Bidirectional modulation of cocaine expectancy by phasic glutamate fluctuations in the nucleus accumbens.

While glutamate in the nucleus accumbens (NAS) contributes to the promotion of drug-seeking by drug-predictive cues, it also appears to play a role in the inhibition of drug-seeking following extinction procedures. Thus we measured extracellular fluctuations of NAS glutamate in response to discriminative stimuli that signaled either cocaine availability or cocaine omission. We trained rats to self-administer intravenous cocaine and then to recognize discriminative odor cues that predicted either sessions where cocaine was available or alternating sessions where it was not (saline substituted for cocaine). Whereas responding in cocaine availability sessions remained stable, responding in cocaine omission sessions progressively declined to chance levels. We then determined the effects of each odor cue on extracellular glutamate in the core and shell subregions of NAS preceding and accompanying lever pressing under an extinction condition. Glutamate levels were elevated in both core and shell by the availability odor and depressed in the core but not the shell by the omission odor. Infusion of kynurenic acid (an antagonist for ionotropic glutamate receptors) into core but not shell suppressed responding associated with the availability odor, but had no effect on the suppression associated with the omission odor. Thus cocaine-predictive cues appear to promote cocaine seeking in part by elevating glutamatergic neurotransmission in the core of NAS, whereas cocaine-omission cues appear to suppress cocaine seeking in part by depressing glutamatergic receptor activation in the same region.

Satiating effects of cocaine are controlled by dopamine actions in the nucleus accumbens core.

Intravenous cocaine intake in laboratory animals is characterized by periods of apparent drug satiety between regularly spaced earned injections. The reinforcing properties of cocaine are linked primarily to dopaminergic neurotransmission in the shell and not the core of nucleus accumbens. To determine whether the satiating effects of cocaine are similarly mediated, we perfused dopamine receptor agonists into the core or the shell during intravenous cocaine self-administrations by rats. Neither D1-type (SKF38393) nor D2-type (quinpirole) agonist was effective when given alone. However, a combination of the two agonists perfused into the core but not the shell significantly increased the time between cocaine self-injections, decreasing the amount of earned intake. Together with previous findings, the current data suggest that the satiating and reinforcing effects of cocaine are mediated by different ventral striatal output neurons.

Cannabidiol Produces Distinct U-Shaped Dose-Response Effects on Cocaine-Induced Conditioned Place Preference and Associated Recruitment of Prelimbic Neurons in Male Rats.

BACKGROUND: Cannabidiol (CBD) has received attention for the treatment of substance use disorders. In preclinical models of relapse, CBD attenuates drug seeking across several drugs of abuse, including cocaine. However, in these models CBD has not been consistently effective. This inconsistency in CBD effects may be related to presently insufficient information on the full spectrum of CBD dose effects on drug-related behaviors. METHODS: We address this issue by establishing a full dose-response profile of CBD's actions using expression of cocaine-induced conditioned place preference as a model for drug-motivated behavior in male rats and by concurrently identifying dose-dependent effects of CBD on underlying neuronal activation and distinct neuronal phenotypes showing dose-dependent activation changes. Additionally, we established CBD levels in plasma and brain samples. RESULTS: CBD produced linear increases in CBD brain/plasma concentrations but suppressed conditioned place preference in a distinct U-shaped manner. In parallel with its behavioral effects, CBD produced U-shaped suppressant effects on neuronal activation in the prelimbic but not infralimbic cortex or nucleus accumbens core and shell. RNAscope in situ hybridization identified suppression of glutamatergic and GABAergic (gamma-aminobutyric acidergic) signaling in the prelimbic cortex as a possible cellular mechanism for the attenuation of cocaine-induced conditioned place preference by CBD. CONCLUSIONS: The findings extend previous evidence on the potential of CBD in preventing drug-motivated behavior. However, CBD's dose-response profile may have important dosing implications for future clinical applications and may contribute to the understanding of discrepant CBD effects on drug seeking reported in the literature.

