Structural plasticity associated with exposure to drugs of abuse

Salience and default mode network dysregulation in chronic cocaine users predict treatment outcome

While chronic cocaine use is associated with abnormalities in both brain structure and function within and interactions between regions, previous studies have been limited to interrogating structure and function independently, and the detected neural differences have not been applied to independent samples to assess the clinical relevance of results. We investigated consequences of structural differences on resting-state functional connectivity in cocaine addiction and tested whether resting-state functional connectivity of the identified circuits predict relapse in an independent cohort. Subjects included 64 non-treatment-seeking cocaine users (NTSCUs) and 67 healthy control subjects and an independent treatment-completed cohort (n = 45) of cocaine-dependent individuals scanned at the end of a 30-day residential treatment programme. Differences in cortical thickness and related resting-state functional connectivity between NTSCUs and healthy control subjects were identified. Survival analysis, applying cortical thickness of the identified regions, resting-state functional connectivity of the identified circuits and clinical characteristics to the treatment cohort, was used to predict relapse. Lower cortical thickness in bilateral insula and higher thickness in bilateral temporal pole were found in NTSCUs versus healthy control subjects. Whole brain resting-state functional connectivity analyses with these four different anatomical regions as seeds revealed eight weaker circuits including within the salience network (insula seeds) and between temporal pole and elements of the default mode network in NTSCUs. Applying these circuits and clinical characteristics to the independent cocaine-dependent treatment cohort, functional connectivity between right temporal pole and medial prefrontal cortex, combined with years of education, predicted relapse status at 150 days with 88% accuracy. Deficits in the salience network suggest an impaired ability to process physiologically salient events, while abnormalities in a temporal pole-medial prefrontal cortex circuit might speak to the social-emotional functional alterations in cocaine addiction. The involvement of the temporal pole-medial prefrontal cortex circuit in a model highly predictive of relapse highlights the importance of social-emotional functions in cocaine dependence, and provides a potential underlying neural target for therapeutic interventions, and for identifying those at high risk of relapse.

Opioid Self-Administration is Attenuated by Early-Life Experience and Gene Therapy for Anti-Inflammatory IL-10 in the Nucleus Accumbens of Male Rats

Early-life conditions can contribute to the propensity for developing neuropsychiatric disease, including substance abuse disorders. However, the long-lasting mechanisms that shape risk or resilience for drug addiction remain unclear. Previous work has shown that a neonatal handling procedure in rats (which promotes enriched maternal care) attenuates morphine conditioning, reduces morphine-induced glial activation, and increases microglial expression of the anti-inflammatory cytokine interleukin-10 (IL-10). We thus hypothesized that anti-inflammatory signaling may underlie the effects of early-life experience on later-life opioid drug-taking. Here we demonstrate that neonatal handling attenuates intravenous self-administration of the opioid remifentanil in a drug-concentration-dependent manner. Transcriptional profiling of the nucleus accumbens (NAc) from handled rats following repeated exposure to remifentanil reveals a suppression of pro-inflammatory cytokine and chemokine gene expression, consistent with an anti-inflammatory phenotype. To determine if anti-inflammatory signaling alters drug-taking behavior, we administered intracranial injections of plasmid DNA encoding IL-10 (pDNA-IL-10) into the NAc of non-handled rats. We discovered that pDNA-IL-10 treatment reduces remifentanil self-administration in a drug-concentration-dependent manner, similar to the effect of handling. In contrast, neither handling nor pDNA-IL-10 treatment alters self-administration of food or sucrose rewards. These collective observations suggest that neuroimmune signaling mechanisms in the NAc are shaped by early-life experience and may modify motivated behaviors for opioid drugs. Moreover, manipulation of the IL-10 signaling pathway represents a novel approach for influencing opioid reinforcement.

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.

Psychostimulants As Cognitive Enhancers in Adolescents: More Risk than Reward?

Methylphenidate and other psychostimulants, originally developed to treat attention deficit-hyperactivity disorder, are increasingly abused by healthy adolescents and adults seeking an advantage in scholastic performance and work productivity. However, how these drugs may affect cognitive performance, especially in the young brain, remains unclear. Here, we review recent literature and emphasize the risks of abuse of psychostimulants in healthy adolescents and young adults. We conclude that while the desire for cognitive enhancement, particularly with rising costs of education and increasingly competitive nature of scholarship programs, is unlikely to diminish in the near future, it is crucial for the scientific community to thoroughly examine the efficacy and safety of these stimulants in healthy populations across development. The current dearth of knowledge on the dose–response curve, metabolism, and cognitive outcomes in adolescents following methylphenidate or other psychostimulant exposure may be perpetuating a perception of these drugs as “safe” when that might not be true for developing brains.

Reversal of Cocaine-Associated Synaptic Plasticity in Medial Prefrontal Cortex Parallels Elimination of Memory Retrieval

Addiction is characterized by abnormalities in prefrontal cortex that are thought to allow drug-associated cues to drive compulsive drug seeking and taking. Identification and reversal of these pathologic neuroadaptations are therefore critical for treatment of addiction. Previous studies using rodents reveal that drugs of abuse cause dendritic spine plasticity in prelimbic medial prefrontal cortex (PL-mPFC) pyramidal neurons, a phenomenon that correlates with the strength of drug-associated memories in vivo. Thus, we hypothesized that cocaine-evoked plasticity in PL-mPFC may underlie cocaine-associated memory retrieval, and therefore disruption of this plasticity would prevent retrieval. Indeed, using patch clamp electrophysiology we find that cocaine place conditioning increases excitatory presynaptic and postsynaptic transmission in rat PL-mPFC pyramidal neurons. This was accounted for by increases in excitatory presynaptic release, paired-pulse facilitation, and increased AMPA receptor transmission. Noradrenergic signaling is known to maintain glutamatergic plasticity upon reactivation of modified circuits, and we therefore next determined whether inhibition of noradrenergic signaling during memory reactivation would reverse the cocaine-evoked plasticity and/or disrupt the cocaine-associated memory. We find that administration of the β-adrenergic receptor antagonist propranolol before memory retrieval, but not after (during memory reconsolidation), reverses the cocaine-evoked presynaptic and postsynaptic modifications in PL-mPFC and causes long-lasting memory impairments. Taken together, these data reveal that cocaine-evoked synaptic plasticity in PL-mPFC is reversible in vivo, and suggest a novel strategy that would allow normalization of prefrontal circuitry in addiction.

Atg5- and Atg7-dependent autophagy in dopaminergic neurons regulates cellular and behavioral responses to morphine

The molecular basis of chronic morphine exposure remains unknown. In this study, we hypothesized that macroautophagy/autophagy of dopaminergic neurons would mediate the alterations of neuronal dendritic morphology and behavioral responses induced by morphine. Chronic morphine exposure caused Atg5 (autophagy-related 5)- and Atg7 (autophagy-related 7)-dependent and dopaminergic neuron-specific autophagy resulting in decreased neuron dendritic spines and the onset of addictive behaviors. In cultured primary midbrain neurons, morphine treatment significantly reduced total dendritic length and complexity, and this effect could be reversed by knockdown of Atg5 or Atg7. Mice deficient for Atg5 or Atg7 specifically in the dopaminergic neurons were less sensitive to developing a morphine reward response, behavioral sensitization, analgesic tolerance and physical dependence compared to wild-type mice. Taken together, our findings suggested that the Atg5- and Atg7-dependent autophagy of dopaminergic neurons contributed to cellular and behavioral responses to morphine and may have implications for the future treatment of drug addiction.

