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

DeltaFosB indirectly regulates Cck promoter activity

Some of the important biochemical, structural, and behavioral changes induced by chronic exposure to drugs of abuse appear to be mediated by the highly stable transcription factor DeltaFosB. Previous work has shown that DeltaFosB overexpression in mice for 2weeks leads to an increase in the expression of numerous genes in striatum, most of which are later downregulated following 8weeks of FosB expression. Interestingly, a large number of these genes were also upregulated in mice overexpressing the transcription factor CREB. It was unclear from this study, however, whether short-term DeltaFosB regulates these genes via CREB. Here, we find that 2weeks of DeltaFosB overexpression increases CREB expression in striatum, an effect that dissipates by 8weeks. The early induction is associated with increased CREB binding to certain target gene promoters in this brain region. Surprisingly, one gene that was a suspected CREB target based on previous reports, cholecystokinin (Cck), was not controlled by CREB in striatum. To further investigate the regulation of Cck following DeltaFosB overexpression, we confirmed that short-term DeltaFosB overexpression increases both Cck promoter activity and gene expression. It also increases binding activity at a putative CREB binding site (CRE) in the Cck promoter. However, while the CRE site is necessary for normal basal expression of Cck, it is not required for DeltaFosB induction of Cck. Taken together, these results suggest that while short-term DeltaFosB induction increases CREB expression and activity at certain gene promoters, this is not the only mechanism by which genes are upregulated under these conditions.

Mice lacking p35 display hyperactivity and paradoxical response to psychostimulants

Cyclin-dependent kinase 5/p35 kinase complex plays a critical role in dopaminergic neurotransmission. Dysregulation of dopamine (DA) signaling is associated with neurological and neuropsychiatric disorders. As cyclin-dependent kinase 5 (Cdk5) requires association with p35 for its proper activation, we hypothesized that dysregulation of Cdk5 activity might have an effect on striatal-mediated behavior. We used a mutant mouse, deficient in p35 protein (p35 KO), which displayed reduced Cdk5 activity. Throughout behavioral and biochemical characterization of naïve and psychostimulant-treated mice, we demonstrated that only juvenile p35 KO mice displayed spontaneous hyperactivity, responded with a paradoxical hypolocomotor effect to psychostimulant drugs and exhibited deficit on proper behavioral inhibition. Strong immunolabeling for tyrosine-hydroxylase and high striatal DA synthesis and contents with a low DA turnover, which were reverted by psychostimulants, were also found in mutant mice. Our results demonstrate that p35 deficiency is critically involved in the expression of a hyperactive behavioral phenotype with hyper-functioning of the dopaminergic system, emphasizing the importance of proper Cdk5 kinase activity for normal motor and emotional features. Thus, p35 KO mice may be another useful animal model for understanding cellular and molecular events underlying attention deficit hyperactivity disorder-like disorders.

Cocainomics: new insights into the molecular basis of cocaine addiction

Until recently, knowledge of the impact of abused 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 nonhuman 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 review summarizes several important genomic and proteomic studies of cocaine abuse/addiction from rodent, nonhuman primate, and human postmortem studies of cocaine abuse and explores how these studies have advanced our understanding of addiction.

Essential role of the histone methyltransferase G9a in cocaine-induced plasticity

Cocaine-induced alterations in gene expression cause changes in neuronal morphology and behavior that may underlie cocaine addiction. In mice, we identified an essential role for histone 3 lysine 9 (H3K9) dimethylation and the lysine dimethyltransferase G9a in cocaine-induced structural and behavioral plasticity. Repeated cocaine administration reduced global levels of H3K9 dimethylation in the nucleus accumbens. This reduction in histone methylation was mediated through the repression of G9a in this brain region, which was regulated by the cocaine-induced transcription factor DeltaFosB. Using conditional mutagenesis and viral-mediated gene transfer, we found that G9a down-regulation increased the dendritic spine plasticity of nucleus accumbens neurons and enhanced the preference for cocaine, thereby establishing a crucial role for histone methylation in the long-term actions of cocaine.

Transient overexpression of alpha-Ca2+/calmodulin-dependent protein kinase II in the nucleus accumbens shell enhances behavioral responding to amphetamine

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is known to contribute to the expression of psychostimulant sensitization by regulating dopamine (DA) overflow from DA neuron terminals in the nucleus accumbens (NAcc). The present experiments explored the contribution of CaMKII in NAcc neurons postsynaptic to these terminals where it is known to participate in a number of signaling pathways that regulate responding to psychostimulant drugs. Exposure to amphetamine transiently increased alphaCaMKII levels in the shell but not the core of the NAcc. Thus, HSV (herpes simplex viral) vectors were used to transiently overexpress alphaCaMKII in NAcc neurons in drug-naive rats, and behavioral responding to amphetamine was assessed. Transiently overexpressing alphaCaMKII in the NAcc shell led to long-lasting enhancement of amphetamine-induced locomotion and self-administration manifested when alphaCaMKII levels were elevated and persisting long after they had returned to baseline. Enhanced locomotion was not observed after infection in the NAcc core or sites adjacent to the NAcc. Transient elevation of NAcc shell alphaCaMKII levels also enhanced locomotor responding to NAcc AMPA and increased phosphorylation levels of GluR1 (Ser831), a CaMKII site, both soon and long after infection. Similar increases in pGluR1 (Ser831) were observed both soon and long after exposure to amphetamine. These results indicate that the transient increase in alphaCaMKII observed in neurons of the NAcc shell after viral-mediated gene transfer and likely exposure to amphetamine leads to neuroadaptations in AMPA receptor signaling in this site that may contribute to the long-lasting maintenance of behavioral and incentive sensitization by psychostimulant drugs like amphetamine.

Mechanisms of locomotor sensitization to drugs of abuse in a two-injection protocol

A single exposure to psychostimulants or morphine is sufficient to induce persistent locomotor sensitization, as well as neurochemical and electrophysiological changes in rodents. Although it provides a unique model to study the bases of long-term behavioral plasticity, sensitization mechanisms remain poorly understood. We investigated in the mouse, a species suited for transgenic studies, the mechanisms of locomotor sensitization showed by the increased response to a second injection of drug (two-injection protocol of sensitization, TIPS). The first cocaine injection induced a locomotor sensitization that was completely context-dependent, increased during the first week, and persisted 3 months later. The induction of sensitized responses to cocaine required dopamine D1 and glutamate NMDA receptors. A single injection of the selective dopamine transporter blocker GBR12783 was sufficient to activate extracellular signal-regulated kinase (ERK) in the striatum to the same level as cocaine and to induce sensitization to cocaine, but not to itself. The induction of sensitization was sensitive to protein synthesis inhibition by anisomycin after cocaine administration. Morphine induced a pronounced context-dependent sensitization that crossed with cocaine. Sensitization to morphine injection was prevented in knockin mutant mice bearing a Thr-34-Ala mutation of DARPP-32, which suppresses its ability to inhibit protein phosphatase-1 (PP1), but not mutation of Thr-75 or Ser-130. These results combined with previous ones show that TIPS in mouse is a context-dependent response, which involves an increase in extracellular dopamine, stimulation of D1 and NMDA receptors, regulation of the cAMP-dependent and ERK pathways, inhibition of PP1, and protein synthesis. It provides a simple and sensitive paradigm to study the mechanisms of long-term effects of drugs of abuse.

