Examination of a role for metabotropic glutamate receptor 5 in the medial prefrontal cortex in cocaine sensitization in rats

Linking of NMDA receptors and mGluR5 in the nucleus accumbens core to repeated cocaine-induced 50-kHz ultrasonic vocalization in rats

Rats express a positive emotional state by emitting 50-kHz ultrasonic vocalization (USV) calls in response to drug exposure. This study demonstrated the linking of glutamate receptors in the nucleus accumbens (NAc) to vocal expression of 50-kHz USV calls after repeated cocaine administration in freely moving rats. Repeated systemic injections of cocaine (20 mg/kg/day, i.p.) for seven consecutive days increased the number of 50-kHz USV calls. Intra-NAc core infusion of the broad-glutamate receptor antagonist, γDGG (50 nmol/side), decreased the repeated cocaine-induced increase in the number of 50-kHz USV calls. Intra-NAc core infusion of the N-methyl-D-aspartate (NMDA) receptor antagonist, MK801 (2 nmol/side), but not α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or kainic acid receptor antagonist, CNQX disodium salt (2 nmol/side), decreased the number of 50-kHz USV calls that had been elevated by repeated exposure to cocaine. Intra-NAc core infusion of the group I metabotropic glutamate receptor subtype 5 (mGluR5), MPEP (0.5 nmol/side), MTEP (15 nmol/side) and inositol-1,4,5-trisphosphate receptor blocker, xestospongin C (0.004 nmol/side) decreased the cocaine-induced increase in the number of USV calls. These data suggest that the NMDA receptor- and mGluR5-dependent increase in intracellular Ca²⁺ concentrations in the NAc core is linked to a positive emotional state after repeated exposure to cocaine in rats.

Metabotropic glutamate type 5 receptor binding availability during dextroamphetamine sensitization in mice and humans

BACKGROUND

Glutamate transmission is implicated in drug-induced behavioural sensitization and the associated long-lasting increases in mesolimbic output. Metabotropic glutamate type 5 (mGlu5) receptors might be particularly important, but most details are poorly understood.

METHODS

We first assessed in mice (n = 51, all male) the effects of repeated dextroamphetamine administration (2.0 mg/kg, i.p.) on locomotor activity and binding of the mGlu5 ligand [3H]ABP688. In a parallel study, in 19 stimulant-drug-naïve healthy human volunteers (14 female) we administered 3 doses of dextroamphetamine (0.3 mg/kg, p.o.) or placebo, followed by a fourth dose 2 weeks later. We measured [11C]ABP688 binding using positron emission tomography before and after the induction phase. We assessed psychomotor and behavioural sensitization using speech rate, eye blink rate and self-report. We measured the localization of mGlu5 relative to synaptic markers in mouse striatum using immunofluorescence.

RESULTS

We observed amphetamine-induced psychomotor sensitization in mice and humans. We did not see group differences in mGlu5 availability following 3 pre-challenge amphetamine doses, but group differences did develop in mice administered 5 doses. In mice and humans, individual differences in mGlu5 binding after repeated amphetamine administration were negatively correlated with the extent of behavioural sensitization. In drug-naïve mice, mGlu5 was expressed at 67% of excitatory synapses on dendrites of striatal medium spiny neur.

LIMITATIONS

Correlational results should be interpreted as suggestive because of the limited sample size. We did not assess sex differences.

CONCLUSION

Together, these results suggest that changes in mGlu5 availability are not part of the earliest neural adaptations in stimulant-induced behavioural sensitization, but low mGlu5 binding might identify a higher propensity for sensitization.

The cognitive cost of reducing relapse to cocaine-seeking with mGlu5 allosteric modulators

RATIONALE

Cocaine use disorder (CUD) remains difficult to treat with no FDA-approved medications to reduce relapse. Antagonism of metabotropic glutamate receptor 5 (mGlu5) has been demonstrated to decrease cocaine-seeking but may also further compromise cognitive function in long-term cocaine users.

OBJECTIVES

Here we assessed the effect of repeated administration of negative or positive allosteric modulators (NAM or PAM) of mGlu5 on both cognitive performance and (context+cue)-primed cocaine-seeking after prolonged abstinence (≥ 45 days).

