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

Cofilin linked to GluN2B subunits of NMDA receptors is required for behavioral sensitization by changing the dendritic spines of neurons in the caudate and putamen after repeated nicotine exposure

BACKGROUND

Nicotine dependence is associated with glutamatergic neurotransmission in the caudate and putamen (CPu) of the forebrain which includes alterations in the structure of dendritic spines at glutamate synapses. These changes after nicotine exposure can lead to the development of habitual behaviors such as smoking. The present study investigated the hypothesis that cofilin, an actin-binding protein that is linked to the GluN2B subunits of N-methyl-D-aspartate (NMDA) receptors regulates the morphology of dendritic spines in the neurons of the CPu after repeated exposure to nicotine.

RESULTS

Adult male rats received subcutaneous injections of nicotine (0.3 mg/kg/day) or vehicle for seven consecutive days. DiI staining was conducted to observe changes in dendritic spine morphology. Repeated subcutaneous injections of nicotine decreased the phosphorylation of cofilin while increasing the formation of thin spines and filopodia in the dendrites of medium spiny neurons (MSN) in the CPu of rats. Bilateral intra-CPu infusion of the cofilin inhibitor, cytochalasin D (12.5 µg/µL/side), restored the thin spines and filopodia from mushroom types after repeated exposure to nicotine. Similar results were obtained from the bilateral intra-CPu infusion of the selective GluN2B subunit antagonist, Ro 25-6981 (4 µM/µL/side). Bilateral intra-CPu infusion of cytochalasin D that interferes with the actin-cofilin interaction attenuated the repeated nicotine-induced increase in locomotor sensitization in rats.

CONCLUSIONS

These findings suggest that active cofilin alters the structure of spine heads from mushroom to thin spine/filopodia by potentiating actin turnover, contributing to behavioral sensitization after nicotine exposure.

Working Memory Performance Predicts, but Does Not Reduce, Cocaine and Cannabinoid Seeking in Adult Male Rats

BACKGROUND

Cognitive deficits reflecting impaired executive function are commonly associated with psychiatric disorders, including substance use. Cognitive training is proposed to improve treatment outcomes for these disorders by promoting neuroplasticity within the prefrontal cortex, enhancing executive control, and mitigating cognitive decline due to drug use. Additionally, brain derived neurotrophic factor (BDNF) can facilitate plasticity in the prefrontal cortex and reduce drug-seeking behaviors. We investigated whether working memory training could elevate BDNF levels in the prefrontal cortex and if this training would predict or protect against cocaine or cannabinoid seeking.

METHODS

Adult male rats were trained to perform a "simple" or "complex" version of a delayed-match-to-sample working memory task. Rats then self-administered cocaine or the synthetic cannabinoid WIN55,212-2 and were tested for cued drug seeking during abstinence. Tissue from the prefrontal cortex and dorsal hippocampus was analyzed for BDNF protein expression.

RESULTS

Training on the working memory task enhanced endogenous BDNF protein levels in the prelimbic prefrontal cortex but not the dorsal hippocampus. Working memory training did not impact self-administration of either drug but predicted the extent of WIN self-administration and cocaine seeking during abstinence.

CONCLUSIONS

These results suggest that working memory training promotes endogenous BDNF but does not alter drug-seeking or drug-taking behavior. However, individual differences in cognitive performance before drug exposure may predict vulnerability to future drug use.

Vagus Nerve Stimulation (VNS) Modulates Synaptic Plasticity in the Infralimbic Cortex via Trk-B Receptor Activation to Reduce Drug-Seeking in Male Rats

Drugs of abuse cause changes in the prefrontal cortex (PFC) and associated regions that impair inhibitory control over drug-seeking. Breaking the contingencies between drug-associated cues and the delivery of the reward during extinction learning reduces relapse. Vagus nerve stimulation (VNS) has previously been shown to enhance extinction learning and reduce drug-seeking. Here we determined the effects of VNS-mediated release of brain-derived neurotrophic factor (BDNF) on extinction and cue-induced reinstatement in male rats trained to self-administer cocaine. Pairing 10 d of extinction training with VNS facilitated extinction and reduced drug-seeking behavior during reinstatement. Rats that received a single extinction session with VNS showed elevated BDNF levels in the medial PFC as determined via an enzyme-linked immunosorbent assay. Systemic blockade of tropomyosin receptor kinase B (TrkB) receptors during extinction, via the TrkB antagonist ANA-12, decreased the effects of VNS on extinction and reinstatement. Whole-cell recordings in brain slices showed that cocaine self-administration induced alterations in the ratio of AMPA and NMDA receptor-mediated currents in Layer 5 pyramidal neurons of the infralimbic cortex (IL). Pairing extinction with VNS reversed cocaine-induced changes in glutamatergic transmission by enhancing AMPAR currents, and this effect was blocked by ANA-12. Our study suggests that VNS consolidates the extinction of drug-seeking behavior by reversing drug-induced changes in synaptic AMPA receptors in the IL, and this effect is abolished by blocking TrkB receptors during extinction, highlighting a potential mechanism for the therapeutic effects of VNS in addiction.

Working memory performance predicts, but does not reduce, cocaine- and cannabinoid-seeking in adult male rats

Background

Cognitive deficits reflecting impaired executive function are commonly associated with psychiatric disorders, including substance use. Cognitive training is proposed to improve treatment outcomes for these disorders by promoting neuroplasticity within the prefrontal cortex, enhancing executive control, and mitigating cognitive decline due to drug use. Additionally, brain derived neurotrophic factor (BDNF) can facilitate plasticity in the prefrontal cortex and reduce drug-seeking behaviors. We investigated whether working memory training could elevate BDNF levels in the prefrontal cortex and if this training would predict or protect against cocaine or cannabinoid seeking.

Methods

Adult male rats were trained to perform a 'simple' or 'complex' version of a delayed- match-to-sample working memory task. Rats then self-administered cocaine or the synthetic cannabinoid WIN55,212-2 and were tested for cued drug-seeking during abstinence. Tissue from the prefrontal cortex and dorsal hippocampus was analyzed for BDNF protein expression.