Epigenetic mediation of environmental influences in major psychotic disorders

The major psychotic disorders schizophrenia and bipolar disorder are etiologically complex involving both heritable and nonheritable factors. The absence of consistently replicated major genetic effects, together with evidence for lasting changes in gene expression after environmental exposures, is consistent with the concept that the biologic underpinnings of these disorders are epigenetic in form rather than DNA sequence based. Psychosis-associated environmental exposures, particularly at key developmental stages, may result in long-lasting epigenetic alterations that impact on the neurobiological processes involved in pathology. Although direct evidence for epigenetic dysfunction in both schizophrenia and bipolar disorder is still limited, methodological technologies in epigenomic profiling have advanced. This means that we are at the exciting stage where it is feasible to start investigating molecular modifications to DNA and histones and examine the mechanisms by which environmental factors can act upon the genome to bring about epigenetic changes in gene expression involved in the etiology of these disorders. Given the dynamic nature of the epigenetic machinery and potential reversibility of epigenetic modifications, the understanding of such mechanisms is of key relevance for clinical psychiatry and for identifying new targets for prevention and/or intervention.

Social and physical environment alter cocaine conditioned place preference and dopaminergic markers in adolescent male rats

This study was done to determine whether social and environmental factors alter cocaine reward and proteins implicated in mediating drug reward in rats during early adolescence. On postnatal day (PND) 23, rats were housed under conditions where both social (number of rats per cage) and environmental (availability of toys) factors were manipulated. Socially isolated rats were housed alone impoverished with no toys (II) or enriched with toys (IE). Social rats were housed two rats/cage with no toys (SI2) or with toys (SE2), or three/cage with (SE3) or without (SI3) toys. On PND 43, cocaine conditioned place preference (CPP) sessions began with the post-test done on PND 47. Cocaine CPP was established in response to 5 or 10 mg/kg cocaine in II rats, and CPP was decreased with the addition of cage mates or toys. No CPP was seen to any dose in SI3 or SE3 rats. Enriched housing (SE3) increased dopamine transporter (DAT) protein in the nucleus accumbens compared to II. There also were differential effects of cocaine on tyrosine hydroxylase and DAT depending on housing, with both increased by cocaine in II but not SE3 rats. DARPP-32 was unchanged by housing or cocaine, while phospho-Thr(34)-DARPP-32 was increased by cocaine treatment across conditions. Thus, both social and environmental enrichment decrease cocaine CPP during adolescence and different housing alters proteins that regulate dopaminergic neurotransmission in a manner that may account for the observed differences in cocaine-induced reward.

Cyclin-dependent kinase 5 inhibitors: inhibition of dopamine transporter activity

Cyclin-dependent kinase (Cdk) 5 reduces the rewarding properties of psychostimulants by dampening postsynaptic dopamine (DA) receptor signaling. Cdk5 is also present in midbrain DA neurons, where the DA transporter (DAT) is localized and limits DA neurotransmission by removing extracellular DA. Here, we tested the hypothesis that Cdk5 could also affect the disposition of DA by regulating DAT activity. Incubation of rat dorsal striatal (dSTR) synaptosomes with the Cdk5 inhibitors roscovitine, olomoucine, and 4-{[(7-oxo-6,7-dihydro-8H-[1,3]thiazolo[5,4-e]indol-8-ylidene)methyl]amino}-N-(2-pyridyl)benzenesulfonamide (GW8510) or the inactive congener iso-olomoucine resulted in a rapid, concentration-dependent inhibition of specific [3H]DA uptake. However, roscovitine was the only inhibitor that did not also decrease [3H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (WIN35,428) binding to dSTR DATs. Roscovitine-induced inhibition of dSTR [3H]DA uptake was explained by decreased maximal uptake velocity, without a change in cell-surface DAT levels. Roscovitine did not enhance [3H]DA release mediated by either DAT reverse-transport or Ca(2+) channels in dSTR slices. Instead, roscovitine enhanced spontaneous [3H]DA outflow and inhibited DAT-mediated [3H]DA reaccumulation into dSTR slices. To explore the involvement of Cdk5 in roscovitine-induced down-regulation of DAT activity, Cdk5 protein was knocked down via Cdk5-small interfering RNA by as much as 86% in porcine aortic endothelial cells stably expressing human (h)DATs. However, Cdk5 depletion did not alter hDAT activity. Taken together, our results suggest that roscovitine inhibits DAT activity independently of Cdk5; therefore, results obtained with such inhibitors should be interpreted with caution. Our study is the first to demonstrate that Cdk5 inhibitors reduce brain DAT activity via a mechanism that is independent of DAT trafficking and reverse-transport.

Early life exposure to a high fat diet promotes long-term changes in dietary preferences and central reward signaling

Overweight and obesity in the United States continues to grow at epidemic rates in large part due to the overconsumption of calorically-dense palatable foods. Identification of factors influencing long-term macronutrient preferences may elucidate points of prevention and behavioral modification. In our current study, we examined the adult macronutrient preferences of mice acutely exposed to a high fat diet during the third postnatal week. We hypothesized that the consumption of a high fat diet during early life would alter the programming of central pathways important in adult dietary preferences. As adults, the early-exposed mice displayed a significant preference for a diet high in fat compared to controls. This effect was not due to diet familiarity as mice exposed to a novel high carbohydrate diet during this same early period failed to show differences in macronutrient preferences as adults. The increased intake of high fat diet in early exposed mice was specific to dietary preferences as no changes were detected for total caloric intake or caloric efficiency. Mechanistically, mice exposed to a high fat diet during early life exhibited significant alterations in biochemical markers of dopamine signaling in the nucleus accumbens, including changes in levels of phospho-dopamine and cyclic AMP-regulated phosphoprotein, molecular weight 32 kDa (DARPP-32) threonine-75, DeltaFosB, and cyclin-dependent kinase 5. These results support our hypothesis that even brief early life exposure to calorically-dense palatable diets alters long-term programming of central mechanisms important in dietary preferences and reward. These changes may underlie the passive overconsumption of high fat foods contributing to the increasing body mass in the western world.

Zebrafish reward mutants reveal novel transcripts mediating the behavioral effects of amphetamine

BACKGROUND

Addiction is a pathological dysregulation of the brain's reward systems, determined by several complex genetic pathways. The conditioned place preference test provides an evaluation of the effects of drugs in animal models, allowing the investigation of substances at a biologically relevant level with respect to reward. Our lab has previously reported the development of a reliable conditioned place preference paradigm for zebrafish. Here, this test was used to isolate a dominant N-ethyl-N-nitrosourea (ENU)-induced mutant, no addiction (nad(dne3256)), which fails to respond to amphetamine, and which we used as an entry point towards identifying the behaviorally relevant transcriptional response to amphetamine.

