Possible Role of the Extracellular Signal-Regulated Kinase (ERK) in Reward-Controlled Learning and Addiction

Nicotinic modulation of hippocampal cell signaling and associated effects on learning and memory

The hippocampus is a key brain structure involved in synaptic plasticity associated with long-term declarative memory formation. Importantly, nicotine and activation of nicotinic acetylcholine receptors (nAChRs) can alter hippocampal plasticity and these changes may occur through modulation of hippocampal kinases and transcription factors. Hippocampal kinases such as cAMP-dependent protein kinase (PKA), calcium/calmodulin-dependent protein kinases (CAMKs), extracellular signal-regulated kinases 1 and 2 (ERK1/2), and c-jun N-terminal kinase 1 (JNK1), and the transcription factor cAMP-response element-binding protein (CREB) that are activated either directly or indirectly by nicotine may modulate hippocampal plasticity and in parallel hippocampus-dependent learning and memory. Evidence suggests that nicotine may alter hippocampus-dependent learning by changing the time and magnitude of activation of kinases and transcription factors normally involved in learning and by recruiting additional cell signaling molecules. Understanding how nicotine alters learning and memory will advance basic understanding of the neural substrates of learning and aid in understanding mental disorders that involve cognitive and learning deficits.

Neurotensin: role in psychiatric and neurological diseases

Neurotensin (NT), an endogenous brain-gut peptide, has a close anatomical and functional relationship with the mesocorticolimbic and neostriatal dopamine system. Dysregulation of NT neurotransmission in this system has been hypothesized to be involved in the pathogenesis of schizophrenia. Additionally, NT containing circuits have been demonstrated to mediate some of the mechanisms of action of antipsychotic drugs, as well as the rewarding and/or sensitizing properties of drugs of abuse. NT receptors have been suggested to be novel targets for the treatment of psychoses or drug addiction.

The lack of A2A adenosine receptors diminishes the reinforcing efficacy of cocaine

Adenosine is an endogenous purine nucleoside, which acts as a neuromodulator in the central nervous system. A2A adenosine and D2 dopamine receptors are colocalized in the same neurons in discrete brain areas, and the dopaminergic transmission plays a crucial role in the addictive properties of drugs of abuse, such as cocaine. In the present study, we have investigated the specific role of A2A adenosine receptors in cocaine-induced behavioral responses related to its addictive properties. For this purpose, we have evaluated the acute locomotor effects produced by cocaine and the development of locomotor sensitization by repeated cocaine administration. In addition, we have also examined cocaine acute rewarding properties using the conditioned place preference. Finally, we used the intravenous drug self-administration paradigm to investigate the acquisition of an operant response maintained by cocaine self-administration and the reinforcing efficacy of the drug in these knockout animals. Acute cocaine induced a similar increase of locomotor activity in mice lacking A2A adenosine receptors and wild-type littermates. Cocaine-induced locomotor sensitization and conditioned place preference were also maintained in A2A knockout mice. Nevertheless, these knockout mice showed a lower rate of cocaine self-administration than wild-type mice in both fixed ratio 1 and 3 schedules of reinforcement. Moreover, a reduction in the maximal effort to obtain a cocaine infusion was found in A2A knockout mice under a progressive ratio schedule. In addition, a vertical shift of the cocaine dose-response curve was observed in mice lacking A2A adenosine receptors in comparison with wild-type littermates. Our study demonstrates that A2A adenosine receptors play an important role in cocaine addictive properties, and these receptors seem to be required to develop the addictive effects of this drug.

