Cocaine can generate a stronger conditioned reinforcer than food despite being a weaker primary reinforcer

Nucleus accumbens neuronal ensembles vary with cocaine reinforcement in male and female rats

Neuronal ensembles in the medial prefrontal cortex mediate cocaine self-administration via projections to the nucleus accumbens. We have recently shown that neuronal ensembles in the prelimbic cortex form rapidly to mediate cocaine self-administration. However, the role of neuronal ensembles within the nucleus accumbens in initial cocaine-seeking behaviour remains unknown. Here, we sought to expand the current literature by testing the necessity of the cocaine self-administration ensemble in the nucleus accumbens core (NAcCore) 1 day after male and female rats acquire cocaine self-administration by using the Daun02 inactivation procedure. We found that disrupting the NAcCore ensembles after a no-cocaine reward-seeking test increased subsequent cocaine seeking, while disrupting NAcCore ensembles following a cocaine self-administration session decreased subsequent cocaine seeking. We then characterized neuronal cell type in the NAcCore using RNAscope in situ hybridization. In the no-cocaine session, we saw reduced dopamine D1 type neuronal activation, while in the cocaine self-administration session, we found preferential dopamine D1 type neuronal activity in the NAcCore.

Drug memory reconsolidation: from molecular mechanisms to the clinical context

Since its rediscovery at the beginning of the 21st Century, memory reconsolidation has been proposed to be a therapeutic target for reducing the impact of emotional memories that can go awry in mental health disorders such as drug addiction (substance use disorder, SUD). Addiction can be conceptualised as a disorder of learning and memory, in which both pavlovian and instrumental learning systems become hijacked into supporting drug-seeking and drug-taking behaviours. The past two decades of research have characterised the details of the molecular pathways supporting the reconsolidation of pavlovian cue-drug memories, with more recent work indicating that the reconsolidation of instrumental drug-seeking memories also relies upon similar mechanisms. This narrative review considers what is known about the mechanisms underlying the reconsolidation of pavlovian and instrumental memories associated with drug use, how these approaches have translated to experimental medicine studies, and the challenges and opportunities for the clinical use of reconsolidation-based therapies.

The Role of Ryanodine Receptor 2 in Drug-Associated Learning

Type-2 ryanodine receptor (RyR2) ion channels facilitate the release of Ca 2+ from stores and serve an important function in neuroplasticity. The role for RyR2 in hippocampal-dependent learning and memory is well established and chronic hyperphosphorylation of RyR2 (RyR2P) is associated with pathological calcium leakage and cognitive disorders, including Alzheimer's disease. By comparison, little is known about the role of RyR2 in the ventral medial prefrontal cortex (vmPFC) circuitry important for working memory, decision making, and reward seeking. Here, we evaluated the basal expression and localization of RyR2 and RyR2P in the vmPFC. Next, we employed an operant model of sucrose, cocaine, or morphine self-administration (SA) followed by a (reward-free) recall test, to reengage vmPFC neurons and reactivate reward-seeking and re-evaluated the expression and localization of RyR2 and RyR2P in vmPFC. Under basal conditions, RyR2 was expressed in pyramidal cells but not regularly detected in PV/SST interneurons. On the contrary, RyR2P was rarely observed in PFC somata and was restricted to a different subcompartment of the same neuron - the apical dendrites of layer-5 pyramidal cells. Chronic SA of drug (cocaine or morphine) and nondrug (sucrose) rewards produced comparable increases in RyR2 protein expression. However, recalling either drug reward impaired the usual localization of RyR2P in dendrites and markedly increased its expression in somata immunoreactive for Fos, a marker of highly activated neurons. These effects could not be explained by chronic stress or drug withdrawal and instead appeared to require a recall experience associated with prior drug SA. In addition to showing the differential distribution of RyR2/RyR2P and affirming the general role of vmPFC in reward learning, this study provides information on the propensity of addictive drugs to redistribute RyR2P ion channels in a neuronal population engaged in drug-seeking. Hence, focusing on the early impact of addictive drugs on RyR2 function may serve as a promising approach to finding a treatment for substance use disorders.

