Repeated nicotine exposure increases the intracellular interaction between ERK-mGluR5 in the nucleus accumbens more in adult than adolescent rats.

Intracellular interactions between protein kinases and metabotropic receptors in the striatum regulate behavioral changes in response to drug exposure. We investigated the difference in the degree of interaction between extracellular signal-regulated kinase (ERK) and metabotropic glutamate receptor subtype 5 (mGluR5) in the nucleus accumbens (NAc) after repeated exposure to nicotine in adult and adolescent rats. The results showed that repeated exposure to nicotine (0.5 mg/kg/day, s.c.) for seven consecutive days increased ERK phosphorylation more in adults than in adolescents. Furthermore, membrane expression of mGluR5 in gamma-aminobutyric acid (GABA) medium spiny neurons was higher in adults than adolescents as a result of repeated exposure to nicotine. Blockade of mGluR5 with MPEP (0.5 nmol/side) decreased the repeated nicotine-induced increase in ERK phosphorylation. Either blockade of mGluR5 or inhibition of ERK with SL327 (150 nmol/side) decreased the repeated nicotine-induced increase in the level of inositol-1,4,5-triphosphate (IP3 ), a key transducer associated with mGluR5-coupled signaling cascades. Similarly, interference of binding between activated ERK and mGluR5 by the blocking peptide, Tat-mGluR5-i (2 nmol/side), decreased the repeated nicotine-induced increases in IP3 and locomotor activity in adults. These findings suggest that the intracellular interaction between ERK and mGluR5 in the NAc is stronger in adult than in adolescent rats, which enhances the understanding of age-associated behavioral changes that occur after repeated exposure to nicotine.

The role of matrix metalloproteinase-9 in negative reinforcement learning and plasticity in alcohol dependence.

A role for matrix metalloproteinases (MMPs) in plasticity-dependent learning has been established. MMPs degrade the extracellular matrix (ECM) when synaptic reorganization is warranted. Previously, we showed that escalation of alcohol self-administration is a learned plasticity-dependent process that requires an intact MMP system. To identify the MMP subtypes within specific brain regions that are associated with plasticity underlying the negative reinforcing effects of alcohol (as measured by escalated alcohol self-administration) during acute withdrawal in alcohol dependence, male Wistar rats were trained to self-administer alcohol in an operant paradigm, subjected to one month of intermittent alcohol vapor exposure to induce alcohol dependence and then allowed to self-administer alcohol during repeated acute withdrawal self-administration sessions. Subsequently, rat brains were extracted after initial or stable escalated alcohol self-administration phases of acute withdrawal and analyzed by immunoblot to detect MMP-2, -3, and -9 levels in the anterior cingulate cortex (ACC), bed nucleus of the stria terminalis, central amygdala (CeA), hippocampus, and nucleus accumbens (NAc). The results showed that MMP-9 expression in the CeA and NAc of alcohol-dependent rats was increased, however, MMP-9 expression in the ACC was decreased during negative reinforcement learning. Subsequently, the importance of plasticity mediated by MMP-9 in escalated alcohol self-administration during acute withdrawal was functionally assessed through site-specific intra-CeA MMP-9 inhibition during repeated acute withdrawal self-administration sessions. MMP-9 inhibition prevented acute withdrawal-induced escalation of alcohol self-administration in a manner that was not confounded by locomotor effects or a permanent inability to learn about the negative reinforcing effects of alcohol.

Glutamine has antidepressive effects through increments of glutamate and glutamine levels and glutamatergic activity in the medial prefrontal cortex.

Emerging evidence has shown the low levels of glutamate (Glu) and glutamine (Gln) and the hypoactivity in the cortex of patients with depression. The hypoactivity is closely related with low frequency of glutamatergic signaling that is affected by the levels of Glu and Gln. Thus, we hypothesized that there might be a causality among low levels of Glu and Gln, hypoactive glutamatergic neurotransmissions, and depressive behaviors. Here, we found low Glu and Gln levels and low frequency of spontaneous excitatory postsynaptic current (sEPSC) of glutamatergic neurons in the medial prefrontal cortex (mPFC) of chronic immobilization stress (CIS)-induced depressed mice. The depressed mice also showed hypoactive Gln synthetase (GS). Inhibition of GS by methionine sulfoximine (MSO) decreased Glu and Gln levels and increased depressive behaviors with low frequency of sEPSC in the mPFC, indicating that Glu and Gln decrements cause hypoactive glutamatergic neurotransmissions and depressive behaviors. Both Glu and Gln could increase sEPSC of glutamatergic neurons in the mPFC on slice patch, but only Gln overcame MSO to increase sEPSC, suggesting that exogenous Gln would recover CIS-induced low frequency of sEPSC caused by hypoactive GS and act as an antidepressant. Expectedly, Gln supplementation showed antidepressant effects against CIS; it increased glutamatergic neurotransmissions with Glu and Gln increment in the mPFC and attenuated depressive behaviors. Moreover, selective glutamatergic activation in the mPFC by optogenetics decreased depressive behavior. In conclusion, depressive behaviors evoked by chronic stress were due to hypoactive glutamatergic neurons in the mPFC caused by low levels of Glu and Gln, and exogenous Gln can be used as an alternative antidepressant to increase glutamatergic neurotransmission.

Psychostimulant-Induced Endoplasmic Reticulum Stress and Neurodegeneration.

