Listening to the Data: Computational Approaches to Addiction and Learning.

Computational approaches hold great promise for identifying novel treatment targets and creating translational therapeutics for substance use disorders. From circuitries underlying decision-making to computationally derived neural markers of drug-cue reactivity, this review is a summary of the approaches to data presented at our 2023 Society for Neuroscience Mini-Symposium. Here, we highlight data- and hypothesis-driven computational approaches that recently afforded advancements in addiction and learning neuroscience. First, we discuss the value of hypothesis-driven algorithmic modeling approaches, which integrate behavioral, neural, and cognitive outputs to refine hypothesis testing. Then, we review the advantages of data-driven dimensionality reduction and machine learning methods for uncovering novel predictor variables and elucidating relationships in high-dimensional data. Overall, this review highlights recent breakthroughs in cognitive mapping, model-based analysis of behavior/risky decision-making, patterns of drug taking, relapse, and neuromarker discovery, and showcases the benefits of novel modeling techniques, across both preclinical and clinical data.

Compulsive-like eating of high-fat high-sugar food is associated with 'addiction-like' glutamatergic dysfunction in obesity prone rats.

Chronic overeating is a core feature of diet-induced obesity. There is increasing evidence that in vulnerable individuals, such overeating could become compulsive, resembling an addictive disorder. The transition to compulsive substance use has been linked with changes at glutamatergic synapses in the nucleus accumbens. In this study, we investigated a potential link between such glutamatergic dysregulation and compulsive-like eating using a rat model of diet-induced obesity. A conditioned suppression task demonstrated that diet-induced obese rats display eating despite negative consequences, as their consumption was insensitive to an aversive cue. Moreover, nucleus accumbens expression of GluA1 and xCT proteins was upregulated in diet-induced obese animals. Lastly, both a computed 'addiction score' (based on performance across three criteria) and weight gain were positively correlated with changes in GluA1 and xCT expression in the nucleus accumbens. These data demonstrate that the propensity for diet-induced obesity is associated with compulsive-like eating of highly palatable food and is accompanied by 'addiction-like' glutamatergic dysregulation in the nucleus accumbens, thus providing neurobiological evidence of addiction-like pathology in this model of obesity.

Regulation of cocaine-related behaviours by estrogen and progesterone.

Women are more sensitive to cocaine craving elicited by stimuli associated with relapse. Ovarian hormones modulate cocaine craving and may therefore function as risk factors or therapeutic agents for the development and treatment of cocaine use disorder, respectively. We review herein the neuropharmacological effects of the steroid hormones 17ß-estradiol, progesterone, and allopregnanolone, a progesterone metabolite, in relation to their effects on cocaine-induced locomotion, behavioural sensitization, conditioned place preference, and reinstatement of cocaine seeking. In general, the literature suggests that female rats are more sensitive to these cocaine-induced behaviours than males and that 17ß-estradiol facilitates the expression of these sex differences. Alternatively, in females, exogenous progesterone attenuates cocaine conditioned place preference, reinstatement, and possibly behavioural sensitization, either on its own or after conversion to allopregnanolone. These opposing effects of 17ß-estradiol and progesterone/allopregnanolone involve endocannabinoid, γ-aminobutyric acid, dopamine, and glutamate transmission in the medial prefrontal cortex and striatum. We conclude that 17ß-estradiol may be a risk factor for various components of cocaine use disorder in women, whereas progesterone and allopregnanolone may be potential treatment options.

Role of prefrontal cortex projections to the nucleus accumbens core in mediating the effects of ceftriaxone on cue-induced cocaine seeking.

