats by use of viral-mediated gene transfer, enhanced food-reinforced instrumental performance and progressive ratio responding. Very similar behavioral effects were found after previous repeated exposure to cocaine, amphetamine, MDMA [(+)-3,4-methylenedioxymethamphetamine], or nicotine in rats. These results reveal the powerful regulation of motivational processes by deltaFosB, and provide evidence that drug-induced alterations in gene expression via induction of deltaFosB within the NAc core may play a critical role in the impact of motivational influences on instrumental behavior.

Chronic cocaine prevents depotentiation at corticostriatal synapses

BACKGROUND

The advanced stages of addiction are characterized by compulsive drug-seeking and drug-taking behaviors despite the loss of the hedonic effect of drug consumption. A pathology of habit forming systems might underlie these features of addiction.

METHODS

We have compared use-dependent plasticity of corticostriatal synapses in saline- and cocaine-treated rats by means of single neuron electrophysiological recordings.

RESULTS

High-frequency stimulation of cortical afferents induced long-term potentiation (LTP) of corticostriatal synapses in treated and untreated animals. Saline- and acute-cocaine-treated rats, however, showed synaptic depotentiation in response to subsequent low-frequency stimulation of the same pathway, whereas chronic cocaine-treated animals were refractory to this process. Depotentiation was also absent in control slices bathed with cocaine, dopamine, or with the D1 receptor agonist SKF38393. The effect of cocaine on depotentiation was prevented by D1 but not D2 dopamine receptor antagonists and was mimicked by pharmacological inhibition of cyclin-dependent kinase 5, to enhance D1-receptor-associated intracellular signaling.

CONCLUSIONS

These results provide the first evidence that cocaine blocks the reversal of LTP in brain circuits. This alteration might be important for the persistence of addictive behavior despite efforts to abstain.

Neuronal nuclear organization is controlled by cyclin-dependent kinase 5 phosphorylation of Ras Guanine nucleotide releasing factor-1

RasGRF1 is a member of the Ras guanine nucleotide exchange factor (RasGEF) family of proteins which are directly responsible for the activation of Ras and Rac GTPases. Originally identified as a phosphoprotein, RasGRF1 has been shown to be phosphorylated by protein kinase A and more recently, by the non-receptor tyrosine kinases Ack1 and Src. In this report we show that RasGRF1 interacts with and is phosphorylated by Cdk5 on serine 731 to regulate its steady state levels in mammalian cells as well as in neurons. Phosphorylation on this site by Cdk5 leads to RasGRF1 degradation through a calpain-dependent mechanism. Additionally, cortical neurons from Cdk5 knockout mice have higher levels of RasGRF1 which are reduced when wild-type Cdk5 is transfected into these neurons. In mitotic cells, nuclei become disorganized when RasGRF1 is overexpressed and this is rescued when RasGRF1 is co-expressed with active Cdk5. When RasGRF1 levels are elevated in neurons through overexpression of either the wild-type RasGRF1, or the phosphorylation mutant of RasGRF1 and by the transfection of a dominant negative Cdk5 construct, nuclei appeared condensed and fragmented. On the other hand, a reduction of RasGRF1 levels through p35/Cdk5 overexpression also leads to nuclear condensation in neurons. These data show that phosphorylation of RasGRF1 by Cdk5 tightly regulates its levels, which is essential for proper cellular organization.

Phosphorylation of WAVE1 regulates actin polymerization and dendritic spine morphology

WAVE1--the Wiskott-Aldrich syndrome protein (WASP)--family verprolin homologous protein 1--is a key regulator of actin-dependent morphological processes in mammals, through its ability to activate the actin-related protein (Arp2/3) complex. Here we show that WAVE1 is phosphorylated at multiple sites by cyclin-dependent kinase 5 (Cdk5) both in vitro and in intact mouse neurons. Phosphorylation of WAVE1 by Cdk5 inhibits its ability to regulate Arp2/3 complex-dependent actin polymerization. Loss of WAVE1 function in vivo or in cultured neurons results in a decrease in mature dendritic spines. Expression of a dephosphorylation-mimic mutant of WAVE1 reverses this loss of WAVE1 function in spine morphology, but expression of a phosphorylation-mimic mutant does not. Cyclic AMP (cAMP) signalling reduces phosphorylation of the Cdk5 sites in WAVE1, and increases spine density in a WAVE1-dependent manner. Our data suggest that phosphorylation/dephosphorylation of WAVE1 in neurons has an important role in the formation of the filamentous actin cytoskeleton, and thus in the regulation of dendritic spine morphology.

