Extracellular signal-regulated mitogen-activated protein kinase inhibitors decrease amphetamine-induced behavior and neuropeptide gene expression in the striatum

Leaky lessons learned: Efflux prone dopamine transporter variant reveals sex and circuit specific contributions of D2 receptor signalling to neuropsychiatric disease

Aberrant dopamine (DA) signalling has been implicated in various neuropsychiatric disorders, including attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder (ASD), schizophrenia, bipolar disorder (BPD) and addiction. The availability of extracellular DA is sculpted by the exocytotic release of vesicular DA and subsequent transporter-mediated clearance, rendering the presynaptic DA transporter (DAT) a crucial regulator of DA neurotransmission. D2-type DA autoreceptors (D2ARs) regulate multiple aspects of DA homeostasis, including (i) DA synthesis, (ii) vesicular release, (iii) DA neuron firing and (iv) the surface expression of DAT and DAT-mediated DA clearance. The DAT Val559 variant, identified in boys with ADHD or ASD, as well as in a girl with BPD, supports anomalous DA efflux (ADE), which we have shown drives tonic activation of D2ARs. Through ex vivo and in vivo studies of the DAT Val559 variant using transgenic knock-in mice, we have uncovered a circuit and sex-specific capacity of D2ARs to regulate DAT, which consequently disrupts DA signalling and behaviour differently in males and females. Our studies reveal the ability of the construct-valid DAT Val559 model to elucidate endogenous mechanisms that support DA signalling, findings that may be of translational and/or therapeutic importance.

Addictive drugs modify neurogenesis, synaptogenesis and synaptic plasticity to impair memory formation through neurotransmitter imbalances and signaling dysfunction

Drug abuse changes neurophysiological functions at multiple cellular and molecular levels in the addicted brain. Well-supported scientific evidence suggests that drugs negatively affect memory formation, decision-making and inhibition, and emotional and cognitive behaviors. The mesocorticolimbic brain regions are involved in reward-related learning and habitual drug-seeking/taking behaviors to develop physiological and psychological dependence on the drugs. This review highlights the importance of specific drug-induced chemical imbalances resulting in memory impairment through various neurotransmitter receptor-mediated signaling pathways. The mesocorticolimbic modifications in the expression levels of brain-derived neurotrophic factor (BDNF) and the cAMP-response element binding protein (CREB) impair reward-related memory formation following drug abuse. The contributions of protein kinases and microRNAs (miRNAs), along with the transcriptional and epigenetic regulation have also been considered in memory impairment underlying drug addiction. Overall, we integrate the research on various types of drug-induced memory impairment in distinguished brain regions and provide a comprehensive review with clinical implications addressing the upcoming studies.

Behaviorally penetrant, anomalous dopamine efflux exposes sex and circuit dependent regulation of dopamine transporters

Virtually all neuropsychiatric disorders display sex differences in prevalence, age of onset, and/or clinical symptomology. Although altered dopamine (DA) signaling is a feature of many of these disorders, sex-dependent mechanisms uniquely responsive to DA that drive sex-dependent behaviors remain unelucidated. Previously, we established that anomalous DA efflux (ADE) is a prominent feature of the DA transporter (DAT) variant Val559, a coding substitution identified in two male-biased disorders: attention-deficit/hyperactivity disorder and autism spectrum disorder. In vivo, Val559 ADE induces activation of nigrostriatal D2-type DA autoreceptors (D2ARs) that magnifies inappropriate, nonvesicular DA release by elevating phosphorylation and surface trafficking of ADE-prone DAT proteins. Here we demonstrate that DAT Val559 mice exhibit sex-dependent alterations in psychostimulant responses, social behavior, and cognitive performance. In a search for underlying mechanisms, we discovered that the ability of ADE to elicit D2AR regulation of DAT is both sex and circuit-dependent, with dorsal striatum D2AR/DAT coupling evident only in males, whereas D2AR/DAT coupling in the ventral striatum is exclusive to females. Moreover, systemic administration of the D2R antagonist sulpiride, which precludes ADE-driven DAT trafficking, can normalize DAT Val559 behavioral changes unique to each sex and without effects on the opposite sex or wildtype mice. Our studies support the sex- and circuit dependent capacity of D2ARs to regulate DAT as a critical determinant of the sex-biased effects of perturbed DA signaling in neurobehavioral disorders. Moreover, our work provides a cogent example of how a shared biological insult drives alternative physiological and behavioral trajectories as opposed to resilience.

Nicotine self-administration and ERK signaling are altered in RasGRF2 knockout mice

Ras/Raf/MEK/ERK (Ras-ERK) signaling has been demonstrated to play a role in the effects of drugs of abuse such as cocaine and alcohol, but has not been extensively examined in nicotine-related reward behaviors. We examined the role of Ras Guanine Nucleotide Releasing Factor 2 (RasGRF2), an upstream mediator of the Ras-ERK signaling pathway, on nicotine self-administration (SA) in RasGRF2 KO and WT mice. We first demonstrated that acute nicotine exposure (0.4 mg/kg) resulted in an increase in phosphorylated ERK1/2 (pERK1/2) in the striatum, consistent with previous reports. We also demonstrated that increases in pERK1/2 resulting from acute (0.4 mg/kg) and repeated (0.4 mg/kg, 10 daily injections) exposure to nicotine in WT mice were not present in RasGRF2 KO mice, confirming that RasGRF2 at least partly regulates the activity of the Ras-ERK signaling pathway following nicotine exposure. We then performed intravenous nicotine SA (0.03 mg/kg/infusion for 10 days) in RasGRF2 KO and WT mice. Consistent with a previous report using cocaine SA, RasGRF2 KO mice demonstrated an increase in nicotine SA relative to WT controls. These findings suggest a role for RasGRF2 in the reinforcing effects of nicotine, and implicate the Ras-ERK signaling pathway as a common mediator of the response to drugs of abuse.

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.