RESULTS

Through the combination of microarray experiments comparing the adult brain transcriptome of mutant and wild-type siblings under normal conditions, as well as their response to amphetamine, we identified genes that correlate with the mutants' altered conditioned place preference behavior. In addition to pathways classically involved in reward, this gene set shows a striking enrichment in transcription factor-encoding genes classically involved in brain development, which later appear to be re-used within the adult brain. We selected a subset of them for validation by quantitative PCR and in situ hybridization, revealing that specific brain areas responding to the drug through these transcription factors include domains of ongoing adult neurogenesis. Finally, network construction revealed functional connections between several of these genes.

CONCLUSIONS

Together, our results identify a new network of coordinated gene regulation that influences or accompanies amphetamine-triggered conditioned place preference behavior and that may underlie the susceptibility to addiction.

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 D(3) dopamine receptor inhibits dopamine release in PC-12/hD3 cells by autoreceptor signaling via PP-2B, CK1, and Cdk-5

The function of the D(3) dopamine (DA) receptor remains ambiguous largely because of the lack of selective D(3) receptor ligands. To investigate the function and intracellular signaling of D(3) receptors, we established a PC-12/hD3 clone, which expresses the human D(3) DA receptor in a DA producing cell line. In this model, we find that the D(3) receptor functions as an autoreceptor controlling neurotransmitter secretion. Pre-treatment with 3,6a,11, 14-tetrahydro-9-methoxy-2 methyl-(12H)-isoquino[1,2-b] pyrrolo[3,2-f][1,3] benzoxanzine-1-carboxylic acid, a D(3) receptor preferring agonist, dose-dependently suppressed K+-evoked [3H]DA release in PC-12/hD3 cells but not in the control cell line. This effect was prevented by D(3) receptor preferring antagonists GR103691 and SB277011-A. Furthermore, activation of D(3) receptors significantly inhibits forskolin-induced cAMP accumulation and leads to transient increases in phosphorylation of cyclin-dependent kinase 5 (Cdk5), dopamine and cAMP-regulated phosphoprotein of M(r) 32 000 and Akt. Because we observed differences in Cdk5 phosphorylation as well as Akt phosphorylation after DA stimulation, we probed the ability of Cdk5 and phosphatidylinositol-3 kinase (PI3K) to influence DA release. Cdk5 inhibitors, roscovitine, or olomoucine, but not the PI3K inhibitor wortmannin, blocked the D(3) receptor inhibition of DA release. In a complimentary experiment, over-expression of Cdk5 potentiated D(3) receptor suppression of DA release. Pertussis toxin, 3-[(2,4,6-trimethoxyphenyl)methylidenyl]-indolin-2-one and cyclosporine A also attenuated D(3) receptor-mediated inhibition of DA release indicating that this phenomenon acts through Gi/oalpha and casein kinase 1, and phosphatase protein phosphatase 2B (calcineurin), respectively. In support of previous data that D(3) DA receptors reduce transmitter release from nerve terminals, the current results demonstrate that D(3) DA receptors function as autoreceptors to inhibit DA release and that a signaling pathway involving Cdk5 is essential to this regulation.

Regulation of hippocampal and behavioral excitability by cyclin-dependent kinase 5

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase that has been implicated in learning, synaptic plasticity, neurotransmission, and numerous neurological disorders. We previously showed that conditional loss of Cdk5 in adult mice enhanced hippocampal learning and plasticity via modulation of calpain-mediated N-methyl-D-aspartic acid receptor (NMDAR) degradation. In the present study, we characterize the enhanced synaptic plasticity and examine the effects of long-term Cdk5 loss on hippocampal excitability in adult mice. Field excitatory post-synaptic potentials (fEPSPs) from the Schaffer collateral CA1 subregion of the hippocampus (SC/CA1) reveal that loss of Cdk5 altered theta burst topography and enhanced post-tetanic potentiation. Since Cdk5 governs NMDAR NR2B subunit levels, we investigated the effects of long-term Cdk5 knockout on hippocampal neuronal excitability by measuring NMDAR-mediated fEPSP magnitudes and population-spike thresholds. Long-term loss of Cdk5 led to increased Mg(2+)-sensitive potentials and a lower threshold for epileptiform activity and seizures. Biochemical analyses were performed to better understand the role of Cdk5 in seizures. Induced-seizures in wild-type animals led to elevated amounts of p25, the Cdk5-activating cofactor. Long-term, but not acute, loss of Cdk5 led to decreased p25 levels, suggesting that Cdk5/p25 may be activated as a homeostatic mechanism to attenuate epileptiform activity. These findings indicate that Cdk5 regulates synaptic plasticity, controls neuronal and behavioral stimulus-induced excitability and may be a novel pharmacological target for cognitive and anticonvulsant therapies.

Caffeine and a selective adenosine A2A receptor antagonist induce reward and sensitization behavior associated with increased phospho-Thr75-DARPP-32 in mice

RATIONALE

Caffeine, an antagonist of adenosine A(1) and A(2A) receptor, is the most widely used psychoactive substance in the world. Evidence indicates that caffeine interacts with the neuronal systems involved in drug reinforcing. Although adenosine A(1) and/or A(2A) receptor have been found to play important roles in the locomotor stimulation and probably reinforcing effect of caffeine, the relative contribution of the A(1) and/or A(2A) receptors to the acute and chronic motor activation and reinforcing effects of caffeine has not been completely investigated.

OBJECTIVE

The roles of adenosine A(1) and/or A(2A) receptor and the association of phospho-Thr75-dopamine- and cAMP-regulated phosphoprotein of molecular weight 32 kDa (DARPP-32) in the motor activation and reinforcing effects of caffeine, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A(1) antagonist, and 5-amino-7-(beta-phenylethyl)-2-(8-furyl) pyrazolol [4,3-e]-1,2,4-triazolol [1,5-c] pyrimidine (SCH58261), a selective A(2A) receptor antagonist were examined.

METHODS

Locomotor stimulation and behavioral sensitization of caffeine, DPCPX, and SCH58261 were studied in C57BL/6 male mice following acute and chronic administration. Conditioned place preference (CPP) paradigm was used to evaluate the drug-seeking potential of these compounds. Furthermore, the expression of phospho-Thr75-DARPP-32 in striatal membrane from behaviorally sensitized mice was analyzed by Western blot.

RESULTS

Caffeine and SCH58261 but not DPCPX induced CPP and locomotor sensitization in C57BL/6 mice. The locomotor sensitization after chronic treatment was associated with increased DARPP-32 phosphorylation at Thr75 in the striatum.

CONCLUSION

Caffeine-induced reinforcing effect and behavioral sensitization are mediated by antagonism at adenosine A(2A) receptor. These effects are associated with phosphorylation of DARPP-32 at Thr75 in the striatum.

Histone acetylation in drug addiction

Regulation of chromatin structure through post-translational modifications of histones (e.g., acetylation) has emerged as an important mechanism to translate a variety of environmental stimuli, including drugs of abuse, into specific changes in gene expression. Since alterations in gene expression are thought to contribute to the development and maintenance of the addicted state, recent efforts are aimed at identifying how drugs of abuse alter chromatin structure and the enzymes which regulate it. This review discusses how drugs of abuse alter histone acetylation in brain reward regions, through which enzymes this occurs, and ultimately what role histone acetylation plays in addiction-related behaviors.