METHODS

Adult male Sprague-Dawley rats underwent 6 days of short-access (1 h/day) and 12 days of long-access (6 h/day) cocaine self-administration. Rats were then trained and tested in a delayed match-to-sample (DMS) task to establish baseline working memory performance over a 5-day block of testing. Next, rats received daily systemic administration of the mGlu5 NAM 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine hydrochloride (MTEP; 3 mg/kg), the mGlu5 PAM 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB; 30 mg/kg) or vehicle prior to DMS testing during a block of 5 days, followed by a 5-day washout DMS testing block.

RESULTS

MTEP and CDPPB decreased drug-seeking in response to cocaine-associated cues after prolonged abstinence. However, repeated treatment with MTEP impaired working memory, while CDPPB had no effects on performance.

CONCLUSIONS

These results emphasize the relevance of evaluating cognitive function within the context of investigating pharmacotherapies to treat CUD. Further research is needed to determine how two mechanistically different pharmacological compounds can exert the same behavioral effects to reduce cocaine-seeking.

Neurobiological substrates of persistent working memory deficits and cocaine-seeking in the prelimbic cortex of rats with a history of extended access to cocaine self-administration

Cocaine use disorder (CUD) is associated with prefrontal cortex dysfunction and cognitive deficits that may contribute to persistent relapse susceptibility. As the relationship between cognitive deficits, cortical abnormalities and drug seeking is poorly understood, development of relevant animal models is of high clinical importance. Here, we used an animal model to characterize working memory and reversal learning in rats with a history of extended access cocaine self-administration and prolonged abstinence. We also investigated immediate and long-term functional changes within the prelimbic cortex (PrL) in relation to cognitive performance and drug-seeking. Adult male rats underwent 6 days of short-access (1 h/day) followed by 12 days of long-access (6 h/day) cocaine self-administration, or received passive saline infusions. Next, rats were tested in delayed match-to-sample (DMS) and (non)match-to-sample (NMS) tasks, and finally in a single context + cue relapse test on day 90 of abstinence. We found that a history of chronic cocaine self-administration impaired working memory, though sparing reversal learning, and that the components of these cognitive measures correlated with later drug-seeking. Further, we found that dysregulated metabolic activity and mGlu5 receptor signaling in the PrL of cocaine rats correlated with past working memory performance and/or drug-seeking, as indicated by the analysis of cytochrome oxidase reactivity, mGlu5 and Homer 1b/c protein expression, as well as Arc mRNA expression in mGlu5-positive cells. These findings advocate for a persistent post-cocaine PrL dysfunction, rooted in ineffective compensatory changes and manifested as impaired working memory performance and hyperreactivity to cocaine cues. Considering the possible interplay between the neural correlates underlying post-cocaine cognitive deficits and drug-seeking, cognitive function should be evaluated and considered when developing neurobiologically-based treatments of cocaine relapse.

MGluR5 activity is required for the induction of ethanol behavioral sensitization and associated changes in ERK MAP kinase phosphorylation in the nucleus accumbens shell and lateral habenula

Metabotropic glutamate receptor subtype-5 (mGluR5) activity regulates a variety of behavioral pathologies associated with alcohol addiction. The main goal of this study was to determine if mGluR5 regulates the induction of ethanol-induced locomotor sensitization, which is a model of experience-dependent plasticity following initial exposure to drugs of abuse. The extracellular signal-regulated kinase (ERK_1/2) pathway is downstream of mGluR5 and implicated in alcohol addiction; however, its role in sensitization remains unexplored. We sought to determine if mGluR5-mediated changes in ethanol-induced sensitization are associated with changes in ERK_1/2 phosphorylation (pERK_1/2) in specific brain regions. Adult male DBA/2 J mice were tested for acute locomotor response to ethanol (0 or 2 g/kg, IP) followed by a 9-day induction period in which the mGluR5 antagonist MPEP (0 or 30 mg/kg, IP) was administered prior to ethanol (0 or 2.5 g/kg, IP). One day later, ethanol (2 g/kg) produced a robust within- and between-group increase in locomotor activity, indicating sensitization in mice that received MPEP (0 mg/kg) during induction. MPEP (30 mg/kg) treatment during induction resulted in locomotor response to ethanol (2 g/kg) challenge that was equivalent to an acute response, indicating full blockade of sensitization. Sensitization was associated with increased pERK_1/2 immunoreactivity (IR) in nucleus accumbens shell (AcbSh) and a reduction in lateral habenula (LHb), both of which were blocked by MPEP treatment during induction. Sensitization was also associated with mGluR5-independent increases in pERK_1/2 IR in the nucleus accumbens core and decreases in the dentate gyrus and lateral septum. These data indicate that mGluR5 activity is required for the induction of ethanol locomotor sensitization and associated changes in ERK_1/2 phosphorylation in the AcbSh and LHb, which raises the hypothesis that mGluR5-mediated cell signaling in these brain regions may mediate the induction of sensitization. Elucidating mechanisms of sensitization may increase understanding of how ethanol hijacks behavioral functions during the development of addiction.

