Methamphetamine promotes habitual action and alters the density of striatal glutamate receptor and vesicular proteins in dorsal striatum

Ceftriaxone reverses diet-induced deficits in goal-directed control

RATIONALE

Obesity is associated with numerous health risks and ever-increasing rates are a significant global concern. However, despite weight loss attempts many people have difficulty maintaining weight loss. Previous studies in animals have shown that chronic access to an obesogenic diet can disrupt goal-directed behavior, impairing the ability of animals to flexibly adjust food-seeking behavior following changes in the value of earned outcomes. Changes in behavioral control have been linked to disruption of glutamate transmission in the dorsal medial striatum (DMS), a region critical for the acquisition and expression of goal-directed behavior.

OBJECTIVES

The goal of this study was to test whether ceftriaxone, a beta-lactam antibiotic shown elsewhere to upregulate the expression of the glutamate transporter GLT-1, would improve goal-directed control following long-term exposure to an obesogenic diet.

METHODS

Male and female rats were given access to either standard chow or chow plus sweetened condensed milk (SCM) for 6 weeks. Access to SCM was ended and rats received daily injections of either ceftriaxone or saline for 6 days. Rats were then trained to press a lever to earn a novel food reward and, finally, were assessed for sensitivity to outcome devaluation. Histological analyses examined changes to GLT-1 protein levels and morphological changes to astrocytes, within the DMS.

RESULTS

We found that ceftriaxone robustly restored goal-directed behavior in animals following long-term exposure to SCM. While we did not observe changes in protein levels of GLT-1 in the DMS, we observed that SCM induced changes in the morphology of astrocytes in the DMS, and that ceftriaxone mitigated these changes.

CONCLUSIONS

These results demonstrate that long-term access to a SCM diet impairs goal-directed behavior while also altering the morphology of astrocytes in the DMS. Furthermore, these results suggest that ceftriaxone administration can reverse the impairment of goal-directed behavior potentially through its actions on astrocytes in decision-making circuitry.

Changes in dorsomedial striatum activity during expression of goal-directed vs. habit-like cue-induced cocaine seeking

A preclinical model of cue exposure therapy, cue extinction, reduces cue-induced cocaine seeking that is goal-directed but not habit-like. Goal-directed and habitual behaviors differentially rely on the dorsomedial striatum (DMS) and dorsolateral striatum (DLS), but the effects of cue extinction on dorsal striatal responses to cue-induced drug seeking are unknown. We used fiber photometry in rats trained to self-administer cocaine paired with an audiovisual cue to examine how dorsal striatal intracellular calcium and extracellular dopamine activity differs between goal-directed and habit-like cue-induced cocaine seeking and how it is impacted by cue extinction. After minimal fixed-ratio training, rats showed enhanced DMS and DLS calcium responses to cue-reinforced compared to unreinforced lever presses. After rats were trained on goal-promoting fixed ratio schedules or habit-promoting second-order schedules of reinforcement, different patterns of dorsal striatal calcium and dopamine responses to cue-reinforced lever presses emerged. Rats trained on habit-promoting second-order schedules showed reduced DMS calcium responses and enhanced DLS dopamine responses to cue-reinforced lever presses. Cue extinction reduced calcium responses during subsequent drug seeking in the DMS, but not in the DLS. Therefore, cue extinction may reduce goal-directed behavior through its effects on the DMS, whereas habit-like behavior and the DLS are unaffected.

Post-transcriptional regulation and subcellular localization of G-protein γ7 subunit: implications for striatal function and behavioral responses to cocaine

The striatal D1 dopamine receptor (D1R) and A2a adenosine receptor (A2aR) signaling pathways play important roles in drug-related behaviors. These receptors activate the Golf protein comprised of a specific combination of αolfβ2γ7 subunits. During assembly, the γ7 subunit sets the cellular level of the Golf protein. In turn, the amount of Golf protein determines the collective output from both D1R and A2aR signaling pathways. This study shows the Gng7 gene encodes multiple γ7 transcripts differing only in their non-coding regions. In striatum, Transcript 1 is the predominant isoform. Preferentially expressed in the neuropil, Transcript 1 is localized in dendrites where it undergoes post-transcriptional regulation mediated by regulatory elements in its 3' untranslated region that contribute to translational suppression of the γ7 protein. Earlier studies on gene-targeted mice demonstrated loss of γ7 protein disrupts assembly of the Golf protein. In the current study, morphological analysis reveals the loss of the Golf protein is associated with altered dendritic morphology of medium spiny neurons. Finally, behavioral analysis of conditional knockout mice with cell-specific deletion of the γ7 protein in distinct populations of medium spiny neurons reveals differential roles of the Golf protein in mediating behavioral responses to cocaine. Altogether, these findings provide a better understanding of the regulation of γ7 protein expression, its impact on Golf function, and point to a new potential target and mechanisms for treating addiction and related disorders.

