Cocaine induction of ERK proteins in dorsal striatum of Fischer rats

An allosteric potentiator of metabotropic glutamate (mGlu) 2 receptors reduces the cocaine-stimulated ERK1/2 phosphorylation in the mouse striatum

Metabotropic glutamate (mGlu) receptors are involved in the experience-dependent neuroplasticity in the mesolimbic reward circuit. A Gαi/o-coupled mGlu2 subtype is distributed presynaptically in the striatum. These autoreceptors may have a significant influence over striatal neurons in their intracellular signaling pathways in response to a psychostimulant. Here we explored the effect of pharmacological potentiation of mGlu2 receptors on cocaine-stimulated phosphorylation (activation) of extracellular signal-regulated kinases (ERK) in the mouse striatum in vivo. We found that an mGlu2 selective positive allosteric modulator (PAM) LY487379 after a systemic injection did not alter basal phosphorylation of ERK1/2 or c-Jun N-terminal kinases in the striatum. However, pretreatment with LY487379 blocked the ERK1/2 phosphorylation induced by cocaine in the two subdivisions of the striatum, i.e., the caudate putamen and nucleus accumbens. LY487379 also blocked the cocaine-induced phosphorylation of Elk-1, a transcription factor downstream to the ERK pathway. Additionally, LY487379 reduced locomotor behavioral responses to cocaine. These results demonstrate that the mGlu2 PAM LY487379 possesses the ability to attenuate the activation of the ERK1/2 pathway in striatal neurons and reduce locomotor activity in response to cocaine in vivo.

Pre-exposure of adolescent mice to morphine results in stronger sensitization and reinstatement of conditioned place preference than pre-exposure to hydrocodone

BACKGROUND

Opioids are commonly prescribed to treat moderate-to-severe pain. However, their use can trigger the development of opioid use disorder. A major problem in treating opioid use disorder remains the high rate of relapse.

AIM

The purpose of this study was to determine whether there are differences among opioids in their ability to trigger relapse after pre-exposure during adolescence.

METHODS

On postnatal day 33, mice were examined for the acute locomotor response to saline, morphine, or hydrocodone (5 mg/kg). They were administered with the corresponding opioid or saline during postnatal days 34-38 (20 mg/kg) and 40-44 (40 mg/kg). On postnatal day 45, they were recorded for the development of locomotor sensitization (5 mg/kg). Starting on postnatal day 55, mice were examined for the acquisition (1, 5, 10, 20, and 40 mg/kg), extinction, and drug-induced reinstatement (1, 2.5, and 5 mg/kg) of conditioned place preference.

RESULTS

There were no significant differences in the acute locomotor response to morphine and hydrocodone. Morphine induced significantly stronger locomotor sensitization as compared to hydrocodone. Pre-exposure to morphine, but not hydrocodone, sensitized the acquisition of conditioned place preference. There were no significant differences in extinction rates. Mice pre-exposed to morphine reinstate conditioned place preference after priming with a 1 mg/kg dose. In contrast, higher priming doses were required for reinstatement in all other experimental groups.

CONCLUSIONS

Adolescent mice administered with morphine develop greater sensitization to its effects and subsequently reinstate conditioned place preference more readily than mice administered with hydrocodone. This suggests higher risk for relapse after pre-exposure to morphine during adolescence as compared to hydrocodone.

Relevance of ERK1/2 Post-retrieval Participation on Memory Processes: Insights in Their Particular Role on Reconsolidation and Persistence of Memories

Back in 1968, Misanin and his group posited that reactivation of consolidated memories could support changes in that trace, similar to what might happen during the consolidation process. Not until 2000, when Nader et al. (2000) studied the behavioral effect of a protein synthesis inhibitor on retrieved memories, could this previous statement be taken under consideration once again; suggesting that consolidated memories can become labile after reactivation. The process of strengthening after memory labilization was named memory reconsolidation. In recent years, many studies pointed towards a critical participation of the extracellular signal-regulated kinase (ERK)/mitogen activated protein kinases (MAPKs) pathway in different memory processes (e.g., consolidation, extinction, reconsolidation, among others). In this review article, we will focus on how this system might be modulating the processes triggered after retrieval of well-consolidated memories in mice.

Inhibition of basal and amphetamine-stimulated extracellular signal-regulated kinase (ERK) phosphorylation in the rat forebrain by muscarinic acetylcholine M4 receptors

The mitogen-activated protein kinase (MAPK), especially its extracellular signal-regulated kinase (ERK) subfamily, is a group of kinases enriched in the mammalian brain. While ERK is central to cell signaling and neural activities, the regulation of ERK by transmitters is poorly understood. In this study, the role of acetylcholine in the regulation of ERK was investigated in adult rat striatum in vivo. We focused on muscarinic M1 and M4 receptors, two principal muscarinic acetylcholine (mACh) receptor subtypes in the striatum. A systemic injection of the M1-preferring antagonist telenzepine did not alter ERK phosphorylation in the two subdivisions of the striatum, the caudate putamen and nucleus accumbens. Similarly, telenzepine did not affect ERK phosphorylation in the medial prefrontal cortex (mPFC), hippocampus, and cerebellum. Moreover, telenzepine had no effect on the ERK phosphorylation induced by dopamine stimulation with the psychostimulant amphetamine. In contrast to telenzepine, the M4-preferring antagonist tropicamide consistently increased ERK phosphorylation in the striatum and mPFC. This increase was rapid and transient. Tropicamide and amphetamine when coadministered at subthreshold doses induced a significant increase in ERK phosphorylation. These results demonstrate that mACh receptors exert a subtype-specific modulation of ERK in striatal and mPFC neurons. While the M1 receptor antagonist has no effect on ERK phosphorylation, M4 receptors inhibit constitutive and dopamine-stimulated ERK phosphorylation in these dopamine-innervated brain regions.