Prenatal cocaine and morphine alter brain cyclin-dependent kinase 5 (Cdk5) activity in rat pups

Pregnant rats received daily injections of saline, cocaine (20 mg/kg), morphine (2 mg/kg), or the combination of both drugs, on days 13-20 of gestation. Cyclin-dependent kinase 5 (Cdk5) activity was then measured in the resulting pups on postnatal days 1, 7, 14 and 28. Cocaine resulted in a time dependent increase in brain Cdk5 activity which peaked on day 14. Morphine, in contrast, induced a decrease in Cdk5 activity which was also maximal on day 14. Combined administration of the two drugs led to smaller increases than those seen after cocaine alone. These findings demonstrate that prenatal drug exposure can modify postnatal activity of Cdk5 in the brain and raise the possibility that alterations in Cdk5 may play a role in some of the neural and behavioral effects produced by these treatments.

Cocaine induces a differential dose-dependent alteration in the expression profile of immediate early genes, transcription factors, and caspases in PC12 cells: a possible mechanism of neurotoxic damage in cocaine addiction

Cocaine is a widely used drug of abuse and psychostimulant that acts on the central nervous system by blocking the dopamine reuptake sites. PC12 cells, a rat pheochromocytoma clonal line, in the presence of nerve growth factor (NGF), multiply and differentiate into competent neurons that can synthesize, store, and secrete the neurotransmitter dopamine (DA). In the present study, we evaluated the effect of increasing doses of cocaine on the expression of immediate early genes (IEGs), c-fos and c-jun, and closely related transcription factors, SP-1 and NF-kbeta, at 24 h after the exposure to cocaine (50, 100, 200, 500, 1000, 2500 microM) in NGF-differentiated PC12 cells. Cocaine (50-500 microM) resulted in significant induction of the expression of c-fos, c-jun, SP-1, and NF-kbeta. However, higher concentrations of cocaine (1000 and 2500 microM) resulted in the downregulation of these expressions after 24 h. To further understand the role of dose-dependent changes in the mechanisms of cell death, we evaluated the protein expression of apoptotic markers. A concentration-dependent increase in the expression of caspase-9 and -3 was observed up to 500 microM cocaine. However, the higher dose did not show any expression. We also evaluated the effect of increasing doses of cocaine on DA concentration and the expression of dopamine transporter (DAT). A significant dose-dependent decrease in the concentration of DA as well as the expression of DAT was observed 24 h after the exposure of PC12 cells to cocaine. Therefore, in the present study, we reported that cocaine has both upstream and downstream regulatory actions on some IEGs and transcription factors that can regulate the mechanism of cell death, and these effects on gene expression are independent of its action on the dopaminergic system.

Reversal of methamphetamine-induced behavioral sensitization by repeated administration of a dopamine D1 receptor agonist

Repeated intermittent administration of methamphetamine (MAP) produces an enduring hypersensitivity to the motor stimulant effect of MAP, termed behavioral sensitization. Dopamine plays a critical role in the development and expression of behavioral sensitization. Here, we investigated whether a dopamine D1 receptor agonist could reverse behavioral sensitization to MAP. Administration of MAP (1.0 mg/kg, i.p.) to rats once every 3 days for a total of 5 times (days 1-13) induced the enhancement of locomotor activity after MAP challenge (0.5 mg/kg, i.p.) on day 20, verifying the development of behavioral sensitization. The MAP-sensitized rats then received a dopamine D1 agonist, R-(+)-SKF38393 (3.0 mg/kg, i.p.), once a day for 7 consecutive days (days 21-27). Behavioral analysis on days 30 and 41 revealed that the enhanced locomotor activity was reversed by repeated R-(+)-SKF38393 administration. Moreover, repeated R-(+)-SKF38393 administration reversed the increased dopamine release in the striatum after MAP challenge on day 41. Thus, repeated administration of the dopamine D1 receptor agonist induces the reversal of established behavioral sensitization to MAP and of increased dopamine release in the striatum, lasting for at least 2 weeks. Dopamine D1 receptor agonists may be useful therapeutic agents for the treatment of psychostimulant addiction.

