Repeated Cocaine Administration Research Articles

Cocaine diminishes functional network robustness and destabilizes the energy landscape of neuronal activity in the medial prefrontal cortex.

We present analysis of neuronal activity recordings from a subset of neurons in the medial prefrontal cortex of rats before and after the administration of cocaine. Using an underlying modern Hopfield model as a description for the neuronal network, combined with a machine learning approach, we compute the underlying functional connectivity of the neuronal network. We find that the functional connectivity changes after the administration of cocaine with both functional-excitatory and functional-inhibitory neurons being affected. Using conventional network analysis, we find that the diameter of the graph, or the shortest length between the two most distant nodes, increases with cocaine, suggesting that the neuronal network is less robust. We also find that the betweenness centrality scores for several of the functional-excitatory and functional-inhibitory neurons decrease significantly, while other scores remain essentially unchanged, to also suggest that the neuronal network is less robust. Finally, we study the distribution of neuronal activity and relate it to energy to find that cocaine drives the neuronal network towards destabilization in the energy landscape of neuronal activation. While this destabilization is presumably temporary given one administration of cocaine, perhaps this initial destabilization indicates a transition towards a new stable state with repeated cocaine administration. However, such analyses are useful more generally to understand how neuronal networks respond to perturbations.

Ectopic myogenesis and fibrosis accompany adipogenesis during thymic involution induced by repeated cocaine administration

We have shown previously that daily cocaine administration for 7–14 days in rats resulted in the acceleration of thymic involution, which is, alike to age-related thymic involution, accompanied by ectopic adipogenesis. Here we show that the thymic involution caused by repeated cocaine administration is accompanied by not only adipogenesis but also ectopic myogenesis and fibrosis. In accordance with fibrosis, we observed an increase of N-cadherin, a marker of mesenchymal cells, as well as a decrease of E-cadherin, an epithelial cell marker, in the thymus in response to cocaine administration, suggesting the occurrence of epithelial-to-mesenchymal transition (EMT). Levels of fibronectin was also increased in the thymus of cocaine group compared to control group. In addition, increases in the levels of the transcription factors for myogenic differentiation, myogenin, MYF5, and MYF6, were observed in the thymus administered cocaine for 14 days. These results indicate that the thymic involution induced by cocaine administration involves not only adipogenesis and fibrosis but also ectopic myogenesis, which is scarcely observed and rather pathological process during thymic involution.

Thymic involution caused by repeated cocaine administration includes apoptotic cell loss followed by ectopic adipogenesis.

Cocaine abuse has a negative impact on the immune system. To investigate the adverse effects of binge cocaine administration on lymphoid organs such as thymus and spleen, we examined the effects of repeated intravenous (i.v.) administration of cocaine on rats. Sprague Dawley rats (male, 8 weeks old) received 20 mg/kg body weight of cocaine hydrochloride per day for 7 or 14 days. In addition to a significant loss in the weight of the spleen, consistent with our previous intraperitoneal (i.p.) injection model of binge cocaine abuse (50 mg/kg cocaine for 7 days), we also found a significant loss of weight as well as apparent shrinkage of the thymus in the cocaine group. Transcriptome analysis of the thymus revealed increased expressions of genes involved in apoptosis, such as Ifi27 and Traf2, as well as decreased expressions of several genes related to lipid metabolism, such as Cd36, Adipoq, Scd1, and Fabp4, in the thymus of the cocaine group (7 days), suggesting an apoptotic loss of thymic cells as well as alterations in lipid metabolism. Paradoxically, cocaine activates PPARγ, a key transcriptional factor activating lipid metabolism, although ectopic adipogenesis was scarcely observed in the thymus. Further analysis of rats administered 20 mg/kg cocaine for 14 days revealed ectopic adipogenesis, which was accompanied with the activation of PPARγ as well as increased expression of Adipoq and Fabp4, in the thymus. Taken together, these results indicate that repeated cocaine administration induces thymic involution, which is initiated by the loss of thymic cells through apoptosis and subsequent ectopic adipocyte development.

An extended amygdala-midbrain circuit controlling cocaine withdrawal-induced anxiety and reinstatement.

