Operant behavior to obtain palatable food modifies ERK activity in the brain reward circuit

Food palatability produces behavioral modifications that resemble those induced by drugs of abuse. Palatability-induced behavioral changes require both, the activation of the endogenous cannabinoid system, and changes in structural plasticity in neurons of the brain reward pathway. The ERK intracellular pathway is activated by CB1 receptors (CB1-R) and plays a crucial role in neuroplasticity. We investigated the activation of the ERK signaling cascade in the mesocorticolimbic system induced by operant training to obtain highly palatable isocaloric food and the involvement of the CB1-R in these responses. Using immunofluorescence techniques, we analyzed changes in ERK intracellular pathway activation in the mesocorticolimbic system of wild-type and CB1 knockout mice (CB1-/-) trained on an operant paradigm to obtain standard, highly caloric or highly palatable isocaloric food. Operant training for highly palatable isocaloric food, but not for standard or highly caloric food, produced a robust activation of the ERK signaling cascade in the same brain areas where this training modified structural plasticity. These changes induced by the operant training were absent in CB1-/-. We can conclude that the activation of the ERK pathway is associated to the neuroplasticity induced by operant training for highly palatable isocaloric food and might be involved in CB1-R mediated alterations in behavior and structural plasticity.

Distribution and compartmental organization of GABAergic medium-sized spiny neurons in the mouse nucleus accumbens

The nucleus accumbens (NAc) is a critical brain region involved in many reward-related behaviors. The NAc comprises major compartments the core and the shell, which encompass several subterritories. GABAergic medium-sized spiny neurons (MSNs) constitute the output neurons of the NAc core and shell. While the functional organization of the NAc core outputs resembles the one described for the dorsal striatum, a simple classification of the NAc shell neurons has been difficult to define due to the complexity of the compartmental segregation of cells. We used a variety of BAC transgenic mice expressing enhanced green fluorescence (EGFP) or the Cre-recombinase (Cre) under the control of the promoter of dopamine D1, D2, and D3 receptors and of adenosine A2a receptor to dissect the microanatomy of the NAc. Moreover, using various immunological markers we characterized in detail the distribution of MSNs in the mouse NAc. In addition, cell-type specific extracellular signal-regulated kinase (ERK) phosphorylation in the NAc subterritories was analyzed following acute administration of SKF81297 (a D1R-like agonist), quinpirole (a D2 receptors (D2R)-like agonist), apomorphine (a non-selective DA receptor agonist), raclopride (a D2R-like antagonist), and psychostimulant drugs, including cocaine and d-amphetamine. Each drug generated a unique topography and cell-type specific activation of ERK in the NAc. Our results show the existence of marked differences in the receptor expression pattern and functional activation of MSNs within the shell subterritories. This study emphasizes the anatomical and functional heterogeneity of the NAc, which will have to be considered in its further study.

The orphan receptor GPR3 modulates the early phases of cocaine reinforcement

BACKGROUND AND PURPOSE

The modulatory activity of the orphan receptor GPR3 in the brain has been related to the control of emotional behaviours. Limbic structures that express GPR3 have been associated with the effects of drug abuse.

EXPERIMENTAL APPROACH

The role of GPR3 in different cocaine-elicited behaviours including locomotor activity, behavioural sensitization, conditioned place preference (CPP) and intravenous self-administration was evaluated in Gpr3-/- mice and their Gpr3+/+ littermates. Cocaine-induced dopamine release in the nucleus accumbens was also evaluated to elucidate the effect of Gpr3 deletion on extracellular levels of dopamine.

KEY RESULTS

Gpr3-/- mice exhibited higher rewarding responses in the CPP paradigm. Gpr3-/- mice self-administered more cocaine, especially during the first days of training. No differences were found between genotypes regarding behavioural sensitization and the maximal effort required to obtain a cocaine infusion. Non-contingent priming injections of cocaine before operant training eliminated enhanced cocaine self-administration in Gpr3-/- mice. Extracellular levels of dopamine in the nucleus accumbens induced by cocaine did not differ between genotypes.

