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

Alcohol Consumption During Adolescence Alters the Cognitive Function in Adult Male Mice by Persistently Increasing Levels of DUSP6

Binge alcohol drinking during adolescence has long-term effects on the adult brain that alter brain structure and behaviors, but the underlying mechanisms remain poorly understood. Extracellular signal-regulated kinase (ERK) is involved in the synaptic plasticity and pathological brain injury by regulating the expression of cyclic adenosine monophosphate response element binding protein (CREB) and brain-derived neurotrophic factor (BDNF). Dual-specificity phosphatase 6 (DUSP6) is a critical effector that dephosphorylates ERK1/2 to control the basal tone, amplitude, and duration of ERK signaling. To explore DUSP6 as a regulator of ERK signaling in the mPFC and its impact on long-term effects of alcohol, a male mouse model of adolescent intermittent alcohol (AIA) exposure was established. Behavioral experiments showed that AIA did not affect anxiety-like behavior or sociability in adulthood, but significantly damaged new object recognition and social recognition memory. Molecular studies further found that AIA reduced the levels of pERK-pCREB-BDNF-PSD95/NR2A involved in synaptic plasticity, while DUSP6 was significantly increased. Intra-mPFC infusion of AAV-DUSP6-shRNA restored the dendritic spine density and postsynaptic density thickness by reversing the level of p-ERK and its downstream molecular expression, and ultimately repaired adult cognitive impairment caused by chronic alcohol exposure during adolescence. These findings indicate that AIA exposure inhibits ERK-CREB-BDNF-PSD95/NR2A by increasing DUSP6 in the mPFC in adulthood that may be associated with long-lasting cognitive deficits.

Protein kinases in natural versus drug reward

Natural and drug rewards act on the same neural pathway, the mesolimbic dopaminergic system. In brain regions such as the nucleus accumbens and ventral tegmental area, drugs of abuse-induced stimulation of signaling pathways can lead to synaptic reshaping within this system. This is believed to be underlying the maladaptive alterations in behaviors associated with addiction. In this review, we discuss animal studies disclosing the implication of several protein kinases, namely protein kinase A (PKA), extracellular signal regulated kinase (ERK) mitogen-activated protein kinases (MAPK), p38 MAPK, and calcium/calmodulin-dependent kinase II (CaMKII), in reward-related brain regions in drug and natural reward. Furthermore, we refer to studies that helped pave the way toward a better understanding of the neurobiology underlying non-drug and drug reward through genetic deletion or brain region-specific pharmacological inhibition of these kinases. Whereas the role of kinases in drug reward has been extensively studied, their implication in natural reward, such as positive social interaction, is less investigated. Discovering molecular candidates, recruited specifically by drug versus natural rewards, can promote the identification of novel targets for the pharmacological treatment of addiction with less off-target effects and being effective when used combined with behavioral-based therapies.

Roles of adenosine A1 receptors in the regulation of SFK activity in the rat forebrain

Adenosine A₁ receptors are widely expressed in the mammalian brain. Through interacting with G_αi/o -coupled A₁ receptors, the neuromodulator adenosine modulates a variety of cellular and synaptic activities. To determine the linkage from A₁ receptors to a key intracellular signaling pathway, we investigated the impact of blocking A₁ receptors on a subfamily of nonreceptor tyrosine kinases, that is, the Src family kinase (SFK), in different rat brain regions in vivo. We found that pharmacological blockade of A₁ receptors by a single systemic injection of the A₁ selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) induced an increase in autophosphorylation of SFKs at a consensus activation site, tyrosine 416 (Y416), in the two subdivisions of the striatum, the caudate putamen and nucleus accumbens. DPCPX also increased SFK Y416 phosphorylation in the medial prefrontal cortex (mPFC) but not the hippocampus. The DPCPX-induced Y416 phosphorylation was time dependent and reversible. In immunopurified Fyn and Src proteins from the striatum, DPCPX elevated SFK Y416 phosphorylation and tyrosine kinase activity in Fyn but not in Src proteins. In the mPFC, DPCPX enhanced Y416 phosphorylation and tyrosine kinase activity in both Fyn and Src immunoprecipitates. DPCPX had no effect on expression of total Fyn and Src proteins in the striatum, mPFC, and hippocampus. These results demonstrate a tonic inhibitory linkage from A₁ receptors to SFKs in the striatum and mPFC. Blocking this inhibitory tone could significantly enhance constitutive SFK Y416 phosphorylation in the rat brain in a region- and time-dependent manner.

