Accumbal and pallidal dopamine, glutamate and GABA overflow during cocaine self-administration and its extinction in rats

Microglia contribute to methamphetamine reinforcement and reflect persistent transcriptional and morphological adaptations to the drug

Methamphetamine use disorder (MUD) is a chronic, relapsing disease that is characterized by repeated drug use despite negative consequences and for which there are currently no FDA-approved cessation therapeutics. Repeated methamphetamine (METH) use induces long-term gene expression changes in brain regions associated with reward processing and drug-seeking behavior, and recent evidence suggests that methamphetamine-induced neuroinflammation may also shape behavioral and molecular responses to the drug. Microglia, the resident immune cells in the brain, are principal drivers of neuroinflammatory responses and contribute to the pathophysiology of substance use disorders. Here, we investigated transcriptional and morphological changes in dorsal striatal microglia in response to methamphetamine-taking and during methamphetamine abstinence, as well as their functional contribution to drug-taking behavior. We show that methamphetamine self-administration induces transcriptional changes associated with protein folding, mRNA processing, immune signaling, and neurotransmission in dorsal striatal microglia. Importantly, many of these transcriptional changes persist through abstinence, a finding supported by morphological analyses. Functionally, we report that microglial ablation increases methamphetamine-taking, possibly involving neuroimmune and neurotransmitter regulation. In contrast, microglial depletion during abstinence does not alter methamphetamine-seeking. Taken together, these results suggest that methamphetamine induces both short and long-term changes in dorsal striatal microglia that contribute to altered drug-taking behavior and may provide valuable insights into the pathophysiology of MUD.

Microglia contribute to methamphetamine reinforcement and reflect persistent transcriptional and morphological adaptations to the drug

Methamphetamine use disorder (MUD) is a chronic, relapsing disease that is characterized by repeated drug use despite negative consequences and for which there are currently no FDA-approved cessation therapeutics. Repeated methamphetamine (METH) use induces long-term gene expression changes in brain regions associated with reward processing and drug-seeking behavior, and recent evidence suggests that methamphetamine-induced neuroinflammation may also shape behavioral and molecular responses to the drug. Microglia, the resident immune cells in the brain, are principal drivers of neuroinflammatory responses and contribute to the pathophysiology of substance use disorders. Here, we investigated transcriptional and morphological changes in dorsal striatal microglia in response to methamphetamine-taking and during methamphetamine abstinence, as well as their functional contribution to drug-taking behavior. We show that methamphetamine self-administration induces transcriptional changes associated with protein folding, mRNA processing, immune signaling, and neurotransmission in dorsal striatal microglia. Importantly, many of these transcriptional changes persist through abstinence, a finding supported by morphological analyses. Functionally, we report that microglial ablation increases methamphetamine-taking, possibly involving neuroimmune and neurotransmitter regulation, and that post-methamphetamine microglial repopulation attenuates drug-seeking following a 21-day period of abstinence. In contrast, microglial depletion during abstinence did not alter methamphetamine-seeking. Taken together, these results suggest that methamphetamine induces both short and long-term changes in dorsal striatal microglia that contribute to altered drug-taking behavior and may provide valuable insights into the pathophysiology of MUD.

Esketamine Inhibits Cocaine-Seeking Behaviour Subsequent to Various Abstinence Conditions in Rats

BACKGROUND

Cocaine use disorder (CUD) is a relapsing brain disease caused by a chronic drug intake that involves neural mechanisms and psychological processes, including depression. Preclinical and clinical studies have demonstrated the promise of pharmacological drugs in controlling the reinstatement of cocaine by targeting the N-methyl-D-aspartate (NMDA) receptor. Recent evidence has revealed that esketamine, a (S) enantiomer of ketamine, shows a high affinity to NMDA receptors and has been used in clinical trials to treat moderate-to-severe depression.

METHODS

In the present paper, we investigated the effects of esketamine in regulating cocaine-seeking behaviour induced through the use of cocaine (10 mg/kg) or the cocaine-associated conditioned cue after a short (10 days)-lasting period of drug abstinence with extinction training, home cage or enrichment environment conditions in male rats. Furthermore, we investigated the acute effects of esketamine on locomotor activity in drug-naïve animals.

RESULTS

Esketamine (2.5-10 mg/kg) administered peripherally attenuated the reinstatement induced with cocaine priming or the drug-associated conditioned cue after different conditions of abstinence.

CONCLUSIONS

These results seem to support esketamine as a candidate for the pharmacological management of cocaine-seeking and relapse prevention; however, further preclinical and clinical research is needed to better clarify esketamine's actions in CUD.

Low Current Intensity Plus an Ineffective Dose of Morphine Affect Conditioning Place Preference Through Different Pathways in the Lateral Habenula

Background

The involvement of lateral habenula and the ineffective dose of morphine on reward-related learning and memory is less well-known. This study looked into the effects of electrical stimulation, GABAB receptor blockade, and a combination of both with morphine on conditioned place preference.

Materials and Methods

In this experiment, male rats were anesthetized with ketamine/xylazine (six rats in each group). A 5-day biased conditioned place preference paradigm was used for the behavioral test. The effects of electrical stimulation and phaclofen plus a low dose of morphine on the acquisition and expression phases were examined during conditioning sessions and before the test phase, respectively.

Results

The conditioning scores were reduced by antagonist injection during the acquisition phase. Interestingly, different intensities exhibited opposite effects on the acquisition phase. Conditioned place preference scores during the acquisition phase were significantly induced by 25 μA electrical stimulation, while conditioning scores were suppressed by electrical stimulation at 150 μA. Phaclofen (2 μg/rat) combined with high intensity induced aversion during the acquisition phase, while inhibiting expression. In contrast, high intensity with phaclofen (1 μg/rat) inhibited only the acquisition session. However, low intensity during the acquisition phase had an additive effect that was prevented by pretreatment with phaclofen (2 μg/rat), but this response was modified by the antagonist's low dose.

Conclusions

A behavioral technique called conditioned place preference is frequently used to evaluate learning that is related to rewards. Therefore, lateral habenula electrical stimulation and phaclofen plus morphine could affect place preference through the involvement of the reward system.

Ventral pallidal regulation of motivated behaviors and reinforcement

The interconnected nuclei of the ventral basal ganglia have long been identified as key regulators of motivated behavior, and dysfunction of this circuit is strongly implicated in mood and substance use disorders. The ventral pallidum (VP) is a central node of the ventral basal ganglia, and recent studies have revealed complex VP cellular heterogeneity and cell- and circuit-specific regulation of reward, aversion, motivation, and drug-seeking behaviors. Although the VP is canonically considered a relay and output structure for this circuit, emerging data indicate that the VP is a central hub in an extensive network for reward processing and the regulation of motivation that extends beyond classically defined basal ganglia borders. VP neurons respond temporally faster and show more advanced reward coding and prediction error processing than neurons in the upstream nucleus accumbens, and regulate the activity of the ventral mesencephalon dopamine system. This review will summarize recent findings in the literature and provide an update on the complex cellular heterogeneity and cell- and circuit-specific regulation of motivated behaviors and reinforcement by the VP with a specific focus on mood and substance use disorders. In addition, we will discuss mechanisms by which stress and drug exposure alter the functioning of the VP and produce susceptibility to neuropsychiatric disorders. Lastly, we will outline unanswered questions and identify future directions for studies necessary to further clarify the central role of VP neurons in the regulation of motivated behaviors. Significance: Research in the last decade has revealed a complex cell- and circuit-specific role for the VP in reward processing and the regulation of motivated behaviors. Novel insights obtained using cell- and circuit-specific interrogation strategies have led to a major shift in our understanding of this region. Here, we provide a comprehensive review of the VP in which we integrate novel findings with the existing literature and highlight the emerging role of the VP as a linchpin of the neural systems that regulate motivation, reward, and aversion. In addition, we discuss the dysfunction of the VP in animal models of neuropsychiatric disorders.

Astrocyte regulation of synaptic signaling in psychiatric disorders

Over the last 15 years, the field of neuroscience has evolved toward recognizing the critical role of astroglia in shaping neuronal synaptic activity and along with the pre- and postsynapse is now considered an equal partner in tripartite synaptic transmission and plasticity. The relative youth of this recognition and a corresponding deficit in reagents and technologies for quantifying and manipulating astroglia relative to neurons continues to hamper advances in understanding tripartite synaptic physiology. Nonetheless, substantial advances have been made and are reviewed herein. We review the role of astroglia in synaptic function and regulation of behavior with an eye on how tripartite synapses figure into brain pathologies underlying behavioral impairments in psychiatric disorders, both from the perspective of measures in postmortem human brains and more subtle influences on tripartite synaptic regulation of behavior in animal models of psychiatric symptoms. Our goal is to provide the reader a well-referenced state-of-the-art understanding of current knowledge and predict what we may discover with deeper investigation of tripartite synapses using reagents and technologies not yet available.