Ceftriaxone is an antibiotic that reliably attenuates the reinstatement of cocaine seeking after extinction while preventing the nucleus accumbens (NA) core glutamate efflux that drives reinstatement. However, when rats undergo abstinence without extinction, ceftriaxone attenuates context-primed cocaine seeking but NA core glutamate efflux still increases. Here, we sought to determine if the same would occur when cocaine seeking is prompted by both context and discrete cues (cue-induced seeking) after cocaine abstinence. Male rats self-administered intravenous cocaine accompanied by drug-associated cues (light + tone) for 2 h/day for 14 days. Rats then experienced abstinence with daily handling but no extinction training for 2 weeks. Ceftriaxone (200 mg/kg IP) or vehicle was administered during the last 6 days of abstinence. During a cue-induced cocaine seeking test, microdialysis procedures were conducted. Rats were perfused at the end of the test for later Fos analysis. A separate cohort of rats was infused with the retrograde tracer cholera toxin B in the NA core and underwent the same self-administration and relapse procedures. Ceftriaxone increased baseline glutamate and attenuated both cue-induced cocaine seeking and NA core glutamate efflux during this test. Ceftriaxone reduced Fos expression in regions sending projections to the NA core (prefrontal cortex, basolateral amygdala, ventral tegmental area) and specifically reduced Fos in prelimbic cortex and not infralimbic cortex neurons projecting to the NA core. Thus, when cocaine seeking is induced by drug-associated cues, ceftriaxone is able to attenuate relapse by preventing NA core glutamate efflux, likely through reducing activity in prelimbic NA core-projecting neurons.

Nucleus accumbens GLT-1a overexpression reduces glutamate efflux during reinstatement of cocaine-seeking but is not sufficient to attenuate reinstatement.

Cocaine use disorder is a chronically relapsing disease without FDA-approved treatments. Using a rodent model of cocaine relapse, we and others have previously demonstrated that the beta-lactam antibiotic ceftriaxone attenuates cue- and cocaine-primed reinstatement of cocaine-seeking. Ceftriaxone restores cocaine-induced deficits in both system xc- and GLT-1 expression and function in the nucleus accumbens core (NAc). We recently demonstrated that restoration of GLT-1 expression in the NAc is necessary for ceftriaxone to attenuate reinstatement of cocaine-seeking. Here we used an adeno-associated virus (AAV) to overexpress GLT-1a in the NAc to investigate whether such restoration is sufficient to attenuate cue- and cocaine-primed reinstatement. Rats self-administered cocaine for two weeks and received injections of either AAV-GFAP-GLT-1a or AAV-GFAP-eGFP in the NAc following the last day of self-administration. Rats then underwent three weeks of extinction training (during which time transduction and expression occurred) before undergoing a cue- or cocaine-primed reinstatement test. Microdialysis for the quantification of glutamate efflux in the NAc was conducted during the cocaine-primed test. Rats that received AAV-GFAP-GLT-1a reinstated cue-primed cocaine-seeking in a similar manner as rats that received the control AAV-GFAP-eGFP. Upregulation of GLT-1a attenuated glutamate efflux during a cocaine-primed reinstatement test, but was not sufficient to attenuate reinstatement. We confirmed that GLT-1a upregulation resulted in functional upregulation of glutamate transport and expression, without affecting sodium-independent glutamate uptake, indicating system xc-was not altered. These results indicate that upregulation of NAc GLT-1 transporters alone is not sufficient to prevent the reinstatement of cocaine-seeking and implicate additional mechanisms in regulating glutamate efflux.

Repeated cycles of chronic intermittent ethanol exposure increases basal glutamate in the nucleus accumbens of mice without affecting glutamate transport.

Repeated cycles of chronic intermittent ethanol (CIE) exposure increase voluntary consumption of ethanol in mice. Previous work has shown that extracellular glutamate in the nucleus accumbens (NAc) is significantly elevated in ethanol-dependent mice and that pharmacologically manipulating glutamate concentrations in the NAc will alter ethanol drinking, indicating that glutamate homeostasis plays a crucial role in ethanol drinking in this model. The present studies were designed to measure extracellular glutamate at a time point in which mice would ordinarily be allowed voluntary access to ethanol in the CIE model and, additionally, to measure glutamate transport capacity in the NAc at the same time point. Extracellular glutamate was measured using quantitative microdialysis procedures. Glutamate transport capacity was measured under Na(+)-dependent and Na(+)-independent conditions to determine whether the function of excitatory amino acid transporters (also known as system XAG) or of system Xc (-) (glial cysteine-glutamate exchanger) was influenced by CIE exposure. The results of the quantitative microdialysis experiment confirm increased extracellular glutamate (approximately twofold) in the NAc of CIE exposed mice (i.e., ethanol-dependent) compared to non-dependent mice in the NAc, consistent with earlier work. However, the increase in extracellular glutamate was not due to altered transporter function in the NAc of ethanol-dependent mice, because neither Na(+)-dependent nor Na(+)-independent glutamate transport was significantly altered by CIE exposure. These findings point to the possibility that hyperexcitability of cortical-striatal pathways underlies the increases in extracellular glutamate found in the ethanol-dependent mice.