Levo-tetrahydropalmatine attenuates the acquisition of fentanyl-induced conditioned place preference and the changes in ERK and CREB phosphorylation expression in mice

Levo-tetrahydropalmatine (L-THP) is the main active ingredient of Corydalis and Stephania and is widely used for its sedative, analgesic, and neuroleptic effects. Though L-THP is an antagonist of dopamine receptors and has been proven to be effective in treating drug addiction, its effect on fentanyl-induced reward learning still remains unclear. This experiment was designed to investigate the effects of L-THP on fentanyl-induced rewarding behavior through conditioned place preference (CPP) in mice. Western blot assays were used to dissect the accompanying changes in the phosphorylation of extracellular signal-regulated kinase (ERK) and cAMP response element binding protein (CREB) in related brain regions, including the hippocampus (Hip), caudate putamen (CPu), prefrontal cortex (PFC), and nucleus accumbens (NAc), which may mediate the effects of L-THP on fentanyl-induced CPP. The results revealed that fentanyl could induce CPP in mice at doses of 0.025 mg/kg, 0.05 mg/kg, 0.1 mg/kg, and 0.2 mg/kg, and L-THP could attenuate the acquisition of fentany-induced CPP at a dose of 10.0 mg/kg. The levels of p-ERK and p-CREB of the saline+fentanyl group (0.05 mg/kg) increased significantly in the Hip, NAc, and PFC compared to the saline+saline group. Furthermore, L-THP (10.0 mg/kg) co-administered with fentanyl during conditioning prevented the enhanced phosphorylation of ERK and CREB in the Hip, NAc, and PFC. Our research revealed that L-THP could suppress the rewarding properties of fentanyl-induced CPP, the inhibitory effect may be related to the suppression of ERK and CREB phosphorylation in the Hip, NAc, and PFC of mice. Thus, L-THP may have therapeutic potential for fentanyl addiction.

The Role of CaMKII and ERK Signaling in Addiction

Nicotine is the predominant addictive compound of tobacco and causes the acquisition of dependence through its interactions with nicotinic acetylcholine receptors and various neurotransmitter releases in the central nervous system. The Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinase (ERK) play a pivotal role in synaptic plasticity in the hippocampus. CaMKII is involved in long-term potentiation induction, which underlies the consolidation of learning and memory; however, the roles of CaMKII in nicotine and other psychostimulant-induced addiction still require further investigation. This article reviews the molecular mechanisms and crucial roles of CaMKII and ERK in nicotine and other stimulant drug-induced addiction. We also discuss dopamine (DA) receptor signaling involved in nicotine-induced addiction in the brain reward circuitry. In the last section, we introduce the association of polyunsaturated fatty acids and cellular chaperones of fatty acid-binding protein 3 in the context of nicotine-induced addiction in the mouse nucleus accumbens and provide a novel target for the treatment of drug abuse affecting dopaminergic systems.

Evidence that a defined population of neurons in lateral amygdala is directly involved in auditory fear learning and memory

Memory is thought to be encoded within networks of neurons within the brain, but the identity of the neurons involved and circuits they form have not been described for any memory. Previously, we used fos-tau-lacZ (FTL) transgenic mice to identify discrete populations of neurons in different regions of the brain which were specifically activated following fear conditioning. This suggested that these populations of neurons form nodes in a network that encodes fear memory. In particular, one population of learning activated neurons was found within a discrete region of the lateral amygdala (LA), a key nucleus required for fear conditioning. In order to provide evidence that this population is directly involved in fear conditioning, we have analysed the expression of a key molecular requirement for fear conditioning in LA, phosphorylated Extracellular Signal Regulated Kinase 1 and 2 (pERK1/2). The only neurons in LA that specifically expressed pERK1/2 following auditory fear conditioning were in the ventrolateral nucleus of the LA (LAvl), in the same discrete region where we found learning specific FTL⁺ neurons. Double labelling experiments in FTL mice showed that a substantial proportion of the learning activated neurons expressed both pERK1/2 and FTL. These experiments provide clear evidence that the learning specific neurons we identified within LAvl are directly involved in auditory fear conditioning. In addition, learning specific expression of pERK1/2 was found in a dense network of dendrites contained within the border region of the LAvl. This network of dendrites may represent an activated dendritic field involved in fear conditioning in LA.

