A circuitry model of the expression of behavioral sensitization to amphetamine-like psychostimulants

Adolescent alcohol binge drinking and withdrawal: behavioural, brain GFAP-positive astrocytes and acute methamphetamine effects in adult female rats

RATIONALE

Alcopop beverages are generally the first alcoholic beverage that young females drink which contain high levels of sugar and alcohol. The over-consumption of these drinks may encourage alcohol co-administration with methamphetamine (METH) impacting on drinking behaviour and glial function.

AIMS

The aims of this study were to evaluate the effect of adolescent binge alcohol exposure on consumption level, anxiety-like behaviour, cross-sensitization with METH and on astrocyte expression in reward related brain regions.

METHODS

Adolescent female Sprague-Dawley rats had daily 1-hour oral alcohol consumption of alcopop (ALCP; with sucrose) or ethanol-only (ETOH; without sucrose), transitioned from 5 to 15% (v/v) ethanol content for 34 days. Water and sucrose groups act as controls. During alcohol withdrawal, rats were tested for anxiety on the elevated plus maze (EPM) and locomotor activity following saline or METH (1 mg/kg i.p) treatment. Brains were then collected to assess astrocyte immunofluorescence for glial fibrillary acidic protein (GFAP) in reward-related brain regions.

RESULTS

Rats pretreated with 5% ALCP consumed significantly more volume and ethanol intake when compared to 5% EtOH rats. Both ALCP and EtOH groups had a higher preference ratio for 5% than 15% alcohol solutions and ALCP rats had greater ethanol intake at 15% than EtOH rats. Alcohol withdrawal showed no significant differences between groups on anxiety, METH cross-sensitization effects or GFAP intensity in the regions studied.

CONCLUSIONS

Overall, the addition of sucrose to alcoholic solutions encouraged female rats to consume larger volumes and greater ethanol intake compared to ethanol-only solutions, yet did not have long lasting effects on behaviour and astrocytes.

Angiotensin II involvement in the development and persistence of amphetamine-induced sensitization: Striatal dopamine reuptake implications

Amphetamine (AMPH) exposure induces behavioural and neurochemical sensitization observed in rodents as hyperlocomotion and increased dopamine release in response to a subsequent dose. Brain Angiotensin II modulates dopaminergic neurotransmission through its AT1 receptors (AT1-R), positively regulating striatal dopamine synthesis and release. This work aims to evaluate the AT1-R role in the development and maintenance of AMPH-induced sensitization. Also, the AT1-R involvement in striatal dopamine reuptake was analysed. The sensitization protocol consisted of daily AMPH administration for 5 days and tested 21 days after withdrawal. An AT1-R antagonist, candesartan, was administered before or after AMPH exposure to evaluate the participation of AT1-R in the development and maintenance of sensitization, respectively. Sensitization was evaluated by locomotor activity and c-Fos immunostaining. Changes in dopamine reuptake kinetics were evaluated 1 day after AT1-R blockade withdrawal treatment, with or without the addition of AMPH in vitro. The social interaction test was performed as another behavioural output. Repeated AMPH exposure induced behavioural and neurochemical sensitization, which was prevented and reversed by candesartan. The AT1-R blockade increased the dopamine reuptake kinetics. Neither the AMPH administration nor the AT1-R blockade altered the performance of social interaction. Our results highlight the AT1-R's crucial role in AMPH sensitization. The enhancement of dopamine reuptake kinetics induced by the AT1-R blockade might attenuate the neuroadaptive changes that lead to AMPH sensitization and its self-perpetuation. Therefore, AT1-R is a prominent candidate as a target for pharmacological treatment of pathologies related to dopamine imbalance, including drug addiction and schizophrenia.

Drosophila learning and memory centers and the actions of drugs of abuse

Drug addiction and the circuitry for learning and memory are intimately intertwined. Drugs of abuse create strong, inappropriate, and lasting memories that contribute to many of their destructive properties, such as continued use despite negative consequences and exceptionally high rates of relapse. Studies in Drosophila melanogaster are helping us understand how drugs of abuse, especially alcohol, create memories at the level of individual neurons and in the circuits where they function. Drosophila is a premier organism for identifying the mechanisms of learning and memory. Drosophila also respond to drugs of abuse in ways that remarkably parallel humans and rodent models. An emerging consensus is that, for alcohol, the mushroom bodies participate in the circuits that control acute drug sensitivity, not explicitly associative forms of plasticity such as tolerance, and classical associative memories of their rewarding and aversive properties. Moreover, it is becoming clear that drugs of abuse use the mushroom body circuitry differently from other behaviors, potentially providing a basis for their addictive properties.

Effects of Stimulant Treatment on Changes in Brain Activation During Reward Notifications in Drug Naïve Youth With ADHD

BACKGROUND

Research examining the potential effects of stimulant exposure in childhood on subsequent development of substance use disorder (SUD) have focused on differences in the brain reward system as a function of risk.

METHODS

18 drug naïve children ages 7 to 12 years (11 High Risk [ADHD + ODD/CD]; 7 Low Risk [ADHD only]), underwent fMRI scans before and after treatment with mixed amphetamine salts, extended release (MAS-XR). We examined correlations between clinical ratings and fMRI activation at baseline and following treatment as a function of risk status.

RESULTS

High Risk children had higher activation than Low Risk children at baseline during both the Reward and Surprising Non-Reward conditions. Treatment produced strong differential effects on brain activation pertinent to group and reward outcome.

CONCLUSIONS

Findings support the hypothesized role of reward mechanisms in SUD risk, and suggest that stimulant treatment may have differential effects on reward processing in relation to SUD risk.

Repeated chemogenetic activation of dopaminergic neurons induces reversible changes in baseline and amphetamine-induced behaviors

RATIONALE

Repeated chemogenetic stimulation is often employed to study circuit function and behavior. Chronic or repeated agonist administration can result in homeostatic changes, but this has not been extensively studied with designer receptors exclusively activated by designer drugs (DREADDs).

OBJECTIVES

We sought to evaluate the impact of repeated DREADD activation of dopaminergic (DA) neurons on basal behavior, amphetamine response, and spike firing. We hypothesized that repeated DREADD activation would mimic compensatory effects that we observed with genetic manipulations of DA neurons.

METHODS

Excitatory hM3D(Gq) DREADDs were virally expressed in adult TH-Cre and WT mice. In a longitudinal design, clozapine N-oxide (CNO, 1.0 mg/kg) was administered repeatedly. We evaluated basal and CNO- or amphetamine (AMPH)-induced locomotion and stereotypy. DA neuronal activity was assessed using in vivo single-unit recordings.

RESULTS

Acute CNO administration increased locomotion, but basal locomotion decreased after repeated CNO exposure in TH-CrehM3Dq mice relative to littermate controls. Further, after repeated CNO administration, AMPH-induced hyperlocomotion and stereotypy were diminished in TH-CrehM3Dq mice relative to controls. Repeated CNO administration reduced DA neuronal firing in TH-CrehM3Dq mice relative to controls. A two-month CNO washout period rescued the decreases in basal locomotion and AMPH response.

CONCLUSIONS

We found that repeated DREADD activation of DA neurons evokes homeostatic changes that should be factored into the interpretation of chronic DREADD applications and their impact on circuit function and behavior. These effects are likely to also be seen in other neuronal systems and underscore the importance of studying neuroadaptive changes with chronic or repeated DREADD activation.

Interaction of Brain-Derived Neurotrophic Factor with the Effects of Chronic Methamphetamine on Prepulse Inhibition in Mice Is Independent of Dopamine D3 Receptors

The aim of the present study was to gain a better understanding of the role of brain-derived neurotrophic factor (BDNF) and dopamine D3 receptors in the effects of chronic methamphetamine (METH) on prepulse inhibition (PPI), an endophenotype of psychosis. We compared the effect of a three-week adolescent METH treatment protocol on the regulation of PPI in wildtype mice, BDNF heterozygous mice (HET), D3 receptor knockout mice (D3KO), and double-mutant mice (DM) with both BDNF heterozygosity and D3 receptor knockout. Chronic METH induced disruption of PPI regulation in male mice with BDNF haploinsufficiency (HET and DM), independent of D3 receptor knockout. Specifically, these mice showed reduced baseline PPI, as well as attenuated disruption of PPI induced by acute treatment with the dopamine receptor agonist, apomorphine (APO), or the glutamate NMDA receptor antagonist, MK-801. In contrast, there were no effects of BDNF heterozygosity or D3 knockout on PPI regulation in female mice. Chronic METH pretreatment induced the expected locomotor hyperactivity sensitisation, where female HET and DM mice also showed endogenous sensitisation. Differential sex-specific effects of genotype and METH pretreatment were observed on dopamine receptor and dopamine transporter gene expression in the striatum and frontal cortex. Taken together, these results show a significant involvement of BDNF in the long-term effects of METH on PPI, particularly in male mice, but these effects appear independent of D3 receptors. The role of this receptor in psychosis endophenotypes therefore remains unclear.

Cocaine-induced sensitization and glutamate plasticity in the nucleus accumbens core: effects of sex

BACKGROUND

The development and persistence of addiction is mediated in part by drug-induced alterations in nucleus accumbens (NAc) function. AMPA-type glutamate receptors (AMPARs) provide the main source of excitatory drive to the NAc and enhancements in transmission of calcium-permeable AMPARs (CP-AMPARs) mediate increased cue-triggered drug-seeking following prolonged withdrawal. Cocaine treatment regimens that result in psychomotor sensitization enhance subsequent drug-seeking and drug-taking behaviors. Furthermore, cocaine-induced locomotor sensitization followed by 14 days of withdrawal results in an increase in glutamatergic synaptic transmission. However, very few studies have examined cocaine-induced alterations in synaptic transmission of females or potential effects of experimenter-administered cocaine on NAc CP-AMPAR-mediated transmission in either sex.