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.

Cocaine self-administration in mice is inversely related to phosphorylation at Thr34 (protein kinase A site) and Ser130 (kinase CK1 site) of DARPP-32

The reinforcing effect of cocaine is associated with increases in dopamine in the striatum. The phosphoprotein DARPP-32 (dopamine- and cAMP-regulated phosphoprotein) has been shown to mediate the intracellular events after activation of dopamine receptors. DARPP-32 is phosphorylated at multiple sites by different protein kinases, but little is known about the functional role of these different sites. Cocaine self-administration and striatal levels of dopamine after acute "binge" cocaine administration were measured in separate lines of mice with alanine mutations introduced into DARPP-32 at either Thr34 (protein kinase A site, Thr34A), Thr75, (cyclin-dependent kinase 5 site, Thr75A), Ser97 (kinase CK2 site, Ser97A), or Ser130 (kinase CK1 site, Ser130A). Acquisition of stable cocaine self-administration required significantly more time in Thr34A-/- mice. Both Thr34A- and Ser130A-DARPP-32 mutant mice self-administered more cocaine than their respective wild-type controls. Also, cocaine-induced increases of dopamine in dorsal striatum were attenuated in the Thr34A- and Ser130A-DARPP-32 phosphomutant mice compared with wild-type mice. Notably, levels of P-Thr34- and P-Ser130-DARPP-32 were reduced after self-administration of cocaine in wild-type mice. Thus, phosphorylation states of Thr34- and Ser130-DARPP-32 play important roles in modulating the reinforcing effects of cocaine.

Cyclin-dependent kinase 5 is essential for neuronal cell cycle arrest and differentiation

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase with significant homology to cell cycle-related Cdks but is not believed to be active in a typical cell cycle. In Cdk5-deficient embryos and Cdk5 chimeras, migration and survival of postmitotic neurons is compromised in a cell-autonomous manner. In the present study, we show that loss of Cdk5 leads to both failure of neuronal differentiation and loss of cell cycle control. Using specific cytoskeletal proteins as indices of neuronal differentiation, we find that Cdk5-deficient neurons are significantly arrested or delayed in their developmental program both in vivo and in vitro. For example, immunocytochemistry of embryonic day 16 (E16) cortex reveals that the expression of microtubule-associated protein 2c (Map-2c), a marker of mature neurons, is nearly absent in Cdk5(-/-) cells that have migrated to the cortical plate while these same cells continue to express nestin. Similarly, in vitro, Map-2-positive cells are rare in cultures from E16 Cdk5(-/-) embryos. Cell cycle control is also deficient in Cdk5(-/-) cells. In vivo, neurons engaged in cell cycle activities are found in the cortical plate, and, in vitro, class III beta-tubulin-positive cells continue to label with bromodeoxyuridine even after 5 d of incubation. Transfection of a wild-type Cdk5 construct reveals that cell cycle control can be regained in Cdk5(-/-) cells by overexpression of Cdk5. These data indicate that Cdk5 is necessary for both neuronal differentiation and cell cycle inhibition.