CONCLUSIONS AND IMPLICATIONS

The increased responsiveness of Gpr3-/- mice to the acute locomotor effects of cocaine and the inconsistency to further increase this effect reflected an 'already maximally sensitized' basal state. Enhanced responsiveness of Gpr3-/- mice to cocaine reward and to early phases of reinforcement suggests that an initial alteration increased vulnerability to this type of drug abuse. Overall, altered signalling pathways of GPR3 could contribute to the neurobiological substrate involved in developing addiction to cocaine.

Operant behavior to obtain palatable food modifies neuronal plasticity in the brain reward circuit

Palatability enhances food intake by hedonic mechanisms that prevail over caloric necessities. Different studies have demonstrated the role of endogenous cannabinoids in the mesocorticolimbic system in controlling food hedonic value and consumption. We hypothesize that the endogenous cannabinoid system could also be involved in the development of food-induced behavioral alterations, such as food-seeking and binge-eating, by a mechanism that requires neuroplastic changes in the brain reward pathway. For this purpose, we evaluated the role of the CB1 cannabinoid receptor (CB1-R) in the behavioral and neuroplastic changes induced by operant training for standard, highly caloric or highly palatable isocaloric food using different genetics, viral and pharmacological approaches. Neuroplasticity was evaluated by measuring changes in dendritic spine density in neurons previously labeled with the dye DiI. Only operant training to obtain highly palatable isocaloric food induced neuroplastic changes in neurons of the nucleus accumbens shell and prefrontal cortex that were associated to changes in food-seeking behavior. These behavioral and neuroplastic modifications induced by highly palatable isocaloric food were dependent on the activity of the CB1-R. Neuroplastic changes induced by highly palatable isocaloric food are similar to those produced by some drugs of abuse and may be crucial in the alteration of food-seeking behavior leading to overweight and obesity.

Striatal cholinergic interneurons display activity-related phosphorylation of ribosomal protein S6

Cholinergic interneurons (CINs) provide the main source of acetylcholine to all striatal regions, and strongly modulate dopaminergic actions through complex regulation of pre- and post-synaptic acetylcholine receptors. Although striatal CINs have a well-defined electrophysiological profile, their biochemical properties are poorly understood, likely due to their low proportion within the striatum (2-3%). We report a strong and sustained phosphorylation of ribosomal protein S6 on its serine 240 and 244 residues (p-Ser²⁴⁰⁻²⁴⁴-S6rp), a protein integrant of the ribosomal machinery related to the mammalian target of the rapamycin complex 1 (mTORC1) pathway, which we found to be principally expressed in striatal CINs in basal conditions. We explored the functional relevance of this cellular event by pharmacologically inducing various sustained physiological activity states in CINs and assessing the effect on the levels of S6rp phosphorylation. Cell-attached electrophysiological recordings from CINs in a striatal slice preparation showed an inhibitory effect of tetrodotoxin (TTX) on action potential firing paralleled by a decrease in the p-Ser²⁴⁰⁻²⁴⁴-S6rp signal as detected by immunofluorescence after prolonged incubation. On the other hand, elevation in extracellular potassium concentration and the addition of apamin generated an increased firing rate and a burst-firing activity in CINs, respectively, and both stimulatory conditions significantly increased Ser²⁴⁰⁻²⁴⁴-S6rp phosphorylation above basal levels when incubated for one hour. Apamin generated a particularly large increase in phosphorylation that was sensitive to rapamycin. Taken together, our results demonstrate for the first time a link between the state of neuronal activity and a biochemical signaling event in striatal CINs, and suggest that immunofluorescence can be used to estimate the cellular activity of CINs under different pharmacological and/or behavioral conditions.

Regulation of the ERK pathway in the dentate gyrus by in vivo dopamine D1 receptor stimulation requires glutamatergic transmission