SUMMARYAlthough midbrain dopamine (DA) circuits are central to motivated behaviors, our knowledge of how experience modifies these circuits to facilitate subsequent behavioral adaptations is limited. Here we demonstrate the selective role of a ventral tegmental area DA projection to the amygdala (VTADA→amygdala) for cocaine-induced anxiety but not cocaine reward or sensitization. Our rabies virus-mediated circuit mapping approach reveals a persistent elevation in spontaneous and task-related activity of inhibitory GABAergic cells from the bed nucleus of the stria terminalis (BNST) and downstream VTADA→amygdala cells that can be detected even after a single cocaine exposure. Activity in BNSTGABA→midbrain cells is related to cocaine-induced anxiety but not reward or sensitization, and silencing this projection prevents development of anxiety during protracted withdrawal after cocaine administration. Finally, we observe that VTADA→amygdala cells are strongly activated after a challenge exposure to cocaine and that activity in these cells is necessary and sufficient for reinstatement of cocaine place preference.

Dopaminylation in Psychostimulant Use Disorder Protects Against Psychostimulant Seeking Behavior by Normalizing Nucleus Accumbens (NAc) Dopamine Expression.

Repeated cocaine administration changes histone acetylation and methylation on Lys residues and Deoxyribonucleic acid (DNA) within the nucleus accumbens (NAc). Recently Nestler's group explored histone Arg (R) methylation in reward processing models. Damez-Werno et al. (2016) reported that during human investigations and animal self-administration experiments, the histone mark protein-R-methyltransferase-6 (PRMT6) and asymmetric dimethylation of R2 on histone H3 (H3R2me2a) decreased in the rodent and cocaine-dependent human NAc. Overexpression of PRMT6 in D2-MSNs in all NAc neurons increased cocaine seeking, whereas PRMT6 overexpression in D1-MSNs protects against cocaine-seeking. The hypothesis is that dopaminylation (H3R2me2a binding) occurs in psychostimulant use disorder (PSU), and the binding inhibitor Srcin1, like the major DRD2 A2 allelic polymorphism, protects against psychostimulant seeking behavior by normalizing nucleus accumbens (NAc) dopamine expression. Numerous publications confirmed the association between the DRD2 Taq A1 allele (30-40 lower D2 receptor numbers) and severe cocaine dependence. Lepack et al. (2020) found that acute cocaine increases dopamine in NAc synapses, and results in histone H3 glutamine 5 dopaminylation (H3Q5dop) and consequent inhibition of D2 expression. The inhibition increases with chronic cocaine use and accompanies cocaine withdrawal. They also found that the Src kinase signaling inhibitor 1 (Srcin1 or p140CAP) during cocaine withdrawal reduced H3R2me2a binding. Consequently, this inhibited dopaminylation induced a "homeostatic brake." The decrease in Src signaling in NAc D2-MSNs, (like the DRD2 Taq A2 allele, a well-known genetic mechanism protective against SUD) normalizes the NAc dopamine expression and decreases cocaine reward and motivation to self-administer cocaine. The Srcin1 may be an important therapeutic target.

Interaction of JNK and mGluR5 in the regulation of psychomotor behaviours after repeated cocaine administration.

Activation of protein kinases after cocaine administration controls psychomotor behaviours by interacting with metabotropic receptors in the brain. This study identified how c-Jun N-terminal kinase (JNK) interacts with metabotropic glutamate receptor 5 (mGluR5) in vitro and in the caudate and putamen (CPu). The potential role of this interaction in the regulation of psychomotor behaviour was also evaluated after administration of cocaine. Active JNK phosphorylates a threonine residue at position 1055 in the carboxyl terminus (CT) of mGluR5 in vitro. The binding of active JNK to the D-motif within CT2 is necessary for that phosphorylation. Interaction of phosphorylated JNK and mGluR5 occurs in the CPu. Unilateral interference of the interaction decreases the repeated cocaine-induced increases in locomotor activity and conditioned place preference. These findings suggest that activation of JNK has the capability to interact with mGluR5 in the CPu. Phosphorylation of mGluR5 following the JNK-mGluR5 interaction may be responsible for the potentiation of behavioural sensitisation and cocaine-wanting behaviour in response to cocaine administration.

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 (2nmol/side), but not α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or kainic acid receptor antagonist, CNQX disodium salt (2nmol/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 (0.5nmol/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.

Repeated cocaine administration upregulates CB2 receptor expression in striatal medium-spiny neurons that express dopamine D1 receptors in mice.