Functional roles of orexin/hypocretin receptors in reward circuit

Since its first discovery in 1998, it has become clear that the orexinergic system plays an important role in regulating a number of functions including food, sex, social connections, and most prominently reward-related behaviors. Orexinergic neurons in the lateral hypothalamus project extensively to other brain areas, two most important of which are the ventral tegmental area and the nucleus accumbens that are involved in reward processing. In this review, we have presented the work in our laboratory along with the work of others and have discussed the possible functions we can infer from the research. We discuss the anatomy of the orexinergic system and its components followed by a presentation of other connected brain areas. The second part of this review discusses observed results from the morphine conditioned place preference test that sheds light on the possible role of the involved areas in reward processing. The complex circuits involved in reward processing are only beginning to be understood and we need to deepen our understanding regarding the nature of the interactions between all brain areas involved.

NMDAR dependent intracellular responses associated with cocaine conditioned place preference behavior

The aim of this study was to investigate the intracellular responses associated with the acquisition and expression of cocaine-context associations. ERK (extracellular regulated kinase), CREB (cAMP responsive element binding protein), FosB and ΔFosB proteins were of particular interest due to their involvement in cocaine reward and in synaptic plasticity underlying learning and memory. We used the conditioned place preference (CPP) paradigm, which employs a Pavlovian conditioning procedure to establish an association between a drug-paired environment and the drug's rewarding effects, to study the role of these signaling pathways in cocaine-context associations. N-methyl-D-aspartate receptor (NMDAR) antagonism prior to cocaine administration during conditioning blocked the acquisition of cocaine CPP and reduced Nucleus Accumbens (NAc) phosphorylated-ERK (pERK) and phosphorylated CREB (pCREB) levels following the CPP test (drug-free). We also show that cocaine-induced increases in Caudate Putamen (CPu) FosB and ΔFosB levels are decreased after MK-801 pre-treatment during conditioning. In addition, our results provide evidence for the involvement of striatal SIRT (Silent Information Regulator of Transcription) proteins in cocaine-CPP. These results will aid in the advancement of general knowledge about the molecular formation and retrieval of cocaine-associated memories that can be used in the future when designing treatments for cocaine addiction that target both prevention and relapse.

Molecular Mechanism: ERK Signaling, Drug Addiction, and Behavioral Effects

Addiction to psychostimulants has been considered as a chronic psychiatric disorder characterized by craving and compulsive drug seeking and use. Over the past two decades, accumulating evidence has demonstrated that repeated drug exposure causes long-lasting neurochemical and cellular changes that result in enduring neuroadaptation in brain circuitry and underlie compulsive drug consumption and relapse. Through intercellular signaling cascades, drugs of abuse induce remodeling in the rewarding circuitry that contributes to the neuroplasticity of learning and memory associated with addiction. Here, we review the role of the extracellular signal-regulated kinase (ERK), a member of the mitogen-activated protein kinase, and its related intracellular signaling pathways in drug-induced neuroadaptive changes that are associated with drug-mediated psychomotor activity, rewarding properties and relapse of drug seeking behaviors. We also discuss the neurobiological and behavioral effects of pharmacological and genetic interferences with ERK-associated molecular cascades in response to abused substances. Understanding the dynamic modulation of ERK signaling in response to drugs may provide novel molecular targets for therapeutic strategies to drug addiction.

Regulation of synaptic MAPK/ERK phosphorylation in the rat striatum and medial prefrontal cortex by dopamine and muscarinic acetylcholine receptors

Dopamine and acetylcholine are two principal transmitters in the striatum and are usually balanced to modulate local neural activity and to maintain striatal homeostasis. This study investigates the role of dopamine and muscarinic acetylcholine receptors in the regulation of a central signaling protein, i.e., the mitogen-activated protein kinase (MAPK). We focus on the synaptic pool of MAPKs because of the fact that these kinases reside in peripheral synaptic structures in addition to their somatic locations. We show that a systemic injection of dopamine D1 receptor (D1R) agonist SKF81297 enhances phosphorylation of extracellular signal-regulated kinases (ERKs), a prototypic subclass of MAPKs, in the adult rat striatum. Similar results were observed in another dopamine-responsive region, the medial prefrontal cortex (mPFC). The dopamine D2 receptor agonist quinpirole had no such effects. Pretreatment with a positive allosteric modulator (PAM) of muscarinic acetylcholine M4 receptors (M4Rs), VU0152100, attenuated the D1R agonist-stimulated ERK phosphorylation in the two regions, whereas the PAM itself did not alter basal ERK phosphorylation. All drug treatments had no effect on phosphorylation of c-Jun N-terminal kinases (JNKs), another MAPK subclass, in the striatum and mPFC. These results demonstrate that dopamine and acetylcholine are integrated to control synaptic ERK but not JNK activation in striatal and mPFC neurons in vivo. Activation of M4Rs exerts an inhibitory effect on the D1R-mediated upregulation of synaptic ERK phosphorylation.