METHODS

Male and female rats were given repeated systemic cocaine injections to induce psychomotor sensitization (15 mg/kg, i.p. 1 injection/day, 8 days). Controls received repeated saline (1 mL/kg, i.p). After 14-16 days of withdrawal brain slices were prepared and whole-cell patch-clamp approaches in the NAc core were used to measure spontaneous excitatory post-synaptic currents (sEPSC), paired pulse ratio, and CP-AMPAR transmission. Additional female rats from this same cohort were also given a challenge injection of cocaine at withdrawal day 14 to assess the expression of sensitization.

RESULTS

Repeated cocaine produced psychomotor sensitization in both sexes. In males this was accompanied by an increase in sEPSC frequency, but not amplitude, and there was no effect on the paired pulse ratio. Males treated with cocaine and saline had similar sensitivity to Naspm. In contrast, in females there were no significant differences between cocaine and saline groups on any measure, despite females showing robust psychomotor sensitization both during the induction and expression phase.

CONCLUSIONS

Overall, these data reveal striking sex differences in cocaine-induced NAc glutamate plasticity that accompany the induction of psychomotor sensitization. This suggests that the neural adaptations that contribute to sensitization vary by sex.

Long-term effects of THC exposure on reward learning and motivated behavior in adolescent and adult male rats

RATIONALE

The endocannabinoid system makes critical contributions to reward processing, motivation, and behavioral control. Repeated exposure to THC or other cannabinoid drugs can cause persistent adaptions in the endocannabinoid system and associated neural circuitry. It remains unclear how such treatments affect the way rewards are processed and pursued.

OBJECTIVE AND METHODS

We examined if repeated THC exposure (5 mg/kg/day for 14 days) during adolescence or adulthood led to long-term changes in rats' capacity to flexibly encode and use action-outcome associations for goal-directed decision making. Effects on hedonic feeding and progressive ratio responding were also assessed.

RESULTS

THC exposure had no effect on rats' ability to flexibly select actions following reward devaluation. However, instrumental contingency degradation learning, which involves avoiding an action that is unnecessary for reward delivery, was augmented in rats with a history of adult but not adolescent THC exposure. THC-exposed rats also displayed more vigorous instrumental behavior in this study, suggesting a motivational enhancement. A separate experiment found that while THC exposure had no effect on hedonic feeding behavior, it increased rats' willingness to work for food on a progressive ratio schedule, an effect that was more pronounced when THC was administered to adults. Adolescent and adult THC exposure had opposing effects on the CB1 receptor dependence of progressive ratio performance, decreasing and increasing sensitivity to rimonabant-induced behavioral suppression, respectively.

CONCLUSIONS

Our findings reveal that exposure to a translationally relevant THC exposure regimen induces long-lasting, age-dependent alterations in cognitive and motivational processes that regulate the pursuit of rewards.

L-type calcium channel regulation of dopamine activity in the ventral tegmental area to nucleus accumbens pathway: Implications for substance use, mood disorders and co-morbidities

L-type calcium channels (LTCCs), including the Cav1.2 and Cav1.3 LTCC subtypes, are important regulators of calcium entry into neurons, which mediates neurotransmitter release and synaptic plasticity. Cav1.2 and Cav1.3 are encoded by the CACNA1C and CACNA1D genes, respectively. These genes are implicated in substance use disorders and depression in humans, as demonstrated by genetic-wide association studies (GWAS). Pre-clinical models have also revealed a critical role of LTCCs on drug and mood related behavior, including the co-morbidity of substance use and mood disorders. Moreover, LTCCs have been shown to regulate the neuronal firing of dopamine (DA) neurons as well as drug and stress-induced plasticity within the ventral tegmental area (VTA) to nucleus accumbens (NAc) pathway. Thus, LTCCs are interesting targets for the treatment of neuropsychiatric diseases. In this review, we provide a brief introduction to voltage-gated calcium channels, specifically focusing on the LTCCs. We place particular emphasis on the ability of LTCCs to regulate DA neuronal activity and downstream signaling in the VTA to NAc pathway, and how such processes mediate substance use and mood disorder-related behavioral responses. We also discuss the bi-directional control of VTA LTCCs on drug and mood-related behaviors in pre-clinical models, with implications for co-morbid psychiatric diagnosis. We conclude with a section on the clinical implications of LTCC blockers, many which are already FDA approved as cardiac medications. Thus, pre-clinical and clinical work should examine the potential of LTCC blockers to be repurposed for neuropsychiatric illness. This article is part of the Special Issue on 'L-type calcium channel mechanisms in neuropsychiatric disorders'.

Methylphenidate cross-sensitization with amphetamine is dose dependent but not age dependent

Psychostimulants such as methylphenidate (MPD) and amphetamine (AMP) are often prescribed to young children and adolescents to treat behavioral disorders, or used to improve their intellectual performance in our competitive society. This is concerning as the temporal effects of how MPD exposure at a young age influences the response to MPD and AMP administration later in adulthood remains unclear. The objective of this study was to test whether MPD has the characteristics of substances that elicit behavioral symptoms of dependence and whether those effects are influenced by the initial age of MPD exposure. Three control and nine experimental groups of male rats were used. They were exposed to repetitive (chronic) 0.6, 2.5, or 10.0 mg/kg MPD in adolescence only, adulthood only, or adolescence and adulthood respectively. Then all groups were subsequently re-challenged with a single AMP dose in adulthood to test whether cross-sensitization between MPD and AMP was expressed, potentially as a result of prior MPD consumption. Exposure to 2.5 mg/kg and 10.0 mg/kg MPD in adolescence and adulthood or in adulthood alone led to cross-sensitization with AMP while exposure to 0.6 mg/kg MPD in adolescence and adulthood or in adulthood alone did not lead to cross-sensitization with AMP. Thus, these results indicate that MPD cross-sensitization with AMP is dose dependent.

The role of the L-arginine-NO-cGMP-dependent pathway in the development of sensitization to mephedrone effects on the locomotor activity in mice

Mephedrone, a popular psychostimulating substance widely used illegally in recreational purposes, exerts in rodents that regularly and intermittently were exposed to it a sensitized response to the drug. Behavioral sensitization is one of experimental models of drug dependency/abuse liability. In the present study we evaluated a potential involvement of the L-arginine-NO-cGMP pathway in the development of sensitization to the mephedrone-induced hyperlocomotion. Locomotor activity was measured automatically and experiments were performed on male Albino Swiss mice. We demonstrated that a 5-day administration of 7-nitroindazole (10 or 20 mg/kg/day) and L-NAME (50 mg/kg/day) suppressed the development of sensitization to the mephedrone-induced hyperlocomotion. As for L-arginine (125 or 250 mg/kg/day) and methylene blue (5 or 10 mg/kg/day) the obtained outcomes are inconclusive. Furthermore, the lower dose of L-NAME (25 mg/kg/day) surprisingly potentiated the development of sensitization to the mephedrone-induced effects on the spontaneous locomotor activity in mice. In conclusion, our data demonstrated that modulators of the L-arginine-NO-cGMP pathway may differently affect the development of sensitization to the locomotor stimulant effects of mephedrone. Inhibition of neuronal nitric oxide synthase (NOS) seems to prevent this process quite profoundly, non-selective inhibition of NOS may have a dual effect, whereas inhibition of soluble guanylate cyclase may only partially suppress the development of sensitization to the mephedrone-induced effects.

The role of metabotropic glutamate receptors in neurobehavioral effects associated with methamphetamine use

Metabotropic glutamate (mGlu) receptors are expressed throughout the central nervous system and act as important regulators of drug-induced neuroplasticity and behavior. Preclinical research suggests that mGlu receptors play a critical role in a spectrum of neural and behavioral consequences arising from methamphetamine (meth) exposure. However, an overview of mGlu-dependent mechanisms linked to neurochemical, synaptic, and behavioral changes produced by meth has been lacking. This chapter provides a comprehensive review of the role of mGlu receptor subtypes (mGlu1-8) in meth-induced neural effects, such as neurotoxicity, as well as meth-associated behaviors, such as psychomotor activation, reward, reinforcement, and meth-seeking. Additionally, evidence linking altered mGlu receptor function to post-meth learning and cognitive deficits is critically evaluated. The chapter also considers the role of receptor-receptor interactions involving mGlu receptors and other neurotransmitter receptors in meth-induced neural and behavioral changes. Taken together, the literature indicates that mGlu5 regulates the neurotoxic effects of meth by attenuating hyperthermia and possibly through altering meth-induced phosphorylation of the dopamine transporter. A cohesive body of work also shows that mGlu5 antagonism (and mGlu2/3 agonism) reduce meth-seeking, though some mGlu5-blocking drugs also attenuate food-seeking. Further, evidence suggests that mGlu5 plays an important role in extinction of meth-seeking behavior. In the context of a history of meth intake, mGlu5 also co-regulates aspects of episodic memory, with mGlu5 stimulation restoring impaired memory. Based on these findings, we propose several avenues for the development of novel pharmacotherapies for Methamphetamine Use Disorder based on the selective modulation mGlu receptor subtype activity.

