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

Psychomotor stimulants and neuronal plasticity

Considerable evidence suggests that neuroadaptations leading to addiction involve the same glutamate-dependent cellular mechanisms that enable learning and memory. Long-term potentiation (LTP) and long-term depression (LTD) have therefore become an important focus of addiction research. This article reviews: (1) basic mechanisms underlying LTP and LTD, (2) the properties of LTP and LTD in ventral tegmental area, nucleus accumbens, dorsal striatum and prefrontal cortex, (3) studies demonstrating that psychomotor stimulants influence LTP or LTD in these brain regions. In addition, we discuss our recent work on cellular mechanisms by which dopamine may influence LTP and LTD. Based on evidence that AMPA receptors are inserted into synapses during LTP and removed during LTD, we investigated the effects of D1 receptor stimulation on AMPA receptor trafficking using primary cultures prepared from nucleus accumbens and prefrontal cortex. Our results suggest that activation of the D1 receptor-protein kinase A signaling pathway leads to externalization of AMPA receptors and promotes LTP. This provides a mechanism to explain facilitation of reward-related learning by dopamine. When this mechanism is activated in an unregulated manner by psychostimulants, maladaptive forms of neuroplasticity may occur that contribute to the transition from casual to compulsive drug use.

Prenatal exposure to cocaine decreases adenylyl cyclase activity in embryonic mouse striatum

Adenylyl cyclase activity was measured in the striatum of naive mice as a function of age and in mice exposed in utero to cocaine. In naive Swiss-Webster mice, basal and forskolin-stimulated adenylyl cyclase activity increased gradually from embryonic day 13 (E13) until 2-3 weeks of age when activity peaked before decreasing slightly to adult levels. The ability of the dopamine D1 receptor agonist, SKF 82958, to stimulate adenylyl cyclase activity also increased in magnitude until P15. In a separate study, pregnant Swiss-Webster mice were injected twice daily with cocaine (15 mg/kg, s.c.) or an equal volume of saline from E10 to E17. Adenylyl cyclase activity was measured in the striatum of E18 embryos. Basal adenylyl cyclase activity was significantly reduced following prenatal exposure to cocaine. Likewise, the ability of forskolin or SKF 82958 to stimulate adenylyl cyclase was attenuated following cocaine exposure. DeltaFosB was not induced, contrary to what is seen in adult mice. These results demonstrate a functional change in a critical signal transduction pathway following chronic in utero exposure to cocaine that might have profound effects of the development of the brain. Alterations in the cAMP system may underlie some of the deficits seen in humans exposed in utero to cocaine.

Cophosphorylation of amphiphysin I and dynamin I by Cdk5 regulates clathrin-mediated endocytosis of synaptic vesicles

It has been thought that clathrin-mediated endocytosis is regulated by phosphorylation and dephosphorylation of many endocytic proteins, including amphiphysin I and dynamin I. Here, we show that Cdk5/p35-dependent cophosphorylation of amphiphysin I and dynamin I plays a critical role in such processes. Cdk5 inhibitors enhanced the electric stimulation-induced endocytosis in hippocampal neurons, and the endocytosis was also enhanced in the neurons of p35-deficient mice. Cdk5 phosphorylated the proline-rich domain of both amphiphysin I and dynamin I in vitro and in vivo. Cdk5-dependent phosphorylation of amphiphysin I inhibited the association with beta-adaptin. Furthermore, the phosphorylation of dynamin I blocked its binding to amphiphysin I. The phosphorylation of each protein reduced the copolymerization into a ring formation in a cell-free system. Moreover, the phosphorylation of both proteins completely disrupted the copolymerization into a ring formation. Finally, phosphorylation of both proteins was undetectable in p35-deficient mice.

Sensitization of adenylate cyclase: a general mechanism of neuroadaptation to persistent activation of Galpha(i/o)-coupled receptors?

Acute activation of Galphas-coupled receptors stimulates cyclic AMP accumulation leading to the activation of downstream signaling cascades. These Galphas-mediated events can be countered by acute activation of inhibitory G proteins (Galpha(i/o)), which inhibit the activity of adenylate cyclase, thereby attenuating cyclic AMP accumulation. Furthermore, an additional, less direct mechanism for Galpha(i/o) proteins modulation of cyclic AMP signaling also has been described. Persistent activation of several Galpha(i/o)-coupled receptors has been shown to result in a subsequent paradoxical enhancement of adenylate cyclase activity in response to drug-stimulated cyclic AMP accumulation. This sensitization of adenylate cyclase likely represents a cellular adaptive response following prolonged activation of inhibitory receptors. Recent advances in our knowledge of G protein signaling, adenylate cyclase regulation, and other cellular signaling mechanisms have extensively increased our insight into this phenomenon. It is now thought that sensitization occurs as part of a compensatory mechanism by which the cell adapts to chronic inhibitory input. Such a mechanism may be involved in modulating Galphas-coupled receptor signaling following neurotransmitter elevations that occur in psychiatric disease states or following the administration of many drugs of abuse. This review will focus on recent advances in the understanding of molecular signaling pathways that are involved in sensitization and describe the potential role of sensitization in neuronal cell function.

Outer dense fibers serve as a functional target for Cdk5.p35 in the developing sperm tail

Cdk5 is ubiquitously expressed in all tissues, but its activators, p35 and p39, are principally found in brain, and Cdk5 activity has mostly been associated with brain development, particularly neuronal differentiation and migration. Here we show that the p35 transcript and protein are also present in the testis, and an active Cdk5.p35 complex exists in this tissue as well. Cdk5 and p35 are prominently observed in elongating spermatid tails, particularly over the tail outer dense fibers (ODF). The appearance of Cdk5.p35 proceeds from the proximal to the distal end of elongating spermatids, coinciding with the proximal to distal assembly of ODF along the length of the tail axoneme. Incidentally, increased Cdk5.p35 activity is observed in isolated elongating spermatids and at a time when elongating spermatids appear in the developing testis, suggesting a role for Cdk5.p35 in spermiogenesis. The presence of Cdk5 and p35 in ODF isolated from rat sperm tails implies a strong association among these proteins. In vitro ODF phosphorylation by Cdk5.p35 and decreased in vivo sperm tail ODF phosphorylation in p35-deficient mice indicate that Cdk5.p35 is an integral component of ODF and that ODF is a functional Cdk5.p35 target in the testis. Our results demonstrate for the first time that Cdk5.p35 may participate in the regulation of sperm tail development via a mechanism involving ODF phosphorylation. Apparently, as in brain development, Cdk5.p35 plays a part in testis development.

Regulation of gene expression and cocaine reward by CREB and ΔFosB

DeltaFosB (a truncated form of FosB) and CREB (cAMP response element binding protein) are transcription factors induced in the brain's reward pathways after chronic exposure to drugs of abuse. However, their mechanisms of action and the genes they regulate remain unclear. Using microarray analysis in the nucleus accumbens of inducible transgenic mice, we found that CREB and a dominant-negative CREB have opposite effects on gene expression, as do prolonged expression of DeltaFosB and the activator protein-1 (AP-1) antagonist DeltacJun. However, unlike CREB, short-term and prolonged DeltaFosB induction had opposing effects on gene expression. Gene expression induced by short-term DeltaFosB and by CREB was strikingly similar, and both reduced the rewarding effects of cocaine, whereas prolonged DeltaFosB expression increased drug reward. Gene expression after a short cocaine treatment was more dependent on CREB, whereas gene expression after a longer cocaine treatment became increasingly DeltaFosB dependent. These findings help define the molecular functions of CREB and DeltaFosB and identify clusters of genes that contribute to cocaine addiction.

