Cocaine increases actin cycling: effects in the reinstatement model of drug seeking

Neuronal pentraxins modulate cocaine-induced neuroadaptations

Neuronal pentraxins (NPs) function in the extracellular matrix to bind alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Three NPs have been described, neuronal activity-regulated pentraxin (Narp), which is regulated as an immediate early gene, NP1, and neuronal pentraxin receptor (NPR). Narp and NP1 enhance synaptogenesis and glutamate signaling by clustering AMPA receptors, whereas NPR contributes to removing AMPA receptors during group I metabotropic glutamate receptor-dependent long-term depression. Here, we examine mice with genetic deletions [knockout (KO)] of each NP to assess their contributions to cocaine-induced neuroplasticity. Consistent with a shared AMPA receptor clustering function for Narp and NP1, deletion of either NP caused similar behavioral alterations. Thus, although both Narp and NP1 deletion promoted cocaine-induced place preference, NPR deletion was without effect. In addition, although Narp and NP1 KO showed reduced time in the center of a novel environment, NPR KO mice spent more time in the center. Finally, although Narp and NP1 KO mice showed blunted locomotion after AMPA microinjection into the accumbens 3 weeks after discontinuing repeated cocaine injections, the AMPA response was augmented in NPR KO. Likewise, endogenous glutamate release elicited less motor activity in Narp KO mice. Consistent with reduced AMPA responsiveness after chronic cocaine in Narp KO mice, glutamate receptor 1 was reduced in the PSD fraction of Narp KO mice withdrawn from cocaine. These data indicate that NPs differentially contribute to cocaine-induced plasticity in a manner that parallels their actions in synaptic plasticity.

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.

Limbic and cortical information processing in the nucleus accumbens

The nucleus accumbens regulates goal-directed behaviors by integrating information from limbic structures and the prefrontal cortex. Here, we review recent studies in an attempt to provide an integrated view of the control of information processing in the nucleus accumbens in terms of the regulation of goal-directed behaviors and how disruption of these functions might underlie the pathological states in drug addiction and other psychiatric disorders. We propose a model that could account for the results of several studies investigating limbic-system interactions in the nucleus accumbens and their modulation by dopamine and provide testable hypotheses for how these might relate to the pathophysiology of major psychiatric disorders.

Drosophila, a genetic model system to study cocaine-related behaviors: a review with focus on LIM-only proteins

In the last decade, the fruit fly Drosophila melanogaster, highly accessible to genetic, behavioral and molecular analyses, has been introduced as a novel model organism to help decipher the complex genetic, neurochemical, and neuroanatomical underpinnings of behaviors induced by drugs of abuse. Here we review these data, focusing specifically on cocaine-related behaviors. Several of cocaine's most characteristic properties have been recapitulated in Drosophila. First, cocaine induces motor behaviors in flies that are remarkably similar to those observed in mammals. Second, repeated cocaine administration induces behavioral sensitization a form of behavioral plasticity believed to underlie certain aspects of addiction. Third, a key role for dopaminergic systems in mediating cocaine's effects has been demonstrated through both pharmacological and genetic methods. Finally, and most importantly, unbiased genetic screens, feasible because of the simplicity and scale with which flies can be manipulated in the laboratory, have identified several novel genes and pathways whose role in cocaine behaviors had not been anticipated. Many of these genes and pathways have been validated in mammalian models of drug addiction. We focus in this review on the role of LIM-only proteins in cocaine-induced behaviors.

Glutamate transmission in addiction

Cortico-striatal glutamate transmission has been implicated in both the initiation and expression of addiction related behaviors, such as locomotor sensitization and drug-seeking. While glutamate transmission onto dopamine cells in the ventral tegmental area undergoes transient plasticity important for establishing addiction-related behaviors, glutamatergic plasticity in the nucleus accumbens is critical for the expression of these behaviors. This information points to the value of exploring pharmacotherapeutic manipulation of glutamate plasticity in treating drug addiction.

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.

Neuroplasticity in the mesolimbic dopamine system and cocaine addiction

The main characteristics of cocaine addiction are compulsive drug use despite adverse consequences and high rates of relapse during periods of abstinence. A current popular hypothesis is that compulsive cocaine use and cocaine relapse is due to drug-induced neuroadaptations in reward-related learning and memory processes, which cause hypersensitivity to cocaine-associated cues, impulsive decision making and abnormal habit-like learned behaviours that are insensitive to adverse consequences. Here, we review results from studies on the effect of cocaine exposure on selected signalling cascades, growth factors and physiological processes previously implicated in neuroplasticity underlying normal learning and memory. These include the extracellular signal-regulated kinase (ERK) signalling pathway, brain-derived neurotrophic factor (BDNF), glutamate transmission, and synaptic plasticity (primarily in the form of long-term potentiation and depression, LTP and LTD). We also discuss the degree to which these cocaine-induced neuroplasticity changes in the mesolimbic dopamine system mediate cocaine psychomotor sensitization and cocaine-seeking behaviours, as assessed in animal models of drug addiction. Finally, we speculate on how these factors may interact to initiate and sustain cocaine psychomotor sensitization and cocaine seeking.