Involvement of GABAA receptors of lateral habenula in the acquisition and expression phases of morphine-induced place preference in male rats

The lateral habenula (LHb) is a critical brain structure involved in the aversive response to drug abuse. It has been determined that the gamma-aminobutyric acid (GABA)-ergic system plays the main role in morphine dependency. The role of GABA type A receptors (GABAARs) in LHb on morphine-induced conditioned place preference (CPP) remains unknown. In this study, the effect of bilateral intra-LHb microinjection of GABAAR agonist and antagonist on the acquisition and expression phases of CPP, utilizing a 5-day CPP paradigm in male rats, was evaluated. Subcutaneous administration of different doses of morphine caused a dose-dependent CPP. Intra-LHb microinjection of the GABAAR agonist, muscimol, in combination with morphine (5 mg/kg; subcutaneously) enhanced CPP scores in the acquisition phase of morphine CPP, whereas the GABAAR antagonist, bicuculline, significantly reduced the conditioning scores in the acquisition phase. Furthermore, pretreatment with a high dose of bicuculline reversed the additive effect of muscimol during the acquisition phase, yet the low dose of antagonist had no significant effect on agonist-induced CPP scores. On the other hand, muscimol (3 µg/rat) significantly increased CPP scores in the expression phase but bicuculline did not induce a significant effect on CPP scores. Bicuculline and muscimol microinjections did not affect locomotor activity in the testing sessions. Our results confirm that GABAARs in LHb play an active role in morphine reward. In addition, microinjections of bicuculline/muscimol may alter the morphine response through the GABAergic system.

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.

Ventral pallidum cellular and pathway specificity in drug seeking

The ventral pallidum (VP) is central to the reinforcing effects across a variety of drugs and relapse to drug seeking. Emerging studies from animal models of reinstatement reveal a complex neurobiology of the VP that contributes to different aspects of relapse to drug seeking. This review builds on classical understanding of the VP as part of the final common pathway of relapse but also discusses the properties of the VP as an independent structure. These include VP neural anatomical subregions, cellular heterogeneity, circuitry, neurotransmitters and peptides. Collectively, this review provides a current understanding of the VP from molecular to circuit level architecture that contributes to both the appetitive and aversive symptoms of drug addiction. We show the complex neurobiology of the VP in drug seeking, emphasizing its critical role in addiction, and review strategic approaches that target the VP to reduce relapse rates.

Treatment with dopamine β-hydroxylase (DBH) inhibitors prevents morphine use and relapse-like behavior in rats

Background

Opioid use disorders are serious contributors to the harms associated with the drug use. Unfortunately, therapeutic interventions for opioid addicts after detoxification have been limited and not sufficiently effective. Recently, several studies have led to promising results with disulfiram (DSF), a dopamine β-hydroxylase (DBH) inhibitor, showing that it is a potent agent against not only alcohol but also addiction to various drugs.

Materials and methods

This study was designed to examine whether DSF and nepicastat (NEP; another DBH inhibitor) modify morphine intake and reinstatement of seeking-behavior using the rat model of intravenous morphine self-administration. Additionally, we intended to estimate the effects of both inhibitors on the locomotor activity as well as on extracellular dopamine and its metabolite levels in the nucleus accumbens using microdialysis in naive rats.

Results

We demonstrated that both DBH inhibitors reduced responding to morphine self-administration. Moreover, DSF and NEP administered acutely before reinstatement test sessions consistently attenuated the reinforcing effects of morphine and a morphine-associated conditioned cue. The observed effects for lower doses (6.25–25 mg/kg; ip) of both DBH inhibitors seem to be independent of locomotor activity reduction and dopamine level in the nucleus accumbens. Neither DSF nor NEP administered daily during morphine abstinence with extinction training sessions had any effect on active lever-responding and changed the reinstatement induced by morphine priming doses. Reinstatement of drug-seeking behavior induced by a conditioned cue previously associated with morphine delivery was attenuated following repeated administration of DSF or NEP during the abstinence period.

Conclusion

These results seem to point to the significance  of DBH inhibition as a potential pharmacotherapy against morphine use disorders.

Graphic abstract

N-acetylcysteine in substance use disorder: a lesson from preclinical and clinical research

Substance use disorder (SUD) is a chronic brain condition, with compulsive and uncontrollable drug-seeking that leads to long-lasting and harmful consequences. The factors contributing to the development of SUD, as well as its treatment settings, are not fully understood. Alterations in brain glutamate homeostasis in humans and animals implicate a key role of this neurotransmitter in SUD, while the modulation of glutamate transporters has been pointed as a new strategy to diminish the excitatory glutamatergic transmission observed after drugs of abuse. N-acetylcysteine (NAC), known as a safe mucolytic agent, is involved in the regulation of this system and may be taken into account as a novel pharmacotherapy for SUD. In this paper, we summarize the current knowledge on the ability of NAC to reduce drug-seeking behavior induced by psychostimulants, opioids, cannabinoids, nicotine, and alcohol in animals and humans. Preclinical studies showed a beneficial effect in animal models of SUD, while the clinical efficacy of NAC has not been fully established. In summary, NAC will be a small add-on to usual treatment and/or psychotherapy for SUD, however, further studies are required.

GPCR-Based Dopamine Sensors-A Detailed Guide to Inform Sensor Choice for In vivo Imaging

Understanding how dopamine (DA) encodes behavior depends on technologies that can reliably monitor DA release in freely-behaving animals. Recently, red and green genetically encoded sensors for DA (dLight, GRAB-DA) were developed and now provide the ability to track release dynamics at a subsecond resolution, with submicromolar affinity and high molecular specificity. Combined with rapid developments in in vivo imaging, these sensors have the potential to transform the field of DA sensing and DA-based drug discovery. When implementing these tools in the laboratory, it is important to consider there is not a 'one-size-fits-all' sensor. Sensor properties, most importantly their affinity and dynamic range, must be carefully chosen to match local DA levels. Molecular specificity, sensor kinetics, spectral properties, brightness, sensor scaffold and pharmacology can further influence sensor choice depending on the experimental question. In this review, we use DA as an example; we briefly summarize old and new techniques to monitor DA release, including DA biosensors. We then outline a map of DA heterogeneity across the brain and provide a guide for optimal sensor choice and implementation based on local DA levels and other experimental parameters. Altogether this review should act as a tool to guide DA sensor choice for end-users.

Extinction Training after Cocaine Self-Administration Influences the Epigenetic and Genetic Machinery Responsible for Glutamatergic Transporter Gene Expression in Male Rat Brain

Glutamate is a key excitatory neurotransmitter in the central nervous system. The balance of glutamatergic transporter proteins allows long-term maintenance of glutamate homeostasis in the brain, which is impaired during cocaine use disorder. The aim of this study was to investigate changes in the gene expression of SLC1A2 (encoding GLT-1), and SLC7A11 (encoding xCT), in rat brain structures after short-term (3 days) and long-term (10 days) extinction training using microarray analysis and quantitative real-time PCR. Furthermore, we analyzed the expression of genes encoding transcription factors, i.e., NFKB1 and NFKB2 (encoding NF-κB), PAX6, (encoding Pax6), and NFE2L2 (encoding Nrf2), to verify the correlation between changes in glutamatergic transporters and changes in their transcriptional factors and microRNAs (miRNAs; miR-124a, miR-543-3p and miR-342-3p) and confirm the epigenetic mechanism. We found reduced GLT-1 transcript and mRNA level in the prefrontal cortex (PFCTX) and dorsal striatum (DSTR) in rats that had previously self-administered cocaine after 3 days of extinction training, which was associated with downregulation of PAX6 (transcript and mRNA) and NFKB2 (mRNA) level in the PFCTX and with upregulation of miR-543-3p and miR-342-3p in the DSTR. The xCT mRNA level was reduced in the PFCTX and DSTR, and NFE2L2 transcript level in the PFCTX was decreased on the 3rd day of extinction training. In conclusion, 3-day drug-free period modulates GLT-1 and xCT gene expression through genetic and epigenetic mechanisms, and such changes in expression seem to be potential molecular targets for developing a treatment for cocaine-seeking behavior.