Glutamate transporter GLT-1 mediates N-acetylcysteine inhibition of cocaine reinstatement.

Both pre-clinical and clinical studies indicate that N-acetylcysteine (NAC) may be useful in treating relapse to addictive drug use. Cocaine self-administration in rats reduces both cystine-glutamate exchange and glutamate transport via GLT-1 in the nucleus accumbens, and NAC treatment normalizes these two glial processes critical for maintaining glutamate homeostasis. However, it is not known if one or both of these actions by NAC is needed to inhibit relapse to cocaine seeking. To determine whether the restoration of GLT-1 and/or cystine-glutamate exchange is required for NAC to inhibit cue-induced reinstatement of cocaine seeking, we utilized the rat self-administration/extinction/reinstatement model of cocaine relapse. Rats were pre-treated in the nucleus accumbens with vivo-morpholino antisense oligomers targeting either GLT-1 or xCT (catalytic subunit of the cystine-glutamate exchanger) overlapping with daily NAC administration during extinction (100 mg/kg, i.p. for the last 5 days). Rats then underwent cue-induced reinstatement of active lever pressing in the absence of NAC, to determine if preventing NAC-induced restoration of one or the other protein was sufficient to block the capacity of chronic NAC to inhibit reinstatement. The vivo-morpholino suppression of xCT reduced cystine-glutamate exchange but did not affect NAC-induced reduction of reinstated cocaine seeking. In contrast, suppressing NAC-induced restoration of GLT-1 not only prevented NAC from inhibiting reinstatement, but augmented the capacity of cues to reinstate cocaine seeking. We hypothesized that the increased reinstatement after inhibiting NAC induction of GLT-1 resulted from increased extracellular glutamate, and show that augmented reinstatement is prevented by blocking mGluR5. Restoring GLT-1, not cystine-glutamate exchange, is a key mechanism whereby daily NAC reduces cue-induced cocaine reinstatement.

Extinction training after cocaine self-administration induces glutamatergic plasticity to inhibit cocaine seeking.

Learning to inhibit drug seeking can be an important strategy for inhibiting relapse, and this can be modeled by extinguishing drug seeking in response to a drug-paired context. Rats were either extinguished or withdrawn without extinction training (abstinence) from cocaine self-administration, and measurements of postsynaptic density proteins in the core and shell subcompartments of the nucleus accumbens were compared with yoked-saline controls. Only extinguished rats had elevations of PSD-95, Homer1b/c, and Narp in the postsynaptic density of the core, whereas no proteins measured were altered in the postsynaptic density of the shell in either extinguished or abstinent rats. Using a biotinylation strategy, it was found that surface expression of mGluR5 was reduced only in the core of extinguished animals. Although both extinguished and abstinent animals showed a reduction in long-term potentiation elicited in the core by stimulating prefrontal cortex, blunted long-term depression was observed only in extinguished rats. These data indicate that the elevation in Homer1b/c in the core may have sequestered mGluR5 away from the membrane surface and that the loss of surface mGluR5 inhibits long-term depression. Accordingly, when Homer1c was overexpressed in the core of cocaine-naive rats with an adenoassociated virus, long-term depression was inhibited. This mechanism may contribute to the inhibition of cocaine seeking by extinction training because overexpression of Homer1c in the core also inhibited cue-induced reinstatement of cocaine seeking. These data identify a cellular mechanism that may contribute to extinction-induced inhibition of cocaine seeking.

Ceftriaxone restores glutamate homeostasis and prevents relapse to cocaine seeking.