Differential Effects of Chronic Methamphetamine Treatment on High-Frequency Oscillations and Responses to Acute Methamphetamine and NMDA Receptor Blockade in Conscious Mice

Dysregulation of high-frequency neuronal oscillations has been implicated in the pathophysiology of schizophrenia. Chronic methamphetamine (METH) use can induce psychosis similar to paranoid schizophrenia. The current study in mice aimed to determine the effect of chronic METH treatment on ongoing and evoked neuronal oscillations. C57BL/6 mice were treated with METH or vehicle control for three weeks and implanted with extradural recording electrodes. Two weeks after the last METH injection, mice underwent three EEG recording sessions to measure ongoing and auditory-evoked gamma and beta oscillatory power in response to an acute challenge with METH (2 mg/kg), the NMDA receptor antagonist MK-801 (0.3 mg/kg), or saline control. A separate group of mice pretreated with METH showed significantly greater locomotor hyperactivity to an acute METH challenge, confirming long-term sensitisation. Chronic METH did not affect ongoing or evoked gamma or beta power. Acute MK-801 challenge reduced ongoing beta power whereas acute METH challenge significantly increased ongoing gamma power. Both MK-801 and METH challenge suppressed evoked gamma power. Chronic METH treatment did not modulate these acute drug effects. There were minor effects of chronic METH and acute METH and MK-801 on selected components of event-related potential (ERP) waves. In conclusion, chronic METH treatment did not exert neuroplastic effects on the regulation of cortical gamma oscillations in a manner consistent with schizophrenia, despite causing behavioural sensitisation.

The collaborative cross strains and their founders vary widely in cocaine-induced behavioral sensitization

Cocaine use and overdose deaths attributed to cocaine have increased significantly in the United States in the last 10 years. Despite the prevalence of cocaine use disorder (CUD) and the personal and societal problems it presents, there are currently no approved pharmaceutical treatments. The absence of treatment options is due, in part, to our lack of knowledge about the etiology of CUDs. There is ample evidence that genetics plays a role in increasing CUD risk but thus far, very few risk genes have been identified in human studies. Genetic studies in mice have been extremely useful for identifying genetic loci and genes, but have been limited to very few genetic backgrounds, leaving substantial phenotypic, and genetic diversity unexplored. Herein we report the measurement of cocaine-induced behavioral sensitization using a 19-day protocol that captures baseline locomotor activity, initial locomotor response to an acute exposure to cocaine and locomotor sensitization across 5 exposures to the drug. These behaviors were measured in 51 genetically diverse Collaborative Cross (CC) strains along with their inbred founder strains. The CC was generated by crossing eight genetically diverse inbred strains such that each inbred CC strain has genetic contributions from each of the founder strains. Inbred CC mice are infinitely reproducible and provide a stable, yet diverse genetic platform on which to study the genetic architecture and genetic correlations among phenotypes. We have identified significant differences in cocaine locomotor sensitivity and behavioral sensitization across the panel of CC strains and their founders. We have established relationships among cocaine sensitization behaviors and identified extreme responding strains that can be used in future studies aimed at understanding the genetic, biological, and pharmacological mechanisms that drive addiction-related behaviors. Finally, we have determined that these behaviors exhibit relatively robust heritability making them amenable to future genetic mapping studies to identify addiction risk genes and genetic pathways that can be studied as potential targets for the development of novel therapeutics.

The effect of environment on cross-sensitization between methylphenidate and amphetamine in female rats

Methylphenidate (MPD) and amphetamine (AMP) are both psychostimulants that are often used to treat behavioral disorders. More recently, it has also been increasingly used illicitly for recreation as well as to improve intellectual performance. Many factors such as age, gender, genetic background, and environment govern the development of behavioral sensitization to MPD and cross-sensitization with other drugs, which are experimental behavioral markers indicating potential of substance dependence and abuse. This study examines the effects of the environment and age when MPD was exposed in adulthood alone as well as in adolescence into adulthood on cross-sensitization with AMP in female SD rats by randomizing animals to either receive the drug in a home cage or a test cage during adolescence, adulthood, or both. In a 34 day experiment, 16 groups of animals starting in adolescence were treated with saline on experimental day one (ED1), followed by a 6 day (ED2-ED7) treatment with either saline, 0.6 mg/kg AMP, 0.6, 2.5, or 10.0 mg/kg MPD. Experimental groups were then subject to a 3-day washout period (ED8-ED10) and then a retreatment with the respective drug on ED11 in adolescence (P-38 to P-49). Experiments continued in the same animal groups now in adulthood (P-60) with a saline treatment (ED1), followed by the same sequence of treatments in adolescence (ED2-ED11;P-61 to P-69). A rechallenge with the same AMP or MPD dose was performed on ED11 (P-70) followed by a single exposure to 0.6 mg/kg AMP on ED12 (P-71) to assess for cross sensitization between MPD and AMP. Animals treated with MPD in both adolescence and adulthood and in the last experimental day of AMP (ED12) showed higher intensity of cross-sensitivity between MPD and AMP as compared to animals treated with MPD only in adulthood. AMP and MPD treatment in adolescence and into adulthood in the home or test cage resulted in significantly higher responses to the drug as compared to those treated only in adulthood. Overall, we conclude that environmental alteration and adolescent exposure to MPD appeared to increase the risk of cross-sensitization to AMP in female SD rats i.e, using MPD in adolescence may increase the probability of becoming dependent on drugs of abuse. This further indicates that age, sex, and environment all influence the response to MPD and AMP, and further work is needed to elucidate the risks associated with MPD and AMP use.

Age differences to methylphenidate-NAc neuronal and behavioral recordings from freely behaving animals

Methylphenidate (MPD) is a psychostimulant that is widely prescribed to treat attention deficit-hyperactivity disorder, but it is abused recreationally as well. The nucleus accumbens (NAc) is part of the motivation circuit implicated in drug-seeking behaviors. The NAc neuronal activity was recorded alongside the behavioral activity from young and adult rats to determine if there are significant differences in the response to MPD. The same dose of MPD elicits behavioral sensitization in some animals and behavioral tolerance in others. In adult animals, higher doses of MPD resulted in a greater ratio of tolerance/sensitization. Animals who responded to chronic MPD with behavioral sensitization usually exhibited further increases in their NAc neuronal firing rates as well. Different upregulations of transcription factors (ΔFOSB/CREB), variable proportions of D1/D2 dopamine receptors, and modulation from other brain areas may predispose certain animals to express behavioral and neuronal sensitization versus tolerance to MPD.

Effects of repeated treatment with the 5-HT1A and 5-HT1B agonists (R)-( +)-8-hydroxy-DPAT and CP-94253 on the locomotor activity and axillary temperatures of preweanling rats: evidence of tolerance and behavioral sensitization

RATIONALE

Drugs that stimulate 5-HT_1A/1B receptors produce both tolerance and behavioral sensitization in adult rats and mice, yet it is unknown whether the same types of plasticity are evident during earlier ontogenetic periods.

OBJECTIVE

The purpose of this study was to determine whether repeated treatment with selective 5-HT_1A and/or 5-HT_1B agonists cause tolerance and/or sensitization in preweanling rats.

METHODS

In Experiments 1 and 2, male and female preweanling rats were given a single pretreatment injection of saline, the 5-HT_1A agonist (R)-( +)-8-hydroxy-DPAT (8-OH-DPAT), or the 5-HT_1B agonist CP-94253 on PD 20. After 48 h, rats received a challenge injection of 8-OH-DPAT or CP-94253, respectively. In Experiment 3, rats were pretreated with saline or DPAT + CP on PD 20 and challenged with the same drug cocktail on PD 22. In Experiment 4, the tolerance- or sensitization-inducing properties of 8-OH-DPAT, CP-94253, or DPAT + CP were tested after a 4-day pretreatment regimen on PD 17-20.

RESULTS

On the first pretreatment day, 8-OH-DPAT, CP-94253, and DPAT + CP increased locomotion and caused hypothermia. Repeated treatment with 8-OH-DPAT (2 or 8 mg/kg) or DPAT + CP caused locomotor sensitization in preweanling rats. In contrast, tolerance to the hypothermic effects of 8-OH-DPAT (8 mg/kg), CP-94253 (5-20 mg/kg), or DPAT + CP was evident after repeated drug treatment.

CONCLUSIONS

During the preweanling period, a single injection of a selective 5-HT_1A or 5-HT_1B agonist is capable of producing drug-induced plasticity. A pretreatment administration of 8-OH-DPAT causes both tolerance (hypothermia) and behavioral sensitization (locomotor activity) in preweanling rats, whereas repeated CP-94253 treatment results in tolerance.

Cannabidiol but not cannabidiolic acid reduces behavioural sensitisation to methamphetamine in rats, at pharmacologically effective doses

RATIONALE

Cannabidiol (CBD) and cannabidiolic acid (CBDA) are non-psychoactive components of the cannabis plant. CBD has been well characterised to have anxiolytic and anticonvulsant activity, whereas the behavioural effects of CBDA are less clear. Preclinical and clinical data suggests that CBD has antipsychotic properties and reduces methamphetamine self-administration in rats. An animal model that is commonly used to mimic the neurochemical changes underlying psychosis and drug dependence is methamphetamine (METH) sensitisation, where repeated administration of the psychostimulant progressively increases the locomotor effects of METH.

OBJECTIVE

The aim of this study was to determine whether CBD or CBDA attenuate METH-induced sensitisation of locomotor hyperactivity in rats.