Region and context-specific intracellular responses associated with cocaine-induced conditioned place preference expression

The development and maintenance of cocaine addiction depend heavily on learned reward-environment associations that can induce drug-seeking behavior and relapse. Understanding the mechanisms underlying these cue-induced conditioned responses is important for relapse prevention. To test whether intracellular responses measured after cocaine conditioned place preference (CPP) expression are context-dependent, we re-exposed cocaine-treated rats (drug-free) to an environment previously paired with cocaine or saline, 24h after the CPP test. After 8 days of cocaine CPP training with one of two cocaine doses (5mg/kg or 20mg/kg, i.p.), CPP was expressed only after conditioning with the higher cocaine dose. In CPP expressing rats, locomotor responses after re-exposure to the cocaine-chamber were greater than in rats re-exposed to the saline-paired chamber. Nucleus Accumbens (NAc) phosphorylated ERK (pERK) levels were increased after re-exposure to the cocaine-paired, but not the saline-paired chamber, regardless of whether or not CPP behavior was expressed. Caudate Putamen (CPu) pERK and FosB protein levels increased after re-exposure to the cocaine chamber only after conditioning with the higher cocaine dose. Conversely, the higher cocaine dose, independent of environment, resulted in increased NAc FosB, ΔFosB and phosphorylated CREB (pCREB) protein levels compared to those conditioned with 5mg/kg cocaine (non-CPP-expressing). Our results suggest that NAc ERK phosphorylation may be involved with retrieving the contextual information of a cocaine-association, without necessarily motivating the expression of CPP behavior. Additionally, we show distinct patterns of intracellular responses in the NAc and CPu indicating a region-specific role for pERK/pCREB/FosB intracellular signaling in the retrieval of cocaine-context associations.

Appetitive cue-evoked ERK signaling in the nucleus accumbens requires NMDA and D1 dopamine receptor activation and regulates CREB phosphorylation

Conditioned stimuli (CS) can modulate reward-seeking behavior. This modulatory effect can be maladaptive and has been implicated in excessive reward seeking and relapse to drug addiction. We previously demonstrated that exposure to an appetitive CS causes an increase in the activation of extracellular signal-regulated kinase (ERK) and cyclic-AMP response-element binding protein (CREB) in the nucleus accumbens (NAc) of rats, and that CS-evoked ERK activation is critical for CS control over reward seeking. To elucidate the mechanism that mediates CS-driven ERK activation in the NAc, we selectively blocked NMDA glutamate or D1 dopamine receptors in the NAc. To determine whether CS-driven ERK and CREB activation are linked, we selectively blocked ERK signaling in the NAc. We found that both NMDA and D1 receptors are critical for CS-driven ERK signaling in the NAc, and that this recruitment of the ERK cascade is responsible for increased CREB activation in the presence of the CS. Our findings suggest that activation of the NMDAR-D1R/ERK/CREB signal transduction pathway plays a critical role in the control of reward-seeking behavior by reward-predictive cues.

Dopamine-induced tyrosine phosphorylation of NR2B (Tyr1472) is essential for ERK1/2 activation and processing of novel taste information

Understanding the heterosynaptic interaction between glutamatergic and neuromodulatory synapses is highly important for revealing brain function in health and disease. For instance, the interaction between dopamine and glutamate neurotransmission is vital for memory and synaptic plasticity consolidation, and it is known to converge on extracellular signal-regulated kinase (ERK)-MAPK signaling in neurons. Previous studies suggest that dopamine induces N-methyl-D-aspartate (NMDA) receptor phosphorylation at the NR2B Y1472 subunit, influencing receptor internalization at the synaptic plasma membrane. However, it is unclear whether this phosphorylation is upstream to and/or necessary for ERK1/2 activation, which is known to be crucial for synaptic plasticity and memory consolidation. Here, we tested the hypothesis that tyrosine phosphorylation of NR2B at Y1472 is correlated with ERK1/2 activation by dopamine and necessary for it as well. We find that dopamine receptor D1, but not D2, activates ERK1/2 and leads to NR2BY1472 phosphorylation in the mature hippocampus and cortex. Moreover, our results indicate that NR2B Y1472 phosphorylation is necessary for ERK1/2 activation. Importantly, application of dopamine or the D1 receptor agonist SKF38393 to hippocampal slices from NR2B F1472 mutant mice did not result in ERK1/2 activation, suggesting this site is not only correlated with ERK1/2 activation by dopamine stimulation, but also necessary for it. In addition, NR2B F1472 mice show impairment in learning of attenuation of taste neophobia but not associative taste learning. Our study shows that the dopaminergic and glutamatergic transmission converge on the NMDA receptor itself, at the Y1472 site of the NR2B subunit, and that this convergence is essential for ERK1/2 activation in the mature brain and for processing new sensory information in the cortex.

The effect of active and passive intravenous cocaine administration on the extracellular signal-regulated kinase (ERK) activity in the rat brain

According to a current hypothesis of learning processes, recent papers pointed out to an important role of the extracellular signal-regulated kinase (ERK), in drug addiction. We employed the Western blotting techniques to examine the ERK activity immediately after cocaine iv self-administration and in different drug-free withdrawal periods in rats. To distinguish motivational vs. pharmacological effects of the psychostimulant intake, a "yoked" procedure was used. Animals were decapitated after 14 daily cocaine self-administration sessions or on the 1st, 3rd or 10th extinction days. At each time point the activity of the ERK was assessed in several brain structures, including the prefrontal cortex, hippocampus, dorsal striatum and nucleus accumbens. Passive, repeated iv cocaine administration resulted in a 45% increase in ERK phosphorylation in the hippocampus while cocaine self-administration did not change brain ERK activity. On the 1st day of extinction, the activity of the ERK in the prefrontal cortex was decreased in rats with a history of cocaine chronic intake: by 66% for "active" cocaine group and by 35% for "yoked" cocaine group. On the 3rd day the reduction in the ERK activity (25-34%) was observed in the hippocampus for both cocaine-treated groups, and also in the nucleus accumbens for "yoked" cocaine group (40%). On the 10th day of extinction there was no significant alteration in ERK activity in any group of rats. Our findings suggest that cortical ERK is involved in cocaine seeking behavior in rats. They also indicate the time and regional adaptations in this enzyme activity after cocaine withdrawal.