BACKGROUND: The cystine-glutamate exchanger is downregulated after chronic cocaine, resulting in reduced extracellular levels of nucleus accumbens glutamate. The importance of cocaine-induced loss of glutamate homeostasis is revealed by N-acetylcysteine restoring cystine-glutamate exchange and attenuating reinstatement to cocaine seeking. Another regulator of extracellular glutamate is the glial glutamate transporter GLT-1. We hypothesized that cocaine self-administration reduces GLT-1 and that GLT-1 upregulation inhibits cocaine seeking. METHODS: We measured [(3)H] glutamate uptake and protein expression of GLT-1 and xCT, the catalytic subunit of the cystine-glutamate exchanger, following cocaine self-administration and 3 weeks of extinction training. We also examined the affect of ceftriaxone (previously shown to increase GLT-1) and N-acetylcysteine treatment on the expression of GLT-1 and xCT. Ceftriaxone was also tested for the capacity to inhibit cue- and cocaine-induced relapse. RESULTS: Cocaine self-administration reduced glutamate uptake and the expression of both GLT-1 and xCT. Ceftriaxone restored GLT-1 and xCT levels and prevented cue- and cocaine-induced reinstatement of drug seeking. N-acetylcysteine also restored GLT-1 and xCT levels. CONCLUSIONS: These results indicate that glutamate transport and cystine-glutamate exchange may be coregulated and provide further evidence that targeting glutamate homeostasis is a potential method for treating cocaine relapse.

Glutamate and reinstatement.

The importance of glutamate in the reinstatement of cocaine-seeking behavior has been established. New molecular and neurochemical adaptations in the glutamatergic system which drive cocaine relapse have been identified, such as the ability of CB1 receptor stimulation to reduce basal glutamate levels and the involvement of the GluR1 receptor subunit in reinstatement. Furthermore, it is apparent that similar glutamatergic neuroadaptations arise after self-administration of cocaine, heroin, nicotine, and alcohol. For example, reinstatement to cocaine, nicotine, and alcohol can be prevented both by the stimulation of group II mGluR receptors and by the blockade of group I mGluR receptors. The similarities in the neurochemistry behind relapse to these varied drug classes indicate that drugs that target the glutamate system could be effective at treating relapse to multiple types of drugs.

The role of cystine-glutamate exchange in nicotine dependence in rats and humans.

BACKGROUND: The present study determined if, akin to cocaine, nicotine self-administration in rats induces adaptations in the expression of glutamate transporters and cystine-glutamate exchangers in brain nuclei implicated in reinforcement and if treating cigarette smokers with a drug that restores cystine-glutamate exchange affects the number of cigarettes smoked. METHODS: Rats self-administered nicotine intravenously for 12 hours/day or received nicotine through osmotic minipumps for 21 days. Somatic signs of withdrawal were measured and immunoblotting was performed 12 hours after the last nicotine exposure to determine if the catalytic subunit of the cystine-glutamate exchanger, xCT, or the glial glutamate transporter, GLT-1, were altered in the ventral tegmental area (VTA), nucleus accumbens, prefrontal cortex, or amygdala. For the smoking reduction study in humans, nicotine-dependent smokers were treated for 4 weeks with N-acetylcysteine (2400 mg daily) to promote cystine-glutamate exchange or placebo. Participants provided weekly ratings of withdrawal symptoms, craving, and carbon monoxide (CO) measurements and logged daily cigarette and alcohol use. RESULTS: Rats receiving nicotine via self-administration or minipumps displayed somatic signs of withdrawal, but only nicotine self-administering rats showed decreased xCT expression in the nucleus accumbens and VTA and decreased GLT-1 expression in the nucleus accumbens. Human smokers treated with N-acetylcysteine reported a reduction in cigarettes smoked, and there was no effect of N-acetylcysteine on estimates of CO levels, craving, or withdrawal. CONCLUSIONS: These results indicate that the cystine-glutamate exchanger and the glial glutamate transporter are downregulated after nicotine self-administration, and augmenting exchanger activity with N-acetylcysteine reduced the number of cigarettes smoked in nicotine-dependent individuals.