Regional Differences in S-Nitrosylation in the Cortex, Striatum, and Hippocampus of Juvenile Male Mice

Nitric oxide (NO) is a multifunctional neurotransmitter that plays a major role in neuronal and synaptic functions. S-nitrosylation (SNO), the NO-mediated protein posttransitional modification (PTM), is known to regulate physiological and pathological processes in the brain. However, the physiological role in different neuroanatomical brain regions has not been well investigated. To understand the role of SNO in the brain of juvenile WT mice, we used SNOTRAP technology. We mapped the SNO-proteome in three different neuroanatomical regions: cortex, striatum, and hippocampus. By conducting systems biology analysis, we found that the three brain regions share similar biological processes (BP) including biogenesis and developmental processes. Exclusive and different BP and molecular functions were found for each of the regions. Unraveling the BP and signaling mechanisms of SNO in the cortex, striatum, and hippocampus may help to understand the functional differences between the three regions under physiological conditions.

Enkephalin as a Pivotal Player in Neuroadaptations Related to Psychostimulant Addiction

Enkephalin expression is high in mesocorticolimbic areas associated with psychostimulant-induced behavioral and neurobiological effects, and may also modulate local neurotransmission in this circuit network. Psychostimulant drugs, like amphetamine and cocaine, significantly increase the content of enkephalin in these brain structures, but we do not yet understand the specific significance of this drug-induced adaptation. In this review, we summarize the neurochemical and molecular mechanism of psychostimulant-induced enkephalin activation in mesocorticolimbic brain areas, and the contribution of this opioid peptide in the pivotal neuroadaptations and long-term behavioral changes underlying psychostimulant addiction. There is evidence suggesting that adaptive changes in enkephalin content in the mesocorticolimbic circuit, induced by acute and chronic psychostimulant administration, may represent a key initial step in the long-term behavioral and neuronal plasticity induced by these drugs.

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.

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.

Dopaminergic dynamics underlying sex-specific cocaine reward

Although both males and females become addicted to cocaine, females transition to addiction faster and experience greater difficulties remaining abstinent. We demonstrate an oestrous cycle-dependent mechanism controlling increased cocaine reward in females. During oestrus, ventral tegmental area (VTA) dopamine neuron activity is enhanced and drives post translational modifications at the dopamine transporter (DAT) to increase the ability of cocaine to inhibit its function, an effect mediated by estradiol. Female mice conditioned to associate cocaine with contextual cues during oestrus have enhanced mesolimbic responses to these cues in the absence of drug. Using chemogenetic approaches, we increase VTA activity to mechanistically link oestrous cycle-dependent enhancement of VTA firing to enhanced cocaine affinity at DAT and subsequent reward processing. These data have implications for sexual dimorphism in addiction vulnerability and define a mechanism by which cellular activity results in protein alterations that contribute to dysfunctional learning and reward processing.

Sex differences in reward processing are at present poorly understood. Calipari and Juarez et al. report oestrous cycle-dependent fluctuations in firing of VTA dopamine neurons that drive alterations in DAT function expressed in terminals in the NAc. These differences underlie enhanced cocaine reward processing during oestrus.

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.

Striatal CB1 and D2 receptors regulate expression of each other, CRIP1A and δ opioid systems

Although biochemical and physiological evidence suggests a strong interaction between striatal CB1 cannabinoid (CB1 R) and D2 dopamine (D2 R) receptors, the mechanisms are poorly understood. We targeted medium spiny neurons of the indirect pathway using shRNA to knockdown either CB1 R or D2 R. Chronic reduction in either receptor resulted in deficits in gene and protein expression for the alternative receptor and concomitantly increased expression of the cannabinoid receptor interacting protein 1a (CRIP1a), suggesting a novel role for CRIP1a in dopaminergic systems. Both CB1 R and D2 R knockdown reduced striatal dopaminergic-stimulated [(35) S]GTPγS binding, and D2 R knockdown reduced pallidal WIN55212-2-stimulated [(35) S]GTPγS binding. Decreased D2 R and CB1 R activity was associated with decreased striatal phosphoERK. A decrease in mRNA for opioid peptide precursors pDYN and pENK accompanied knockdown of CB1 Rs or D2 Rs, and over-expression of CRIP1a. Down-regulation in opioid peptide mRNAs was followed in time by increased DOR1 but not MOR1 expression, leading to increased [D-Pen2, D-Pen5]-enkephalin-stimulated [(35) S]GTPγS binding in the striatum. We conclude that mechanisms intrinsic to striatal medium spiny neurons or extrinsic via the indirect pathway adjust for changes in CB1 R or D2 R levels by modifying the expression and signaling capabilities of the alternative receptor as well as CRIP1a and the DELTA opioid system.

Dopamine transporter phosphorylation site threonine 53 regulates substrate reuptake and amphetamine-stimulated efflux

In the central nervous system, levels of extraneuronal dopamine are controlled primarily by the action of the dopamine transporter (DAT). Multiple signaling pathways regulate transport activity, substrate efflux, and other DAT functions through currently unknown mechanisms. DAT is phosphorylated by protein kinase C within a serine cluster at the distal end of the cytoplasmic N terminus, whereas recent work in model cells revealed proline-directed phosphorylation of rat DAT at membrane-proximal residue Thr(53). In this report, we use mass spectrometry and a newly developed phospho-specific antibody to positively identify DAT phosphorylation at Thr(53) in rodent striatal tissue and heterologous expression systems. Basal phosphorylation of Thr(53) occurred with a stoichiometry of ~50% and was strongly increased by phorbol esters and protein phosphatase inhibitors, demonstrating modulation of the site by signaling pathways that impact DAT activity. Mutations of Thr(53) to prevent phosphorylation led to reduced dopamine transport V(max) and total apparent loss of amphetamine-stimulated substrate efflux, supporting a major role for this residue in the transport kinetic mechanism.