Results

Training on the working memory task enhanced endogenous BDNF protein levels in the prelimbic prefrontal cortex but not the dorsal hippocampus. Working memory training did not impact self-administration of either drug but predicted the extent of WIN self-administration and cocaine seeking during abstinence.

Conclusions

These results suggest that working memory training promotes endogenous BDNF but does not alter drug-seeking or drug-taking behavior. However, individual differences in cognitive performance prior to drug exposure may predict vulnerability to future drug use.

Vagus nerve stimulation (VNS) modulates synaptic plasticity in the rat infralimbic cortex via Trk-B receptor activation to reduce drug-seeking

Drugs of abuse cause changes in the prefrontal cortex (PFC) and associated regions that impair inhibitory control over drug-seeking. Breaking the contingencies between drug-associated cues and the delivery of the reward during extinction learning reduces relapse. Vagus nerve stimulation (VNS) has previously been shown to enhance extinction learning and reduce drug-seeking. Here we determined the effects of VNS-mediated release of brain-derived neurotrophic factor (BDNF) on extinction and cue-induced reinstatement in rats trained to self-administer cocaine. Pairing 10 days of extinction training with VNS facilitated extinction and reduced drug-seeking behavior during reinstatement. Rats that received a single extinction session with VNS showed elevated BDNF levels in the medial PFC as determined via an enzyme-linked immunosorbent assay (ELISA). Systemic blockade of Tropomyosin receptor kinase B (TrkB) receptors during extinction, via the TrkB antagonist ANA-12, decreased the effects of VNS on extinction and reinstatement. Whole-cell recordings in brain slices showed that cocaine self-administration induced alterations in the ratio of AMPA and NMDA receptor-mediated currents in layer 5 pyramidal neurons of the infralimbic cortex (IL). Pairing extinction with VNS reversed cocaine-induced changes in glutamatergic transmission by enhancing AMPAR currents, and this effect was blocked by ANA-12. Our study suggests that VNS consolidates extinction of drug-seeking behavior by reversing drug-induced changes in synaptic AMPA receptors in the IL, and this effect is abolished by blocking TrkB receptors during extinction, highlighting a potential mechanism for the therapeutic effects of VNS in addiction.

Cellular mechanisms of contextual fear memory reconsolidation: Role of hippocampal SFKs, TrkB receptors and GluN2B-containing NMDA receptors

Memories are stored into long-term representations through a process that depends on protein synthesis. However, a consolidated memory is not static and inflexible and can be reactivated under certain circumstances, the retrieval is able to reactivate memories and destabilize them engaging a process of restabilization known as reconsolidation. Although the molecular mechanisms that mediate fear memory reconsolidation are not entirely known, so here we investigated the molecular mechanisms in the hippocampus involved in contextual fear conditioning memory (CFC) reconsolidation in male Wistar rats. We demonstrated that the blockade of Src family kinases (SFKs), GluN2B-containing NMDA receptors and TrkB receptors (TrkBR) in the CA1 region of the hippocampus immediately after the reactivation session impaired contextual fear memory reconsolidation. These impairments were blocked by the neurotrophin BDNF and the NMDAR agonist, D-Serine. Considering that the study of the link between synaptic proteins is crucial for understanding memory processes, targeting the reconsolidation process may provide new ways of disrupting maladaptive memories, such as those seen in post-traumatic stress disorder. Here we provide new insights into the cellular mechanisms involved in contextual fear memory reconsolidation, demonstrating that SFKs, GluN2B-containing NMDAR, and TrkBR are necessary for the reconsolidation process. Our findings suggest a link between BDNF and SFKs and GluN2B-containing NMDAR as well as a link between NMDAR and SFKs and TrkBR in fear memory reconsolidation. These preliminary pharmacological findings provide new evidence of the mechanisms involved in the reconsolidation of fear memory and have the potential to contribute to the development of treatments for psychiatric disorders involving maladaptive memories.

Intrahippocampal injection of a selective blocker of NMDA receptors containing the GluN2B subunit, Ro25-6981, increases glutamate neurotransmission and induces antidepressant-like effects

Major depressive disorder (MDD) is a serious public health problem, as it is the most common psychiatric disorder worldwide. Antidepressant drugs increase adult hippocampal neurogenesis, which is required to induce some behavioral effects of antidepressants. Adult-born granule cells in the dentate gyrus (DG) and the glutamate receptors subunits 2 (GluN2B) subunit of N-methyl-D-aspartate (NMDA) ionotropic receptors play an important role in these effects. However, the precise neurochemical role of the GluN2B subunit of the NMDA receptor on adult-born GCs for antidepressant-like effects has yet to be elucidated. The present study aims to explore the contribution of the GluN2B-containing NMDA receptors in the ventral dentate gyrus (vDG) to the antidepressant drug treatment using a pharmacological approach. Thus, (αR)-(4-hydroxyphenyl)-(βS)-methyl-4-(phenylmethyl)-1-piperidinepropanol (Ro25-6981), a selective antagonist of the GluN2B subunit, was acutely administered locally into the ventral DG (vDG, 1 μg each side) following a chronic fluoxetine (18 mg/kg/day) treatment-known to increase adult hippocampal neurogenesis-in a mouse model of anxiety/depression. Responses in a neurogenesis-dependent task, the novelty suppressed feeding (NSF), and neurochemical consequences on extracellular glutamate and gamma-aminobutyric acid (GABA) levels in the vDG were measured. Here, we show a rapid-acting antidepressant-like effect of local Ro25-6981 administration in the NSF independent of fluoxetine treatment. Furthermore, we revealed a fluoxetine-independent increase in the glutamatergic transmission in the vDG. Our results suggest behavioral and neurochemical effects of GluN2B subunit independent of serotonin reuptake inhibition.