Cannabinoid CB2 receptors (CB2R) are importantly involved in drug reward and addiction. However, the cellular mechanisms underlying CB2R action remain unclear. We have previously reported that cocaine self-administration upregulates CB2R expression in midbrain dopamine (DA) neurons. In the present study, we investigated whether cocaine or heroin also alters CB2R expression in striatal medium-spiny neuronsthat express dopamineD1 or D2 receptors (D1-MSNs, D2-MSNs) and microglia. Due to the concern of CB2R antibody specificity, we developed three mouse CB2-specific probes to detect CB2R mRNA using quantitative RT-PCR and RNAscope in situ hybridization (ISH) assays. We found that a single injection of cocaine failed to alter, while repeated cocaine injections or self-administration dose-dependently upregulated CB2R gene expression in both brain (cortex and striatum) and periphery (spleen). In contrast, repeated administration ofheroin produced a dose-dependent reduction in striatal CB2 mRNA expression. RNAscope ISH assays detected CB2R mRNA in striatal D1- and D2-MSNs, not in microglia. We then used transgenic CX3CR1eGFP/+ microglia reporter mice and D1- or D2-Cre-RiboTag mice to purify striatal microglia or ribosome-associated mRNAs from CX3CR1eGFP/+,D1-MSNs, or D2-MSNs, respectively. We found that CB2R upregulation occurred mainly in D1-MSNs, not in D2-MSNs or microglia, in the nucleus accumbens rather than the dorsal striatum. These findings indicate that repeated cocaine exposure may upregulate CB2R expression in both brain and spleen, with regional and cell type-specific profiles. In the striatum, CB2R upregulation occurs mainly in D1-MSNs in the nucleus accumbens. Given the important role of D1-MSNs in brain reward function, the present findings provide new insight into mechanisms by which brain CB2Rs modulate cocaine action.

Sustained splenic contraction after daily cocaine administration in rats.

The purpose of this study is to examine the effect of repeated cocaine administration on the whole body of rats. Rats (male, 6 weeks old, Sprague Dawley) were injected intraperitoneally with cocaine (50 mg/kg) once a day for 1, 3 or 7 days, and major organs (heart, liver, lung, brain, kidney, spleen) were excised from the sacrificed animals. During autopsy, we found a reduction in spleen size, but not other organs, in cocaine-administered rats as compared to control rats. This reduction became to be noticed at 3 day and easily perceived at 7 day. No marked changes were observed in other organs examined. H&E and EMG staining showed a tendency for a decrease in the number of red blood cells (RBCs) as well as an increase in collagen fibers in the spleens of rats treated repeatedly with cocaine. Transcriptome analysis indicated that repeated cocaine administration depletes RBCs from the spleen. Immunoblot analysis showed that cocaine increases the phosphorylation of myosin light chain (MYL) as well as the levels of transgelin, both of which are involved in the contraction of myofibrils. Collectively, these results show that repeated cocaine administration results in sustained contraction of the spleen, which leads to the release of RBCs from the spleen into circulation.

Serum- and glucocorticoid-inducible kinase 1 activity in ventral tegmental area dopamine neurons regulates cocaine conditioned place preference but not cocaine self-administration.

Drugs of abuse regulate the activity of the mesolimbic dopamine (DA) system, and drug-induced changes in ventral tegmental area (VTA) cellular activity and gene regulation are linked to behavioral outputs associated with addiction. Previous work from our lab determined that VTA serum- and glucocorticoid-inducible kinase 1 (SGK1) transcription and catalytic activity were increased by repeated cocaine administration; however, it was unknown if these biochemical changes contributed to cocaine-elicited behaviors. Using transgenic and viral-mediated manipulations, we investigated the role of VTA SGK1 catalytic activity in regulating cocaine conditioned place preference and self-administration. We showed intra-VTA infusion of a catalytically inactive SGK1 mutant (K127Q) significantly decreased cocaine conditioned place preference (CPP). Further, we found that K127Q expression in VTA DA neurons significantly decreased cocaine CPP, while this same manipulation in VTA GABA neurons had no effect. However, blunted VTA DA SGK1 catalytic activity did not alter cocaine self-administration. Altogether, these studies identify the specific VTA cells critical for SGK1-mediated effects on cocaine CPP but not self-administration.

Cocaine-Dependent Acquisition of Locomotor Sensitization and Conditioned Place Preference Requires D1 Dopaminergic Signaling through a Cyclic AMP, NCS-Rapgef2, ERK, and Egr-1/Zif268 Pathway.