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.

ELK1 transcription factor linked to dysregulated striatal mu opioid receptor signaling network and OPRM1 polymorphism in human heroin abusers

BACKGROUND

Abuse of heroin and prescription opiate medications has grown to disturbing levels. Opioids mediate their effects through mu opioid receptors (MOR), but minimal information exists regarding MOR-related striatal signaling relevant to the human condition. The striatum is a structure central to reward and habitual behavior and neurobiological changes in this region are thought to underlie the pathophysiology of addiction disorders.

METHODS

We examined molecular mechanisms related to MOR in postmortem human brain striatal specimens from a homogenous European Caucasian population of heroin abusers and control subjects and in an animal model of heroin self-administration. Expression of ets-like kinase 1 (ELK1) was examined in relation to polymorphism of the MOR gene OPRM1 and drug history.

RESULTS

A characteristic feature of heroin abusers was decreased expression of MOR and extracellular regulated kinase signaling networks, concomitant with dysregulation of the downstream transcription factor ELK1. Striatal ELK1 in heroin abusers associated with the polymorphism rs2075572 in OPRM1 in a genotype dose-dependent manner and correlated with documented history of heroin use, an effect reproduced in an animal model that emphasizes a direct relationship between repeated heroin exposure and ELK1 dysregulation. A central role of ELK1 was evidenced by an unbiased whole transcriptome microarray that revealed ~20% of downregulated genes in human heroin abusers are ELK1 targets. Using chromatin immune precipitation, we confirmed decreased ELK1 promoter occupancy of the target gene Use1.

CONCLUSIONS

ELK1 is a potential key transcriptional regulatory factor in striatal disturbances associated with heroin abuse and relevant to genetic mutation of OPRM1.

Sexually dimorphic intracellular responses after cocaine-induced conditioned place preference expression

Sex differences in cocaine's mechanisms of action and behavioral effects have been widely reported. However, little is known about how sex influences intracellular signaling cascades involved with drug-environment associations. We investigated whether ERK/CREB intracellular responses in the mesocorticolimbic circuitry underlying cocaine environmental associations are sexually dimorphic. We used a standard 4 day conditioned place preference (CPP) paradigm using 20mg/kg cocaine-a dose that induced CPP in male and female Fischer rats. In the nucleus accumbens (NAc) following CPP expression, cocaine treated animals showed increased phosphorylated ERK (pERK), phosphorylated CREB (pCREB) and ΔFosB protein levels. In the hippocampus (HIP) and caudate putamen (CPu), pERK and FosB/ΔFosB levels were also increased, respectively. Cocaine females had a larger change in HIP pERK and CPu ΔFosB levels than cocaine males; partly due to lower protein levels in saline female rats when compared to saline males. Prefrontal cortex (PfC) pCREB levels increased in cocaine males, but not females, whereas PfC pERK levels were increased in cocaine females, but not males. CPP scores were positively correlated to NAc pERK, HIP pERK and CPu FosB protein levels, suggesting that similar to males, the ERK/CREB intracellular pathway in mesocorticolimbic regions undergoes cocaine induced neuroplasticity in female rats. However, there seem to be intrinsic (basal) sexual dimorphisms in this pathway that may contribute to responses expressed after cocaine-CPP. Taken together, our results suggest that cellular responses associated with the expression of learned drug-environment associations may play an important role in sex differences in cocaine addiction and relapse.

Amphetamine increases phosphorylation of MAPK/ERK at synaptic sites in the rat striatum and medial prefrontal cortex

Mitogen-activated protein kinases (MAPKs) play a central role in cell signaling. Extracellular signal-regulated kinase (ERK) is a prototypic subclass of MAPKs and is densely expressed in postmitotic neurons of adult mammalian brains. Active ERK translocates into the nucleus to regulate gene expression. Additionally, ERK is visualized in neuronal peripheries, such as distal synaptic structures. While nuclear ERK is a known sensitive target of psychostimulants, little is known about the responsiveness of synaptic ERK to stimulants. In this study, we focused on ERK at synaptic versus extrasynaptic sites and investigated its responses to the psychostimulant amphetamine in the adult rat striatum and medial prefrontal cortex (mPFC) in vivo. We used a pre-validated biochemical fractionation procedure to isolate synapse- and extrasynapse-enriched membranes. We found that two common ERK isoforms (ERK1 and ERK2) were concentrated more in extrasynaptic fractions than in synaptic fractions in striatal and cortical neurons under normal conditions. At synaptic sites, ERK2 was noticeably more abundant than ERK1. Acute injection of amphetamine induced an increase in ERK2 phosphorylation in the synaptic fraction of striatal neurons, while the drug did not alter extrasynaptic ERK2 phosphorylation. Similar results were observed in the mPFC. In both synaptic and extrasynaptic compartments, total ERK1/2 proteins remained stable in response to amphetamine. Our data establish the subsynaptic distribution pattern of MAPK/ERK in striatal and cortical neurons. Moreover, the synaptic pool of ERK2 in these neurons can be selectively activated by amphetamine.