Prenatal cocaine exposure alters glycogen synthase kinase-3beta (GSK3beta) pathway in select rabbit brain areas

Prenatal cocaine exposure in rabbits induces cerebrocortical structural abnormalities. Glycogen synthase kinase-3beta (GSK3beta) plays an important role in neuronal development and survival. This study was designed to examine the effect of prenatal cocaine on brain GSK3beta. Rabbits exposed in utero to cocaine and assessed on postnatal day 20 had increased basal levels of phospho-GSK3beta (ser-9) in frontal cortex (FCX) and striatum, but not hippocampus (HP). However, no changes in GSK3beta expression were detected in the brain regions of treated rabbits. Consistent with the change in GSK3beta activity, levels of beta-catenin, a downstream substrate of GSK3beta, increased in FCX but not in HP of cocaine offspring. Administration of a D(1) dopamine receptor agonist inhibited GSK3beta activity in FCX and HP of control rabbits but not in cocaine offspring. This loss of GSK3beta inhibition is in accord with the previously demonstrated dysfunction of this receptor in in utero cocaine-exposed animals. The results indicate that prenatal cocaine exposure alters GSK3beta pathway in select brain areas and may underlie the structural changes noted in these animals.

Sustained elevation of extracellular dopamine causes motor dysfunction and selective degeneration of striatal GABAergic neurons

Dopamine is believed to contribute to the degeneration of dopamine-containing neurons in the brain. However, whether dopamine affects the survival of other neuronal populations has remained unclear. Here we document that mice with persistently elevated extracellular dopamine, resulting from inactivation of the dopamine transporter gene, sporadically develop severe symptoms of dyskinesia concomitant with apoptotic death of striatal dopamine-responsive gamma-aminobutyric acidergic neurons. Chronic inhibition of dopamine synthesis prevents the appearance of motor dysfunction. The neuronal death is associated with overactivation of dopaminergic signaling as evidenced by the robust up-regulation of striatal DeltaFosB, cyclin-dependent kinase 5, and p35. Moreover, hyperphosphorylation of the tau protein, a phenomenon associated with the activation of cyclin-dependent kinase 5 in several neurodegenerative disorders, is observed in symptomatic mice. These findings provide in vivo evidence that, in addition to its proposed role in the degeneration of dopamine neurons, dopamine can also contribute to the selective death of its target neurons via a previously unappreciated mechanism.

Differential regulation by stimulants of neocortical expression of mrt1, arc, and homer1a mRNA in the rats treated with repeated methamphetamine

The present work was conducted to obtain clues for the possible roles of a novel stimulant-inducible gene mrt1 (methamphetamine-responsive transcript 1) encoding a PDZ-PX protein in stimulant-induced behavioral sensitization. In the young adult rats, repeated daily treatment with methamphetamine (4 mg/kg, intraperitoneally, once a day) for 5 days caused an enhanced behavioral response to methamphetamine: behavioral sensitization. The 5-day intermittent administration of MAP upregulated the basal expression of mrt1 transcripts and eliminated the increasing effects of a challenge dose of MAP (1.6 mg/kg, i.p.) or cocaine (30 mg/kg, i.p.) on mrt1 expression on day 14 of withdrawal in the neocortex that has been considered to be composed of a neuron circuit implicated in the sensitization phenomenon. In contrast, the basal expression of other stimulant-inducible and plasticity-related genes arc and homer1a and the ability of MAP or cocaine challenge to augment the amounts of their transcripts were not affected by the repeated MAP regimen in the cortical area. These findings suggest the differential regulation by stimulant of neocortical mrt1, arc, and homer1a expression in the behaviorally sensitized animals and supports the view that stimulant induction of mrt1 may be involved in the early molecular signalings for stimulant sensitization.

Identification of a novel, membrane-associated neuronal kinase, cyclin-dependent kinase 5/p35-regulated kinase

Here we characterize a novel neuronal kinase, cyclin-dependent kinase 5 (cdk5)/p35-regulated kinase (cprk). Cprk is a member of a previously undescribed family of kinases that are predicted to contain two N-terminal membrane-spanning domains and a long C terminus, which harbors a dual-specificity serine/threonine/tyrosine kinase domain. Cprk was isolated in a yeast two-hybrid screen using the neuronal cdk5 activator p35 as "bait." Cprk interacts with p35 in the yeast-two hybrid system, binds to p35 in glutathione S-transferase fusion pull-down assays, and colocalizes with p35 in cultured neurons and transfected cells. In these cells, cprk is present with p35 in the Golgi apparatus. Cprk is expressed in a number of tissues but is enriched in brain and muscle and within the brain is found in a wide range of neuronal populations. Cprk displays catalytic activity in in vitro kinase assays and is itself phosphorylated by cdk5/p35. Cdk5/p35 inhibits cprk activity. Cdk5/p35 may therefore regulate cprk function in the brain.

Cdk5 is essential for synaptic vesicle endocytosis

Synaptic vesicle endocytosis (SVE) is triggered by calcineurin-mediated dephosphorylation of the dephosphin proteins. SVE is maintained by the subsequent rephosphorylation of the dephosphins by unidentified protein kinases. Here, we show that cyclin-dependent kinase 5 (Cdk5) phosphorylates dynamin I on Ser 774 and Ser 778 in vitro, which are identical to its endogenous phosphorylation sites in vivo. Cdk5 antagonists and expression of dominant-negative Cdk5 block phosphorylation of dynamin I, but not of amphiphysin or AP180, in nerve terminals and inhibit SVE. Thus Cdk5 has an essential role in SVE and is the first dephosphin kinase identified in nerve terminals.

Brain levels of CDK5 activator p25 are not increased in Alzheimer's or other neurodegenerative diseases with neurofibrillary tangles

Elevated levels of p25 and constitutive activation of CDK5 have been observed in AD brains. This has led to the hypothesis that increased p25 levels could promote neurofibrillary tangles (NFT) through CDK5-mediated hyperphosphorylation of tau, the principal component of NFTs. We examined p25 immunoreactivity in brains from sporadic and familial AD cases, as well as other neurologic diseases that exhibit NFT, such as Down's syndrome (DS), Pick's disease (Pick), corticobasal degeneration (CBD), progressive supranuclear palsy (PSP), frontotemporal dementia (FTD). Neither the p25 immunoreactivity nor the p25/p35 ratio was elevated in the AD brains or in the other tauopathies (n = 34) compared with controls (n = 11). Although Abeta peptides have been suggested to activate calpain-mediated cleavage of p35 to p25 in cultured neurons, p25 levels in brains of TgCRND8 mice, which express high levels of brain Abeta peptides, were similar to those of non-Tg littermates. Our data suggest that high Abeta levels in brain do not activate p35 proteolysis, and p25 is unlikely to be a causative agent for NFT formation in AD or other tauopathies.

