Regulation of NMDA receptor subunits and nitric oxide synthase expression during cocaine withdrawal

Further proof on the role of accumbal nNOS in cocaine-seeking behavior in rats

BACKGROUND

Cocaine use disorder (CUD) remains a severe health problem with no effective pharmacological therapy. One of the potential pharmacological strategies for CUD pharmacotherapy includes manipulations of the brain glutamatergic (Glu) system which is particularly involved in drug withdrawal and relapse. Previous research indicated a pivotal role of ionotropic N-methyl-D-aspartate (NMDA) receptors or metabotropic receptors' type 5 (mGlu5) receptors in controlling the reinstatement of cocaine. Stimulation of the above molecules results in the activation of the downstream signaling targets such as neuronal nitric oxide synthase (nNOS) and the release of nitric oxide.

METHODS

In this paper, we investigated the molecular changes in nNOS in the prefrontal cortex and nucleus accumbens following 3 and 10 days of cocaine abstinence as well as the effectiveness of nNOS blockade with the selective enzyme inhibitor N-ω-propyl-L-arginine hydrochloride (L-NPA) on cocaine seeking in male rats. The effect of L-NPA on locomotor activity in drug-naïve animals was investigated.

RESULTS

Ten-day (but not 3-day) cocaine abstinence from cocaine self-administration increased nNOS gene and protein expression in the nucleus accumbens, but not in the prefrontal cortex. L-NPA (0.5-5 mg/kg) administered peripherally did not change locomotor activity but attenuated the reinstatement induced with cocaine priming or the drug-associated conditioned cue.

CONCLUSIONS

Our findings support accumbal nNOS as an important molecular player for cocaine seeking while its inhibitors could be considered as anti-cocaine pharmacological tools in male rats.

Region-specific alterations in the expression and phosphorylation of NMDA receptor subunits in the rat prefrontal cortex and dorsal striatum accompanying behavioral sensitization induced by cocaine and ethanol

Behavioral sensitization is defined as the enhanced behavioral response to drugs of abuse after repeated exposure, which can serve as a behavioral model of addiction. Our previous study demonstrated that behavioral cross-sensitization occurs between cocaine and ethanol, suggesting commonalities between these drugs. N-methyl-d-aspartate (NMDA) receptors play important roles in synaptic plasticity, learning, memory, and addiction-associated behaviors. However, little is known about whether NMDA receptor-mediated signaling regulation is a common feature following behavioral sensitizations induced by cocaine and ethanol. Thus, the present study examined the expression of phospho-S896-NR1, NR2A, and NR2B subunits in the prefrontal cortex and dorsal striatum following reciprocal cross-sensitization between cocaine and ethanol. We also examined the mRNA expression of the NR2A and NR2B subunits. In the ethanol-sensitized state, phosphorylation of NR1 and expression of NR2A and NR2B subunits were increased in both the prefrontal cortex and dorsal striatum. In the cocaine-sensitized state, phosphorylation of NR1 and expression of the NR2A and NR2B subunits were increased in the prefrontal cortex but not in the dorsal striatum. Corresponding changes in mRNA expression were observed in the ethanol-sensitized state but not in the cocaine-sensitized state. Acute treatment with either cocaine or ethanol had no effect on the phosphorylation and expression of NMDA receptor subunits in either the prefrontal cortex or dorsal striatum, regardless of the sensitization state. These results indicate a partially overlapping neural mechanism for cocaine and ethanol that may induce the development of behavioral sensitization.

The effect of self-administered methamphetamine on GABAergic interneuron populations and functional connectivity of the nucleus accumbens and prefrontal cortex

INTRODUCTION

Methamphetamine (METH, "ice") is a potent and addictive psychostimulant. Abuse of METH perturbs neurotransmitter systems and induces neurotoxicity; however, the neurobiological mechanisms which underlie addiction to METH are not fully understood, limiting the efficacy of available treatments. Here we investigate METH-induced changes to neuronal nitric oxide synthase (nNOS), parvalbumin and calretinin-expressing GABAergic interneuron populations within the nucleus accumbens (NAc), prefrontal cortex (PFC) and orbitofrontal cortex (OFC). We hypothesise that dysfunction or loss of these GABAergic interneuron populations may disrupt the excitatory/inhibitory balance within the brain.

METHODS

Male Long Evans rats (N = 32) were trained to lever press for intravenous METH or received yoked saline infusions. Following 14 days of behavioural extinction, animals were given a non-contingent injection of saline or METH (1 mg/kg, IP) to examine drug-primed reinstatement to METH-seeking behaviours. Ninety minutes post-IP injection, animals were culled and brain sections were analysed for Fos, nNOS, parvalbumin and calretinin immunoreactivity in eight distinct subregions of the NAc, PFC and OFC.

RESULTS

METH exposure differentially affected GABAergic populations, with METH self-administration increasing nNOS immunoreactivity at distinct locations in the prelimbic cortex and decreasing parvalbumin immunoreactivity in the NAc. METH self-administration triggered reduced calretinin immunoreactivity, whilst acute METH administration produced a significant increase in calretinin immunoreactivity. As expected, non-contingent METH-priming treatment increased Fos immunoreactivity in subregions of the NAc and PFC.

CONCLUSION

Here we report that METH exposure in this model may alter the function of GABAergic interneurons in more subtle ways, such as alterations in neuronal firing or synaptic connectivity.

Cocaine-induced Changes in the Expression of NMDA Receptor Subunits

Cocaine use disorder is manifested by repeated cycles of drug seeking and drug taking. Cocaine exposure causes synaptic transmission in the brain to exhibit persistent changes, which are poorly understood, while the pharmacotherapy of this disease has not been determined. Multiple potential mechanisms have been indicated to be involved in the etiology of co-caine use disorder. The glutamatergic system, especially N-methyl-D-aspartate (NMDA) receptors, may play a role in sever-al physiological processes (synaptic plasticity, learning and memory) and in the pathogenesis of cocaine use disorder. The composition of the NMDA receptor subunits changes after contingent and noncontingent cocaine administration and after drug abstinence in a region-specific and time-dependent manner, as well as depending on the different protocols used for co-caine administration. Changes in the expression of NMDA receptor subunits may underlie the transition from cocaine abuse to dependence, as well as the transition from cocaine dependence to cocaine withdrawal. In this paper, we summarize the cur-rent knowledge regarding neuroadaptations within NMDA receptor subunits and scaffolding proteins observed following voluntary and passive cocaine intake, as well as the effects of NMDA receptor antagonists on cocaine-induced behavioral changes during cocaine seeking and relapse.

The role of neuronal nitric oxide synthase in cocaine place preference and mu opioid receptor expression in the nucleus accumbens

RATIONALE

There is evidence that central mu opioid receptors (MORs) are implicated in several aspects of cocaine addiction, and that MOR expression is elevated by cocaine in vitro and in the nucleus accumbens (NAc) when administered in vivo.