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

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

Pharmacological Modulation of Endogenous Opioid Activity to Attenuate Neuropathic Pain in Rats

We showed previously that spinal metabotropic glutamate receptor 1 (mGluR₁) signaling suppresses or facilitates (depending on the stage of estrous cycle) analgesic responsiveness to intrathecal endomorphin 2, a highly mu-opioid receptor-selective endogenous opioid. Spinal endomorphin 2 antinociception is suppressed during diestrus by mGluR₁ when it is activated by membrane estrogen receptor alpha (mERα) and is facilitated during proestrus when mGluR₁ is activated by glutamate. In the current study, we tested the hypothesis that in female rats subjected to spinal nerve ligation (SNL), the inhibition of spinal estrogen synthesis or blockade of spinal mERα/mGluR₁ would be antiallodynic during diestrus, whereas during proestrus, mGluR₁ blockade would worsen the mechanical allodynia. As postulated, following SNL, aromatase inhibition or mERα/mGluR₁ blockade during diestrus markedly lessened the mechanical allodynia. This was observed only on the paw ipsilateral to SNL and was eliminated by naloxone, implicating endogenous opioid mediation. In contrast, during proestrus, mGluR₁ blockade worsened the SNL-induced mechanical allodynia of the ipsilateral paw. Findings suggest menstrual cycle stage-specific drug targets for and the putative clinical utility of harnessing endogenous opioids for chronic pain management in women, as well as the value of, if not the necessity for, considering menstrual cycle stage in clinical trials thereof. PERSPECTIVE: Intrathecal treatments that enhance spinal endomorphin 2 analgesic responsiveness under basal conditions lessen mechanical allodynia in a chronic pain model. Findings provide a foundation for developing drugs that harness endogenous opioid antinociception for chronic pain relief, lessening the need for exogenous opioids and thus prescription opioid abuse.

Age-Related Declines in Prefrontal Cortical Expression of Metabotropic Glutamate Receptors that Support Working Memory

Glutamate signaling is essential for the persistent neural activity in prefrontal cortex (PFC) that enables working memory. Metabotropic glutamate receptors (mGluRs) are a diverse class of proteins that modulate excitatory neurotransmission via both presynaptic regulation of extracellular glutamate levels and postsynaptic modulation of ion channels on dendritic spines. This receptor class is of significant therapeutic interest for treatment of cognitive disorders associated with glutamate dysregulation. Working memory impairment and cortical hypoexcitability are both associated with advanced aging. Whether aging modifies PFC mGluR expression, and the extent to which any such alterations are regionally or subtype specific, however, is unknown. Moreover, it is unclear whether specific mGluRs in PFC are critical for working memory, and thus, whether altered mGluR expression in aging or disease is sufficient to play a causative role in working memory decline. Experiments in the current study first evaluated the effects of age on medial PFC (mPFC) mGluR expression using biochemical and molecular approaches in rats. Of the eight mGluRs examined, only mGluR5, mGluR3, and mGluR4 were significantly reduced in the aged PFC. The reductions in mGluR3 and mGluR5 (but not mGluR4) were observed in both mRNA and protein and were selectively localized to the prelimbic (PrL), but not infralimbic (IL), subregion of mPFC. Finally, pharmacological blockade of mGluR5 or mGluR2/3 using selective antagonists directed to PrL significantly impaired working memory without influencing non-mnemonic aspects of task performance. Together, these data implicate attenuated expression of PFC mGluR5 and mGluR3 in the impaired working memory associated with advanced ages.