The reinstatement model of drug relapse: history, methodology and major findings

RATIONAL AND OBJECTIVES

The reinstatement model is currently used in many laboratories to investigate mechanisms underlying relapse to drug seeking. Here, we review briefly the history of the model and describe the different procedures that have been used to study the phenomenon of reinstatement of drug seeking. The results from studies using pharmacological and neuroanatomical techniques to determine the neuronal events that mediate reinstatement of heroin, cocaine and alcohol seeking by acute priming injections of drugs, drug-associated cues and environmental stressors are summarized. In addition, several issues are discussed, including (1) the concordance between the neuronal mechanisms involved in drug-induced reinstatement and those involved in drug reward and discrimination, (2) the role of drug withdrawal states and periods in reinstatement of drug seeking, (3) the role of neuronal adaptations induced by exposure to drugs in relapse, and (4) the degree to which the rat reinstatement model provides a suitable preclinical model of relapse to drug taking.

CONCLUSIONS

The data derived from studies using the reinstatement model suggest that the neuronal events that mediate drug-, cue- and stress-induced reinstatement of drug seeking are not identical, that the mechanisms underlying drug-induced reinstatement are to some degree different from those mediating drug discrimination or reward, and that the duration of the withdrawal period following cocaine and heroin self-administration has a profound effect on reinstatement induced by drug cues and stress. Finally, there appears to be a good correspondence between the events that induce reinstatement in laboratory animals and those that provoke relapse in humans.

Amphetamine withdrawal modulates FosB expression in mesolimbic dopaminergic target nuclei: effects of different schedules of administration

Different patterns of psychostimulant intake can elicit widely varying behavioral and neurochemical consequences. Accordingly, rats were studied during withdrawal from either of two schedules of amphetamine administration, one consisting of 6 days of low-dose (1.5 mg/kg, i.p.) daily intermittent (INT) amphetamine (AMPH) injections, and the other of 6 days of moderately high-dose (1-5 mg/kg, i.p.) escalating (ESC) AMPH injections, for the effects of these treatments on numbers of FosB-positive nuclei and monoamine utilization in dopaminergic target areas. Withdrawal from AMPH pretreatment according to the ESC schedule markedly increased FosB expression in the nucleus accumbens shell and basolateral amygdala. In contrast, withdrawal from INT-AMPH administration did not increase FosB expression in any of the regions examined. Post-mortem neurochemical analyses of these same brain regions did not reveal effects of withdrawal from either INT or ESC administration of AMPH. These results suggest that withdrawal from a moderately high-dose AMPH regimen modifies patterns of gene expression in mesocorticolimbic dopaminergic target nuclei without significantly affecting basal monoamine levels. The strength of these effects in the nucleus accumbens shell and basolateral nucleus of the amygdala are consistent with behavioral and clinical data indicating the importance of these areas in the neuroadaptive changes which characterize addiction and withdrawal states.

Molecular neuroadaptations in the accumbens and ventral tegmental area during the first 90 days of forced abstinence from cocaine self-administration in rats

Cocaine self-administration is associated with a propensity to relapse in humans and reinstatement of drug seeking in rats after prolonged withdrawal periods. These behaviors are hypothesized to be mediated by molecular neuroadaptations within the mesolimbic dopamine system. However, in most studies of drug-induced neuroadaptations, cocaine was experimenter-delivered and molecular measurements were performed after short withdrawal periods. In the present study, rats were trained to self-administer intravenous cocaine or oral sucrose (a control non-drug reward) for 10 days (6-h/day) and were killed following 1, 30, or 90 days of reward withdrawal. Tissues from the accumbens and ventral tegmental area (VTA) were assayed for candidate molecular neuroadaptations, including enzyme activities of cAMP-dependent protein kinase (PKA) and adenylate cyclase (AC), and protein expression of cyclin-dependent kinase 5 (cdk5), tyrosine hydroxylase (TH) and glutamate receptor subunits (GluR1, GluR2 and NMDAR1). In the accumbens of cocaine-trained rats, GluR1 and NMDAR1 levels were increased on days 1 and 90, while GluR2 levels were increased on days 1 and 30, but not day 90; PKA activity levels were increased on days 1 and 30, but not day 90, while AC activity, TH and cdk5 levels were unaltered. In the VTA of cocaine-trained rats, NMDAR1 levels were increased for up to 90 days, while GluR2 levels were increased only on day 1; TH and Cdk5 levels were increased only on day 1, while PKA and AC activity levels were unaltered. Cocaine self-administration produces long-lasting molecular neuroadaptations in the VTA and accumbens that may underlie cocaine relapse during periods of abstinence.

Regulation of contextual fear conditioning by baseline and inducible septo-hippocampal cyclin-dependent kinase 5

In this work, we confirm the novel role of cyclin-dependent kinase (Cdk) 5 in associative learning by demonstrating that injection of the Cdk5 inhibitor butyrolactone I into the lateral septum or hippocampus profoundly impaired context-dependent fear conditioning of C57BL/6J mice. However, unlike the inducible up-regulation of Cdk5 and its regulator p35 observed in Balb/c mice, high baseline levels, which were not affected by fear conditioning, were found in C57BL/6J mice. Surprisingly, microinjections of butyrolactone I into the lateral septum or hippocampus significantly decreased baseline Cdk5 activity within the entire septo-hippocampal circuitry, suggesting a functional link between septal and hippocampal Cdk5 activity. Significantly higher levels of the transcription factor Sp4 in the septo-hippocampal system of C57BL/6J mice may account for the high baseline Cdk5/p35 production. On the other hand, the stronger cFos production observed in the lateral septum of fear conditioned Balb/c mice may be responsible, at least in part, for the inducible up-regulation of Cdk5 in this strain. These results suggest that the role of Cdk5 in memory consolidation is strain independent and functionally related to the septo-hippocampal circuitry. However, the molecular regulation of baseline and inducible Cdk5 protein might be different among individual mouse strains and possibly other species.

Downregulation of neuronal cdk5/p35 in opioid addicts and opiate-treated rats: relation to neurofilament phosphorylation

Neuronal cyclin-dependent kinase-5 (Cdk5) and its neuron-specific activator p35 play a major role in regulating the cytoskeleton dynamics. Since opioid addiction was associated with hyperphosphorylation of neurofilament (NF) in postmortem human brains, this study was undertaken to assess the status of the cdk5/p35 complex and its relation with NF-H phosphorylation in brains of chronic opioid abusers. Decreased immunodensities of cdk5 (18%) and p35 (26-44%) were found in the prefrontal cortex of opioid addicts compared with matched controls. In the same brains, the densities of p25 (a truncated neurotoxic form of p35), phosphatase PP2Ac and mu-calpain were found unaltered. Acute treatment of rats with morphine (30 mg/kg, 2 h) increased the density of cdk5 (35%), but not that of p35, in the cerebral cortex. In contrast, chronic morphine (10-100 mg/kg for 5 days) induced marked decreases in cdk5 (40%) and p35 (47%) in rat brain. In brains of opioid addicts, the density of phosphorylated NF-H was increased (43%) as well as the ratio of phosphorylated to nonphosphorylated NF-H forms (two-fold). In these brains, phosphorylated NF-H significantly correlated with p35 (r=0.58) but not with cdk5 (r=0.03). The results suggest that opiate addiction is associated with downregulation of cdk5/p35 levels in the brain. This downregulation and the aberrant hyperphosphorylation of NF-H proteins might have important consequences in the development of neural plasticity associated with opiate addiction in humans.