Involvement of neural substrates in reward and aversion to methamphetamine addiction: Testing the reward comparison hypothesis and the paradoxical effect hypothesis of abused drugs

Clinical studies of drug addiction focus on the reward impact of abused drugs that produces compulsive drug-seeking behavior and drug dependence. However, a small amount of research has examined the opposite effect of aversion to abused drugs to balance the reward effect for drug taking. An aversive behavioral model of abused drugs in terms of conditioned taste aversion (CTA) was challenged by the reward comparison hypothesis (Grigson, 1997). To test the reward comparison hypothesis, the present study examined the rewarding or aversive neural substrates involved in methamphetamine-induced conditioned suppression. The behavioral data showed that methamphetamine induced conditioned suppression on conditioning and reacquisition but extinguished it on extinction. A higher level of stressful aversive corticosterone occurred on conditioning and reacquisition but not extinction. The c-Fos or p-ERK immunohistochemical activity showed that the cingulated cortex area 1 (Cg1), infralimbic cortex (IL), prelimbic cortex (PrL), basolateral amygdala (BLA), nucleus accumbens (NAc), and dentate gyrus (DG) of the hippocampus were overexpressed in aversive CTA induced by methamphetamine. These data may indicate that the Cg1, IL, PrL, BLA, NAc, and DG probably mediated the paradoxical effect-reward and aversion. Altogether, our data conflicted with the reward comparison hypothesis, and methamphetamine may simultaneously induce the paradoxical effect of reward and aversion in the brain to support the paradoxical effect hypothesis of abused drugs. The present data implicate some insights for drug addiction in clinical aspects.

The ERK Pathway: Molecular Mechanisms and Treatment of Depression

Major depressive disorder is a chronic debilitating mental illness. Its pathophysiology at cellular and molecular levels is incompletely understood. Increasing evidence supports a pivotal role of the mitogen-activated protein kinase (MAPK), in particular the extracellular signal-regulated kinase (ERK) subclass of MAPKs, in the pathogenesis, symptomatology, and treatment of depression. In humans and various chronic animal models of depression, the ERK signaling was significantly downregulated in the prefrontal cortex and hippocampus, two core areas implicated in depression. Inhibiting the ERK pathway in these areas caused depression-like behavior. A variety of antidepressants produced their behavioral effects in part via normalizing the downregulated ERK activity. In addition to ERK, the brain-derived neurotrophic factor (BDNF), an immediate upstream regulator of ERK, the cAMP response element-binding protein (CREB), a transcription factor downstream to ERK, and the MAPK phosphatase (MKP) are equally vulnerable to depression. While BDNF and CREB were reduced in their activity in the prefrontal cortex and hippocampus of depressed animals, MKP activity was enhanced in parallel. Chronic antidepressant treatment readily reversed these neurochemical changes. Thus, ERK signaling in the depression-implicated brain regions was disrupted during the development of depression, which contributes to the long-lasting and transcription-dependent neuroadaptations critical for enduring depression-like behavior and the therapeutic effect of antidepressants.

Amphetamine-induced Conditioned Place Preference and Changes in mGlu1/5 Receptor Expression and Signaling in the Rat Medial Prefrontal Cortex