NMDARs control object recognition memory destabilization and reconsolidation

Object recognition memory (ORM) allows identification of previously encountered items and is therefore crucial for remembering episodic information. In rodents, reactivation during recall in the presence of a novel object destabilizes ORM and initiates a Zif268 and protein synthesis-dependent reconsolidation process in the hippocampus that links the memory of this object to the reactivated recognition trace. Hippocampal NMDA receptors (NMDARs) modulate Zif268 expression and protein synthesis and regulate memory stability but their possible involvement in the ORM destabilization/reconsolidation cycle has yet to be analyzed in detail. We found that, in adult male Wistar rats, intra dorsal-CA1 administration of the non-subunit selective NMDAR antagonist AP5, or of the GluN2A subunit-containing NMDAR antagonist TCN201, 5min after an ORM reactivation session in the presence of a novel object carried out 24h post-training impaired retention 24h later. In contrast, pre-reactivation administration of the GluN2B subunit-containing NMDAR antagonist RO25-6981 had no effect on ORM recall or retention but impeded the amnesia caused by Zif268 silencing and protein synthesis inhibition in dorsal CA1. Our results indicate that GluN2B-containing hippocampal NMDARs are necessary for ORM destabilization whereas GluN2A-containing NMDARs are involved in ORM reconsolidation, and suggest that modulation of the relative activity of these receptor subtypes during recall regulates ORM persistence.

Identification of miRNA-mediated gene regulatory networks in L-methionine exposure counteracts cocaine-conditioned place preference in mice

Background and Aims: Methionine has been proven to inhibit addictive behaviors of cocaine dependence. This study aimed to identify the potential mechanisms of MET relating to its inhibitory effects on cocaine induced cellular and behavioral changes. Methods: MRNA and miRNA high-throughput sequencing of the prefrontal cortex in a mouse model of cocaine conditioned place preference (CPP) combined with L-methionine was performed. Differentially expressed miRNAs (DE-miRNAs) and differentially expressed genes (DEGs) regulated by cocaine and inhibited by L-methionine were identified. DEGs were mapped to STRING database to construct a protein-protein interaction (PPI) network. Then, the identified DEGs were subjected to the DAVID webserver for functional annotation. Finally, miRNA-mRNA regulatory network and miRNA-mRNA-TF regulatory networks were established to screen key DE-miRNAs and coregulation network in Cytoscape. Results: Sequencing data analysis showed that L-methionine reversely regulated genes and miRNAs affected by cocaine. Pathways associated with drug addiction only enriched in CS-down with MC-up genes targeted by DE-miRNAs including GABAergic synapse, Glutamatergic synapse, Circadian entrainment, Axon guidance and Calcium signaling pathway. Drug addiction associated network was formed of 22 DEGs including calcium channel (Cacna1c, Cacna1e, Cacna1g and Cacng8), ephrin receptor genes (Ephb6 and Epha8) and ryanodine receptor genes (Ryr1 and Ryr2). Calcium channel gene network were identified as a core gene network modulated by L-methionine in response to cocaine dependence. Moreover, it was predicted that Grin1 and Fosb presented in TF-miRNA-mRNA coregulation network with a high degree of interaction as hub genes and interacted calcium channels. Conclusion: These identified key genes, miRNA and coregulation network demonstrated the efficacy of L-methionine in counteracting the effects of cocaine CPP. To a certain degree, it may provide some hints to better understand the underlying mechanism on L-methionine in response to cocaine abuse.

BDNF as a therapeutic candidate for cocaine use disorders

Cocaine self-administration disturbs intracellular signaling in multiple reward circuitry neurons that underlie relapse to drug seeking. Cocaine-induced deficits in prelimbic (PL) prefrontal cortex change during abstinence, resulting in different neuroadaptations during early withdrawal from cocaine self-administration than after one or more weeks of abstinence. Infusion of brain-derived neurotrophic factor (BDNF) into the PL cortex immediately following a final session of cocaine self-administration attenuates relapse to cocaine seeking for an extended period. BDNF affects local (PL) and distal subcortical target areas that mediate cocaine-induced neuroadaptations that lead to cocaine seeking. Blocking synaptic activity selectively in the PL projection to the nucleus accumbens during early withdrawal prevents BDNF from decreasing subsequent relapse. In contrast, blocking synaptic activity selectively in the PL projection to the paraventricular thalamic nucleus by itself decreases subsequent relapse and prior intra-PL BDNF infusion prevents the decrease. Infusion of BDNF into other brain structures at different timepoints after cocaine self administration differentially alters cocaine seeking. Thus, the effects of BDNF on drug seeking are different depending on the brain region, the timepoint of intervention, and the specific pathway that is affected.

The Src-Kinase Fyn is Required for Cocaine-Associated Memory Through Regulation of Tau

Drug-associated context-induced relapse of cocaine-seeking behaviour requires the retrieval of drug-associated memory. Studies exploring the underlying neurobiological mechanism of drug memory formation will likely contribute to the development of treatments for drug addiction and the prevention of relapse. In our study, we applied a cocaine-conditioned place preference (CPP) paradigm and a self-administration paradigm (two drug-associated memory formation model) to confirm the hypothesis that the Src kinase Fyn critically regulates cocaine-associated memory formation in the hippocampus. For this experiment, we administered the Src kinase inhibitor PP2 into the bilateral hippocampus before cocaine-CPP and self-administration training, and the results showed that pharmacological manipulation of the Src kinase Fyn activity significantly attenuated the response to cocaine-paired cues in the cocaine-CPP and self-administration paradigms, indicating that hippocampal Fyn activity contributes to cocaine-associated memory formation. In addition, the regulation of cocaine-associated memory formation by Fyn depends on Tau expression, as restoring Tau to normal levels disrupted cocaine memory formation. Together, these results indicate that hippocampal Fyn activity plays a key role in the formation of cocaine-associated memory, which underlies cocaine-associated contextual stimulus-mediated regulation of cocaine-seeking behaviour, suggesting that Fyn represents a promising therapeutic target for weakening cocaine-related memory and treating cocaine addiction.