Inducible, brain region-specific expression of a dominant negative mutant of c-Jun in transgenic mice decreases sensitivity to cocaine

Administration of cocaine induces the Fos family of transcription factors in the striatum, including the nucleus accumbens (NAc), a brain region important for the rewarding effects of addictive drugs. Several Fos proteins are induced acutely by cocaine, with stable isoforms of DeltaFosB predominating after chronic drug administration. However, it has been difficult to study the functional consequences of these Fos responses in vivo. Fos proteins heterodimerize with members of the Jun family to form active AP-1 transcription factor complexes. In the present study, we took advantage of this property and generated transgenic mice, using the tetracycline gene regulation system, that support the inducible, brain region-specific expression of a dominant negative mutant form of c-Jun (Deltac-Jun), which can antagonize the actions of Fos proteins. Expression of Deltac-Jun in the striatum and certain other brain regions of adult mice decreases their development of cocaine-induced conditioned place preference, suggesting reduced sensitivity to the rewarding effects of cocaine. In contrast, Deltac-Jun expression had no effect on cocaine-induced locomotor activity or sensitization. However, expression of Deltac-Jun in adult mice blocked the ability of chronic cocaine administration to induce three known targets for AP-1 in the NAc: the AMPA glutamate receptor subunit GluR2, the cyclin-dependent protein kinase Cdk5, and the transcription factor nuclear factor-kappaB (NFkappaB), without affecting several other proteins examined for comparison. Taken together, these results provide further support for an important role of AP-1-mediated transcription in some of the behavioral and molecular mechanisms underlying cocaine addiction.

Striatal cell type-specific overexpression of DeltaFosB enhances incentive for cocaine

The transcription factor DeltaFosB accumulates in substance P-dynorphin-containing striatal neurons with repeated cocaine use. Here, we show that inducible transgenic DeltaFosB overexpression in this same striatal cell type facilitates acquisition of cocaine self-administration at low-threshold doses, consistent with increased sensitivity to the pharmacological effects of the drug. Importantly, DeltaFosB also enhances the degree of effort mice will exert to maintain self-administration of higher doses on a progressive ratio schedule of reinforcement, whereas levels of cocaine intake are not altered on less demanding fixed-ratio schedules. Acquisition and extinction of behavior reinforced by food pellets is not altered in DeltaFosB-overexpressing mice, indicating that DeltaFosB does not alter the capacity to learn an instrumental response or cause response perseveration in the absence of reinforcement. These data suggest that accumulation of DeltaFosB contributes to drug addiction by increasing the incentive properties of cocaine, an effect that could increase the risk for relapse long after cocaine use ceases.

A developmentally regulated and psychostimulant-inducible novel rat gene mrt1 encoding PDZ-PX proteins isolated in the neocortex

Single or repeated exposure to psychostimulants such as amphetamines and cocaine after postnatal week 3 leads to an enduring enhancement in the psychotomimetic responses elicited by a subsequent challenge of a stimulant in rodents. This behavioral sensitization phenomenon has been considered to be the neural consequences of stimulant-induced alterations in gene expression in the brain after a critical period of postnatal development. Using a differential cloning technique, RNA arbitrarily primed PCR, we have now identified from the rat neocortex a novel and developmentally regulated methamphetamine (MAP)-inducible gene mrt1 (MAP responsive transcript 1). mrt1 encodes two major types of PDZ- and PX-domains containing proteins of approximately 62 kDa in size with different carboxy termini, Mrt1a and Mrt1b. The mrt1 mRNAs for Mrt1a, mrt1a, and for Mrt1b, mrt1b, are predominantly expressed in various brain regions and the testes, respectively. Acute MAP injection upregulated mrt1b expression in the neocortex after postnatal week 3 in a D1 receptor antagonist-sensitive manner without affecting mrt1a expression. This upregulation was mimicked by another stimulant, cocaine, whereas pentobarbital and D1 antagonist failed to change the mrt1b transcript levels. Moreover, repeated daily treatment of MAP, but not MAP plus D1 antagonist, for 5 days caused an augmentation of the basal expression of mrt1b 2 and 3 weeks after the drug discontinuation. These late-developing, cocaine-crossreactive, D1 antagonist-sensitive and long-term regulations of mrt1b by MAP are similar to the pharmacological profiles of stimulant-induced behavioral sensitization, and therefore may be associated with the initiation and/or maintenance of the long-term neuronal adaptation.

Cyclin-dependent kinase 5 (CDK5) and neuronal cell death

Many neurological disorders like Parkinson's and Alzheimer's disease, amyotrophic lateral sclerosis (ALS) or stroke have in common a definite loss of CNS neurons due to apoptotic or necrotic neuronal cell death. Previous studies suggested that proapoptotic stimuli may trigger an abortive and, therefore, eventually fatal cell cycle reentry in postmitotic neurons. Neuroprotective effects of small molecule inhibitors of cyclin-dependent kinases (CDKs), which are key regulators of cell cycle progression, support the cell cycle theory of neuronal apoptosis. However, growing evidence suggests that deregulated CDK5, which is not involved in cell cycle control, rather than cell cycle relevant members of the CDK family, promotes neuronal cell death. Here we summarize the current knowledge about the involvement of CDK5 in neuronal cell death and discuss possible up- or downstream partners of CDK5. Moreover, we discuss potential therapeutic options that might arise from the identification of CDK5 as an important upstream element of neuronal cell death cascades.

Cocaine-induced proliferation of dendritic spines in nucleus accumbens is dependent on the activity of cyclin-dependent kinase-5

Repeated exposure to cocaine produces an enduring increase in dendritic spine density in adult rat nucleus accumbens. It has been shown previously that chronic cocaine administration increases the expression of cyclin-dependent kinase-5 in this brain region and that this neuronal protein kinase regulates cocaine-induced locomotor activity. Moreover, cyclin-dependent kinase-5 has been implicated in neuronal function and synaptic plasticity. Therefore, we studied the involvement of this enzyme in cocaine's effect on dendritic spine density. Adult male rats, receiving intra-accumbens infusion of the cyclin-dependent kinase-5 inhibitor roscovitine or saline, were administered a 28-day cocaine treatment regimen. Animals were killed 24-48 h after the final cocaine injection and their brains removed and processed for Golgi-Cox impregnation. Our findings demonstrate that roscovitine attenuates cocaine-induced dendritic spine outgrowth in nucleus accumbens core and shell and such inhibition reduces spine density in nucleus accumbens shell of control animals. These data indicate that cyclin-dependent kinase-5 is involved in regulation of, as well as cocaine-induced changes in, dendritic spine density.

Repeated exposure to amphetamine disrupts dopaminergic modulation of excitatory synaptic plasticity and neurotransmission in nucleus accumbens

The mesolimbic dopamine system is essential for reward-seeking behavior, and drugs of abuse perturb the normal functioning of this pathway. The nucleus accumbens (NAc) is a major terminal field of the mesolimbic dopamine neurons and modifications in neuronal structure and function in NAc accompany repeated exposure to psychomotor stimulants and other addictive drugs. Glutamatergic afferents to the NAc are thought to be crucial to the development of several aspects of addictive behavior, including behavioral sensitization and relapse to cocaine self-administration. Here we examine glutamatergic neurotransmission and synaptic plasticity in NAc neurons in vitro before and after repeated amphetamine treatment in vivo. We find that dopamine attenuates the response of NAc neurons to repetitive activation of glutamatergic afferents and thereby blocks long-term potentiation (LTP) induced by high-frequency afferent stimulation. Dopamine's effects are mimicked by dopamine receptor agonists and by amphetamine. In a second set of experiments, animals were treated with amphetamine daily for 6 days and brain slices were prepared after 8-10 days of withdrawal. In these slices, LTP in the NAc appears normal. However, acute exposure of such slices to amphetamine no longer modulates synaptic transmission or LTP induction. Thus, repeated exposure to amphetamine produces long-lasting changes in the modulation of glutamatergic synaptic transmission by amphetamine in the NAc. Our results support the notion that after psychostimulant exposure, excitatory synapses on NAc neurons alter their response to further psychostimulant for long periods of time.