RGS4 overexpression in the rat dorsal striatum modulates mGluR5- and amphetamine-mediated behavior and signaling

RATIONALE

Regulator of G-protein signaling 4 (RGS4) is a brain-enriched negative modulator of G-protein-coupled receptor signaling. Decreased availability of RGS4 in the frontal cortex and striatum has been described in animal models of schizophrenia and drug addiction. However, cellular and behavioral consequences of dysregulated RGS4-dependent receptor signaling in the brain remain poorly understood.

OBJECTIVE

This study aims to investigate whether RGS4, through inhibiting the function of mGluR5 receptors in the dorsal striatum (dSTR), regulates cellular and behavioral responses to acute amphetamine.

METHODS

After herpes simplex virus-RGS4 was infused into the dSTR, RGS4 overexpression as well as binding of recombinant RGS4 to mGluR5 was assessed. The effect of RGS4 overexpression on behavioral activity induced by the intrastriatal mGluR5 agonist, DHPG, or amphetamine was recorded. Activation of extracellular signal-regulated kinase (ERK) and Akt (protein kinase B) was measured in the dSTR tissue at the end of each behavioral experiment.

RESULTS

RGS4 overexpressed in the dSTR coimmunoprecipitated with mGluR5 receptors and suppressed both behavioral activity and phospho-ERK levels induced by DHPG. RGS4 overexpression or the mGluR5 antagonist, 3-((2-methyl-4-thiazolyl)ethynyl)pyridine (MTEP), attenuated amphetamine-induced phospho-ERK (but not phospho-Akt) levels. RGS4 suppressed amphetamine-induced vertical activity and augmented horizontal activity over 90 min. Similarly, MTEP augmented amphetamine-induced horizontal activity, but did not affect vertical activity.

CONCLUSIONS

The present data demonstrate that RGS4 in the dSTR attenuates amphetamine-induced ERK signaling and decreases the behavioral efficacy of acute amphetamine likely by limiting mGluR5 function.

Rescue of dopamine transporter function in hypoinsulinemic rats by a D2 receptor-ERK-dependent mechanism

The dopamine (DA) transporter (DAT) is a major target for abused drugs and a key regulator of extracellular DA. A rapidly growing literature implicates insulin as an important regulator of DAT function. We showed previously that amphetamine (AMPH)-evoked DA release is markedly impaired in rats depleted of insulin with the diabetogenic agent streptozotocin (STZ). Similarly, functional magnetic resonance imaging experiments revealed that the blood oxygenation level-dependent signal following acute AMPH administration in STZ-treated rats is reduced. Here, we report that these deficits are restored by repeated, systemic administration of AMPH (1.78 mg/kg, every other day for 8 d). AMPH stimulates DA D(2) receptors indirectly by increasing extracellular DA. Supporting a role for D(2) receptors in mediating this "rescue," the effect was completely blocked by pre-treatment of STZ-treated rats with the D(2) receptor antagonist raclopride before systemic AMPH. D(2) receptors regulate DAT cell surface expression through ERK1/2 signaling. In ex vivo striatal preparations, repeated AMPH injections increased immunoreactivity of phosphorylated ERK1/2 (p-ERK1/2) in STZ-treated but not control rats. These data suggest that repeated exposure to AMPH can rescue, by activating D(2) receptors and p-ERK signaling, deficits in DAT function that result from hypoinsulinemia. Our data confirm the idea that disorders influencing insulin levels and/or signaling, such as diabetes and anorexia, can degrade DAT function and that insulin-independent pathways are present that may be exploited as potential therapeutic targets to restore normal DAT function.

Sex and temporally-dependent effects of methamphetamine toxicity on dopamine markers and signaling pathways

Methamphetamine induces a greater neurodegenerative effect in male versus female mice. In order to investigate this sex difference we studied the involvement of Akt and extracellular signal-regulated kinase (ERK1/2) in methamphetamine toxicity as a function of time post-treatment (30 min, 1 and 3 days). Methamphetamine-induced decreases in dopamine concentrations and dopamine transporter (DAT) specific binding in the medial striatum were similar in female and male mice when evaluated 1 day post-methamphetamine (40 mg/kg). At 3 days post-methamphetamine, striatal dopamine concentration and DAT specific binding continued to decline in males, whereas females showed a recovery with increases in dopamine content and DAT specific binding in medial striatum at day 3 versus day 1 post-methamphetamine. The reduction in striatal vesicular monoamine transporter 2 specific binding observed at 1 and 3 days post-methamphetamine showed neither a sex- nor temporal-dependent effect. Under the present experimental conditions, methamphetamine treatments had modest effects on dopamine markers measured in the substantia nigra. Proteins assessed by Western blots showed similar reductions in both female and male mice for DAT proteins at 1 and 3 days post-methamphetamine. An increase in the phosphorylation of striatal Akt (after 1 day), glycogen synthase kinase 3β (at 1 and 3 days) and ERK1/2 (30 min post-methamphetamine) was only observed in females. Striatal glial fibrillary acidic protein levels were augmented in both females and males at 3 days post-methamphetamine. These results reveal some of the sex- and temporally-dependent effects of methamphetamine toxicity on dopaminergic markers and suggest some of the signaling pathways associated with these responses.

Laminin-β1 impairs spatial learning through inhibition of ERK/MAPK and SGK1 signaling

Laminin is a major structural element of the basal lamina consisting of an α-chain, a β-chain, and a γ-chain arranged in a cross-like structure, with their C-terminal inter-coiled. Laminin is abundantly expressed in the hippocampus of mature brain and is implicated in several psychiatric disorders, but its possible role involved in learning and memory function is not known. This issue was examined here. Our results revealed that water maze training significantly decreased laminin-β1 (LB1) expression in the rat hippocampal CA1 area. Transfection of LB1 WT plasmid to hippocampal CA1 neurons impaired water maze performance in rats. Meanwhile, it decreased the phosphorylation level of ERK/MAPK and protein kinase serum- and glucocorticoid-inducible kinase-1 (SGK1). By contrast, knockdown of endogenous LB1 expression using RNA interference (LB1 siRNA) enhanced water maze performance. Meanwhile, it increased the phosphorylation level of ERK/MAPK and SGK1. The enhancing effect of LB1 siRNA on spatial learning and on the phosphorylation of ERK/MAPK and SGK1 was blocked by co-treatment with the MEK inhibitor U0126 at a concentration that did not apparently affect spatial learning and ERK/MAPK phosphorylation alone. Further, the enhancing effect of LB1 siRNA on spatial learning and SGK1 phosphorylation was similarly blocked by co-transfection with SGK1 siRNA at a concentration that did not markedly affect spatial learning and SGK1 expression alone. These results together indicate that LB1 negatively regulates spatial learning in rats. In addition, LB1 impairs spatial learning through decreased activation of the ERK/MAPK-SGK1 signaling pathway in the rat hippocampus.