Double dissociation of perirhinal nicotinic acetylcholine receptors and dopamine D2 receptors in modulation of object memory consolidation by nicotine, cocaine and their conditioned stimuli

There are indications that drug conditioned stimuli (CS) may activate neurochemical systems of memory modulation that are activated by the drugs themselves. To directly test this hypothesis, a cholinergic nicotinic receptor antagonist (mecamylamine; MEC: 0, 10 or 30 µg/side) and a dopamine D2 receptor antagonist (l-741,626: 0, 0.63, 2.5 µg/side) were infused in the perirhinal cortex (PRh) to block modulation of object recognition memory consolidation induced by 0.4 mg/kg nicotine, 20 mg/kg cocaine, or their CSs. To establish these CSs, male Sprague-Dawley rats were confined for 2 h in a chamber, the CS+, after injections of 0.4 mg/kg nicotine, or 20 mg/kg cocaine, and in another chamber, the CS-, after injections of vehicle. This was repeated over 10 days (5 drug/CS+ and 5 vehicle/CS- pairings in total). It was found that the memory enhancing action of post-sample nicotine was blocked by intra-PRh infusions of both MEC doses, and 30 µg/side MEC also blocked the memory enhancing action of the nicotine CS. Interestingly, intra-PRh MEC did not block the memory enhancing effect of cocaine, nor that of the cocaine CS. In contrast, the memory enhancing action of post-sample cocaine administration was blocked by both l-741,626 doses, and 2.5 µg/side also blocked the effect of the cocaine CS, but not the memory effects of nicotine or of the nicotine CS. This functional double dissociation strongly indicates that drug CSs modulate memory consolidation by activating neural systems that are activated by the drugs themselves.

Large-scale brain correlates of sweet versus cocaine reward in rats

Cocaine induces many supranormal changes in neuronal activity in the brain, notably in learning- and reward-related regions, in comparison with nondrug rewards-a difference that is thought to contribute to its relatively high addictive potential. However, when facing a choice between cocaine and a nondrug reward (e.g., water sweetened with saccharin), most rats do not choose cocaine, as one would expect from the extent and magnitude of its global activation of the brain, but instead choose the nondrug option. We recently showed that cocaine, though larger in magnitude, is also an inherently more delayed reward than sweet water, thereby explaining why it has less value during choice and why rats opt for the more immediate nondrug option. Here, we used a large-scale Fos brain mapping approach to measure brain responses to each option in saccharin-preferring rats, with the hope to identify brain regions whose activity may explain the preference for the nondrug option. In total, Fos expression was measured in 142 brain levels corresponding to 52 brain subregions and composing 5 brain macrosystems. Overall, our findings confirm in rats with a preference for saccharin that cocaine induces more global brain activation than the preferred nondrug option does. Only very few brain regions were uniquely activated by saccharin. They included regions involved in taste processing (i.e., anterior gustatory cortex) and also regions involved in processing reward delay and intertemporal choice (i.e., some components of the septohippocampal system and its connections with the lateral habenula).

Intermittent access training produces greater motivation for a non-drug reinforcer than long access training

It has recently been proposed that the intermittent access (IntA) drug self-administration procedure better produces behavioral changes relevant to addiction than the long access (LgA) procedure. In this version of the IntA procedure, the drug is made available for a 5-min period during each half hour of a 6-h session. In contrast, on the LgA procedure, the drug is available continuously for 6 h. Previous studies have found that IntA drug self-administration produces greater drug motivation, measured by increased progressive ratio breakpoints, than LgA self-administration. It has been hypothesized that this effect is due to the rapid, "spiking" brain levels of the drug, and consequent neuroadaptations, experienced by rats during IntA sessions. However, no study has compared the effects of IntA versus LgA training on reinforcer motivation when using a non-drug reinforcer. The present study compared motivation for a saccharin reinforcer after IntA or LgA training. In Experiment 1, separate groups of rats lever-pressed for saccharin on the IntA or LgA procedures. In Experiment 2, a within-subjects design was used where rats pressed one lever on the IntA procedure and another lever on the LgA procedure for saccharin. In both experiments, IntA training produced greater breakpoints than LgA training. As no drug was used here, spiking drug levels could not have been responsible for the increased saccharin motivation observed after IntA training. Instead, it is proposed that differences in stimulus-reinforcer associations learned during IntA versus LgA training may be responsible for the effect. Future research is needed to determine the extent to which such learning factors may contribute to the increased motivation observed after IntA training with drug reinforcers.