OBJECTIVE

To understand the cellular mechanisms involved in regulating MOR expression, this study explored whether neuronal nitric oxide synthase (nNOS) modulates the neurochemical and behavioral effects of acute and repeated cocaine administration.

METHODS

Male Sprague-Dawley rats received a single cocaine injection (20 mg/kg, i.p.) in combination with the selective nNOS inhibitor 7-nitroindazole (7-NI) (0, 25, or 50 mg/kg, i.p.), and the expression of MOR and nNOS messenger RNA (mRNA) and protein levels in the NAc were measured. In a separate conditioned place preference (CPP) experiment, 7-NI (0, 25, or 50 mg/kg, i.p.) was administered prior to cocaine (0 or 20 mg/kg, i.p.) conditioning sessions, and levels of MOR and nNOS mRNA and protein in the NAc were measured following CPP test.

RESULTS

Acute cocaine administration significantly enhanced nNOS and MOR mRNA and protein expression in the NAc, and this increase in MOR expression was blocked by 7-NI. Furthermore, in 7-NI pre-treated rats, cocaine-induced CPP was not statistically significant and the increase in MOR mRNA expression in the NAc in these animals was attenuated.

CONCLUSIONS

These findings suggest that nNOS modulates MOR expression following acute cocaine administration, and that cocaine CPP and associated upregulation of MOR expression involve both nNOS-dependent and independent mechanisms. Elucidation of these molecular events may identify useful therapeutic target for cocaine addiction.

Methamphetamine promotes habitual action and alters the density of striatal glutamate receptor and vesicular proteins in dorsal striatum

Goal-directed actions are controlled by the value of the consequences they produce and so increase when what they produce is valuable and decrease when it is not. With continued invariant practice, however, goal-directed actions can become habits, controlled not by their consequences but by antecedent, reward-related states and stimuli. Here, we show that pre-exposure to methamphetamine (METH) caused abnormally rapid development of habitual control. Furthermore, these drug-induced habits differed strikingly from conventional habits; we found that they were insensitive both to changes in reward value and to the effects of negative feedback. In addition to these behavioral changes, METH exposure produced bidirectional changes to synaptic proteins in the dorsal striatum. In the dorsomedial striatum, a structure critical for goal-directed action, METH exposure was associated with a reduction in glutamate receptor and glutamate vesicular proteins, whereas in the dorsolateral striatum, a region that has previously been implicated in habit learning, there was an increase in these proteins. Together, these results indicate that METH exposure promotes habitual control of action that appears to be the result of bidirectional changes in glutamatergic transmission in the circuits underlying goal-directed and habit-based learning.

Toluene inhalation in adolescent rats reduces flexible behaviour in adulthood and alters glutamatergic and GABAergic signalling

Toluene is a commonly abused inhalant that is easily accessible to adolescents. Despite the increasing incidence of use, our understanding of its long-term impact remains limited. Here, we used a range of techniques to examine the acute and chronic effects of toluene exposure on glutameteric and GABAergic function, and on indices of psychological function in adult rats after adolescent exposure. Metabolomics conducted on cortical tissue established that acute exposure to toluene produces alterations in cellular metabolism indicative of a glutamatergic and GABAergic profile. Similarly, in vitro electrophysiology in Xenopus oocytes found that acute toluene exposure reduced NMDA receptor signalling. Finally, in an adolescent rodent model of chronic intermittent exposure to toluene (10 000 ppm), we found that, while toluene exposure did not affect initial learning, it induced a deficit in updating that learning when response-outcome relationships were reversed or degraded in an instrumental conditioning paradigm. There were also group differences when more effort was required to obtain the reward; toluene-exposed animals were less sensitive to progressive ratio schedules and to delayed discounting. These behavioural deficits were accompanied by changes in subunit expression of both NMDA and GABA receptors in adulthood, up to 10 weeks after the final exposure to toluene in the hippocampus, prefrontal cortex and ventromedial striatum; regions with recognized roles in behavioural flexibility and decision-making. Collectively, our data suggest that exposure to toluene is sufficient to induce adaptive changes in glutamatergic and GABAergic systems and in adaptive behaviour that may underlie the deficits observed following adolescent inhalant abuse, including susceptibility to further drug-use.

Cocaine Self-Administration Elevates GluN2B within dmPFC Mediating Heightened Cue-Elicited Operant Responding

Cue-elicited drug-craving correlates with hyperactivity within prefrontal cortex (PFC), which is theorized to result from dysregulated excitatory neurotransmission. The NMDA glutamate receptor is highly implicated in addiction-related neuroplasticity. As NMDA receptor function is regulated critically by its GluN2 subunits, herein, we assayed the relation between incubated cue-elicited cocaine-seeking following extended access to intravenous cocaine (6 h/d; 0.25 mg/infusion for 10 d) and the expression of GluN2A/B receptor subunits within PFC sub regions during early versus late withdrawal (respectively, 3 vs. 30 days). Cocaine-seeking rats exhibited elevated GluN2B expression within the dorsomedial aspect of the PFC (dmPFC); this effect was apparent at both 3 and 30 days withdrawal and occurred in cocaine-experienced rats, regardless of experiencing an extinction test or not. Thus, elevated dmPFC GluN2B expression appears to reflect a pharmacodynamic response to excessive cocaine intake that is independent of the duration of drug withdrawal or re-exposure to drug-taking context. The functional relevance of elevated dmPFC GluN2B expression for drug-seeking was assessed by the local infusion of the prototypical GluN2B-selective antagonist ifenprodil (1.0 µg/side). Ifenprodil did not alter cue-elicited responding in animals with a history of saline self-administration. In contrast, ifenprodil lowered cue-elicited cocaine-seeking, while potentiating cue-elicited sucrose-seeking. Thus, the effects of an intra-dmPFC ifenprodil infusion upon cue reactivity are reinforcer-specific, arguing in favor of targeting GluN2B-containing NMDA receptors as a pharmacological strategy for reducing behavioral reactivity to drug-associated cues with the potential benefit of heightening the reinforcing properties of cues associated with non-drug primary rewards.

Synaptic Cytoskeletal Plasticity in the Prefrontal Cortex Following Psychostimulant Exposure

Addiction is characterized by maladaptive decision-making, a loss of control over drug consumption and habit-like drug seeking despite adverse consequences. These cognitive changes may reflect the effects of drugs of abuse on prefrontal cortical neurobiology. Here, we review evidence that amphetamine and cocaine fundamentally remodel the structure of excitatory neurons in the prefrontal cortex. We summarize evidence in particular that these psychostimulants have opposing effects in the medial and orbital prefrontal cortices ('mPFC' and 'oPFC', respectively). For example, amphetamine and cocaine increase dendrite length and spine density in the mPFC, while dendrites are impoverished and dendritic spines are eliminated in the oPFC. We will discuss evidence that certain cytoskeletal regulatory proteins expressed in the oPFC and implicated in postnatal (adolescent) neural development also regulate behavioral sensitivity to cocaine. These findings potentially open a window of opportunity for the identification of novel pharmacotherapeutic targets in the treatment of drug abuse disorders in adults, as well as in drug-vulnerable adolescent populations. Finally, we will discuss the behavioral implications of drug-related dendritic spine elimination in the oPFC, with regard to reversal learning tasks and tasks that assess the development of reward-seeking habits, both used to model aspects of addiction in rodents.