Basal Ganglia disorders associated with imbalances in the striatal striosome and matrix compartments

The striatum is composed principally of GABAergic, medium spiny striatal projection neurons (MSNs) that can be categorized based on their gene expression, electrophysiological profiles, and input–output circuits. Major subdivisions of MSN populations include (1) those in ventromedial and dorsolateral striatal regions, (2) those giving rise to the direct and indirect pathways, and (3) those that lie in the striosome and matrix compartments. The first two classificatory schemes have enabled advances in understanding of how basal ganglia circuits contribute to disease. However, despite the large number of molecules that are differentially expressed in the striosomes or the extra-striosomal matrix, and the evidence that these compartments have different input–output connections, our understanding of how this compartmentalization contributes to striatal function is still not clear. A broad view is that the matrix contains the direct and indirect pathway MSNs that form parts of sensorimotor and associative circuits, whereas striosomes contain MSNs that receive input from parts of limbic cortex and project directly or indirectly to the dopamine-containing neurons of the substantia nigra, pars compacta. Striosomes are widely distributed within the striatum and are thought to exert global, as well as local, influences on striatal processing by exchanging information with the surrounding matrix, including through interneurons that send processes into both compartments. It has been suggested that striosomes exert and maintain limbic control over behaviors driven by surrounding sensorimotor and associative parts of the striatal matrix. Consistent with this possibility, imbalances between striosome and matrix functions have been reported in relation to neurological disorders, including Huntington’s disease, L-DOPA-induced dyskinesias, dystonia, and drug addiction. Here, we consider how signaling imbalances between the striosomes and matrix might relate to symptomatology in these disorders.

Amphetamine-induced ERM Proteins Phosphorylation Is through PKCβ Activation in PC12 Cells

Amphetamine, a synthetic psychostimulant, is transported by the dopamine transporter (DAT) to the cytosol and increases the exchange of extracellular amphetamine by intracellular dopamine. Recently, we reported that the phosphorylation levels of ezrin-radixin-moesin (ERM) proteins are regulated by psychostimulant drugs in the nucleus accumbens, a brain area important for drug addiction. However, the significance of ERM proteins phosphorylation in response to drugs of abuse has not been fully investigated. In this study, using PC12 cells as an in vitro cell model, we showed that amphetamine increases ERM proteins phosphorylation and protein kinase C (PKC) β inhibitor, but not extracellular signal-regulated kinase (ERK) or phosphatidylinositol 3-kinases (PI3K) inhibitors, abolished this effect. Further, we observed that DAT inhibitor suppressed amphetamine-induced ERM proteins phosphorylation in PC12 cells. These results suggest that PKCβ-induced DAT regulation may be involved in amphetmaine-induced ERM proteins phosphorylation.

Sex differences in methamphetamine toxicity in mice: effect on brain dopamine signaling pathways

Male mice were reported to display greater methamphetamine-induced neurotoxicity than females. The present study evaluated the involvement of phosphatidylinositol-3 kinase (PI3K)/Akt and extracellular signal-regulated kinase (ERK1/2) pathways in this sex-dependent methamphetamine toxicity. Intact female and male mice were administered methamphetamine (20 or 40mg/kg) and euthanized a week later. Dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) autoradiography in the lateral striatum showed a greater sensitivity in male mice treated with 20mg/kg methamphetamine compared to female mice. Striatal dopamine concentration and DAT autoradiography showed a more extensive depletion in male mice given 40mg/kg methamphetamine compared to female mice. Mice administered 40mg/kg methamphetamine showed no sex difference in striatal VMAT2 autoradiography. In the substantia nigra, DAT specific binding was decreased only in male mice treated with 40mg/kg methamphetamine and DAT mRNA levels decreased in methamphetamine-treated female and male mice. Methamphetamine-treated male mice presented a dose-dependent decrease of VMAT2 mRNA levels. Methamphetamine reduced insulin-like growth factor 1 receptor levels in females at both methamphetamine doses tested whereas it elevated G protein-coupled estrogen receptor 1 (GPER1) only in male mice. Phosphorylated Akt levels decreased only in male mice treated with 40mg/kg methamphetamine. Glycogen synthase kinase 3β levels were reduced in male mice at both methamphetamine doses tested and in females receiving 40mg/kg. Bcl-2 levels were increased in male mice treated with methamphetamine, whereas ERK1/2 and BAD levels were unchanged. These results implicate some of the signaling pathways associated with the sex differences in methamphetamine-induced toxicity.