Cocaine and amphetamine-like psychostimulants: neurocircuitry and glutamate neuroplasticity

Although the pharmacology of amphetamine-like psychostimulants at dopamine transporters is well understood, addiction to this class of drugs has proven difficult to deal with. The reason for this disconnection is that while the molecular mechanism of amphetamine action is critical to reinforce drug use, it is only the first step in a sequence of widespread neuroplastic events in brain circuitry. This review outlines the affect of psychostimulants on mesocorticolimbic dopamine projections that mediate their reinforcing effect, and how this action ultimately leads to enduring pathological neuroplasticity in glutamatergic projections from the prefrontal cortex to the nucleus accumbens. Molecular neuroadaptations induced by psychostimulant abuse are described in glutamate neurotransmission, and from this information potential pharmacotherapeutic targets are identified, based upon reversing or countermanding psychostimulant-induced neuroplasticity.

Homers regulate drug-induced neuroplasticity: implications for addiction

Drug addiction is a chronic, relapsing disorder, characterized by an uncontrollable motivation to seek and use drugs. Converging clinical and preclinical observations implicate pathologies within the corticolimbic glutamate system in the genetic predisposition to, and the development of, an addicted phenotype. Such observations pose cellular factors regulating glutamate transmission as likely molecular candidates in the etiology of addiction. Members of the Homer family of proteins regulate signal transduction through, and the trafficking of, glutamate receptors, as well as maintain and regulate extracellular glutamate levels in corticolimbic brain regions. This review summarizes the existing data implicating the Homer family of protein in acute behavioral and neurochemical sensitivity to drugs of abuse, the development of drug-induced neuroplasticity, as well as other behavioral and cognitive pathologies associated with an addicted state.

Drug addiction as a pathology of staged neuroplasticity

Using addictive drugs can evolve from controlled social use into the compulsive relapsing disorder that characterizes addiction. This transition to addiction results from genetic, developmental, and sociological vulnerabilities, combined with pharmacologically induced plasticity in brain circuitry that strengthens learned drug-associated behaviors at the expense of adaptive responding for natural rewards. Advances over the last decade have identified the brain circuits most vulnerable to drug-induced changes, as well as many associated molecular and morphological underpinnings. This growing knowledge has contributed to an expanded understanding of how drugs usurp normal learning circuitry to create the pathology of addiction, as evidenced by involuntary activation of reward circuits in response to drug-associated cues and simultaneous reports of drug craving. This new understanding provides unprecedented potential opportunities for novel pharmacotherapeutic targets in treating addiction. There appears to be plasticity associated with the addiction phenomenon in general as well as changes produced by addiction to a specific class of addicting drugs. These findings also provide the basis for the current understanding of addiction as a chronic, relapsing disease of the brain with changes that persist long after the last use of the drug. Here, we describe the neuroplasticity in brain circuits and cell function induced by addictive drugs that is thought to underlie the compulsions to resume drug-taking, and discuss how this knowledge is impelling exploration and testing of novel addiction therapies.

Methamphetamine-induced structural plasticity in the dorsal striatum

Repeated exposure to psychostimulant drugs produces long-lasting changes in dendritic structure, presumably reflecting a reorganization in patterns of synaptic connectivity, in brain regions that mediate the psychomotor activating and incentive motivational effects of these drugs, including the nucleus accumbens and prefrontal cortex. However, repeated exposure to psychostimulant drugs also facilitates a transition in the control of some behaviors from action-outcome associations to behavior controlled by stimulus-response (S-R) habits. This latter effect is thought to be due to increasing engagement and control over behavior by the dorsolateral (but not dorsomedial) striatum. We hypothesized therefore that repeated exposure to methamphetamine would differentially alter the density of dendritic spines on medium spiny neurons (MSNs) in the dorsolateral vs. dorsomedial striatum. Rats were treated with repeated injections of methamphetamine, and 3 months later dendrites were visualized using Sindbis virus-mediated green fluorescent protein (GFP) expression in vivo. We report that prior exposure to methamphetamine produced a significant increase in mushroom and thin spines on MSNs in the dorsolateral striatum, but a significant decrease in mushroom spines in the dorsomedial striatum. This may be due to changes in the glutamatergic innervation of these two subregions of the dorsal striatum. Thus, we speculate that exposure to psychostimulant drugs may facilitate the development of S-R habits because this reorganizes patterns of synaptic connectivity in the dorsal striatum in a way that increases control over behavior by the dorsolateral striatum.