Cocaine falls into oblivion during volitional initiation of choice trials

When facing a choice, most animals quit drugs in favour of a variety of nondrug alternatives. We recently found, rather unexpectedly, that choice of the nondrug alternative is in fact inflexible and habitual. One possible contributing factor to habitual choice is the intermittency and uncontrollability of choice trials in previous studies. Here, we asked whether and to what extent volitional control over the occurrence of choice trials could change animals' preference by preventing habitual choice. To do so, rats were trained to nosepoke in a hole to trigger the presentation of two operant levers: one associated with cocaine, the other with saccharin. Rats were then free to choose among the two levers to obtain the corresponding reward, after which both levers retracted until rats self-initiated the next choice trial. Overall, we found that volitional control over choice trials did not change preference. Most rats preferred saccharin over cocaine and selected this option almost exclusively. Intriguingly, after repeated choice and consumption of saccharin, rats transiently lost interest in this option (i.e., due to sensory-specific satiety), but they did not switch to cocaine, preferring instead to pause during long periods of time before reinitiating a choice trial for saccharin. This finding suggests that during volitional initiation of a choice trial, rats fail to consider cocaine as an option. We discuss a possible associative mechanism to explain this perplexing behaviour.

Animal Models of the Behavioral Symptoms of Substance Use Disorders

To more effectively manage substance use disorders, it is imperative to understand the neural, genetic, and psychological underpinnings of addictive behavior. To contribute to this understanding, considerable efforts have been made to develop translational animal models that capture key behavioral characteristics of addiction on the basis of DSM5 criteria of substance use disorders. In this review, we summarize empirical evidence for the occurrence of addiction-like behavior in animals. These symptoms include escalation of drug use, neurocognitive deficits, resistance to extinction, exaggerated motivation for drugs, increased reinstatement of drug seeking after extinction, preference for drugs over nondrug rewards, and resistance to punishment. The occurrence of addiction-like behavior in laboratory animals has opened the opportunity to investigate the neural, genetic, and psychological background of key aspects of addiction, which may ultimately contribute to the prevention and treatment of substance use disorders.

Associative processes in addiction relapse models: A review of their Pavlovian and instrumental mechanisms, history, and terminology

Animal models of relapse to drug-seeking have borrowed heavily from associative learning approaches. In studies of relapse-like behaviour, animals learn to self-administer drugs then receive a period of extinction during which they learn to inhibit the operant response. Several triggers can produce a recovery of responding which form the basis of a variety of models. These include the passage of time (spontaneous recovery), drug availability (rapid reacquisition), extinction of an alternative response (resurgence), context change (renewal), drug priming, stress, and cues (reinstatement). In most cases, the behavioural processes driving extinction and recovery in operant drug self-administration studies are similar to those in the Pavlovian and behavioural literature, such as context effects. However, reinstatement in addiction studies have several differences with Pavlovian reinstatement, which have emerged over several decades, in experimental procedures, associative mechanisms, and terminology. Interestingly, in cue-induced reinstatement, drug-paired cues that are present during acquisition are omitted during lever extinction. The unextinguished drug-paired cue may limit the model’s translational relevance to cue exposure therapy and renders its underlying associative mechanisms ambiguous. We review major behavioural theories that explain recovery phenomena, with a particular focus on cue-induced reinstatement because it is a widely used model in addiction. We argue that cue-induced reinstatement may be explained by a combination of behavioural processes, including reacquisition of conditioned reinforcement and Pavlovian to Instrumental Transfer. While there are important differences between addiction studies and the behavioural literature in terminology and procedures, it is clear that understanding associative learning processes is essential for studying relapse.