Drug withdrawal conceptualized as a stressor

Drug withdrawal is often conceptualized as an aversive state that motivates drug-seeking and drug-taking behaviors in humans. Stress is more difficult to define, but is also frequently associated with aversive states. Here we describe evidence for the simple theory that drug withdrawal is a stress-like state, on the basis of common effects on behavioral, neurochemical, and molecular endpoints. We also describe data suggesting a more complex relationship between drug withdrawal and stress. As one example, we will highlight evidence that, depending on drug class, components of withdrawal can produce effects that have characteristics consistent with mood elevation. In addition, some stressors can act as positive reinforcers, defined as having the ability to increase the probability of a behavior that produces it. As such, accumulating evidence supports the general principles of opponent process theory, whereby processes that have an affective valence are followed in time by an opponent process that has the opposite valence. Throughout, we identify gaps in knowledge and propose future directions for research. A better understanding of the similarities, differences, and overlaps between drug withdrawal and stress will lead to the development of improved treatments for addiction, as well as for a vast array of neuropsychiatric conditions that are triggered or exacerbated by stress.

Altered energy production, lowered antioxidant potential, and inflammatory processes mediate CNS damage associated with abuse of the psychostimulants MDMA and methamphetamine

Central nervous system (CNS) damage associated with psychostimulant dependence may be an ongoing, degenerative process with adverse effects on neuropsychiatric function. However, the molecular mechanisms regarding how altered energy regulation affects immune response in the context of substance use disorders are not fully understood. This review summarizes the current evidence regarding the effects of psychostimulant [particularly 3,4-methylenedioxy-N-methylamphetamine (MDMA) and methamphetamine] exposure on brain energy regulation, immune response, and neuropsychiatric function. Importantly, the neuropsychiatric impairments (e.g., cognitive deficits, depression, and anxiety) that persist following abstinence are associated with poorer treatment outcomes - increased relapse rates, lower treatment retention rates, and reduced daily functioning. Qualifying the molecular changes within the CNS according to the exposure and use patterns of specifically abused substances should inform the development of new therapeutic approaches for addiction treatment.

Cocaine-induced changes in NMDA receptor signaling

Addictive states are often thought to rely on lasting modification of signaling at relevant synapses. A long-standing theory posits that activity at N-methyl-D-aspartate receptors (NMDARs) is a critical component of long-term synaptic plasticity in many brain areas. Indeed, NMDAR signaling has been found to play a role in the etiology of addictive states, in particular, following cocaine exposure. However, no consensus is apparent with respect to the specific effects of cocaine exposure on NMDARs. Part of the difficulty lies in the fact that NMDARs interact extensively with multiple membrane proteins and intracellular signaling cascades. This allows for highly heterogeneous patterns of NMDAR regulation by cocaine in distinct brain regions and at distinct synapses. The picture is further complicated by findings that cocaine effects on NMDARs are sensitive to the behavioral history of cocaine exposure such as the mode of cocaine administration. This review provides a summary of evidence for cocaine-induced changes in NMDAR expression, cocaine-induced alterations in NMDAR function, and cocaine effects on NMDAR control of intracellular signaling cascades.

Extrasynaptic targeting of NMDA receptors following D1 dopamine receptor activation and cocaine self-administration

We previously showed that after repeated exposure to cocaine, D1-like dopamine receptor (D1DR) stimulation reverses plastic changes of AMPA receptor-mediated signaling in the nucleus accumbens shell. However, there is little information on the impact of cocaine self-administration on D1-NMDA receptor interactions in this brain region. Here, using whole-cell patch-clamp recordings, we assessed whether cocaine self-administration alters the effects of D1DR stimulation on synaptic and extrasynaptic NMDA receptors (NMDARs). In slices from cocaine-naive rats, pretreatment with a D1DR agonist decreased synaptic NMDAR-mediated currents and increased the contribution of extrasynaptic NMDARs. In contrast, neither cocaine self-administration alone nor cocaine experience followed by D1DR stimulation had an effect on synaptic or extrasynaptic NMDAR signaling. Activation of extrasynaptic NMDARs relies on the availability of extracellular glutamate, which is regulated primarily by glutamate transporters. In cocaine-experienced animals, relative to cocaine-naive rats, administration of a glutamate reuptake blocker, DL-threo-β-benzyloxyaspartic acid, revealed increased extrasynaptic NMDAR activity and stronger baseline activity of glutamate uptake transporters. In cocaine-naive rats, the D1DR-mediated increase in extrasynaptic NMDAR signaling was independent of the activity of glutamate reuptake transporters. Together, these results indicate that cocaine experience blunts the influence of D1DRs on synaptic and extrasynaptic NMDAR signaling. Additionally, prior cocaine self-administration limits activation of the extrasynaptic NMDAR pool by increasing glutamate reuptake. These findings outline a pattern of adaptive interactions between D1DRs and NMDARs in the nucleus accumbens shell and demonstrate upregulation of extrasynaptic NMDAR signaling as a novel consequence of cocaine self-administration.

Reduced D-serine levels in the nucleus accumbens of cocaine-treated rats hinder the induction of NMDA receptor-dependent synaptic plasticity

Cocaine seeking behaviour and relapse have been linked to impaired potentiation and depression at excitatory synapses in the nucleus accumbens, but the mechanism underlying this process is poorly understood. We show that, in the rat nucleus accumbens core, D-serine is the endogenous coagonist of N-methyl-D-aspartate receptors, and its presence is essential for N-methyl-D-aspartate receptor-dependent potentiation and depression of synaptic transmission. Nucleus accumbens core slices obtained from cocaine-treated rats after 1 day of abstinence presented significantly reduced D-serine concentrations, increased expression of the D-serine degrading enzyme, D-amino acid oxidase, and downregulated expression of serine racemase, the enzyme responsible for D-serine synthesis. The D-serine deficit was associated with impairment of potentiation and depression of glutamatergic synaptic transmission, which was restored by slice perfusion with exogenous D-serine. Furthermore, in vivo administration of D-serine directly into the nucleus accumbens core blocked behavioural sensitization to cocaine. These results provide evidence for a critical role of D-serine signalling in synaptic plasticity relevant to cocaine addiction.