Neither Amphetamine nor Sub-Anesthetic Ketamine Treatment during Adolescence Impairs Devaluation in Rats Tested during Adulthood

BACKGROUND

Much of the existing animal literature on the devaluation task suggests that prior repeated exposure to drugs of abuse during adulthood can impair goal-directed action, but the literature on human drug users is mixed. Also, the initiation of drug use often occurs during adolescence, but examinations of the effects of drug exposure during adolescence on behavior in the devaluation task are lacking.

METHODS

We examined whether repeated exposure during adolescence to amphetamine (3 mg/kg injections every-other day from post-natal day 27-45) or ketamine (twice daily 30 mg/kg injections from post-natal day 35-44) would impair behavior in a devaluation test when tested drug-free in adulthood. Rats were trained to press a left lever with a steady cue-light above it for one reinforcer and a right lever with a flashing cue-light above it for a different reinforcer. We tested whether any impairments in goal-directed action could be overcome by compensation between strategies by giving rats information based on lever-location and cue-lights during the test that was either congruent (allowing compensation) or incongruent (preventing compensation between strategies) with the configurations during training.

RESULTS

Our results provided no evidence for impairment of goal-directed action during adulthood after adolescent amphetamine or ketamine exposure.

CONCLUSIONS

We discuss possible reasons for this discrepancy with the prior literature, including (1) the age of exposure and (2) the pattern in the previous literature that most previous demonstrations of drug exposure impairing devaluation in laboratory animals may be attributed to either drug-associated cues present in the testing environment and/or accelerated habit learning in tasks that predispose laboratory animals towards habit formation with extended training (with training procedures that should resist the formation of habits in the current experiment). However, additional research is needed to examine the effects of these factors, as well a potential role for the particular doses and washout periods to determine the cause of our finding of no devaluation impairment after drug exposure.

Impairments in expression of devaluation in a Pavlovian goal-tracking task, but not a free operant devaluation task, after fentanyl exposure in female rats

In laboratory animals, there are numerous demonstrations that past exposure to drugs of abuse can lead to devaluation impairments weeks after the final drug exposure, with the majority of these demonstrations examining effects of exposure to psychostimulants. There has been minimal investigation into whether prior exposure to opiates can lead to devaluation impairments. Here, we first trained female rats that two separate cuelights predicted two different foods and measured Pavlovian goal-tracking responses (Experiment 1) or trained female rats to press two levers to earn two different foods and measured this operant response (Experiment 2). In both experiments, we subsequently gave the rats injections of fentanyl twice daily for 6 days, and then tested rats for conditioned responses after satiation on one of the foods 48-h after the final injection. We found that rats were impaired in the expression of devaluation in the Pavlovian task after fentanyl exposure, but were unimpaired in the expression of devaluation in the operant task. The pattern of results is most consistent with an impairment in lateral orbitofrontal cortex function, but additional research is needed to determine the neurobiological cause of this pattern of results.

Incubation of methamphetamine craving in punishment-resistant individuals is associated with activation of specific gene networks in the rat dorsal striatum

Methamphetamine use disorder (MUD) is characterized by loss of control over compulsive drug use. Here, we used a self-administration (SA) model to investigate transcriptional changes associated with the development of early and late compulsivity during contingent footshocks. Punishment initially separated methamphetamine taking rats into always shock-resistant (ASR) rats that continued active lever pressing and shock-sensitive (SS) rats that reduced their lever pressing. At the end of the punishment phase, rats underwent 15 days of forced abstinence at the end of which they were re-introduced to the SA paradigm followed by SA plus contingent shocks. Interestingly, 36 percent of the initial SS rats developed delayed shock-resistance (DSR). Of translational relevance, ASR rats showed more incubation of methamphetamine craving than DSR and always sensitive (AS) rats. RNA sequencing revealed increased striatal Rab37 and Dipk2b mRNA levels that correlated with incubation of methamphetamine craving. Interestingly, Bdnf mRNA levels showed HDAC2-dependent decreased expression in the AS rats. The present SA paradigm should help to elucidate the molecular substrates of early and late addiction-like behaviors.