Striatal glutamatergic mechanisms and extrapyramidal movement disorders

The nonphysiologic stimulation of striatal dopaminergic receptors, as a result of disease- or drug-related denervation or intermittent excitation, triggers adaptive responses in the basal ganglia which contribute to the appearance of parkinsonian symptoms and later to the dyskinesias and other alterations in motor response associated with dopaminergic therapy. Current evidence suggests that these altered responses involve activation of signal transduction cascades in striatal medium spiny neurons linking dopaminergic to coexpressed ionotropic glutamatergic receptors of the N-methyl-D-aspartate (NMDA) and Alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) classes. These intraneuronal signaling pathways appear capable of modifying the phosphorylation state of NMDA and AMPA receptor subunits; resultant sensitization enhances cortical glutamatergic input which in turn modifies striatal output in ways that compromise motor behavior. The regulation of these spiny neuron glutamate receptors can also be affected by the activation state of coexpressed nondopaminergic receptors as well as by changes associated with Huntington's disease. These observations lend new insight into molecular mechanisms contributing to the integration of synaptic inputs to spiny neurons. They also suggest novel approaches to the pharmacotherapy of extrapyramidal motor dysfunction.

Gene expression profiling: methodological challenges, results, and prospects for addiction research

This review describes the current methods used to profile gene expression. These methods include microarrays, spotted arrays, serial analysis of gene expression (SAGE), and massive parallel signature sequencing (MPSS). Methodological and statistical problems in interpreting microarray and spotted array experiments are also discussed. Methods and formats such as minimum information about microarray experiments (MIAME) needed to share gene expression data are described. The last part of the review provides an overview of the application of gene-expression profiling technology to substance abuse research and discusses future directions.

D1 dopamine receptor stimulation increases GluR1 surface expression in nucleus accumbens neurons

The goal of this study was to understand how dopamine receptors, which are activated during psychostimulant administration, might influence glutamate-dependent forms of synaptic plasticity that are increasingly recognized as important to drug addiction. Regulation of the surface expression of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit GluR1 plays a critical role in long-term potentiation, a well-characterized form of synaptic plasticity. Primary cultures of rat nucleus accumbens neurons were used to examine whether dopamine receptor stimulation influences cell surface expression of GluR1, detected using antibody to the extracellular portion of GluR1 and fluorescence microscopy. Surface GluR1 labeling on processes of medium spiny neurons and interneurons was increased by brief (5-15 min) incubation with a D1 agonist (1 microm SKF 81297). This effect was attenuated by the D1 receptor antagonist SCH 23390 (10 microm) and reproduced by the adenylyl cyclase activator forskolin (10 microm). Labeling was decreased by glutamate (10-50 microm, 15 min). These results are the first to demonstrate modulation of AMPA receptor surface expression by a non-glutamatergic G protein-coupled receptor. Normally, this may enable ongoing regulation of AMPA receptor transmission in response to changes in the activity of dopamine projections to the nucleus accumbens. When dopamine receptors are over-stimulated during chronic drug administration, this regulation may be disrupted, leading to inappropriate plasticity in neuronal circuits governing motivation and reward.

Experimental genetic approaches to addiction

Drugs of abuse are able to elicit compulsive drug-seeking behaviors upon repeated administration, which ultimately leads to the phenomenon of addiction. Evidence indicates that the susceptibility to develop addiction is influenced by sources of reinforcement, variable neuroadaptive mechanisms, and neurochemical changes that together lead to altered homeostasis of the brain reward system. Addiction is hypothesized to be a cycle of progressive dysregulation of the brain reward system that results in the compulsive use and loss of control over drug taking and the initiation of behaviors associated with drug seeking. The view that addiction represents a pathological state of reward provides an approach to identifying the factors that contribute to vulnerability, addiction, and relapse in genetic animal models.

Delta FosB regulates wheel running

DeltaFosB is a transcription factor that accumulates in a region-specific manner in the brain after chronic perturbations. For example, repeated administration of drugs of abuse increases levels of DeltaFosB in the striatum. In the present study, we analyzed the effect of spontaneous wheel running, as a model for a natural rewarding behavior, on levels of DeltaFosB in striatal regions. Moreover, mice that inducibly overexpress DeltaFosB in specific subpopulations of striatal neurons were used to study the possible role of DeltaFosB on running behavior. Lewis rats given ad libitum access to running wheels for 30 d covered what would correspond to approximately 10 km/d and showed increased levels of DeltaFosB in the nucleus accumbens compared with rats exposed to locked running wheels. Mice that overexpress DeltaFosB selectively in striatal dynorphin-containing neurons increased their daily running compared with control littermates, whereas mice that overexpress DeltaFosB predominantly in striatal enkephalin-containing neurons ran considerably less than controls. Data from the present study demonstrate that like drugs of abuse, voluntary running increases levels of DeltaFosB in brain reward pathways. Furthermore, overexpression of DeltaFosB in a distinct striatal output neuronal population increases running behavior. Because previous work has shown that DeltaFosB overexpression within this same neuronal population increases the rewarding properties of drugs of abuse, results of the present study suggest that DeltaFosB may play a key role in controlling both natural and drug-induced reward.

Repeated cocaine self-administration causes multiple changes in rat frontal cortex gene expression

Repeated cocaine administration produces changes in gene expression that are thought to contribute to the behavioral alterations observed with cocaine abuse. This study focuses on gene expression changes in the frontal cortex, a component of reinforcement, sensory, associative, and executive circuitries. Changes in frontal cortex gene expression after repeated cocaine self-administration may lead to changes in the behaviors associated with this brain region. Rats self-administered cocaine for 10 days in a continuous access, discrete trial paradigm (averaging 100 mg/kg/day) and were examined for changes in relative frontal cortex mRNA abundance by cDNA hybridization arrays. Among the changes observed following array analysis, increased nerve-growth-factor-induced B (NGFI-B), adenylyl cyclase type VIII (AC VIII), and reduced cysteine-rich protein 2 (CRP2) mRNA were confirmed by quantitative RT-PCR. These changes share commonalities and exhibit differences with previous reports of gene expression changes in the frontal cortex after noncontingent cocaine administration.