Neuronal Subset-Specific Migration and Axonal Wiring Mechanisms in the Developing Midbrain Dopamine System

The midbrain dopamine (mDA) system is involved in the control of cognitive and motor behaviors, and is associated with several psychiatric and neurodegenerative diseases. mDA neurons receive diverse afferent inputs and establish efferent connections with many brain areas. Recent studies have unveiled a high level of molecular and cellular heterogeneity within the mDA system with specific subsets of mDA neurons displaying select molecular profiles and connectivity patterns. During mDA neuron development, molecular differences between mDA neuron subsets allow the establishment of subset-specific afferent and efferent connections and functional roles. In this review, we summarize and discuss recent work defining novel mDA neuron subsets based on specific molecular signatures. Then, molecular cues are highlighted that control mDA neuron migration during embryonic development and that facilitate the formation of selective patterns of efferent connections. The review focuses largely on studies that show differences in these mechanisms between different subsets of mDA neurons and for which in vivo data is available, and is concluded by a section that discusses open questions and provides directions for further research.

How methamphetamine exposure during different neurodevelopmental stages affects social behavior of adult rats?

Social behavior involves complex of different forms of interactions between individuals that is essential for healthy mental and physical development throughout lifespan. Psychostimulants, including methamphetamine (MA), have neurotoxic effect, especially, if they are targeting CNS during its critical periods of development. The present study was aimed on evaluation of changes in social interactions (SI) following scheduled prenatal/neonatal MA treatment in combination with acute application in adulthood. Eight groups of male and eight groups of female rats were tested in adulthood: rats, whose mothers were exposed to MA (5mg/ml/kg) or saline (SA, 1ml/kg) during the first half of gestation (ED 1-11), the second half of gestation (ED 12-22) and neonatal period (PD 1-11). To do this, we compared indirect neonatal applications via the exposed dams with group of rat pups that received MA or SA directly through injections. In adulthood, half animals from each group were injected with MA (1mg/kg), second half with saline 45min prior to the Social Interaction Test. Females and males were observed for social and nonsocial activities of two unfamiliar individuals of the same sex and treatment in a familiar Open field arena. The present study demonstrated that prenatal/neonatal MA exposure leads to decrease the time spent in genital investigation, following and nonsocial activity. Acute dose of MA leads to a decrease in all SI patterns and to an increase in nonsocial activities relative to acute SA. Females were more active than males. Animals exposed to prenatal/neonatal treatment during the second half of gestation (ED 12-22) and throughout lactation period (PD 1-11 indirect/direct) had fewer SI and greater exploratory behavior than animals exposed during the first half of gestation (ED 1-11).

Nicotine-induced and D1-receptor-dependent dendritic remodeling in a subset of dorsolateral striatum medium spiny neurons

Nicotine is one of the most addictive substances known, targeting multiple memory systems, including the ventral and dorsal striatum. One form of neuroplasticity commonly associated with nicotine is dendrite remodeling. Nicotine-induced dendritic remodeling of ventral striatal medium spiny neurons (MSNs) is well-documented. Whether MSN dendrites in the dorsal striatum undergo a similar pattern of nicotine-induced structural remodeling is unknown. A morphometric analysis of Golgi-stained MSNs in rat revealed a natural asymmetry in dendritic morphology across the mediolateral axis, with larger, more complex MSNs found in the dorsolateral striatum (DLS). Chronic nicotine produced a lasting (at least 21day) expansion in the dendritic complexity of MSNs in the DLS, but not dorsomedial striatum (DMS). Given prior evidence that MSN subtypes can be distinguished based on dendritic morphology, MSNs were segregated into morphological subpopulations based on the number of primary dendrites. Analysis of these subpopulations revealed that DLS MSNs with more primary dendrites were selectively remodeled by chronic nicotine exposure and remodeling was specific to the distal-most portions of the dendritic arbor. Co-administration of the dopamine D1 receptor (D1R) antagonist SCH23390 completely reversed the selective effects of nicotine on DLS MSN dendrite morphology, supporting a causal role for dopamine signaling at D1 receptors in nicotine-induced dendrite restructuring. Considering the functional importance of the DLS in shaping and expressing habitual behavior, these data support a model in which nicotine induces persistent and selective changes in the circuit connectivity of the DLS that may promote and sustain addiction-related behavior.

Does effect from developmental methamphetamine exposure on spatial learning and memory depend on stage of neuroontogeny?

Psychostimulants, including methamphetamine (MA), have neurotoxic effect, especially, if they are targeting CNS during its critical periods of development. The present study was aimed to examine cognitive changes after prenatal and neonatal MA treatment in combination with chronic MA exposure in adulthood of male rats. Eight groups of male rats were tested in adulthood: males whose mothers were exposed to MA (5 mg/kg) or saline (SA, 1 ml/kg) during the first half of gestation period (GD 1-11), the second half of gestation period (GD 12-22) and neonatal period (PD 1-11). In addition, we compared indirect neonatal application via the breast milk with the group of rat pups that received MA or SA directly by injection (PD 1-11). Males were tested in adulthood for cognitive changes in the Morris Water Maze (MWM). MWM experiment lasted for 12 days: Learning (Day 1-6), Probe test (Day 8) and Retrieval Memory test (Day 12). Each day of the MWM animals were injected with MA (1 mg/kg) or SA (1 ml/kg). Prenatal MA exposure did not induce changes in learning abilities of male rats, but neonatal exposure to MA leads to an increase search errors and latencies to find the hidden platform. Prenatal and also neonatal MA exposure impaired cognitive ability to remember the position of the platform in Retrieval Memory test in adulthood. Animals exposed to the prenatal treatment within the second half of gestation (ED 12-22) swam longer, slower and spent more time to find the hidden platform in Retrieval Memory test than animals exposed throughout other periods. The present study demonstrated that stage of development is crucial for determination the cognitive deficits induced by prenatal or neonatal MA exposure.

The effect of crack cocaine addiction and age on the microstructure and morphology of the human striatum and thalamus using shape analysis and fast diffusion kurtosis imaging

The striatum and thalamus are subcortical structures intimately involved in addiction. The morphology and microstructure of these have been studied in murine models of cocaine addiction (CA), showing an effect of drug use, but also chronological age in morphology. Human studies using non-invasive magnetic resonance imaging (MRI) have shown inconsistencies in volume changes, and have also shown an age effect. In this exploratory study, we used MRI-based volumetric and novel shape analysis, as well as a novel fast diffusion kurtosis imaging sequence to study the morphology and microstructure of striatum and thalamus in crack CA compared to matched healthy controls (HCs), while investigating the effect of age and years of cocaine consumption. We did not find significant differences in volume and mean kurtosis (MKT) between groups. However, we found significant contraction of nucleus accumbens in CA compared to HCs. We also found significant age-related changes in volume and MKT of CA in striatum and thalamus that are different to those seen in normal aging. Interestingly, we found different effects and contributions of age and years of consumption in volume, displacement and MKT changes, suggesting that each measure provides different but complementing information about morphological brain changes, and that not all changes are related to the toxicity or the addiction to the drug. Our findings suggest that the use of finer methods and sequences provides complementing information about morphological and microstructural changes in CA, and that brain alterations in CA are related cocaine use and age differently.