Cocaine Self-Administration and Abstinence Modulate NMDA Receptor Subunits and Active Zone Proteins in the Rat Nucleus Accumbens

Cocaine-induced plasticity in the glutamatergic transmission and its N-methyl-d-aspartate (NMDA) receptors are critically involved in the development of substance use disorder. The presynaptic active zone proteins control structural synaptic plasticity; however, we are still far from understanding the molecular determinants important for cocaine seeking behavior. The aim of this study was to investigate the effect of cocaine self-administration and different conditions of cocaine forced abstinence on the composition of the NMDA receptor subunits and on the levels of active zone proteins, i.e., Ras-related protein 3A (Rab3A), Rab3 interacting molecules 1 (RIM1) and mammalian uncoordinated protein 13 (Munc13) in the rat nucleus accumbens. We found an up-regulation of the accumbal levels of GluN1 and GluN2A following cocaine self-administration that was paralleled by an increase of Munc13 and RIM1 levels. At the same time, we also demonstrated that different conditions of cocaine abstinence abolished changes in NMDA receptor subunits (except for higher GluN1 levels after cocaine abstinence with extinction training), while an increase in the Munc13 concentration was shown in rats housed in an enriched environment. In conclusion, cocaine self-administration is associated with the specific up-regulation of the NMDA receptor subunit composition and is related with new presynaptic targets controlling neurotransmitter release. Moreover, changes observed in cocaine abstinence with extinction training and in an enriched environment in the levels of NMDA receptor subunit and in the active zone protein, respectively, may represent a potential regulatory step in cocaine-seeking behavior.

Adenosine A2AReceptors in Substance Use Disorders: A Focus on Cocaine

Several psychoactive drugs can evoke substance use disorders (SUD) in humans and animals, and these include psychostimulants, opioids, cannabinoids (CB), nicotine, and alcohol. The etiology, mechanistic processes, and the therapeutic options to deal with SUD are not well understood. The common feature of all abused drugs is that they increase dopamine (DA) neurotransmission within the mesocorticolimbic circuitry of the brain followed by the activation of DA receptors. D₂ receptors were proposed as important molecular targets for SUD. The findings showed that D₂ receptors formed heteromeric complexes with other GPCRs, which forced the addiction research area in new directions. In this review, we updated the view on the brain D₂ receptor complexes with adenosine (A)2A receptors (A_2AR) and discussed the role of A_2AR in different aspects of addiction phenotypes in laboratory animal procedures that permit the highly complex syndrome of human drug addiction. We presented the current knowledge on the neurochemical in vivo and ex vivo mechanisms related to cocaine use disorder (CUD) and discussed future research directions for A_2AR heteromeric complexes in SUD.

Acute cocaine treatment enhances the antagonistic allosteric adenosine A2A-dopamine D2 receptor-receptor interactions in rat dorsal striatum without increasing significantly extracellular dopamine levels

BACKGROUND

Antagonistic adenosine A2A receptor (A2AR)-dopamine D2 receptor (D2R) receptor-receptor interactions have previously been demonstrated in A2AR-D2R heteroreceptor complexes in the rat dorsal striatum. They mainly involve a reduction of affinity in the high-affinity component of the D2R agonist binding site upon activation in vivo of the A2AR by an A2AR agonist. Upon cocaine self-administration, this antagonistic A2AR-D2R interaction disappeared in the dorsal striatum.

METHODS

In the current experiments, it was tested whether such modifications in the antagonistic A2AR-D2R receptor-receptor interactions can develop also after an acute systemic injection of a low cocaine dose (1 mg/kg; sc).

RESULTS

Microdialysis experiments indicated that acute cocaine did not significantly alter the extracellular dopamine levels in the dorsal striatum of the awake Wistar rats. Competition dopamine receptor binding experiments demonstrated that in the acute cocaine group, the A2AR agonist CGS-21680 produced significantly larger increases in the D2R K_{i, High} values (reduction of high-affinity) versus the saline-injected (i.e. control) group. Furthermore, in the dorsal striatum membrane preparation from acute cocaine-injected rats, CGS-21680 also produced significant increases in the D2R K_{i, Low} values (reduction of low-affinity) and in the proportion of D2Rs in the high-affinity state (RH). Such significant effects were not observed with CGS-21680 in the control group.

CONCLUSIONS

The molecular mechanism involved in the acute cocaine-induced increase in the antagonistic allosteric A2AR-D2R receptor-receptor interactions may be an increased formation of higher-order complexes A2AR-D2R-sigma1R in which cocaine by binding to the sigma1R protomer also allosterically enhances the inhibitory A2AR-D2R interaction in this receptor complex.

A2AR Transmembrane 2 Peptide Administration Disrupts the A2AR-A2AR Homoreceptor but Not the A2AR-D2R Heteroreceptor Complex: Lack of Actions on Rodent Cocaine Self-Administration

It was previously demonstrated that rat adenosine A2AR transmembrane V peptide administration into the nucleus accumbens enhances cocaine self-administration through disruption of the A2AR-dopamine (D2R) heteroreceptor complex of this region. Unlike human A2AR transmembrane 4 (TM4) and 5 (TM5), A2AR TM2 did not interfere with the formation of the A2AR-D2R heteroreceptor complex in cellular models using BRET¹ assay. A2AR TM2 was proposed to be part of the of the receptor interface of the A2AR homomer instead and was therefore tested in the current article for effects on rat cocaine self-administration using rat A2AR synthetic TM2 peptide bilaterally injected into the nucleus accumbens. The injected A2AR TM2 peptide failed to significantly counteract the inhibitory action of the A2AR agonist CGS 21680 (0.1 mg/Kg) on cocaine self-administration. In line with these results, the microinjected A2AR TM2 peptide did not reduce the number of proximity ligation assay blobs identifying A2AR-D2R heteroreceptor complexes in the nucleus accumbens. In contrast, the A2AR TM2 peptide significantly reduced the number of A2AR-A2AR homoreceptor complexes in the nucleus accumbens. As to effects on the receptor–receptor interactions in the A2AR-D2R heteroreceptor complexes, the A2AR TM2 peptide did not alter the significant increase in the D2R Ki, high values produced by the A2AR agonist CGS 21680 ex vivo in the ventral striatum. The results indicate that the accumbal A2AR-A2AR homomeric complexes are not involved in mediating the A2AR agonist-induced inhibition of cocaine self-administration.

Extinction training following cocaine or MDMA self-administration produces discrete changes in D2-like and mGlu5 receptor density in the rat brain

BACKGROUND

Several studies strongly support the role of the dopamine D2-like and glutamate mGlu5 receptors in psychostimulant reward and relapse.

METHODS

The present study employed cocaine or MDMA self-administration with yoked-triad procedure in rats to explore whether extinction training affects the drug-seeking behavior and the D2-like and mGlu5 receptor Bmax and Kd values in several regions of the animal brain.

RESULTS

Both cocaine and MDMA rats developed maintenance of self-administration, but MDMA evoked lower response rates and speed of self-administration acquisition. During reinstatement tests, cocaine or MDMA seeking behavior was produced by either exposure to the drug-associated cues or drug-priming injections. The extinction training after cocaine self-administration did not alter significantly D2-like receptor expression the in the limbic and subcortical brain areas, while MDMA yoked rats showed a decrease of the D2-like binding density in the nucleus accumbens and increase in the hippocampus and a rise of affinity in the striatum and hippocampus. Interestingly, in the prefrontal cortex a reduction in the mGlu5 receptor density in cocaine- or MDMA-abstinent rats was demonstrated, with significant effects being observed after previous MDMA exposure. Moreover, rats self-administered cocaine showed a rise in the density of mGlu5 receptor for the nucleus accumbens.

CONCLUSION

This study first time shows that abstinence followed extinction training after cocaine or MDMA self- or passive-injections changes the D2-like and mGlu5 density and affinity. The observed changes in the expression of both receptors are brain-region specific and related to either pharmacological and/or motivational features of cocaine or MDMA.

Role of dopamine D2-like receptors and their modulation by adenosine receptor stimulation in the reinstatement of methamphetamine seeking

RATIONALE AND OBJECTIVE

Previous work has demonstrated that dopamine and adenosine receptors are involved in drug-seeking behaviors, yet the pharmacological interactions between these receptors in methamphetamine (MA) seeking are not well characterized. The present studies examined the role of the dopamine D₂-like receptors in MA seeking and identified the interactive effects of adenosine receptor stimulation.

METHODS

Adult male Sprague-Dawley rats were trained to lever press for MA in daily 2-h self-administration sessions on a fixed-ratio 1 schedule for 10 consecutive days. After 1 day of abstinence, lever pressing was extinguished in six daily extinction sessions. Treatments were administered systemically prior to a 2-h reinstatement test session.