Impaired striatal glutamate/GABA regulation in violent offenders with antisocial personality disorder and psychopathy

Men with antisocial personality disorder (ASPD) with or without psychopathy (+/-P) are responsible for most violent crime in society. Development of effective treatments is hindered by poor understanding of the neurochemical underpinnings of the condition. Men with ASPD with and without psychopathy demonstrate impulsive decision-making, associated with striatal abnormalities in functional neuroimaging studies. However, to date, no study has directly examined the potential neurochemical underpinnings of such abnormalities. We therefore investigated striatal glutamate: GABA ratio using Magnetic Resonance Spectroscopy in 30 violent offenders (16 ASPD-P, 14 ASPD + P) and 21 healthy non-offenders. Men with ASPD +/- P had a significant reduction in striatal glutamate : GABA ratio compared to non-offenders. We report, for the first time, striatal Glutamate/GABA dysregulation in ASPD +/- P, and discuss how this may be related to core behavioral abnormalities in the disorders.

Changes in dorsomedial striatum activity mediate expression of goal-directed vs. habit-like cue-induced cocaine seeking

A preclinical model of cue exposure therapy, cue extinction, reduces cue-induced cocaine seeking when drug seeking is goal-directed but not habitual. Goal-directed and habitual behaviors differentially rely on the dorsomedial striatum (DMS) and dorsolateral striatum (DLS), but the effects of cue extinction on dorsal striatal responses to cue-induced drug seeking are unknown. We used fiber photometry to examine how dorsal striatal intracellular calcium and extracellular dopamine activity differs between goal-directed and habitual cue-induced cocaine seeking and how it is impacted by cue extinction. Rats trained to self-administer cocaine paired with an audiovisual cue on schedules of reinforcement that promote goal-directed or habitual cocaine seeking had different patterns of dorsal striatal calcium and dopamine responses to cue-reinforced lever presses. Cue extinction reduced calcium and dopamine responses during subsequent drug seeking in the DMS, but not in the DLS. Therefore, cue extinction may reduce goal-directed behavior through its effects on the DMS, whereas habitual behavior and the DLS are unaffected.

NMDA receptor blockade in the dorsolateral striatum impairs consolidation but not retrieval of habit memory

The present study investigated whether N-methyl-d-aspartate (NMDA) receptors in the dorsolateral striatum (DLS) mediate consolidation and retrieval of habit memory. Adult male Long-Evans rats were trained in a response learning version of a water plus-maze task in which rats were reinforced to make a habitual and consistent body-turn response at the maze choice point in order to mount a hidden escape platform. Prior research indicates that acquisition, consolidation, and retrieval in this task requires DLS function. The present study consisted of two experiments. In Experiment 1, rats received intra-DLS post-training injections of the NMDA receptor antagonist 2-amino-5- phosphonopentanoic acid (AP5; 2 µg/side) to examine the role of NMDA receptors in consolidation of habit memory. In Experiment 2, different groups of rats received a single pre-retrieval injection of AP5 in the DLS (AP5; 2 µg/side) during the last day of maze training to examine the potential role of NMDA receptors in retrieval of habit memory. Results indicated that post-training intra-DLS AP5 injections impaired memory consolidation. However, administration of AP5 at the same dose that impaired consolidation had no effect on memory retrieval. The findings are consistent with previous research indicating a role for NMDA receptors in the DLS in memory consolidation, and suggest that NMDA-dependent synaptic activity in the DLS may not be a critical component of habit memory retrieval.

Habitual behaviour associated with exposure to high-calorie diet is prevented by an orexin-receptor-1 antagonist

Habitual actions, which are associated with addictive behaviours, contribute to the loss of control of food seeking seen following exposure to calorie-dense foods in rats. Antagonism of orexin-receptor-1 (ORX-R1) has been shown to reduce a range of stimulus-driven feeding behaviours, but have yet to be implicated in the regulation of habitual actions. In the current study, male Long-Evans rats were given 'binge-like' access to high-calorie diet (HCD) or standard chow diet, and were subsequently trained to press a lever for food outcome. When lever responses were tested following outcome devaluation, chow-fed rats displayed goal-directed actions, whereas HCD-exposed rats displayed habitual actions. In study 1, it was shown that systemic administration of the ORX-R1 antagonist, SB-334867, prior to test restored goal-directed behaviour in HCD-exposed rats. In study 2, intra-nigral administration of SB-334867 similarly restored goal-directed behaviour, thereby implicating the substantia nigra as an important site for this effect. This study demonstrates that targeting ORX-R1 reduces habitual food seeking in male rats which may be important for understanding and treating compulsive feeding, obesity and binge eating disorder. This study also implicates the lateral hypothalamus, where ORX is produced, in mediating the expression of habits for the first time, and thus extends on the neurocircuits known to regulate habitual actions. Further investigation is required to determine whether the same effects are also seen in female rats, given that there are recognised sexual dimorphisms in feeding behaviour and a higher incidence of disordered eating in female than male populations.