The Role of BDNF/TrkB Signaling in Acute Amphetamine-Induced Locomotor Activity and Opioid Peptide Gene Expression in the Rat Dorsal Striatum

Exposure to psychostimulants increases brain-derived neurotrophic factor (BDNF) mRNA and protein levels in the cerebral cortex and subcortical structures. Because BDNF is co-localized with dopamine and glutamate in afferents to the striatum of rats, it may be co-released with those neurotransmitters upon stimulation. Further, there may be an interaction between the intracellular signaling cascades activated by dopamine, glutamate, and TrkB receptors in medium spiny striatal neurons. In the present study, the effect of acute amphetamine administration on TrkB phosphorylation, as an indirect indicator of activation, and striatal gene expression, was evaluated. In Experiment 1, 15 min or 2 h after a single saline or amphetamine (2.5 mg/kg, i.p.) injection, the caudate–putamen (CPu), nucleus accumbens (NAc), and dorsomedial prefrontal cortex (dmPFC) were extracted and processed for phospho (p)-TrkB immunoreactivity. Immunoprecipitation analyses indicated that neither the tyrosine phosphorylation (p-Tyr) or autophosphorylation sites of TrkB (706) were changed in NAc, CPu, or dmPFC 15 min after amphetamine administration. In contrast, p-Tyr and the PLCγ phosphorylation site of TrkB (816) were increased in the NAc and CPu 2 h after amphetamine. In Experiment 2, intra-striatal infusion of the tyrosine kinase inhibitor, K252a, increased amphetamine-induced vertical activity but not total distance traveled. In addition, K252a inhibited amphetamine-induced preprodynorphin, but not preproenkephalin, mRNA expression in the striatum. These data indicate that acute amphetamine administration induces p-TrkB activation and signaling in a time- and brain region-dependent manner and that TrkB/BDNF signaling plays an important role in amphetamine-induced behavior and striatal gene expression.

The suppressive effect of an intra-prefrontal cortical infusion of BDNF on cocaine-seeking is Trk receptor and extracellular signal-regulated protein kinase mitogen-activated protein kinase dependent

Cocaine-mediated neuroadaptations in the prefrontal cortical-nucleus accumbens pathway underlie drug-seeking in animals with a cocaine self-administration (SA) history. Neuroplasticity in the cortico-accumbens pathway is regulated, in part, by the expression and availability of neurotrophic factors, such as BDNF. We have previously demonstrated that infusion of BDNF into the dorsomedial prefrontal cortex (dmPFC) immediately after the last of 10 cocaine SA sessions attenuates contextual, cue- and cocaine prime-induced reinstatement of cocaine-seeking (Berglind et al., 2007) and normalizes cocaine-induced disruption of glutamatergic transmission in the nucleus accumbens (Berglind et al., 2009). In the present study, the suppressive effect of intra-dmPFC BDNF on cocaine-seeking is shown to depend on Trk receptor-mediated activation of extracellular signal-regulated kinase (ERK) signaling in the dmPFC. The tyrosine kinase inhibitor, K252a, and the mitogen-activated protein/extracellular signal-regulated kinase kinase inhibitor, U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene), prevented BDNF's suppressive effects on cocaine-seeking. Vehicle-infused rats with a cocaine SA history showed significant decreases in ERK and cyclic AMP response element binding protein (CREB), but not Akt, phosphorylation after the final cocaine SA session that were reversed by intra-dmPFC BDNF. Additionally, BDNF's ability to normalize cocaine-mediated decreases in ERK and CREB phosphorylation was blocked by U0126, demonstrating that ERK/MAPK activation mediated the behavioral effects. This study elucidates a mechanism whereby BDNF/TrkB (tropomyosin receptor kinase B) activates ERK-regulated CREB phosphorylation in the dmPFC to counteract the neuroadaptations induced by cocaine SA and subsequent relapse to cocaine-seeking.

Metabotropic glutamate mGluR5 receptor blockade opposes abnormal involuntary movements and the increases in glutamic acid decarboxylase mRNA levels induced by l-DOPA in striatal neurons of 6-hydroxydopamine-lesioned rats

The present study examined the effect of a subchronic systemic administration of the glutamate metabotropic mGluR5 receptor antagonist MPEP on l-DOPA-induced dyskinesias and striatal gene expression in adult rats with a unilateral 6-OHDA lesion of dopamine neurons. The daily systemic administration of l-DOPA for 2 weeks induced a gradual increase in limb dyskinesia and axial dystonia. The subchronic systemic co-administration of MPEP reduced the severity of limb dyskinesia and axial dystonia over the whole duration of l-DOPA treatment. Subchronic l-DOPA administration was paralleled by a significant increase in mRNA levels of the two isoforms of the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD67 and GAD65) and preprodynorphin (PPD). Single cell analysis on emulsion radioautographs indicated that l-DOPA-induced increases in GAD67 occurred predominantly in preproenkephalin-unlabeled striatonigral and, to a lesser extent, in preproenkephalin-labeled striatopallidal neurons. MPEP completely reversed the effects of l-DOPA on GAD67 and reduced the increases in GAD65 and PPD mRNA levels in striatonigral neurons. MPEP also reversed the small l-DOPA-induced increase in GAD67 mRNA levels in striatopallidal neurons. Altogether, the findings support the idea that the relative efficacy of mGluR5 receptor antagonists to oppose l-DOPA-induced abnormal involuntary movements involves an ability to oppose increases in GAD gene expression and GABA-mediated signaling in striatonigral and striatopallidal neurons. The results also confirm the potential usefulness of antagonists of mGluR5 receptors as adjuncts in the treatment of l-DOPA-induced dyskinesia in patients with Parkinson's disease.

Neuroprotective actions of sex steroids in Parkinson's disease

The sex difference in Parkinson's disease, with a higher susceptibility in men, suggests a modulatory effect of sex steroids in the brain. Numerous studies highlight that sex steroids have neuroprotective properties against various brain injuries. This paper reviews the protective effects of sex hormones, particularly estradiol, progesterone and androgens, in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) animal model of Parkinson's disease as compared to methamphetamine toxicity. The molecular mechanisms underlying beneficial actions of sex steroids on the brain have been investigated showing steroid, dose, timing and duration specificities and presently focus is on the dopamine signaling pathways, the next frontier. Both genomic and non-genomic actions of estrogen converge to promote survival factors and show sex differences. Neuroprotection by estrogen involves activation of signaling molecules such as the phosphatidylinositol-3 kinase/Akt and the mitogen-activated protein kinase pathways. Interaction with growth factors, such as insulin-like growth factor 1, also contributes to protective actions of estrogen.

Increased operant responding for ethanol in male C57BL/6J mice: specific regulation by the ERK1/2, but not JNK, MAP kinase pathway

RATIONALE

Extracellular signal-regulated protein kinase (ERK(1/2)) is a member of the mitogen-activated protein kinase (MAPK) signaling pathway and a key molecular target for ethanol (EtOH) and other drugs of abuse.