Increase in A2A receptors in the nucleus accumbens after extended cocaine self-administration and its disappearance after cocaine withdrawal

Effects of extended cocaine self-administration and its withdrawal have been studied on A(2A) and D(2) receptor binding characteristics and expression in the nucleus accumbens and the anterior and posterior dorsal striatum of the rat (Rattus norvegicus). Biochemical binding techniques have been used with the D(2)-like receptor antagonist radioligand [(3)H]-Raclopride and the A(2A) receptor antagonist radioligand [(3)H]-ZM 241385 and immunoblots to study their expression. A substantial and significant increase in functional A(2A), but not in functional D(2) receptors, was observed in the nucleus accumbens immediately following 10 days of cocaine self-administration which returned to normal levels after 7 days of drug withdrawal. In contrast, in the posterior dorsal striatum significant reductions in A(2A) expression were observed immediately after cocaine self-administration which was associated with a trend for a reduction of the A(2A) receptor antagonist binding sites. In cocaine withdrawal groups, significant increases in the density and K(d) value of D(2)-like antagonist binding sites were observed in the nucleus accumbens in the absence of changes in D(2) expression, suggesting an up-regulation of D(3) receptors in this region after cocaine withdrawal. A(2A) receptor increases in the nucleus accumbens induced by cocaine may represent a compensatory up-regulation to counteract cocaine-induced increases in D(2) signaling and D(3) signaling which is in line with its disappearance in the 7-day withdrawal group displaying increased reinforcing efficacy of cocaine. A(2A) agonists may therefore represent cocaine antagonist drugs to be used in treatment of cocaine addiction acting inter alia by antagonizing signaling in accumbens A(2A)/D(2) and A(2A)/D(3) heteromers.

Adaptive plasticity of NMDA receptors and dendritic spines: implications for enhanced vulnerability of the adolescent brain to alcohol addiction

It is now known that brain development continues into adolescence and early adulthood and is highly influenced by experience-dependent adaptive plasticity during this time. Behaviorally, this period is also characterized by increased novelty seeking and risk-taking. This heightened plasticity appears to be important in shaping behaviors and cognitive processes that contribute to proper development of an adult phenotype. However, increasing evidence has linked these same experience-dependent learning mechanisms with processes that underlie drug addiction. As such, the adolescent brain appears to be particularly susceptible to experience-dependent learning processes associated with consumption of alcohol and addictive drugs. At the level of the synapse, homeostatic changes during ethanol consumption are invoked to counter the destabilizing effects of ethanol on neural networks. This homeostatic response may be especially pronounced in the adolescent and young adult brain due to its heightened capacity to undergo experience-dependent changes, and appears to involve increased synaptic targeting of NMDA receptors. Interestingly, recent work from our lab also indicates that the enhanced synaptic localization of NMDA receptors promotes increases in the size of dendritic spines. This increase may represent a structural-based mechanism that supports the formation and stabilization of maladapted synaptic connections that, in a sense, "fix" the addictive behavior in the adolescent and young adult brain.

Altered attention and prefrontal cortex gene expression in rats after binge-like exposure to cocaine during adolescence

Illicit use of drugs frequently begins and escalates during adolescence, with long-term adverse consequences. Because it is increasingly accepted that neural development continues through adolescence, addiction research has become more invested in understanding the behavioral and molecular consequences of early exposure to drugs of abuse. In a novel binge administration paradigm designed to model the pattern of human adolescent drug use, we administered ascending doses of cocaine or saline during a 12-d developmental period [postnatal day 35 (P35) to P46] corresponding to human adolescence. During adulthood (P70), rats treated with this regimen displayed increased responsiveness to the stimulant effects of cocaine. Adult rats also displayed abnormally rapid shifts in attention when performing an attentional set-shifting task, which measures the ability to shift attention between stimuli and whose performance requires an intact prefrontal cortex (PFC). Treatment with cocaine during adolescence also caused acute alterations in the expression of genes encoding cell adhesion molecules and transcription factors within the PFC. Furthermore, we observed decreases in histone methylation, which may indicate a role for chromatin remodeling in the observed changes in gene expression patterns. These findings suggest that exposure to cocaine during adolescence has far-reaching molecular and behavioral consequences in the rat PFC that develop over time and endure long after drug administration has ceased.

Exciting inhibition in psychostimulant addiction

Neuroplasticity induced in the nucleus accumbens by repeated psychostimulant administration is thought to underlie the vulnerability to relapse in addicts. Electrophysiological research presents a contradictory portrait of psychostimulant-induced neuroplasticity, reflecting both increases and decreases in excitatory transmission. Drug-induced adaptations of ionic conductances decrease the intrinsic excitability of individual nucleus accumbens spiny neurons but, in the context of the circuitry in which these neurons are embedded, such reduced intrinsic excitability increases the salience of excitatory drive that is elicited by drug-associated stimuli. Thus, we propose that reduced basal excitability, combined with enhanced excitatory drive by drug-associated stimuli, contributes to the two cardinal features of addiction: reduced responding to natural reward and enduring vulnerability to relapse.

Corticostriatal Up-Regulation of Activity-Regulated Cytoskeletal-Associated Protein Expression after Repeated Exposure to Cocaine

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