Abnormal Brain Networks Related to Drug and Nondrug Reward Anticipation and Outcome Processing in Stimulant Use Disorder: A Functional Connectomics Approach

BACKGROUND

Drug addiction is associated with blunted neural responses to nondrug rewards, such as money, but heightened responses to drug cues that predict drug-reward outcomes. This dissociation underscores the role of incentive context in the attribution of motivational salience, which may reflect a narrowing toward drug-related goals. This hypothesis, however, has scarcely been investigated.

METHODS

To address this important scientific gap, the current study performed an empirical assessment of differences in salience attribution by comparing patients with stimulant use disorder (SUD) (n = 41) with control participants (n = 48) on network connectivity related to anticipation and outcome processing using a modified monetary incentive delay task. We hypothesized increased task-related activation and connectivity to drug rewards in patients with SUD, and reduced task-related activation and connectivity to monetary rewards during incentive processing across brain networks.

RESULTS

In the presence of behavioral and regional brain activation similarities, we found that patients with SUD showed significantly less connectivity involving three separate distributed networks during monetary reward anticipation, and drug and monetary reward outcome processing. No group connectivity differences for drug reward anticipation were identified. Additional graph theory analyses revealed that patients with SUD had longer path lengths across these networks, all of which positively correlated with the duration of stimulant drug use.

CONCLUSIONS

Specific disruptions in connectivity in networks related to the anticipation of nondrug reward together with more general dysconnectivity in the processing of rewarding outcomes suggest an insensitivity to consequences. These observations support the notion of a predominance of habitual control in patients with SUD.

Common and distinguishing genetic factors for substance use behavior and disorder: an integrated analysis of genomic and transcriptomic studies from both human and animal studies

BACKGROUND AND AIMS

Genomic and transcriptomic findings greatly broaden the biological knowledge regarding substance use. However, systematic convergence and comparison evidence of genome-wide findings is lacking for substance use. Here, we combined all the genome-wide findings from both substance use behavior and disorder (SUBD) and identified common and distinguishing genetic factors for different SUBDs.

METHODS

Systemic literature search for genome-wide association (GWAS) and RNA-seq studies of alcohol/nicotine/drug use behavior (partially meets or not reported diagnostic criteria) and alcohol use behavior and disorder (AUBD), nicotine use behavior and disorder (NUBD) and drug use behavior and disorder (DUBD) was performed using PubMed and the GWAS catalog. Drug use was focused upon cannabis, opioid, cocaine and methamphetamine use. GWAS studies required case-control or case/cohort samples. RNA-seq studies were based on brain tissues. The genes which contained significant single nucleotide polymorphism (P ≤ 1 × 10^-6 ) in GWAS and reported as significant in RNA-seq studies were extracted. Pathway enrichment was performed by using Metascape. Gene interaction networks were identified by using the Protein Interaction Network Analysis database.

RESULTS

Total SUBD-related 2910 genes were extracted from 75 GWAS studies (2 773 889 participants) and 17 RNA-seq studies. By overlapping the genes and pathways of AUBD, NUBD and DUBD, four shared genes (CACNB2, GRIN2B, PLXDC2 and PKNOX2), four shared pathways [two Gene Ontology (GO) terms of 'modulation of chemical synaptic transmission', 'regulation of trans-synaptic signaling', two Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways of 'dopaminergic synapse', 'cocaine addiction'] were identified (significantly higher than random, P < 1 × 10^-5 ). The top shared KEGG pathways (Benjamini-Hochberg-corrected P-value < 0.05) in the pairwise comparison of AUBD versus DUBD, NUBD versus DUBD, AUBD versus NUBD were 'Epstein-Barr virus infection', 'protein processing in endoplasmic reticulum' and 'neuroactive ligand-receptor interaction', respectively. We also identified substance-specific genetic factors: i.e. ADH1B and ALDH2 were unique for AUBD, while CHRNA3 and CHRNA4 were unique for NUBD.