Aberrant expression of BDNF might serve as a candidate target for cocaine-induced psychosis: insights from bioinformatics analysis and microarray validation

Background

Cocaine use disorder (CUD) and associated psychosis are major public health issues worldwide, along with high relapse outcome and limited treatment options. Exploring the molecular mechanisms underlying cocaine-induced psychosis (CIP) could supply integrated insights for understanding the pathogenic mechanism and potential novel therapeutic targets.

Aims

The aim of the study was to explore common alterations of CUD-schizophrenia-target genes and identify core risk genes contributing to CIP through data mining and network pharmacology approach.

Methods

Target genes of CUD were obtained from GeneCards, Comparative Toxicogenomics Database, Swiss Target Prediction platform and PubChem. Schizophrenia-related target genes were derived from DisGeNET, GeneCards, MalaCards and Online Mendelian Inheritance in Man databases. Then, the overlap genes of these two sets were regarded as risk genes contributing to CIP. Based on these CUD-schizophrenia-target genes, functional annotation and pathway analysis were performed using the clusterProfiler package in R. Protein–protein interaction network construction and module detection were performed based on the Search Tool for the Retrieval of Interacting Genes (STRING) database and Cytoscape software. Gene expression datasets GSE54839 and GSE93577 were applied for data validation and diagnostic capacity evaluation of interested hub genes.

Results

A total of 165 CUD-schizophrenia-target genes were obtained. These genes were mainly contributing to chemical synaptic transmission, neuropeptide hormone activity, postsynaptic membrane and neuroactive ligand–receptor interaction pathway. Network analysis and validation analysis indicated that BDNF might serve as an important risk gene in mediating CIP.

Conclusions

This study generates a holistic view of CIP and provides a basis for the identification of potential CUD-schizophrenia-target genes involved in the development of CIP. The abnormal expression of BDNF would be a candidate therapeutic target underlying the pathogenesis of CUD and associated CIP.

Enhancement of the GluN2B subunit of glutamatergic NMDA receptors in rat brain areas after cocaine abstinence

BACKGROUND

Cocaine use disorder is associated with compulsive drug-seeking and drug-taking, whereas relapse may be induced by several factors, including stress, drug-related places, people, and cues. Recent observations strongly support the involvement of the N-methyl-D-aspartate (NMDA) receptors in cocaine use disorders and abstinence, whereas withdrawal in different environments may affect the intensification of relapse.

METHODS

The aim of this study was to examine the GluN2B subunit expression and its association with the postsynaptic density protein 95 (PSD95) in several brain structures in rats with a history of cocaine self-administration and housed either in an enriched environment or in an isolated condition. Furthermore, a selective antagonist of the GluN2B subunit-CP 101,606 (10 and 20 mg/kg) administered during exposure to cocaine or a drug-associated conditional stimulus (a cue) was used to evaluate seeking behavior in rats.

RESULTS

In rats previously self-administering cocaine, we observed an increase in the GluN2B expression in the total homogenate from the dorsal hippocampus under both enriched environment and isolation. Cocaine abstinence under isolation conditions increased the GluN2B and GluN2B/PSD95 complex levels in the PSD fraction of the prelimbic cortex in rats previously self-administering cocaine. Administration of CP 101,606 attenuated cue-induced cocaine-seeking behavior only in isolation-housed rats.

CONCLUSION

In summary, in this study we showed region-specific changes in both the expression of GluN2B subunit and NMDA receptor trafficking during cocaine abstinence under different housing conditions. Furthermore, we showed that the pharmacological blockade of the GluN2B subunit may be useful in attenuating cocaine-seeking behavior.

Inhibition of CSF1R, a receptor involved in microglia viability, alters behavioral and molecular changes induced by cocaine

Different data suggest that microglia may participate in the drug addiction process as these cells respond to neurochemical changes induced by the administration of these substances. In order to study the role of microglia in drug abuse, Swiss mice aged 8-9 weeks were treated with the CSF1R inhibitor PLX3397 (40 mg/kg, p.o.) and submitted to behavioral sensitization or conditioned place preference (CPP) induced by cocaine (15 mg/kg, i.p.). Thereafter, brains were used to evaluate the effects of CSF1R inhibition and cocaine administration on morphological, biochemical and molecular changes. CSF1R inhibition attenuated behavioral sensitization, reduced the number of Iba-1+ cells and increased ramification and lengths of the branches in the remaining microglia. Additionally, both cocaine and PLX3397 increased the cell body to total cell size ratio of Iba-1+ cells, as well as CD68+ and GFAP+ stained areas, suggesting an activated pattern of the glial cells. Besides, CSF1R inhibition increased CX3CL1 levels in the striatum, prefrontal cortex and hippocampus, as well as reduced CX3CR1 expression in the hippocampus. In this region, cocaine also reduced BDNF levels, an effect that was enhanced by CSF1R inhibition. In summary, our results suggest that microglia participate in the behavioral and molecular changes induced by cocaine. This study contributes to the understanding of the role of microglia in cocaine addiction.

The NMDA Receptor Subunit (GluN1 and GluN2A) Modulation Following Different Conditions of Cocaine Abstinence in Rat Brain Structures

Different neuronal alterations within glutamatergic system seem to be crucial for developing of cocaine-seeking behavior. Cocaine exposure provokes a modulation of the NMDA receptor subunit expression in rodents, which probably contributes to cocaine-induced behavioral alterations. The aim of this study was to examine the composition of the NMDA receptor subunits in the brain structures in rats with the history of cocaine self-administration after cocaine abstinence (i) in an enriched environment, (ii) in an isolated condition, (iii) with extinction training, or (iv) without instrumental task, as well as the Grin1 (encoding GluN1) and Grin2A (encoding GluN2A) gene expression were evaluated after 10-day extinction training in rat brain structures. In the present study, we observed changes only following cocaine abstinence with extinction training, when the increased GluN2A subunit levels were seen in the postsynaptic density fraction but not in the whole homogenate of the prelimbic cortex (PLC) and dorsal hippocampus (dHIP) in rats previously self-administered cocaine. At the same time, extinction training did not change the Grin1 and Grin2A gene expression in these structures. In conclusion, NMDA receptor subunit modulation observed following cocaine abstinence with extinction training may represent a potential target in cocaine-seeking behavior.