METHODS

Eighty-six male Sprague Dawley rats underwent METH sensitisation protocol where they were subjected to daily METH (1 mg/kg on days 2 and 8, 5 mg/kg on days 3-7; i.p.) injections for 7 days. After 21 days of withdrawal, rats were given a prior injection of CBD (0, 40 and 80 mg/kg; i.p.) or CBDA (0, 0.1, 10 and 1000 µg/kg; i.p.) and challenged with acute METH (1 mg/kg; i.p.). Locomotor activity was then measured for 60 min.

RESULTS

Rats displayed robust METH sensitisation as evidenced by increased locomotor activity to METH challenge in METH-pretreated versus SAL-pretreated rats. CBD (40 and 80 mg/kg) reduced METH-induced sensitisation. There was no effect of any CBDA doses on METH sensitisation or acute METH-induced hyperactivity.

CONCLUSION

These results demonstrate that CBD, but not CBDA, reduces METH sensitisation of locomotor activity in rats at pharmacologically effective doses, thus reinforcing evidence that CBD has anti-addiction and antipsychotic properties.

Susceptibility to express amphetamine locomotor sensitization correlates with dorsolateral striatum bursting activity and GABAergic synapses in the globus pallidus

Repeated psychostimulant administration results in behavioral sensitization, a process that is relevant in the early phases of drug addiction. Critically, behavioral sensitization is not observed in all subjects. Evidence shows that differential neuronal activity in the dorsolateral striatum (DLS) accompanies the expression of amphetamine (AMPH) locomotor sensitization. However, whether individual differences in DLS activity previous to AMPH administration can predict the expression of locomotor sensitization has not been assessed. Here, we examined DLS neuronal activity before and after repeated AMPH administration and related it to the susceptibility of rats to sensitize. For that, single-unit recordings on DLS medium spiny neurons (MSNs) were carried out in freely moving male Sprague Dawley rats during repeated AMPH administration. We also examined differences in neurostructure that could accompany sensitization. We quantified the density of the inhibitory postsynaptic marker gephyrin (Geph) in the entopeduncular nucleus (EP) and globus pallidus (GP). A higher burst firing and a lower percentage of correlation between MSNs post-Saline firing rate vs. locomotion predicted the expression of locomotor sensitization. Moreover, during the AMPH challenge, we observed that burst firing decreased in sensitized rats, in contrast to non-sensitized rats in which burst firing was maintained. Finally, a higher Geph density on GP but not EP was observed in non-sensitized rats after AMPH challenge. These results indicate that initial differences in DLS burst firing might underlie the susceptibility to express locomotor sensitization and suggest that the potentiation of dorsal striatum indirect pathway could be considered a protective mechanism to locomotor sensitization.

The abuse potential of prolintane in rodents: Behavioral pharmacology approaches

Prolintane (1-Phenyl-2-pyrrolidinylpentane), a synthetic central nervous system (CNS) stimulant, is structurally similar to amphetamine but pharmacologically acts as a dopamine reuptake inhibitor like cocaine. While several case studies reported adverse effects and recreational use of prolintane, the abuse potential of the drug has not been systemically examined yet. In the present study, we evaluated the behavioral effects of prolintane regarding its abuse liability in rodents using locomotor activity, conditioned place preference (CPP), self-administration (SA), and drug discrimination paradigms, as well as in-vivo microdialysis experiment. First, acute prolintane (10 and 20 mg/kg, intraperitoneal injection) increased locomotor activity (distance traveled, cm) in mice but to a lesser degree than methamphetamine (as a positive control). We also found that a single and solitary injection of prolintane (20 mg/kg, IP) significantly increased extracellular dopamine in the striatum. The following result suggests that its stimulatory effects might be associated with the mesolimbic dopaminergic pathway. Further, prolintane produced a significant drug-paired place preference at doses of both 10 and 20 mg/kg. In the SA experiment, the mice that self-administered prolintane intravenously (4 mg/kg/inf) showed a higher infusion and active lever responses but not inactive lever responses. Additionally, cumulative doses of prolintane partially elicited cocaine-appropriate lever responses (38.57% at doses up to 10 mg/kg) in rats. These results implied that prolintane has not only rewarding and reinforcing effects but also interoceptive stimulus properties, which are similar to cocaine at a moderate level. Taken together, this study was the first to show, to our knowledge, that prolintane has a certain level of abuse potential and should be considered carefully as a valuable basis for legal restrictions on use.

Npas4 regulates medium spiny neuron physiology and gates cocaine-induced hyperlocomotion

We show here that the transcription factor Npas4 is an important regulator of medium spiny neuron spine density and electrophysiological parameters and that it determines the magnitude of cocaine-induced hyperlocomotion in mice. Npas4 is induced by synaptic stimuli that cause calcium influx, but not dopaminergic or PKA-stimulating input, in mouse medium spiny neurons and human iPSC-derived forebrain organoids. This induction is independent of ubiquitous kinase pathways such as PKA and MAPK cascades, and instead depends on calcineurin and nuclear calcium signalling. Npas4 controls a large regulon containing transcripts for synaptic molecules, such as NMDA receptors and VDCC subunits, and determines in vivo MSN spine density, firing rate, I/O gain function and paired-pulse facilitation. These functions at the molecular and cellular levels control the locomotor response to drugs of abuse, as Npas4 knockdown in the nucleus accumbens decreases hyperlocomotion in response to cocaine in male mice while leaving basal locomotor behaviour unchanged.

Anxiety in Parkinson's Disease: A Systematic Review of Neuroimaging Studies

OBJECTIVE

The mechanisms and neuronal networks associated with anxiety in Parkinson's disease (PD) are incompletely understood. One of the best tools for investigating both component function and neuronal networks associated with psychiatric symptoms is functional MRI (fMRI). Unlike structural scans, functional scans, whether task-based or resting-state, are more likely to be clinically relevant and sensitive to changes related to treatment. The investigators provide a comprehensive review of and present results for imaging studies of anxiety in PD.

METHODS

A systematic review of the literature on fMRI and anxiety in PD was conducted, and the quality of all included studies was simultaneously assessed. Eighteen studies were included: 15 studies assessed anxiety directly, and three evaluated emotional processing. Imaging methodology and behavioral assessments varied across studies, preventing direct comparison of results in most cases.

RESULTS

There was a convergence in findings across methods, implicating involvement of the amygdala, caudate, and putamen in association with anxiety in PD. For both task-based activation and resting-state connectivity, dopamine medication status was associated with differences in activation and behavioral function.

CONCLUSIONS

Although there is little consensus in the current fMRI literature studying anxiety in PD, these results suggest an overlap between structures classically involved in the brain's fear circuit (particularly the amygdala) and the alterations in the nigro-striatal system (e.g., the caudate and putamen and on-off dopamine findings) related to PD and its dopaminergic treatments.

Stress-induced vulnerability to develop cocaine addiction depends on cofilin modulation

Actin dynamics in dendritic spines can be associated with the neurobiological mechanisms supporting the comorbidity between stress exposure and cocaine increase rewards. The actin cytoskeleton remodeling in the nucleus accumbens (NA) has been implicated in the expression of stress-induced cross-sensitization with cocaine. The present study evaluates the involvement of cofilin, a direct regulator of actin dynamics, in the impact of stress on vulnerability to cocaine addiction. We assess whether the neurobiological mechanisms that modulate repeated-cocaine administration also occur in a chronic restraint stress-induced cocaine self-administration model. We also determine if chronic stress induces alterations in dendritic spines through dysregulation of cofilin activity in the NA core. Here, we show that the inhibition of cofilin expression in the NA core using viral short-hairpin RNA is sufficient to prevent the cocaine sensitization induced by chronic stress. The reduced cofilin levels also impede a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor surface expression enhancement and promote the reduction of head diameter in animals pre-exposed to stress after a cocaine challenge in the NA core. Moreover, downregulation of cofilin expression prevents facilitation of the acquisition of cocaine self-administration (SA) in male rats pre-exposed to chronic stress without modifying performance in sucrose SA. These findings reveal a novel, crucial role for cofilin in the neurobiological mechanisms underpinning the comorbidity between stress exposure and addiction-related disorders.

Transgenerational evidence of increases in dopamine D2 receptor sensitivity in rodents: Impact on sensorimotor gating, the behavioral response to nicotine and BDNF

BACKGROUND/AIMS

Neonatal quinpirole (NQ) treatment to rats increases dopamine D₂ (DAD₂) receptor sensitivity in adult animals. We investigated if increased DAD₂ sensitivity would be passed to the next (F1) generation, and if these animals demonstrated sensorimotor gating deficits and enhanced behavioral responses to nicotine.

METHODS

Male and female rats were intraperitoneal (IP) administered quinpirole (1 mg/kg) or saline (NS) from postnatal day (P)1-21. Animals were either behaviorally tested (F0) or raised to P60 and mated, creating F1 offspring.

RESULTS

Experiment 1 revealed that F1 generation animals that were the offspring of at least one NQ-treated founder increased yawning behavior, a DAD₂-mediated behavioral event, in response to acute quinpirole (0.1 mg/kg). F1 generation rats also demonstrated increased striatal β arrestin-2 and decreased phospho-AKT signaling, consistent with increased G-protein independent DAD₂ signaling, which was equal to F0 NQ-treated founders, although this was not observed in all groups. RNA-Seq analysis revealed significant gene expression changes in the F1 generation that were offspring of both NQ-treated founders compared to F0 NQ founders and controls, with enrichment in sensitivity to stress hormones and cell signaling pathways. In Experiment 2, all F1 generation offspring demonstrated sensorimotor gating deficits compared to controls, which were equivalent to F0 NQ-treated founders. In Experiment 3, all F1 generation animals demonstrated enhanced nicotine behavioral sensitization and nucleus accumbens (NAcc) brain-derived neurotrophic factor (BDNF) protein. Further, F1 generation rats demonstrated enhanced adolescent nicotine conditioned place preference equivalent to NQ-treated founders conditioned with nicotine.