Expression profiling to understand actions of NMDA/glutamate receptor antagonists in rat brain

Agents acting as noncompetitive N-methyl-D-aspartate (NMDA)/glutamate receptor antagonists induce the expression of several genes in limbic cortical regions, such as the cingulate, retrosplenial, and entorhinal cortices. These include important regulatory genes such as the neurotrophin brain-derived neurotrophic factor (BDNF), its receptor trkB, and c-fos. We applied expression profiling methods to find genes coregulated with BDNF following treatment with the prototypical NMDA/glutamate receptor antagonist MK-801. Expression profiling provides a useful technique for describing the molecular and transcriptional level events that follow various processes. We illustrate the utility of microarrays to find novel ESTs regulated by MK-801. We also used expression profiling with microarrays to characterize the levels of transcription factor cAMP response element modulator (CREM) and inducible cAMP early repressor (ICER) isoforms that are induced by MK-801. These factors may act as the eventual repressors for BDNF expression via competition and heterodimerization with phosphorylated CREB, a transcription factor important for BDNF expression. Finally, we find and confirm the regulation of Erp29, RTNI, and an ABC transporter by antagonism of NMDA/glutamate receptors as potential stress related molecules in brain. The emerging picture generated by using these expression profiling approaches, identifies several of what likely will be many molecules that take part in the complex events that occur during BDNF signaling mediated by blockade of NMDA/ glutamate receptors.

Effects of acute "binge" cocaine on preprodynorphin, preproenkephalin, proopiomelanocortin, and corticotropin-releasing hormone receptor mRNA levels in the striatum and hypothalamic-pituitary-adrenal axis of mu-opioid receptor knockout mice

Cocaine administration increases activity at dopamine receptors, increases preprodynorphin (ppDyn) gene expression in the caudate-putamen (CPu), and activates the stress responsive hypothalamic-pituitary-adrenal (HPA) axis. To examine the hypothesis that mu-opioid receptors (MOR) may play roles in these cocaine effects, we tested the effects of acute "binge" pattern cocaine administration in mice with targeted disruption of the MOR gene. Wild-type (+/+) and homozygous MOR-deficient (-/-) mice received three injections of 15 mg/kg cocaine at 1-h intervals. Mice were sacrificed 30 min after the last injection and mRNAs for ppDyn and preproenkephalin (ppEnk) in the CPu and nucleus accumbens (NAc), and for type I corticotropin-releasing hormone receptor (CRH(1) receptor) and pro-opiomelanocortin (POMC) in the hypothalamus and pituitary, were measured by solution hybridization RNase protection assays. Cocaine elevated ppDyn mRNA in the CPu, but not NAc, of both the MOR -/- and wild-type mice. ppEnk mRNA in the CPu, but not NAc, was lower in MOR -/- mice than in wild-type mice following cocaine administration. Hypothalamic CRH(1) receptor and POMC mRNAs were expressed at similar levels in untreated and in cocaine-treated mice of each genotype. However, there were lower basal levels of CRH(1) receptor mRNA in the anterior pituitary of the MOR -/- mice than in wild-type mice and the MOR -/- mice failed to show the cocaine-induced decreases in CRH(1) receptor mRNA found in the wild-type mice. Cocaine activated the HPA axis similarly in MOR -/- and wild-type mice, as reflected in similar increases in plasma corticosterone levels in both genotypes. These results support a specific role for MORs in acute cocaine effects on striatal ppEnk gene expression and fail to support critical roles for these receptors in acute cocaine's effects on either ppDyn gene expression or HPA activation. MOR -/- mice are useful models for studying cocaine effects on ppEnk gene expression that could aid interpretation of the similar postmortem phenomena found in human cocaine addicts.

Pharmacological inhibitors of cyclin-dependent kinases

Cyclin-dependent kinases (CDKs) regulate the cell division cycle, apoptosis, transcription and differentiation in addition to functions in the nervous system. Deregulation of CDKs in various diseases has stimulated an intensive search for selective pharmacological inhibitors of these kinases. More than 50 inhibitors have been identified, among which >20 have been co-crystallized with CDK2. These inhibitors all target the ATP-binding pocket of the catalytic site of the kinase. The actual selectivity of most known CDK inhibitors, and thus the underlying mechanism of their cellular effects, is poorly known. Pharmacological inhibitors of CDKs are currently being evaluated for therapeutic use against cancer, alopecia, neurodegenerative disorders (e.g. Alzheimer's disease, amyotrophic lateral sclerosis and stroke), cardiovascular disorders (e.g. atherosclerosis and restenosis), glomerulonephritis, viral infections (e.g. HCMV, HIV and HSV) and parasitic protozoa (Plasmodium sp. and Leishmania sp.).

Repeated cocaine administration alters the expression of genes in corticolimbic circuitry after a 3-week withdrawal: a DNA macroarray study

Addiction to psychostimulants elicits behavioral and biochemical changes that are assumed to be mediated by alterations of gene expression in the brain. The changes in gene expression after 3 weeks of withdrawal from chronic cocaine treatment were evaluated in the nucleus accumbens core and shell, dorsal prefrontal cortex and caudate using a complementary DNA (cDNA) array. The level of mRNA encoded by several genes was identified as being up- or down-regulated in repeated cocaine versus saline subjects. The results from the cDNA array were subsequently confirmed at the protein level with immunoblotting. Of particular interest, parallel up-regulation in protein and mRNA was found for the adenosine A1 receptor in the accumbens core, neuroglycan C in the accumbens shell, and the GluR5 glutamate receptor subtype in dorsal prefrontal cortex. However, there was an increase in TrkB protein in the nucleus accumbens core of cocaine-treated rats without a corresponding alteration in mRNA. These changes of gene expression in corticolimbic circuitry may contribute to the psychostimulant-induced behavioral changes associated with addiction.

Repeated ventral tegmental area amphetamine administration alters dopamine D1 receptor signaling in the nucleus accumbens

Neuroadaptations of the mesoaccumbens dopamine (DA) system likely underlie the emergence of locomotor sensitization following the repeated intermittent systemic administration of amphetamine (AMPH). In the nucleus accumbens (NAc), such neuroadaptations include enhanced DA overflow in response to a subsequent AMPH challenge as well as increased sensitivity to the inhibitory effects of D1 DA receptor (D1R) activation and an altered profile of D1R-dependent induction of immediate early genes (IEGs). Previous results indicate that AMPH acts in the ventral tegmental area (VTA) to initiate those changes leading to sensitization of the locomotor activity and NAc DA overflow produced by systemic administration of this drug. These observations are intriguing, given that acute infusion of AMPH into the VTA does not stimulate locomotor activity or, as we report presently, increase extracellular NAc DA concentrations. Two experiments, therefore, assessed the ability of repeated VTA AMPH to produce adaptations in D1R signaling in the NAc. Rats were administered three bilateral VTA infusions of saline or AMPH (2.5 microg/0.5 microl/side, one every third day). In the first experiment, in vivo extracellular electrophysiological recordings revealed that previous exposure to VTA AMPH enhanced the sensitivity of NAc neurons to the inhibitory effects of iontophoretic application of the D1R agonist SKF 38393. This effect was observed early (2-3 days) and at 1 month of withdrawal, but not after 2 months. Similarly, in the second experiment it was found that the D1R-dependent induction by AMPH of Fos, FosB, and JunB, but not NGFI-A, in the NAc was enhanced in rats exposed 1 week earlier to repeated VTA AMPH. These findings indicate that repeated VTA AMPH administration initiates relatively long-lasting adaptations in D1R signaling in the NAc that may, together with presynaptic adaptations affecting DA overflow, contribute to the expression of locomotor sensitization by this drug.