The role of the serotonin transporter in prefrontal cortex glutamatergic signaling following short- and long-access cocaine self-administration

Vulnerability to drug addiction relies on substantial individual differences. We previously demonstrated that serotonin transporter knockout (SERT^−/−) rats show increased cocaine intake and develop signs of compulsivity. However, the underlying neural mechanisms are not fully understood. Given the pivotal role of glutamate and prefrontal cortex in cocaine‐seeking behavior, we sought to investigate the expression of proteins implicated in glutamate neurotransmission in the prefrontal cortex of naïve and cocaine‐exposed rats lacking SERT. We focused on the infralimbic (ILc) and prelimbic (PLc) cortices, which are theorized to exert opposing effects on the control over subcortical brain areas. SERT^−/− rats, which compared to wild‐type (SERT^+/+) rats show increased ShA and LgA intake short‐access (ShA) and long‐access (LgA) cocaine intake, were sacrificed 24 h into withdrawal for ex vivo molecular analyses. In the ILc homogenate of SERT^−/− rats, we observed a sharp increase in glial glutamate transporter 1 (GLT‐1) after ShA, but not LgA, cocaine intake. This was paralleled by ShA‐induced increases in GluN1, GluN2A, and GluN2B NMDA receptor subunits and their scaffolding protein SAP102 in the ILc homogenate, but not postsynaptic density, of these knockout animals. In the PLc, we found no major changes in the homogenate; conversely, the expression of GluN1 and GluN2A NMDA receptor subunits was increased in the postsynaptic density under ShA conditions and reduced under LgA conditions. These results point to SERT as a critical regulator of glutamate homeostasis in a way that differs between the subregions investigated, the duration of cocaine exposure as well as the cellular compartment analyzed.

GluN2B and GluN2A-containing NMDAR are differentially involved in extinction memory destabilization and restabilization during reconsolidation

Extinction memory destabilized by recall is restabilized through mTOR-dependent reconsolidation in the hippocampus, but the upstream pathways controlling these processes remain unknown. Hippocampal NMDARs drive local protein synthesis via mTOR signaling and may control active memory maintenance. We found that in adult male Wistar rats, intra dorsal-CA1 administration of the non-subunit selective NMDAR antagonist AP5 or of the GluN2A subunit-containing NMDAR antagonist TCN201 after step down inhibitory avoidance (SDIA) extinction memory recall impaired extinction memory retention and caused SDIA memory recovery. On the contrary, pre-recall administration of AP5 or of the GluN2B subunit-containing NMDAR antagonist RO25-6981 had no effect on extinction memory recall or retention per se but hindered the recovery of the avoidance response induced by post-recall intra-CA1 infusion of the mTOR inhibitor rapamycin. Our results indicate that GluN2B-containing NMDARs are necessary for extinction memory destabilization whereas GluN2A-containing NMDARs are involved in its restabilization, and suggest that pharmacological modulation of the relative activation state of these receptor subtypes around the moment of extinction memory recall may regulate the dominance of extinction memory over the original memory trace.

MeBib Suppressed Methamphetamine Self-Administration Response via Inhibition of BDNF/ERK/CREB Signal Pathway in the Hippocampus

Methamphetamine (MA) is one of the most commonly abused drugs in the world by illegal drug users. Addiction to MA is a serious public health problem and effective therapies do not exist to date. It has also been reported that behavior induced by psychostimulants such as MA is related to histone deacetylase (HDAC). MeBib is an HDAC6 inhibitor derived from a benzimidazole scaffold. Many benzimidazole-containing compounds exhibit a wide range of pharmacological activity. In this study, we investigated whether HDAC6 inhibitor MeBib modulates the behavioral response in MA self-administered rats. Our results demonstrated that the number of active lever presses in MA self-administered rats was reduced by pretreatment with MeBib. In the hippocampus of rats, we also found MA administration promotes GluN2B, an NMDA receptor subunit, expression, which results in sequential activation of ERK/CREB/BDNF pathway, however, MeBib abrogated it. Collectively, we suggest that MeBib prevents the MA seeking response induced by MA administration and therefore, represents a potent candidate as an MA addiction inhibitor.

Cocaine-induced Changes in the Expression of NMDA Receptor Subunits

Cocaine use disorder is manifested by repeated cycles of drug seeking and drug taking. Cocaine exposure causes synaptic transmission in the brain to exhibit persistent changes, which are poorly understood, while the pharmacotherapy of this disease has not been determined. Multiple potential mechanisms have been indicated to be involved in the etiology of co-caine use disorder. The glutamatergic system, especially N-methyl-D-aspartate (NMDA) receptors, may play a role in sever-al physiological processes (synaptic plasticity, learning and memory) and in the pathogenesis of cocaine use disorder. The composition of the NMDA receptor subunits changes after contingent and noncontingent cocaine administration and after drug abstinence in a region-specific and time-dependent manner, as well as depending on the different protocols used for co-caine administration. Changes in the expression of NMDA receptor subunits may underlie the transition from cocaine abuse to dependence, as well as the transition from cocaine dependence to cocaine withdrawal. In this paper, we summarize the cur-rent knowledge regarding neuroadaptations within NMDA receptor subunits and scaffolding proteins observed following voluntary and passive cocaine intake, as well as the effects of NMDA receptor antagonists on cocaine-induced behavioral changes during cocaine seeking and relapse.