CONCLUSIONS

This systematic review identifies the shared and unique genes and pathways for alcohol, nicotine and drug use behaviors and disorders at the genome-wide level and highlights critical biological processes for the common and distinguishing vulnerability of substance use behaviors and disorders.

Sustained inhibitory transmission but dysfunctional dopamine D2 receptor signaling in dorsal striatal subregions following protracted abstinence from amphetamine

Behavioral sensitization to amphetamine is a complex phenomenon that engages several neurotransmitter systems and brain regions. While dysregulated signaling in the mesolimbic dopamine system repeatedly has been linked to behavioral sensitization, later research has implicated dorsal striatal circuits and GABAergic neurotransmission in contributing to behavioral transformation elicited by amphetamine. The aim of this study was thus to determine if repeated amphetamine exposure followed by abstinence would alter inhibitory neurotransmission in dorsal striatal subregions. To this end, male Wistar rats received amphetamine (2.0 mg/kg) in an intermittent manner for a total of five days. Behavioral sensitization to amphetamine was measured in locomotor-activity boxes, while neuroadaptations were recorded in the dorsolateral (DLS) and dorsomedial striatum (DMS) using ex vivo electrophysiology at different timepoints of amphetamine abstinence (2 weeks, 4-5 weeks, 10-11 weeks). Data show that repeated drug-exposure produces behavioral sensitization to the locomotor-stimulatory properties of amphetamine, which sustains for at least ten weeks. Electrophysiological recordings demonstrated a long-lasting suppression of evoked population spikes in both striatal subregions. Furthermore, following ten weeks of abstinence, the responsiveness to a dopamine D2 receptor agonist was significantly impaired in brain slices from rats previously receiving amphetamine. However, neither the frequency nor the amplitude of spontaneous inhibitory currents was affected by treatment at any of the time points analyzed. In conclusion, passive administration of amphetamine initiates long-lasting neuroadaptations in brain regions associated with goal-directed behavior and habitual performance, but these transformations do not appear to be driven by changes in GABAergic neurotransmission.

Diet-induced deficits in goal-directed control are rescued by agonism of group II metabotropic glutamate receptors in the dorsomedial striatum

Many overweight or obese people struggle to sustain the behavioural changes necessary to achieve and maintain weight loss. In rodents, obesogenic diet can disrupt goal-directed control of responding for food reinforcers, which may indicate that diet can disrupt brain regions associated with behavioural control. We investigated a potential glutamatergic mechanism to return goal-directed control to rats who had been given an obesogenic diet prior to operant training. We found that an obesogenic diet reduced goal-directed control and that systemic injection of LY379268, a Group II metabotropic glutamate receptor (mGluR2/3) agonist, returned goal-directed responding in these rats. Further, we found that direct infusion of LY379268 into the dorsomedial striatum, a region associated with goal-directed control, also restored goal-directed responding in the obesogenic-diet group. This indicates that one mechanism through which obesogenic diet disrupts goal-directed control is glutamatergic, and infusion of a mGluR2/3 agonist into the DMS is sufficient to ameliorate deficits in goal-directed control.

Deconstructing and reconstructing behaviour relevant to mental health disorders: The benefits of a psychological approach, with a focus on addiction

RUTHERFORD, L.G. and Milton, A.L. Deconstructing and reconstructing behaviour relevant to mental health disorders: what can psychology offer? NEUROSCI BIOBEHAV REV XX(X)XXX-XXX, 2021. - Current treatments for mental health disorders are successful only for some patients, and there is an unmet clinical need for new treatment development. One challenge for treatment development has been how best to model complex human conditions in animals, where mechanism can be more readily studied with a range of neuroscientific techniques. We suggest that an approach to modelling based on associative animal learning theory provides a good framework for deconstructing complex mental health disorders such that they can be studied in animals. These individual simple models can subsequently be used in combination to 'reconstruct' a more complex model of the mental health disorder of interest. Using examples primarily from the field of drug addiction, we explore the 'psychological approach' and suggest that in addition to facilitating translation and backtranslation of tasks between animal models and patients, it is also highly concordant with the concept of triangulation.