Elk-1 a transcription factor with multiple facets in the brain

The ternary complex factor (TCF) Elk-1 is a transcription factor that regulates immediate early gene (IEG) expression via the serum response element (SRE) DNA consensus site. Elk-1 is associated with a dimer of serum response factor (SRF) at the SRE site, and its phosphorylation occurs at specific residues in response to mitogen-activated protein kinases (MAPKs), including c-Jun-N terminal kinase (JNK), p38/MAPK, and extracellular-signal regulated kinase (ERK). This phosphorylation event is critical for triggering SRE-dependent transcription. Although MAPKs are fundamental actors for the instatement and maintenance of memory, and much investigation of their downstream signaling partners have been conducted, no data yet clearly implicate Elk-1 in these processes. This is partly due to the complexity of Elk-1 sub-cellular localization, and hence functions, within neurons. Elk-1 is present in its resting state in the cytoplasm, where it colocalizes with mitochondrial proteins or microtubules. In this particular sub-cellular compartment, overexpression of Elk-1 is toxic for neuronal cells. When phosphorylated by the MAPK/ERK, Elk-1 translocates to the nucleus where it is implicated in regulating chromatin remodeling, SRE-dependent transcription, and neuronal differentiation. Another post-translational modification is the conjugation to SUMO (Small Ubiquitin-like MOdifier), which relocalizes Elk-1 in the cytoplasm. Thus, Elk-1 plays a dual role in neuronal functions: pro-apoptotic within the cytoplasm, and pro-differentiation within the nucleus. To address the role of Elk-1 in the brain, one must be aware of its multiple facets, and design molecular tools that will shut down Elk-1 expression, trafficking, or activation, in specific neuronal compartments. We summarize in this review the known molecular functions of Elk-1, its regulation in neuronal cells, and present evidence of its possible implication in model systems of synaptic plasticity, learning, but also in neurodegenerative diseases.

Changes in phosphorylation of CREB, ERK, and c-fos induction in rat ventral tegmental area, hippocampus and prefrontal cortex after conditioned place preference induced by chemical stimulation of lateral hypothalamus

Experimental evidence indicates that chemical stimulation of lateral hypothalamus (LH) by carbachol can produce conditioned place preference (CPP) in rats. Several lines of evidence have shown that cAMP-response element binding protein (CREB), extracellular signal-regulated kinase (ERK), and c-fos have pivotal role in CPP induced by drugs of abuse, such as morphine, cocaine, nicotine, and alcohol. Therefore, in the present study, we investigated the changes in phosphorylated-CREB (p-CREB) and -ERK (p-ERK), and c-fos induction within ventral tegmental area (VTA), hippocampus and prefrontal cortex (PFC) after the acquisition of CPP induced by intra-LH administration of carbachol. Animals were unilaterally implanted by cannula into LH. For chemical stimulation of LH, carbachol (250 nmol/0.5 μl saline) was microinjected once each day, during 3-day conditioning phase (acquisition period) of CPP paradigm. After the acquisition period, the brains were removed, and p-CREB and p-ERK, and c-fos induction in the ipsilateral VTA, hippocampus and PFC were measured by Western blot analysis. The results indicated a significant increase in level of phosphorylated CREB (P<0.01) in VTA, and PFC (P<0.05), during LH stimulation-induced CPP, while its level decreased in hippocampus (P<0.05). Also, in aforementioned regions, an increase in c-fos level was observed, but this enhancement in PFC was not significant. Moreover, p-ERK changed in these areas, but not significantly. Our findings suggest that studying the intracellular signals and their changes, such as phosphorylated-CREB, can elucidate a functional relationship between LH and other brain structures involved in reward processing in rats.

Cyclic adenosine monophosphate-independent tyrosine phosphorylation of NR2B mediates cocaine-induced extracellular signal-regulated kinase activation

BACKGROUND

Activation of the extracellular signal-regulated kinase (ERK) in the striatum is crucial for long-term behavioral alterations induced by drugs of abuse. In response to cocaine, ERK phosphorylation (i.e., activation) is restricted to medium-sized spiny neurons expressing dopamine D1 receptor (D1R) and depends on a concomitant stimulation of D1R and glutamate N-methyl-D-aspartate receptor (NMDAR). However, the mechanisms responsible for this activation, especially the respective contribution of D1R and NMDAR, remain unknown.

METHODS

We studied striatal neurons in culture stimulated with D1R agonist and/or glutamate and wild-type or genetically modified mice treated with cocaine. Biochemical, immunohistochemical, and imaging studies were performed. Mice were also subjected to behavioral experiments.

RESULTS

Stimulation of D1R cannot activate ERK by itself but potentiates glutamate-mediated calcium influx through NMDAR that is responsible for ERK activation. Potentiation of NMDAR by D1R depends on a cyclic adenosine monophosphate-independent signaling pathway, which involves tyrosine phosphorylation of the NR2B subunit of NMDAR by Src family kinases. We also demonstrate that the D1R/Src family kinases/NR2B pathway is responsible for ERK activation by cocaine in vivo. Inhibition of this pathway abrogates cocaine-induced locomotor sensitization and conditioned place preference.

CONCLUSIONS

Our results show that potentiation of NR2B-containing NMDAR by D1R is necessary and sufficient to trigger cocaine-induced ERK activation. They highlight a new cyclic adenosine monophosphate-independent pathway responsible for the integration of dopamine and glutamate signals by the ERK cascade in the striatum and for long-term behavioral alterations induced by cocaine.

Elevated BDNF after cocaine withdrawal facilitates LTP in medial prefrontal cortex by suppressing GABA inhibition

Medial prefrontal cortex (mPFC) is known to be involved in relapse after cocaine withdrawal, but the underlying cellular mechanism remains largely unknown. Here, we report that after terminating repeated cocaine exposure in rats, a gradual increase in the expression of brain-derived neurotrophic factor (BDNF) in the mPFC facilitates activity-induced long-term potentiation (LTP) of excitatory synapses on layer V pyramidal neurons. This enhanced synaptic plasticity could be attributed to BDNF-induced suppression of GABAergic inhibition in the mPFC by reducing the surface expression of GABA(A) receptors. The BDNF effect was mediated by BDNF-TrkB-phosphatase 2A signaling pathway. Downregulating TrkB expression bilaterally in the mPFC reduced the locomotor hypersensitivity to cocaine 8 days after cocaine withdrawal. Thus, elevated BDNF expression after cocaine withdrawal sensitizes the excitatory synapses in the mPFC to undergo activity-induced persistent potentiation that may contribute to cue-induced drug craving and drug-seeking behavior.