Acute systemic administration of the dopamine D1/D5 receptors (D1Rs) agonist, SKF81297, activates the extracellular signal-regulated protein kinases (ERK) pathway selectively in the granule cells of the dentate gyrus. In this study, we examined the mechanisms involved in this regulation and investigated the molecular components that could promote ERK-dependent transcription and translation. SKF81297 induced phosphorylation of ERK and histone H3 required intact glutamatergic transmission. Blockade of glutamate release achieved by the mGluR2/3 agonist, LY354740 or the selective adenosine A1R agonist, CCPA as well as neurotoxic lesions of lateral entorhinal cortex reduced the ability of SKF81297 to induce ERK activation in the dentate gyrus. This activation required the combined stimulation of NR2B-containing NMDARs, mGluR1 and mGluR5. SKF81297 evoked phosphorylation of the ribosomal protein S6 (rpS6) selectively at the Ser235/236 site while the Ser240/244 site remains unchanged. The SKF81297 induced increased phosphorylation of rpS6 was dependent on PKC and ERK/p90RSK activation. Surprisingly, administration of D1Rs agonist suppressed mTORC1/p70S6K pathway suggesting an mTOR-independent regulation of rpS6 phosphorylation. Taken together, our results show that intact glutamatergic transmission plays a major role in the regulation of ERK-dependent phosphorylation of histone H3 and rpS6 observed in the mouse dentate gyrus after systemic administration of SKF81297.

Combinatorial topography and cell-type specific regulation of the ERK pathway by dopaminergic agonists in the mouse striatum

Therapeutic agents and drugs of abuse regulate the extracellular signal-regulated kinase (ERK) cascade signaling in the medium-sized spiny neurons (MSNs) of the striatum. However, whether this regulation is associated with specific cortical and thalamic inputs has never been studied. We used Drd2-EGFP BAC-transgenic mice to undertake a topographical and cell-type specific analysis of ERK phosphorylation and two of its downstream targets histone H3 and ribosomal protein S6 (rS6) in the dorsal striatum following injection of SKF81297 (D1R-like agonist), quinpirole (D2R-like agonist) or apomorphine (non selective DA receptor agonist). In striatal areas receiving inputs from the cingulate/prelimbic, visual and auditory cortex, SKF81297 treatment increased phosphorylation of ERK, histone H3 and rS6 selectively in EGFP-negative MSNs of Drd2-EGFP mice. In contrast, no regulation was found in striatal region predominantly targeted by the sensorimotor and motor cortex. Apomorphine slightly enhanced ERK and rS6, but not histone H3 phosphorylation. This regulation occurred exclusively in EGFP-negative neurons mostly in striatal sectors receiving connections from the insular, visual and auditory cortex. Quinpirole administration inhibited basal ERK activation but did not change histone H3 and rS6 phosphorylation throughout the rostrocaudal axis of the dorsal striatum. This anatomo-functional study indicates that D1R and D2R agonists produce a unique topography and cell-type specific regulation of the ERK cascade signaling in the mouse striatum, and that those patterns are closely associated with particular cortical and thalamic inputs. This work evidences the need of a precise identification of the striatal areas under study to further understand striatal plasticity.

Combination of group I mGlu receptors antagonist with dopaminergic agonists strengthens the synaptic transmission at corticostriatal synapses in culture

Restoring synaptic plasticity in neurodegenerative diseases could prevent neuronal degeneration, as well as motor and cognitive disorders. In Parkinson's disease, synaptic plasticity at corticostriatal synapses is altered. Dendrites of striatal medium spiny neurons (MSNs) receive dopaminergic inputs from the substantia nigra and glutamatergic cortical afferents. Because both glutamate and dopamine are required to induce and sustain MSNs plasticity, the particular molecular mechanisms involved at this synaptic triad are difficult to understand. In the present work, we established a convenient in vitro model of the corticostriatal synapse to study synaptic plasticity. We focused on long-term depression involving group I metabotropic glutamate (mGlu) receptors. We found that in striatal neurons co-cultured with cortical neurons, the absence of dopaminergic stimuli favored the excess of glutamatergic drive from cortical neuron terminals, thus resulting in a constitutive depression of the corticostriatal glutamatergic transmission. Indeed, concomitant blockade of group I mGlu receptors and activation of dopaminergic receptors stably reduced the depression of the synaptic transmission. Thus the dependence on glutamate and dopamine balance of the corticostriatal synapse responsiveness validates the accuracy of this manageable in vitro model to depict the molecular pathways involved in the plasticity at corticostriatal synapses and to test restorative therapeutic approaches in Parkinson's disease. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Convulsant doses of a dopamine D1 receptor agonist result in Erk-dependent increases in Zif268 and Arc/Arg3.1 expression in mouse dentate gyrus