Structural alterations in the prefrontal cortex mediate the relationship between Internet gaming disorder and depressed mood

Adaptive gaming use has positive effects, whereas depression has been reported to be prevalent in Internet gaming disorder (IGD). However, the neural correlates underlying the association between depression and Internet gaming remain unclear. Moreover, the neuroanatomical profile of the striatum in IGD is relatively less clear despite its important role in addiction. We found lower gray matter (GM) density in the left dorsolateral prefrontal cortex (DLPFC) in the IGD group than in the Internet gaming control (IGC) group and non-gaming control (NGC) group, and the GM density was associated with lifetime usage of Internet gaming, depressed mood, craving, and impulsivity in the gaming users. Striatal volumetric analysis detected a significant reduction in the right nucleus accumbens (NAcc) in the IGD group and its association with lifetime usage of gaming and depression. These findings suggest that alterations in the brain structures involved in the reward system are associated with IGD-related behavioral characteristics. Furthermore, the DLPFC, involved in cognitive control, was observed to serve as a mediator in the association between prolonged gaming and depressed mood. This finding may provide insight into an intervention strategy for treating IGD with comorbid depression.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

Specific behavioral and cellular adaptations induced by chronic morphine are reduced by dietary omega-3 polyunsaturated fatty acids

Opiates, one of the oldest known drugs, are the benchmark for treating pain. Regular opioid exposure also induces euphoria making these compounds addictive and often misused, as shown by the current epidemic of opioid abuse and overdose mortalities. In addition to the effect of opioids on their cognate receptors and signaling cascades, these compounds also induce multiple adaptations at cellular and behavioral levels. As omega-3 polyunsaturated fatty acids (n-3 PUFAs) play a ubiquitous role in behavioral and cellular processes, we proposed that supplemental n-3 PUFAs, enriched in docosahexanoic acid (DHA), could offset these adaptations following chronic opioid exposure. We used an 8 week regimen of n-3 PUFA supplementation followed by 8 days of morphine in the presence of this diet. We first assessed the effect of morphine in different behavioral measures and found that morphine increased anxiety and reduced wheel-running behavior. These effects were reduced by dietary n-3 PUFAs without affecting morphine-induced analgesia or hyperlocomotion, known effects of this opiate acting at mu opioid receptors. At the cellular level we found that morphine reduced striatal DHA content and that this was reversed by supplemental n-3 PUFAs. Chronic morphine also increased glutamatergic plasticity and the proportion of Grin2B-NMDARs in striatal projection neurons. This effect was similarly reversed by supplemental n-3 PUFAs. Gene analysis showed that supplemental PUFAs offset the effect of morphine on genes found in neurons of the dopamine receptor 2 (D2)-enriched indirect pathway but not of genes found in dopamine receptor 1(D1)-enriched direct-pathway neurons. Analysis of the D2 striatal connectome by a retrogradely transported pseudorabies virus showed that n-3 PUFA supplementation reversed the effect of chronic morphine on the innervation of D2 neurons by the dorsomedial prefontal and piriform cortices. Together these changes outline specific behavioral and cellular effects of morphine that can be reduced or reversed by dietary n-3 PUFAs.

Individual differences in orexin-I receptor modulation of motivation for the opioid remifentanil

Orexin-1 receptors (Ox1Rs) have been implicated in the motivation for drugs of abuse. Here, we utilized a within-session behavioral-economics threshold procedure to screen for individual differences in economic demand for the ultra-short-acting opioid remifentanil and to test whether antagonism of Ox1Rs reduces remifentanil demand. The behavioral-economics procedure revealed robust individual differences in free consumption of remifentanil (Q₀ parameter; hedonic set point). Rats with low baseline Q₀ (low takers) displayed high demand elasticity (α parameter; reduced responding as drug price increased indicating low motivation for drug), whereas subjects with a higher Q₀ (high takers) exhibit low demand elasticity (low α) by continuing to self-administer remifentanil despite increased cost (reflecting higher motivation for drug). In a punished responding paradigm utilizing footshock, subjects that were classified as high takers at baseline withstood twice as much shock as low takers to continue self-administering remifentanil. Interestingly, Ox1R antagonism with SB-334867 reduced Q₀ and increased α in low takers but not in high takers. Similarly, the Ox1R antagonist attenuated cue-induced, but not drug-induced, reinstatement of remifentanil seeking in low takers but had no significant effect on reinstatement of drug seeking in high takers. Together, these data reveal a novel role of orexins in demand for remifentanil: Ox1Rs modulate demand in low takers but not in individuals that exhibit addictive-like behaviors (high takers). Finally, the behavioral assays in this study can serve as a novel laboratory model for studying individual differences in opioid use disorders.

Liking, wanting, and the incentive-sensitization theory of addiction

Rewards are both "liked" and "wanted," and those 2 words seem almost interchangeable. However, the brain circuitry that mediates the psychological process of "wanting" a particular reward is dissociable from circuitry that mediates the degree to which it is "liked." Incentive salience or "wanting," a form of motivation, is generated by large and robust neural systems that include mesolimbic dopamine. By comparison, "liking," or the actual pleasurable impact of reward consumption, is mediated by smaller and fragile neural systems, and is not dependent on dopamine. The incentive-sensitization theory posits the essence of drug addiction to be excessive amplification specifically of psychological "wanting," especially triggered by cues, without necessarily an amplification of "liking." This is because of long-lasting changes in dopamine-related motivation systems of susceptible individuals, called "neural sensitization." A quarter-century after its proposal, evidence has continued to grow in support the incentive-sensitization theory. Further, its scope is now expanding to include diverse behavioral addictions and other psychopathologies. (PsycINFO Database Record

Dynamic Alterations of Rat Nucleus Accumbens Dendritic Spines over 2 Months of Abstinence from Extended-Access Cocaine Self-Administration

Chronic cocaine exposure influences the density and morphology of dendritic spines on medium spiny neurons (MSNs) in the nucleus accumbens (NAc), a critical brain region for cocaine craving. However, the relationship between spine plasticity and craving remains unclear. To study this relationship, we trained rats to self-administer cocaine using an extended-access regimen (6 h per day, 10 days); controls self-administered saline. Previously, a time-dependent intensification (incubation) of cue-induced cocaine craving has been demonstrated after withdrawal from this regimen; furthermore, Ca²⁺-permeable AMPA receptors (CP-AMPARs) increase in the NAc core after ~1 month of withdrawal and thereafter mediate the expression of incubated craving. Although neither craving nor CP-AMPAR levels were measured in the present study, we killed rats at four withdrawal day (WD) time-points (WD14, WD25, WD36, or WD60) selected to span the rising phase of incubation and the transition from low to high CP-AMPAR levels. MSNs were iontophoretically filled with Lucifer yellow and spines were analyzed with NeuronStudio software. Compared with saline controls, cocaine rats showed no changes in spine density or morphology in the NAc core on WD14 or WD25. On WD36, approximately the withdrawal time when stable elevation of CP-AMPAR levels is detected, the cocaine group exhibited increased density of thin spines in the NAc core. By WD60, however, this effect had reversed: the density of thin spines was lower in cocaine rats compared with saline rats. In contrast, craving and CP-AMPAR levels remain high on WD60. We also assessed spine density on WD36 in the dorsolateral striatum, a region that is not implicated in incubation of cocaine craving and does not undergo CP-AMPAR plasticity. Here, the cocaine group exhibited a small leftward shift in the distribution of spine densities plotted as a cumulative distribution, opposite to the effect found in the NAc core. Overall, our results demonstrate changes in NAc core spines over 2 months of withdrawal but no simple relationship between the time dependency of these spine changes and the previously demonstrated time course of incubation of cocaine craving. However, they raise the possibility that CP-AMPAR accumulation in the NAc core occurs in a population of thin spines that emerges after ~1 month of withdrawal.

Nicotine-induced enhancement of Pavlovian alcohol-seeking behavior in rats

RATIONALE

Nicotine enhances responding elicited by Pavlovian cues that predict positive outcomes.

OBJECTIVES

We tested the hypothesis that nicotine acting at nicotinic acetylcholine receptors (nAChRs) would augment Pavlovian alcohol-seeking.