Repeated cocaine exposure dysregulates BDNF expression and signaling in the mesocorticolimbic pathway of the adolescent rat

Objectives: Long-term abstinence following cocaine exposure up-regulates brain-derived neurotrophic factor (BDNF) expression in the mesocorticolimbic pathway. Given the increased vulnerability to drug abuse typical of adolescence, we hypothesized that changes in BDNF expression may become manifest early after the end of cocaine treatment in the adolescent brain.Methods: Rats received cocaine injections from postnatal day 28 (PND28) to PND42 and the mesocorticolimbic expression of BDNF was measured by real-time PCR and Western blotting at PND43.Results: In the ventral tegmental area, BDNF-tropomyosin receptor kinase B (TrΚB) expression and phosphorylation are enhanced while the intracellular signaling is unaltered. In the nucleus accumbens (NAc) shell and core, BDNF and its signaling were down-regulated. In the prelimbic (PL) cortex, we found reduced BDNF expression and increased phosphoprylation of TrΚB, ERK and AKT. In the infralimbic (IL) cortex, increased BDNF expression was coupled with reduced activity and expression of its downstream targets. To evaluate the role of glutamate on BDNF-independent changes, we investigated the expression of the transporter GLT-1 and the activation of the NMDA receptor subunit GluN2B, which were both increased in the PL cortex while reduced in the IL cortex.Conclusions: Our results show that adolescent cocaine exposure modulates BDNF system early after treatment in the mesocorticolimbic pathway, identifying a complex but specific set of changes that could provide clues for treatment.

Interactions of Glutamatergic Neurotransmission and Brain-Derived Neurotrophic Factor in the Regulation of Behaviors after Nicotine Administration

Nicotine causes tobacco dependence, which may result in fatal respiratory diseases. The striatum is a key structure of forebrain basal nuclei associated with nicotine dependence. In the striatum, glutamate release is increased when α7 nicotinic acetylcholine receptors expressed in the glutamatergic terminals are exposed to nicotine, and over-stimulates glutamate receptors in gamma amino-butyric acid (GABA)ergic neurons. These receptor over-stimulations in turn potentiate GABAergic outputs to forebrain basal nuclei and contribute to the increase in psychomotor behaviors associated with nicotine dependence. In parallel with glutamate increases, nicotine exposure elevates brain-derived neurotrophic factor (BDNF) release through anterograde and retrograde targeting of the synapses of glutamatergic terminals and GABAergic neurons. This article reviews nicotine-exposure induced elevations of glutamatergic neurotransmission, the bidirectional targeting of BDNF in the striatum, and the potential regulatory role played by BDNF in behavioral responses to nicotine exposure.

Positive and Negative Allosteric Modulators of N-Methyl-d-aspartate (NMDA) Receptors: Structure-Activity Relationships and Mechanisms of Action

Excitatory activity in the CNS is predominately mediated by l-glutamate through several families of l-glutamate neurotransmitter receptors. Of these, the N-methyl-d-aspartate receptor (NMDAR) family has many critical roles in CNS function and in various neuropathological and psychiatric conditions. Until recently, the types of compounds available to regulate NMDAR function have been quite limited in terms of mechanism of action, subtype selectivity, and biological effect. However, several new classes of NMDAR agents have now been identified that are positive or negative allosteric modulators (PAMs and NAMs, respectively) with various patterns of NMDAR subtype selectivity. These new agents act at several newly recognized binding sites on the NMDAR complex and offer significantly greater pharmacological control over NMDAR activity than previously available agents. The purpose of this review is to summarize the structure-activity relationships for these new NMDAR modulator drug classes and to describe the current understanding of their mechanisms of action.

Effects of GluN2B-selective antagonists on delay and probability discounting in male rats: Modulation by delay/probability presentation order

The contribution of the GluN2B subunit of the NMDA receptor to impulsivity has recently been examined. Ro 63-1908, a highly selective antagonist for the GluN2B, decreases impulsive choice. Because the order in which delays are presented modulates drug effects in discounting procedures, one goal of the current study was to determine the effects of Ro 63-1908 in delay discounting procedures in which the delays to obtaining the large reinforcer either increase or decrease across the session. We also determined if Ro 63-1908 differentially alters risky choice in probability discounting procedures that use ascending/descending schedules. Male rats were trained in either delay (n = 24) or probability (n = 24) discounting in which the delay to/odds against reinforcement were presented in either ascending or descending order (n = 12 each schedule). Following training, rats received the GluN2B antagonists Ro 63-1908 (0-1.0 mg/kg) and CP-101,606 (0-3.0 mg/kg). In delay discounting, Ro 63-1908 (1.0 mg/kg), but not CP-101,606, decreased choice for the large reinforcer, but only when the delays decreased across the session. In probability discounting, Ro 63-1908 (0.3 mg/kg)/CP-101,606 (1.0 mg/kg) increased choice for the large reinforcer when the probability of obtaining this alternative decreased across the session, but Ro 63-1908 (1.0 mg/kg)/CP-101,606 (3.0 mg/kg) decreased choice when the probabilities increased. These results show that the GluN2B is a mediator of impulsive/risky choice, but the effects of GluN2B antagonists are dependent on the order in which delays/probabilities are presented. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Biphasic effect of abstinence duration following cocaine self-administration on spine morphology and plasticity-related proteins in prelimbic cortical neurons projecting to the nucleus accumbens core

Cocaine self-administration (SA) in rats dysregulates glutamatergic signaling in the prelimbic (PrL) cortex and glutamate release in the nucleus accumbens (NA) core, promoting cocaine seeking. PrL adaptations that affect relapse to drug seeking emerge during the first week of abstinence, switching from an early (2 h) hypoglutamatergic state to a later (7 days) hyperglutamatergic state. Different interventions that normalize glutamatergic signaling in PrL cortex at each timepoint are necessary to suppress relapse. We hypothesized that plasticity-related proteins that regulate glutamatergic neurotransmission as well as dendritic spine morphology would be biphasically regulated during these two phases of abstinence in PrL cortical neurons projecting to the NA core (PrL-NA core). A combinatorial viral approach was used to selectively label PrL-NA core neurons with an mCherry fluorescent reporter. Male rats underwent 2 weeks of cocaine SA or received yoked-saline infusions and were perfused either 2 h or 7 days after the final SA session. Confocal microscopy and 3D reconstruction analyses were performed for Fos and pCREB immunoreactivity (IR) in the nucleus of layer V PrL-NA core neurons and GluA1-IR and GluA2-IR in apical dendritic spines of the same neurons. Here, we show that cocaine SA decreased PrL-NA core spine head diameter, nuclear Fos-IR and pCREB-IR, and GluA1-IR and GluA2-IR in putative mushroom-type spines 2 h after the end of cocaine SA, whereas the opposite occurred following 1 week of abstinence. Our findings reveal biphasic, abstinence duration-dependent alterations in structural plasticity and relapse-related proteins in the PrL-NA core pathway after cocaine SA.