OBJECTIVE

The aim of the study was to assess the role of two MAPK pathways, ERK(1/2) and c-Jun N-terminal kinase (JNK), on the modulation of EtOH and sucrose self-administration.

MATERIALS AND METHODS

C57BL/6J mice were trained to lever press on a fixed-ratio 4 schedule with 9% EtOH/2% sucrose, or 2% sucrose, as the reinforcer. In experiments 1 and 2, mice were injected with the MEK(1/2) inhibitor SL 327 (0-100 mg/kg) and the JNK inhibitor AS 6012452 (0-56 mg/kg) prior to self-administration. In experiment 3, SL 327 (0-100 mg/kg) was administered prior to performance on a progressive ratio (PR) schedule of EtOH reinforcement. In experiment 4, SL 327 and AS 601245 were injected 2 h before a locomotor test.

RESULTS

SL 327 (30 mg/kg) significantly increased EtOH self-administration without affecting locomotion. Higher doses of SL 327 and AS 601245 reduced EtOH-reinforced responding and locomotor activity. Reductions of both ligands on sucrose self-administration were due to decreases in motor activity. SL 327 pretreatment had no effect on PR responding.

CONCLUSIONS

ERK(1/2) activity is more directly involved in modulating the reinforcing properties of EtOH than JNK activity due to its selective potentiation of EtOH-reinforced responding. The specificity of this effect to EtOH self-administration, rather than sucrose self-administration, suggests that the mechanism by which ERK(1/2) increases EtOH-reinforced responding does not generalize to all reinforcing solutions and is not due to increased motivation to consume EtOH.

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.

A substrate trapping mutant form of striatal-enriched protein tyrosine phosphatase prevents amphetamine-induced stereotypies and long-term potentiation in the striatum

BACKGROUND

Chronic, intermittent exposure to psychostimulant drugs results in striatal neuroadaptations leading to an increase in an array of behavioral responses on subsequent challenge days. A brain-specific striatal-enriched tyrosine phosphatase (STEP) regulates synaptic strengthening by dephosphorylating and inactivating several key synaptic proteins. This study tests the hypothesis that a substrate-trapping form of STEP will prevent the development of amphetamine-induced stereotypies.

METHODS

A substrate-trapping STEP protein, TAT-STEP (C-S), was infused into the ventrolateral striatum on each of 5 consecutive exposure days and 1 hour before amphetamine injection. Animals were challenged to see whether sensitization to the stereotypy-producing effects of amphetamine developed. The same TAT-STEP (C-S) protein was used on acute striatal slices to determine the impact on long-term potentiation and depression.

RESULTS

Infusion of TAT-STEP (C-S) blocks the increase of amphetamine-induced stereotypies when given during the 5-day period of sensitization. The TAT-STEP (C-S) has no effect if only infused on the challenge day. Treatment of acute striatal slices with TAT-STEP (C-S) blocks the induction of long-term potentiation and potentates long-term depression.

CONCLUSIONS

A substrate trapping form of STEP blocks the induction of amphetamine-induced neuroplasticity within the ventrolateral striatum and supports the hypothesis that STEP functions as a tonic break on synaptic strengthening.

Effects of the MEK inhibitor, SL-327, on rewarding, motor- and cellular-activating effects of D-amphetamine and SKF-82958, and their augmentation by food restriction in rat

RATIONALE

Food restriction (FR) enhances learned and unlearned behavioral responses to drugs of abuse and increases D-1 dopamine (DA) receptor-mediated activation of extracellular signal-regulated kinases (ERK) 1/2 MAP kinase in nucleus accumbens (NAc). While a role has been established for ERK signaling in drug-mediated associative learning, it is not clear whether ERK regulates unconditioned behavioral effects of abused drugs.

OBJECTIVES

The purpose of this study was to determine whether blockade of ERK signaling, using the brain-penetrant MEK inhibitor, SL-327, decreases behavioral or NAc cellular responses to acute drug treatment and their augmentation by FR.

MATERIALS AND METHODS

Separate experiments assessed the effects of SL-327 (50 mg/kg, intraperitoneally) on (1) the reward-potentiating effect of D-amphetamine in an intracranial self-stimulation protocol, (2) the locomotor-activating effect of the D-1 agonist, SKF-82958, and (3) Fos-immunostaining induced in the NAc by SKF-82958.

RESULTS

FR rats displayed enhanced responses to drug treatment on all measures. SL-327 had no effect on sensitivity to rewarding brain stimulation or the reward-potentiating effect of D-amphetamine. The MEK inhibitor, U0126, microinjected into the NAc was also without effect. The locomotor-activating effect of SKF-82958 was unaffected by SL-327. In contrast, SL-327 decreased NAc Fos-immunostaining and abolished the difference between feeding groups.

CONCLUSIONS

These results support the conclusion that ERK signaling does not mediate unlearned behavioral responses to drug treatment. However, the upregulation of ERK and downstream transcriptional responses to acute drug treatment may underlie the reported enhancement of reward-related learning in FR subjects.

Quantification of isotope encoded proteins in 2-D gels using surface enhanced resonance Raman

A strategy for quantification of multiple protein isoforms from a complex sample background is demonstrated, combining isotopomeric rhodamine 6G (R6G) labels and surface-enhanced Raman in polyacrylamide matrix. The procedure involves isotope-encoding by lysine-labeling with (R6G) active ester reagents, isoform separation by 2-DGE, fluorescence quantification using internal standardization to water, and silver nanoparticle deposition followed by surface-enhanced Raman detection. R6G sample encoding and standardization enabled the determination of total protein concentration and the distribution of specific isoforms using the combined detection approach of water-referenced fluorescence spectral imaging and ratiometric quantification. A detection limit of approximately 13.5 picomolar R6G-labeled protein was determined for the surface-enhanced Raman in a gel matrix (15-fold lower than fluorescence). High quantification accuracies for small differences in protein populations at low nanogram abundance were demonstrated for human GMP synthetase (hGMPS) either as purified protein samples in a single-point determination mode (3% relative standard deviation, RSD%) or as HCT116 human cancer cellular lysate in an imaging application (with 16% RSD%). These results represent a prototype for future applications of isotopic surface-enhanced resonance Raman scatter to quantification of protein distributions.