Genome-wide analysis of chromatin regulation by cocaine reveals a role for sirtuins

Changes in gene expression contribute to the long-lasting regulation of the brain's reward circuitry seen in drug addiction; however, the specific genes regulated and the transcriptional mechanisms underlying such regulation remain poorly understood. Here, we used chromatin immunoprecipitation coupled with promoter microarray analysis to characterize genome-wide chromatin changes in the mouse nucleus accumbens, a crucial brain reward region, after repeated cocaine administration. Our findings reveal several interesting principles of gene regulation by cocaine and of the role of DeltaFosB and CREB, two prominent cocaine-induced transcription factors, in this brain region. The findings also provide comprehensive insight into the molecular pathways regulated by cocaine-including a new role for sirtuins (Sirt1 and Sirt2)-which are induced in the nucleus accumbens by cocaine and, in turn, dramatically enhance the behavioral effects of the drug.

Molecular mechanisms of psychostimulant-induced structural plasticity

Drug addiction is characterized by persistent behavioral and cellular plasticity throughout the brain's reward regions. Among the many neuroadaptations that occur following repeated drug administration are alterations in cell morphology including changes in dendritic spines. While this phenomenon has been well documented, the underlying molecular mechanisms are poorly understood. Here, within the context of drug abuse, we review and integrate several of the established pathways known to regulate synaptic remodeling, and discuss the contributions of neurotrophic and dopamine signaling in mediating this structural plasticity. Finally, we discuss how such upstream mechanisms could regulate actin dynamics, the common endpoint involved in structural remodeling in neurons.

Nuclear factor kappa B signaling regulates neuronal morphology and cocaine reward

Although chronic cocaine-induced changes in dendritic spines on nucleus accumbens (NAc) neurons have been correlated with behavioral sensitization, the molecular pathways governing these structural changes, and their resulting behavioral effects, are poorly understood. The transcription factor, nuclear factor kappa B (NFkappaB), is rapidly activated by diverse stimuli and regulates expression of many genes known to maintain cell structure. Therefore, we evaluated the role of NFkappaB in regulating cocaine-induced dendritic spine changes on medium spiny neurons of the NAc and the rewarding effects of cocaine. We show that chronic cocaine induces NFkappaB-dependent transcription in the NAc of NFkappaB-Lac transgenic mice. This induction of NFkappaB activity is accompanied by increased expression of several NFkappaB genes, the promoters of which show chromatin modifications after chronic cocaine exposure consistent with their transcriptional activation. To study the functional significance of this induction, we used viral-mediated gene transfer to express either a constitutively active or dominant-negative mutant of Inhibitor of kappa B kinase (IKKca or IKKdn), which normally activates NFkappaB signaling, in the NAc. We found that activation of NFkappaB by IKKca increases the number of dendritic spines on NAc neurons, whereas inhibition of NFkappaB by IKKdn decreases basal dendritic spine number and blocks the increase in dendritic spines after chronic cocaine. Moreover, inhibition of NFkappaB blocks the rewarding effects of cocaine and the ability of previous cocaine exposure to increase an animal's preference for cocaine. Together, these studies establish a direct role for NFkappaB pathways in the NAc to regulate structural and behavioral plasticity to cocaine.

Basal and cocaine-induced sex differences in the DARPP-32-mediated signaling pathway

RATIONALE

Behavioral and dopamine responses to cocaine are sexually dimorphic: Female rats exhibit higher levels of locomotor and reward-associated behaviors after cocaine administration and dopamine release than do males. Activation of the dopamine- and cAMP-regulated phosphoprotein of Mr 32 kDa (DARPP-32) intracellular cascade mediates responses to cocaine.

OBJECTIVE

To examine the possibility that acute cocaine administration alters the DARPP-32 cascade in a sexually dimorphic pattern.

MATERIALS AND METHODS

Male and female rats received either saline or cocaine (30 mg/kg). Protein levels of DARPP-32, phosphorylation of DARPP-32 at the Thr34 site (P-Thr34-DARPP-32), protein phosphatase 1 (PP-1), and protein phosphatase 2B (PP-2B) in nucleus accumbens were measured via Western blot analysis.

RESULTS

Females had higher protein levels of DARPP-32, P-Thr34-DARPP-32, calcineurin A (CaN-A; catalytic subunit of PP-2B), and calcineurin B (CaN-B; regulatory subunit of PP-2B) than males 5 min after saline treatment. In females, CaN-A protein levels were also higher at 15 min and PP-1 protein levels were higher 30 min after saline administration than males. In male rats, cocaine significantly increased CaN-A protein levels at 30 min and CaN-B protein levels at 15 min. In females, cocaine administration significantly decreased protein levels of DARPP-32, P-Thr34-DARPP-32, and CaN-A at 45 min but increased PP-1 protein levels at 30 min. Overall, males had higher activation of the DARPP-32 pathway after cocaine administration than did females.

CONCLUSION

These novel results show that basal and cocaine-induced sex differences in the DARPP-32/PP-1 cascade may be responsible for the sexual dimorphism in acute cocaine-induced behavioral responses.

Methylphenidate-induced dendritic spine formation and DeltaFosB expression in nucleus accumbens

Methylphenidate is the psychostimulant medication most commonly prescribed to treat attention deficit hyperactivity disorder (ADHD). Recent trends in the high usage of methylphenidate for both therapeutic and nontherapeutic purposes prompted us to investigate the long-term effects of exposure to the drug on neuronal adaptation. We compared the effects of chronic methylphenidate or cocaine (15 mg/kg, 14 days for both) exposure in mice on dendritic spine morphology and DeltaFosB expression in medium-sized spiny neurons (MSN) from ventral and dorsal striatum. Chronic methylphenidate increased the density of dendritic spines in MSN-D1 (MSN-expressing dopamine D1 receptors) from the core and shell of nucleus accumbens (NAcc) as well as MSN-D2 (MSN-expressing dopamine D2 receptors) from the shell of NAcc. In contrast, cocaine increased the density of spines in both populations of MSN from all regions of striatum. In general, the effect of methylphenidate on the increase of shorter spines (class 2) was less than that of cocaine. Interestingly, the methylphenidate-induced increase in the density of relatively longer spines (class 3) in the shell of NAcc was bigger than that induced by cocaine. Furthermore, methylphenidate exposure increased expression of DeltaFosB only in MSN-D1 from all areas of striatum, and surprisingly, the increase was greater than that induced by cocaine. Thus, our results show differential effects of methylphenidate and cocaine on neuronal adaptation in specific types of MSN in reward-related brain regions.