Activation of dopamine D1 receptors (D1Rs) has been shown to induce epileptiform activity. We studied the molecular changes occurring in the hippocampus in response to the administration of the D1-type receptor agonist, SKF 81297. SKF 81297 at 2.5 and 5.0 mg/kg induced behavioural seizures. Electrophysiological recordings in the dentate gyrus revealed the presence of epileptiform discharges peaking at 30-45 min post-injection and declining by 60 min. Seizures were prevented by the D1-type receptor antagonist, SCH 23390, or the cannabinoid CB1 receptor agonist, CP 55,940. The effect of SKF 81297 was accompanied by increased phosphorylation of the extracellular signal-regulated protein kinases 1 and 2 (ERK), in the granule cells of the dentate gyrus. This effect was also observed in response to administration of other D1-type receptor agonists, such as SKF83822 and SKF83959. In addition, SKF 81297 increased the phosphorylation of the ribosomal protein S6 and histone H3, two downstream targets of ERK. These effects were prevented by genetic inactivation of D1Rs, or by pharmacological inhibition of ERK. SKF 81297 was also able to enhance the levels of Zif268 and Arc/Arg3.1, two immediate early genes involved in transcriptional regulation and synaptic plasticity. These changes may be involved in forms of activity-dependent plasticity linked to the manifestation of seizures and to the ability of dopamine to affect learning and memory.

What is the Degree of Segregation between Striatonigral and Striatopallidal Projections?

In contrast to most other brain regions, in the striatum the output neurons (the medium-sized spiny neurons, MSNs) are GABAergic and act by inhibiting their targets. The standard model of the basal ganglia is built on the segregation of information processing in the direct and indirect pathways, which act in opposing directions to control movement. The MSNs participating in these two pathways can be identified according to their projection sites and the proteins they express. The differential expression of two of the five known dopamine receptor subtypes, D1 and D2, in the two populations of MSNs is of particular importance, since it confers to dopamine the ability to exert opposite functional modulation on the direct and indirect pathways. However, beyond this simple view of the striatal output organization, anatomical studies questioned the segregation of direct and indirect projections to the SNr, while other studies disclosed variable degrees of overlapping expression of dopamine receptor subtypes in striatal MSNs. New ways to address these issues have emerged recently, using mouse models in which specific populations of striatal neurons are genetically tagged. Here, we review classical and recent studies supporting the segregation of striatonigral and striatopallidal neurons. We also consider this issue at a functional level by focusing on the regulation of striatal signaling pathways in the two populations of MSNs, which clearly emphasize their profound differences. We discuss the anatomical and functional evidence challenging some aspects of this segregation and outline questions that are still to be addressed.

mTORC1 signaling in Parkinson's disease and L-DOPA-induced dyskinesia: A sensitized matter

Parkinson disease is caused by the progressive loss of dopamine innervation to the basal ganglia and is commonly treated with the dopamine precursor, L-DOPA. Prolonged administration of L-DOPA results in the development of severe motor complications or dyskinesia, which seriously hamper its clinical use. Recent evidence indicates that L-DOPA-induced dyskinesia (LID) is associated with persistent activation of the mammalian target of rapamycin complex 1 (mTORC1) in the medium spiny neurons (MSNs) of the striatum, the main component of the basal ganglia. This phenomenon is secondary to the development of a strong sensitization at the level of dopamine D1 receptors, which are abundantly expressed in a subset of MSNs. Such sensitization confers to dopaminergic drugs (including L-DOPA) the ability to activate the extracellular signal-regulated protein kinases 1/2, which, in turn promote mTORC1 signaling. Using a mouse model of LID, we recently showed that administration of the allosteric mTORC1 inhibitor, rapamycin, reduces dyskinesia. This finding is discussed with respect to underlying mechanisms and potential significance for the development of future therapeutic interventions.