The endoplasmic reticulum (ER) is a subcellular organelle that ensures proper protein folding process. The ER stress is defined as cellular conditions that disturb the ER homeostasis, resulting in accumulation of unfolded and/or misfolded proteins in the lumen of the ER. The presence of these proteins within the ER activates the ER stress response, known as unfolded protein response (UPR), to restore normal functions of the ER. However, under the severe and/or prolonged ER stress, UPR initiates apoptotic cell death. Psychostimulants such as cocaine, amphetamine, and methamphetamine cause the ER stress and/or apoptotic cell death in regions of the brain related to drug addiction. Recent studies have shown that the ER stress in response to psychostimulants is linked to behavioral sensitization and that the psychostimulant-induced ER stress signaling cascades are closely associated with the pathogenesis of the neurodegenerative diseases. Therefore, this review was conducted to improve understanding of the functional role of the ER stress in the addiction as well as neurodegenerative diseases. This would be helpful to facilitate development of new therapeutic strategies for the drug addiction and/or neurodegenerative diseases caused or exacerbated by exposure to psychostimulants.

Glutamatergic neurotransmission in the prefrontal cortex mediates the suppressive effect of intra-prelimbic cortical infusion of BDNF on cocaine-seeking.

Cocaine self-administration induces dysfunctional neuroadaptations in the prefrontal cortex that underlie relapse to cocaine-seeking. Cocaine self-administration disturbs glutamatergic transmission in the nucleus accumbens that is prevented by infusion of brain-derived neurotrophic factor (BDNF) into the prelimbic area of the prefrontal cortex. Intra-prelimbic infusion of BDNF decreases cocaine-seeking in a TrkB-ERK MAP kinase-dependent manner. Neuronal activity triggers an interaction between TrkB receptors and NMDA receptors, leading to ERK activation. In the present study, infusion of the GluN2A-containing NMDA receptor antagonist, TCN-201, or the GluN2B-containing NMDA receptor antagonist, Ro-25-6981, into the prelimbic cortex of rats blocked the suppressive effect of BDNF on cocaine-seeking. During early withdrawal from cocaine self-administration, tyrosine phosphorylation of ERK, GluN2A, and GluN2B in the prelimbic cortex was reduced and this reduction of phospho-proteins was prevented by intra-prelimbic BDNF infusion. TCN-201 infusion into the prelimbic cortex inhibited the BDNF-mediated increase in pERK and pGluN2A whereas Ro-25-6981 infusion into the prelimbic cortex blocked BDNF-induced elevation of pERK and pGluN2B, indicating that both GluN2A- and GluN2B-containing NMDA receptors underlie BDNF-induced ERK activation. These data demonstrate that BDNF-mediated activation of GluN2A- and GluN2B-containing NMDA receptors underlies ERK activation in the prelimbic cortex during early withdrawal, preventing subsequent relapse to cocaine-seeking.

Linking cocaine to endoplasmic reticulum in striatal neurons: role of glutamate receptors.

The endoplasmic reticulum (ER) controls protein folding. Accumulation of unfolded and misfolded proteins in the ER triggers an ER stress response to accelerate normal protein folding or if failed to cause apoptosis. The ER stress response is a conserved cellular response in mammalian cells and is sensitive to various physiological or pathophysiological stimuli. Recent studies unravel that this response in striatal neurons is subject to the tight modulation by psychostimulants. Cocaine and amphetamines markedly increased expression of multiple ER stress reporter proteins in the dorsal striatum (caudate putamen) and other basal ganglia sites. This evoked ER stress response is mediated by activation of group I metabotropic glutamate receptors and N-methyl-D-aspartate receptors. Converging Ca(2+) signals derived from activation of these receptors activate the c-Jun N-terminal kinase pathway to evoke ER stress responses. The discovery of robust ER stress responses to stimulant exposure establishes a previously unrecognized stimulant-ER coupling. This inducible coupling seems to contribute to neurotoxicity of stimulants related to various neuropsychiatric and neurodegenerative illnesses. Elucidating cellular mechanisms linking cocaine and other stimulants to ER is therefore important for the development of therapeutic agents for treating neurological disorders resulted from stimulant toxicity.

Activation of c-Jun N-terminal kinase is required for the regulation of endoplasmic reticulum stress response in the rat dorsal striatum following repeated cocaine administration.

Repeated exposure to cocaine upregulates endoplasmic reticulum (ER) stress response and c-Jun N-terminal kinase (JNK) phosphorylation is associated with the ER stress response in neurons. In this study, we investigated the involvement of JNK in the regulation of the ER stress response following repeated cocaine administration in the dorsal striatum in vivo. The results showed that systemic injections of cocaine (20 mg/kg) for seven consecutive days increased the induction of p46 JNK (JNK) phosphorylation, immunoglobulin heavy chain binding protein (BiP), the ER stress-associated protein caspase-12, and behavioral locomotor activity. This enhancement of BiP and caspase-12 expression and locomotor response was reduced by inhibiting JNK. Similar reduction of elevated JNK phosphorylation was induced by blocking dopamine D1 receptors, N-methyl-D-aspartate (NMDA) receptors, and group I metabotropic glutamate receptors (mGluRs). These data suggest that JNK activation following repeated cocaine administration is required for the regulation of the ER stress protein expression and behavioral alteration in the dorsal striatum. Stimulation of dopamine D1 receptors, NMDA receptors or group I mGluRs participates in the regulation of JNK activation.