Cocaine-induced dendritic spine formation in D1 and D2 dopamine receptor-containing medium spiny neurons in nucleus accumbens

Psychostimulant-induced alteration of dendritic spines on dopaminoceptive neurons in nucleus accumbens (NAcc) has been hypothesized as an adaptive neuronal response that is linked to long-lasting addictive behaviors. NAcc is largely composed of two distinct subpopulations of medium-sized spiny neurons expressing high levels of either dopamine D1 or D2 receptors. In the present study, we analyzed dendritic spine density after chronic cocaine treatment in distinct D1 or D2 receptor-containing medium-sized spiny neurons in NAcc. These studies made use of transgenic mice that expressed EGFP under the control of either the D1 or D2 receptor promoter (Drd1-EGFP or Drd2-EGFP). After 28 days of cocaine treatment and 2 days of withdrawal, spine density increased in both Drd1-EGFP- and Drd2-EGFP-positive neurons. However, the increase in spine density was maintained only in Drd1-EGFP-positive neurons 30 days after drug withdrawal. Notably, increased DeltaFosB expression also was observed in Drd1-EGFP- and Drd2-EGFP-positive neurons after 2 days of drug withdrawal but only in Drd1-EGFP-positive neurons after 30 days of drug withdrawal. These results suggest that the increased spine density observed after chronic cocaine treatment is stable only in D1-receptor-containing neurons and that DeltaFosB expression is associated with the formation and/or the maintenance of dendritic spines in D1 as well as D2 receptor-containing neurons in NAcc.

Chromatin remodeling is a key mechanism underlying cocaine-induced plasticity in striatum

Given that cocaine induces neuroadaptations through regulation of gene expression, we investigated whether chromatin remodeling at specific gene promoters may be a key mechanism. We show that cocaine induces specific histone modifications at different gene promoters in striatum, a major neural substrate for cocaine's behavioral effects. At the cFos promoter, H4 hyperacetylation is seen within 30 min of a single cocaine injection, whereas no histone modifications were seen with chronic cocaine, consistent with cocaine's ability to induce cFos acutely, but not chronically. In contrast, at the BDNF and Cdk5 promoters, genes that are induced by chronic, but not acute, cocaine, H3 hyperacetylation was observed with chronic cocaine only. DeltaFosB, a cocaine-induced transcription factor, appears to mediate this regulation of the Cdk5 gene. Furthermore, modulating histone deacetylase activity alters locomotor and rewarding responses to cocaine. Thus, chromatin remodeling is an important regulatory mechanism underlying cocaine-induced neural and behavioral plasticity.

Opposing roles of transient and prolonged expression of p25 in synaptic plasticity and hippocampus-dependent memory

While deregulation of cyclin-dependent kinase 5 (Cdk5) has been implicated in neurodegenerative diseases, its precise role in synaptic plasticity and memory remains elusive. Proteolytic cleavage of p35, a regulatory subunit of Cdk5, by calpain results in the generation of the truncated p25 protein, which causes hyperactivation of Cdk5. Using region-specific and inducible transgenic mice, we show that transiently increased p25 expression in the hippocampus enhanced long-term potentiation (LTP) and facilitated hippocampus-dependent memory. Moreover, p25 expression increased the number of dendritic spines and synapses. Importantly, enhanced memory achieved by a transient expression of p25 followed by its repression did not cause neurodegeneration. In contrast, prolonged p25 production caused severe cognitive deficits, which were accompanied by synaptic and neuronal loss and impaired LTP. Our data suggest a role for p25 in synaptic plasticity, synaptogenesis, learning, and memory and provide a model whereby deregulation of a plasticity factor can contribute to neurodegeneration.

c-Fos and ΔFosB expression are differentially altered in distinct subregions of the nucleus accumbens shell in cocaine-sensitized rats