Changes in rat frontal cortex gene expression following chronic cocaine

Alterations in gene expression caused by repeated cocaine administration have been implicated in the long-term behavioral aspects of cocaine abuse. The frontal cortex mediates reinforcement, sensory, associative, and executive functions and plays an important role in the mesocortical dopamine reinforcement system. Repeated cocaine administration causes changes in frontal cortex gene expression that may lead to changes in the behaviors subserved by this brain region. Rats treated non-contingently with a binge model of cocaine (45 mg/kg/day, i.p.) for 14 days were screened for changes in relative mRNA abundance in the frontal cortex by cDNA hybridization arrays. To confirm changes, immunoreactive protein was measured (via protein-specific immunoblots) in a second group of identically-treated animals. Protein levels of protein tyrosine kinase 2 (PYK2), activity-regulated cytoskeletal protein (ARC), as well as an antigen related to nerve growth factor I-B (NGFI-B-RA) were shown to be significantly induced after cocaine administration. Levels of NGFI-B mRNA were confirmed by real-time RT-PCR to be increased with cocaine administration. These observations are similar to previously reported cocaine-responsive changes in gene expression but novel to the frontal cortex. This study also validates the use of hybridization arrays for screening of neuronal gene expression changes and the utility of relative protein quantification as a post-hoc confirmation tool.

D(1) dopamine receptor stimulation increases GluR1 phosphorylation in postnatal nucleus accumbens cultures

Postsynaptic interactions between dopamine and glutamate receptors in the nucleus accumbens are critical for acute responses to drugs of abuse and for neuroadaptations resulting from their chronic administration. We tested the hypothesis that D(1) dopamine receptor stimulation increases phosphorylation of the AMPA receptor subunit GluR1 at the protein kinase A phosphorylation site (Ser845). Nucleus accumbens cell cultures were prepared from postnatal day 1 rats. After 14 days in culture, GluR1 phosphorylation was measured by western blotting using phosphorylation site-specific antibodies. The D(1) receptor agonist SKF 81297 increased Ser845 phosphorylation in a concentration- dependent manner, with marked increases occurring within 5 min. This was prevented by the D(1) receptor antagonist SCH 23390 and the protein kinase A inhibitor H89, and reproduced by forskolin. The D(2) receptor agonist quinpirole attenuated the response to D(1) receptor stimulation. Neither D(1) nor D(2) receptor agonists altered GluR1 phosphorylation at Ser831, the site phosphorylated by protein kinase C and calcium/calmodulin-dependent protein kinase II. In other systems, phosphorylation of GluR1 at Ser845 is associated with enhancement of AMPA receptor currents. Thus, the present results suggest that AMPA receptor transmission in the nucleus accumbens may be augmented by concurrent D(1) receptor stimulation.

Stimulant-induced psychosis and schizophrenia: the role of sensitization

Three different conditions, psychostimulant-induced behavioral sensitization in rodents, psychostimulant-induced psychoses in human, and chronic schizophrenia show similar longitudinal alternations, progressively enhanced susceptibility to abnormal behaviors, psychotic state, and relapse. Sensitization phenomena to the drugs or endogenous dopamine should be involved in the mechanisms underlying the development of such susceptibility. Recently, an enhanced dopamine release in vivo by amphetamine administration in the striatum has been shown in schizophrenics, which is a replication of that previously proven in the behavioral sensitization in rats. Accordingly, common molecular mechanisms of sensitization phenomena must develop in these three conditions, and are overviewed in this review

Functional genomics in neuropsychiatric disorders and in neuropharmacology

The rapidly accumulating amount of information concerning gene and protein expression patterns produced by functional genomics, proteomics and bioinformatics is presently providing new targets for drug development. Furthermore, the analysis of gene expression in cells and tissues affected by a disease may reveal the underlying metabolic pathways and cellular processes affected. Finally, changes in gene expression may be used in either diagnostics or the monitoring of drug responses. This review focuses on advances in the use of functional genomics in neurological and neuropsychiatric diseases and neuropsychopharmacology. Although the number of published studies in this field is still limited, it already appears that this strategy may become a fruitful means in the analysis of the aetiology of neuropsychiatric disorders and the search for novel neuropharmacological drugs.

Cyclin-dependent kinase 5 is required for associative learning

Transient stressful experiences may persistently facilitate associative and nonassociative learning, possibly through alterations of gene expression. Here we identify, by subtractive hybridization, differential expression of the Cdk5 gene in response to stress. The Cdk5 protein is selectively induced in the fibers of septohippocampal cholinergic neurons but not in other regions of prominent Cdk5 production. This upregulation is accompanied by increased Cdk5 kinase activity, which is blocked completely by the Cdk5 inhibitor butyrolactone I. Microinjection of butyrolactone I into the lateral septum and hippocampus prevents the acquisition of conditioned context-dependent fear as well as its stress-induced facilitation. By demonstrating that a transient increase of Cdk5 activity within the septohippocampal system is required for associative learning, an important novel role of Cdk5 has been identified.

Calpain-mediated cleavage of the cyclin-dependent kinase-5 activator p39 to p29

The activity of cyclin-dependent kinase-5 (Cdk5) is tightly regulated by binding of its neuronal activators p35 and p39. Upon neurotoxic insults, p35 is cleaved to p25 by the Ca(2+)-dependent protease calpain. p25 is accumulated in ischemic brains and in brains of patients with Alzheimer's disease. p25 deregulates Cdk5 activity by causing prolonged activation and mislocalization of Cdk5. It is unknown whether p39, which is expressed throughout the adult rat brain, is cleaved by calpain, and whether this contributes to deregulation of Cdk5. Here, we show that calpain cleaved p39 in vitro, resulting in generation of a C-terminal p29 fragment. In vivo, p29 was generated in ischemic brain concomitant with increased calpain activity. In fresh brain lysates, generation of p29 was Ca(2+)-dependent, and calpain inhibitors abolished p29 production. The Ca(2+) ionophore ionomycin and the excitotoxin glutamate induced production of p29 in cultures of cortical neurons in a calpain-dependent manner. Like p25, p29 was more stable than p39 and caused redistribution of Cdk5 in cortical neurons. Our data suggest that neurotoxic insults lead to calpain-mediated conversion of p39 to p29, which might contribute to deregulation of Cdk5.

Cocaine-responsive gene expression changes in rat hippocampus

Chronic cocaine use is known to elicit changes in the pattern of gene expression within the brain. The hippocampus plays a critical role in learning and memory and may also play a role in mediating behaviors associated with cocaine abuse. To profile the gene expression response of the hippocampus to chronic cocaine treatment, cDNA hybridization arrays were used to illuminate cocaine-regulated genes in rats treated non-contingently with a binge model of cocaine (45 mg/kg/day, i.p.) for 14 days. Validation of mRNA changes illuminated by hybridization array analysis was accomplished by measuring immunoreactive protein (via specific immunoblots). The induction of protein kinase Calpha, potassium channel 1.1, and metabotropic glutamate receptor 5 seen by hybridization arrays was confirmed at the level of protein. Immunoblot screening of previously described cocaine-responsive genes demonstrated increased levels of protein tyrosine kinase 2, beta-catenin, and protein kinase Cepsilon. While some of these changes exist in previously described cocaine-responsive models, others are novel to any model of cocaine use. The inductions of potassium channel 1.1, protein tyrosine kinase 2 and metabotropic glutamate receptor 5 are novel findings to hippocampal cocaine-responsive gene expression. These proteins have been shown to subserve learning and memory and/or long-term potentiation functions within the hippocampus. Additionally, these genes are known to interact with one another, forming a more complex pattern of gene expression changes. The findings suggest altered expression of genes with a number of different functions in the rat hippocampus after a 'binge' style of non-contingent cocaine administration. These changes in gene expression may play roles in neuronal plasticity and the behavioral phenomena associated with cocaine abuse.