Context, attention, and the switch between habit and goal-direction in behavior

This article reviews recent findings from the author’s laboratory that may provide new insights into how habits are made and broken. Habits are extensively practiced behaviors that are automatically evoked by antecedent cues and performed without their goal (or reinforcer) “in mind.” Goal-directed actions, in contrast, are instrumental behaviors that are performed because their goal is remembered and valued. New results suggest that actions may transition to habit after extended practice when conditions encourage reduced attention to the behavior. Consistent with theories of attention and learning, a behavior may command less attention (and become habitual) as its reinforcer becomes well-predicted by cues in the environment; habit learning is prevented if presentation of the reinforcer is uncertain. Other results suggest that habits are not permanent, and that goal-direction can be restored by several environmental manipulations, including exposure to unexpected reinforcers or context change. Habits are more context-dependent than goal-directed actions are. Habit learning causes retroactive interference in a way that is reminiscent of extinction: It inhibits, but does not erase, goal-direction in a context-dependent way. The findings have implications for the understanding of habitual and goal-directed control of behavior as well as disordered behaviors like addictions.

Dopamine sensitization by methamphetamine treatment prior to instrumental training delays the transition into habit in female rats

Early in instrumental learning, behavior is goal-directed and sensitive to changes in the value of the instrumental outcome. With sufficient repetition, responding becomes insensitive to changes in outcome value, or habitual. We have previously found that females transition into habit over a distinct range of training from 120 to 160 reinforced responses. This low level of instrumental training is markedly less than what has been shown to support habitual responding in male rats. To begin to investigate the early development of habit in females, we conducted a series of experiments in which we pretreated female rats with methamphetamine (METH) with the aim of sensitizing central dopamine, a major modulator of striatal function, prior to instrumental nose-poke training at the beginning and at the endpoint of the transition range in females. Following training, we tested for sensitivity to reinforcer devaluation (RD), which was conducted by repeatedly pairing reinforcers previously earned during training with lithium chloride (LiCl)-induced illness. As a counterpoint, a series of similar experiments was conducted separately in male rats. Additionally, in order to ascertain the validity of using nose-poke as an instrumental response, we compared sensitivity to devaluation between the Pavlovian approach towards the food magazine and the nose-poke response. In females, Vehicle groups responded in a habitual manner at both training levels (120 and 160 reinforced responses), whereas METH groups remained sensitive to devaluation. This suggests that increasing central dopamine delays habit formation in female rats. In male rats, Vehicle groups demonstrated goal-directed responding following training with 120 and 320 reinforced responses, and marginally goal-directed responding,with 160. METH-pretreated males were sensitive to devaluation at the 120 and 160 training levels, however, following more extended training to 320 reinforced responses, METH-pretreated males responded in a habitual manner, indicating that increasing central dopamine can advance habit formation in male rats. Overall, these results suggest that METH pretreatment maintains goal-directed responding in female rats when they are typically transitioning to habitual control of instrumental behavior and can advance habit formation in male rats given sufficient instrumental training. In addition, we found differential RD sensitivity of the nose-poke response used during instrumental training compared to Pavlovian approach towards the food magazine, confirming that there is a distinction between these two behaviors and that nose-poking is a valid instrumental response.

Effects of the GluN2B-selective antagonist Ro 63-1908 on acquisition and expression of methamphetamine conditioned place preference in male and female rats

BACKGROUND

Methamphetamine abuse has increased significantly in recent years. Currently, there are no FDA-approved pharmacotherapies for the treatment of methamphetamine use disorder. The goal of the current study was to determine if the N-methyl-d-aspartate (NMDA) GluN2B-selective antagonist Ro 63-1908 can block the conditioned rewarding effects of methamphetamine as assessed in conditioned place preference (CPP).

METHODS

Two main experiments were conducted. In the first experiment, male (n = 24) and female (n = 24) rats received either vehicle or Ro 63-1908 (1.0-10.0 mg/kg) 30 min prior to the posttest to determine if blocking the GluN2B subunit attenuates expression of methamphetamine CPP. In the second experiment, male (n = 18) and female (n = 18) rats received either vehicle or Ro 63-1908 (1.0 or 3.0 mg/kg) 30 min prior to each conditioning session to determine if blocking the GluN2B subunit attenuates acquisition of methamphetamine CPP.

RESULTS

Ro 63-1908 (3.0 mg/kg) blocked acquisition of methamphetamine CPP in male rats, but only attenuated CPP in female rats. Ro 63-1908 did not alter expression of CPP in either sex. Increasing the dose of Ro 63-1908 (10.0 mg/kg) failed to block acquisition of CPP in an additional group of female rats (n = 6). A control experiment showed that Ro 63-1908 (3.0 mg/kg) did not produce CPP or conditioned place aversion in male rats (n = 6) or in female rats (n = 6).