The medial prefrontal cortex (mPFC) is implicated in the rewarding effect of psychostimulants, although molecular mechanisms underlying the rewarding properties of stimulants in this region are poorly understood. Group I metabotropic glutamate (mGlu) receptors (mGlu1/5 subtypes) are believed to be critical in this event. We thus in this study investigated changes in mGlu1/5 receptor expression and function in the rat mPFC in response to conditioned place preference (CPP) induced by amphetamine. Repeated amphetamine administration (2.5 mg/kg, once daily on alternate days for 10 days) induced reliable CPP. In the mPFC, surface expression of mGlu5 receptors was elevated in rats after amphetamine conditioning. mGlu5 receptors were also increased at synaptic and extrasynaptic sites in amphetamine-conditioned rats. Expression of mGlu1 receptors was stable in surface and synaptic compartments, while it was elevated in the extrasynaptic location. In mPFC neurons, the mGlu1/5 agonist-stimulated phosphoinositide signaling pathway was upregulated in its efficacy following amphetamine conditioning. The mGlu1/5 agonist-stimulated Src kinase phosphorylation was also augmented in rats treated with amphetamine. These results demonstrate the sensitivity of mPFC mGlu1/5 receptors to amphetamine-induced CPP. Amphetamine conditioning results in the upregulation of mGlu1/5 receptor expression at subcellular and/or subsynaptic levels and mGlu1/5-mediated postreceptor signaling in mPFC neurons.

Methamphetamine and its immune-modulating effects

The recreational use of methamphetamine (METH, or ice) is a global burden. It pervades and plagues contemporary society; it has been estimated that there are up to 35 million users worldwide. METH is a highly addictive psychotropic compound which acts on the central nervous system, and chronic use can induce psychotic behavior. METH has the capacity to modulate immune cells, giving the drug long-term effects which may manifest as neuropsychiatric disorders, and that increase susceptibility to communicable diseases, such as HIV. In addition, changes to the cytokine balance have been associated with compromise of the blood-brain barrier, resulting to alterations to brain plasticity, creating lasting neurotoxicity. Immune-related signaling pathways are key to further evaluating how METH impacts host immunity through these neurological and peripheral modifications. Combining this knowledge with current data on inflammatory responses will improve understanding of how the adaptive and innate immunity responds to METH, how this can activate premature-ageing processes and how METH exacerbates disturbances that lead to non-communicable age-related diseases, including cardiovascular disease, stroke, depression and dementia.

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.

Alterations in mGlu5 receptor expression and function in the striatum in a rat depression model

Major depressive disorder is a common form of mental illness. Many brain regions are implicated in the pathophysiology and symptomatology of depression. Among key brain areas is the striatum that controls reward and mood and is involved in the development of core depression-like behavior in animal models of depression. While molecular mechanisms in this region underlying depression-related behavior are poorly understood, the glutamatergic input to the striatum is believed to play a role. In this study, we investigated changes in metabotropic glutamate (mGlu) receptor expression and signaling in the striatum of adult rats in response to prolonged (10-12 weeks) social isolation, a pre-validated animal paradigm modeling depression in adulthood. We found that mGlu5 receptor protein levels in the striatum were increased in rats that showed typical depression- and anxiety-like behavior after chronic social isolation. This increase in mGlu5 receptor expression was seen in both subdivisions of the striatum, the nucleus accumbens and caudate putamen. At subcellular and subsynaptic levels, mGlu5 receptor expression was elevated in surface membranes at synaptic sites. In striatal neurons, the mGlu5-associated phosphoinositide signaling pathway was augmented in its efficacy after prolonged social isolation. These data indicate that the mGlu5 receptor is a sensitive substrate of depression. Adulthood social isolation leads to the up-regulation of mGlu5 receptor expression and function in striatal neurons.

Local substrates of non-receptor tyrosine kinases at synaptic sites in neurons

Several non-receptor tyrosine kinase (nRTK) members are expressed in neurons of mammalian brains. Among these neuron-enriched nRTKs, two Src family kinase members (Src and Fyn) are particularly abundant at synaptic sites and have been most extensively studied for their roles in the regulation of synaptic activity and plasticity. Increasing evidence shows that the synaptic subpool of nRTKs interacts with a number of local substrates, including glutamate receptors (both ionotropic and metabotropic glutamate receptors), postsynaptic scaffold proteins, presynaptic proteins, and synapse-enriched enzymes. By phosphorylating specific tyrosine residues in the intracellular domains of these synaptic proteins either constitutively or in an activity-dependent manner, nRTKs regulate these substrates in trafficking, surface expression, and function. Given the high sensitivity of nRTKs to changing synaptic input, nRTKs are considered to act as a critical regulator in the determination of the strength and efficacy of synaptic transmission.