Pharmacokinetics trumps pharmacodynamics during cocaine choice: a reconciliation with the dopamine hypothesis of addiction

Cocaine is known to increase brain dopamine at supranormal levels in comparison to alternative nondrug rewards. According to the dopamine hypothesis of addiction, this abnormally large dopamine response would explain why cocaine use is initially highly rewarding and addictive. Though resting on solid neuroscientific foundations, this hypothesis has nevertheless proven difficult to reconcile with research on cocaine choice in experimental animals. When facing a choice between an intravenous bolus of cocaine and a nondrug alternative (e.g., sweet water), both delivered immediately after choice, rats do not choose the drug, as would be predicted, but instead develop a strong preference for the nondrug alternative. Here we report evidence that reconciles this finding with the dopamine hypothesis of addiction. First, a systematic literature analysis revealed that the delays of effects of intravenous cocaine on nucleus accumbens dopamine are of the order of tens of seconds and are considerably longer than those of nondrug reward. Second, this was confirmed by measuring response times to cocaine omission during self-administration as a behavioral proxy of these delays. Finally, when the influence of the drug delays was reduced during choice by adding an increasing delay to both the drug and nondrug rewards, rats shifted their choice to cocaine. Overall, this study suggests that cocaine is indeed supranormal in reward magnitude, as postulated by the dopamine hypothesis of addiction, but is less preferred during choice because its pharmacokinetics makes it an inherently more delayed reward than the alternative. Reframing previous drug choice studies in rats as intertemporal choice studies reveals that the discounting effects of delays spare no rewards, including supranormal ones, and that during choice, pharmacokinetics trumps pharmacodynamics.

Cocaine and sucrose rewards recruit different seeking ensembles in the nucleus accumbens core

Poorly regulated reward seeking is a central feature of substance use disorder. Recent research shows that rewarding drug-related experiences induce synchronous activation of a discrete number of neurons in the nucleus accumbens that are causally linked to reward-related contexts. Here we comprehensively characterize the specific ensemble of neurons built through experience that are linked to seeking behavior. We additionally address the question of whether or not addictive drugs usurp the neuronal networks recruited by natural rewards by evaluating cocaine- and sucrose-associated ensembles within the same mouse. We used FosCreERT2/+/Ai14 transgenic mice to tag cells activated by and potentially encoding cocaine and sucrose seeking. We tagged ~1% of neurons in the core subregion of the accumbens (NAcore) activated during cue-induced seeking for cocaine or sucrose. The majority of tagged cells in the seeking ensembles were D1-MSNs, and specifically activated during seeking, not during extinction or when mice remained in the home cage. To compare different reward-specific ensembles within the same mouse, we used a dual cocaine and sucrose self-administration protocol allowing reward-specific seeking. Using this model, we found ~70% distinction between the cells constituting the cocaine- compared to the sucrose-seeking ensemble. Establishing that cocaine recruits an ensemble of NAcore neurons largely distinct from neurons recruited into an ensemble coding for sucrose seeking suggest a finely tuned specificity of ensembles. The findings allow further exploration of the mechanisms that transform reward-based positive reinforcement into maladaptive drug seeking.

Heroin choice depends on income level and economy type

RATIONALE

In a previous study, investigating choice between heroin and a non-drug alternative in animals and reductions in income (i.e., choices/day) caused the percentage of income spent on heroin to progressively decrease. In contrast, another study found that humans with opioid use disorder spent the majority of their income on heroin even though they had little income. Comparison of these two studies suggests that the seemingly conflicting results could be explained by differences in the underlying economy types of the choice alternatives.