RESULTS

An increase in MA seeking was observed following the administration of the dopamine D₂-like agonist, quinpirole, or the D₃ receptor agonist, 7-OH-DPAT. Stimulation of D₂ or D₄ receptors was ineffective at inducing MA seeking. Quinpirole-induced MA seeking was inhibited by D₃ receptor antagonism (SB-77011A or PG01037), an adenosine A₁ agonist, CPA, and an adenosine A_2A agonist, CGS 21680. MA seeking induced by a MA priming injection or D₃ receptor stimulation was inhibited by a pretreatment with the adenosine A₁ agonist, CPA, but not the adenosine A_2A agonist, CGS 21680.

CONCLUSIONS

These results demonstrate the sufficiency of dopamine D₃ receptors to reinstate MA seeking that is inhibited when combined with adenosine A₁ receptor stimulation.

Disruption of A2AR-D2R Heteroreceptor Complexes After A2AR Transmembrane 5 Peptide Administration Enhances Cocaine Self-Administration in Rats

Antagonistic allosteric A2AR-D2R receptor-receptor interactions in heteroreceptor complexes counteract cocaine self-administration and cocaine seeking in rats as seen in biochemical and behavioral experiments. It was shown that the human A2AR transmembrane five (TM5) was part of the interface of the human A2AR-D2R receptor heteromer. In the current paper, the rat A2AR synthetic TM5 (synthTM5) peptide disrupts the A2AR-D2R heteroreceptor complex in HEK293 cells as shown by the bioluminescence resonance energy transfer method. Rat A2AR synthTM5 peptide, microinjected into the nucleus accumbens, produced a complete counteraction of the inhibitory effects of the A2AR agonist CGS21680 on cocaine self-administration. It was linked to a disappearance of the accumbal A2AR-D2R heteroreceptor complexes and the A2AR agonist induced inhibition of D2R recognition using proximity ligation assay and biochemical binding techniques. However, possible effects of the A2AR synthTM5 peptide on accumbal A2AR-D3R and A2AR-D4R heteroreceptor complexes remain to be excluded. Evidence is provided that accumbal A2AR-D2R-like heteroreceptor complexes with their antagonistic receptor-receptor interactions can be major targets for treatment of cocaine use disorder.

Effects of Cocaine Self-Administration and Its Extinction on the Rat Brain Cannabinoid CB1 and CB2 Receptors

The aim of this study was to evaluate changes in the expression of cannabinoid type 1 (CB1) and 2 (CB2) receptor proteins in several brain regions in rats undergoing cocaine self-administration and extinction training. We used a triad-yoked procedure to distinguish between the motivational and pharmacological effects of cocaine. Using immunohistochemistry, we observed a significant decrease in CB1 receptor expression in the prefrontal cortex, dorsal striatum, and the basolateral and basomedial amygdala following cocaine (0.5 mg/kg/infusion) self-administration. Increased CB1 receptor expression in the ventral tegmental area in rats with previous cocaine exposure was also found. Following cocaine abstinence after 10 days of extinction training, we detected increases in the expression of CB1 receptors in the substantia nigra in both cocaine groups and in the subregions of the amygdala for only the yoked cocaine controls, while any method of cocaine exposure resulted in a decrease in CB2 receptor expression in the prefrontal cortex (p < 0.01), nucleus accumbens (p < 0.01), and medial globus pallidus (p < 0.01). Our findings further support the idea that the eCB system and CB1 receptors are involved in cocaine-reinforced behaviors. Moreover, we detected a cocaine-evoked adaptation in CB2 receptors in the amygdala, prefrontal cortex, and globus pallidus.

The Role of Adenosine Receptors in Psychostimulant Addiction

Adenosine receptors (AR) are a family of G-protein coupled receptors, comprised of four members, named A1, A2A, A2B, and A3 receptors, found widely distributed in almost all human body tissues and organs. To date, they are known to participate in a large variety of physiopathological responses, which include vasodilation, pain, and inflammation. In particular, in the central nervous system (CNS), adenosine acts as a neuromodulator, exerting different functions depending on the type of AR and consequent cellular signaling involved. In terms of molecular pathways and second messengers involved, A1 and A3 receptors inhibit adenylyl cyclase (AC), through Gi/o proteins, while A2A and A2B receptors stimulate it through Gs proteins. In the CNS, A1 receptors are widely distributed in the cortex, hippocampus, and cerebellum, A2A receptors are localized mainly in the striatum and olfactory bulb, while A2B and A3 receptors are found at low levels of expression. In addition, AR are able to form heteromers, both among themselves (e.g., A1/A2A), as well as with other subtypes (e.g., A2A/D2), opening a whole range of possibilities in the field of the pharmacology of AR. Nowadays, we know that adenosine, by acting on adenosine A1 and A2A receptors, is known to antagonistically modulate dopaminergic neurotransmission and therefore reward systems, being A1 receptors colocalized in heteromeric complexes with D1 receptors, and A2A receptors with D2 receptors. This review documents the present state of knowledge of the contribution of AR, particularly A1 and A2A, to psychostimulants-mediated effects, including locomotor activity, discrimination, seeking and reward, and discuss their therapeutic relevance to psychostimulant addiction. Studies presented in this review reinforce the potential of A1 agonists as an effective strategy to counteract psychostimulant-induced effects. Furthermore, different experimental data support the hypothesis that A2A/D2 heterodimers are partly responsible for the psychomotor and reinforcing effects of psychostimulant drugs, such as cocaine and amphetamine, and the stimulation of A2A receptor is proposed as a potential therapeutic target for the treatment of drug addiction. The overall analysis of presented data provide evidence that excitatory modulation of A1 and A2A receptors constitute promising tools to counteract psychostimulants addiction.

Prolonged withdrawal from cocaine self-administration affects prefrontal cortex- and basolateral amygdala-nucleus accumbens core circuits but not accumbens GABAergic local interneurons

Withdrawal from extended-access cocaine self-administration leads to progressive intensification ('incubation') of cocaine craving. After prolonged withdrawal (1-2 months), when craving is high, expression of incubation depends on strengthening of excitatory inputs to medium spiny neurons (MSN) of the nucleus accumbens (NAc). These excitatory inputs interact with the intra-NAc GABAergic 'microcircuit', composed of MSN axon collaterals and GABAergic interneurons. Here, we investigated whether the increased glutamatergic neurotransmission observed after prolonged withdrawal is accompanied by altered GABAergic neurotransmission, focusing on NAc core. Rats self-administered cocaine or saline (6 hours/day) and then underwent >40 days of withdrawal. First, we investigated parvalbumin positive (PV+) interneurons, GABAergic fast-spiking interneurons that regulate MSN activity. Immunohistochemical studies revealed no significant change in PV signal intensity or the number of PV+ cells in cocaine rats versus saline controls. We then screened PV and other interneuron markers using immunoblotting. We detected no changes in levels of PV, calretinin, calbindin or neuronal nitric oxide synthase. Because expression of these markers is activity dependent, our results suggest no marked changes in interneuron activity. Finally, we utilized local field potential recording, which can detect GABA-mediated alterations at the circuit level, to investigate potential changes in two circuits implicated in cocaine craving: prelimbic prefrontal cortex to NAc core and basolateral amygdala to NAc core. We detected differential adaptations in these circuits, some of which may involve GABA. Overall, our results suggest that alterations in GABA transmission may accompany incubation of cocaine craving, but they are circuit specific and less pronounced than alterations in glutamate transmission.

Molecular and synaptic mechanisms regulating drug-associated memories: Towards a bidirectional treatment strategy

The successful treatment of substance use disorders is dependent on the establishment of a long-term abstinent state. Relapse can be suppressed by interfering with memories of drug use that are evoked by re-exposure to drug-associated contexts and cues. Two strategies for accomplishing this goal are either to prevent drug-memory reconsolidation or to induce the formation of a competing, extinction memory. However, clinical attempts to prolong abstinence by behavioral modification of drug-related memories have had limited success. One approach to improve behavioral treatment strategies is to identify the molecular mechanisms that regulate these memory processes and then use pharmacological tools as supplements to improve efficacy. Still, due to the involvement of several overlapping signaling cascades in both reconsolidation and extinction, it is difficult to specifically modify one of the two processes. For example, attempting to elicit extinction may instead initiate reconsolidation, resulting in the unintentional strengthening of drug-related memories. A better approach is to identify diverging components of the two processes, whereby a single medication would simultaneously weaken reconsolidation and enhance extinction. This review will provide an overview of the neural substrates that are involved in the regulation of drug-associated memories, and will discuss emerging approaches to pharmacologically weaken these memories, including recent efforts to precisely and bidirectionally target reconsolidation and extinction. Ultimately, pharmacologically-enhanced memory-based approaches have the potential to produce more informed relapse-prevention therapies.