Decoding the epigenetic language of neuronal plasticity

Neurons are submitted to an exceptional variety of stimuli and are able to convert these into high-order functions, such as storing memories, controlling behavior, and governing consciousness. These unique properties are based on the highly flexible nature of neurons, a characteristic that can be regulated by the complex molecular machinery that controls gene expression. Epigenetic control, which largely involves events of chromatin remodeling, appears to be one way in which transcriptional regulation of gene expression can be modified in neurons. This review will focus on how epigenetic control in the mature nervous system may guide dynamic plasticity processes and long-lasting cellular neuronal responses. We outline the molecular pathways underlying chromatin transitions, propose the presence of an "epigenetic indexing code," and discuss how central findings accumulating at an exponential pace in the field of epigenetics are conceptually changing our perspective of adult brain function.

Transient enhanced expression of Cdk5 activator p25 after acute and chronic d-amphetamine administration

The cellular and molecular mechanisms of sensitization in the addictive process are still unclear. Recently, chronic treatment with cocaine has been shown to upregulate the expression of cyclin-dependent kinase 5 (cdk5) and its specific activator, p35, in the striatum, as a downstream target gene of DeltaFosB, and has been implicated in compensatory adaptive changes associated with psychostimulants. Cdk5 is a serine/threonine kinase and its activation is achieved through association with a regulatory subunit, either p35 or p39. P35 is cleaved by the protease calpain, which results in the generation of a truncated product termed p25, which contains all elements necessary for cdk5 activation. The cdk5/p35 complex plays an essential role in neuronal development and survival. It has also been involved in neuronal trafficking and transport and in dopaminergic transmission, indicating its role either in presynaptic and postsynaptic signaling. In this study we report that the cdk5/p35 complex participates in acute and chronic d-amphetamine (AMPH)-evoked behavioral events, and we show a surprisingly transient enhanced expression of p25 and a lasting increased expression of p35 in dorsal striatal synaptosomes after acute and chronic AMPH administration. Pak1, a substrate for cdk5, is also enriched in the synaptosomal fraction of acute AMPH-treated rats. Our data suggest that the transient upregulation of p25 may regulate the activity of cdk5 in phosphorylating particular substrates, such as Pak1, implicated in the compensatory adaptive morphophysiologic changes associated with the process of behavioral sensitization to psychostimulants.

Delta FosB-mediated alterations in dopamine signaling are normalized by a palatable high-fat diet

BACKGROUND

Sensitivity to reward has been implicated as a predisposing factor for behaviors related to drug abuse as well as overeating. However, the underlying mechanisms contributing to reward sensitivity are unknown. We hypothesized that a dysregulation in dopamine signaling might be an underlying cause of heightened reward sensitivity whereby rewarding stimuli could act to normalize the system.

METHODS

We used a genetic mouse model of increased reward sensitivity, the Delta FosB-overexpressing mouse, to examine reward pathway changes in response to a palatable high-fat diet. Markers of reward signaling in these mice were examined both basally and following 6 weeks of palatable diet exposure. Mice were examined in a behavioral test following high-fat diet withdrawal to assess the vulnerability of this model to removal of rewarding stimuli.

RESULTS

Our results demonstrate altered reward pathway activation along the nucleus accumbens-hypothalamic-ventral tegmental area circuitry resulting from overexpression of Delta FosB in the nucleus accumbens and striatal regions. Levels of phosphorylated cyclic adenosine monophosphate (cAMP) response element binding protein (pCREB), brain-derived neurotrophic factor (BDNF), and dopamine and cyclic adenosine monophosphate regulated phosphoprotein with a molecular mass of 32 kDa (DARPP-32) in the nucleus accumbens were reduced in Delta FosB mice, suggestive of reduced dopamine signaling. Six weeks of high-fat diet exposure completely ameliorated these differences, revealing the potent rewarding capacity of a palatable diet. Delta FosB mice also showed a significant increase in locomotor activity and anxiety-related responses 24 hours following high-fat withdrawal.

CONCLUSIONS

These results establish an underlying sensitivity to changes in reward related to dysregulation of Delta FosB and dopamine signaling that can be normalized with palatable diets and may be a predisposing phenotype in some forms of obesity.

Striatal dysregulation of Cdk5 alters locomotor responses to cocaine, motor learning, and dendritic morphology

Motor learning and neuro-adaptations to drugs of abuse rely upon neuronal signaling in the striatum. Cyclin-dependent kinase 5 (Cdk5) regulates striatal dopamine neurotransmission and behavioral responses to cocaine. Although the role for Cdk5 in neurodegeneration in the cortex and hippocampus and in hippocampal-dependent learning has been demonstrated, its dysregulation in the striatum has not been examined. Here we show that strong activation of striatal NMDA receptors produced p25, the truncated form of the Cdk5 co-activator p35. Furthermore, inducible overexpression of p25 in the striatum prevented locomotor sensitization to cocaine and attenuated motor coordination and learning. This corresponded with reduced dendritic spine density, increased neuro-inflammation, altered dopamine signaling, and shifted Cdk5 specificity with regard to physiological and aberrant substrates, but no apparent loss of striatal neurons. Thus, dysregulation of Cdk5 dramatically affects striatal-dependent brain function and may be relevant to non-neurodegenerative disorders involving dopamine neurotransmission.

Review. Transcriptional mechanisms of addiction: role of DeltaFosB

Regulation of gene expression is considered a plausible mechanism of drug addiction, given the stability of behavioural abnormalities that define an addicted state. Among many transcription factors known to influence the addiction process, one of the best characterized is DeltaFosB, which is induced in the brain's reward regions by chronic exposure to virtually all drugs of abuse and mediates sensitized responses to drug exposure. Since DeltaFosB is a highly stable protein, it represents a mechanism by which drugs produce lasting changes in gene expression long after the cessation of drug use. Studies are underway to explore the detailed molecular mechanisms by which DeltaFosB regulates target genes and produces its behavioural effects. We are approaching this question using DNA expression arrays coupled with the analysis of chromatin remodelling--changes in the posttranslational modifications of histones at drug-regulated gene promoters--to identify genes that are regulated by drugs of abuse via the induction of DeltaFosB and to gain insight into the detailed molecular mechanisms involved. Our findings establish chromatin remodelling as an important regulatory mechanism underlying drug-induced behavioural plasticity, and promise to reveal fundamentally new insight into how DeltaFosB contributes to addiction by regulating the expression of specific target genes in brain reward pathways.

Cdk5 and the non-catalytic arrest of the neuronal cell cycle

Cyclin-dependent kinase 5 (Cdk5) is a nontraditional Cdk that is primarily active in postmitotic neurons. An important core function of Cdk5 involves regulating the migration and maturation of embryonic post-mitotic neurons. These developmental roles are dependent on its kinase activity. Initially, there was little evidence indicating a role for Cdk5 in normal cell cycle regulation. Recent data from our lab, however, suggest that Cdk5 plays a crucial role as a cell cycle suppressor in normal post-mitotic neurons and neuronal cell lines. It performs this foundation in a kinase independent manner. Cdk5 normally found in both nucleus and cytoplasm, but it exits the nucleus in neurons risk to death in an AD patient's brain. The shift in sub-cellular location is accompanied by cell cycle re-entry and neuronal death. This "new" function of Cdk5 raises cautions in the design of Cdk5-directed drugs for the therapy of neurodegenerative diseases.