GC box-binding transcription factors control the neuronal specific transcription of the cyclin-dependent kinase 5 regulator p35

Cyclin-dependent kinase 5 (cdk5)/p35 kinase activity is highest in post-mitotic neurons of the central nervous system and is critical for development and function of the brain. The neuronal specific activity of the cdk5/p35 kinase is achieved through the regulated expression of p35 mRNA. We have identified a small 200-bp fragment of the p35 promoter that is sufficient for high levels of neuronal specific expression. Mutational analysis of this TATA-less promoter has identified a 17-bp GC-rich element, present twice, that is both required for promoter activity and sufficient for neuronal specific transcription. A GC box within the 17-bp element is critical for both promoter activity and protein-DNA complex formation. The related transcription factors Sp1, Sp3, and Sp4 constitute most of the GC box DNA binding activity in neurons. We have found that both the relative contribution of the Sp family proteins to GC box binding and the transcriptional activity of these proteins is regulated during neuronal differentiation. Thus, our data show that the GC box-binding Sp proteins contribute to the regulation of p35 expression in neurons, suggesting changes in the Sp transcription factors level and activity may contribute to cell type-specific expression of many genes in the central nervous system.

Prenatal exposure to cocaine disrupts D1A dopamine receptor function via selective inhibition of protein phosphatase 1 pathway in rabbit frontal cortex

Previous work has demonstrated that in utero cocaine exposure induces an uncoupling of brain D(1A) dopamine receptors (D(1A)DARs) from G(s)-protein. The present work is an attempt to define the mechanism underlying the uncoupling. We detected a significant elevation of phosphoserine in frontal cortical D(1A)DARs of rabbits that were exposed prenatally to cocaine compared with saline controls. This increase in phosphorylation is observed at gestational day 22 and persists to postnatal day 20. The hyperphosphorylation of the D(1A)DAR is accompanied by a 45% inhibition in frontal cortex (FCX) protein phsphatase-1 (PP1) activity that appears to be mediated via DARPP-32 (dopamine and cAMP-regulated phosphoprotein) as indicated by elevated FCX phospho-DARPP-32 (Thr(34)). Furthermore, we demonstrated in both FCX and in PC2 cells that express D(1A)DARs that PP1 is physically associated with D(1A)DARs. We also observed a dramatic decrease in D(1A)DAR-associated PP1 activity in FCX of prenatal cocaine-exposed rabbits, indicating that the reduction in PP1 activity may be responsible for the hyperphosphorylation of the receptor. Furthermore, pretreatment of cortical membranes obtained from cocaine-exposed animals with exogenous PP1 dephosphorylated the phosphorylated D(1A)DAR and significantly reversed the impaired receptor-G(alphas) coupling. This work indicates (1) that D(1A)DAR dephosphorylation via PP1 is essential for receptor resensitization or reactivation and (2) an alteration in the DARPP-32/PP1 cascade appears to be a primary event responsible for D(1A)DAR dysfunction in in utero cocaine-exposed rabbit progeny. The present finding of an altered DARPP-32/PP1 cascade in association with a dysfunction in D(1A)DAR signal transmission in the prenatal cocaine-exposed rabbit brain may implicate novel strategies for the prevention and treatment for in utero cocaine-induced developmental and behavioral abnormalities.

Cdk5 sinks into ALS

Recent research points to an involvement of deregulated cdk5 activity in the pathogenesis of mutant SOD1-mediated disease. In addition, inhibition of this activity might promote motor neuron survival. These observations have opened the door to further research into the role of cdk5 in ALS and other neurodegenerative diseases.

Cdk5 behind the wheel: a role in trafficking and transport?

Cdk5, a serine/threonine kinase in the cyclin-dependent kinase (Cdk) family, is an important regulator of neuronal positioning during brain development. Cdk5 might also play a role in synaptogenesis and neurotransmission. Loss of Cdk5 in mice is perinatal lethal, and overactive Cdk5 induces apoptosis in cultured cells, indicating that strict regulation of kinase activity is crucial. Indeed, activity depends on the stability of activating partners, subcellular localization and the phosphorylation state of the enzyme itself. Deregulated kinase activity has been linked to neurodegenerative diseases such as Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS). This review focuses on links between Cdk5 activity and components of cytoskeletal, membrane and adhesion systems that allow us to postulate a role for Cdk5 in directing intracellular traffic in neurons.

Psychogenomics: opportunities for understanding addiction

The term psychogenomics is used here to describe the process of applying the powerful tools of genomics and proteomics to achieve a better understanding of the biological substrates of normal behavior and of diseases of the brain that manifest themselves as behavioral abnormalities. Applying psychogenomics to the study of drug addiction will lead to the identification of genes and their protein products that control the reward pathways of the brain and their adaptations to drugs of abuse, as well as variations in these genes that confer genetic risk for addiction and related disorders. The ultimate goal is to use this information to develop more effective treatments for these disorders as well as objective diagnostic tools, preventive measures, and eventually cures.

Long-term depression in the nucleus accumbens: a neural correlate of behavioral sensitization to cocaine

A compelling model of experience-dependent plasticity is the long-lasting sensitization to the locomotor stimulatory effects of drugs of abuse. Adaptations in the nucleus accumbens (NAc), a component of the mesolimbic dopamine system, are thought to contribute to this behavioral change. Here we examine excitatory synaptic transmission in NAc slices prepared from animals displaying sensitization 10-14 days after repeated in vivo cocaine exposure. The ratio of AMPA (alpha-amino-3-hydroxy-5-methyl-4- isoxazole propionic acid) receptor- to NMDA (N-methyl-d-aspartate) receptor-mediated excitatory postsynaptic currents (EPSCs) was decreased at synapses made by prefrontal cortical afferents onto medium spiny neurons in the shell of the NAc. The amplitude of miniature EPSCs at these synapses also was decreased, as was the magnitude of long-term depression. These data suggest that chronic in vivo administration of cocaine elicits a long-lasting depression of excitatory synaptic transmission in the NAc, a change that may contribute to behavioral sensitization and addiction.

The neurobiology of slow synaptic transmission

Nerve cells communicate with each other through two mechanisms, referred to as fast and slow synaptic transmission. Fast-acting neurotransmitters, e.g., glutamate (excitatory) and gamma-aminobutyric acid (GABA) (inhibitory), achieve effects on their target cells within one millisecond by virtue of opening ligand-operated ion channels. In contrast, all of the effects of the biogenic amine and peptide neurotransmitters, as well as many of the effects of glutamate and GABA, are achieved over hundreds of milliseconds to minutes by slow synaptic transmission. This latter process is mediated through an enormously more complicated sequence of biochemical steps, involving second messengers, protein kinases, and protein phosphatases. Slow-acting neurotransmitters control the efficacy of fast synaptic transmission by regulating the efficiency of neurotransmitter release from presynaptic terminals and by regulating the efficiency with which fast-acting neurotransmitters produce their effects on postsynaptic receptors.