Cocaine self-administration specifically increases A2AR-D2R and D2R-sigma1R heteroreceptor complexes in the rat nucleus accumbens shell. Relevance for cocaine use disorder

Adenosine 2A receptor (A2AR) agonists were indicated to reduce cocaine reward and cocaine seeking mainly through activation of antagonistic allosteric A2AR-dopamine D2R (D2R) interactions in A2AR-D2R heteroreceptor complexes. Furthermore, it was shown that modulation of cocaine reward involves antagonistic A2AR-D2R interactions in the ventral but not the dorsal striatum in rats. In the current work the proximity ligation assay (PLA) was used to further study the A2AR-D2R heteroreceptor complexes in the nucleus accumbens shell and core as well as the dorsal striatum under the influence of cocaine self-administration in rats. A significant increase in the A2AR-D2R PLA positive clusters was observed in the nucleus accumbens shell but not in the other regions vs yoked saline controls using the duolink software. Additionally, cocaine self-administration evoked a selective and significant increase in the density of D2R-sigma1R positive clusters in the nucleus accumbens shell vs yoked saline controls, while a significant reduction of the density of the D2R-sigma1R positive clusters was found in the dorsal part of the dorsal striatum. The results suggest that cocaine self-administration can reorganize A2AR and D2R into increased A2AR-D2R heteroreceptor complexes in the nucleus accumbens shell associated with increases in the D2R-sigma1R heteroreceptor complexes in this region. This reorganization can contribute to the demonstrated anti-cocaine actions of A2A receptor agonists and the putative formation of A2AR-D2R-sigma1R heterocomplexes.

Gamma-Aminobutyric Acidergic Projections From the Dorsal Raphe to the Nucleus Accumbens Are Regulated by Neuromedin U

BACKGROUND

Neuromedin U (NMU) is a neuropeptide enriched in the nucleus accumbens shell (NAcSh), a brain region associated with reward. While NMU and its receptor, NMU receptor 2 (NMUR2), have been studied for the ability to regulate food reward, NMU has not been studied in the context of drugs of abuse (e.g., cocaine). Furthermore, the neuroanatomical pathways that express NMUR2 and its ultrastructural localization are unknown.

METHODS

Immunohistochemistry was used to determine the synaptic localization of NMUR2 in the NAcSh and characterize which neurons express this receptor (n = 17). The functional outcome of NMU on NMUR2 was examined using microdialysis (n = 16). The behavioral effects of NMU microinjection directly to the NAcSh were investigated using cocaine-evoked locomotion (n = 93). The specific effects of NMUR2 knockdown on cocaine-evoked locomotion were evaluated using viral-mediated RNA interference (n = 40).

RESULTS

NMUR2 is localized to presynaptic gamma-aminobutyric acidergic nerve terminals in the NAcSh originating from the dorsal raphe nucleus. Furthermore, NMU microinjection to the NAcSh decreased local gamma-aminobutyric acid concentrations. Next, we evaluated the effects of NMU microinjection on behavioral sensitization to cocaine. When repeatedly administered throughout the sensitization regimen, NMU attenuated cocaine-evoked hyperactivity. Additionally, small hairpin RNA-mediated knockdown of presynaptic NMUR2 in the NAcSh using a retrograde viral vector potentiated cocaine sensitization.

CONCLUSIONS

Together, these data reveal that NMUR2 modulates a novel gamma-aminobutyric acidergic pathway from the dorsal raphe nucleus to the NAcSh to influence behavioral responses to cocaine.

Cocaine self-administration, extinction training and drug-induced relapse change metabotropic glutamate mGlu5 receptors expression: Evidence from radioligand binding and immunohistochemistry assays

Several behavioral findings highlight the importance of glutamatergic transmission and its metabotropic receptor type 5 (mGlu₅) in the controlling of cocaine reward and seeking behaviors. The molecular or neurochemical nature of such interactions is not well recognized, so in the present paper we determine if cocaine self-administration and extinction/reinstatement models with the yoked triad control procedure alter mGlu₅ receptor density in rats. [³H]MPEP was used to evaluate mGlu₅ receptors density and affinity in selected brain structures, while immunofluorescence analysis was used to detect changes in mGlu₅ receptors' brain location. Cocaine self-administration and yoked cocaine delivery evoked a significant elevation in mGlu₅ receptors' density in the dorsal striatum, while receptor protein expression was importantly elevated in the substantia nigra and reduced in the nucleus accumbens shell. Cocaine administration followed by 10 extinction training sessions resulted in biphasic mGlu₅ receptor density changes in the prefrontal cortex-nucleus accumbens pathway. mGlu₅ receptors' up-regulation was noted for cocaine self-administration and extinction training in the hippocampus and in yoked cocaine controls following drug abstinence in the dorsal striatum. A cocaine priming dose (but not a saline priming) resulted in a significant decrease of mGlu₅ receptors' density in the nucleus accumbens of rats previously treated with the drug and in the hippocampus of rats previously self-administered cocaine. The latter decrease in mGlu₅ receptors' density and protein expression in the hippocampus was parallel to an increase in [³H]MPEP affinity and opposite to a rise observed after single cocaine administration (ip) to drug-naïve yoked saline controls. Additionally, we also observed a significant elevation in the protein expression of the tested receptors in the limbic cortex in both cocaine groups. The present results shown modality dependent and brain-region specific changes in mGlu₅ receptors' localization and membrane specific binding.

Understanding the Functional Plasticity in Neural Networks of the Basal Ganglia in Cocaine Use Disorder: A Role for Allosteric Receptor-Receptor Interactions in A2A-D2 Heteroreceptor Complexes

Our hypothesis is that allosteric receptor-receptor interactions in homo- and heteroreceptor complexes may form the molecular basis of learning and memory. This principle is illustrated by showing how cocaine abuse can alter the adenosine A2AR-dopamine D2R heterocomplexes and their receptor-receptor interactions and hereby induce neural plasticity in the basal ganglia. Studies with A2AR ligands using cocaine self-administration procedures indicate that antagonistic allosteric A2AR-D2R heterocomplexes of the ventral striatopallidal GABA antireward pathway play a significant role in reducing cocaine induced reward, motivation, and cocaine seeking. Anticocaine actions of A2AR agonists can also be produced at A2AR homocomplexes in these antireward neurons, actions in which are independent of D2R signaling. At the A2AR-D2R heterocomplex, they are dependent on the strength of the antagonistic allosteric A2AR-D2R interaction and the number of A2AR-D2R and A2AR-D2R-sigma1R heterocomplexes present in the ventral striatopallidal GABA neurons. It involves a differential cocaine-induced increase in sigma1Rs in the ventral versus the dorsal striatum. In contrast, the allosteric brake on the D2R protomer signaling in the A2AR-D2R heterocomplex of the dorsal striatopallidal GABA neurons is lost upon cocaine self-administration. This is potentially due to differences in composition and allosteric plasticity of these complexes versus those in the ventral striatopallidal neurons.

Persistent strengthening of the prefrontal cortex - nucleus accumbens pathway during incubation of cocaine-seeking behavior

High rates of relapse after prolonged abstinence are often triggered by exposure to drug-associated cues that induce drug craving. Incubation of drug craving is a phenomenon that consists of time-dependent increases in cue-induced drug craving during withdrawal. Plasticity mechanisms in the nucleus accumbens (NAc) underlie drug-seeking responses and involve changes in excitatory synaptic transmission's efficacy. In particular, the prefrontal cortex (PFC) glutamatergic input to the NAc core has been well characterized regarding cocaine-evoked plasticity following non-contingent versus contingent exposure to cocaine or alternatively after protracted abstinence. Still, the synaptic strength during the course of withdrawal compared to drug-naïve condition is unknown, since electrophysiological characterizations are mainly performed in brain slices or focus on distinct time points during cocaine-evoked plasticity in vivo. Here we used an incubation paradigm, in which rats had extended accessed to cocaine self-administration, and underwent cue-induced reinstatement at withdrawal day 1 and 30. Longitudinal in vivo field potential recordings in awake rats showed that chronic contingent exposure to cocaine strengthened the prelimbic PFC to NAc core pathway when compared to pre-cocaine condition. This strengthening was associated with decreased paired-pulse ratios (PPR), indicative of presynaptic enhancement of glutamate release, which persisted throughout withdrawal. Moreover, both field potential increase and PPR reduction after chronic cocaine exposure correlated with the number of cocaine infusions received during training. The present results together with previous findings of withdrawal-dependent postsynaptic enhancement of the PFC-NAc core pathway, suggest an additional presynaptic strengthening that is initiated during self-administration and maintained throughout abstinence in drug-seeking rats. These cocaine-driven neuroadaptations may provide a neural substrate for maladaptive processing of cues that can ultimately trigger craving and relapse.