Repeated cocaine administration in rats can lead to sensitization as evidenced by an increased locomotor response to a subsequent exposure (challenge) dose of cocaine even after a drug-free period. Expression of the immediate early gene product, c-Fos, differs among distinct subregions of the nucleus accumbens shell. This would suggest that these subregions may be differentially involved in sensitization. The present study quantified c-Fos- and deltaFosB-immunoreactive nuclei in subterritories of the nucleus accumbens in animals behaviorally sensitized to cocaine. Rats received a sensitization-inducing regimen of cocaine (twice-daily injections of 15 mg/kg i.p. for five consecutive days). Fourteen days following the last injection, rats were given a challenge injection of cocaine (15 mg/kg i.p.), and killed 2 h later. Sections through the nucleus accumbens were processed for tyrosine hydroxylase and either c-Fos or deltaFosB. The number of immunoreactive nuclei was quantified in five subregions of the nucleus accumbens shell: the vertex, arch, cone, intermediate zone and ventrolateral zone, which can be identified by differential histological staining for tyrosine hydroxylase. Repeated cocaine administration resulted in robust sensitization that was associated with more deltaFosB in the vertex, arch, and cone compared with saline-treated controls. As previously reported, c-Fos immunoreactivity was increased in the intermediate zone in cocaine-sensitized rats. deltaFosB was significantly elevated in rats that did not receive a cocaine challenge, attesting to the long half-life of this transcription factor. These results provide further evidence suggesting distinct anatomical neuroadaptations within the nucleus accumbens shell that may play a functional role in psychomotor-stimulant sensitization.

Assessment of genome and proteome profiles in cocaine abuse

Until recently, knowledge of the impact of abuse drugs on gene and protein expression in the brain was limited to less than 100 targets. With the advent of high-throughput genomic and proteomic techniques investigators are now able to evaluate changes across the entire genome and across thousands of proteins in defined brain regions and generate expression profiles of vulnerable neuroanatomical substrates in rodent and non-human primate drug abuse models and in human post-mortem brain tissue from drug abuse victims. The availability of gene and protein expression profiles will continue to expand our understanding of the short- and long-term consequences of drug addiction and other addictive disorders and may provide new approaches or new targets for pharmacotherapeutic intervention. This chapter will review gene expression data from rodent, non-human primate and human post-mortem studies of cocaine abuse and will provide a preliminary proteomic profile of human cocaine abuse and explore how these studies have advanced our understanding of addiction.

DARPP-32 phosphorylation opposes the behavioral effects of nicotine

BACKGROUND

The addictive properties of nicotine are mediated via dopaminergic pathways and their post-synaptic neurons in the striatum. Because post-synaptic neurons within the striatum contain high levels of the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), we hypothesized that DARPP-32 may functionally contribute to the behavioral effects of nicotine.

METHODS

We examined the behavioral effects of nicotine and the phosphorylation state of DARPP-32 in wild-type (WT) and DARPP-32 knockout (KO) mice. In one experiment, we assessed voluntary nicotine intake (0-50 microg/ml) of WT and KO mice in a two-bottle choice paradigm. In a separate experiment, the motor-depressant effects of acute and repeated nicotine injections (0-.8 mg/kg, subcutaneously [SC]) were assessed. The phosphorylation of DARPP-32 at threonine34 and threonine75 were examined using Western blotting.

RESULTS

A heightened responsiveness to nicotine was seen in KO mice when compared with WT mice in oral intake and motor depression. The enhanced responsiveness in KO mice was not due to alterations in taste sensations, fluid intake, or blood nicotine or cotinine levels. Systemic injections of nicotine resulted in increased striatal DARPP-32 phosphorylation at threonine34 and threonine75.

CONCLUSIONS

DARPP-32 opposes the behavioral effects of nicotine possibly via concurrent phosphorylation at the two threonine sites.

The "ups and downs" of signaling cascades in addiction

Drug addiction is a chronic disease characterized by compulsive drug use despite the severe negative consequences associated with it. Repeated exposure to drugs of abuse results in molecular adaptations in neuronal signaling pathways, which eventually manifest in the complex behavioral alterations that characterize addiction. These include tolerance, sensitization, dependence, drug craving, and relapse. In this Review, we focus on recent studies highlighting signaling cascades initiated by cocaine, as a representative of a drug of abuse with a defined site of action, and alcohol, as a drug with an undefined primary site of action. Specifically, we describe recent studies that emphasize the role of protein-protein interactions, phosphorylation, and compartmentalization in the molecular mechanisms that result in the cellular and behavioral adaptations that underlie addiction. Signaling cascades that contribute to addiction, as well as those that protect or delay the development of addiction, are presented.

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.