Elucidation of the mechanism of dopamine signaling to ERK that underlies plasticity in dopamine D1 receptor-expressing neurons leading to acquired cocaine preference is incomplete. NCS-Rapgef2 is a novel cAMP effector, expressed in neuronal and endocrine cells in adult mammals, that is required for D1 dopamine receptor-dependent ERK phosphorylation in mouse brain. In this report, we studied the effects of abrogating NCS-Rapgef2 expression on cAMP-dependent ERK→Egr-1/Zif268 signaling in cultured neuroendocrine cells; in D1 medium spiny neurons of NAc slices; and in either male or female mouse brain in a region-specific manner. NCS-Rapgef2 gene deletion in the NAc in adult mice, using adeno-associated virus-mediated expression of cre recombinase, eliminated cocaine-induced ERK phosphorylation and Egr-1/Zif268 upregulation in D1-medium spiny neurons and cocaine-induced behaviors, including locomotor sensitization and conditioned place preference. Abrogation of NCS-Rapgef2 gene expression in mPFC and BLA, by crossing mice bearing a floxed Rapgef2 allele with a cre mouse line driven by calcium/calmodulin-dependent kinase IIα promoter also eliminated cocaine-induced phospho-ERK activation and Egr-1/Zif268 induction, but without effect on the cocaine-induced behaviors. Our results indicate that NCS-Rapgef2 signaling to ERK in dopamine D1 receptor-expressing neurons in the NAc, but not in corticolimbic areas, contributes to cocaine-induced locomotor sensitization and conditioned place preference. Ablation of cocaine-dependent ERK activation by elimination of NCS-Rapgef2 occurred with no effect on phosphorylation of CREB in D1 dopaminoceptive neurons of NAc. This study reveals a new cAMP-dependent signaling pathway for cocaine-induced behavioral adaptations, mediated through NCS-Rapgef2/phospho-ERK activation, independently of PKA/CREB signaling.SIGNIFICANCE STATEMENT ERK phosphorylation in dopamine D1 receptor-expressing neurons exerts a pivotal role in psychostimulant-induced neuronal gene regulation and behavioral adaptation, including locomotor sensitization and drug preference in rodents. In this study, we examined the role of dopamine signaling through the D1 receptor via a novel pathway initiated through the cAMP-activated guanine nucleotide exchange factor NCS-Rapgef2 in mice. NCS-Rapgef2 in the NAc is required for activation of ERK and Egr-1/Zif268 in D1 dopaminoceptive neurons after acute cocaine administration, and subsequent enhanced locomotor response and drug seeking behavior after repeated cocaine administration. This novel component in dopamine signaling provides a potential new target for intervention in psychostimulant-shaped behaviors, and new understanding of how D1-medium spiny neurons encode the experience of psychomotor stimulant exposure.

Dopamine D1-D2 receptor heteromer expression in key brain regions of rat and higher species: Upregulation in rat striatum after cocaine administration

BackgroundDopamine receptors interact with other receptors to form heterooligomers. One such complex, the D1-D2 heteromer, demonstrated in cultured striatal neurons and rat striatum has been linked to drug addiction, Parkinson's disease, schizophrenia, depression and anhedonia. MethodsD1-D2 heteromer expression was evaluated using in situ proximity ligation assay, in parallel with cellular colocalization of D1 and D2 mRNA using in situ hybridization in 19 different key rat brain regions. Expression in higher species and changes in rat striatum after repeated cocaine administration were evaluated. ResultsDifferences in D1-D2 heteromer expression in striatal subregions are documented in higher species with nonhuman primate and human demonstrating higher density of heteromer-expressing neurons compared to rodents. All species had higher density of D1-D2 neurons in nucleus accumbens compared to dorsal striatum. Multiple other brain regions are identified where D1-D2 heteromer is expressed, prominently in cerebral cortical subregions including piriform, medial prefrontal, orbitofrontal and others; subcortical regions such as claustrum, amygdala and lateral habenula. Three categories of regions are identified: D1-D2 heteromer expressed despite little to no observed D1/D2 mRNA colocalization, likely representing heteromer on neuronal projections from other brain regions; D1-D2 heteromer originating locally with the density of neurons expressing heteromer matching neurons with colocalized D1/D2 mRNA; regions with both a local origin and targeted inputs projecting from other regions. Repeated cocaine administration significantly increased density of neurons expressing D1-D2 heteromer and D1/D2 mRNA colocalization in rat striatum, with changes in both direct and indirect pathway neurons. ConclusionThe dopamine D1-D2 heteromer is expressed in key brain cortical and subcortical regions of all species examined. Species differences in striatum revealed greater abundance in human>nonhuman-primate>rat>mouse, suggesting an evolutionary biologic role for the D1-D2 heteromer in higher CNS function. Its upregulation in rat striatum following cocaine points to regulatory significance with possible relevance for clinical disorders such as drug addiction. The dopamine D1-D2 receptor heteromer may represent a potential target for neuropsychiatric and neurodegenerative disorders, given its distribution in highly relevant brain regions.