Regulation of cocaine-induced activator protein 1 transcription factors by the extracellular signal-regulated kinase pathway

Extracellular signal-regulated kinases and activator protein 1 transcription factor have been functionally linked to addiction. It has also been shown that extracellular signal-regulated kinase activation can regulate cocaine-induced expression of c-Fos and FosB, two possible components of activator protein 1. A direct link between extracellular signal-regulated kinases and activator protein 1 activation has, however, remained unexplored. In this study, we investigated the role of extracellular signal-regulated kinases in the regulation of DNA-binding activity and composition of activator protein 1 induced in the mouse caudate putamen by cocaine treatment. We have found that pre-treatment with SL327, a selective inhibitor the extracellular signal-regulated kinase pathway, has no influence on cocaine-induced DNA-binding activity of activator protein 1, when examined one hour after an acute cocaine treatment. This phenomenon results from simultaneous decrease of c-Fos protein level and increases in JunB and deltaFosB protein levels. SL327 pre-treatment, however, reduces the DNA-binding activity of the activator protein 1 complex induced six hours after an acute cocaine treatment as well as one hour after the last of the chronic cocaine injections, a phenomenon that results from the concomitant reduction of all cocaine-induced proteins (c-Fos, FosB, deltaFosB, JunB). In conclusion, we have found that extracellular signal-regulated kinase inhibition may not only interfere with cocaine-induced gene expression and activator protein 1 complex activation, but may also disturb the time-course of gene expression and composition of activator protein 1 complex. Our results support the notion that inhibitors of the extracellular signal-regulated kinase pathway could be valuable tools to obliterate cocaine-induced molecular changes and the development of addiction.

Extracellular signal-regulated kinases (ERKs) modulate cocaine-induced gene expression in the mouse amygdala

It is known that acute cocaine administration activates the extracellular signal-regulated kinase (ERK) pathway in the striatum, and results in transcription and translation of immediate early genes (IEGs). In the present study we investigated a possible involvement of ERK in the regulation of IEG expression in the amygdala, another brain structure known to be related to an addicted state. The patterns of cocaine-induced c-Fos, JunB and Zif268 protein expression were investigated, using an immunohistochemical approach, within distinct nuclei of the amygdala, either in the presence or absence of a selective inhibitor of the ERK pathway, SL327. Although these IEGs were similarly activated in the various nuclei of the amygdala after acute administration of cocaine, they showed different patterns after chronic injections. They also showed selective sensitivities to ERK inhibition. In particular, whereas c-Fos and JunB expressions were augmented following chronic cocaine treatment, as compared with acute treatment, Zif268 expression was decreased by this chronic treatment. Additionally, chronic blocking of ERK activation affected cocaine-induced c-Fos and JunB but not Zif268 expression. Thus, the differential involvement of ERK in chronic vs. acute regulation of IEGs may account for its specific role in addiction-related behavioral alterations, such as sensitization and tolerance.

Regulation of a protein phosphatase cascade allows convergent dopamine and glutamate signals to activate ERK in the striatum

Many drugs of abuse exert their addictive effects by increasing extracellular dopamine in the nucleus accumbens, where they likely alter the plasticity of corticostriatal glutamatergic transmission. This mechanism implies key molecular alterations in neurons in which both dopamine and glutamate inputs are activated. Extracellular signal-regulated kinase (ERK), an enzyme important for long-term synaptic plasticity, is a good candidate for playing such a role. Here, we show in mouse that d-amphetamine activates ERK in a subset of medium-size spiny neurons of the dorsal striatum and nucleus accumbens, through the combined action of glutamate NMDA and D1-dopamine receptors. Activation of ERK by d-amphetamine or by widely abused drugs, including cocaine, nicotine, morphine, and Delta(9)-tetrahydrocannabinol was absent in mice lacking dopamine- and cAMP-regulated phosphoprotein of M(r) 32,000 (DARPP-32). The effects of d-amphetamine or cocaine on ERK activation in the striatum, but not in the prefrontal cortex, were prevented by point mutation of Thr-34, a DARPP-32 residue specifically involved in protein phosphatase-1 inhibition. Regulation by DARPP-32 occurred both upstream of ERK and at the level of striatal-enriched tyrosine phosphatase (STEP). Blockade of the ERK pathway or mutation of DARPP-32 altered locomotor sensitization induced by a single injection of psychostimulants, demonstrating the functional relevance of this regulation. Thus, activation of ERK, by a multilevel protein phosphatase-controlled mechanism, functions as a detector of coincidence of dopamine and glutamate signals converging on medium-size striatal neurons and is critical for long-lasting effects of drugs of abuse.