Mechanisms of locomotor sensitization to drugs of abuse in a two-injection protocol

A single exposure to psychostimulants or morphine is sufficient to induce persistent locomotor sensitization, as well as neurochemical and electrophysiological changes in rodents. Although it provides a unique model to study the bases of long-term behavioral plasticity, sensitization mechanisms remain poorly understood. We investigated in the mouse, a species suited for transgenic studies, the mechanisms of locomotor sensitization showed by the increased response to a second injection of drug (two-injection protocol of sensitization, TIPS). The first cocaine injection induced a locomotor sensitization that was completely context-dependent, increased during the first week, and persisted 3 months later. The induction of sensitized responses to cocaine required dopamine D1 and glutamate NMDA receptors. A single injection of the selective dopamine transporter blocker GBR12783 was sufficient to activate extracellular signal-regulated kinase (ERK) in the striatum to the same level as cocaine and to induce sensitization to cocaine, but not to itself. The induction of sensitization was sensitive to protein synthesis inhibition by anisomycin after cocaine administration. Morphine induced a pronounced context-dependent sensitization that crossed with cocaine. Sensitization to morphine injection was prevented in knockin mutant mice bearing a Thr-34-Ala mutation of DARPP-32, which suppresses its ability to inhibit protein phosphatase-1 (PP1), but not mutation of Thr-75 or Ser-130. These results combined with previous ones show that TIPS in mouse is a context-dependent response, which involves an increase in extracellular dopamine, stimulation of D1 and NMDA receptors, regulation of the cAMP-dependent and ERK pathways, inhibition of PP1, and protein synthesis. It provides a simple and sensitive paradigm to study the mechanisms of long-term effects of drugs of abuse.

Ras-guanine nucleotide-releasing factor 1 (Ras-GRF1) controls activation of extracellular signal-regulated kinase (ERK) signaling in the striatum and long-term behavioral responses to cocaine

BACKGROUND

Ras-extracellular signal-regulated kinase (Ras-ERK) signaling is central to the molecular machinery underlying cognitive functions. In the striatum, ERK1/2 kinases are co-activated by glutamate and dopamine D1/5 receptors, but the mechanisms providing such signaling integration are still unknown. The Ras-guanine nucleotide-releasing factor 1 (Ras-GRF1), a neuronal specific activator of Ras-ERK signaling, is a likely candidate for coupling these neurotransmitter signals to ERK kinases in the striatonigral medium spiny neurons (MSN) and for modulating behavioral responses to drug abuse such as cocaine.

METHODS

We used genetically modified mouse mutants for Ras-GRF1 as a source of primary MSN cultures and organotypic slices, to perform both immunoblot and immunofluorescence studies in response to glutamate and dopamine receptor agonists. Mice were also subjected to behavioral and immunohistochemical investigations upon treatment with cocaine.

RESULTS

Phosphorylation of ERK1/2 in response to glutamate, dopamine D1 agonist, or both stimuli simultaneously is impaired in Ras-GRF1-deficient striatal cells and organotypic slices of the striatonigral MSN compartment. Consistently, behavioral responses to cocaine are also affected in mice deficient for Ras-GRF1 or overexpressing it. Both locomotor sensitization and conditioned place preference are significantly attenuated in Ras-GRF1-deficient mice, whereas a robust facilitation is observed in overexpressing transgenic animals. Finally, we found corresponding changes in ERK1/2 activation and in accumulation of FosB/DeltaFosB, a well-characterized marker for long-term responses to cocaine, in MSN from these animals.

CONCLUSIONS

These results strongly implicate Ras-GRF1 in the integration of the two main neurotransmitter inputs to the striatum and in the maladaptive modulation of striatal networks in response to cocaine.

Striatal medium-sized spiny neurons: identification by nuclear staining and study of neuronal subpopulations in BAC transgenic mice