Alpha-1 Adrenergic Receptors Modulate Glutamate and GABA Neurotransmission onto Ventral Tegmental Dopamine Neurons during Cocaine Sensitization.

The ventral tegmental area (VTA) plays an important role in the reward and motivational processes that facilitate the development of drug addiction. Presynaptic α1-AR activation modulates glutamate and Gamma-aminobutyric acid (GABA) release. This work elucidates the role of VTA presynaptic α1-ARs and their modulation on glutamatergic and GABAergic neurotransmission during cocaine sensitization. Excitatory and inhibitory currents (EPSCs and IPSCs) measured by a whole cell voltage clamp show that α1-ARs activation increases EPSCs amplitude after 1 day of cocaine treatment but not after 5 days of cocaine injections. The absence of a pharmacological response to an α1-ARs agonist highlights the desensitization of the receptor after repeated cocaine administration. The desensitization of α1-ARs persists after a 7-day withdrawal period. In contrast, the modulation of α1-ARs on GABA neurotransmission, shown by decreases in IPSCs’ amplitude, is not affected by acute or chronic cocaine injections. Taken together, these data suggest that α1-ARs may enhance DA neuronal excitability after repeated cocaine administration through the reduction of GABA inhibition onto VTA dopamine (DA) neurons even in the absence of α1-ARs’ function on glutamate release and protein kinase C (PKC) activation. α1-AR modulatory changes in cocaine sensitization increase our knowledge of the role of the noradrenergic system in cocaine addiction and may provide possible avenues for therapeutics.

Pharmacological NOS-1 Inhibition Within the Hippocampus Prevented Expression of Cocaine Sensitization: Correlation with Reduced Synaptic Transmission.

Behavioral sensitization to psychostimulants hyperlocomotor effect is a useful model of addiction and craving. Particularly, cocaine sensitization in rats enhanced synaptic plasticity within the hippocampus, an important brain region for the associative learning processes underlying drug addiction. Nitric oxide (NO) is a neurotransmitter involved in both, hippocampal synaptic plasticity and cocaine sensitization. It has been previously demonstrated a key role of NOS-1/NO/sGC/cGMP signaling pathway in the development of cocaine sensitization and in the associated enhancement of hippocampal synaptic plasticity. The aim of the present investigation was to determine whether NOS-1 inhibition after development of cocaine sensitization was able to reverse it, and to characterize the involvement of the hippocampus in this phenomenon. Male Wistar rats were administered only with cocaine (15mg/kg/dayi.p.) for 5days. Then, animals received 7-nitroindazole (NOS-1 inhibitor) either systemically for the next 5days or a single intra-hippocampal administration. Development of sensitization and its expression after withdrawal were tested, as well as threshold for long-term potentiation in hippocampus, NOS-1, and CREB protein levels and gene expression. The results showed that NOS-1 protein levels and gene expression were increased only in sensitized animals as well as CREB gene expression. NOS-1 inhibition after sensitization reversed behavioral expression and the highest level of hippocampal synaptic plasticity. In conclusion, NO signaling within the hippocampus is critical for the development and expression of cocaine sensitization. Therefore, NOS-1 inhibition or NO signaling pathways interferences during short-term withdrawal after repeated cocaine administration may represent plausible pharmacological targets to prevent or reduce susceptibility to relapse.

Early-life stress influences acute and sensitized responses of adult mice to cocaine by interacting with GABAA α2 receptor expression.