Behavioral History of Withdrawal Influences Regulation of Cocaine Seeking by Glutamate Re-Uptake

Withdrawal from cocaine regulates expression of distinct glutamate re-uptake transporters in the nucleus accumbens (NAc). In this study, we examined the cumulative effect of glutamate re-uptake by multiple excitatory amino acid transporters (EAATs) on drug-seeking at two different stages of withdrawal from self-administered cocaine. Rats were trained on fixed ratio 1 (FR1), progressing to FR5 schedule of reinforcement. After one day of withdrawal, microinfusion of a broad non-transportable EAAT antagonist, DL-threo-beta-benzyloxyaspartate (DL-TBOA), into the NAc shell dose-dependently attenuated self-administration of cocaine. Sucrose self-administration was not affected by DL-TBOA, indicating an effect specific to reinforcing properties of cocaine. The attenuating effect on cocaine seeking was not due to suppression of locomotor response, as DL-TBOA was found to transiently increase spontaneous locomotor activity. Previous studies have established a role for EAAT2-mediated re-uptake on reinstatement of cocaine seeking following extended withdrawal and extinction training. We found that blockade of NAc shell EAATs did not affect cocaine-primed reinstatement of cocaine seeking. These results indicate that behavioral history of withdrawal influences the effect of re-uptake mediated glutamate clearance on cocaine seeking. Dynamic regulation of glutamate availability by re-uptake mechanisms may impact other glutamate signaling pathways to account for such differences.

N-acetylcysteine amide (AD4) reduces cocaine-induced reinstatement

RATIONALE

Chronic exposure to drugs of abuse changes glutamatergic transmission in human addicts and animal models. N-acetylcysteine (NAC) is a cysteine prodrug that indirectly activates cysteine-glutamate antiporters. In the extrasynaptic space, NAC restores basal glutamate levels during drug abstinence and normalizes increased glutamatergic tone in rats during reinstatement to drugs of abuse. In initial clinical trials, repeated NAC administration seems to be promising for reduced craving in cocaine addicts.

OBJECTIVE

In this study, NAC-amide, called AD4 or NACA, was examined in intravenous cocaine self-administration and extinction/reinstatement procedures in rats. We investigated the behavioral effects of AD4 in the olfactory bulbectomized (OBX) rats, considered an animal model of depression. Finally, we tested rats injected with AD4 or NAC during 10-daily extinction training sessions to examine subsequent cocaine seeking.

RESULTS

AD4 (25-75 mg kg(-1)) given acutely did not alter the rewarding effects of cocaine in OBX rats and sham-operated controls. However, at 6.25-50 mg kg(-1), AD4 decreased dose-dependently cocaine seeking and relapse triggered by cocaine priming or drug-associated conditioned cues in both phenotypes. Furthermore, repeated treatment with AD4 (25 mg kg(-1)) or NAC (100 mg kg(-1)) during daily extinction trials reduced reinstatement of drug-seeking behavior in sham-operated controls. In the OBX rats only, AD4 effectively blocked cocaine-seeking behavior.

CONCLUSIONS

Our results demonstrate that AD4 is effective at blocking cocaine-seeking behavior, highlighting its potential clinical use toward cocaine use disorder.

Beneficial impact of intracerebroventricular fractalkine administration on behavioral and biochemical changes induced by prenatal stress in adult rats: Possible role of NLRP3 inflammasome pathway

Several lines of evidence indicate that adverse experience in early life may be a triggering factor for pathological inflammatory processes and lead to the development of depression. Fractalkine (CX3CL1), a chemokine, plays an important role not only in the migration, differentiation and proliferation of neuronal and glial cells but also in the regulation of neuronal-microglial signaling and the production of pro-inflammatory factors. In the present study, we examined the impact of a prenatal stress procedure on the expression of fractalkine in the hippocampus and frontal cortex of young and adult male rats. Furthermore, we measured the age-dependent effect of stress during pregnancy on the expression of pro-inflammatory factors IL-1β, IL-18, TNF-α, IL-6, and CCL2 in both brain structures. Next, to illustrate the link between fractalkine signaling and the behavioral and biochemical changes induced by prenatal stress, adult prenatally stressed offspring were injected intracerebroventricularly (icv) with exogenous fractalkine. We reported that prenatal stress leads to long-lasting deficits in fractalkine signaling and enhanced inflammatory activation. The study demonstrates that icv administration of fractalkine attenuates the behavioural changes evoked by prenatal stress procedure in adult animals. Moreover, fractalkine administration, exhibits anti-inflammatory action, mainly in the frontal cortex of adult prenatally stressed rats. The effect of fractalkine is related to inhibition of NLRP3 inflammasome. However, its action on the other members of NOD-like receptor family (NLR) cannot be excluded. These findings provide new in vivo evidence that the behavioral and inflammatory disturbances observed in adult prenatally stressed rats may be related to long-lasting malfunctions in fractalkine signaling.

Surface expression of GABAA receptors in the rat nucleus accumbens is increased in early but not late withdrawal from extended-access cocaine self-administration

It is well established that cocaine-induced changes in glutamate receptor expression in the nucleus accumbens (NAc) play a significant role in animal models of cocaine addiction. Far less is known about cocaine-induced changes in GABA transmission, despite its importance in regulating NAc output via local interneurons and medium spiny neuron (MSN) axon collaterals (GABA 'microcircuit'). Here we investigated whether GABAA receptor surface or total expression is altered following an extended-access cocaine self-administration regimen that produces a time-dependent intensification (incubation) of cue-induced cocaine craving in association with strengthening of AMPA receptor (AMPAR) transmission onto MSN. Rats self-administered cocaine or saline (control condition) 6h/day for 10 days. NAc tissue was obtained and surface proteins biotinylated on three withdrawal days (WD) chosen to span incubation of craving and associated AMPAR plasticity: WD2, WD25 and WD48. Immunoblotting was used to measure total and surface expression of three GABAA receptor subunits (α1, α2, and α4) that are strongly expressed in the NAc. We found a transient increase in surface, but not total, expression of the α2 subunit on WD2 from cocaine self-administration, an effect that was no longer observed by WD25. The expression of α1 and α4 subunits was not altered at these withdrawal times. On WD48, when AMPAR transmission is significantly potentiated, we did not find any alteration in GABAA receptor surface or total expression. Our findings suggest that the strengthening of AMPAR-mediated glutamate transmission in the NAc is not accompanied by compensatory strengthening of GABAergic transmission through insertion of additional GABAA receptors.

Cocaine self-administration differentially affects allosteric A2A-D2 receptor-receptor interactions in the striatum. Relevance for cocaine use disorder

In the current study behavioral and biochemical experiments were performed to study changes in the allosteric A2AR-D2R interactions in the ventral and dorsal striatum after cocaine self-administration versus corresponding yoked saline control. By using ex vivo [(3)H]-raclopride/quinpirole competition experiments, the effects of the A2AR agonist CGS 21680 (100 nM) on the KiH and KiL values of the D2-like receptor (D2-likeR) were determined. One major result was a significant reduction in the D2-likeR agonist high affinity state observed with CGS 21680 after cocaine self-administration in the ventral striatum compared with the yoked saline group. The results therefore support the hypothesis that A2AR agonists can at least in part counteract the motivational actions of cocaine. This action is mediated via the D2-likeR by targeting the A2AR protomer of A2AR-D2-like R heteroreceptor complexes in the ventral striatum, which leads to the reduction of D2-likeR protomer recognition through the allosteric receptor-receptor interaction. In contrast, in the dorsal striatum the CGS 21680-induced antagonistic modulation in the D2-likeR agonist high affinity state was abolished after cocaine self-administration versus the yoked saline group probably due to a local dysfunction/disruption of the A2AR-D2-like R heteroreceptor complexes. Such a change in the dorsal striatum in cocaine self-administration can contribute to the development of either locomotor sensitization, habit-forming learning and/or the compulsive drug seeking by enhanced D2-likeR protomer signaling. Potential differences in the composition and stoichiometry of the A2AR-D2R heteroreceptor complexes, including differential recruitment of sigma 1 receptor, in the ventral and dorsal striatum may explain the differential regional changes observed in the A2A-D2-likeR interactions after cocaine self-administration.