CONCLUSIONS

This represents the first demonstration of transgenerational effects of increased DAD₂ sensitivity in a rodent model.

Fluoxetine Potentiates Oral Methylphenidate-Induced Gene Regulation in the Rat Striatum

Methylphenidate (MP) is combined with selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine (FLX) to treat various disorders. MP, a dopamine reuptake inhibitor, helps manage attention-deficit hyperactivity disorder (ADHD) and is abused as a cognitive enhancer; it has a reduced addiction liability. We showed that combining FLX (serotonin) with MP potentiates MP-induced gene regulation in the striatum. These studies used intraperitoneal drug administration, which is relevant for MP abuse. Clinically, MP and FLX are taken orally (slower bioavailability). Here, we investigated whether chronic oral administration of MP and FLX also altered striatal gene regulation. MP (30/60 mg/kg/day), FLX (20 mg/kg/day), and MP + FLX were administered in rats' drinking water for 8 h/day over 4 weeks. We assessed the expression of dynorphin and substance P (both markers for striatal direct pathway neurons) and enkephalin (indirect pathway) by in situ hybridization histochemistry. Chronic oral MP alone produced a tendency for increased dynorphin and substance P expression and no changes in enkephalin expression. Oral FLX alone did not increase gene expression. In contrast, when given together, FLX greatly enhanced MP-induced expression of dynorphin and substance P and to a lesser degree enkephalin. Thus, FLX potentiated oral MP-induced gene regulation predominantly in direct pathway neurons, mimicking cocaine effects. The three functional domains of the striatum were differentially affected. MP + SSRI concomitant therapies are indicated in ADHD/depression comorbidity and co-exposure occurs with MP misuse as a cognitive enhancer by patients on SSRIs. Our findings indicate that MP + SSRI combinations, even given orally, may enhance addiction-related gene regulation.

Altered Grooming Syntax and Amphetamine-Induced Dopamine Release in EAAT3 Overexpressing Mice

The excitatory amino acid transporter EAAT3 plays an important role in the neuronal uptake of glutamate regulating the activation of glutamate receptors. Polymorphisms in the gene-encoding EAAT3 have been associated with obsessive-compulsive disorder (OCD), although the mechanisms underlying this relationship are still unknown. We recently reported that mice with increased EAAT3 expression in forebrain neurons (EAAT3 ^{g lo} /CMKII) display behavioral and synaptic features relevant to OCD, including increased grooming, higher anxiety-like behavior and altered cortico-striatal synaptic function. The dopamine neurotransmitter system is implicated in ritualistic behaviors. Indeed, dopaminergic neurons express EAAT3, and mice lacking EAAT3 exhibit decreased dopamine release and decreased expression of the dopamine D1 receptor. Moreover, EAAT3 plays a role on the effect of the psychostimulant amphetamine. As such, we sought to determine if the OCD-like behavior in EAAT3 ^{g lo} /CMKII mice is accompanied by altered nigro-striatal dopaminergic transmission. The aim of this study was to analyze dopamine transmission both in basal conditions and after an acute challenge of amphetamine, using behavioral, neurochemical, molecular, and cellular approaches. We found that in basal conditions, EAAT3 ^{g lo} /CMKII mice performed more grooming events and that they remained in phase 1 of the grooming chain syntax compared with control littermates. Administration of amphetamine increased the number of grooming events in control mice, while EAAT3 ^{g lo} /CMKII mice remain unaffected. Interestingly, the grooming syntax of amphetamine-control mice resembled that of EAAT3 ^{g lo} /CMKII mice in basal conditions. Using in vivo microdialysis, we found decreased basal dopamine levels in EAAT3 ^{g lo} /CMKII compared with control mice. Unexpectedly, we found that after acute amphetamine, EAAT3 ^{g lo} /CMKII mice had a higher release of dopamine compared with that of control mice, suggesting that EAAT3 overexpression leads to increased dopamine releasability. To determine postsynaptic effect of EAAT3 overexpression over dopamine transmission, we performed Western blot analysis of dopaminergic proteins and found that EAAT3 ^{g lo} /CMKII mice have higher expression of D2 receptors, suggesting a higher inhibition of the indirect striatal pathway. Together, the data indicate that EAAT3 overexpression impacts on dopamine transmission, making dopamine neurons more sensitive to the effect of amphetamine and leading to a disbalance between the direct and indirect striatal pathways that favors the performance of repetitive behaviors.

The Paraventricular Thalamus as a Critical Node of Motivated Behavior via the Hypothalamic-Thalamic-Striatal Circuit

In this review, we highlight evidence that supports a role for the paraventricular nucleus of the thalamus (PVT) in motivated behavior. We include a neuroanatomical and neurochemical overview, outlining what is known of the cellular makeup of the region and its most prominent afferent and efferent connections. We discuss how these connections and distinctions across the anterior-posterior axis correspond to the perceived function of the PVT. We then focus on the hypothalamic-thalamic-striatal circuit and the neuroanatomical and functional placement of the PVT within this circuit. In this regard, the PVT is ideally positioned to integrate information regarding internal states and the external environment and translate it into motivated actions. Based on data that has emerged in recent years, including that from our laboratory, we posit that orexinergic (OX) innervation from the lateral hypothalamus (LH) to the PVT encodes the incentive motivational value of reward cues and thereby alters the signaling of the glutamatergic neurons projecting from the PVT to the shell of the nucleus accumbens (NAcSh). The PVT-NAcSh pathway then modulates dopamine activity and resultant cue-motivated behaviors. As we and others apply novel tools and approaches to studying the PVT we will continue to refine the anatomical, cellular, and functional definitions currently ascribed to this nucleus and further elucidate its role in motivated behaviors.

Locomotor sensitization modulates voluntary self-administration of methamphetamine in Drosophila melanogaster

As complexities of addictive behaviors cannot be fully captured in laboratory studies, scientists use simple addiction-associated phenotypes and measure them in laboratory animals. Locomotor sensitization, characterized by an increased behavioral response to the same dose of the drug, has been extensively used to elucidate the genetic basis and molecular mechanisms of neuronal plasticity. However, to what extent it contributes to the development of addiction is not completely clear. We tested if the development of locomotor sensitization to methamphetamine affects voluntary self-administration, and vice versa, in order to investigate how two drug-associated phenotypes influence one another. In our study, we used the genetically tractable model organism, Drosophila melanogaster, and quantified locomotor sensitization and voluntary self-administration to methamphetamine using behavioral tests that were developed and adapted in our laboratory. We show that flies express robust locomotor sensitization to the second dose of volatilized methamphetamine, which significantly lowers preferential self-administration of methamphetamine. Naive flies preferentially self-administer food with methamphetamine over plain food. Exposing flies to volatilized methamphetamine after voluntary self-administration abolishes locomotor sensitization. We tested period null (per⁰¹ ) mutant flies and showed that they do not develop locomotor sensitization, nor do they show preferential self-administration of methamphetamine. Our results suggest that there may be partially overlapping neural circuitry that regulates the expression of locomotor sensitization and preferential self-administration to methamphetamine and that this circuitry requires a functional per gene.

Comparison of the sensitizing effects of cocaine and ethanol on histone deacetylase isoforms in the rat brain

Behavioral sensitization, an animal model of drug addiction, persists for a prolonged period after repeated exposure to drugs of abuse. The persistence of an addiction behavioral phenotype suggests long-lasting changes in gene regulation at the epigenetic level. We measured the expression of histone deacetylases (HDACs) isoforms in the prefrontal cortex and dorsal striatum following the development of sensitization to cocaine (15 mg/kg, administered five times) and ethanol (0.5 g/kg, administered 15 times) to investigate the epigenetic changes that mediate sensitization. Animals sensitized to ethanol exhibited augmented locomotor activity in response to the cocaine challenge. Similarly, those sensitized to cocaine exhibited increased locomotor activity in response to an ethanol challenge. These findings indicate cross-sensitization between ethanol and cocaine and suggest that a common molecular mechanism underlying the cross-sensitization. In animals sensitized to cocaine or ethanol, mRNA levels of class II HDACs (HDAC4 and HDAC5) were decreased in the prefrontal cortex and dorsal striatum, whereas acute treatments with either drug had no effect on the expression of class II HDACs. By contrast, class I HDACs (HDAC1 and HDAC2) responded to the acute cocaine challenge, whereas sensitization itself did not have a consistent effect on class I HDAC levels. These findings support the hypothesis of a common epigenetic mechanism underlying persistent behavioral sensitization induced by different drugs, which may be mediated by the altered expression of class II HDACs.

Dopamine receptors regulate preference between high-effort and high-risk rewards

RATIONALE

Optimal decision-making necessitates evaluation of multiple rewards that are each offset by distinct costs, such as high effort requirement or high risk of failure. The neurotransmitter dopamine is fundamental toward these cost-benefit analyses, and D1-like and D2-like dopamine receptors differently modulate the reward-discounting effects of both effort and risk. However, measuring the role of dopamine in regulating decision-making between options associated with distinct costs exceeds the scope of traditional rodent economic decision-making paradigms.

OBJECTIVES

We developed the effort vs probability economic conflict task (EvP) to model multimodal economic decision-making in rats. This task measures choice between two rewards of uniform magnitude associated with either a high effort requirement or risk of reward omission. We then tested the modulatory effects of systemic cocaine and D1/D2 blockade or activation on the preference between high-effort and high-risk alternatives.