Arg kinase regulates prefrontal dendritic spine refinement and cocaine-induced plasticity

Adolescence is characterized by vulnerability to the development of neuropsychiatric disorders including drug addiction, as well as prefrontal cortical refinement that culminates in structural stability in adulthood. Neuronal refinement and stabilization are hypothesized to confer resilience to poor decision making and addictive-like behaviors, although intracellular mechanisms are largely unknown. We characterized layer V prefrontal dendritic spine development and refinement in adolescent wild-type mice and mice lacking the cytoskeletal regulatory protein Abl-related gene (Arg) kinase. Relative to hippocampal CA1 pyramidal neurons, which exhibited a nearly linear increase in spine density up to postnatal day 60 (P60), wild-type prefrontal spine density peaked at P31, and then declined by 18% by P56-P60. In contrast, dendritic spines in mice lacking Arg destabilized by P31, leading to a net loss in both structures. Destabilization corresponded temporally to the emergence of exaggerated psychomotor sensitivity to cocaine. Moreover, cocaine reduced dendritic spine density in wild-type orbitofrontal cortex and enlarged remaining spine heads, but arg(-/-) spines were unresponsive. Local application of Arg or actin polymerization inhibitors exaggerated cocaine sensitization, as did reduced gene dosage of the Arg substrate, p190RhoGAP. Genetic and pharmacological Arg inhibition also retarded instrumental reversal learning and potentiated responding for reward-related cues, providing evidence that Arg regulates both psychomotor sensitization and decision-making processes implicated in addiction. These findings also indicate that structural refinement in the adolescent orbitofrontal cortex mitigates psychostimulant sensitivity and support the emerging perspective that the structural response to cocaine may, at any age, have behaviorally protective consequences.

Immunogold electron microscopic evidence of differential regulation of GluN1, GluN2A, and GluN2B, NMDA-type glutamate receptor subunits in rat hippocampal CA1 synapses during benzodiazepine withdrawal

Benzodiazepine withdrawal-anxiety is associated with enhanced α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR)-mediated glutamatergic transmission in rat hippocampal CA1 synapses due to enhanced synaptic insertion and phosphorylation of GluA1 homomers. Interestingly, attenuation of withdrawal-anxiety is associated with a reduction in N-methyl-D-aspartate receptor (NMDAR)-mediated currents and subunit expression, secondary to AMPA receptor potentiation. Therefore, in this study ultrastructural evidence for possible reductions in NMDAR GluN1, GluN2A, and GluN2B subunits was sought at CA1 stratum radiatum synapses in proximal dendrites using postembedding immunogold labeling of tissues from rats withdrawn for 2 days from 1-week daily oral administration of the benzodiazepine, flurazepam (FZP). GluN1-immunogold density and the percentage of immunopositive synapses were significantly decreased in tissues from FZP-withdrawn rats. Similar decreases were observed for GluN2B subunits; however, the relative lateral distribution of GluN2B-immunolabeling within the postsynaptic density did not change after BZ withdrawal. In contrast to the GluN2B subunit, the percentage of synapses labeled with the GluN2A subunit antibody and the density of immunogold labeling for this subunit was unchanged. The spatial localization of immunogold particles associated with each NMDAR subunit was consistent with a predominantly postsynaptic localization. The data therefore provide direct evidence for reduced synaptic GluN1/GluN2B receptors and preservation of GluN1/GluN2A receptors in the CA1 stratum radiatum region during BZ withdrawal. Based on collective findings in this benzodiazepine withdrawal-anxiety model, we propose a functional model illustrating the changes in glutamate receptor populations at excitatory synapses during benzodiazepine withdrawal.

Cocaine-induced metaplasticity in the nucleus accumbens: silent synapse and beyond

The neuroadaptation theory of addiction suggests that, similar to the development of most memories, exposure to drugs of abuse induces adaptive molecular and cellular changes in the brain which likely mediate addiction-related memories or the addictive state. Compared to other types of memories, addiction-related memories develop fast and last extremely long, suggesting that the cellular and molecular processes that mediate addiction-related memories are exceptionally adept and efficient. We recently demonstrated that repeated exposure to cocaine generated a large portion of "silent" glutamatergic synapses within the nucleus accumbens (NAc). Silent glutamatergic synapses are synaptic connections in which only N-methyl-D-aspartic acid receptor (NMDAR)-mediated responses are readily detected whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are absent or highly labile. Extensive experimental evidence suggests that silent synapses are conspicuously efficient plasticity sites at which long-lasting plastic changes can be more easily induced and maintained. Thus, generation of silent synapses can be regarded as a process of metaplasticity, which primes the NAc for subsequent durable and robust plasticity for addiction-related memories. Focusing on silent synapse-based metaplasticity, this review discusses how key brain regions, such as the NAc, utilize the metaplasticity mechanism to optimize the plasticity machineries to achieve fast and durable plastic changes following exposure to cocaine. A summary of recent related results suggests that upon cocaine exposure, newly generated silent synapses may prime excitatory synapses within the NAc for long-term potentiation (LTP), thus setting the direction of future plasticity. Furthermore, because cocaine-generated silent synapses are enriched in NMDARs containing the NR2B subunit, the enhanced NR2B-signaling may set up a selective recruitment of certain types of AMPARs. Thus, silent synapse-based metaplasticity may lead to not only quantitative but also qualitative alterations in excitatory synapses within the NAc. This review is one of the first systematic analyses regarding the hypothesis that drugs of abuse induce metaplasticity, which regulates the susceptibility, the direction, and the molecular details of subsequent plastic changes. Taken together, metaplasticity ultimately serves as a key step in mediating cascades of addiction-related plastic alterations.

Cocaine-induced neuroadaptations in glutamate transmission: potential therapeutic targets for craving and addiction

A growing body of evidence indicates that repeated exposure to cocaine leads to profound changes in glutamate transmission in limbic nuclei, particularly the nucleus accumbens. This review focuses on preclinical studies of cocaine-induced behavioral plasticity, including behavioral sensitization, self-administration, and the reinstatement of cocaine seeking. Behavioral, pharmacological, neurochemical, electrophysiological, biochemical, and molecular biological changes associated with cocaine-induced plasticity in glutamate systems are reviewed. The ultimate goal of these lines of research is to identify novel targets for the development of therapies for cocaine craving and addiction. Therefore, we also outline the progress and prospects of glutamate modulators for the treatment of cocaine addiction.

The role of glutamate receptor redistribution in locomotor sensitization to cocaine

alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR) surface expression in the nucleus accumbens (NAc) is enhanced after withdrawal from repeated cocaine exposure. However, it is unclear whether this contributes to the expression of locomotor sensitization and whether similar changes can be observed in other striatal regions. In this study we examined the relationship between AMPAR surface expression in the NAc and locomotor sensitization. We also examined AMPAR distribution in the dorsolateral striatum (DS) and NMDA receptor (NMDAR) distribution in the NAc and DS. Trends but no significant changes in NMDAR distribution were found in the NAc after withdrawal. No NMDAR changes were observed in the DS. AMPAR surface expression was increased in the NAc 15 days after the last exposure to cocaine, but decreased in the DS. Re-exposure to cocaine on withdrawal day 14 decreased AMPAR surface expression in the NAc 24 h, but not 30 min, after challenge, but increased it in the DS 24 h and 30 min after challenge. Locomotor sensitization was evaluated at times associated with increased or decreased AMPAR surface expression in the NAc. The magnitude of sensitization did not vary with changes in the level of AMPAR surface expression, nor was it significantly reduced by decreasing AMPAR transmission through intra-NAc infusion of CNQX before cocaine challenge. On the basis of our results, and other findings, we suggest that the expression of sensitization has no clear relationship to altered AMPAR surface expression in the NAc, although the latter may have a role in the enhanced pursuit and self-administration of drugs observed in sensitized rats.