Estrogens synthesized and acting within a spinal oligomer suppress spinal endomorphin 2 antinociception: ebb and flow over the rat reproductive cycle

The magnitude of antinociception elicited by intrathecal endomorphin 2 (EM2), an endogenous mu-opioid receptor (MOR) ligand, varies across the rat estrous cycle. We now report that phasic changes in analgesic responsiveness to spinal EM2 result from plastic interactions within a novel membrane-bound oligomer containing estrogen receptors (mERs), aromatase (aka estrogen synthase), metabotropic glutamate receptor 1 (mGluR1), and MOR. During diestrus, spinal mERs, activated by locally synthesized estrogens, act with mGluR1 to suppress spinal EM2/MOR antinociception. The emergence of robust spinal EM2 antinociception during proestrus results from the loss of mER-mGluR1 suppression, a consequence of altered interactions within the oligomer. The chemical pairing of aromatase with mERs within the oligomer containing MOR and mGluR1 allows estrogens to function as intracellular messengers whose synthesis and actions are confined to the same signaling oligomer. This form of estrogenic signaling, which we term "oligocrine," enables discrete, highly compartmentalized estrogen/mER-mGluR1 signaling to regulate MOR-mediated antinociception induced by EM2. Finally, spinal neurons were observed not only to coexpress MOR, mERα, aromatase, and mGluR1 but also be apposed by EM2 varicosities. This suggests that modulation of spinal analgesic responsiveness to exogenous EM2 likely reflects changes in its endogenous analgesic activity. Analogous suppression of spinal EM2 antinociception in women (eg, around menses, comparable with diestrus in rats) as well as the (pathological) inability to transition out of that suppressed state at other menstrual cycle stages could underlie, at least in part, the much greater prevalence and severity of chronic pain in women than men.

Ventral tegmental area dopamine and GABA neurons: Physiological properties and expression of mRNA for endocannabinoid biosynthetic elements

The ventral tegmental area (VTA) is involved in adaptive reward and motivation processing and is composed of dopamine (DA) and GABA neurons. Defining the elements regulating activity and synaptic plasticity of these cells is critical to understanding mechanisms of reward and addiction. While endocannabinoids (eCBs) that potentially contribute to addiction are known to be involved in synaptic plasticity mechanisms in the VTA, where they are produced is poorly understood. In this study, DA and GABAergic cells were identified using electrophysiology, cellular markers, and a transgenic mouse model that specifically labels GABA cells. Using single-cell RT-qPCR and immunohistochemistry, we investigated mRNA and proteins involved in eCB signaling such as diacylglycerol lipase α, N-acyl-phosphatidylethanolamine-specific phospholipase D, and 12-lipoxygenase, as well as type I metabotropic glutamate receptors (mGluRs). Our results demonstrate the first molecular evidence of colocalization of eCB biosynthetic enzyme and type I mGluR mRNA in VTA neurons. Further, these data reveal higher expression of mGluR1 in DA neurons, suggesting potential differences in eCB synthesis between DA and GABA neurons. These data collectively suggest that VTA GABAergic and DAergic cells have the potential to produce various eCBs implicated in altering neuronal activity or plasticity in adaptive motivational reward or addiction.

Molecular changes in the medial prefrontal cortex and nucleus accumbens are associated with blocking the behavioral sensitization to cocaine

Previous studies have demonstrated that cocaine-induced behavioral sensitization is associated with persistent functional and structural alterations in the medial prefrontal cortex (mPFC) and nucleus accumbens (NAc); however, the molecular mechanisms underlying these changes have not been elucidated. In this study, the behavioral sensitization to cocaine was established in Sprague Dawley rats and was measured by locomotion and behavioral rating. The brain tissue homogenization was used for measuring the level of brain-derived neurotrophic factor (BDNF), the expression and activity of integrin-linked kinase (ILK), level of protein kinase B (Akt) phosphorylation at serine 473 and threonine 308, and the expression of p75(NTR), TrkA, and TrkB protein. The Results showed that cocaine sensitization was associated with increased BDNF, ILK activity, phospho-Akt Ser(473), p75(NTR), and TrkB protein levels in the mPFC and NAc core. The combination of pergolide and ondansetron normalized not only behavioral sensitization, but also the increases in these molecular markers. Dual immunofluoresence staining showed that ILK expression is co-distributed with p75(NTR) and TrkA expression in both the mPFC and NAc core. Results suggested that the BDNF-TrkA/p75(NTR)-ILK-Akt signaling pathway may be active in cocaine sensitization and associated neural plasticity in the mPFC and NAc core.