Depolarization activates ERK and proline-rich tyrosine kinase 2 (PYK2) independently in different cellular compartments in hippocampal slices

In the hippocampus, extracellular signal-regulated kinase (ERK) and the non-receptor protein proline-rich tyrosine kinase 2 (PYK2) are activated by depolarization and involved in synaptic plasticity. Both are also activated under pathological conditions following ischemia, convulsions, or electroconvulsive shock. Although in non-neuronal cells PYK2 activates ERK through the recruitment of Src-family kinases (SFKs), the link between these pathways in the hippocampus is not known. We addressed this question using K(+)-depolarized rat hippocampal slices. Depolarization increased the phosphorylation of PYK2, SFKs, and ERK. These effects resulted from Ca(2+) influx through voltage-gated Ca(2+) channels and were diminished by GF109203X, a protein kinase C inhibitor. Inhibition of SFKs with PP2 decreased PYK2 tyrosine phosphorylation dramatically, but not its autophosphorylation on Tyr-402. Moreover, PYK2 autophosphorylation and total tyrosine phosphorylation were profoundly altered in fyn-/- mice, revealing an important functional relationship between Fyn and PYK2 in the hippocampus. In contrast, ERK activation was unaltered by PP2, Fyn knock-out, or LY294002, a phosphatidyl-inositol-3-kinase inhibitor. ERK activation was prevented by MEK inhibitors that had no effect on PYK2. Immunofluorescence of hippocampal slices showed that PYK2 and ERK were activated in distinct cellular compartments in somatodendritic regions and nerve terminals, respectively, with virtually no overlap. Activation of ERK was critical for the rephosphorylation of a synaptic vesicle protein, synapsin I, following depolarization, underlining its functional importance in nerve terminals. Thus, in hippocampal slices, in contrast to cell lines, depolarization-induced activation of non-receptor tyrosine kinases and ERK occurs independently in distinct cellular compartments in which they appear to have different functional roles.

Addictive and non-addictive drugs induce distinct and specific patterns of ERK activation in mouse brain

A major goal of research on addiction is to identify the molecular mechanisms of long-lasting behavioural alterations induced by drugs of abuse. Cocaine and delta-9-tetrahydrocannabinol (THC) activate extracellular signal-regulated kinase (ERK) in the striatum and blockade of the ERK pathway prevents establishment of conditioned place preference to these drugs. However, it is not known whether activation of ERK in the striatum is specific for these two drugs and/or this brain region. We studied the appearance of phospho-ERK immunoreactive neurons in CD-1 mouse brain following acute administration of drugs commonly abused by humans, cocaine, morphine, nicotine and THC, or of other psychoactive compounds including caffeine, scopolamine, antidepressants and antipsychotics. Each drug generated a distinct regional pattern of ERK activation. All drugs of abuse increased ERK phosphorylation in nucleus accumbens, lateral bed nucleus of the stria terminalis, central amygdala and deep layers of prefrontal cortex, through a dopamine D1 receptor-dependent mechanism. Although some non-addictive drugs moderately activated ERK in a few of these areas, they never induced this combined pattern of strong activation. Antidepressants and caffeine activated ERK in hippocampus and cerebral cortex. Typical antipsychotics mildly activated ERK in dorsal striatum and superficial prefrontal cortex, whereas clozapine had no effect in the striatum, but more widespread effects in cortex and amygdala. Our results outline a subset of structures in which ERK activation might specifically contribute to the long-term effects of drugs of abuse, and suggest mapping ERK activation in brain as a way to identify potential sites of action of psychoactive drugs.