Early-life stress (ELS) is known to exert long-term effects on brain function, with resulting deleterious consequences for several aspects of mental health, including the development of addiction to drugs of abuse. One potential mechanism in humans is suggested by findings that ELS interacts with polymorphisms of the GABRA2 gene, encoding α2 subunits of GABAA receptors, to increase the risk for both post-traumatic stress disorder and vulnerability to cocaine addiction. We used a mouse model, in which the amount of material for nest building was reduced during early postnatal life, to study interactions between ELS and expression of α2-containing GABAA receptors in influencing cocaine-related behaviour. Breeding of parents heterozygous for a deletion of α2 resulted in litters containing homozygous knockout (α2), heterozygous knockout (α2) and wild-type (α2) offspring. Following the ELS procedure, the mice were allowed to develop to adulthood before being tested for the acute effect of cocaine on locomotor stimulation, behavioural sensitization to repeated cocaine and to cocaine-conditioned activity. Exposure to ELS resulted in increased acute locomotor stimulant effects of cocaine across all genotypes, with the most marked effects in α2 mice (which also showed increased activity following vehicle). Repeated cocaine administration to nonstressed mice resulted in sensitization in α2 and α2 mice, but, in keeping with previous findings, not in α2 mice. Previous exposure to ELS reduced sensitization in α2 mice, albeit not significantly, and abolished sensitization in α2 mice. Conditioned activity was elevated following ELS in all animals, independently of genotype. Thus, while the enhanced acute effects of cocaine following ELS being most marked in α2 mice suggests a function of α2-containing GABAA receptors in protecting against stress, the interaction between ELS and genotype in influencing sensitization may be more in keeping with ELS reducing expression of α2-containing GABAA receptors. The ability of ELS to increase cocaine-conditioned locomotor activity appears to be independent of α2-containing GABAA receptors.

Impact of acute and repeated administration of cocaine on expression of proteins involved in the cerebral metabolism of cholesterol

Impact of acute and repeated administration of cocaine on expression of proteins involved in the cerebral metabolism of cholesterol

Prolonged-access to cocaine induces distinct Homer2 DNA methylation, hydroxymethylation, and transcriptional profiles in the dorsomedial prefrontal cortex of Male Sprague-Dawley rats

Repeated cocaine administration induces many long-term structural and molecular changes in the dorsal medial prefrontal cortex (dmPFC) and are known to underlie aspects of cocaine-seeking behavior. DNA methylation is a key long-lasting epigenetic determinant of gene expression and is implicated in neuroplasticity, however, the extent to which this epigenetic modification is involved in the neuroplasticity associated with drug addiction has received limited attention. Here, we examine the relation between DNA methylation and gene expression within the dorsal medial prefrontal cortex (dmPFC) following limited cocaine self-administration (1 h/day), prolonged cocaine self-administration (6 h/day), and saline self-administration (1 h/day). Rats were fitted with intravenous catheters and allowed to lever press for saline or cocaine (0.25 mg/kg/0.1 mL infusion) in the different access conditions for 20 days. Prolonged-access rats exhibited escalation in cocaine intake over the course of training, while limited-access rats did not escalate cocaine intake. Additionally, limited-access and prolonged-access rats exhibited unique Homer2 epigenetic profiles and mRNA expression. In prolonged-access rats, Homer2 mRNA levels in the dmPFC were increased, which was accompanied by decreased DNA methylation and p300 binding within the Homer2 promoter. Limited-access animals exhibited decreased DNA methylation, decreased DNA hydroxymethylation, and increased p300 binding within the Homer2 promoter. These data indicate that distinct epigenetic profiles are induced by limited-versus prolonged-access self-administration conditions that contribute to transcriptional profiles and lend support to the notion that covalent modification of DNA is implicated in addiction-like changes in cocaine-seeking behavior.

Early-life adversity selectively impairs α2-GABAA receptor expression in the mouse nucleus accumbens and influences the behavioral effects of cocaine