Precise identification of neuronal populations is a major challenge in neuroscience. In the striatum, more than 95% of neurons are GABAergic medium-sized spiny neurons (MSNs), which form two intermingled populations distinguished by their projections and protein content. Those expressing dopamine D₁-receptors (D1Rs) project preferentially to the substantia nigra pars reticulata (SNr), whereas those expressing dopamine D₂- receptors (D2Rs) project preferentially to the lateral part of the globus pallidus (LGP). The degree of segregation of these populations has been a continuous subject of debate, and the recent introduction of bacterial artificial chromosome (BAC) transgenic mice expressing fluorescent proteins driven by specific promoters was a major progress to facilitate striatal neuron identification. However, the fraction of MSNs labeled in these mice has been recently called into question, casting doubt on the generality of results obtained with such approaches. Here, we performed an in-depth quantitative analysis of striatal neurons in drd1a-EGFP and drd2-EGFP mice. We first quantified neuronal and non-neuronal populations in the striatum, based on nuclear staining with TO-PRO-3, and immunolabeling for NeuN, DARPP-32 (dopamine- and cAMP-regulated phosphoprotein Mr∼32,000), and various markers for interneurons. TO-PRO-3 staining was sufficient to identify MSNs by their typical nuclear morphology and, with a good probability, interneuron populations. In drd1a-EGFP/drd2-EGFP double transgenic mice all MSNs expressed EGFP, which was driven in about half of them by drd1a promoter. Retrograde labeling showed that all MSNs projecting to the SNr expressed D1R and very few D2R (<1%). In contrast, our results were compatible with the existence of some D1R-EGFP-expressing fibers giving off terminals in the LGP. Thus, our study shows that nuclear staining is a simple method for identifying MSNs and other striatal neurons. It also unambiguously confirms the degree of segregation of MSNs in the mouse striatum and allows the full exploitation of results obtained with BAC-transgenic mice.

A phosphatase cascade by which rewarding stimuli control nucleosomal response

Dopamine orchestrates motor behaviour and reward-driven learning. Perturbations of dopamine signalling have been implicated in several neurological and psychiatric disorders, and in drug addiction. The actions of dopamine are mediated in part by the regulation of gene expression in the striatum, through mechanisms that are not fully understood. Here we show that drugs of abuse, as well as food reinforcement learning, promote the nuclear accumulation of 32-kDa dopamine-regulated and cyclic-AMP-regulated phosphoprotein (DARPP-32). This accumulation is mediated through a signalling cascade involving dopamine D1 receptors, cAMP-dependent activation of protein phosphatase-2A, dephosphorylation of DARPP-32 at Ser 97 and inhibition of its nuclear export. The nuclear accumulation of DARPP-32, a potent inhibitor of protein phosphatase-1, increases the phosphorylation of histone H3, an important component of nucleosomal response. Mutation of Ser 97 profoundly alters behavioural effects of drugs of abuse and decreases motivation for food, underlining the functional importance of this signalling cascade.

Parkinson's disease: levodopa-induced dyskinesia and signal transduction

l-3,4-Dihydroxyphenylalanine (L-dopa) remains the most effective pharmacological treatment for relief of the severe motor impairments of Parkinson's disease. It is very effective in controlling parkinsonian symptoms in the initial phase of the disease, but its action wanes with time. Such 'wearing-off' imposes an escalation in the dosage of the drug, which ultimately fails to provide stable control of motor symptoms and results in the appearance of abnormal involuntary movements or dyskinesia. 'Peak-dose'l-dopa-induced dyskinesia (LID) currently represents one of the major challenges in the treatment of Parkinson's disease. Accumulating evidence suggests that LID derives from overstimulation of dopamine receptors located on the GABAergic medium spiny neurons (MSNs) of the dorsal striatum. These neurons form two distinct projection pathways, which exert opposite effects on motor activity: the direct, striatonigral pathway promotes locomotion, whereas the indirect, striatopallidal pathway depresses locomotion. In order to understand the mechanisms underlying LID, it is important to identify molecular adaptations produced by chronic administration of L-dopa, at the level of one or the other of these two neuronal populations. This review summarizes the results of recent studies indicating that LID is associated with abnormal dopamine D1 receptor signaling affecting the MSNs of the direct pathway. The role of this pathological adaptation and of the consequent changes in signaling in the development and expression of LID are discussed.