Divergent Prelimbic Cortical Pathways Interact with BDNF to Regulate Cocaine-seeking

A single BDNF microinfusion into prelimbic (PrL) cortex immediately after the last cocaine self-administration session decreases relapse to cocaine-seeking. The BDNF effect is blocked by NMDAR antagonists. To determine whether synaptic activity in putative excitatory projection neurons in PrL cortex is sufficient for BDNF's effect on relapse, the PrL cortex of male rats was infused with an inhibitory Designer Receptor Exclusively Activated by Designer Drugs (DREADD) viral vector driven by an αCaMKII promoter. Immediately after the last cocaine self-administration session, rats were injected with clozapine-N-oxide 30 min before an intra-PrL BDNF microinfusion. DREADD-mediated inhibition of the PrL cortex blocked the BDNF-induced decrease in cocaine-seeking after abstinence and cue-induced reinstatement after extinction. Unexpectedly, DREADD inhibition of PrL neurons in PBS-infused rats also reduced cocaine-seeking, suggesting that divergent PrL pathways affect relapse. Next, using a cre-dependent retroviral approach, we tested the ability of DREADD inhibition of PrL projections to the NAc core or the paraventricular thalamic nucleus (PVT) to alter cocaine-seeking in BDNF- and PBS-infused rats. Selective inhibition of the PrL-NAc pathway at the end of cocaine self-administration blocked the BDNF-induced decrease in cocaine-seeking but had no effect in PBS-infused rats. In contrast, selective inhibition of the PrL-PVT pathway in PBS-infused rats decreased cocaine-seeking, and this effect was prevented in BDNF-infused rats. Thus, activity in the PrL-NAc pathway is responsible for the therapeutic effect of BDNF on cocaine-seeking whereas inhibition of activity in the PrL-pPVT pathway elicits a similar therapeutic effect in the absence of BDNF.SIGNIFICANCE STATEMENT The major issue in cocaine addiction is the high rate of relapse. However, the neuronal pathways governing relapse remain unclear. Using a pathway-specific chemogenetic approach, we found that BDNF differentially regulates two key prelimbic pathways to guide long-term relapse. Infusion of BDNF in the prelimbic cortex during early withdrawal from cocaine self-administration decreases relapse that is prevented when neurons projecting from the prelimbic cortex to the nucleus accumbens core are inhibited. In contrast, BDNF restores relapse when neurons projecting from the prelimbic cortex to the posterior paraventricular thalamic nucleus are inhibited. This study demonstrates that two divergent cortical outputs mediate relapse that is regulated in opposite directions by infusing BDNF in the prelimbic cortex during early withdrawal from cocaine.

Increasing Brain-Derived Neurotrophic Factor (BDNF) in medial prefrontal cortex selectively reduces excessive drinking in ethanol dependent mice

The neurotrophin Brain-Derived Neurotrophic Factor (BDNF) has been implicated in a number of neuropsychiatric disorders, including alcohol use disorder. Studies have shown that BDNF activity in cortical regions, such as the medial prefrontal cortex (mPFC) mediates various ethanol-related behaviors. We previously reported a significant down-regulation in Bdnf mRNA in mPFC following chronic ethanol exposure compared to control mice. The present study was conducted to extend these findings by examining whether chronic ethanol treatment reduces BDNF protein expression in mPFC and whether reversing this deficit via direct injection of BDNF or viral-mediated overexpression of BDNF in mPFC alters voluntary ethanol consumption in dependent and nondependent mice. Repeated cycles of chronic intermittent ethanol (CIE) exposure was employed to model ethanol dependence, which produces robust escalation of ethanol intake. Results indicated that CIE treatment significantly increased ethanol intake and this was accompanied by a significant decrease in BDNF protein in mPFC that lasted at least 72 h after CIE exposure. In a separate study, once dependence-related increased drinking was established, bilateral infusion of BDNF (0, 0.25, 0.50 μg) into mPFC significantly decreased ethanol intake in a dose-related manner in dependent mice but did not affect moderate drinking in nondependent mice. In a third study, viral-mediated overexpression of BDNF in mPFC prevented escalation of drinking in dependent mice but did not alter intake in nondependent mice. Collectively, these results provide evidence that adaptations in cortical (mPFC) BDNF activity resulting from chronic ethanol exposure play a role in mediating excessive ethanol drinking associated with dependence.

Intra-prelimbic cortical inhibition of striatal-enriched tyrosine phosphatase suppresses cocaine seeking in rats

Cocaine self-administration in rats results in dysfunctional neuroadaptations in the prelimbic (PrL) cortex during early abstinence. Central to these adaptations is decreased phospho-extracellular signal-regulated kinase (p-ERK), which plays a key role in cocaine seeking. Normalizing ERK phosphorylation in the PrL cortex immediately after cocaine self-administration decreases subsequent cocaine seeking. The disturbance in ERK phosphorylation is accompanied by decreased phosphorylation of striatal-enriched protein tyrosine phosphatase (STEP), indicating increased STEP activity. STEP is a well-recognized ERK phosphatase but whether STEP activation during early abstinence mediates the decrease in p-ERK and is involved in relapse is unknown. Here, we show that a single intra-PrL cortical microinfusion of the selective STEP inhibitor, TC-2153, immediately after self-administration suppressed post-abstinence context-induced relapse under extinction conditions and cue-induced reinstatement, but not cocaine prime-induced drug seeking or sucrose seeking. Moreover, an intra-PrL cortical TC-2153 microinfusion immediately after self-administration prevented the cocaine-induced decrease in p-ERK within the PrL cortex during early abstinence. Interestingly, a systemic TC-2153 injection at the same timepoint failed to suppress post-abstinence context-induced relapse or cue-induced reinstatement, but did suppress cocaine prime-induced reinstatement. These data indicate that the STEP-induced ERK dephosphorylation in the PrL cortex during early abstinence is a critical neuroadaptation that promotes relapse to cocaine seeking and that systemic versus intra-PrL cortical inhibition of STEP during early abstinence differentially suppresses cocaine seeking.