Dopamine transporter phosphorylation site threonine 53 is stimulated by amphetamines and regulates dopamine transport, efflux, and cocaine analog binding

The dopamine transporter (DAT) controls the spatial and temporal dynamics of dopamine neurotransmission through reuptake of extracellular transmitter and is a target for addictive compounds such as cocaine, amphetamine (AMPH), and methamphetamine (METH). Reuptake is regulated by kinase pathways and drug exposure, allowing for fine-tuning of clearance in response to specific conditions, and here we examine the impact of transporter ligands on DAT residue Thr-53, a proline-directed phosphorylation site previously implicated in AMPH-stimulated efflux mechanisms. Our findings show that Thr-53 phosphorylation is stimulated in a transporter-dependent manner by AMPH and METH in model cells and rat striatal synaptosomes, and in striatum of rats given subcutaneous injection of METH. Rotating disc electrode voltammetry revealed that initial rates of uptake and AMPH-induced efflux were elevated in phosphorylation-null T53A DAT relative to WT and charge-substituted T53D DATs, consistent with functions related to charge or polarity. These effects occurred without alterations of surface transporter levels, and mutants also showed reduced cocaine analog binding affinity that was not rescued by Zn²⁺ Together these findings support a role for Thr-53 phosphorylation in regulation of transporter kinetic properties that could impact DAT responses to amphetamines and cocaine.

Low-anxiety rats are more sensitive to amphetamine in comparison to high-anxiety rats

This study utilised the two injection protocol of sensitisation (TIPS) and the conditioned place preference test to validate and extend previous findings on the effects of amphetamine on positive reinforcement-related 50 kHz ultrasonic vocalisation (USV) in rats. We also examined changes in the expression of c-Fos and the NMDA receptor 2B (GluN2B) subunit, markers of neuronal activity and plasticity, in brain regions of rats in response to TIPS. We used low anxiety-responsive (LR) and high anxiety-responsive (HR) rats, which are known to exhibit different fear-conditioned response strengths, different susceptibilities to amphetamine in the TIPS procedure and different amphetamine-dependent 50 kHz USV responses. The LR rats, compared to the HR rats, not only vocalised much more intensely but also spent significantly more time in the amphetamine-paired compartment. After the second dose of amphetamine, the LR rats exhibited more c-Fos and GluN2B activation in layers II and III of the M1/M2 motor cortex area and prefrontal cortex (PRE, PRL, IL) and also presented with more GluN2B activation in the basal amygdala. These data reveal that HR and LR rats exhibit different levels of reactivity in the cortical-limbic pathway, which controls reward-related motivational processes. These findings contribute to the general hypothesis that heterogeneity in emotional processes is one of the causes of sensitisation to amphetamine and drug addiction.

Antagonism of Muscarinic Acetylcholine Receptors Alters Synaptic ERK Phosphorylation in the Rat Forebrain

Acetylcholine (ACh) is a key transmitter in the mesocorticolimbic circuit. By interacting with muscarinic ACh receptors (mAChR) enriched in the circuit, ACh actively regulates various neuronal and synaptic activities. The extracellular signal-regulated kinase (ERK) is one of members of the mitogen-activated protein kinase family and is subject to the regulation by dopamine receptors, although the regulation of ERKs by limbic mAChRs is poorly understood. In this study, we investigated the role of mAChRs in the regulation of ERK phosphorylation (activation) in the mesocorticolimbic system of adult rat brains in vivo. We targeted a sub-pool of ERKs at synaptic sites. We found that a systemic injection of the mAChR antagonist scopolamine increased phosphorylation of synaptic ERKs in the striatum (caudate putamen and nucleus accumbens) and medial prefrontal cortex (mPFC). Increases in ERK phosphorylation in both forebrain regions were rapid and transient. Notably, pretreatment with a dopamine D1 receptor (D1R) antagonist SCH23390 blocked the scopolamine-stimulated ERK phosphorylation in these brain regions, while a dopamine D2 receptor antagonist eticlopride did not. Scopolamine and SCH23390 did not change the amount of total ERK proteins. These results demonstrate that mAChRs inhibit synaptic ERK phosphorylation in striatal and mPFC neurons under normal conditions. Blockade of this inhibitory mAChR tone leads to the upregulation of ERK phosphorylation likely through a mechanism involving the level of D1R activity.