CONCLUSIONS

The results of this study show that Ro 63-1908 is able to decrease the conditioned rewarding effects of methamphetamine.

Reduced Glutamate Turnover in the Putamen Is Linked With Automatic Habits in Human Cocaine Addiction

BACKGROUND

The balance between goal-directed behavior and habits has been hypothesized to be biased toward the latter in individuals with cocaine use disorder (CUD), suggesting possible neurochemical changes in the putamen, which may contribute to their compulsive behavior.

METHODS

We assessed habitual behavior in 48 patients with CUD and 42 healthy control participants using a contingency degradation paradigm and the Creature of Habit Scale. In a subgroup of this sample (CUD: n = 21; control participants: n = 22), we also measured glutamate and glutamine concentrations in the left putamen using ultra-high-field (7T) magnetic resonance spectroscopy. We hypothesized that increased habitual tendencies in patients with CUD would be associated with abnormal glutamatergic metabolites in the putamen.

RESULTS

Compared with their non-drug-using peers, patients with CUD exhibited greater habitual tendencies during contingency degradation, which correlated with increased levels of self-reported daily habits. We further identified a significant reduction in glutamate concentration and glutamate turnover (glutamate-to-glutamine ratio) in the putamen in patients with CUD, which was significantly related to the level of self-reported daily habits.

CONCLUSIONS

Patients with CUD exhibit enhanced habitual behavior, as assessed both by questionnaire and by a laboratory paradigm of contingency degradation. This automatic habitual tendency is related to a reduced glutamate turnover in the putamen, suggesting a dysregulation of habits caused by chronic cocaine use.

Dorsolateral striatum dopamine-dependent cocaine seeking is resistant to pavlovian cue extinction in male and female rats

Cue exposure therapy (CET) reduces craving induced by drug-associated cues in individuals with substance use disorders. A preclinical model of CET, cue extinction, similarly reduces cue-induced cocaine seeking in rodent self-administration models; however, those models may not capture the habitual or compulsive aspects of drug use. Thus, the effectiveness of cue extinction was tested in male and female rats trained to self-administer cocaine using second-order (SO) or fixed-ratio (FR) schedules of reinforcement to facilitate dorsolateral striatum (DLS) dopamine-dependent or -independent response strategies, respectively. Cue extinction significantly reduced drug seeking in FR-trained rats, replicating prior results, but was ineffective in SO-trained rats. SO-trained rats also showed increased indices of glutamate signaling in the DLS relative to FR-trained rats, despite comparable levels of cocaine intake. Furthermore, in SO-trained rats, antagonism of AMPA receptors in the DLS rescued the efficacy of cue extinction to reduce drug seeking. Finally, the effectiveness of cue extinction was also revealed in SO-trained rats when they were taught to perform a new, non-habitual response for cocaine cue presentation. Overall, our findings indicate that habit-like drug seeking leads to plasticity in the DLS and behavior that is resistant to cue extinction, but that the effects of cue extinction are restored when DLS glutamatergic signaling is blocked. These results have implications for the effectiveness of clinical cue exposure therapy.

K369I Tau Mice Demonstrate a Shift Towards Striatal Neuron Burst Firing and Goal-directed Behaviour

Pathological forms of the microtubule-associated protein tau are involved in a large group of neurodegenerative diseases named tauopathies, including frontotemporal lobar degeneration (FTLD-tau). K369I mutant tau transgenic mice (K3 mice) recapitulate neural and behavioural symptoms of FTLD, including tau aggregates in the cortex, alterations to nigrostriatum, memory deficits and parkinsonism. The aim of this study was to further characterise the K3 mouse model by examining functional alterations to the striatum. Whole-cell patch-clamp electrophysiology was used to investigate the properties of striatal neurons in K3 mice and wildtype controls. Additionally, striatal-based instrumental learning tasks were conducted to assess goal-directed versus habitual behaviours (i.e., by examining sensitivity to outcome devaluation and progressive ratios). The K3 model demonstrated significant alterations in the discharge properties of striatal neurons relative to wildtype mice, which manifested as a shift in neuronal output towards a burst firing state. K3 mice acquired goal-directed responding faster than control mice and were goal-directed at test unlike wildtype mice, which is likely to indicate reduced capacity to develop habitual behaviour. The observed pattern of behaviour in K3 mice is suggestive of deficits in dorsal lateral striatal function and this was supported by our electrophysiological findings. Thus, both the electrophysiological and behavioural alterations indicate that K3 mice have early deficits in striatal function. This finding adds to the growing literature which indicate that the striatum is impacted in tau-related neuropathies such as FTLD, and further suggests that the K3 model is a unique mouse model for investigating FTLD especially with striatal involvement.