Quantitative trait locus analysis identifies rat genomic regions related to amphetamine-induced locomotion and Galpha(i3) levels in nucleus accumbens

Identification of the genetic factors that underlie stimulant responsiveness in animal models has significant implications for better understanding and treating stimulant addiction in humans. F(2) progeny derived from parental rat strains F344/NHsd and LEW/NHsd, which differ in responses to drugs of abuse, were used in quantitative trait locus (QTL) analyses to identify genomic regions associated with amphetamine-induced locomotion (AIL) and G-protein levels in the nucleus accumbens (NAc). The most robust QTLs were observed on chromosome 3 (maximal log ratio statistic score (LRS(max))=21.3) for AIL and on chromosome 2 (LRS(max)=22.0) for Galpha(i3). A 'suggestive' QTL (LRS(max)=12.5) was observed for AIL in a region of chromosome 2 that overlaps with the Galpha(i3) QTL. Novelty-induced locomotion (NIL) showed different QTL patterns from AIL, with the most robust QTL on chromosome 13 (LRS(max)=12.2). Specific unique and overlapping genomic regions influence AIL, NIL, and inhibitory G-protein levels in the NAc. These findings suggest that common genetic mechanisms influence certain biochemical and behavioral aspects of stimulant responsiveness.

c-Fos is an intracellular regulator of cocaine-induced long-term changes

Development of drug addiction is accompanied by the induction of long-lasting neurobiological changes. Dopamine D1 receptors are involved in mediating cocaine-induced neuroadaptation, yet the underlying intracellular mechanisms remain less clear. Using a genetically modified mouse in which Fos is primarily mutated in D1 receptor-bearing neurons in the brain, we examined a potential role of the immediate early gene Fos, which is rapidly induced by cocaine via D1 receptors, in mediating cocaine-induced persistent neurobiological changes. We found that the composition of AP-1 transcription complexes and expression levels of AP-1 complexes, and several transcription factors, neurotransmitter receptors as well as intracellular signaling molecules following repeated cocaine administration are altered in Fos-deficient brains. Moreover, dendritic reorganization of medium spiny neurons induced by repeated exposure to cocaine is attenuated in the mutant brains. The mutant mice also exhibit reduced behavioral sensitization after repeated cocaine administration. These findings suggest that c-Fos expressed in D1 receptor-bearing neurons mediates cocaine-induced persistent changes.

Protein kinases and addiction

Although drugs of abuse have different chemical structures and interact with different protein targets, all appear to usurp common neuronal systems that regulate reward and motivation. Addiction is a complex disease that is thought to involve drug-induced changes in synaptic plasticity due to alterations in cell signaling, gene transcription, and protein synthesis. Recent evidence suggests that drugs of abuse interact with and change a common network of signaling pathways that include a subset of specific protein kinases. The best studied of these kinases are reviewed here and include extracellular signal-regulated kinase, cAMP-dependent protein kinase, cyclin-dependent protein kinase 5, protein kinase C, calcium/calmodulin-dependent protein kinase II, and Fyn tyrosine kinase. These kinases have been implicated in various aspects of drug addiction including acute drug effects, drug self-administration, withdrawal, reinforcement, sensitization, and tolerance. Identifying protein kinase substrates and signaling pathways that contribute to the addicted state may provide novel approaches for new pharmacotherapies to treat drug addiction.

Genetic NMDA receptor deficiency disrupts acute and chronic effects of cocaine but not amphetamine

NMDA receptor-mediated glutamate transmission is required for several forms of neuronal plasticity. Its role in the neuronal responses to addictive drugs is an ongoing subject of investigation. We report here that the acute locomotor-stimulating effect of cocaine is absent in NMDA receptor-deficient mice (NR1-KD). In contrast, their acute responses to amphetamine and to direct dopamine receptor agonists are not significantly altered. The striking attenuation of cocaine's acute effects is not likely explained by alterations in the dopaminergic system of NR1-KD mice, since most parameters of pre- and postsynaptic dopamine function are unchanged. Consistent with the behavioral findings, cocaine induces less c-Fos expression in the striatum of these mice, while amphetamine-induced c-Fos expression is intact. Furthermore, chronic cocaine-induced sensitization and conditioned place preference are attenuated and develop more slowly in mutant animals, but amphetamine's effects are not altered significantly. Our results highlight the importance of NMDA receptor-mediated glutamatergic transmission specifically in cocaine actions, and support a hypothesis that cocaine and amphetamine elicit their effects through differential actions on signaling pathways.

Cocaine regulates MEF2 to control synaptic and behavioral plasticity

Repeated exposure to cocaine causes sensitized behavioral responses and increased dendritic spines on medium spiny neurons of the nucleus accumbens (NAc). We find that cocaine regulates myocyte enhancer factor 2 (MEF2) transcription factors to control these two processes in vivo. Cocaine suppresses striatal MEF2 activity in part through a mechanism involving cAMP, the regulator of calmodulin signaling (RCS), and calcineurin. We show that reducing MEF2 activity in the NAc in vivo is required for the cocaine-induced increases in dendritic spine density. Surprisingly, we find that increasing MEF2 activity in the NAc, which blocks the cocaine-induced increase in dendritic spine density, enhances sensitized behavioral responses to cocaine. Together, our findings implicate MEF2 as a key regulator of structural synapse plasticity and sensitized responses to cocaine and suggest that reducing MEF2 activity (and increasing spine density) in NAc may be a compensatory mechanism to limit long-lasting maladaptive behavioral responses to cocaine.

Epigenetic mechanisms in drug addiction

Changes in gene expression in brain reward regions are thought to contribute to the pathogenesis and persistence of drug addiction. Recent studies have begun to focus on the molecular mechanisms by which drugs of abuse and related environmental stimuli, such as drug-associated cues or stress, converge on the genome to alter specific gene programs. Increasing evidence suggests that these stable gene expression changes in neurons are mediated in part by epigenetic mechanisms that alter chromatin structure on specific gene promoters. This review discusses recent findings from behavioral, molecular and bioinformatic approaches being used to understand the complex epigenetic regulation of gene expression by drugs of abuse. This novel mechanistic insight might open new avenues for improved treatments of drug addiction.

Involvement of septal Cdk5 in the emergence of excessive anxiety induced by stress

The aim of the present study was to evaluate whether the activation of Cdk5, a protein that has been suggested to participate in higher cognitive functions, is required for the onset of a sensitized anxiety-related behavior induced by stress. The exposure to restraint enhanced both Cdk5 expression in certain subareas of the septohippocampal system, principally in the lateral septum (LS) and septal Cdk5 kinase activity in rats. Behaviorally, restrained wild type mice showed a behavior indicative of enhanced anxiety in the elevated plus maze (EPM). In contrast, unstressed mice and stressed knockout mice, which lacked the p35 protein, the natural activator of Cdk5, displayed similar anxiety-like behavior in the EPM. Finally, the intra-LS infusion of olomoucine - a Cdk5 inhibitor - blocked the enhanced anxiety in the EPM induced by prior stress in rats. All these data provide evidence that septal Cdk5 is required in the emergence of a sensitized emotional process induced by stress.