Neural substrates of psychostimulant withdrawal-induced anhedonia

Psychostimulant drugs have powerful reinforcing and hedonic properties and are frequently abused. Cessation of psychostimulant administration results in a withdrawal syndrome characterized by anhedonia (i.e., an inability to experience pleasure). In humans, psychostimulant withdrawal-induced anhedonia can be debilitating and has been hypothesized to play an important role in relapse to drug use. Hence, understanding the neural substrates involved in psychostimulant withdrawal-induced anhedonia is essential. In this review, we first summarize the theoretical perspectives of psychostimulant withdrawal-induced anhedonia. Experimental procedures and measures used to assess anhedonia in experimental animals are also discussed. The review then focuses on neural substrates hypothesized to play an important role in anhedonia experienced after termination of psychostimulant administration, such as with cocaine, amphetamine-like drugs, and nicotine. Both neural substrates that have been extensively investigated and some that need further evaluation with respect to psychostimulant withdrawal-induced anhedonia are reviewed. In the context of reviewing the various neurosubstrates of psychostimulant withdrawal, we also discuss pharmacological medications that have been used to treat psychostimulant withdrawal in humans. This literature review indicates that great progress has been made in understanding the neural substrates of anhedonia associated with psychostimulant withdrawal. These advances in our understanding of the neurobiology of anhedonia may also shed light on the neurobiology of nondrug-induced anhedonia, such as that seen as a core symptom of depression and a negative symptom of schizophrenia.

Extended daily access to cocaine results in distinct alterations in Homer 1b/c and NMDA receptor subunit expression within the medial prefrontal cortex

Human cocaine addicts show altered function within the basal ganglia and the medial prefrontal cortex (mPFC) and altered glutamate function within these areas is postulated to be critical for cocaine addiction. The present project utilized a highly valid animal model of cocaine addiction, to test whether excessive use of cocaine alters glutamate function within these brain areas. Rats were trained to lever-press for i.v. saline vehicle or cocaine (0.25 mg/infusion) over seven 1-h daily sessions, after which, saline controls and half of cocaine self-administering animals (brief access condition) received 10 more 1-h daily sessions, whereas the remaining cocaine animals received 10 additional 6-h daily sessions (extended access condition). One, 14, or 60 days after the last self-administration session, animals were sacrificed. Tissue samples from the ventral tegmental area (VTA), nucleus accumbens (N.Acc) core and shell, and mPFC were analyzed by immunoblotting for expression of Homer1b/c, Homer2a/b, mGluR1, mGluR5, NR2a, and NR2b subunits of the NMDA receptor. Brief and extended access to cocaine failed to alter protein levels within the VTA, and produced transient and similar changes in N.Acc protein expression, which were more pronounced in the core subregion. In contrast, extended access to cocaine resulted in distinct and long lasting alterations of protein expression within the mPFC that included: increased levels of Homer1b/c at 1 day, NR2b at 14 days, and NR2a at 60 days, of withdrawal. These data support the notion that altered NMDA function within the mPFC may contribute, in part, to the transition to excessive uncontrolled consumption of cocaine.

Plasticity of addiction: a mesolimbic dopamine short-circuit?

The development of drug addiction progresses along a continuum from acute drug use to compulsive use and drug seeking behavior. Many researchers have focused on identifying the physiological mechanisms involved in drug addiction in order to develop effective pharmacotherapies. Neuroplasticity, the putative mechanism underlying learning and memory, is modified by drugs of abuse and may contribute to the development of the eventual addicted state. Innovative treatments directly targeting these drug-induced changes in brain reward components and circuits may be efficacious in reducing drug use and relapse.

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

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

Stability of surface NMDA receptors controls synaptic and behavioral adaptations to amphetamine

Plastic changes in glutamatergic synapses that lead to enduring drug craving and addiction are poorly understood. By focusing on the turnover and trafficking of NMDA receptors, we found that chronic exposure to the psychostimulant amphetamine induces selective downregulation of NMDA receptor NR2B subunits in the confined surface membrane pool of rat striatal neurons at synaptic sites. Remarkably, this downregulation is a long-lived event and results from the destabilization of surface-expressed NR2B due to accelerated ubiquitination and degradation of crucial NR2B-anchoring proteins by the ubiquitin-proteasome system. The biochemical loss of synaptic NR2B further translates to the significant modulation of synaptic plasticity in the form of long-term depression at cortico-accumbal glutamatergic synapses. Behaviorally, genetic disruption of NR2B induces, whereas restoration of NR2B loss prevents, behavioral sensitization to amphetamine. Our data identify NR2B as a key regulator in the remodeling of excitatory synapses and persistent psychomotor plasticity in response to amphetamine.

Role of NMDA receptors in dopamine neurons for plasticity and addictive behaviors

A single exposure to drugs of abuse produces an NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) of AMPA receptor (AMPAR) currents in DA neurons; however, the importance of LTP for various aspects of drug addiction is unclear. To test the role of NMDAR-dependent plasticity in addictive behavior, we genetically inactivated functional NMDAR signaling exclusively in DA neurons (KO mice). Inactivation of NMDARs results in increased AMPAR-mediated transmission that is indistinguishable from the increases associated with a single cocaine exposure, yet locomotor responses to multiple drugs of abuse were unaltered in the KO mice. The initial phase of locomotor sensitization to cocaine is intact; however, the delayed sensitization that occurs with prolonged cocaine withdrawal did not occur. Conditioned behavioral responses for cocaine-testing environment were also absent in the KO mice. These findings provide evidence for a role of NMDAR signaling in DA neurons for specific behavioral modifications associated with drug seeking behaviors.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Deprenyl treatment attenuates long-term pre- and post-synaptic changes evoked by chronic methamphetamine