METHODS

Male, Long-Evans rats with unrestricted access to food and water were acclimated to drinking 15% ethanol in their home cages and then given Pavlovian conditioning sessions in which each trial of a 15-s conditioned stimulus (CS, 12 trials/session) was paired with 0.2 ml of ethanol (unconditioned stimulus, US, 2.4 ml/session). Entries into a port where ethanol was delivered were used to assess conditioning. Control groups received explicitly unpaired trials of the CS and US. In experiment 1, systemic injections of saline (1 ml/kg) or nicotine (0.4 mg/kg, freebase) were administered before each session. In experiments 2 and 3, an identical regimen of saline or nicotine injections was administered before the start of Pavlovian conditioning sessions.

RESULTS

All paired groups acquired conditioned port-entry responding to the CS, indicative of Pavlovian alcohol-seeking, whereas unpaired control group did not. Pre-session nicotine injections increased CS port-entries relative to saline, only in the paired group. This nicotine-induced enhancement of Pavlovian alcohol-seeking was blocked by pre-treatment with the nAChR antagonist mecamylamine. Prior exposure to nicotine did not influence the subsequent acquisition of Pavlovian alcohol-seeking.

CONCLUSIONS

These findings highlight for the first time that nicotine acting at nAChRs augments Pavlovian alcohol-seeking, specifically in non-restricted rats. Individuals who smoke and drink may thus be particularly susceptible to alcohol cues that could trigger further drinking.

Suppression of inhibitory G protein signaling in forebrain pyramidal neurons triggers plasticity of glutamatergic neurotransmission in the nucleus accumbens core

Cocaine and other drugs of abuse trigger long-lasting adaptations in excitatory and inhibitory neurotransmission in the mesocorticolimbic system, and this plasticity has been implicated in several key facets of drug addiction. For example, glutamatergic neurotransmission mediated by AMPA receptors (AMPAR) is strengthened in medium spiny neurons (MSNs) in the NAc core and shell during withdrawal following repeated in vivo cocaine administration. Repeated cocaine administration also suppresses inhibitory signaling mediated by G protein-gated inwardly rectifying K⁺ (GIRK) channels in pyramidal neurons of the prelimbic cortex, an important source of glutamatergic input to the NAc core that has been implicated in cocaine-seeking and behavioral sensitization. Here, we tested the hypothesis that suppression of GIRK channel activity in forebrain pyramidal neurons can promote plasticity of glutamatergic signaling in MSNs. Using novel conditional knockout mouse lines, we report that GIRK channel ablation in forebrain pyramidal neurons is sufficient to enhance AMPAR-dependent neurotransmission in D₁R-expressing MSNs in the NAc core, while also increasing motor-stimulatory responses to cocaine administration. A similar increase in AMPAR-dependent signaling was seen in both D₁R- and D₂R-expressing MSNs in the NAc core during withdrawal from repeated cocaine administration in normal mice. Collectively, these data are consistent with the premise that the cocaine-induced suppression of GIRK-dependent signaling in glutamatergic inputs to the NAc core contributes to some of the electrophysiological and behavioral hallmarks associated with repeated cocaine administration.

Visualizing Changes in Neuronal Dendritic Morphology in Response to Stress and Pharmacological Challenge

This unit outlines a protocol for Golgi staining, which has been used extensively to reliably and quantitatively assess alterations in dendritic arborization and spine density as a result of a variety of factors, including chronic administration of glucocorticoids, chronic stress, and pharmacological manipulations. The method stains neurons in their entirety, allowing for sophisticated analyses of branch lengths and numbers as well as patterns of dendritic branching. Advantages of the technique include its usefulness in multisite collaborations and its utility in visualizing neurons in multiple regions within the same brain. Given that it typically labels approximately one in one hundred neurons, many neurons per region of interest can be sampled per animal, greatly increasing the ability to obtain a representative sample of neurons. Limitations include its time-consuming nature, the hazardous chemicals employed, and the inability to use the stain to identify discrete subpopulations of neurons based on their projections, activation, or protein expression. © 2017 by John Wiley & Sons, Inc.

Corticostriatal circuitry in regulating diseases characterized by intrusive thinking

Intrusive thinking triggers clinical symptoms in many neuropsychiatric disorders. Using drug addiction as an exemplar disorder sustained in part by intrusive thinking, we explore studies demonstrating that impairments in corticostriatal circuitry strongly contribute to intrusive thinking. Neuroimaging studies have long implicated this projection in cue-induced craving to use drugs, and preclinical models show that marked changes are produced at corticostriatal synapses in the nucleus accumbens during a relapse episode. We delineate an accumbens microcircuit that mediates cue-induced drug seeking becoming an intrusive event. This microcircuit harbors many potential therapeutic targets. We focus on preclinical and clinical studies, showing that administering N-acetylcysteine restores uptake of synaptic glutamate by astroglial glutamate transporters and thereby inhibits intrusive thinking. We posit that because intrusive thinking is a shared endophenotype in many disorders, N-acetylcysteine has positive effects in clinical trials for a variety of neuropsychiatric disorders, including drug addiction, gambling, trichotillomania, and depression.

Rat-strain dependent changes of dendritic and spine morphology in the hippocampus after cocaine self-administration

We previously showed that cocaine self-administration increases spine density in CA1 hippocampal neurons in Lewis (LEW) but not in Fischer 344 (F344) rats. Dendritic spine morphology is intimately related to its function. Thus, we conducted a 3D morphological analysis of CA1 dendrites and dendritic spines in these two strains of rats. Strain-specific differences were observed prior to cocaine self-administration: LEW rats had significantly larger dendritic diameters but lower spine density than the F344 strain. After cocaine self-administration, proximal dendritic volume, dendritic surface area and spine density were increased in LEW rats, where a higher percentage of larger spines were also observed. In addition, we found a strong positive correlation between dendritic volume and spine morphology, and a moderate correlation between dendritic volume and spine density in cocaine self-administered LEW rats, an effect that was not evident in any other condition. By contrast, after cocaine self-administration, F334 rats showed decreased spine head volumes. Our findings suggest that genetic differences could play a key role in the structural plasticity induced by cocaine in CA1 pyramidal neurons. These cocaine-induced alterations could be related to differences in the memory processing of drug reward cues that could potentially explain differential individual vulnerability to cocaine addiction.

Effects of an acute cannabidiol treatment on cocaine self-administration and cue-induced cocaine seeking in male rats

Cannabidiol is a non-psychoactive compound that is the second most abundant component of cannabis. It has been shown to have a potential therapeutic value for a wide range of disorders, including anxiety, psychosis, and depression. Recently, it was suggested that cannabidiol might be a potential treatment for heroin craving and relapse. Here we investigated the effects of an acute treatment with cannabidiol on cocaine self-administration and cue-induced cocaine seeking in rats. Rats were trained to press a lever to self-administer cocaine (0.5 mg/kg/infusion), first under a fixed interval 20 s (FI-20 s) and then under a progressive ratio (PR) schedule of reinforcement. Cocaine self-administration under a PR schedule of reinforcement was not attenuated by cannabidiol injections (5.0 mg/kg and 10.0 mg/kg; i.p.) when tested 30 min and 24 h after treatment. Cannabidiol treatment (5.0 mg/kg or 10.0 mg/kg) also did not attenuate cue-induced cocaine seeking in rats after a withdrawal period of 14 days. In contrast, treatment with cannabidiol (10.0 mg/kg; i.p.) resulted in a statistically significant anxiolytic effect in the elevated plus-maze. Our findings suggest that, under the conditions described here, an acute cannabidiol treatment has a minimal effect on a rat model of cocaine intake and relapse.