Synaptic ERK2 Phosphorylates and Regulates Metabotropic Glutamate Receptor 1 In Vitro and in Neurons

A synaptic pool of extracellular signal-regulated kinases (ERK) controls synaptic transmission, although little is known about its underlying signaling mechanisms. Here, we found that synaptic ERK2 directly binds to postsynaptic metabotropic glutamate receptor 1a (mGluR1a). This binding is direct and the ERK-binding site is located in the intracellular C-terminus (CT) of mGluR1a. Parallel with this binding, ERK2 phosphorylates mGluR1a at a cluster of serine residues in the distal part of mGluR1a-CT. In rat cerebellar neurons, ERK2 interacts with mGluR1a at synaptic sites, and active ERK constitutively phosphorylates mGluR1a under normal conditions. This basal phosphorylation is critical for maintaining adequate surface expression of mGluR1a. ERK is also essential for controlling mGluR1a signaling in triggering distinct postreceptor signaling transduction pathways. In summary, we have demonstrated that mGluR1a is a sufficient substrate of ERK2. ERK that interacts with and phosphorylates mGluR1a is involved in the regulation of the trafficking and signaling of mGluR1.

Extracellular Signal-Regulated Kinase in Nucleus Accumbens Mediates Propofol Self-Administration in Rats

Clinical and animal studies have indicated that propofol has potential for abuse, but the specific neurobiological mechanism underlying propofol reward is not fully understood. The purpose of this study was to investigate the role of extracellular signal-regulated kinase (ERK) signal transduction pathways in the nucleus accumbens (NAc) in propofol self-administration. We tested the expression of p-ERK in the NAc following the maintenance of propofol self-administration in rats. We also assessed the effect of administration of SCH23390, an antagonist of the D1 dopamine receptor, on the expression of p-ERK in the NAc in propofol self-administering rats, and examined the effects of intra-NAc injection of U0126, an MEK inhibitor, on propofol reinforcement in rats. The results showed that the expression of p-ERK in the NAc increased significantly in rats maintained on propofol, and pre-treatment with SCH23390 inhibited the propofol self-administration and diminished the expression of p-ERK in the NAc. Moreover, intra-NAc injection of U0126 (4 µg/side) attenuated the propofol self-administration. The data suggest that ERK signal transduction pathways coupled with D1 dopamine receptors in the NAc may be involved in the maintenance of propofol self-administration and its rewarding effects.

Integrated regulation of AMPA glutamate receptor phosphorylation in the striatum by dopamine and acetylcholine

Dopamine (DA) and acetylcholine (ACh) signals converge onto protein kinase A (PKA) in medium spiny neurons of the striatum to control cellular and synaptic activities of these neurons, although underlying molecular mechanisms are less clear. Here we measured phosphorylation of the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) at a PKA site (S845) as an indicator of AMPAR responses in adult rat brains in vivo to explore how DA and ACh interact to modulate AMPARs. We found that subtype-selective activation of DA D1 receptors (D1Rs), D2 receptors (D2Rs), or muscarinic M4 receptors (M4Rs) induced specific patterns of GluA1 S845 responses in the striatum. These defined patterns support a local multitransmitter interaction model in which D2Rs inhibited an intrinsic inhibitory element mediated by M4Rs to enhance the D1R efficacy in modulating AMPARs. Consistent with this, selective enhancement of M4R activity by a positive allosteric modulator resumed the cholinergic inhibition of D1Rs. In addition, D1R and D2R coactivation recruited GluA1 and PKA preferentially to extrasynaptic sites. In sum, our in vivo data support an existence of a dynamic DA-ACh balance in the striatum which actively modulates GluA1 AMPAR phosphorylation and trafficking. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Regulation of Group I Metabotropic Glutamate Receptors by MAPK/ERK in Neurons