Deprenyl, used clinically in Parkinson's disease, has multiple pharmacological effects which make it a good candidate to treat neurotoxicity. Thus, we investigated deprenyl's ability to attenuate methamphetamine-induced dopamine neurotoxicity. We also examined deprenyl's effect in changing markers associated with psychostimulant sensitization. A potential therapeutic effect on either pathological domain would be a boon in developing novel treatments for methamphetamine abuse. Adult male Sprague-Dawley rats were split into 6 groups. Three groups received a 7-day saline minipump with saline, 0.05 or 0.25 mg/kg SC deprenyl injections given for 10 days before, during and 5 days after the 7-day saline minipump implant. Similarly, 3 groups received methamphetamine pumps (25 mg/kg/day) with escalating daily injections of methamphetamine (0-6 mg/kg) in addition to the minipump treatment. These rats also received saline, 0.05 or 0.25 mg/kg deprenyl injections given before, during and the 7-day minipump treatment. Rats were killed on day 28 of withdrawal and brain samples taken. HPLC analysis for dopamine and 3,4-Dihydroxy-Phenylacetic Acid (DOPAC) revealed a loss of dopamine in the caudate and accumbens which was partially reversed by high dose deprenyl. Tyrosine hydroxylase immunostaining in the midbrain was unaffected by methamphetamine, suggesting that dopamine neurotoxicity was localized to the caudate. Western blot analysis of the caudate after methamphetamine revealed little change in Alpha-Amino-3-Hydroxy-5-Methyl-4-Isoxazole Propionic Acid (AMPA) GluR1 or N-Methyl-d-Aspartate (NMDA) NR2B subunits, or their phosphorylation state. However, methamphetamine increased levels of GluR1 and its phosphorylation state in the prefrontal cortex (PFC), and these increases were attenuated by deprenyl. Methamphetamine also increased levels of PFC NR2B subunit, but these increases were not attenuated by deprenyl. We suggest that deprenyl may be effective in reducing the neurotoxic effects of methamphetamine and may also attenuate changes in prefrontal AMPA receptor function, presumably more associated with addiction rather than neurotoxicity.

Cocaine experience controls bidirectional synaptic plasticity in the nucleus accumbens

Plasticity of glutamatergic synapses is a fundamental mechanism through which experience changes neural function to impact future behavior. In animal models of addiction, glutamatergic signaling in the nucleus accumbens (NAc) exerts powerful control over drug-seeking behavior. However, little is known about whether, how or when experience with drugs may trigger synaptic plasticity in this key nucleus. Using whole-cell synaptic physiology in NAc brain slices, we demonstrate that a progression of bidirectional changes in glutamatergic synaptic strength occurs after repeated in vivo exposure to cocaine. During a protracted drug-free period, NAc neurons from cocaine-experienced mice develop a robust potentiation of AMPAR-mediated synaptic transmission. However, a single re-exposure to cocaine during extended withdrawal becomes a potent stimulus for synaptic depression, abruptly reversing the initial potentiation. These enduring modifications in AMPAR-mediated responses and plasticity may provide a neural substrate for disrupted processing of drug-related stimuli in drug-experienced individuals.

Animal models and treatments for addiction and depression co-morbidity

The high rates of co-morbidity of drug addiction with depression may be attributable to shared neurobiology. Here, we discuss shared neurobiological substrates in drug withdrawal and depression, with an emphasis on changes in brain reward circuitry that may underlie anhedonia, a core symptom of depression and drug withdrawal. We explored experimentally whether clinical antidepressant medications or other treatments would reverse the anhedonia observed in rats undergoing spontaneous nicotine or amphetamine withdrawal, defined operationally as elevated brain reward thresholds. The co-administration of selective serotonin reuptake inhibitors with a serotonin-1A receptor antagonist, or the tricyclic antidepressant desipramine, or the atypical antidepressant bupropion ameliorated nicotine or amphetamine withdrawal in rats. Thus, increases in monoaminergic neurotransmission, or neuroadaptations induced by increased monoaminergic neurotransmission, ameliorated depression-like aspects of drug withdrawal. Further, chronic pretreatment with the atypical antipsychotic clozapine, that has some efficacy in the treatment of the depression-like symptoms of schizophrenia, attenuated nicotine and amphetamine withdrawal. Finally, a metabotropic glutamate 2/3 receptor antagonist reversed threshold elevations associated with nicotine withdrawal. The effects of these pharmacological manipulations are consistent with the altered neurobiology observed in drug withdrawal and depression. Thus, these data support the hypothesis of common substrates mediating the depressive symptoms of drug withdrawal and those seen in psychiatric patients. Accordingly, the anhedonic state associated with drug withdrawal can be used to study the neurobiology of anhedonia, and thus contribute to the identification of novel targets for the treatment of depression-like symptoms seen in various psychiatric and neurological disorders.

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

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

Long-term potentiation (LTP) in the central amygdala (CeA) is enhanced after prolonged withdrawal from chronic cocaine and requires CRF1 receptors

The amygdala is part of the brain reward circuitry that plays a role in cocaine-seeking and abstinence in animals and cocaine craving and relapse in humans. Cocaine-seeking is elicited by cocaine-associated cues, and the basolateral amygdala (BLA) and CeA are essential in forming and communicating drug-related associations that are thought to be critical in long-lasting relapse risk associated with drug addiction. Here we simulated a cue stimulus with high-frequency stimulation (HFS) of the BLA-CeA pathway to examine mechanisms that may contribute to drug-related associations. We found enhanced long-term potentiation (LTP) after 14-day but not 1-day withdrawal from 7-day cocaine treatment mediated through N-methyl-d-aspartate (NMDA) receptors (NRs), L-type voltage-gated calcium channels (L-VGCCs), and corticotropin-releasing factor (CRF)(1) receptors; this was accompanied by increased phosphorylated NR1 and CRF(1) protein not associated with changes in NMDA/AMPA ratios in amygdalae from cocaine-treated animals. We suggest that these signaling mechanisms may provide therapeutic targets for the treatment of cocaine cravings.

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

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

Differential regulation of ionotropic glutamate receptor subunits following cocaine self-administration

Previous examination of binge cocaine self-administration and 2 week withdrawal from cocaine self-administration on ionotropic glutamate receptor subunit (iGluRs) protein levels revealed significant alterations in iGluR protein levels that differed between the mesocorticolimbic and nigrostriatal pathways. The present study was undertaken to extend the examination of cocaine-induced alterations in iGluR protein expression by assessing the effects of acute withdrawal (15-16 h) from limited access cocaine self-administration (8 h/day, 15 days). Western blotting was used to compare levels of iGluR protein expression (NR1-3B, GluR1-7, KA2) in the mesolimbic (ventral tegmental area, VTA; nucleus accumbens, NAc; and prefrontal cortex, PFC) and nigrostriatal pathways (substantia nigra, SN and dorsal caudate-putamen, CPu). Within the mesolimbic pathway, reductions were observed in NR1 and GluR5 immunoreactivity in the VTA although no significant alterations were observed in any iGluR subunits in the NAc. In the PFC, NR1 was significantly upregulated while GluR2/3, GluR4, GluR5, GluR6/7, and KA2 were decreased. Within the nigrostriatal pathway, NR1, NR2A, NR2B, GluR1, GluR6/7 and KA2 were increased in the dorsal CPu, whereas no significant changes were observed in the SN. The results demonstrate region- and pathway-specific alterations in iGluR subunit expression following limited cocaine self-administration and suggest the importance for the activation of pathways that are substrates of the reinforcing and motoric effects of cocaine.