Protein kinases and addiction

Although drugs of abuse have different chemical structures and interact with different protein targets, all appear to usurp common neuronal systems that regulate reward and motivation. Addiction is a complex disease that is thought to involve drug-induced changes in synaptic plasticity due to alterations in cell signaling, gene transcription, and protein synthesis. Recent evidence suggests that drugs of abuse interact with and change a common network of signaling pathways that include a subset of specific protein kinases. The best studied of these kinases are reviewed here and include extracellular signal-regulated kinase, cAMP-dependent protein kinase, cyclin-dependent protein kinase 5, protein kinase C, calcium/calmodulin-dependent protein kinase II, and Fyn tyrosine kinase. These kinases have been implicated in various aspects of drug addiction including acute drug effects, drug self-administration, withdrawal, reinforcement, sensitization, and tolerance. Identifying protein kinase substrates and signaling pathways that contribute to the addicted state may provide novel approaches for new pharmacotherapies to treat drug addiction.

Amphetamine-induced locomotion and gene expression are altered in BDNF heterozygous mice

Administration of amphetamine overstimulates medium spiny neurons (MSNs) by releasing dopamine and glutamate from afferents in the striatum. However, these afferents also release brain-derived neurotrophic factor (BDNF) that protects striatal MSNs from overstimulation. Intriguingly, all three neurochemicals increase opioid gene expression in MSNs. In contrast, striatal opioid expression is less in naive BDNF heterozygous (BDNF(+/-)) vs. wild-type (WT) mice. This study was designed to determine whether partial genetic depletion of BDNF influences the behavioral and molecular response to an acute amphetamine injection. An acute injection of amphetamine [5 mg/kg, intraperitoneal (i.p.)] or saline was administered to WT and BDNF(+/-) mice. WT and BDNF(+/-) mice exhibited similar locomotor activity during habituation, whereas BDNF(+/-) mice exhibited more prolonged locomotor activation during the third hour after injection of amphetamine. Three hours after amphetamine injection, there was an increase of preprodynorphin mRNA in the caudate putamen and nucleus accumbens (Acb) and dopamine D(3) receptor mRNA levels were increased in the Acb of BDNF(+/-) and WT mice. Striatal/cortical trkB and BDNF, and mesencephalic tyrosine hydroxylase mRNA levels were only increased in WT mice. These results indicate that BDNF modifies the locomotor responses of mice to acute amphetamine and differentially regulates amphetamine-induced gene 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.

Extracellular signal-regulated kinases (ERK) and protein kinase C (PKC) activities are involved in the modulation of Nur77 and Nor-1 expression by dopaminergic drugs

The dopamine system is the main target of antipsychotic and psychostimulant drugs. These drugs induce intracellular events that culminate in the transcription of immediate early genes, such as c-fos. Another class of transcription factors, namely, the nuclear receptor subgroup called Nurs (Nur77, Nurr1 and Nor-1), has recently been associated with behavioral and biochemical effects mediated by dopamine. However, the signaling cascade leading to modulation of Nur mRNA levels in the brain has never been investigated. In the present study, we explore in vivo using specific kinase inhibitors the role of mitogen-associated and extracellular signal-regulated kinases (MEK) and protein kinase C (PKC) in the modulation of Nur expression induced by dopamine receptor drugs. Modulation of Nur77 expression by a dopamine D(2) receptor antagonist is associated with MEK and PKC activities, whereas only the PKC activity participates in the modulation of Nor-1 expression. Both MEK and PKC activities also participate in the modulation of Nur77 mRNA levels induced by dopamine receptor agonists, whereas a selective MEK activity is associated with the modulation of Nor-1 mRNA levels. Interestingly, modulation of dopamine drug-induced locomotor activities by kinase inhibitors is in accordance with the effects on Nur77, but not Nor-1, expression. Taken together, the results indicate that signaling events leading to modulation of Nur77 and Nor-1 expression following dopamine receptor interacting drugs are distinct. Considering that orphan nuclear receptors of the Nur subgroup display an important ligand-independent constitutive activity, characterization of the signaling cascades involved in the regulation of their expression represents an important step for understanding their role in dopamine system physiology and pathophysiology.

Behavioral and biochemical responses to d-amphetamine in MCH1 receptor knockout mice

The melanin-concentrating hormone (MCH) system is anatomically and functionally interlaced with the mesocorticolimbic dopamine system. Therefore, we investigated whether MCH(1) receptor knockout (KO) mice are more susceptible than wild-type (WT) mice to psychostimulant-induced locomotor stimulation and sensitization, dopamine receptor-mediated phosphorylation events and c-fos expression within the frontal cortex and ventral striatum. MCH(1) receptor KO mice have 20% higher basal locomotor activity, are hypersensitive to the locomotor activating effects of d-amphetamine (1 mg/kg), and develop behavioral sensitization to a regimen of repeated d-amphetamine administration that does not induce sensitization in WT mice. In addition, d-amphetamine-mediated regulation of p44-mitogen activated protein kinase (MAPK) phosphorylation within the frontal cortex was significantly enhanced in MCH(1) receptor KO mice, when compared with WT mice. No significant genotype difference in the effects of d-amphetamine on MAPK phosphorylation events within the ventral striatum, phosphorylation at Ser(897) of the NR1 subunit of the NMDA receptor or Ca(2+) and cyclic AMP response-element binding-protein (CREB) at Ser(133) in the frontal cortex was detected. d-Amphetamine (3 mg/kg) increased c-fos expression within the frontal cortex in MCH(1) receptor KO mice, but not WT mice. There were no d-amphetamine-induced changes in c-fos expression within the ventromedial striatum in KO or WT mice. Overall, MCH(1) receptor KO mice are hypersensitive to the behavioral and molecular effects of the dopaminergic psychostimulant d-amphetamine. Increased frontal cortical MAPK phosphorylation and c-fos expression in MCH(1) receptor KO mice indicates that the MCH(1) receptor may be an important target for treating neuropsychiatric disorders characterized by frontal cortex dysfunction, including depression, attention deficit hyperactivity disorder (ADHD) and schizophrenia.