METHODS

In the EvP, two reinforcers of equal magnitude are associated with either (1) an effort requirement that increases throughout the session (1, 5, 10, and 20 lever presses), or (2) a low probability of reward receipt (25% of probabilistic choices). Critically, the reinforcer for each choice is comparable (one pellet), which eliminates the influence of magnitude discrimination on the decision-making process. After establishing the task, the dopamine transporter blocker cocaine and D1/D2 antagonists and agonists were administered prior to EvP performance.

RESULTS

Preference shifted away from either effortful or probabilistic choice when either option became more costly, and this preference was highly variable between subjects and stable over time. Cocaine, D1 activation, and D2 blockade produced limited, dose-dependent shifts in choice preference contingent on high or low effort conditions. In contrast, D2 activation across multiple doses evoked a robust shift from effortful to risky choice that was evident even when clearly disadvantageous.

CONCLUSIONS

The EvP clearly demonstrates that rats can evaluate distinct effortful or risky costs associated with rewards of comparable magnitude, and shift preference away from either option with increasing cost. This preference is more tightly linked to D2 than D1 receptor manipulation, suggesting D2-like receptors as a possible therapeutic target for maladaptive biases toward risk-taking over effort.

Plasticity in the Hippocampus, Neurogenesis and Drugs of Abuse

Synaptic plasticity in the hippocampus assists with consolidation and storage of long-lasting memories. Decades of research has provided substantial information on the cellular and molecular mechanisms underlying synaptic plasticity in the hippocampus, and this review discusses these mechanisms in brief. Addiction is a chronic relapsing disorder with loss of control over drug taking and drug seeking that is caused by long-lasting memories of drug experience. Relapse to drug use is caused by exposure to context and cues associated with the drug experience, and is a major clinical problem that contributes to the persistence of addiction. This review also briefly discusses some evidence that drugs of abuse alter plasticity in the hippocampus, and that development of novel treatment strategies that reverse or prevent drug-induced synaptic alterations in the hippocampus may reduce relapse behaviors associated with addiction.

Long-term effects of young-adult methamphetamine on dorsal raphe serotonin systems in mice: Role of brain-derived neurotrophic factor

To assess the long-term effects of chronic adolescent methamphetamine (METH) treatment on the serotonin system in the brain, we used serotonin-1A receptor (5-HT_1A) and serotonin transporter (SERT) autoradiography, and quantitative tryptophan-hydroxylase 2 (TPH2) immunohistochemistry in the raphe nuclei of mice. Because of the modulatory role of brain-derived neurotrophic factor (BDNF) on the serotonin system and the effects of METH, we included both BDNF heterozygous (HET) mice and wildtype (WT) controls. Male and female mice of both genotypes were treated with an escalating METH dose regimen from the age of 6-9 weeks. At least two weeks later, acute locomotor hyperactivity induced by a 5 mg/kg D-amphetamine challenge was significantly enhanced in METH-pretreated mice, showing long-term sensitisation. METH pretreatment caused a small, but significant decrease of 5-HT_1A receptor binding in the dorsal raphe nucleus (DRN) of males independent of genotype, but there were no changes in the median raphe nucleus (MRN) or in SERT binding density. METH treatment reduced the number of TPH2 positive cells in ventral subregions of the rostral and medial DRN independent of genotype. METH treatment selectively reduced DRN cell counts in BDNF HET mice compared to wildtype mice in medial and caudal ventrolateral subregions previously associated with panic-like behaviour. The data increase our understanding of the long-term and selective effects of METH on brain serotonin systems. These findings could be relevant for some of the psychosis-like symptoms associated with long-term METH use.

Ganoderma lucidum polysaccharides ameliorated depression-like behaviors in the chronic social defeat stress depression model via modulation of Dectin-1 and the innate immune system

Major depressive disorder (MDD) is a prevalent, chronic, and recurrent disease. At least one-third of patients have treatment-resistant depression; therefore, there is an urgent need for novel drug development. Cumulative studies have suggested an inflammatory mechanism for the pathophysiology of MDD. Ganoderma lucidum polysaccharides (GLP) is an anti-inflammatory and immunomodulatory agent. Here, we found that an injection of GLP led to a rapid and robust antidepressant effect after 60 min in the tail suspension test. This antidepressant effect remained after 5 days of treatment with GLP in the forced swim test. Unlike psychostimulants, GLP did not show a hyperactive effect in the open field test. After 60 min or 5 days of treatment, GLP exhibited an antidepressant effect in a chronic social defeat stress (CSDS) depression animal model. Moreover, after 5 days of treatment, GLP attenuated the expression of the proinflammatory cytokines IL-1β and TNF-α, enhanced the expression of the anti-inflammatory cytokine IL-10 and the neurotrophic factor BDNF, and inhibited the activation of microglia and proliferation of astrocytes in the hippocampus of CSDS mice. In addition, after 5 days of treatment, GLP significantly enhanced GluA1 S845 phosphorylation as well as GluA1 and GluA2 expression levels in the hippocampus of CSDS mice. To determine whether the antidepressant effect was mediated by Dectin-1, we found that GLP treatment enhanced Dectin-1 expression in the hippocampus in CSDS mice, and the Dectin-1-specific inhibitor laminarin almost completely blocked the antidepressant effect of GLP. This study identified GLP, an agonist of Dectin-1, as a novel and rapid antidepressant with clinical potential and multiple beneficial mechanisms, particularly in regulating the neuroimmune system and, subsequently, AMPA receptor function.

CXCR4 inhibition with AMD3100 attenuates amphetamine induced locomotor activity in adolescent Long Evans male rats

Adolescent psychostimulant abuse has been on the rise over the past decade. This trend has demonstrable ramifications on adolescent behavior and brain morphology, increasing risk for development of addiction during adolescence and in later adulthood. Neuroimmune substrates are implicated in the etiology of substance use disorders. To add to this body of work, the current study was developed to explore the role of a chemokine receptor, CXC Chemokine Receptor 4 (CXCR4), in the development of amphetamine (AMPH) sensitization. We targeted CXCR4 as it is implicated in developmental processes, dopaminergic transmission, neuroimmune responses, and the potentiation of psychostimulant abuse pathology. To evaluate the role of CXCR4 activity on the development of AMPH sensitization, a CXCR4 antagonist (Plerixafor; AMD3100) was administered to rats as a pretreatment variable. Specifically, adolescent Long Evans male rats (N = 37) were divided into four groups: (1) AMD3100 (IP, 4.0 mg/kg) + AMPH (IP, 4.0 mg/kg), (2) saline (SAL; 0.9% NaCl) + AMPH, (3) AMD3100 + SAL, and (4) SAL + SAL. Animals were first habituated to locomotor activity (LMA) chambers, then injected with a pretreatment drug (AMD3100 or SAL) followed by AMPH or SAL every other for four days. After a one-week withdrawal period, all animals were administered a low challenge dose of AMPH (IP, 1.0 mg/kg). AMPH-injected rats displayed significantly more locomotor activity compared to controls across all testing days. CXCR4 antagonism significantly attenuated AMPH-induced locomotor activity. On challenge day, AMD3100 pre-treated animals exhibited diminutive AMPH-induced locomotor activity compared to SAL pre-treated animals. Postmortem analyses of brain tissue revealed elevated CXCR4 protein levels in the striatum of all experimental groups. Our results implicate CXCR4 signaling in the development of AMPH sensitization and may represent an important therapeutic target for future research in psychostimulant abuse.

Phosphorylation of GluN2B subunits of N-methyl-d-aspartate receptors in the frontal association cortex involved in morphine-induced conditioned place preference in mice

Morphine is one of the most abused drugs in the world, which has resulted in serious social problems. The frontal association cortex (FrA) has been shown to play a key role in memory formation and drug addiction. N-Methyl-d-aspartate receptors (NMDARs) are abundant in the prefrontal cortex (PFc) and much evidence indicates that GluN2B-containing NMDARs are involved in morphine-induced conditioned place preference (CPP). However, the function of GluN2B in the FrA during morphine-induced CPP has yet to be fully investigated. In the present work, a CPP animal model was employed to measure the expression of phosphorylated (p-) GluN2B (Serine; Ser 1303) in the FrA and NAc in different phases of morphine-induced CPP. We found that p-GluN2B (Ser 1303) was increased in the FrA during the development and reinstatement phases but unchanged in the extinction phase. The use of ifenprodil, a GluN2B-specific antagonist, to block the activity of GluN2B in the two phases attenuated morphine-induced CPP and reinstatement. Furthermore, ifenprodil also blocked morphine-induced upregulation of p-GluN2B (Ser 1303) in the FrA in both phases. These results indicate that GluN2B-containing NMDARs in the FrA may be involved in the regulation of morphine-induced CPP and reinstatement.