Group I metabotropic glutamate receptors (mGluR1 and mGluR5 subtypes) are regulated by protein kinases. A recent focus is mitogen-activated protein kinases (MAPK). A prototypic subclass of MAPKs, extracellular signal-regulated kinases (ERK), is densely expressed in adult brain postmitotic neurons. This kinase resides in not only the cytoplasm around the nucleus, also the neuronal peripheral structures such as synapses. Recombinant ERK2 binds to C terminal tails of mGluR1a in vitro and native ERK1/2 forms complexes with mGluR1/5 in neurons in vivo. Association of ERK with mGluR1/5 enables the kinase to phosphorylate mGluR1/5 at a cluster of serine sites in the distal C terminus, including a serine residue within the Homer binding site. The ERK-mediated phosphorylation of mGluR1/5 promotes surface expression of mGluR1a in cerebellar neurons. ERK also regulates mGluR1/5 signaling and functions. Among different functional outputs surveyed, ERK exerts an output-specific role in either potentiating or inhibiting their activities. In sum, synaptic group I mGluRs are sufficient substrates of MAPK/ERK. Phosphorylation of mGluR1/5 by ERK has a significant impact on subcellular expression and function of phospho-modified receptors.

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.

Synaptically Localized Mitogen-Activated Protein Kinases: Local Substrates and Regulation

Mitogen-activated protein kinases (MAPKs) are expressed in postmitotic neurons and act as important regulators in intracellular signaling. In addition to their nuclear distribution and roles in regulating gene expression, MAPKs, especially the extracellular signal-regulated kinase (ERK) subclass, reside in peripheral dendritic spines and synapses, including the postsynaptic density (PSD) microdomain. This peripheral pool of MAPKs/ERKs is either constitutively active or sensitive to changing synaptic input. Active MAPKs directly interact with and phosphorylate local substrates to alter their trafficking and subcellular/subsynaptic distributions, through which MAPKs regulate function of substrates and contribute to long-lasting synaptic plasticity. A number of physiologically relevant substrates of MAPKs have been identified at synaptic sites. Central among them are key synaptic scaffold proteins (PSD-95 and PSD-93), cadherin-associated proteins (δ-catenin), Kv4.2 K⁺ channels, and metabotropic glutamate receptors. Through a reversible phosphorylation event, MAPKs rapidly and efficiently modulate the function of these substrates and thus determine the strength of synaptic transmission. This review summarizes the recent progress in cell biology of synaptic MAPKs and analyzes roles of this specific pool of MAPKs in regulating local substrates and synaptic plasticity.

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.

Upregulation of D site of albumin promoter binding protein in the brain of patients with intractable epilepsy

The mechanisms that underlie the pathogenesis of intractable epilepsy (IE) remain to be elucidated. The present study aimed to investigate the expression of D site of albumin promoter binding protein (DBP) and mitogen‑activated protein kinases (MAPKs) in the temporal lobes of patients with IE, in order to examine the possible roles of DBP in the pathogenesis of IE. The expression of DBP and MAPK was detected by immunohistochemistry and double‑label immunofluorescence staining against DBP/MAPK in 35 patients with IE, and the data were compared with those of the 15 controls. The results demonstrated that DBP expression in IE group (0.31±0.03) was significantly higher compared with that in the controls (0.18±0.02; P<0.05) and MAPK expression in the IE group (0.19±0.03) was also higher compared with that in the controls (0.12±0.02; P<0.05). DBP and MAPK were mainly expressed in the cytoplasm of neurons and the double‑label immunofluorescence staining demonstrated that DBP and MAPK expression occurred in the same neurons. Therefore, the expression of DBP and MAPK in epilepsy patients was upregulated, suggesting a possible pathogenetic role in IE.

Transcriptional and epigenetic substrates of methamphetamine addiction and withdrawal: evidence from a long-access self-administration model in the rat