Regulation of psychostimulant-induced signaling and gene expression in the striatum

Amphetamine (AMPH) and cocaine are indirect dopamine agonists that activate multiple signaling cascades in the striatum. Each cascade has a different subcellular location and duration of action that depend on the strength of the drug stimulus. In addition to activating D1 dopamine-Gs-coupled-protein kinase A signaling, acute psychostimulant administration activates extracellular-regulated kinase transiently in striatal cells; conversely, inhibition of extracellular-regulated kinase phosphorylation decreases the ability of psychostimulants to elevate locomotor behavior and opioid peptide gene expression. Moreover, a drug challenge in rats with a drug history augments and prolongs striatal extracellular-regulated kinase phosphorylation, possibly contributing to behavioral sensitization. In contrast, AMPH activates phosphoinositide-3 kinase substrates, like protein kinase B/Akt, only in the nuclei of striatal cells but this transient increase induced by AMPH is followed by a delayed decrease in protein kinase B/Akt phosphorylation whether or not the rats have a drug history, suggesting that the phosphoinositide-3 kinase pathway is not essential for AMPH-induced behavioral sensitization. Chronic AMPH or cocaine also alters the regulation of inhibitory G protein-coupled receptors in the striatum, as evident by a prolonged decrease in the level of regulator of G protein signaling 4 after non-contingent or contingent (self-administered) drug exposure. This decrease is exacerbated in behaviorally sensitized rats and reversed by re-exposure to a cocaine-paired environment. A decrease in regulator of G protein signaling 4 levels may weaken its interactions with metabotropic glutamate receptor 5, Galphaq, and phospholipase C beta that may enhance drug-induced signaling. Alteration of these protein-protein interactions suggests that the striatum responds to psychostimulants with a complex molecular repertoire that both modulates psychomotor effects and leads to long-term neuroadaptations.

Dynorphin is a downstream effector of striatal BDNF regulation of ethanol intake

We recently identified brain-derived neurotrophic factor (BDNF) in the dorsal striatum to be a major component of a homeostatic pathway controlling ethanol consumption. We hypothesized that ethanol-mediated activation of the BDNF signaling cascade is required for the ethanol-related function of the neurotrophic factor. Here, we demonstrate that exposure of striatal neurons to ethanol results in the activation of the BDNF receptor TrkB, leading to the activation of the mitogen-activated protein kinase (MAP kinase) signaling pathway and the subsequent increase in the expression of preprodynorphin (Pdyn) via BDNF. Finally, we show that activation of the dynorphin receptor, the kappa opioid receptor (KOR), is required for the BDNF-mediated decrease in ethanol intake, illustrating a function of dynorphin in BDNF's homeostatic control of ethanol consumption. Taken together, these results demonstrate that BDNF regulates ethanol intake by initiation of MAP kinase signaling and the ensuing production of downstream gene products, including Pdyn.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

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.

ERK-FosB signaling in dorsal MPOA neurons plays a major role in the initiation of parental behavior in mice

During mouse parental behavior, neurons in the dorsal medial preoptic area (MPOAd) are activated and express transcription factors such as c-Fos and FosB. FosB-knockout mice are reported to be defective in parental care. To clarify molecular signaling responsible for parental behavior, we investigated gene expression profiles in the MPOAd of parental versus nonparental mice. We identified upregulation of NGFI-B, SPRY1, and Rad in parental mice, together with c-Fos and FosB. A common inducer of these genes, the extracellular signal regulated kinase (ERK) was phosphorylated in MPOAd neurons upon pup exposure. Pharmacological blockade of ERK phosphorylation inhibited the FosB upregulation in MPOAd, and the initiation of pup retrieving in virgin female mice, but did not affect the established parenting in parous females. Furthermore, induction of SPRY1 and Rad was impaired in MPOAd of nonparental FosB-knockout mice. These results suggest the pivotal role of ERK-FosB signaling in the initiation of parental care.

Repeated amphetamine treatment increases phosphorylation of extracellular signal-regulated kinase, protein kinase B, and cyclase response element-binding protein in the rat striatum

Extracellular signal-regulated kinases, protein kinase B/Akt and cyclase response element-binding protein play important roles in drug-induced neuroadaptations. Acute psychostimulant exposure rapidly alters the phosphorylation of these proteins in the striatum but less is known about their responses to repeated stimulant administration. In this study the phosphorylated state of these proteins in rat striatum was analyzed by immunoblotting 15 min and 2 h after amphetamine (AMPH)-induced behavioral sensitization. Two weeks after the last dose of 5 mg/kg, i.p. AMPH once daily for 5 days, rats were challenged with 1 mg/kg, i.p. AMPH or saline and sacrificed 15 min or 2 h later. Sensitization to AMPH-induced behavioral activity was observed in AMPH pre-treated rats after AMPH on the challenge day. Phosphorylation of all three proteins was significantly greater 15 min after AMPH in AMPH-pre-treated than in saline-pre-treated rats. Two hours after AMPH challenge in AMPH-pre-treated rats, phospho-extracellular signal-regulated kinase and phospho-cAMP response element-binding protein immunoreactivity was still significantly elevated but not after AMPH injection in saline-pre-treated rats. In contrast, phospho-Akt was down-regulated to the same extent 2 h after acute AMPH or repeated AMPH with an AMPH challenge. These data implicate differential regulation of phospho-extracellular signal-regulated kinase, phospho-cAMP response element-binding protein versus phospho-Akt in sensitized responses to AMPH.

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.