Regulation of Kalirin by Cdk5

Kalirin, one of the few Rho guanine nucleotide exchange factors (GEFs) that contains spectrin-like repeats, plays a critical role in axon extension and maintenance of dendritic spines. PC12 cells were used to determine whether Cdk5, a critical participant in both processes, regulates the action of Kalirin. Expression of Kalirin-7 in nondifferentiated PC12 cells caused GEF-activity-dependent extension of broad cytoplasmic protrusions; coexpression of dominant-negative Cdk5 largely eliminated this response. The spectrin-like repeat region of Kalirin plays an essential role in this response, which is not mimicked by the GEF domain alone. Thr1590, which follows the first GEF domain of Kalirin, is the only Cdk5 phosphorylation site in Kalirin-7. Although mutant Kalirin-7 with Ala1590 retains GEF activity, it is unable to cause extension of protrusions. Kalirin-7 with an Asp1590 mutation has slightly increased GEF activity and dominant-negative Cdk5 fails to block its ability to cause extension of protrusions. Phosphorylation of Thr1590 causes a slight increase in GEF activity and Kalirin-7 solubility. Dendritic spines formed by cortical neurons in response to the expression of Kalirin-7 with Ala1590 differ in shape from those formed in response to wild-type Kalirin-7 or Kalirin-7 containing Asp1590. The presence of Thr1590 in each major Kalirin isoform would allow Cdk5 to regulate Kalirin function throughout development.

Psychiatric research: psychoproteomics, degradomics and systems biology

While proteomics has excelled in several disciplines in biology (cancer, injury and aging), neuroscience and psychiatryproteomic studies are still in their infancy. Several proteomic studies have been conducted in different areas of psychiatric disorders, including drug abuse (morphine, alcohol and methamphetamine) and other psychiatric disorders (depression, schizophrenia and psychosis). However, the exact cellular and molecular mechanisms underlying these conditions have not been fully investigated. Thus, one of the primary objectives of this review is to discuss psychoproteomic application in the area of psychiatric disorders, with special focus on substance- and drug-abuse research. In addition, we illustrate the potential role of degradomic utility in the area of psychiatric research and its application in establishing and identifying biomarkers relevant to neurotoxicity as a consequence of drug abuse. Finally, we will discuss the emerging role of systems biology and its current use in the field of neuroscience and its integral role in establishing a comprehensive understanding of specific brain disorders and brain function in general.

Delta FosB mediates epigenetic desensitization of the c-fos gene after chronic amphetamine exposure

The molecular mechanisms underlying the transition from recreational drug use to chronic addiction remain poorly understood. One molecule implicated in this process is DeltaFosB, a transcription factor that accumulates in striatum after repeated drug exposure and mediates sensitized behavioral responses to psychostimulants and other drugs of abuse. The downstream transcriptional mechanisms by which DeltaFosB regulates drug-induced behaviors are incompletely understood. We reported previously the chromatin remodeling mechanisms by which DeltaFosB activates the expression of certain genes; however, the mechanisms underlying DeltaFosB-mediated gene repression remain unknown. Here, we identify c-fos, an immediate early gene rapidly induced in striatum after acute psychostimulant exposure, as a novel downstream target that is repressed chronically by DeltaFosB. We show that accumulation of DeltaFosB in striatum after chronic amphetamine treatment desensitizes c-fos mRNA induction to a subsequent drug dose. DeltaFosB desensitizes c-fos expression by recruiting histone deacetylase 1 (HDAC1) to the c-fos gene promoter, which, in turn, deacetylates surrounding histones and attenuates gene activity. Accordingly, local knock-out of HDAC1 in striatum abolishes amphetamine-induced desensitization of the c-fos gene. In concert, chronic amphetamine increases histone H3 methylation on the c-fos promoter, a chromatin modification also known to repress gene activity, as well as expression levels of the H3 histone methyltransferase, KMT1A (lysine methyltransferase 1A, formerly SUV39H1). This study reveals a novel epigenetic pathway through which DeltaFosB mediates distinct transcriptional programs that may ultimately alter behavioral plasticity to chronic amphetamine exposure.

Neurotrophic factors and structural plasticity in addiction

Drugs of abuse produce widespread effects on the structure and function of neurons throughout the brain's reward circuitry, and these changes are believed to underlie the long-lasting behavioral phenotypes that characterize addiction. Although the intracellular mechanisms regulating the structural plasticity of neurons are not fully understood, accumulating evidence suggests an essential role for neurotrophic factor signaling in the neuronal remodeling which occurs after chronic drug administration. Brain-derived neurotrophic factor (BDNF), a growth factor enriched in brain and highly regulated by several drugs of abuse, regulates the phosphatidylinositol 3'-kinase (PI3K), mitogen-activated protein kinase (MAPK), phospholipase Cgamma (PLCgamma), and nuclear factor kappa B (NFkappaB) signaling pathways, which influence a range of cellular functions including neuronal survival, growth, differentiation, and structure. This review discusses recent advances in our understanding of how BDNF and its signaling pathways regulate structural and behavioral plasticity in the context of drug addiction.

Repeated administration of a dopamine D1 receptor agonist reverses the increased proportions of striatal dopamine D1High and D2High receptors in methamphetamine-sensitized rats

Repeated administration of psychostimulants produces a behavioural sensitization. Amphetamine-sensitized animals are known to have a higher proportion of high-affinity states of dopamine D2 receptors (D2(High) receptors) in the striatum. We recently reported that repeated administration of a dopamine D1 receptor agonist, R-(+)-SKF38393, reverses the established behavioural sensitization to methamphetamine (MAP). To investigate the mechanisms for reversal of behavioural sensitization, we examined the effect of repeated administration of the dopamine D1 receptor agonist on the proportions of D2(High) receptors and the high-affinity states of dopamine D1 receptors (D1(High) receptors) in the striatum. In the striatum from the MAP-sensitized rats, the proportions of D1(High) and D2(High) receptors (28.5 +/- 1.96 and 57.5 +/- 3.58%) were higher than those in the saline-control rats (12.0 +/- 1.01 and 21.9 +/- 1.60%, respectively). Repeated administration of R-(+)-SKF38393 to the MAP-sensitized rats reduced the increased proportions of D1(High) and D2(High) receptors to 12.4 +/- 1.57 and 31.0 +/- 2.14%, respectively, which were similar to the proportions in the saline-control rats. The total densities of dopamine D1 and D2 receptors were not altered in each treatment condition. The results demonstrate that the proportions of D1(High) and D2(High) receptors in the striatum are elevated in MAP-sensitized rats, and that repeated administration of the dopamine D1 receptor agonist to the MAP-sensitized rats reverses the increased proportions of D1(High) and D2(High) receptors. The findings reveal postsynaptic mechanisms for the development of behavioural sensitization to MAP and the reversal of established sensitization by repeated administration of the dopamine D1 receptor agonist.