Calcineurin is essential for depolarization-induced nuclear translocation and tyrosine phosphorylation of PYK2 in neurons

Proline-rich tyrosine kinase 2 (PYK2) is a non-receptor tyrosine kinase expressed in many cell types and enriched in neurons. PYK2 is a cytoplasmic enzyme activated by increases in cytosolic free Ca(2+) through an unknown mechanism. We report that depolarization or electrical stimulation of hippocampal slices induced a rapid and transient nuclear accumulation of PYK2. Depolarization of cultured neurons or PC12 cells also triggered a Ca(2+)-dependent nuclear accumulation of PYK2, much more pronounced than that induced by blockade of nuclear export with leptomycin B. Src-family kinase activity, PYK2 autophosphorylation and kinase activity were not required for its nuclear translocation. Depolarization induced a slight decrease in PYK2 apparent molecular mass, compatible with a Ca(2+)-activated dephosphorylation. Pretreatment of PC12 cells with inhibitors of calcineurin (protein phosphatase 2B), cyclosporin A and FK506, prevented depolarization-induced nuclear translocation and tyrosine phosphorylation of PYK2. Transfection with dominant-negative and constitutively active calcineurin-A confirmed the role of calcineurin in the regulation of PYK2 tyrosine phosphorylation and nuclear accumulation. Our results show that depolarization independently induces nuclear translocation and tyrosine phosphorylation of PYK2, and that both responses require calcineurin activation. We suggest that PYK2 exerts some of its actions in the nucleus and that the effects of calcineurin inhibitors may involve PYK2 inhibition.

Role of cannabinoid type 1 receptors in locomotor activity and striatal signaling in response to psychostimulants

A single administration of cocaine or D-amphetamine produces acute hyperlocomotion and long-lasting increased sensitivity to subsequent injections. This locomotor sensitization reveals the powerful ability of psychostimulants to induce brain plasticity and may participate in the alterations that underlie addiction. We investigated the role of cannabinoid receptor type 1 (CB1-R) in the effects of a single injection of psychostimulants. The acute locomotor response to cocaine was normal in mice pretreated with the CB1-R inverse agonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251), whereas no sensitization was observed in response to a second administration a week later. Locomotor responses to cocaine and D-amphetamine were decreased in CB1-R-deficient mice, and sensitization was impaired. To determine how CB1-R controls long-lasting effects of psychostimulants, we studied cocaine-activated signaling pathways. Cocaine-induced cAMP-dependent phosphorylation of glutamate receptor 1 was altered in the striatum of CB1-R mutant mice but not of AM251-treated mice. In contrast, cocaine-induced phosphorylation of extracellular signal-regulated kinase (ERK) was blocked in both CB1-R mutant and antagonist-pretreated mice. Conditional deletion of CB1-R in forebrain principal neurons or GABAergic neurons prevented cocaine-induced ERK activation in dorsal striatum and nucleus accumbens. Our results provide strong evidence for the role of the endocannabinoid system in regulating neuronal circuits critical for long-lasting effects of cocaine, presumably by acting on CB1-R located on terminals of striatal medium spiny neurons.

Critical involvement of cAMP/DARPP-32 and extracellular signal-regulated protein kinase signaling in L-DOPA-induced dyskinesia

The molecular basis of L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia (LID), one of the major hindrances in the current therapy for Parkinson's disease, is still unclear. We show that attenuation of cAMP signaling in the medium spiny neurons of the striatum, achieved by genetic inactivation of the dopamine and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), reduces LID. We also show that, in dyskinetic mice, sensitized cAMP/cAMP-dependent protein kinase/DARPP-32 signaling leads to phosphorylation/activation of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). The increase in ERK1/2 phosphorylation associated with dyskinesia results in activation of mitogen- and stress-activated kinase-1 (MSK-1) and phosphorylation of histone H3, two downstream targets of ERK involved in transcriptional regulation. In line with these observations, we found that c-Fos expression is abnormally elevated in the striata of mice affected by LID. Persistent enhancement of the ERK signaling cascade is implicated in the generation of LID. Thus, pharmacological inactivation of ERK1/2 achieved using SL327 (alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzeneacetonitrile), an inhibitor of the mitogen-activated kinase/ERK kinase, MEK, during chronic L-DOPA treatment counteracts the induction dyskinesia. Together, these results indicate that a significant proportion of the abnormal involuntary movements developed in response to chronic L-DOPA are attributable to hyperactivation in striatal medium spiny neurons of a signaling pathway including sequential phosphorylation of DARPP-32, ERK1/2, MSK-1, and histone H3.