Regulation of miR-128 in the nucleus accumbens affects methamphetamine-induced behavioral sensitization by modulating proteins involved in neuroplasticity

Methamphetamine (METH) -induced behavioral sensitization depends on long-term neuroplasticity in the mesolimbic dopamine system, especially in the nucleus accumbens (NAc). miR-128, a brain enriched miRNA, was found to have abilities in regulating neuronal excitability and formation of fear-extinction memory. Here, we aim to identify the role of miR-128 on METH-induced locomotor sensitization of male mice. We identified a significant increase of miR-128 in the NAc of mice upon repeated-intermittent METH exposure but not acute METH administration. Microinjection of adeno-associated virus (AAV)-miR-128 over-expression and inhibition constructs into the NAc of mice resulted in enhanced METH-induced locomotor sensitization and attenuated effects of METH respectively. Isobaric tags for relative and absolute quantification (iTRAQ) technology and ingenuity pathway analysis (IPA) were carried out to uncover the potential molecular mechanisms underlying miR-128-regulated METH sensitization. Differentially expressed proteins, including 25 potential targets for miR-128 were annotated in regulatory pathways that modulate dendritic spines, synaptic transmission and neuritogenesis. Of which, Arf6, Cpeb3 and Nlgn1, were found to be participating in miR-128-regulated METH sensitization. Consistently, METH-induced abnormal changes of Arf6, Cpeb3 and Nlgn1 in the NAc of mice were also detected by qPCR and validated by western blot analysis. Thus, miR-128 may contribute to METH sensitization through controlling neuroplasticity. Our study suggested miR-128 was an important regulator of METH- induced sensitization and also provided the potential molecular networks of miR-128 in regulating METH-induced sensitization.

Morphine coordinates SST and PV interneurons in the prelimbic cortex to disinhibit pyramidal neurons and enhance reward

Opioids, such as morphine, are clinic analgesics which induce euphoria. Morphine exposure modifies the excitability and functional interactions between neurons, while the underlying cellular and molecular mechanisms, especially how morphine assembles heterogeneous interneurons (INs) in prelimbic cortex (PrL) to mediate disinhibition and reward, are not clear. Using approaches of optogenetics, electrophysiology, and cell type-specific RNA-seq, we show that morphine attenuates the inhibitory synaptic transmission from parvalbumin⁺ (PV)-INs onto pyramidal neurons in PrL via μ-opioid receptor (MOR) in PV-INs. Meanwhile, morphine enhances the inhibitory inputs from somatostatin⁺ (SST)-INs onto PV-INs, and thus disinhibits pyramidal neurons via δ-opioid receptor (DOR)-dependent Rac1 upregulation in SST-INs. We show that MOR in PV-INs is required for morphine-induced behavioral sensitization, while DOR as well as Rac1 activity in SST-INs is required for morphine-induced conditioned place preference and hyper-locomotion. These results reveal that SST- and PV-INs, functioning in PrL as a disinhibitory architecture, are coordinated by morphine via different opioid receptors to disinhibit pyramidal neurons and enhance reward.

Confirmation of a Causal Taar1 Allelic Variant in Addiction-Relevant Methamphetamine Behaviors

Sensitivity to rewarding and reinforcing drug effects has a critical role in initial use, but the role of initial aversive drug effects has received less attention. Methamphetamine effects on dopamine re-uptake and efflux are associated with its addiction potential. However, methamphetamine also serves as a substrate for the trace amine-associated receptor 1 (TAAR1). Growing evidence in animal models indicates that increasing TAAR1 function reduces drug self-administration and intake. We previously determined that a non-synonymous single nucleotide polymorphism (SNP) in Taar1 predicts a conformational change in the receptor that has functional consequences. A Taar1 ^m1J mutant allele existing in DBA/2J mice expresses a non-functional receptor. In comparison to mice that possess one or more copies of the reference Taar1 allele (Taar1 ^+/+ or Taar1 ^+/m1J ), mice with the Taar1 ^m1J/m1J genotype readily consume methamphetamine, express low sensitivity to aversive effects of methamphetamine, and lack sensitivity to acute methamphetamine-induced hypothermia. We used three sets of knock-in and control mice in which one Taar1 allele was exchanged with the alternative allele to determine if other methamphetamine-related traits and an opioid trait are impacted by the same Taar1 SNP proven to affect MA consumption and hypothermia. First, we measured sensitivity to conditioned rewarding and aversive effects of methamphetamine to determine if an impact of the Taar1 SNP on these traits could be proven. Next, we used multiple genetic backgrounds to study the consistency of Taar1 allelic effects on methamphetamine intake and hypothermia. Finally, we studied morphine-induced hypothermia to confirm prior data suggesting that a gene in linkage disequilibrium with Taar1, rather than Taar1, accounts for prior observed differences in sensitivity. We found that a single SNP exchange reduced sensitivity to methamphetamine conditioned reward and increased sensitivity to conditioned aversion. Profound differences in methamphetamine intake and hypothermia consistently corresponded with genotype at the SNP location, with only slight variation in magnitude across genetic backgrounds. Morphine-induced hypothermia was not dependent on Taar1 genotype. Thus, Taar1 genotype and TAAR1 function impact multiple methamphetamine-related effects that likely predict the potential for methamphetamine use. These data support further investigation of their potential roles in risk for methamphetamine addiction and therapeutic development.

Prolonged functional cerebral asymmetry as a consequence of dysfunctional parvocellular paraventricular hypothalamic nucleus signaling: An integrative model for the pathophysiology of bipolar disorder

Approximately 45 million people worldwide are diagnosed with bipolar disorder (BD). While there are many known risk factors and models of the pathologic processes influencing BD, the exact neurologic underpinnings of BD are unknown. We attempt to integrate the existing literature and create a unifying hypothesis regarding the pathophysiology of BD with the hope that a concrete model may potentially facilitate more specific diagnosis, prevention, and treatment of BD in the future. We hypothesize that dysfunctional signaling from the parvocellular neurons of the paraventricular hypothalamic nucleus (PVN) results in the clinical presentation of BD. Functional damage to this nucleus and its signaling pathways may be mediated by myriad factors (e.g. immune dysregulation and auto-immune processes, polygenetic variation, dysfunctional interhemispheric connections, and impaired or overactivated hypothalamic axes) which could help explain the wide variety of clinical presentations along the BD spectrum. The neurons of the PVN regulate ultradian rhythms, which are observed in cyclic variations in healthy individuals, and mediate changes in functional hemispheric lateralization. Theoretically, dysfunctional PVN signaling results in prolonged functional hemispheric dominance. In this model, prolonged right hemispheric dominance leads to depressive symptoms, whereas left hemispheric dominance correlated to the clinical picture of mania. Subsequently, physiologic processes that increase signaling through the PVN (hypothalamic-pituitaryadrenal axis, hypothalamic- pituitary-gonadal axis, and hypothalamic-pituitary-thyroid axis activity, suprachiasmatic nucleus pathways) as well as, neuro-endocrine induced excito-toxicity, auto-immune and inflammatory flairs may induce mood episodes in susceptible individuals. Potentially, ultradian rhythms slowing with age, in combination with changes in hypothalamic axes and maturation of neural circuitry, accounts for BD clinically presenting more frequently in young adulthood than later in life.

Oridonin is an antidepressant molecule working through the PPAR-γ/AMPA receptor signaling pathway

Oridonin is a diterpene compound that regulates the activity of PPAR-γ (peroxisome proliferator-activated receptor gamma) transcription factor. Cumulative evidence indicates that AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-type glutamate receptors (AMPARs) play an important role in the treatment of depression. In the article, we found that after treatment with oridonin, the immobility time of mice was significantly reduced in the tail suspension test (TST) and the forced-swim test (FST). After five consecutive days of treatment in mice, oridonin significantly enhanced the expression of PPAR-γ, GluA1 (Ser845) phosphorylation, and GluA1 in the total protein extract of the prefrontal cortex (PFC). Blocking PPAR-γ was able to block antidepressant effects of oridonin. In synaptosome fractions of the PFC, oridonin treatment also significantly increased the GluA1 (Ser845) phosphorylation and GluA1 levels. Moreover, antidepressant actions of oridonin were blocked by AMPA receptor-specific antagonist GYKI 52466. This study demonstrates that oridonin regulates PPAR-γ/AMPA receptor signaling in the prefrontal cortex, and that oridonin can be identified as a novel antidepressant with clinical potential.

Cocaine-induced changes in behaviour and DNA methylation in rats are influenced by inter-individual differences in spontaneous exploratory activity

BACKGROUND

Individual differences in behavioural traits influence susceptibility to addictive disorders. Drug addiction involves changes in gene expression, proposed to occur via DNA methylation (DNAm).

AIMS

To investigate DNAm changes in reward-related brain structures (nucleus accumbens (NAc), lateral habenula (LHb)) in response to cocaine exposure in rats differing in spontaneous exploratory activity.

METHODS

Rats were observed in the exploration box and categorised as high- (HE) or low explorers (LE). Rats were administered vehicle or cocaine (12 mg/kg, i.p.) for 7 days, followed by a 14-day withdrawal period and cocaine challenge (7 mg/kg); horizontal locomotor activity was recorded. Brain tissue was dissected after 24 h; we analysed messenger RNA (mRNA) and activity levels of epigenetic DNA modifiers (DNMTs and TETs) as well as mRNA and promoter methylation levels at selected genes previously linked to addictive behaviours.

RESULTS

The cocaine challenge dose stimulated locomotor activity in both LE- and HE rats only when administered after a repeated cocaine schedule, suggesting development of behavioural sensitisation. Quantitative polymerase chain reaction analyses demonstrated higher basal expression of Dnmt3a, Tet2 and Tet3 in the LHb of HE- vs. LE rats, and we observed differential effects of cocaine exposure on the expression and activity of epigenetic DNA modifiers in the NAc and LHb of HE- and LE rats. Furthermore, cocaine exposure differentially altered promoter methylation levels of A_2AR, Ppp1cc, and Taar7b in the NAc and LHb of HE- and LE rats.

CONCLUSIONS

DNAm might play a role in the HE- and LE phenotypes as well as mediate behavioural effects of LE- and HE rats in response to drugs of abuse.