Prolonged Induction of miR-212/132 and REST Expression in Rat Striatum Following Cocaine Self-Administration

Chronic exposure to cocaine in vivo induces long-term synaptic plasticity associated with the brain’s circuitry that underlies development of repetitive and automatic behaviors called habits. In fact, prolonged drug consumption results in aberrant expression of protein-coding genes and small regulatory RNAs, including miRNAs that are involved in synaptic plasticity and neuroadaptations. However, the mechanisms mediating cocaine use disorder are still not fully understood. The present study is designed to examine the expression of miR-124, miR-132, miR-134, and miR-212, as well as the levels of the Ago2, Pum2, and REST mRNAs and proteins implicated in their regulation. We applied rat cocaine self-administration (SA) and extinction training procedures with a yoked triad to assess the changes in the levels of four miRNAs and three protein-coding genes and corresponding proteins in the dorsal striatum. We demonstrated that elevated expression of mature miR-212 and miR-132 is long-lasting and persists in the drug-free period (till 10-day abstinence). Moreover, mRNA and protein of REST, a regulator of neuronal transcription, was raised selectively in cocaine self-administering rats and Ago2 transcript decreased after cocaine treatment. Unexpectedly, the expression level of Ago2 and Pum2 proteins changed only in the active cocaine-receiving animals. These results point out the important aspects of long-lasting alterations in microRNAs, genes, and protein expressions involved in the control of synaptic plasticity associated with reward and motivation learning related to cocaine addiction.

The Nociceptin Receptor as an Emerging Molecular Target for Cocaine Addiction

Cocaine addiction is a global public health and socioeconomic issue that requires pharmacological and cognitive therapies. Currently there are no FDA-approved medications to treat cocaine addiction. However, in preclinical studies, interventions ranging from herbal medicine to deep-brain stimulation have shown promise for the therapy of cocaine addiction. Recent developments in molecular biology, pharmacology, and medicinal chemistry have enabled scientists to identify novel molecular targets along the pathways involved in drug addiction. In 1994, a receptor that showed a great deal of homology to the traditional opioid receptors was characterized. However, endogenous and exogenous opioids failed to bind to this receptor, which led scientists to name it opioid receptor-like receptor, now referred to as the nociceptin receptor. The endogenous ligand of NOPr was identified a year later and named orphanin FQ/nociceptin. Nociceptin and NOPr are widely distributed throughout the CNS and are involved in many physiological responses, such as food intake, nociceptive processing, neurotransmitter release, etc. Furthermore, exogenous nociceptin has been shown to regulate the activity of mesolimbic dopaminergic neurons, glutamate, and opioid systems, and the stress circuit. Importantly, exogenous nociceptin has been shown to reduce the rewarding and addictive actions of a number of drugs of abuse, such as psychostimulants, alcohol, and opioids. This paper reviews the existing literature on the role of endogenous nociceptin in the rewarding and addictive actions of cocaine. The effect of exogenous nociceptin on these processes is also reviewed. Furthermore, the effects of novel small-molecule NOPr ligands on these actions of cocaine are discussed. Overall, a review of the literature suggests that NOPr could be an emerging target for cocaine addiction pharmacotherapy.

Reward related neurotransmitter changes in a model of depression: An in vivo microdialysis study

OBJECTIVES

The self-medication hypothesis assumes that symptoms related to potential monoaminergic deficits in depression may be relieved by drug abuse. The aim of this study was to elucidate the neurotransmitter changes in a rat model of depression by measuring their levels in the nucleus accumbens shell, which is typically involved in the drug of abuse acquisition mechanism.

METHODS

Depression was modelled by the olfactory bulbectomy (OBX) in Wistar male rats. In vivo microdialysis was performed, starting from the baseline and following after a single methamphetamine injection and behaviour was monitored. The determination of neurotransmitters and their metabolites was performed by high-performance liquid chromatography combined with mass spectrometry.

RESULTS

OBX animals had lower basal levels of dopamine and serotonin and their metabolites. However, γ-aminobutyric acid (GABA) and glutamate levels were increased. The methamphetamine injection induced stronger dopamine and serotonin release in the OBX rats and lower release of glutamate in comparison with sham-operated rats; GABA levels did not differ significantly.

CONCLUSIONS

This study provides an evidence of mesolimbic neurotransmitter changes in the rat model of depression which may elucidate mechanisms underlying intravenous self-administration studies in which OBX rats were demonstrated to have higher drug intake in comparison to intact controls.

The ventral pallidum: Subregion-specific functional anatomy and roles in motivated behaviors

The ventral pallidum (VP) plays a critical role in the processing and execution of motivated behaviors. Yet this brain region is often overlooked in published discussions of the neurobiology of mental health (e.g., addiction, depression). This contributes to a gap in understanding the neurobiological mechanisms of psychiatric disorders. This review is presented to help bridge the gap by providing a resource for current knowledge of VP anatomy, projection patterns and subregional circuits, and how this organization relates to the function of VP neurons and ultimately behavior. For example, ventromedial (VPvm) and dorsolateral (VPdl) VP subregions receive projections from nucleus accumbens shell and core, respectively. Inhibitory GABAergic neurons of the VPvm project to mediodorsal thalamus, lateral hypothalamus, and ventral tegmental area, and this VP subregion helps discriminate the appropriate conditions to acquire natural rewards or drugs of abuse, consume preferred foods, and perform working memory tasks. GABAergic neurons of the VPdl project to subthalamic nucleus and substantia nigra pars reticulata, and this VP subregion is modulated by, and is necessary for, drug-seeking behavior. Additional circuits arise from nonGABAergic neuronal phenotypes that are likely to excite rather than inhibit their targets. These subregional and neuronal phenotypic circuits place the VP in a unique position to process motivationally relevant stimuli and coherent adaptive behaviors.

Increased Sensitivity to Cocaine Self-Administration in HIV-1 Transgenic Rats is Associated with Changes in Striatal Dopamine Transporter Binding

Cocaine abuse in HIV patients accelerates the progression and severity of neuropathology, motor impairment and cognitive dysfunction compared to non-drug using HIV patients. Cocaine and HIV interact with the dopamine transporter (DAT); however, the effect of their interaction on DAT binding remains understudied. The present study compared the dose-response functions for intravenous self-administration of cocaine and heroin between male HIV-1 transgenic (HIV-1 Tg) and Fischer 344 rats. The cocaine and heroin dose-response functions exhibit an inverted U-shape for both HIV-1 Tg and F344 rats. For cocaine, the number of infusions for each dose on the ascending limb was greater for HIV-1 Tg versus F344 rats. No significant changes in the heroin dose-response function were observed in HIV-1 Tg animals. Following the conclusion of self-administration experiments, DAT binding was assessed in striatal membranes. Saturation binding of the cocaine analog [(125)I] 3β-(4-iodophenyl)tropan-2β-carboxylic acid methyl ester ([(125)I]RTI-55) in rat striatal membranes resulted in binding curves that were best fit to a two-site binding model, allowing for calculation of dissociation constant (Kd) and binding density (Bmax) values that correspond to high- and low-affinity DAT binding sites. Control HIV-1 Tg rats exhibited a significantly greater affinity (i.e., decrease in Kd value) in the low-affinity DAT binding site compared to control F344 rats. Furthermore, cocaine self-administration in HIV-1 Tg rats increased low-affinity Kd (i.e., decreased affinity) compared to levels observed in control F344 rats. Cocaine also increased low-affinity Bmax in HIV-1 Tg rats as compared to controls, indicating an increase in the number of low-affinity DAT binding sites. F344 rats did not exhibit any change in high- or low-affinity Kd or Bmax values following cocaine or heroin self-administration. The increase in DAT affinity in cocaine HIV-1 Tg rats is consistent with the leftward shift of the ascending limb of the cocaine dose-response curve observed in HIV-1 Tg vs. F344 rats, and has major implications for the function of cocaine binding to DAT in HIV patients. The absence of HIV-related changes in heroin intake are likely due to less dopaminergic involvement in the mediation of heroin reward, further emphasizing the preferential influence of HIV on dopamine-related behaviors.