Extinction training regulates neuroadaptive responses to withdrawal from chronic cocaine self-administration

Cocaine produces multiple neuroadaptations with chronic repeated use. Many of these neuroadaptations can be reversed or normalized by extinction training during withdrawal from chronic cocaine self-administration in rats. This article reviews our past and present studies on extinction-induced modulation of the neuroadaptive response to chronic cocaine in the mesolimbic dopamine system, and the role of this modulation in addictive behavior in rats. Extinction training normalizes tyrosine hydroxylase levels in the nucleus accumbens (NAc) shell, an effect that could help ameliorate dysphoria and depression associated with withdrawal from chronic cocaine use. Extinction training also increases levels of GluR1 and GluR2/3 AMPA receptor subunits, while normalizing deficits in NR1 NMDA receptor subunits, in a manner consistent with long-term potentiation of excitatory synapses in the NAc shell. Our results suggest that extinction-induced increases in AMPA and NMDA receptors may restore deficits in cortico-accumbal neurotransmission in the NAc shell and facilitate inhibitory control over cocaine-seeking behavior. Other changes identified by gene expression profiling, including up-regulation in the AMPA receptor aggregating protein Narp, suggest that extinction training induces extensive synaptic reorganization. These studies highlight potential benefits for extinction training procedures in the treatment of drug addiction.

Psychostimulant withdrawal as an inducing condition in animal models of depression

A large body of evidence indicates that the withdrawal from high doses of psychostimulant drugs in humans induces a transient syndrome, with symptoms that appear isomorphic to those of major depressive disorder. Pharmacological treatment strategies for psychostimulant withdrawal in humans have focused mainly on compounds with antidepressant properties. Animal models of psychostimulant withdrawal have been shown to demonstrate a wide range of deficits, including changes in homeostatic, affective and cognitive behaviors, as well as numerous physiological changes. Many of these behavioral and physiological sequelae parallel specific symptoms of major depressive disorder, and have been reversed by treatment with antidepressant drugs. These combined findings provide strong support for the use of psychostimulant withdrawal as an inducing condition in animal models of depression. In the current review we propound that the psychostimulant withdrawal model displays high levels of predictive and construct validity. Recent progress and limitations in the development of this model, as well as future directions for research, are evaluated and discussed.

Alterations in ionotropic glutamate receptor subunits during binge cocaine self-administration and withdrawal in rats

Chronic cocaine use in humans and animal models is known to lead to pronounced alterations in glutamatergic function in brain regions associated with reinforcement. Previous studies have examined ionotropic glutamate receptor (iGluR) subunit protein level changes following acute and chronic experimenter-administered cocaine or after withdrawal periods from experimenter-administered cocaine. To evaluate whether alterations in expression of iGluRs are associated with cocaine reinforcement, protein levels were assessed after binge (8 h/day, 15 days; 24-h access, days 16-21) cocaine self-administration and following 2 weeks of abstinence from this binge. Western blotting was used to compare levels of iGluR protein expression (NR1-3B, GluR1-7, KA2) in the ventral tegmental area (VTA), substantia nigra (SN), nucleus accumbens (NAc), striatum and prefrontal cortex (PFC) of rats. iGluR subunits were altered in a time-dependent manner in all brain regions studied; however, selective alterations in certain iGluR subtypes appeared to be associated with binge cocaine self-administration and withdrawal in a region-specific manner. In the SN and VTA, alterations in iGluR protein levels compared with controls occurred only following binge access, whereas in the striatum and PFC, iGluR alterations occurred with binge access and following withdrawal. In the NAc, GluR2/3 levels were increased following withdrawal compared with binge access, and were the only changes observed in this region. Because subunit composition determines the functional properties of iGluRs, the observed changes may indicate alterations in the excitability of dopamine transmission underlying long-term biochemical and behavioral effects of cocaine.

Nerve terminal glutamate immunoreactivity in the rat nucleus accumbens and ventral tegmental area after a short withdrawal from cocaine

Cocaine administration has been shown to alter glutamate transmission in numerous studies. Using quantitative electron microscopic immunogold labeling, our laboratory has previously reported that nerve terminal glutamate immunoreactivity is transiently altered following cocaine administration. The present study was undertaken to examine presynaptic nerve terminal glutamate immunoreactivity at shorter time points after withdrawal from cocaine. Animals received saline or cocaine for 7 days followed 3 days later by a cocaine or saline challenge. Most (>75%) cocaine-challenged animals had a heightened locomotor response to cocaine compared to the first day of cocaine and were considered behaviorally sensitized. One day after the challenge, glutamate immunogold-labeling was quantified in nerve terminals making asymmetrical synaptic contacts within the core and shell of the nucleus accumbens and ventral tegmental area. A single dose of cocaine did not alter the density of presynaptic nerve terminal glutamate immunoreactivity in the nucleus accumbens (NAc) or ventral tegmental area (VTA). The density of nerve terminal glutamate immunoreactivity in the shell, but not the core, was significantly increased in the animals receiving repeated cocaine. In the VTA the density of nerve terminal glutamate immunoreactivity did not change in the cocaine-sensitized group, but was significantly increased in the nonsensitized group. The finding that repeated cocaine treatment increased glutamate nerve terminal immunolabeling within the nucleus accumbens shell, but not the core, supports the hypothesis that glutamate synapses in the core and shell are differentially sensitive to repeated cocaine administration. Overall, our study does not support a role for changes in presynaptic glutamate in the development of behavioral sensitization.

Discrete cell gene profiling of ventral tegmental dopamine neurons after acute and chronic cocaine self-administration

Chronic cocaine administration induces a number of biochemical alterations within the mesolimbic dopamine system that may mediate various aspects of the addictive process such as sensitization, craving, withdrawal, and relapse. In the present study, rats were allowed to self-administer cocaine (0.5 mg/infusion) for 1 or 20 days. Tyrosine hydroxylase immunopositive cells were microdissected from the ventral tegmental area (VTA) using laser capture microdissection, and changes in the abundances of 95 mRNAs were assessed using cDNA macroarrays. Five GABA-A receptor subunit mRNAs (alpha4, alpha6, beta2, gamma2, and delta) were down-regulated at both 1 and 20 days of cocaine self-administration. In contrast, the catalytic subunit of protein phosphatase 2A (PP2alpha), GABA-A alpha1, and Galphai2 were significantly increased at both time points. Additionally, calcium/calmodulin-dependent protein kinase IIalpha mRNA levels were increased initially followed by a slight decrease after 20 days, whereas neuronal nitric-oxide synthase mRNA levels were initially decreased but returned to near control levels by day 20. These results indicate that alterations of specific GABA-A receptor subtypes and other signal transduction transcripts seem to be specific neuroadaptations associated with cocaine self-administration. Moreover, as subunit composition determines the functional properties of GABA-A receptors, the observed changes may indicate alterations in the excitability of dopamine transmission underlying long-term biochemical and behavioral effects of cocaine.