Deletion of Gαq in the telencephalon alters specific neurobehavioral outcomes

G(αq) -coupled receptors are ubiquitously expressed throughout the brain and body, and it has been shown that these receptors and associated signaling cascades are involved in a number of functional outputs, including motor function and learning and memory. Genetic alterations to G(αq) have been implicated in neurodevelopmental disorders such as Sturge-Weber syndrome. Some of these associated disease outcomes have been modeled in laboratory animals, but as G(αq) is expressed in all cell types, it is difficult to differentiate the underlying circuitry or causative neuronal population. To begin to address neuronal cell type diversity in G(αq) function, we utilized a conditional knockout mouse whereby G(αq) was eliminated from telencephalic glutamatergic neurons. Unlike the global G(αq) knockout mouse, we found that these conditional knockout mice were not physically different from control mice, nor did they exhibit any gross motor abnormalities. However, similarly to the constitutive knockout animal, G(αq) conditional knockout mice demonstrated apparent deficits in spatial working memory. Loss of G(αq) from glutamatergic neurons also produced enhanced sensitivity to cocaine-induced locomotion, suggesting that cortical G(αq) signaling may limit behavioral responses to psychostimulants. Screening for a variety of markers of forebrain neuronal architecture revealed no obvious differences in the conditional knockouts, suggesting that the loss of G(αq) in telencephalic excitatory neurons does not result in major alterations in brain structure or neuronal differentiation. Taken together, our results define specific modulation of spatial working memory and psychostimulant responses through disruptions in G(αq) signaling within cerebral cortical glutamatergic neurons.

AMN082, a metabotropic glutamate receptor 7 allosteric agonist, attenuates locomotor sensitization and cross-sensitization induced by cocaine and morphine in mice

Previous studies have indicated that metabotropic glutamate receptors 7 (mGluR7s) are involved in drug addiction. However, the role of these receptors in drug-induced behavioral sensitization is unknown. The aim of the present study was to determine whether systemic injection of AMN082, a selective mGluR7 allosteric agonist, reduces the cocaine- and morphine-induced hyperactivity and the development and expression of locomotor sensitization, and also affects the reciprocal cross-sensitization to the stimulant effect of cocaine and morphine in mice. AMN082 (1.25-10.0 mg/kg, i.p.) did not have an impact on locomotion of naive mice and did not affect the acute cocaine- or morphine-induced hyperactivity, except the dose of 10 mg/kg that suppressed the locomotor effect of both drugs. Repeated exposure to cocaine or morphine (10 mg/kg, 5× every 3 days) gradually increased locomotion during induction of sensitization and after 4 (cocaine) or 7 day (morphine) withdrawal phase when challenged with cocaine (10 mg/kg, i.p.) or morphine (10 mg/kg, i.p.) on day 17 or 20, respectively. Pretreatment of animals with the lower doses of AMN082 (1.25-5.0 mg/kg, i.p.), 30 min before every cocaine or morphine injection during repeated drug administration or before cocaine or morphine challenge, dose-dependently attenuated the development, as well as the expression of cocaine or morphine locomotor sensitization. AMN082 also inhibited the reciprocal cross-sensitization between these drugs. Prior to administration of MMPIP (10 mg/kg, i.p.), a selective mGluR7 antagonist reversed the inhibitory effect of AMN082 on the development or expression of cocaine or morphine sensitization. These data indicate that AMN082 attenuated the development and expression of cocaine and morphine sensitization, and the reciprocal cross-sensitization via a mechanism that involves mGluR7s. Thus, AMN082 might have therapeutic implications not only in the treatment of cocaine or opioid addiction but also in the treatment of cocaine/opioid polydrug-abusers.

Group I metabotropic glutamate receptor-mediated activation of PKC gamma in the nucleus accumbens core promotes the reinstatement of cocaine seeking

Emerging evidence indicates that type I metabotropic glutamate receptors (mGluRs) in the nucleus accumbens play a critical role in cocaine seeking. The present study sought to determine the role of accumbens core mGluR1, mGluR5 and protein kinase C (PKC) in cocaine priming-induced reinstatement of drug seeking. Here, we show that intra-accumbens core administration of the mGluR1/5 agonist DHPG (250 μM) promoted cocaine seeking in rats. Consistent with these results, administration of an mGluR1 (50.0 μM YM 298198) or mGluR5 (9.0 μM MPEP) antagonist directly into the accumbens core prior to a priming injection of cocaine (10 mg/kg) attenuated the reinstatement of drug seeking. mGluR1/5 stimulation activates a signaling cascade including PKC. Intracore microinjection of PKC inhibitors (10 μM Ro 31-8220 or 30.0 μM chelerythrine) also blunted cocaine seeking. In addition, cocaine priming-induced reinstatement of drug seeking was associated with increased phosphorylation of PKCγ, but not PKCα or PKCβII, in the core. There were no effects of pharmacological inhibition of mGluR1, mGluR5 or PKC in the accumbens core on sucrose seeking. Together, these findings indicate that mGluR1 and mGluR5 activation in the accumbens core promotes cocaine seeking and that these effects are reinforcer specific. Furthermore, stimulation of mGluR1 and mGluR5 in the accumbens core may regulate cocaine seeking, in part, through activation of PKCγ.