Linking drug and food addiction via compulsive appetite.

BACKGROUND AND PURPOSE: 'Food addiction' is the subject of intense public and research interest. However, this nosology based on neurobehavioural similarities among obese individuals, patients with eating disorders and those with substance use disorders (drug addiction) remains controversial. We thus sought to determine which aspects of disordered eating are causally linked to preclinical models of drug addiction. We hypothesized that extensive drug histories, known to cause addiction-like brain changes and drug motivation in rats, would also cause addiction-like food motivation. EXPERIMENTAL APPROACH: Rats underwent extensive cocaine, alcohol, caffeine or obesogenic diet histories and were subsequently tested for punishment-resistant food self-administration or 'compulsive appetite', as a measure of addiction-like food motivation. KEY RESULTS: Extensive cocaine and alcohol (but not caffeine) histories caused compulsive appetite that persisted long after the last drug exposure. Extensive obesogenic diet histories also caused compulsive appetite, although neither cocaine nor alcohol histories caused excess calorie intake and bodyweight during abstinence. Hence, compulsive appetite and obesity appear to be dissociable, with the former sharing common mechanisms with preclinical drug addiction models. CONCLUSION AND IMPLICATIONS: Compulsive appetite, as seen in subsets of obese individuals and patients with binge-eating disorder and bulimia nervosa (eating disorders that do not necessarily result in obesity), appears to epitomize 'food addiction'. Because different drug and obesogenic diet histories caused compulsive appetite, overlapping dysregulations in the reward circuits, which control drug and food motivation independently of energy homeostasis, may offer common therapeutic targets for treating addictive behaviours across drug addiction, eating disorders and obesity.

Anti-relapse neurons in the infralimbic cortex of rats drive relapse-suppression by drug omission cues.

Drug addiction is a chronic relapsing disorder of compulsive drug use. Studies of the neurobehavioral factors that promote drug relapse have yet to produce an effective treatment. Here we take a different approach and examine the factors that suppress-rather than promote-relapse. Adapting Pavlovian procedures to suppress operant drug response, we determined the anti-relapse action of environmental cues that signal drug omission (unavailability) in rats. Under laboratory conditions linked to compulsive drug use and heightened relapse risk, drug omission cues suppressed three major modes of relapse-promotion (drug-predictive cues, stress, and drug exposure) for cocaine and alcohol. This relapse-suppression is, in part, driven by omission cue-reactive neurons, which constitute small subsets of glutamatergic and GABAergic cells, in the infralimbic cortex. Future studies of such neural activity-based cellular units (neuronal ensembles/memory engram cells) for relapse-suppression can be used to identify alternate targets for addiction medicine through functional characterization of anti-relapse mechanisms.

Mechanistic Resolution Required to Mediate Operant Learned Behaviors: Insights from Neuronal Ensemble-Specific Inactivation.

Many learned behaviors are directed by complex sets of highly specific stimuli or cues. The neural mechanisms mediating learned associations in these behaviors must be capable of storing complex cue information and distinguishing among different learned associations-we call this general concept "mechanistic resolution". For many years, our understanding of the circuitry of these learned behaviors has been based primarily on inactivation of specific cell types or whole brain areas regardless of which neurons were activated during the cue-specific behaviors. However, activation of all cells or specific cell types in a brain area do not have enough mechanistic resolution to encode or distinguish high-resolution learned associations in these behaviors. Instead, these learned associations are likely encoded within specific patterns of sparsely distributed neurons called neuronal ensembles that are selectively activated by the cues. This review article focuses on studies of neuronal ensembles in operant learned responding to obtain food or drug rewards. These studies suggest that the circuitry of operant learned behaviors may need to be re-examined using ensemble-specific manipulations that have the requisite level of mechanistic resolution.

Distinct memory engrams in the infralimbic cortex of rats control opposing environmental actions on a learned behavior.