The effects of sodium butyrate, an inhibitor of histone deacetylase, on the cocaine- and sucrose-maintained self-administration in rats

In order to substantiate the concept that cocaine behavioral effects may be influenced by histone modification, rats were trained to self-administer cocaine intravenously (0.75 mg/(kginjection)), and were systemically pretreated with sodium butyrate (NaBu), a potent histone deacetylase inhibitor, before the test session during the maintenance phase. The effect of NaBu on a control reinforcer (sucrose)-induced self-administration was also assessed. NaBu (100-200 mg/kg) was inactive in altering the cocaine (0.75 mg/(kg injection))-maintained responding and at the highest dose (400 mg/kg) it did increase cocaine-induced lever presses during the maintenance phase. On the other hand, sucrose-reinforcing potential was not altered when NaBu was given at the highest dose (400 mg/kg). These findings extend previous observations that changes in histone acetylation are relevant to cocaine-induced behavioral effects. Given that histone acetylase inhibitor enhances cocaine-induced behavioral plasticity, the therapeutic benefits of histone acetyltransferase inhibitors warrant further investigation in the experimental models of cocaine abuse.

An unusual member of the Cdk family: Cdk5

The proline-directed serine threonine kinase, Cdk5, is an unusual molecule that belongs to the well-known large family of proteins, cyclin-dependent kinases (Cdks). While it has significant homology with the mammalian Cdk2 and yeast cdc2, unlike the other Cdks, it has little role to play in cell cycle regulation and is activated by non-cyclin proteins, p35 and p39. It phosphorylates a spectrum of proteins, most of them associated with cell morphology and motility. A majority of known substrates of Cdk5 are cytoskeletal elements, signalling molecules or regulatory proteins. It also appears to be an important player in cell-cell communication. Highly conserved, Cdk5 is most abundant in the nervous system and is of special interest to neuroscientists as it appears to be indispensable for normal neural development and function. In normal cells, transcription and activity of Cdk5 is tightly regulated. Present essentially in post-mitotic neurons, its normal activity is obligatory for migration and differentiation of neurons in developing brain. Deregulation of Cdk5 has been implicated in Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease and acute neuronal injury. Regulators of Cdk5 activity are considered as potential therapeutic molecules for degenerative diseases. This review focuses on the role of Cdk5 in neural cells as regulator of cytoskeletal elements, axonal guidance, membrane transport, synaptogenesis and cell survival in normal and pathological conditions.

Valproate and acetyl-L-carnitine prevent methamphetamine-induced behavioral sensitization in mice

This study deals with the possible inhibitory role played by acetyl-l-carnitine (ALC) against methamphetamine (METH)-induced behavioral sensitization. Because valproate (VAL) inhibits the behavioral sensitization exerted by different psychostimulants, we investigated ALC's potential to prevent the amplification of METH-mediated psychomotor effects. We therefore evaluated the locomotor effects of VAL or ALC alone or in combination with METH after acute (day 1) as well as repeated (day 7) drug challenge. Finally, to assess the induction of behavioral sensitization, we also recorded the METH-mediated locomotor response after 7 days of drug suspension (day 15). Results showed that both VAL and ALC prevented the METH-induced sensitization. Another interesting observation was the significantly higher METH-induced hyperactivity at day 15 (after a 7-day drug-free period), indicating that behavioral sensitization developed during the washout period. Results also showed that both the acute and repeated coadministration of METH with either VAL or ALC inhibited METH-induced hyperactivity. We present different hypotheses concerning similar but also peculiar mechanisms that might underlie the preventive action of VAL and ALC. These data add to a growing body of literature that illustrates the potential of ALC in protecting against the insult of dysfunctional mitochondrial metabolism and psychostimulant-mediated neurotoxicity. By demonstrating an in vivo action against one of the most abused drugs, these results raise the possibility of beneficial effects of ALC in abuse behavior.

Involvement of AMPA receptor phosphorylation in antidepressant actions with special reference to tianeptine

Depression is associated with abnormal neuronal plasticity. AMPA receptors mediate transmission and plasticity at excitatory synapses in a manner which is positively regulated by phosphorylation at Ser831-GluR1, a CaMKII/PKC site, and Ser845-GluR1, a PKA site. Treatment with the selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitor fluoxetine increases P-Ser845-GluR1 but not P-Ser831-GluR1. Here, it was found that treatment with another antidepressant, tianeptine, increased P-Ser831-GluR1 in the frontal cortex and the CA3 region of hippocampus and P-Ser845-GluR1 in the CA3 region of hippocampus. A receptorome profile detected no affinity for tianeptine at any monaminergic receptors or transporters, confirming an atypical profile for this compound. Behavioural analyses showed that mice bearing point mutations at both Ser831- and Ser845-GluR1, treated with saline, exhibited increased latency to enter the centre of an open field and increased immobility in the tail-suspension test compared to their wild-type counterparts. Chronic tianeptine treatment increased open-field locomotion and reduced immobility in wild-type mice but not in phosphomutant GluR1 mice. P-Ser133-CREB was reduced in the CA3 region of hippocampus in phosphomutant mice, and tianeptine decreased P-Ser133-CREB in this region in wild-type, but not in phosphomutant, mice. Tianeptine increased P-Ser133-CREB in the CA1 region in wild-type mice but not in phosphomutant GluR1 mice. There were higher basal P-Ser133-CREB and c-fos levels in frontal and cingulate cortex in phosphomutant GluR1 mice; these changes in level were counteracted by tianeptine in a GluR1-independent manner. Using phosphorylation assays and phosphomutant GluR1 mice, this study provides evidence that AMPA receptor phosphorylation mediates certain explorative and antidepressant-like actions under basal conditions and following tianeptine treatment.

Trk: a neuromodulator of age-specific behavioral and neurochemical responses to cocaine in mice

Responses to psychostimulants vary with age, but the molecular etiologies of these differences are largely unknown. The goal of the present research was to identify age-specific behavioral and molecular adaptations to cocaine and to elucidate the mechanisms involved therein. Postweanling, periadolescent, and adult male CD-1 mice were exposed to cocaine (20 mg/kg) for 7 d. The rewarding effects of cocaine were assessed, as were the response to a Trk antagonist and the regulation of dopamine and cAMP-regulated phosphoprotein, 32 kDa (DARPP-32). Cocaine was rewarding in both periadolescent and adult mice using a conditioned place preference procedure. In contrast, postweanling mice failed to demonstrate significant cocaine-induced place preference. Because components of the neurotrophin system including brain-derived neurotrophic factor and TrkB are developmentally regulated, their role in the age-specific effects of cocaine was determined using the Trk receptor antagonist K252a. Postweanling mice that received K252a before daily cocaine showed a significant place preference to the cocaine-paired environment that was not seen in the absence of K252a. DARPP-32 protein levels were significantly upregulated in the lateral region of the caudate-putamen exclusively in postweanling mice after chronic cocaine. Daily pretreatment with K252a attenuated the induction of DARPP-32 in the postweanling striatum. These data indicate that Trk neurotransmission plays a role in age-specific behavioral and molecular responses to cocaine and concurrently modulates DARPP-32 levels.