Genome-Wide Association of Heroin Dependence in Han Chinese

Drug addiction is a costly and recurring healthcare problem, necessitating a need to understand risk factors and mechanisms of addiction, and to identify new biomarkers. To date, genome-wide association studies (GWAS) for heroin addiction have been limited; moreover they have been restricted to examining samples of European and African-American origin due to difficulty of recruiting samples from other populations. This is the first study to test a Han Chinese population; we performed a GWAS on a homogeneous sample of 370 Han Chinese subjects diagnosed with heroin dependence using the DSM-IV criteria and 134 ethnically matched controls. Analysis using the diagnostic criteria of heroin dependence yielded suggestive evidence for association between variants in the genes CCDC42 (coiled coil domain 42; p = 2.8x10^-7) and BRSK2 (BR serine/threonine 2; p = 4.110⁻⁶). In addition, we found evidence for risk variants within the ARHGEF10 (Rho guanine nucleotide exchange factor 10) gene on chromosome 8 and variants in a region on chromosome 20q13, which is gene-poor but has a concentration of mRNAs and predicted miRNAs. Gene-based association analysis identified genome-wide significant association between variants in CCDC42 and heroin addiction. Additionally, when we investigated shared risk variants between heroin addiction and risk of other addiction-related and psychiatric phenotypes using polygenic risk scores, we found a suggestive relationship with variants predicting tobacco addiction, and a significant relationship with variants predicting schizophrenia. Our genome wide association study of heroin dependence provides data in a novel sample, with functionally plausible results and evidence of genetic data of value to the field.

Reduction of Cocaine-Induced Locomotor Effects by Enriched Environment Is Associated with Cell-Specific Accumulation of ΔFosB in Striatal and Cortical Subregions

BACKGROUND

Early exposure to enriched environments has been shown to decrease the locomotor effects induced by repeated injections of cocaine and modify basal and cocaine-induced total protein levels of the transcription factor ΔFosB in the whole striatum of mice. In this study, we aimed at characterizing whether the profile of ΔFosB accumulation induced by enriched environments and cocaine would be similar or different in terms of brain areas and cell type.

METHODS

We used mice expressing the eGFP protein in D1 receptor positive (D1R(+)) neurons to determine whether Δ FosB induced by enriched environment or cocaine injections (5×15 mg/kg) would occur in selective subpopulations of neurons in several subregions of the striatum and prefrontal cortex.

RESULTS

We found that: (1) exposure to enriched environment reduces cocaine-induced locomotor activation, confirming our previous findings; (2) exposure to enriched environment by itself increases the accumulation of Δ FosB mostly in D1R(-) cells in the shell part of the nucleus accumbens and dorsal striatum, whereas in the nucleus accumbens core, Δ FosB accumulates in both D1R(+) and D1R(-) neurons; (3) in standard environment mice, cocaine induces accumulation of Δ FosB selectively in D1R(+) cells in the nucleus accumbens, dorsal striatum, and infralimbic cortex; and (4) the effects of enriched environments and cocaine on accumulation of Δ FosB were reciprocally blocked by their combination.

CONCLUSIONS

Altogether, these results suggest that the enriched environment-induced reduction in behavioral effects of cocaine might result from 2 distinct effects on ΔFosB in striatal medium-sized spiny neurons belonging to the direct and indirect pathways.

Role of Wnt/β-catenin pathway in the nucleus accumbens in long-term cocaine-induced neuroplasticity: a possible novel target for addiction treatment

Cocaine addiction is a chronic relapsing disorder characterized by the loss of control over drug-seeking and taking, and continued drug use regardless of adverse consequences. Despite years of research, effective treatments for psycho-stimulant addiction have not been identified. Persistent vulnerability to relapse arises from a number of long-lasting adaptations in the reward circuitry that mediate the enduring response to the drug. Recently, we reported that the activity of the canonical or Wnt/β-catenin pathway in the prefrontal cortex (PFC) is very important in the early stages of cocaine-induced neuroadaptations. In the present work, our main goal was to elucidate the relevance of this pathway in cocaine-induced long-term neuroadaptations that may underlie relapse. We found that a cocaine challenge, after a period of abstinence, induced an increase in the activity of the pathway which is revealed as an increase in the total and nuclear levels of β-catenin (final effector of the pathway) in the nucleus accumbens (NAcc), together with a decrease in the activity of glycogen synthase kinase 3β (GSK3β). Moreover, we found that the pharmacological modulation of the activity of the pathway has long-term effects on the cocaine-induced neuroplasticity at behavioral and molecular levels. All the results imply that changes in the Wnt/β-catenin pathway effectors are long-term neuroadaptations necessary for the behavioral response to cocaine. Even though more research is needed, the present results introduce the Wnt canonical pathway as a possible target to manage cocaine long-term neuroadaptations.

Exposure to Ketamine Anesthesia Affects Rat Impulsive Behavior

Introduction: Ketamine is a general anesthetic (GA) that activates several neurotransmitter pathways in various part of the brain. The acute effects as GA are the most well-known and sought-after: to induce loss of responsiveness and to produce immobility during invasive procedures. However, there is a concern that repeated exposure might induce behavioral changes that could outlast their acute effect. Most research in this field describes how GA affects cognition and memory. Our work is to access if general anesthesia with ketamine can disrupt the motivational behavior trait, more specifically measuring impulsive behavior. Methods: Aiming to evaluate the effects of exposure to repeat anesthetic procedures with ketamine in motivational behavior, we tested animals in a paradigm of impulsive behavior, the variable delay-to-signal (VDS). In addition, accumbal and striatal medium spiny neurons morphology was assessed. Results: Our results demonstrated that previous exposure to ketamine deep-anesthesia affects inhibitory control (impulsive behavior). Specifically, ketamine exposed animals maintain a subnormal impulsive rate in the initial periods of the delays. However, in longer delays while control animals progressively refrain their premature unrewarded actions, ketamine-exposed animals show a different profile of response with higher premature unrewarded actions in the last seconds. Animals exposed to multiple ketamine anesthesia also failed to show an increase in premature unrewarded actions between the initial and final periods of 3 s delays. These behavioral alterations are paralleled by an increase in dendritic length of medium spiny neurons of the nucleus accumbens (NAc). Conclusions: This demonstrates that ketamine anesthesia acutely affects impulsive behavior. Interestingly, it also opens up the prospect of using ketamine as an agent with the ability to modulate impulsivity trait.

Midbrain functional connectivity and ventral striatal dopamine D2-type receptors: link to impulsivity in methamphetamine users

Stimulant use disorders are associated with deficits in striatal dopamine receptor availability, abnormalities in mesocorticolimbic resting-state functional connectivity (RSFC) and impulsivity. In methamphetamine-dependent research participants, impulsivity is correlated negatively with striatal D2-type receptor availability, and mesocorticolimbic RSFC is stronger than that in controls. The extent to which these features of methamphetamine dependence are interrelated, however, is unknown. This question was addressed in two studies. In Study 1, 19 methamphetamine-dependent and 26 healthy control subjects underwent [¹⁸F]fallypride positron emission tomography to measure ventral striatal dopamine D2-type receptor availability, indexed by binding potential (BP_ND), and functional magnetic resonance imaging (fMRI) to assess mesocorticolimbic RSFC, using a midbrain seed. In Study 2, an independent sample of 20 methamphetamine-dependent and 18 control subjects completed the Barratt Impulsiveness Scale in addition to fMRI. Study 1 showed a significant group by ventral striatal BP_ND interaction effect on RSFC, reflecting a negative relationship between ventral striatal BP_ND and RSFC between the midbrain and striatum, orbitofrontal cortex and insula in methamphetamine-dependent participants, but a positive relationship in the control group. In Study 2, an interaction of the group with RSFC on impulsivity was observed. Methamphetamine-dependent users exhibited a positive relationship of midbrain RSFC to the left ventral striatum with cognitive impulsivity, whereas a negative relationship was observed in healthy controls. The results indicate that ventral striatal D2-type receptor signaling may affect the system-level activity within the mesocorticolimbic system, providing a functional link that may help explain high impulsivity in methamphetamine-dependent individuals.