Conflicting evidence exists regarding the role of infralimbic cortex (IL) in the environmental control of appetitive behavior. Inhibition of IL, irrespective of its intrinsic neural activity, attenuates not only the ability of environmental cues predictive of reward availability to promote reward seeking, but also the ability of environmental cues predictive of reward omission to suppress this behavior. Here we report that such bidirectional behavioral modulation in rats is mediated by functionally distinct units of neurons (neural ensembles) that are concurrently localized within the same IL brain area but selectively reactive to different environmental cues. Ensemble-specific neural activity is thought to function as a memory engram representing a learned association between environment and behavior. Our findings establish the causal evidence for the concurrent existence of two distinct engrams within a single brain site, each mediating opposing environmental actions on a learned behavior.

Sensitization of midbrain dopamine neuron reactivity promotes the pursuit of amphetamine.

Stimulant drugs such as amphetamine are readily self-administered by humans and laboratory animals by virtue of their actions on dopamine (DA) neurons of the midbrain. Repeated exposure to this drug systemically or exclusively in the cell body region of these neurons in the ventral tegmental area (VTA) leads to long-lasting changes in dopaminergic function that can be assessed by increased locomotor activity and enhanced DA overflow in the nucleus accumbens (NAcc) after re-exposure to the drug. Three experiments were conducted to evaluate the possibility that this enduring sensitized reactivity underlies compulsive drug self-administration. In all experiments, rats were pre-exposed to amphetamine and, starting 10 d later, their intravenous self-administration of the drug was assessed. In the first experiment, rats previously exposed to amphetamine systemically or exclusively in the VTA subsequently worked harder than untreated animals to obtain the drug when the work required to obtain successive infusions was increased progressively. In the second experiment, this progressively increasing workload was found to decrease the magnitude of amphetamine-induced DA overflow observed with successive infusions until responding ceased. Rats previously exposed to amphetamine were more resistant to this decline and more apt to maintain responding. Finally, in experiment three, a noncontingent priming injection of the drug produced a greater NAcc DA response and a greater parallel increase in lever pressing in drug compared with saline pre-exposed rats. Together, these results demonstrate a direct relation between drug-induced sensitization of midbrain dopamine neuron reactivity and the excessive pursuit and self-administration of an abused substance.

Previous exposure to VTA amphetamine enhances cocaine self-administration under a progressive ratio schedule in an NMDA, AMPA/kainate, and metabotropic glutamate receptor-dependent manner.

Previous exposure to amphetamine (AMPH) in the ventral tegmental area (VTA) enhances cocaine self-administration in a D(1) dopamine receptor-dependent manner. The present study examined the contribution of VTA NMDA, AMPA/kainate, and metabotropic glutamate (mGlu) receptors to this effect. Rats in different groups received three intra-VTA injections, one every third day, of either saline (0.5 microl/side), AMPH (2.5 microg/0.5 microl/side), AMPH+CPP (NMDA receptor antagonist; 10 microM or 100 microM/0.5 microl/side), AMPH+CNQX (AMPA/kainate receptor antagonist; 0.3 mM or 1 mM/0.5 microl/side), AMPH+MCPG (mGlu receptor antagonist; 0.5 mM or 50 mM/0.5 microl/side), or the glutamate receptor antagonists alone. Starting 7-10 days after the last pre-exposure injection, rats were trained to self-administer cocaine (0.3 mg/kg/infusion) and then tested under a progressive ratio (PR) schedule of reinforcement for 6 consecutive days. As reported previously, VTA AMPH pre-exposed rats worked more and obtained more infusions of cocaine than saline pre-exposed animals. Coadministration of CPP, CNQX, or MCPG with AMPH during pre-exposure dose-dependently blocked this enhancement of cocaine self-administration. Rats pre-exposed to the glutamate receptor antagonists alone did not differ on the test days from the saline pre-exposed controls. These results indicate that, in a manner paralleling the induction of sensitization of the locomotor stimulating effects of AMPH, activation of NMDA, AMPA/kainate, and mGlu receptors during pre-exposure to AMPH in the VTA is necessary for the enhancement of cocaine self-administration to develop.