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.

Differential effect of dopamine deficiency on the expression of NMDA receptor subunits in the weaver mouse brain

The weaver mutant mouse is characterized by degeneration of the dopaminergic mesencephalic neurons. The role of the dopaminergic system in the regulation of N-methyl-d-aspartate (NMDA) receptor subunit expression was addressed in the present study. In situ hybridization experiments were conducted to determine the expression levels of the NMDA receptor subunit mRNAs, z1, epsilon1 and epsilon2, in striatum, nucleus accumbens, olfactory tubercle and cerebral cortical regions of 26-day-, 3- and 6-month-old weaver mice. Data indicated statistically significant increases in z1 and epsilon2 mRNA levels in 6-month-old weaver striatum, whereas at the same age epsilon1 mRNA expression was decreased in all striatal regions, as well as in the cortex. In the 26-day-old weaver striatum and nucleus accumbens, statistically significant increases were observed in epsilon1 mRNA levels, whereas no changes were observed in the other two subunits. In the somatosensory cortex of 26-day-old weaver brain an increased expression of all three subunits was observed. The upregulation of NMDA receptor subunit expression observed in the somatosensory cortex can be attributed to a decreased activity of the glutamatergic thalamocortical pathway, following the degeneration of the nigrostriatal dopaminergic fibres. In the striatum, the present results demonstrate a differential control on the expression of z1 and epsilon2 subunits on the one hand, and epsilon1 subunit on the other. It is suggested that dopamine exerts a negative control on the expression of z1 and epsilon2 subunits, through a downregulation of transcription factors associated with the AP1 regulatory site, which is mediated by the activation of striatal dopamine D2 receptors.

Molecular profiling of midbrain dopamine regions in cocaine overdose victims

Chronic cocaine use in humans and animal models is known to lead to pronounced alterations in neuronal function in brain regions associated with drug reinforcement. To evaluate whether the alterations in gene expression in cocaine overdose victims are associated with specific dopamine populations in the midbrain, cDNA arrays and western blotting were used to compare gene and protein expression patterns between cocaine overdose victims and age-matched controls in the ventral tegmental area (VTA) and lateral substantia nigra (l-SN). Array analysis revealed significant up-regulation of numerous transcripts in the VTA, but not in the l-SN, of cocaine overdose victims including NMDAR1, GluR2, GluR5 and KA2 receptor mRNA (p < 0.05). No significant alterations between overdose victims and controls were observed for GluR1, R3 or R4 mRNA levels. Correspondingly, western blot analysis revealed VTA-selective up-regulation of CREB (p < 0.01), NMDAR1 (p < 0.01), GluR2 (p < 0.05), GluR5 (p < 0.01) and KA2 (p < 0.05) protein levels of cocaine overdose victims. The present results indicate that selective alterations of CREB and certain ionotropic glutamate receptor (iGluR) subtypes appear to be associated with chronic cocaine use in humans in a region-specific manner. Moreover, as subunit composition determines the functional properties of iGluRs, the observed changes may indicate alterations in the excitability of dopamine transmission underlying long-term biochemical and behavioral effects of cocaine in humans.

The N-methyl-D-aspartate receptor subunit NR2B: localization, functional properties, regulation, and clinical implications

The N-methyl-D-aspartate (NMDA) receptor is an example of a heteromeric ligand-gated ion channel that interacts with multiple intracellular proteins by way of different subunits. NMDA receptors are composed of seven known subunits (NR1, NR2A-D, NR3A-B). The present review focuses on the NR2B subunit of the receptor. Over the last several years, an increasing number of reports have demonstrated the importance of the NR2B subunit in a variety of synaptic signaling events and protein-protein interactions. The NR2B subunit has been implicated in modulating functions such as learning, memory processing, pain perception, and feeding behaviors, as well as being involved in a number of human disorders. The following review provides a summary of recent findings regarding the structural features, localization, functional properties, and regulation of the NR2B subunit. The review concludes with a section discussing the role of NR2B in human diseases.

Repeated cocaine administration differentially affects NMDA receptor subunit (NR1, NR2A-C) mRNAs in rat brain

We investigated the effects of intermittent intraperitoneal (i.p.) injections of cocaine (20 mg/kg) on subunit mRNAs of N-methyl-D-aspartate (NMDA) receptors (NR1/NR2A-2C) in the rat brain by in situ hybridization using phosphor screen analysis. The level of NR1 subunit mRNA significantly increased in hippocampal complexes 1 h after a single i.p. injection of cocaine. After repeated cocaine injection, the mean scores of stereotyped behavior were increased with the number of injections. The level of NR1 subunit mRNA was obviously decreased in the striatum and cortices 24 h (early withdrawal) after a final injection following 14 days of subchronic administration. During the early withdrawal period, the amount of the NR1 subunit decreased in the nucleus accumbens, globus pallidus, and subiculum. In the dentate gyrus, the NR1 mRNA level significantly increased during early withdrawal in rats subchronically treated with cocaine. Levels of NR2B subunit mRNA were reduced in the cortices and striatum. During late withdrawal from cocaine, the level of NR2C subunit mRNA in the cerebellum was also reduced. These findings suggest that the disruption of NR1, NR2B, and NR2C subunits in the discrete brain regions occurs under the cocaine-related behavioral abnormalities and would be closely implicated in the initiation and expression of behavioral sensitization induced by repeated cocaine administration. Further studies on the changes in non-NMDA receptors are required to elucidate the biological significance of glutamate receptors for the mechanisms underlying the development of behavioral sensitization.

Dose-related alteration in nitric oxide synthase mRNA expression induced by amphetamine and the full D1 dopamine receptor agonist SKF-82958 in mouse striatum

Neuronal nitric oxide synthase (nNOS) is normally expressed in one population of intrinsic interneurons of the striatum. Production of nitric oxide in the nNOS-containing neurons is sensitive to dopamine stimulation. Using quantitative in situ hybridization, the present study investigated the alteration in basal nNOS mRNA expression in striatal nNOS-containing neurons of mice treated with the psychostimulant amphetamine or a full D1 dopamine receptor agonist, SKF-82958. A single systemic injection of amphetamine induced a dose-related change in striatal nNOS mRNA expression. Whereas amphetamine at 4 mg/kg decreased basal levels of nNOS mRNA in both the dorsal (caudoputamen) and ventral (nucleus accumbens) striatum, the drug at a higher dose (12 mg/kg) increased nNOS expression in the two regions. Similarly, an acute systemic injection of SKF-82958 decreased and increased nNOS mRNA levels in the dorsal and ventral striatum at 2 and 4 mg/kg, respectively. These data indicate that constitutive nNOS expression in nitric oxide-producing neurons of the mouse striatum is regulated by dopaminergic transmission. Altered nNOS expression may result in changes in nitric oxide synthesis and thus contribute to biological actions of dopamine stimulants.