The good and bad news about glutamate in drug addiction

In 1998 we published a perspective review describing how drug-induced neuroadaptations might serve towards understanding drug craving. We proposed experimental perspectives to help discern data relevant to long-lasting brain changes, and to distinguish dopamine-related changes that were largely pharmacological from glutamatergic changes that were based on drug-environment associations. These perspectives are embedded in drug abuse research, and the last 18 years has witnessed marked development in understanding addiction-associated corticostriatal glutamate plasticity. Here we propose three new perspectives on how the field might approach integrating and using the emerging data on glutamatergic adaptations. (1) Consider adaptations produced in kind across drug classes as most useful towards understanding shared characteristics of addiction, such as relapse. (2) Consider how drug-induced changes in glia and the extracellular matrix may contribute to synaptic alterations. (3) Make measurements not only at late withdrawal, but also during drug seeking events to capture transient changes that mediate active drug seeking that are shared across drug classes.

Estradiol impacts the endocannabinoid system in female rats to influence behavioral and structural responses to cocaine

Compared with men, women show enhanced responses to drugs of abuse, and consequently are thought to be more vulnerable to addiction. The ovarian hormone estradiol has emerged as a key facilitator in the heightened development of addiction in females. These actions of estradiol appear mediated by estrogen receptor (ER) activation of metabotropic glutamate receptor type 5 (mGluR5). However, the downstream effectors of this ER/mGluR5 signaling pathway are unknown. Here we investigate whether cannabinoid 1 receptor (CB1R) activation is a part of the mechanism whereby estradiol influences behavioral and synaptic correlates of addiction. Following repeated cocaine administration, estradiol-treated ovariectomized rats exhibited both sensitized locomotor responses and decreases in the dendritic spine density of nucleus accumbens core medium-spiny neurons in comparison to oil-treated controls. Both effects of estradiol were blocked by AM251, a CB1R inverse agonist. These results indicate that part of the signaling mechanism through which estradiol impacts behavioral and synaptic correlates of addiction in female rats requires activation of CB1Rs.

Acquisition of morphine conditioned place preference increases the dendritic complexity of nucleus accumbens core neurons

Contexts associated with opioid reward trigger craving and relapse in opioid addiction. Effects of reward-context associative learning on nucleus accumbens (NAc) dendritic morphology were studied using morphine conditioned place preference (CPP). Morphine-conditioned mice received saline and morphine 10 mg/kg subcutaneous (s.c.) on alternate days. Saline-conditioned mice received saline s.c. each day. Morphine-conditioned and saline-conditioned groups received injections immediately before each of eight daily conditioning sessions. Morphine homecage controls had no CPP training, but received saline and morphine in the homecage concomitantly with the morphine-conditioned group. Morphine conditioning produced greater place preference than saline conditioning. Mice were sacrificed 1 day after CPP expression. Dendritic changes were studied using Golgi-Cox staining and digital tracing of NAc core and shell neurons. In the NAc core, morphine homecage administration increased spine density, while morphine conditioning increased dendritic complexity, as defined by increased dendritic count, length and intersections. Place preference positively correlated with dendritic length and intersections in the NAc core. The core may mediate reward consolidation and determine how context-related signals from the shell lead to motor behavior. The combination of drug and conditioning in the morphine-conditioned group produced unique morphological effects different from the effects of drug or conditioning procedures by themselves. An additional study found no differences in neuron morphology between saline-conditioned mice, trained as described earlier, and mice that were not conditioned, but received saline in the homecage. The unique effect of morphine reward learning on NAc core dendrites reflects a brain substrate that could be targeted for therapeutic intervention in addiction.

Morphine-induced locomotor sensitization produces structural plasticity in the mesocorticolimbic system dependent on CB1-R activity

Changes in structural plasticity produced by the chronic exposure to drugs of abuse, such as alterations in dendritic spine densities, participate in the development of maladaptive learning processes leading to drug addiction. Understanding the neurobiological mechanisms involved in these aberrant changes is crucial to clarify the neurobiological substrate of addiction. Drug-induced locomotor sensitization has been widely accepted as a useful animal model to study these mechanisms related to drug addiction. We have evaluated the changes in structural plasticity in the mesocorticolimbic system involved in morphine-induced locomotor sensitization. The role of the cannabinoid receptor type 1 (CB1-R) in these neuroplastic alterations has also been studied using CB1-R-deficient (CB1-R KO) mice. Structural plasticity changes promoted by morphine are a highly dynamic phenomenon that evolves during the entire time course of the behavioral sensitization in wild-type (WT) animals. The different phases of the sensitization process were related to specific changes in connectivity between neurons revealed by modifications in dendritic spines in specific areas of the mesocorticolimbic system. Moreover, the lack of morphine-induced locomotor sensitization in CB1-R KO mice was accompanied by abnormal alterations in structural plasticity in the same mesocorticolimbic areas. These specific structural plasticity changes mediated by CB1-R activity seem necessary for the normal progression of morphine-induced locomotor sensitization and could play a critical role in the addictive process.

Transactivation of TrkB by Sigma-1 receptor mediates cocaine-induced changes in dendritic spine density and morphology in hippocampal and cortical neurons

Cocaine is a highly addictive narcotic associated with dendritic spine plasticity in the striatum. However, it remains elusive whether cocaine modifies spines in a cell type-specific or region-specific manner or whether it alters different types of synapses in the brain. In addition, there is a paucity of data on the regulatory mechanism(s) involved in cocaine-induced modification of spine density. In the current study, we report that cocaine exposure differentially alters spine density, spine morphology, and the types of synapses in hippocampal and cortical neurons. Cocaine exposure in the hippocampus resulted in increased spine density, but had no significant effect on cortical neurons. Although cocaine exposure altered spine morphology in both cell types, the patterns of spine morphology were distinct for each cell type. Furthermore, we observed that cocaine selectively affects the density of excitatory synapses. Intriguingly, in hippocampal neurons cocaine-mediated effects on spine density and morphology involved sigma-1 receptor (Sig-1 R) and its downstream TrkB signaling, which were not the case in cortical neurons. Furthermore, pharmacological inhibition of Sig-1 R prevented cocaine-induced TrkB activation in hippocampal neurons. Our findings reveal a novel mechanism by which cocaine induces selective changes in spine morphology, spine density, and synapse formation, and could provide insights into the cellular basis for the cognitive impairment observed in cocaine addicts.

Glucocorticoid Homeostasis in the Dentate Gyrus Is Essential for Opiate Withdrawal-Associated Memories

Drug-withdrawal-associated aversive memories might trigger relapse to drug-seeking behavior. However, changes in structural and synaptic plasticity, as well as epigenetic mechanisms, which may be critical for long-term aversive memory, have yet to be elucidated. We used male Wistar rats and performed conditioned-place aversion (CPA) paradigm to uncover the role of glucocorticoids (GCs) on plasticity-related processes that occur within the dentate gyrus (DG) during opiate-withdrawal conditioning (memory formation-consolidation) and after reactivation by re-exposure to the conditioned environment (memory retrieval). Rats subjected to conditioned morphine-withdrawal robustly expressed CPA, while adrenalectomy impaired naloxone-induced CPA. Importantly, while activity-regulated cytoskeletal-associated protein (Arc) expression was induced in sham- and ADX-dependent animals during the conditioning phase, Arc and early growth response 1 (Egr-1) induction was restricted to sham-dependent rats following memory retrieval. Moreover, we found a correlation between Arc induction and CPA score, and Arc was selectively expressed in the granular zone of the DG in dopaminoceptive, glutamatergic and GABAergic neurons. We further found that brain-derived neurotrophic factor was regulated in the opposite way during the test phase. Our results also suggest a role for epigenetic regulation on the expression of glucocorticoid receptors and Arc following memory retrieval. Our data provide the first evidence that GC homeostasis is important for the expression of long-term morphine-withdrawal memories. Moreover, our results support the idea that targeting Arc and Egr-1 in the DG may provide important insights into the role of these signaling cascades in withdrawal-context memory re-consolidation. Together, disrupting these processes in the DG might lead to effective treatments in drug addiction thereby rapidly and persistently reducing invasive memories and subsequent drug seeking.