Role of MEK-ERK pathway in morphine-induced conditioned place preference in ventral tegmental area of rats

A major goal of research on drug addiction is to develop the effective treatments to deal with the long-term behavioral disorders especially reinstatement induced by the addictive drugs such as opiates, cocaine, and cannabinoid. The molecular mechanisms underlying these substance-related disorders remain unclear so far. Here we used the model of morphine-induced conditioned place preference (CPP) in rats to mimic the progress of drug-taking, withdrawal and relapse in human. The tissue of ventral tegmental area (VTA), one of the most important brain structures associated with abused drug-related disorders, was taken and two-dimensional electrophoresis (2-DE) was performed to analyze and compare the changes of protein expression patterns during the different stages of morphine-induced CPP. First, we found that there were 80 proteins identified to be changed in the process of morphine-induced CPP. Furthermore, as the mitogen-activated protein kinase kinase 1 (MAPKK1) was increased significantly in the stages of establishment and reinstatement, we confirmed the change of activated extracellular signal-regulated kinase (ERK) by Western blotting in VTA tissue and cultured cell. The results demonstrated that the activated MEK-ERK pathway by chronic morphine treatment in VTA was involved in morphine-induced reinstatement. Moreover, inhibition of MEK-ERK pathway by infusion the MEK inhibitor U0126 in VTA blocked the establishment of morphine-induced CPP. The present study found significant changes in a group of protein expressions in VTA during morphine-induced CPP and further confirmed the role of MEK-ERK cell signaling pathway of VTA in morphine addiction.

Dopamine D1 and N-methyl-D-aspartate receptors and extracellular signal-regulated kinase mediate neuronal morphological changes induced by repeated cocaine administration

The development of drug addiction involves persistent cellular and molecular changes in the CNS. The brain dopamine and glutamate systems play key roles in mediating drug-induced neuroadaptation. Changes in dendritic morphology in medium spiny neurons (MSNs) in the nucleus accumbens (NAc) and caudate putamen (CPu) accompany drug-induced enduring behavioral and molecular changes. We have investigated the potential involvement of dopamine D1 and D2 receptors, the N-methyl-D-aspartate (NMDA) receptor, and the extracellular signal-regulated kinase (ERK) in dendritic morphological changes induced by repeated cocaine administration. We show that either a genetic mutation or pharmacological blockade of dopamine D1 receptors attenuated cocaine-induced changes in both dendritic branching and spine density of MSNs in the shell of the NAc and CPu. In contrast, antagonism of dopamine D2 receptors had no obvious effect on changes in dendritic branching but had a partial effect on changes in spine density of MSNs in these brain regions following repeated cocaine injections. Pharmacological inhibition of either NMDA receptors or ERK attenuated cocaine-induced changes in both dendritic branching and spine density of MSNs in the shell of the NAc and CPu. These results suggest that dopamine D1 and NMDA receptors and ERK contribute significantly to neuronal morphological changes induced by repeated exposure to cocaine.

Brain-derived neurotrophic factor signaling modulates cocaine induction of reward-associated ultrasonic vocalization in rats

Cocaine exhibits high liability for inducing addictive behaviors, but the mechanisms of neuroplasticity underlying the behavioral effects remain unclear. As a crucial mediator of neuroplasticity in diverse functional models, brain-derived neurotrophic factor (BDNF) could contribute to the mechanisms of addiction-related neuroplasticity. Here, we addressed the hypothesis that cocaine increases synaptic dopamine, which induces BDNF protein expression to initiate addiction-related behavior in the rat. An enzyme-linked immunosorbent assay was used to measure BDNF protein expression in rat striatal tissues. For behavioral readout, we used a noninvasive measurement system to measure the emission of 50-kHz ultrasonic vocalization (USV), a response that correlates with electrical brain stimulation and conditioned place preference behavior in rodents. A single injection of cocaine significantly increased BDNF protein expression, but this effect was not further augmented by repeated cocaine administration. A single administration of cocaine elicited significant and dose-related USV responses, and the magnitude of the behavior increased with repeated drug administration. R-(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH23390), but not raclopride, significantly attenuated cocaine-induced BDNF protein expression, whereas either the D(1)-like or D(2)-like receptor antagonist blocked cocaine-induced USV behavior. Furthermore, significant USV behavior was elicited by the nonselective dopamine agonist, apomorphine, but not by agonists that are selective for D(1)-like or D(2)-like receptors. Intracerebroventricular injection of the neurotrophin TrkB receptor inhibitor, K252a, blocked cocaine-induced USV behavior but not locomotor activity. These results suggest that neurotrophin signaling downstream of dopamine receptor function probably constitutes a crucial link in cocaine induction of USV behavior and may contribute to the mechanisms underlying the development of addiction-related behaviors.