Molecular and circuit mechanisms regulating cocaine memory

Risk of relapse is a major challenge in the treatment of substance use disorders. Several types of learning and memory mechanisms are involved in substance use and have implications for relapse. Associative memories form between the effects of drugs and the surrounding environmental stimuli, and exposure to these stimuli during abstinence causes stress and triggers drug craving, which can lead to relapse. Understanding the neural underpinnings of how these associations are formed and maintained will inform future advances in treatment practices. A large body of research has expanded our knowledge of how associative memories are acquired and consolidated, how they are updated through reactivation and reconsolidation, and how competing extinction memories are formed. This review will focus on the vast literature examining the mechanisms of cocaine Pavlovian associative memories with an emphasis on the molecular memory mechanisms and circuits involved in the consolidation, reconsolidation, and extinction of these memories. Additional research elucidating the specific signaling pathways, mechanisms of synaptic plasticity, and epigenetic regulation of gene expression in the circuits involved in associative learning will reveal more distinctions between consolidation, reconsolidation, and extinction learning that can be applied to the treatment of substance use disorders.

Accelerated habitual learning resulting from L-dopa exposure in rats is prevented by N-acetylcysteine

Instrumental actions are initially goal-directed and driven by their associated outcome. However, with repeated experience habitual actions develop which are automated and efficient, as they are instead driven by antecedent stimuli. Dopamine is thought to facilitate the transition from goal-directed to habitual actions. This idea has been largely derived from evidence that psychostimulants accelerate the development of habitual actions. In the current study, we examined the impact of L-dopa (levodopa or L-dihydroxyphenylalanine), which also potentiates dopamine activity, on habitual learning. L-dopa was systemically administered prior to training rats to press a lever for a food outcome. When tested, L-dopa exposed animals were insensitive to changes in the value of the food outcome, and hence demonstrated accelerated habitual behavioral control compared to control animals that remained goal directed. We also showed that when N-acetylcysteine (NAC), an antioxidant and regulator of glutamate activity, was co-administered with L-dopa, it prevented the transition to habitual behavior; an effect demonstrated previously for cocaine. Therefore, this study establishes similarities between L-dopa and psychostimulants in both the development and prevention of habitual actions, and supports the notion that excess dopamine potentiates habitual learning. This finding extends the limited existing knowledge of the impact of L-dopa on learning and behavior, and has implications for neurological disorders where L-dopa is the primary treatment.

Individual differences in the neuropsychopathology of addiction

Drug addiction or substance-use disorder is a chronically relapsing disorder that progresses through binge/intoxication, withdrawal/negative affect and preoccupation/anticipation stages. These stages represent diverse neurobiological mechanisms that are differentially involved in the transition from recreational to compulsive drug use and from positive to negative reinforcement. The progression from recreational to compulsive substance use is associated with downregulation of the brain reward systems and upregulation of the brain stress systems. Individual differences in the neurobiological systems that underlie the processing of reward, incentive salience, habits, stress, pain, and executive function may explain (i) the vulnerability to substance-use disorder; (ii) the diversity of emotional, motivational, and cognitive profiles of individuals with substance-use disorders; and (iii) heterogeneous responses to cognitive and pharmacological treatments. Characterization of the neuropsychological mechanisms that underlie individual differences in addiction-like behaviors is the key to understanding the mechanisms of addiction and development of personalized pharmacotherapy.

Individual differences in the neuropsychopathology of addiction

Drug addiction or substance-use disorder is a chronically relapsing disorder that progresses through binge/intoxication, withdrawal/negative affect and preoccupation/anticipation stages. These stages represent diverse neurobiological mechanisms that are differentially involved in the transition from recreational to compulsive drug use and from positive to negative reinforcement. The progression from recreational to compulsive substance use is associated with downregulation of the brain reward systems and upregulation of the brain stress systems. Individual differences in the neurobiological systems that underlie the processing of reward, incentive salience, habits, stress, pain, and executive function may explain (i) the vulnerability to substance-use disorder; (ii) the diversity of emotional, motivational, and cognitive profiles of individuals with substance-use disorders; and (iii) heterogeneous responses to cognitive and pharmacological treatments. Characterization of the neuropsychological mechanisms that underlie individual differences in addiction-like behaviors is the key to understanding the mechanisms of addiction and development of personalized pharmacotherapy.