Haplotypes of the Gabra2 gene encoding the α2-subunit of the GABAA receptor (GABAAR) are associated with drug abuse, suggesting that α2-GABAARs may play an important role in the circuitry underlying drug misuse. The genetic association of Gabra2 haplotypes with cocaine addiction appears to be evident primarily in individuals who had experienced childhood trauma. Given this association of childhood trauma, cocaine abuse and the Gabra2 haplotypes, we have explored in a mouse model of early life adversity (ELA) whether such events influence the behavioral effects of cocaine and if, as suggested by the human studies, α2-GABAARs in the nucleus accumbens (NAc) are involved in these perturbed behaviors. In adult mice prior ELA caused a selective decrease of accumbal α2-subunit mRNA, resulting in a selective decrease in the number and size of the α2-subunit (but not the α1-subunit) immunoreactive clusters in NAc core medium spiny neurons (MSNs). Functionally, in adult MSNs ELA decreased the amplitude and frequency of GABAAR-mediated miniature inhibitory postsynaptic currents (mIPSCs), a profile similar to that of α2 “knock-out” (α2−/−) mice. Behaviourally, adult male ELA and α2−/− mice exhibited an enhanced locomotor response to acute cocaine and blunted sensitisation upon repeated cocaine administration, when compared to their appropriate controls. Collectively, these findings reveal a neurobiological mechanism which may relate to the clinical observation that early trauma increases the risk for substance abuse disorder (SAD) in individuals harbouring haplotypic variations in the Gabra2 gene.

Effects of repeated cocaine administration on dopamine D1 receptor modulation of mesocorticolimbic GABA and glutamate transmission

Repeated cocaine exposure alters medial prefrontal cortex (mPFC) function to allow for enhanced excitatory transmission to the nucleus accumbens and ventral tegmental area (VTA). Previous studies have demonstrated changes in receptor function in the mPFC in animals repeatedly exposed to cocaine that produced increased excitatory output. The present report tested the hypothesis that daily injections of cocaine would enhance D1 receptor responsiveness by infusing the D1 receptor agonist SKF 38393 into the mPFC and monitoring glutamate and/or GABA release in the mPFC, nucleus accumbens and VTA of saline- and cocaine-pretreated animals using in vivo microdialysis. The data demonstrated that intra-mPFC SKF 38393 reduced GABA and glutamate levels in the mPFC in control animals. Intra-mPFC SKF 38393 had no effect on glutamate levels in animals 1 day after daily cocaine treatments, increased mPFC glutamate at 7 days of withdrawal and reverted to decreasing glutamate at 30 days of withdrawal. SKF 38393 induced reduction in mPFC GABA is lost at 7 and 30 days of withdrawal. Intra-mPFC SKF 38393 did not alter glutamate levels in the nucleus accumbens or VTA of control animals. Infusion of SKF 38393 into the mPFC of animals previously exposed to cocaine increased and reduced glutamate release in the nucleus accumbens after 7 and 30 days of withdrawal, respectively and increased glutamate levels in the VTA 7 and 30 days after daily cocaine injections. The data suggest that repeated cocaine exposure alters D1 receptor function in the mPFC that could contribute to enhanced behavioral responses that occur following repeated cocaine.

Activation of Protein Kinase G After Repeated Cocaine Administration Is Necessary for the Phosphorylation of α-Amino-3-Hydroxy-5-Methyl-4-Isoxazolepropionic Acid Receptor GluA1 at Serine 831 in the Rat Nucleus Accumbens.

Phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the striatum plays a crucial role in regulating the receptor-coupled signaling cascades leading to behavioral changes associated with psychostimulant exposure. The present study determined if activation of protein kinase G (PKG) contributes to the phosphorylation of AMPA receptor GluA1 subunit at the position of serine 831 (GluA1-S831) in the rat nucleus accumbens (NAc) after repeated cocaine administration. The results demonstrated that repeated intraperitoneal (i.p.) injections of cocaine (20 mg/kg) once a day for seven consecutive days significantly increased the level of phosphorylated (p)GluA1-S831. This increase was decreased by the intra-NAc infusion of either the cyclic guanosine monophosphate (cGMP) analog, Rp-8-Br-PET-cGMPS (5 nmol/1 μL), or the PKG inhibitor, KT5823 (2 nmol/1 μL). Repeated cocaine administration increased PKG binding activity to GluA1. This increase in GluA1-S831 phosphorylation after repeated cocaine administration was decreased by the intra-NAc infusion of the synthetic peptide (Tat-tagged interfering peptide (Tat-GluA1-i)), that interferes with the binding of PKG to GluA1. Intra-NAc infusion of the interfering peptide also reduced the repeated cocaine-induced increase in locomotor activity. These findings suggest that activated PKG, after repeated exposure to cocaine, binds to AMPA receptor GluA1 and is required for the phosphorylation of S831, contributing to behavioral changes.