Cocaine effects on dopamine and NMDA receptors interactions in the striatum of Fischer rats

Numerous studies have shown that biochemical and behavioral effects of cocaine are mediated by dopamine D1 receptor (D1R) and NMDA R1 receptor (NR1)-mediated transmission. In this study, we investigated the physical interactions between D1R and NR1 in response to acute cocaine administration in a time course of 5-60 min. In the caudate-putamen (CPu) of male Fischer rats, a single cocaine injection (30 mg/kg) reduced D1R-NR1 protein-protein interactions 30 min after treatment. In addition, activation or blockade of the NMDA receptor using NMDA (25mg/kg) or MK-801 (0.25mg/kg), respectively, also reduced the D1R-NR1 physical interactions. Acute cocaine administration did not alter total D1R or NR1 protein levels in our time course of study. These results indicate that D1R-NR1 physical interaction rather than total protein levels may regulate the intracellular signaling after acute cocaine administration.

Amphetamine alters Ras-guanine nucleotide-releasing factor expression in the rat striatum in vivo

Ras-guanine nucleotide-releasing factors (Ras-GRFs) are densely expressed in neurons of the mammalian brain. As a Ras-specific activator predominantly concentrated at synaptic sites, Ras-GRFs activate the Ras-mitogen-activated protein kinase (Ras-MAPK) cascade in response to changing synaptic inputs, thereby modifying a variety of cellular and synaptic activities. While the Ras-MAPK cascade in the limbic reward circuit is well-known to be sensitive to dopamine inputs, the sensitivity of its upstream activator (Ras-GRFs) to dopamine remains to be investigated. In this study, the response of Ras-GRFs in their protein expression to dopamine stimulation was evaluated in the rat striatum in vivo. A single systemic injection of the psychostimulant amphetamine produced an increase in Ras-GRF1 protein levels in both the dorsal (caudoputamen) and ventral (nucleus accumbens) striatum. The increase in Ras-GRF1 proteins was dose-dependent. The reliable increase was seen 2.5h after drug injection and returned to normal levels by 6h. In contrast to Ras-GRF1, protein levels of Ras-GRF2 in the striatum were not altered by amphetamine. In addition to the striatum, the medial prefrontal cortex is another forebrain site where amphetamine induced a parallel increase in Ras-GRF1 but not Ras-GRF2. No significant change in Ras-GRF1/2 proteins was observed in the hippocampus. These data demonstrate that Ras-GRF1 is a susceptible and selective target of amphetamine in striatal and cortical neurons. Its protein expression is subject to the modulation by acute exposure of amphetamine.

Molecular mechanisms of psychostimulant-induced structural plasticity

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

Effects of dopamine and NMDA receptors on cocaine-induced Fos expression in the striatum of Fischer rats

Cocaine is an addictive psychostimulant that induces immediate early gene (IEG) expression by activating dopamine (DA) D1 and glutamate NMDA receptors in the striatum. In this study, we show that a single cocaine administration (30 mg/kg) time-dependently increases ERK phosphorylation, c-Fos and FosB protein expression, and MKP-1 phosphorylation (p-MKP-1), in the caudate-putamen (CPu) and nucleus accumbens (NAc) of Fischer rats. In the CPu, 1 h after cocaine injection, the increase in c-Fos and FosB protein expressions is totally abolished by pre-administration of DA-D1 receptor antagonist, SCH23390. In the NAc, SCH23390 also inhibits cocaine-induced c-Fos protein expression. The pre-treatment of NMDA receptor antagonist, MK801, partially reduces cocaine-activated c-Fos protein expression in the CPu. Furthermore, the escalation of p-MKP-1 after acute cocaine administration is dependent on both DA-D1 and NMDA receptor activation in both brain regions examined. Our data suggest that cocaine may modulate ERK pathway signaling through the activation of DA-D1 and NMDA receptors, subsequently influencing the IEG protein expression.

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.

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.

Drug-induced alterations in the extracellular signal-regulated kinase (ERK) signalling pathway: implications for reinforcement and reinstatement

Drug addiction, characterized by high rates of relapse, is recognized as a kind of neuroadaptive disorder. Since the extracellular signal-regulated kinase (ERK) pathway is critical to neuroplasticity in the adult brain, understanding the role this pathway plays is important for understanding the molecular mechanism underlying drug addiction and relapse. Here, we review previous literatures that focus on the effects of exposure to cocaine, amphetamine, Delta(9)-tetrahydrocannabinol (THC), nicotine, morphine, and alcohol on ERK signaling in the mesocorticolimbic dopamine system; these alterations of ERK signaling have been thought to contribute to the drug's rewarding effects and to the long-term maladaptation induced by drug abuse. We then discuss the possible upstreams of the ERK signaling pathway activated by exposure of drugs of abuse and the environmental cues previously paired with drugs. Finally, we argue that since ERK activation is a key molecular process in reinstatement of conditioned place preference and drug self-administration, the pharmacological manipulation of the ERK pathway is a potential treatment strategy for drug addiction.

Brain-derived neurotrophic factor and its intracellular signaling pathways in cocaine addiction

Cocaine addiction is one of the severest health problems faced by western countries, where there is an increasing prevalence of lifelong abuse. The most challenging aspects in the treatment of cocaine addiction are craving and relapse, especially in view of the fact that, at present, there is a lack of effective pharmacological treatment for the disorder. What is required are new pharmacological approaches based on our current understanding of the neurobiological bases of drug addiction. Within the context of the behavioral and neurochemical actions of cocaine, this paper considers the contribution of brain-derived neurotrophic factor (BDNF) and its main intracellular signaling mechanisms, including mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK) and phosphatidylinositol 3-kinase (PI3K), in psychostimulant addiction. Repeated cocaine administration leads to an increase in BDNF levels and enhanced activity in the intracellular pathways (PI3K and MAPK/ERK) in the reward-related brain areas, which applies especially several days following withdrawal. It has been hypothesized that these neurochemical changes contribute to the enduring synaptic plasticity that underlies sensitized responses to psychostimulants and drug-conditioned memories leading to compulsive drug use and frequent relapse after withdrawal. Nevertheless, increased BDNF levels could also have a role as a protection factor in addiction. The inhibition of the intracellular pathways, ERK and PI3K, leads to a disruption in sensitized responses and conditioned memories associated with cocaine addiction and suggests new, potential therapeutic strategies to explore in the dependence on psychostimulants.