Cocaine-induced behavioral sensitization is associated with changes in the expression of endocannabinoid and glutamatergic signaling systems in the mouse prefrontal cortex

BACKGROUND

Endocannabinoids modulate the glutamatergic excitatory transmission by acting as retrograde messengers. A growing body of studies has reported that both signaling systems in the mesocorticolimbic neural circuitry are involved in the neurobiological mechanisms underlying drug addiction.

METHODS

We investigated whether the expression of both endocannabinoid and glutamatergic systems in the prefrontal cortex (PFC) were altered by an acute and/or repeated cocaine administration schedule that resulted in behavioral sensitization. We measured the protein and mRNA expression of the main endocannabinoid metabolic enzymes and the cannabinoid receptor type 1 (CB1). We also analyzed the mRNA expression of relevant components of the glutamate-signaling system, including glutamate-synthesizing enzymes, metabotropic receptors, and ionotropic receptors.

RESULTS

Although acute cocaine (10 mg/kg) produced no significant changes in the endocannabinoid-related proteins, repeated cocaine administration (20 mg/kg daily) induced a pronounced increase in the CB1 receptor expression. In addition, acute cocaine administration (10 mg/kg) in cocaine-sensitized mice (referred to as cocaine priming) induced a selective increase in the endocannabinoid-degrading enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL). These protein changes were accompanied by an overall decrease in the ratios of endocannabinoid synthesis/degradation, especially the N-acyl phosphatidylethanolamine phospholipase D/FAAH and diacylglycerol lipase alpha/MAGL ratios. Regarding mRNA expression, while acute cocaine administration produced a decrease in CB1 receptors and N-acyl phosphatidylethanolamine phospholipase D, repeated cocaine treatment enhanced CB1 receptor expression. Cocaine-sensitized mice that were administered priming injections of cocaine mainly displayed an increased FAAH expression. These endocannabinoid changes were associated with modifications in glutamatergic transmission-related genes. An overall decrease was observed in the mRNA expression of the glutamate-synthesizing gene kidney-type glutaminase (KGA), the metabotropic glutamate receptors (mGluR3 and GluR), and subunits of NMDA ionotropic receptors (NR1, NR2A, NR2B and NR2C) after acute cocaine administration, while mice repeatedly exposed to cocaine only displayed an increase in NR2C. However, in cocaine-sensitized mice primed with cocaine, this inhibition was reversed and a strong increase was detected in the mGluR5, NR2 subunits, and both GluR1 and GluR3.

CONCLUSIONS

These findings indicate that cocaine sensitization is associated with an endocannabinoid downregulation and a hyperglutamatergic state in the PFC that, overall, contribute to an enhanced glutamatergic input into PFC-projecting areas.

Effects of acute versus repeated cocaine exposure on the expression of endocannabinoid signaling-related proteins in the mouse cerebellum