Exercise as a Sex-Specific Treatment for Substance Use Disorder

PURPOSE OF REVIEW

Exercise is a promising treatment for substance use disorder that may reduce withdrawal symptoms and prevent relapse. In this review, we discuss recent evidence from clinical and preclinical studies for its efficacy, from a behavioral to a molecular level, in order to understand the exercise conditions that lead to beneficial effects. We also highlight the few recent findings of sex-specific differences.

RECENT FINDINGS

Clinical and preclinical findings show that exercise decreases withdrawal symptoms, including craving, in both males and females. Evidence from clinical studies support the efficacy of exercise to prevent relapse to smoking, although further research is needed to examine sex differences, establish long-term efficacy, and to determine if effects extend to other substance use disorders. Preclinical findings also support the potential utility of exercise to prevent relapse with evidence suggesting that its efficacy is enhanced in males, and mediated by blocking drug-induced adaptations that occur during early abstinence.

SUMMARY

Sex differences and timing of exercise availability during abstinence should be considered in future studies examining exercise as an intervention for relapse. A better understanding of the neurobiological mechanisms underlying the efficacy of exercise to reduce withdrawal symptoms and prevent relapse is needed to guide its development as a sex-specific treatment.

Cross-talk between the epigenome and neural circuits in drug addiction

Drug addiction is a behavioral disorder characterized by dysregulated learning about drugs and associated cues that result in compulsive drug seeking and relapse. Learning about drug rewards and predictive cues is a complex process controlled by a computational network of neural connections interacting with transcriptional and molecular mechanisms within each cell to precisely guide behavior. The interplay between rapid, temporally specific neuronal activation, and longer-term changes in transcription is of critical importance in the expression of appropriate, or in the case of drug addiction, inappropriate behaviors. Thus, these factors and their interactions must be considered together, especially in the context of treatment. Understanding the complex interplay between epigenetic gene regulation and circuit connectivity will allow us to formulate novel therapies to normalize maladaptive reward behaviors, with a goal of modulating addictive behaviors, while leaving natural reward-associated behavior unaffected.

Role of Src Family Kinases in BDNF-Mediated Suppression of Cocaine-Seeking and Prevention of Cocaine-Induced ERK, GluN2A, and GluN2B Dephosphorylation in the Prelimbic Cortex

Models of relapse have demonstrated that neuroadaptations in reward circuits following cocaine self-administration (SA) underlie reinstatement of drug-seeking. Dysregulation of the pathway from the prelimbic (PrL) cortex to the nucleus accumbens is implicated in reinstatement. A single BDNF infusion into the PrL cortex following a final cocaine SA session results in attenuation of reinstatement of cocaine-seeking. Inhibiting BDNF's receptor, TrkB, ERK/MAP kinase activation, or NMDA receptors blocks this attenuating effect, indicating that the interaction between glutamate-mediated synaptic activity and TrkB signaling is imperative to BDNF's suppressive effect on drug-seeking. Src family kinases (SFKs) are involved in both NMDA-mediated activation of TrkB- and TrkB-mediated tyrosine phosphorylation of NMDA receptors. We hypothesized that infusion of the SFK inhibitor, PP2, into the PrL cortex prior to a BDNF infusion, immediately after the end of the last cocaine SA session, would block BDNF's ability to suppress reinstatement of cocaine-seeking in rats with a cocaine SA history. PP2, but not the negative control, PP3, blocked BDNF's suppressive effect on context-induced relapse after 1 week of abstinence and cue-induced reinstatement after extinction. As previously reported, infusion of BDNF into the PrL cortex blocked cocaine SA-induced dephosphorylation of ERK, GluN2A, and GluN2B-containing receptors. Inhibition of SFKs using PP2 blocked BDNF-mediated phosphorylation of GluN2A, GluN2B, and ERK. These data indicate that SFK activity is necessary for BDNF-mediated suppression of cocaine-seeking and reversal of cocaine-induced dephosphorylation of key phosphoproteins in the prefrontal cortex related to synaptic plasticity.

Suppression of Methamphetamine Self-Administration by Ketamine Pre-treatment Is Absent in the Methylazoxymethanol (MAM) Rat Model of Schizophrenia

Ketamine may prove to be a potential candidate in treating the widespread drug addiction/substance abuse epidemic among patients with schizophrenia. Clinical studies have shown ketamine to reduce cocaine and heroin cravings. However, the use of ketamine remains controversial as it may exacerbate the symptoms of schizophrenia. Therefore, the aim of this study is to characterize the effects of ketamine on drug addiction in schizophrenia using the methylazoxymethanol (MAM) acetate rat model on operant IV methamphetamine (METH) self-administration. MAM was administered intraperitoneally (22 mg/kg) on gestational day 17. Locomotor activity test and later IV self-administration (IVSA) were then performed in the male offspring followed by a period of forced abstinence and relapse of METH taking. After reaching stable intakes in the relapse phase, ketamine (5 mg/kg) was administered intraperitoneally 30 min prior to the self-administration session. As documented previously, the MAM rats showed a lack of habituation in the locomotor activity test but developed stable maintenance of METH self-administration with no difference in operant behaviour to control animals. Results show that ketamine treatment significantly reduced the METH intake in the control animals but not in MAM animals. Ketamine effect on METH self-administration may be explained by increased glutamatergic signalling in the prefrontal cortex caused by the N-methyl-D-aspartate antagonism and disinhibition of GABA interneurons which was shown to be impaired in the MAM rats. This mechanism may at least partly explain the clinically proven anti-craving potential of ketamine and allow development of more specific anti-craving medications with fewer risks.