Previous exposure to psychostimulants enhances the reinstatement of cocaine seeking by nucleus accumbens AMPA.

The effect of previous exposure to psychostimulants on the subsequent self-administration of cocaine as well as reinstatement of this behavior by priming infusions of AMPA into the nucleus accumbens (NAcc) was examined. Rats were exposed to five injections, one injection every third day, of either saline or amphetamine (AMPH: 1.5 mg/kg, i.p.). Starting 10 days later, they were trained to self-administer cocaine (0.3 mg/kg/infusion, i.v.) and subsequently tested under a progressive ratio (PR) schedule for 4 consecutive days. As expected, rats exposed to AMPH worked more and obtained more cocaine infusions than saline exposed controls on the PR test sessions. Following daily extinction sessions during which saline was substituted for cocaine, the effect of priming infusions of AMPA (0.0, 0.08, or 0.8 nmol/0.5 microl/side) into the NAcc was then examined on two tests: one conducted 4 days after the last cocaine PR test session (2-3 weeks after the last AMPH exposure injection) and the next 4 weeks later. Consistent with previous reports, NAcc AMPA dose-dependently reinstated cocaine seeking on both tests regardless of exposure condition. Importantly, this priming effect of NAcc AMPA was significantly enhanced in AMPH compared to saline exposed rats on the first test conducted 2-3 weeks after AMPH. On the second test, conducted 4 weeks after cocaine, reinstatement was similarly enhanced in both groups to levels observed on the first test in AMPH exposed rats. These results indicate that both noncontingent (AMPH) and contingent (cocaine) exposure to psychostimulants enhances the reinstatement of cocaine seeking by NAcc AMPA and appears to do so in a time-dependent manner.

Cocaine self-administration is not dependent upon mesocortical α1 noradrenergic signaling.

The rewarding properties of psychomotor stimulants are traditionally thought to be independent of norepinephrine. Recent findings, however, suggest that local noradrenergic signaling through α1 receptors in the medial prefrontal cortex and the ventral tegmental area - brain regions critically important in natural and drug rewards - is in a position to influence stimulant reward. Despite this controversy, the contribution of this targeted signaling to stimulant self-administration has not been directly assessed. We have thus examined whether pharmacological blockade of α1 receptors in the medial prefrontal cortex and ventral tegmental area alters cocaine self-administration. Rats were trained to lever-press for cocaine (1.0 mg/kg/infusion) under a fixed ratio 1 schedule of reinforcement for 10 days. After training, the rats received a bilateral microinjection of an α1 noradrenergic antagonist (terazosin: 1.0, 5.0, or 10 mM/side), a D1 dopaminergic antagonist (SCH23390: 12.3 mM/side), or saline into either the medial prefrontal cortex or ventral tegmental area immediately before a cocaine self-administration session. Although SCH23390 significantly increased cocaine self-administration when injected into either brain region, terazosin, at all doses and sites tested, failed to alter this behavior. Thus, the maintenance of cocaine self-administration appears to be under the influence of D1 dopaminergic, rather than α1 noradrenergic, signaling at these mesocortical sites.

Dorsal as well as ventral striatal lesions affect levels of intravenous cocaine and morphine self-administration in rats.