Growing awareness of cerebellar involvement in addiction is based on the cerebellum's intermediary position between motor and reward, potentially acting as an interface between motivational and cognitive functions. Here, we examined the impact of acute and repeated cocaine exposure on the two main signaling systems in the mouse cerebellum: the endocannabinoid (eCB) and glutamate systems. To this end, we investigated whether eCB signaling-related gene and protein expression {cannabinoid receptor type 1 receptors and enzymes that produce [diacylglycerol lipase alpha/beta (DAGLα/β) and N-acyl phosphatidylethanolamine phospholipase D (NAPE-PLD)] and degrade [monoacylglycerol lipase (MAGL) and fatty acid amino hydrolase (FAAH)] eCB} were altered. In addition, we analyzed the gene expression of relevant components of the glutamate signaling system [glutamate synthesizing enzymes liver-type glutaminase isoform (LGA) and kidney-type glutaminase isoform (KGA), metabotropic glutamatergic receptor (mGluR3/5), NMDA-ionotropic glutamatergic receptor (NR1/2A/2B/2C) and AMPA-ionotropic receptor subunits (GluR1/2/3/4)] and the gene expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis, because noradrenergic terminals innervate the cerebellar cortex. Results indicated that acute cocaine exposure decreased DAGLα expression, suggesting a down-regulation of 2-arachidonylglycerol (2-AG) production, as well as gene expression of TH, KGA, mGluR3 and all ionotropic receptor subunits analyzed in the cerebellum. The acquisition of conditioned locomotion and sensitization after repeated cocaine exposure were associated with an increased NAPE-PLD/FAAH ratio, suggesting enhanced anandamide production, and a decreased DAGLβ/MAGL ratio, suggesting decreased 2-AG generation. Repeated cocaine also increased LGA gene expression but had no effect on glutamate receptors. These findings indicate that acute cocaine modulates the expression of the eCB and glutamate systems. Repeated cocaine results in normalization of glutamate receptor expression, although sustained changes in eCB is observed. We suggest that cocaine-induced alterations to cerebellar eCB should be considered when analyzing the adaptations imposed by psychostimulants that lead to addiction.

Stimulation of mGluR5 in the accumbens shell promotes cocaine seeking by activating PKC gamma

Recent studies indicate a critical role for metabotropic glutamate receptor 5 (mGluR5) in the reinstatement of cocaine seeking. However, the signal transduction pathways through which mGluR5s regulate cocaine seeking have not been identified. Here, we show that intra-accumbens shell administration of an mGluR5 (9.0 μm MPEP), but not mGluR1 (50.0 μm YM 298198), antagonist before a priming injection of cocaine (10 mg/kg) attenuated the reinstatement of drug seeking in rats. Consistent with these results, intra-shell microinjection of the mGluR1/5 agonist DHPG (250 μm) promoted cocaine seeking. Intra-shell administration of a phospholipase C (PLC) inhibitor (40.0 μm U73122) or a protein kinase C (PKC) inhibitor (10.0 μm Ro 31-8220 or 30.0 μm chelerythrine chloride) attenuated cocaine seeking. Pharmacological inhibition of PKC in the shell also blocked intra-shell DHPG-induced reinstatement of cocaine seeking. In addition, cocaine priming-induced reinstatement of drug seeking was associated with increased phosphorylation of PKCγ, but not PKCα or PKCβII, in the shell. Cocaine seeking previously was linked to increased phosphorylation of GluA2 at Ser880, a PKC phosphorylation site, which promotes the endocytosis of GluA2-containing AMPA receptors via interactions with Protein Associated with C Kinase (PICK1). The present results indicated that inhibition of PICK1 (100 μm FSC-231) in the shell attenuated cocaine seeking. There were no effects of any drug treatment in the shell on sucrose seeking. Together, these findings indicate that accumbens shell mGluR5 activation promotes cocaine seeking, in part, through activation of PLC and PKCγ. Moreover, the endocytosis of shell GluA2-containing AMPARs during cocaine seeking may depend on interactions with PKCγ and PICK1.

Group I metabotropic glutamate receptors in the medial prefrontal cortex: role in mesocorticolimbic glutamate release in cocaine sensitization

Cocaine sensitization is associated with increased excitability of pyramidal projection neurons in the medial prefrontal cortex. Such hyperexcitability is presumed to increase glutamatergic input to the nucleus accumbens and ventral tegmental area. This study examined the effects of medial prefrontal cortex Group I metabotropic glutamate receptor activation on glutamate levels in the medial prefrontal cortex, nucleus accumbens, and ventral tegmental area in sensitized and control animals. Male Sprague-Dawley rats received four daily injections of cocaine (15 mg/kg, i.p.) or saline (1 mL/kg i.p.). One, 7, or 21 days from the fourth injection, dual-probe microdialysis experiments were performed wherein Group I metabotropic glutamate receptor agonist DHPG was infused into the medial prefrontal cortex and glutamate levels in this region as well as the nucleus accumbens or ventral tegmental area were examined. Intra-mPFC DHPG infusion increased glutamate levels in the medial prefrontal cortex at 1 and 7 days withdrawal, and in the nucleus accumbens at 21 days withdrawal in sensitized rats. These results suggest Group I metabotropic glutamate receptor activation may contribute to the increased excitability of medial prefrontal cortex pyramidal neurons in sensitized animals.