Opposite Effects of mGluR1a and mGluR5 Activation on Nucleus Accumbens Medium Spiny Neuron Dendritic Spine Density

The group I metabotropic glutamate receptors (mGluR1a and mGluR5) are important modulators of neuronal structure and function. Although these receptors share common signaling pathways, they are capable of having distinct effects on cellular plasticity. We investigated the individual effects of mGluR1a or mGluR5 activation on dendritic spine density in medium spiny neurons in the nucleus accumbens (NAc), which has become relevant with the potential use of group I mGluR based therapeutics in the treatment of drug addiction. We found that systemic administration of mGluR subtype-specific positive allosteric modulators had opposite effects on dendritic spine densities. Specifically, mGluR5 positive modulation decreased dendritic spine densities in the NAc shell and core, but was without effect in the dorsal striatum, whereas increased spine densities in the NAc were observed with mGluR1a positive modulation. Additionally, direct activation of mGluR5 via CHPG administration into the NAc also decreased the density of dendritic spines. These data provide insight on the ability of group I mGluRs to induce structural plasticity in the NAc and demonstrate that the group I mGluRs are capable of producing not just distinct, but opposing, effects on dendritic spine density.

The brain at risk: the sepsis syndrome and lessons from preclinical experiments

There is a growing awareness of the chronic brain injury that results from the sepsis syndrome. We review experiments in several animal models of sepsis and show in one model, cecal ligation and puncture (CLP), that permanent structural pathology matures after the initial event. Specifically, we observed after exposure to CLP significant decreased spine density on the apical tree, but not the basal tree, of dendrites in the CA1 region of the dorsal hippocampus that was accompanied by a significantly diminished arbor of the apical dendrites, by 8 weeks, but not after 2 weeks. These novel data from dendritic arborizations elaborate information about a cohort of mice that had behaved in spatial memory tasks. These results raise questions about the relationship between long-term behavioral consequences and intervention strategies.

Longitudinal magnetic resonance imaging reveals striatal hypertrophy in a rat model of long-term stimulant treatment

Stimulant treatment is highly effective in mitigating symptoms associated with attention-deficit/hyperactivity disorder (ADHD), though the neurobiological underpinnings of this effect have not been established. Studies using anatomical magnetic resonance imaging (MRI) in children with ADHD have suggested that long-term stimulant treatment may improve symptoms of ADHD in part by stimulating striatal hypertrophy. This conclusion is limited, however, as these studies have either used cross-sectional sampling or did not assess the impact of treatment length on their dependent measures. We therefore used longitudinal anatomical MRI in a vehicle-controlled study design to confirm causality regarding stimulant effects on striatal morphology in a rodent model of clinically relevant long-term stimulant treatment. Sprague Dawley rats were orally administered either lisdexamfetamine (LDX, 'Vyvanse') or vehicle (N=12 per group) from postnatal day 25 (PD25, young juvenile) until PD95 (young adult), and imaged one day before and one day after the 70-day course of treatment. Our LDX dosing regimen yielded blood levels of dextroamphetamine comparable to those documented in patients. Longitudinal analysis of striatal volume revealed significant hypertrophy in LDX-treated animals when compared to vehicle-treated controls, with a significant treatment by time point interaction. These findings confirm a causal link between long-term stimulant treatment and striatal hypertrophy, and support utility of longitudinal MRI in rodents as a translational approach for bridging preclinical and clinical research. Having demonstrated comparable morphological effects in both humans and rodents using the same imaging technology, future studies may now use this rodent model to identify the underlying cellular mechanisms and behavioral consequences of stimulant-induced striatal hypertrophy.

Neonatal Masculinization Blocks Increased Excitatory Synaptic Input in Female Rat Nucleus Accumbens Core

Steroid sex hormones and genetic sex regulate the phenotypes of motivated behaviors and relevant disorders. Most studies seeking to elucidate the underlying neuroendocrine mechanisms have focused on how 17β-estradiol modulates the role of dopamine in striatal brain regions, which express membrane-associated estrogen receptors. Dopamine action is an important component of striatal function, but excitatory synaptic neurotransmission has also emerged as a key striatal substrate and target of estradiol action. Here, we focus on excitatory synaptic input onto medium spiny neurons (MSNs) in the striatal region nucleus accumbens core (AcbC). In adult AcbC, miniature excitatory postsynaptic current (mEPSC) frequency is increased in female compared with male MSNs. We tested whether increased mEPSC frequency in female MSNs exists before puberty, whether this increased excitability is due to the absence of estradiol or testosterone during the early developmental critical period, and whether it is accompanied by stable neuron intrinsic membrane properties. We found that mEPSC frequency is increased in female compared with male MSNs before puberty. Increased mEPSC frequency in female MSNs is abolished after neonatal estradiol or testosterone exposure. MSN intrinsic membrane properties did not differ by sex. These data indicate that neonatal masculinization via estradiol and/or testosterone action is sufficient for down-regulating excitatory synaptic input onto MSNs. We conclude that excitatory synaptic input onto AcbC MSNs is organized long before adulthood via steroid sex hormone action, providing new insight into a mechanism by which sex differences in motivated behavior and other AbcC functions may be generated or compromised.

Cocaine-Induced Synaptic Alterations in Thalamus to Nucleus Accumbens Projection

Exposure to cocaine induces addiction-associated behaviors partially through remodeling neurocircuits in the nucleus accumbens (NAc). The paraventricular nucleus of thalamus (PVT), which projects to the NAc monosynaptically, is activated by cocaine exposure and has been implicated in several cocaine-induced emotional and motivational states. Here we show that disrupting synaptic transmission of select PVT neurons with tetanus toxin activated via retrograde trans-synaptic transport of cre from NAc efferents decreased cocaine self-administration in rats. This projection underwent complex adaptations after self-administration of cocaine (0.75 mg/kg/infusion; 2 h/d × 5 d, 1d overnight training). Specifically, 1d after cocaine self-administration, we observed increased levels of AMPA receptor (AMPAR)-silent glutamatergic synapses in this projection, accompanied by a decreased ratio of AMPAR-to-NMDA receptor (NMDAR)-mediated EPSCs. Furthermore, the decay kinetics of NMDAR EPSCs was significantly prolonged, suggesting insertion of new GluN2B-containing NMDARs to PVT-to-NAc synapses. After 45-d withdrawal, silent synapses within this projection returned to the basal levels, accompanied by a return of the AMPAR/NMDAR ratio and NMDAR decay kinetics to the basal levels. In amygdala and infralimbic prefrontal cortical projections to the NAc, a portion of cocaine-generated silent synapses becomes unsilenced by recruiting calcium-permeable AMPARs (CP-AMPARs) after drug withdrawal. However, the sensitivity of PVT-to-NAc synapses to CP-AMPAR-selective antagonists was not changed after withdrawal, suggesting that CP-AMPAR trafficking is not involved in the evolution of cocaine-generated silent synapses within this projection. Meanwhile, the release probability of PVT-to-NAc synapses was increased after short- and long-term cocaine withdrawal. These results reveal complex and profound alterations at PVT-to-NAc synapses after cocaine exposure and withdrawal.