While the ventral striatum has long been implicated in the rewarding properties of psychomotor stimulants and opiates, little attention has been paid to the possible contribution of more dorsal regions of the striatum. We have thus examined the effects of lesions in three different striatal subregions on cocaine and morphine self-administration. Different groups of rats were trained to self-administer intravenous cocaine (1.0mg/kg/infusion) or morphine (0.5mg/kg/infusion) first under fixed ratio (FR) and then under progressive ratio (PR) schedules of reinforcement. Upon completion of the training, independent groups received bilateral electrolytic or sham lesions of the dorsal portion of the caudate-putamen (dCPu), the ventral portion of the caudate-putamen (vCPu) or the more ventral nucleus accumbens (NAS). Following recovery, they were tested for self-administration of cocaine (0.25, 0.5, 1.0 and 1.5mg/kg/infusion) or morphine (0.125, 0.25, 0.5 and 0.75mg/kg/infusion) under the PR schedule. The PR responding for each drug was significantly reduced in a dose-dependent manner following lesions of dCPu, vCPu and NAS. While the relative effectiveness of these lesions is likely to be specific to the conditions of this experiment, NAS lesions reduced self-administration of each drug to a greater extent than did dCPu or vCPu lesions.

Extracellular fluctuations of dopamine and glutamate in the nucleus accumbens core and shell associated with lever-pressing during cocaine self-administration, extinction, and yoked cocaine administration.

RATIONALE: Dopamine and glutamate in the nucleus accumbens (NAS) are differentially implicated in cocaine-directed behavior. OBJECTIVES: We sought to compare extracellular fluctuations of dopamine and glutamate in core and shell of NAS associated with operant responding during cocaine self-administration, extinction, and yoked cocaine administration. METHODS: Rats were trained to lever-press for cocaine or saline under FR1 before undergoing microdialysis testing during cocaine self-administration, extinction, or yoked cocaine administration. Microdialysis samples were collected every 20 min and were analyzed for dopamine and glutamate with high-performance liquid chromatography. RESULTS: Rats actively lever-pressed during cocaine self-administration and extinction. However, lever-pressing was minimal during yoked cocaine administration in both cocaine-trained and saline-trained rats. Dopamine was elevated throughout cocaine self-administration and yoked cocaine administration. The extent of cocaine-evoked dopamine was greater in shell than in core, greater in cocaine-trained than in saline-trained rats, and greater during self-administration than during yoked administration. Dopamine was also elevated in core (first 60 min) and in shell (first 40 min) during extinction. Basal concentration of glutamate, but not dopamine, was lower in cocaine-trained than in saline-trained rats. In cocaine-trained rats, glutamate was elevated during cocaine self-administration and extinction but was depressed below baseline during yoked cocaine administration. The extent and direction of glutamate fluctuation was similar between core and shell. In saline-trained rats, glutamate was not affected by yoked cocaine. CONCLUSION: Distinct patterns of dopamine and glutamate fluctuations in core and shell appear to underlie characteristic patterns of lever-pressing associated with cocaine self-administration, extinction, and yoked cocaine administration.

Control of within-binge cocaine-seeking by dopamine and glutamate in the core of nucleus accumbens.

RATIONALE: Dopamine and glutamate are thought to interact in the ventral striatum and to play important roles there in the cocaine-seeking of cocaine-experienced animals. OBJECTIVES: We sought to determine the relative roles of the two transmitters in the two major zones of the nucleus accumbens (NAS), the core and shell subregions. METHODS: We assessed the effects of dopamine and glutamate receptor blockade in the core and shell on intravenous cocaine self-administration in rats. Trained animals were allowed to self-administer cocaine for an initial hour, and then D1-type or D2-type dopamine receptor blockers or NMDA-type or AMPA-type glutamate receptor blockers were infused by reverse microdialysis into one of the two regions for an additional 3 h of testing. RESULTS: The D1-type antagonist SCH23390 and the D2-type antagonist raclopride each increased cocaine intake whereas the AMPA-type antagonist CNQX decreased responding when infused into the core. SCH23390 increased cocaine intake less strongly when infused into the shell, while raclopride and CNQX were each ineffective when infused into the shell. The NMDA-antagonist CPP failed to affect cocaine self-administration when infused into either site. CONCLUSIONS: These findings implicate the core of NAS in the maintenance of established cocaine self-administration in trained animals, despite the fact that the reinforcement of responding in untrained animals appears to results from cocaine actions in the olfactory tubercle and medial shell and not the core of accumbens.