Lithium Pharmacology and a Potential Role of Lithium on Methamphetamine Abuse and Dependence

BACKGROUND

The effectiveness of lithium salts in neuropsychiatric disorders such as bipolar disorder, Alzheimer's disease, and treatment-resistant depression has been documented in an extensive scientific literature. Lithium inhibits inositol monophosphatase, inositol polyphosphate 1- phosphatase, and glycogen synthase kinase-3 and decreases expression level of tryptophan hydroxylase 2, conceivably underlying the mood stabilizing effects of lithium, as well as procognitive and neuroprotective effects. However, the exact molecular mechanisms of action of lithium on mood stabilizing and pro-cognitive effects in humans are still largely unknown.

OBJECTIVE

On the basis of the known aspects of lithium pharmacology, this review will discuss the possible mechanisms underlying the therapeutic effects of lithium on positive symptoms of methamphetamine abuse and dependence.

CONCLUSION

It is possible that lithium treatment reduces the amount of newly synthesized phosphatidylinositol, potentially preventing or reversing neuroadaptations contributing to behavioral sensitization induced by methamphetamine. In addition, it is suggested that exposure to repeated doses of methamphetamine induces hyperactivation of glycogen synthase kinase-3β in the nucleus accumbens and in dorsal hippocampus, resulting in a long-term alterations in synaptic plasticity underlying behavioral sensitization as well as other behavioral deficits in memory-related behavior. Therefore it is clear that glycogen synthase kinase-3β inhibitors can be considered as a potential candidate for the treatment of methamphetamine abuse and dependence.

Molecular changes in the nucleus accumbens and prefrontal cortex associated with the locomotor sensitization induced by coca paste seized samples

RATIONALE

In previous studies, we have demonstrated that seized samples of a smokable form of cocaine, also known as coca paste (CP), induced behavioral sensitization in rats. Interestingly, this effect was accelerated and enhanced when the samples were adulterated with caffeine. While the cocaine phenomenon is associated with persistent functional and structural alterations in the prefrontal cortex (PFC) and nucleus accumbens (NAc), the molecular mechanisms underlying the CP sensitization and the influence of caffeine remains still unknown.

OBJECTIVE

We examined the gene expression in NAc and mPFC after the expression caffeine-adulterated and non-adulterated CP locomotor sensitization.

METHODS

The locomotor sensitization was established in C57BL/6 mice, repeatedly treated with a CP-seized sample adulterated with caffeine (CP-2) and a non-adulterated one (CP-1). We then assessed the mRNA expression of receptor subunits of the dopaminergic and glutamatergic systems in the medial PFC (mPFC) and NAc. Other molecular markers (e.g., adenosinergic, endocannabinoid receptor subunits, and synaptic plasticity-associated genes) were also analyzed.

RESULTS

Only CP-2-treated mice expressed locomotor sensitization. This phenomenon was associated with increased Drd1a, Gria1, Cnr1, and Syn mRNA expression levels in the NAc. Drd3 mRNA expression levels were only significantly increased in mPFC of CP-2-treated group.

CONCLUSIONS

Our results demonstrated that caffeine actively collaborates in the induction of the molecular changes underlying CP sensitization. The present study provides new knowledge on the impact of active adulterants to understand the early dependence induced by CP consumption.

Locomotor sensitization in male Sprague-Dawley rats following repeated concurrent treatment with 4-methylmethcathinone and 3,4-methylenedioxymethamphetamine

Recreational abuse of illicit synthetic cathinones is an ongoing public health concern. Recent studies indicate that the methcathinone derivative 4-methylmethcathinone (4-MMC) produces behavioral and neurochemical effects similar to the entactogen 3,4-methylenedioxymethamphetamine (MDMA). Whereas polysubstance abuse is common, most preclinical studies of drug abuse liability only evaluate the effects of single drugs. Utilizing the locomotor sensitization paradigm, the present study assessed the combined locomotor stimulant effects of 4-MMC and MDMA for induction of sensitization following repeated administration and for expression of sensitization to a challenge dose of either substance alone after a 10-day period of drug abstinence. Male Sprague-Dawley rats received once daily intraperitoneal injections of saline, 4-MMC (1.0 mg/kg or 5.0 mg/kg), MDMA (3.0 mg/kg), or a mixture containing 4-MMC (1.0 mg/kg or 5.0 mg/kg) + MDMA (3.0 mg/kg) for 7 consecutive days. Following a 10-day drug-free period, rats were given a single intraperitoneal injection of either saline, 4-MMC (1.0 or 5.0 mg/kg), or 3.0 mg/kg MDMA. Activity was recorded for 1 h immediately before and 1 h immediately after injections on days 1, 7, and 17. 4-MMC treatment failed to induce locomotor sensitization, but, when combined with MDMA, sensitization was induced to a greater extent than with MDMA alone. Furthermore, the expression of sensitization to a subsequent challenge dose of MDMA was observed only in animals previously exposed to MDMA or a 5.0 mg/kg 4-MMC + MDMA mixture. In consideration of these findings along with the fact that 4-MMC has similar neurochemical actions to MDMA, further research may be warranted to determine the abuse liability of drug mixtures including 4-MMC and MDMA.

Effects of exposure to chronic uncertainty and a sensitizing regimen of amphetamine injections on locomotion, decision-making, and dopamine receptors in rats

Gambling disorder (GD) is a behavioral addiction that may be linked to alterations in dopamine (DA) systems. Gambling involves chronic exposure to uncertain reward, which can sensitize the activity of DA systems. Here we explored how combinations of Pavlovian and instrumental uncertainty impact DA sensitization and risky decision-making. Experiment 1: 40 rats underwent 66 uncertainty exposure (UE) sessions during which they responded for saccharin. Animal responding was reinforced according to a fixed or variable (FR/VR) ratio schedule that turned on a conditioned stimulus (CS; light), which predicted saccharin on 50% or 100% of trials. Animals responded under one of the four conditions: FR-CS100% (no uncertainty), VR-CS100%, FR-CS50%, and VR-CS50% (maximal uncertainty). DA sensitization was inferred from an enhanced locomotor response to d-amphetamine (d-AMPH; 0.5 mg/kg) challenge. The rat gambling task (rGT) was used to assess decision-making. Experiment 2: 24 rats received 5 weeks of sensitizing d-AMPH or saline doses, followed by locomotor activity and rGT testing. Experiment 3: Effects of UE and a sensitizing d-AMPH regimen on DA D1, D2, and D3 receptor binding were assessed in 44 rats using autoradiography. Compared to FR-CS100%, VR-CS100% and VR-CS50% rats displayed a greater locomotor response to d-AMPH, and VR-CS50% rats demonstrated riskier decision-making. Chronic d-AMPH-treated rats mirrored the effects of VR-CS50% groups on these two indices. Both VR-CS50% and d-AMPH-treated groups had increased striatal DA D2 receptor binding. These results suggest that chronic uncertainty exposure, similar to exposure to a sensitizing d-AMPH regimen, sensitized the function of DA systems and increased risky decision-making.

Brain Reward Function after Chronic and Binge Methamphetamine Regimens in Mice Expressing the HIV-1 TAT Protein

BACKGROUND

Methamphetamine abuse and human immunodeficiency virus (HIV) are common comorbidities. HIV-associated proteins, such as the regulatory protein TAT, may contribute to brain reward dysfunction, inducing an altered sensitivity to methamphetamine reward and/or withdrawal in this population.

OBJECTIVE

These studies examined the combined effects of TAT protein expression and, chronic and binge methamphetamine regimens on brain reward function.

METHODS

Transgenic mice with inducible brain expression of the TAT protein were exposed to either saline, a chronic, or a binge methamphetamine regimen. TAT expression was induced via doxycycline treatment during the last week of methamphetamine exposure. Brain reward function was assessed daily throughout the regimens, using the intracranial self-stimulation procedure, and after a subsequent acute methamphetamine challenge.

RESULTS

Both methamphetamine regimens induced withdrawal-related decreases in reward function. TAT expression substantially, but not significantly increased the withdrawal associated with exposure to the binge regimen compared to the chronic regimen, but did not alter the response to acute methamphetamine challenge. TAT expression also led to persistent changes in adenosine 2B receptor expression in the caudate putamen, regardless of methamphetamine exposure. These results suggest that TAT expression may differentially affect brain reward function, dependent on the pattern of methamphetamine exposure.

CONCLUSION

The subtle effects observed in these studies highlight that longer-term TAT expression, or its induction at earlier stages of methamphetamine exposure, may be more consequential at inducing behavioral and neurochemical effects.

Social Isolation in Male Rats During Adolescence Inhibits the Wnt/β-Catenin Pathway in the Prefrontal Cortex and Enhances Anxiety and Cocaine-Induced Plasticity in Adulthood

In adult animals, it is well established that stress has a proactive effect on psychostimulant responses. However, whether only a short period of stress during adolescence can also affect cocaine responses later in life and what mechanisms are involved are unknown. Here, we showed that 5 days of social isolation during rat adolescence had a long-term impact on anxiety-like behaviors, cocaine-induced conditioned place preference, and the expression of sensitization during adulthood. At the molecular level, social isolation decreased the activity of the Wnt/β-catenin pathway in the prefrontal cortex (PFC). Furthermore, after the expression of cocaine sensitization, isolated rats showed an increase in this pathway in the nucleus accumbens. Together, these findings suggest that, adolescent social isolation by altering the Wnt/β-catenin pathway in the developing PFC might increase the cocaine responses during adulthood, introducing this pathway as a novel neuroadaptation in the cortical-accumbens connection that may mediate a stress-induced increase in vulnerability to drugs.