Glutamatergic mechanisms of comorbidity between acute stress and cocaine self-administration

There is substantial comorbidity between stress disorders and substance use disorders (SUDs), and acute stress augments the locomotor stimulant effect of cocaine in animal models. Here we endeavor to understand the neural underpinnings of comorbid stress disorders and drug use by determining whether the glutamatergic neuroadaptations that characterize cocaine self-administration are induced by acute stress. Rats were exposed to acute (2 h) immobilization stress, and 3 weeks later the nucleus accumbens core was examined for changes in glutamate transport, glutamate-mediated synaptic currents and dendritic spine morphology. We also determined whether acute stress potentiated the acquisition of cocaine self-administration. Acute stress produced an enduring reduction in glutamate transport and potentiated excitatory synapses on medium spiny neurons. Acute stress also augmented the acquisition of cocaine self-administration. Importantly, by restoring glutamate transport in the accumbens core with ceftriaxone the capacity of acute stress to augment the acquisition of cocaine self-administration was abolished. Similarly, ceftriaxone treatment prevented stress-induced potentiation of cocaine-induced locomotor activity. However, ceftriaxone did not reverse stress-induced synaptic potentiation, indicating that this effect of stress exposure did not underpin the increased acquisition of cocaine self-administration. Reversing acute stress-induced vulnerability to self-administer cocaine by normalizing glutamate transport poses a novel treatment possibility for reducing comorbid SUDs in stress disorders.

Regulator of G-Protein Signaling 7 Regulates Reward Behavior by Controlling Opioid Signaling in the Striatum

BACKGROUND

Morphine mediates its euphoric and analgesic effects by acting on the μ-opioid receptor (MOR). MOR belongs to the family of G-protein coupled receptors whose signaling efficiency is controlled by the regulator of G-protein signaling (RGS) proteins. Our understanding of the molecular diversity of RGS proteins that control MOR signaling, their circuit specific actions, and underlying cellular mechanisms is very limited.

METHODS

We used genetic approaches to ablate regulator of G-protein signaling 7 (RGS7) both globally and in specific neuronal populations. We used conditioned place preference and self-administration paradigms to examine reward-related behavior and a battery of tests to assess analgesia, tolerance, and physical dependence to morphine. Electrophysiology approaches were applied to investigate the impact of RGS7 on morphine-induced alterations in neuronal excitability and plasticity of glutamatergic synapses. At least three animals were used for each assessment.

RESULTS

Elimination of RGS7 enhanced reward, increased analgesia, delayed tolerance, and heightened withdrawal in response to morphine administration. RGS7 in striatal neurons was selectively responsible for determining the sensitivity of rewarding and reinforcing behaviors to morphine without affecting analgesia, tolerance, and withdrawal. In contrast, deletion of RGS7 in dopaminergic neurons did not influence morphine reward. RGS7 exerted its effects by controlling morphine-induced changes in excitability of medium spiny neurons in nucleus accumbens and gating the compositional plasticity of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid and N-methyl-D-aspartate receptors.

CONCLUSIONS

This study identifies RGS7 as a novel regulator of MOR signaling by dissecting its circuit specific actions and pinpointing its role in regulating morphine reward by controlling the activity of nucleus accumbens neurons.

Aggregated single-walled carbon nanotubes attenuate the behavioural and neurochemical effects of methamphetamine in mice

Methamphetamine (METH) abuse is a serious social and health problem worldwide. At present, there are no effective medications to treat METH addiction. Here, we report that aggregated single-walled carbon nanotubes (aSWNTs) significantly inhibited METH self-administration, METH-induced conditioned place preference and METH- or cue-induced relapse to drug-seeking behaviour in mice. The use of aSWNTs alone did not significantly alter the mesolimbic dopamine system, whereas pretreatment with aSWNTs attenuated METH-induced increases in extracellular dopamine in the ventral striatum. Electrochemical assays suggest that aSWNTs facilitated dopamine oxidation. In addition, aSWNTs attenuated METH-induced increases in tyrosine hydroxylase or synaptic protein expression. These findings suggest that aSWNTs may have therapeutic effects for treatment of METH addiction by oxidation of METH-enhanced extracellular dopamine in the striatum.

Signals from the Fourth Dimension Regulate Drug Relapse

Despite the enormous societal burden of alcohol and drug addiction and abundant research describing drug-induced maladaptive synaptic plasticity, there are few effective strategies for treating substance use disorders. Recent awareness that synaptic plasticity involves astroglia and the extracellular matrix is revealing new possibilities for understanding and treating addiction. We first review constitutive corticostriatal adaptations that are elicited by and shared between all abused drugs from the perspective of tetrapartite synapses, and integrate recent discoveries regarding cell type-specificity in striatal neurons. Next, we describe recent discoveries that drug-seeking is associated with transient synaptic plasticity that requires all four synaptic elements and is shared across drug classes. Finally, we prognosticate how considering tetrapartite synapses can provide new treatment strategies for addiction.

Quantitative EEG and Low-Resolution Electromagnetic Tomography (LORETA) Imaging of Patients Undergoing Methadone Treatment for Opiate Addiction

Methadone maintenance treatment (MMT) has been used as a treatment for opiate dependence since the mid-1960s. Evidence suggests that methadone binds to mu opiate receptors as do other opiates and induces changes in neurophysiological function. However, little is known, about how neural activity within the higher frequency gamma band (>30 Hz) while at rest changes in those stabilized on MMT despite its association with the excitation-inhibition balance within pyramidal-interneuron networks. Our study investigated differences in resting gamma power (37-41 Hz) between patients undergoing MMT for opiate dependence, illicit opiate users, and healthy controls subjects. Electroencephalographic data were recorded from 26 sites according to the international 10-20 system. Compared with the healthy controls subjects, people either undergoing MMT (mean difference [MD] = 0.32, 95% CI = 0.09-0.55, P < .01) or currently using illicit opiates (MD = 0.31, 95% CI = 0.06-0.56, P = .01) exhibited significant increased gamma power. The sLORETA (standardized low-resolution electromagnetic tomography) between-group comparison revealed dysfunctional neuronal activity in the occipital, parietal, and frontal lobes in the patients undergoing MMT. A more severe profile of dysfunction was observed in those using illicit opiates. Our findings suggest that long-term exposure to opioids is associated with disrupted resting state network, which may be reduced after MMT.

Drug-Paired Contextual Stimuli Increase Dendritic Spine Dynamics in Select Nucleus Accumbens Neurons

Repeated exposure to amphetamine leads to both associative conditioning and nonassociative sensitization. Here we assessed the contribution of neuronal ensembles in the nucleus accumbens (NAcc) to these behaviors. Animals exposed to amphetamine IP or in the ventral tegmental area (VTA) showed a sensitized locomotor response when challenged with amphetamine weeks later. Both exposure routes also increased ΔFosB levels in the NAcc. Further characterization of these ΔFosB+ neurons, however, revealed that amphetamine had no effect on dendritic spine density or size, indicating that these neurons do not undergo changes in dendritic spine morphology that accompany the expression of nonassociative sensitization. Additional experiments determined how neurons in the NAcc contribute to the expression of associative conditioning. A discrimination learning procedure was used to expose rats to IP or VTA amphetamine either Paired or Unpaired with an open field. As expected, compared with Controls, Paired rats administered IP amphetamine subsequently showed a conditioned locomotor response when challenged with saline in the open field, an effect accompanied by an increase in c-Fos+ neurons in the medial NAcc. Further characterization of these c-Fos+ cells revealed that Paired rats showed an increase in the density of dendritic spines and the frequency of medium-sized spines in the NAcc. In contrast, Paired rats previously exposed to VTA amphetamine showed neither conditioned locomotion nor conditioned c-Fos+ expression. Together, these results suggest a role for c-Fos+ neurons in the medial NAcc and rapid changes in the morphology of their dendritic spines in the expression of conditioning evoked by amphetamine-paired contextual stimuli.