Effects of acute cocaine on ERK and DARPP-32 phosphorylation pathways in the caudate-putamen of Fischer rats

Activation of extracellular signal-regulated kinase (ERK) and dopamine- and cAMP-regulated phosphoprotein (DARPP-32) pathways has been implicated in biochemical and behavioral effects induced by various drugs of abuse. In this study, we investigated the phosphorylation pathways of these two proteins in response to acute cocaine administration. A single cocaine administration (30 mg/kg) increased ERK-mediated signaling proteins, phosphoryation of cAMP response element-binding protein (CREB) kinase, pp90 ribosomal S6 kinase (RSK), and c-Fos protein levels in the caudate/putamen of Fischer rats. Acute cocaine administration also induced phosphorylation of the striatal-enriched protein tyrosine phosphatase (STEP) and decreased the phosphorylation of DARPP-32 protein at the Thr-75 site. The phosphorylation states of these inhibitors of ERK and DARPP-32 proteins may thus contribute to the effects of cocaine on ERK- and DARPP-32-mediated cascades, on gene expression and on behaviors.

Region-specific tolerance to cocaine-regulated cAMP-dependent protein phosphorylation following chronic self-administration

Chronic cocaine self-administration can produce either tolerance or sensitization to certain cocaine-regulated behaviours, but whether differential alterations develop in the biochemical response to cocaine is less clear. We measured cocaine-induced phosphorylation of multiple cAMP-dependent and -independent protein substrates in mesolimbic dopamine terminal regions following chronic self-administration. Changes in self-administering rats were compared to changes produced by passive yoked injection to identify reinforcement-related regulation, whereas acute and chronic yoked groups were compared to identify the development tolerance or sensitization in the biochemical response to cocaine. Microwave-fixed brain tissue was collected immediately following 4 h of intravenous cocaine administration, and subjected to Western blot analysis of phosphorylated and total protein substrates. Chronic cocaine produced region- and substrate-specific tolerance to cAMP-dependent protein phosphorylation, including GluR1(S845) phosphorylation in striatal and amygdala subregions and NR1(S897) phosphorylation in the CA1 subregion of the hippocampus. Tolerance also developed to cAMP-independent GluR1(S831) phosphorylation in the prefrontal cortex. In contrast, sensitization to presynaptic regulation of synapsin(S9) phosphorylation developed in the hippocampal CA3 subregion while cAMP-dependent tyrosine hydroxylase(S40) phosphorylation decreased in striatal dopamine terminals. Cocaine-induced ERK and CREB(S133) phosphorylation were dissociated in many brain regions and failed to develop either tolerance or sensitization with chronic administration. Positive reinforcement-related correlations between cocaine intake and protein phosphorylation were found only in self-administering animals, while negative dose-related correlations were found primarily with yoked administration. These regional- and substrate-specific adaptations in cocaine-induced protein phosphorylation are discussed in view of their potential impact on the development of cocaine addiction.

Role of ERK in cocaine addiction

Cocaine addiction is characterized by compulsive drug-taking behavior and high rates of relapse. According to recent theories, this addiction is due to drug-induced adaptations in the cellular mechanisms that underlie normal learning and memory. Such mechanisms involve signaling by extracellular signal-regulated kinase (ERK). As we review here, evidence from rodent studies also implicates ERK in cocaine psychomotor sensitization, cocaine reward, consolidation and reconsolidation of memories for cocaine cues, and time-dependent increases in cocaine seeking after withdrawal (incubation of cocaine craving). The role of ERK in these behaviors involves long-term stable alterations in synaptic plasticity that result from repeated cocaine exposure, and also rapidly induced alterations in synaptic transmission events that acutely control cocaine-seeking behaviors. Pharmacological manipulations that decrease the extent to which cocaine and cocaine cues induce ERK activity might therefore be considered as potential treatments for cocaine addiction.

Regulation of mitogen-activated protein kinases by glutamate receptors

Glutamate receptors regulate gene expression in neurons by activating intracellular signaling cascades that phosphorylate transcription factors within the nucleus. The mitogen-activated protein kinase (MAPK) cascade is one of the best characterized cascades in this regulatory process. The Ca(2+)-permeable ionotropic glutamate receptor, mainly the NMDA receptor subtype, activates MAPKs through a biochemical route involving the Ca(2+)-sensitive Ras-guanine nucleotide releasing factor, Ca(2+)/calmodulin-dependent protein kinase II, and phosphoinositide 3-kinase. The metabotropic glutamate receptor (mGluR), however, activates MAPKs primarily through a Ca(2+)-insensitve pathway involving the transactivation of receptor tyrosine kinases. The adaptor protein Homer also plays a role in this process. As an information superhighway between surface glutamate receptors and transcription factors in the nucleus, active MAPKs phosphorylate specific transcription factors (Elk-1 and CREB), and thereby regulate distinct programs of gene expression. The regulated gene expression contributes to the development of multiple forms of synaptic plasticity related to long-lasting changes in memory function and addictive properties of drugs of abuse. This review, by focusing on new data from recent years, discusses the signaling mechanisms by which different types of glutamate receptors activate MAPKs, features of each MAPK cascade in regulating gene expression, and the importance of glutamate/MAPK-dependent synaptic plasticity in memory and addiction.