Serotonin-2C receptor agonists decrease potassium-stimulated GABA release in the nucleus accumbens

The serotonin 5-HT2C receptor has shown promise in vivo as a pharmacotherapeutic target for alcoholism. For example, recently, a novel 4-phenyl-2-N,N-dimethylaminotetralin (PAT) drug candidate, that demonstrates 5-HT2C receptor agonist activity together with 5-HT2A/2B receptor inverse agonist activity, was shown to reduce operant responding for ethanol after peripheral administration to rats. Previous studies have shown that the 5-HT2C receptor is found throughout the mesoaccumbens pathway and that 5-HT2C receptor agonism causes activation of ventral tegmental area (VTA) GABA neurons. It is unknown what effect 5-HT2C receptor modulation has on GABA release in the nucleus accumbens core (NAcc). To this end, microdialysis coupled to capillary electrophoresis with laser-induced fluorescence was used to quantify extracellular neurotransmitter concentrations in the NAcc under basal and after potassium stimulation conditions, in response to PAT analogs and other 5-HT2C receptor modulators administered by reverse dialysis to rats. 5-HT2C receptor agonists specifically attenuated stimulated GABA release in the NAcc while 5-HT2C antagonists or inverse agonists had no effect. Agents with activity at 5-HT2A receptors had no effect on GABA release. Thus, in contrast to results reported for the VTA, current results suggest 5-HT2C receptor agonists decrease stimulated GABA release in the NAcc, and provide a possible mechanism of action for 5HT2C -mediated negative modulation of ethanol self-administration.

A single brain-derived neurotrophic factor infusion into the dorsomedial prefrontal cortex attenuates cocaine self-administration-induced phosphorylation of synapsin in the nucleus accumbens during early withdrawal

BACKGROUND

Dysregulation in the prefrontal cortex-nucleus accumbens pathway has been implicated in cocaine addiction. We have previously demonstrated that one intra-dorsomedial prefrontal cortex brain-derived neurotrophic factor (BDNF) infusion immediately following the last cocaine self-administration session caused a long-lasting inhibition of cocaine-seeking and normalized the cocaine-induced disturbance of glutamate transmission in the nucleus accumbens after extinction and a cocaine prime. However, the molecular mechanism mediating the brain-derived neurotrophic factor effect on cocaine-induced alterations in extracellular glutamate levels is unknown.

METHODS

In the present study, we determined the effects of brain-derived neurotrophic factor on cocaine-induced changes in the phosphorylation of synapsin (p-synapsin), a family of presynaptic proteins that mediate synaptic vesicle mobilization, in the nucleus accumbens during early withdrawal.

RESULTS

Two hours after cocaine self-administration, p-synapsin Ser9 and p-synapsin Ser62/67, but not p-synapsin Ser603, were increased in the nucleus accumbens. At 22 hours, only p-synapsin Ser9 was still elevated. Elevations at both time points were attenuated by an intra-dorsomedial prefrontal cortex brain-derived neurotrophic factor infusion immediately after the end of cocaine self-administration. Brain-derived neurotrophic factor also reduced cocaine self-administration withdrawal-induced phosphorylation of the protein phosphatase 2A C-subunit, suggesting that brain-derived neurotrophic factor disinhibits protein phosphatase 2A C-subunit, consistent with p-synapsin Ser9 dephosphorylation. Further, co-immunoprecipitation demonstrated that protein phosphatase 2A C-subunit and synapsin are associated in a protein-protein complex that was reduced after 2 hours of withdrawal from cocaine self-administration and reversed by brain-derived neurotrophic factor.

CONCLUSIONS

Taken together, these findings demonstrate that brain-derived neurotrophic factor normalizes the cocaine self-administration-induced elevation of p-synapsin in nucleus accumbens that may underlie a disturbance in the probability of neurotransmitter release or represent a compensatory neuroadaptation in response to the hypofunction within the prefrontal cortex-nucleus accumbens pathway during cocaine withdrawal.

On the role of A₂A and D₂ receptors in control of cocaine and food-seeking behaviors in rats

Recent studies indicate that adenosine may influence dopamine neurotransmission via A2A receptors which antagonistically interact with D2 receptor-mediated signaling in the brain. We examined the effects of selective A2A receptor ligands such as the agonist CGS 21680 and the antagonists KW 6002 or SCH 58261 as well as of the D2-like receptor antagonist raclopride on reinstatement of cocaine seeking induced by cocaine, the cocaine-conditioned cue, or the D2-like receptor agonist quinpirole in rats. For comparison, effects of the A2A receptor ligands on reinstatement of food seeking were also studied. CGS 21680 significantly attenuated the reinstatement of cocaine (ip) seeking, and even more potently it reduced quinpirole (ip) or the cue-induced relapse of cocaine seeking as well as cue-induced food seeking. A potent reduction toward the cocaine-, quinpirole-, or cue-induced reinstatement of cocaine seeking was seen with raclopride. Pretreatment with KW 6002 or SCH 58261 reinstated cocaine seeking, and such increases were blocked by raclopride. In the higher doses, KW 6002 or SCH 58261 evoked food-seeking. In combination with the subthreshold dose of cocaine (2.5 mg/kg) or with the cue, low doses of KW 6002 but not SCH 58261 reinstated cocaine-seeking behavior, while none of the A2A receptor antagonists affected the cue-induced food-seeking behavior. The results indicate that A2A activation and D2-like receptor blockade counteract cocaine and food relapse. It is proposed that A2A receptor- and D2 receptor-mediated adenosine and dopamine signaling antagonistically interact in the striato-pallidal GABA neurons to regulate cocaine and food-seeking behavior.

On the role of adenosine (A)₂A receptors in cocaine-induced reward: a pharmacological and neurochemical analysis in rats

RATIONALE

Several studies have suggested the inhibitory control of adenosine (A)2A receptor stimulation in cocaine-induced behavioral actions.

OBJECTIVES

A combination of systemic or local drug injections and in vivo neurochemical analysis investigated A2A receptors in cocaine and food reward.

METHODS

Rats, trained to self-administer cocaine or food, were tested with the selective A2A receptor antagonists KW 6002 and SCH 58261 or the selective A2A receptor agonist CGS 21680. Extracellular dopamine, glutamate, and GABA levels in the nucleus accumbens and ventral pallidum were determined following intra-accumbal CGS 21680 administration during cocaine self-administration.

RESULTS

Neither KW 6002 nor SCH 58261 (0.25-1 mg/kg) altered cocaine self-administration (0.125-0.5 mg/kg/infusion), while CGS 21680 (0.2-0.4 mg/kg) produced a downward shift in the cocaine dose-response curve under a fixed ratio schedule of reinforcement and decreased the cocaine breaking point. CGS 21680 blocked also operant responding for food, while the A2A receptor antagonists were inactive. Local steady-state infusion of CGS 21680 (10 μM) during cocaine self-administration increased the active level pressing that was accompanied with reduced dopamine and increased GABA in the nucleus accumbens in the absence of changes in GABA and glutamate levels in the ventral pallidum. Pretreatment with systemic KW 6002 counteracted the increases in number of cocaine infusions seen after intra-accumbal administration of CGS 21680.

CONCLUSION

The findings support a role of A2A receptors in modulating goal-maintained behaviors. They also indicate that increased accumbal GABA release involving an antagonistic A2A-D2 receptor interaction can participate in mediating the inhibitory effects of the A2A agonist on cocaine reward.

Performance on a strategy set shifting task in rats following adult or adolescent cocaine exposure

RATIONALE

Neuropsychological testing is widespread in adult cocaine abusers, but lacking in teens. Animal models may provide insight into age-related neuropsychological consequences of cocaine exposure.

OBJECTIVES

The objective of the present study is to determine whether developmental plasticity protects or hinders behavioral flexibility after cocaine exposure in adolescent vs. adult rats.

METHODS

Using a yoked-triad design, one rat controlled cocaine delivery and the other two passively received cocaine or saline. Rats controlling cocaine delivery (1.0 mg/kg) self-administered for 18 sessions (starting P37 or P77), followed by 18 drug-free days. Rats next were tested in a strategy set shifting task, lasting 11-13 sessions.

RESULTS

Cocaine self-administration did not differ between age groups. During initial set formation, adolescent-onset groups required more trials to reach criterion and made more errors than adult-onset groups. During the set shift phase, rats with adult-onset cocaine self-administration experience had higher proportions of correct trials and fewer perseverative + regressive errors than age-matched yoked-controls or rats with adolescent-onset cocaine self-administration experience. During reversal learning, rats with adult-onset cocaine experience (self-administered or passive) required fewer trials to reach criterion, and the self-administering rats made fewer perseverative + regressive errors than yoked-saline rats. Rats receiving adolescent-onset yoked-cocaine had more trial omissions and longer lever press reaction times than age-matched rats self-administering cocaine or receiving yoked-saline.

CONCLUSIONS

Prior cocaine self-administration may impair memory to reduce proactive interference during set shifting and reversal learning in adult-onset but not adolescent-onset rats (developmental plasticity protective). Passive cocaine may disrupt aspects of executive function in adolescent-onset but not adult-onset rats (developmental plasticity hinders).