Methamphetamine use disorder is a chronic neuropsychiatric disorder characterized by recurrent binge episodes, intervals of abstinence, and relapses to drug use. Humans addicted to methamphetamine experience various degrees of cognitive deficits and other neurological abnormalities that complicate their activities of daily living and their participation in treatment programs. Importantly, models of methamphetamine addiction in rodents have shown that animals will readily learn to give themselves methamphetamine. Rats also accelerate their intake over time. Microarray studies have also shown that methamphetamine taking is associated with major transcriptional changes in the striatum measured within a short or longer time after cessation of drug taking. After a 2-h withdrawal time, there was increased expression of genes that participate in transcription regulation. These included cyclic AMP response element binding (CREB), ETS domain-containing protein (ELK1), and members of the FOS family of transcription factors. Other genes of interest include brain-derived neurotrophic factor (BDNF), tyrosine kinase receptor, type 2 (TrkB), and synaptophysin. Methamphetamine-induced transcription was found to be regulated via phosphorylated CREB-dependent events. After a 30-day withdrawal from methamphetamine self-administration, however, there was mostly decreased expression of transcription factors including junD. There was also downregulation of genes whose protein products are constituents of chromatin-remodeling complexes. Altogether, these genome-wide results show that methamphetamine abuse might be associated with altered regulation of a diversity of gene networks that impact cellular and synaptic functions. These transcriptional changes might serve as triggers for the neuropsychiatric presentations of humans who abuse this drug. Better understanding of the way that gene products interact to cause methamphetamine addiction will help to develop better pharmacological treatment of methamphetamine addicts.

Rapid and sustained GluA1 S845 phosphorylation in synaptic and extrasynaptic locations in the rat forebrain following amphetamine administration

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor is a major ionotropic glutamate receptor subtype in the mammalian brain. Like other glutamate receptors, the AMPA receptor is regulated by phosphorylation. By phosphorylating specific serine resides in AMPA receptor subunits (GluA1 and GluA2), various protein kinases regulate subcellular/subsynaptic expression and function of the receptor. In this study, we conducted a time course study to evaluate the temporal property of responses of phosphorylation at those sites to dopamine stimulation with the psychostimulant amphetamine in the adult rat striatum and medial prefrontal cortex (mPFC) in vivo. We focused on biochemically-enriched AMPA receptors from synaptic and extrasynaptic compartments. We found that acute injection of amphetamine induced a rapid and relatively sustained increase in GluA1 S845 phosphorylation at both synaptic and extrasynaptic sites in the striatum. Similar results were observed in the mPFC. In contrast to S845, amphetamine did not induce a significant change in GluA1 S831 phosphorylation in synaptic and extrasynaptic pools in the striatum and mPFC. GluA2 S880 phosphorylation in synaptic and extrasynaptic fractions in the two brain regions also remained stable in response to amphetamine. These results support S845 to be a principal site on AMPA receptors sensitive to acute stimulant exposure. Its phosphorylation levels are rapidly upregulated by amphetamine in the two defined subsynaptic microdomains (synaptic versus extrasynaptic locations) in striatal and cortical neurons.

Regulation of phosphorylation of synaptic and extrasynaptic GluA1 AMPA receptors in the rat forebrain by amphetamine

The AMPA receptor is regulated by phosphorylation. Two major phosphorylation sites (S831 and S845) are located in the intracellular C-terminal tail of GluA1 subunits. The phosphorylation on these sites controls receptor expression and function and is subject to the regulation by psychostimulants. In this study, we further characterized the regulation of S831 and S845 phosphorylation by amphetamine (AMPH) in the adult rat striatum and medial prefrontal cortex (mPFC) in vivo. We focused on the specific fraction of GluA1/AMPA receptors enriched from synaptic and extrasynaptic membranes, using a pre-validated biochemical fractionation procedure. We found that acute AMPH administration elevated GluA1 S845 phosphorylation in the defined synaptic membrane from the striatum in a dose-dependent manner. AMPH also induced a comparable increase in S845 phosphorylation in the extrasynaptic fraction of striatal GluA1. Similar increases in S845 phosphorylation in both synaptic and extrasynaptic pools were observed in the mPFC. In contrast, S831 phosphorylation was not altered in synaptic and extrasynaptic GluA1 in striatal neurons and synaptic GluA1 in mPFC neurons in response to AMPH, although a moderate increase in S831 phosphorylation was seen in extrasynaptic GluA1 in the mPFC after an AMPH injection at a high dose. Total synaptic and extrasynaptic GluA1 expression remained stable in the two regions after AMPH administration. Our data demonstrate the differential sensitivity of S845 and S831 phosphorylation to dopamine stimulation. S845 is a primary site where phosphorylation of GluA1 is upregulated by AMPH in striatal and mPFC neurons at both synaptic and extrasynaptic compartments.