OBJECTIVE

The present experiment tested the hypothesis that the effect of income changes on choice between heroin and a non-drug alternative depends on economy type.

METHODS

Rats chose between heroin and saccharin under three income levels. For the Closed group, the choice session was the only opportunity to obtain these reinforcers. For the Heroin Open group and the Saccharin Open group, choice sessions were followed by 3-h periods of unlimited access to heroin or saccharin, respectively.

RESULTS

As income decreased, the Closed and Heroin Open groups, but not the Saccharin Open group, spent an increasingly greater percentage of income on saccharin than on heroin. The Saccharin Open group, compared to the other groups, spent a greater percentage of income on heroin as income decreased.

CONCLUSIONS

Results confirm that the effects of income and economy type can interact and this may explain the apparently discrepant results of earlier studies. More generally, findings suggest that situations where heroin choice has little consequence for consumption of non-drug alternatives may promote heroin use.

Non-pharmacological factors that determine drug use and addiction

Based on their pharmacological properties, psychoactive drugs are supposed to take control of the natural reward system to finally drive compulsory drug seeking and consumption. However, psychoactive drugs are not used in an arbitrary way as pure pharmacological reinforcement would suggest, but rather in a highly specific manner depending on non-pharmacological factors. While pharmacological effects of psychoactive drugs are well studied, neurobiological mechanisms of non-pharmacological factors are less well understood. Here we review the emerging neurobiological mechanisms beyond pharmacological reinforcement which determine drug effects and use frequency. Important progress was made on the understanding of how the character of an environment and social stress determine drug self-administration. This is expanded by new evidence on how behavioral alternatives and opportunities for drug instrumentalization generate different patterns of drug choice. Emerging evidence suggests that the neurobiology of non-pharmacological factors strongly determines pharmacological and behavioral drug action and may, thus, give rise for an expanded system's approach of psychoactive drug use and addiction.

The effect of economy type on heroin and saccharin essential value

According to behavioral economics, reinforcer value should be lower in an open economy than in a closed economy. An animal model was used to determine how economy type affected the value of heroin and saccharin. In a first phase, separate groups of rats worked for heroin or saccharin. The price of these reinforcers increased over sessions. For rats in the open heroin or open saccharin economies, the work period of each session was followed by a postwork period where a cheaper source of heroin or saccharin was available for three hours. For rats in the closed economies, the work period was their only opportunity to obtain the reinforcer. Rats in the open saccharin economy worked less hard to defend consumption of saccharin as price increased than rats in the closed saccharin economy. That is, opening the saccharin economy reduced its essential value. In contrast, economy type had no effect on heroin's essential value. In a second phase, rats were allowed to choose between heroin and saccharin. The majority of rats strongly preferred saccharin over heroin regardless of economy type. The finding that economy type changed the essential value of saccharin, but not heroin, adds to previous findings suggesting that the value of drug reinforcers is unaffected by future drug availability. The difference in effect of economy type on drug versus nondrug reinforcers could be relevant to addiction. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Understanding Addiction Using Animal Models

Drug addiction is a neuropsychiatric disorder with grave personal consequences that has an extraordinary global economic impact. Despite decades of research, the options available to treat addiction are often ineffective because our rudimentary understanding of drug-induced pathology in brain circuits and synaptic physiology inhibits the rational design of successful therapies. This understanding will arise first from animal models of addiction where experimentation at the level of circuits and molecular biology is possible. We will review the most common preclinical models of addictive behavior and discuss the advantages and disadvantages of each. This includes non-contingent models in which animals are passively exposed to rewarding substances, as well as widely used contingent models such as drug self-administration and relapse. For the latter, we elaborate on the different ways of mimicking craving and relapse, which include using acute stress, drug administration or exposure to cues and contexts previously paired with drug self-administration. We further describe paradigms where drug-taking is challenged by alternative rewards, such as appetitive foods or social interaction. In an attempt to better model the individual vulnerability to drug abuse that characterizes human addiction, the field has also established preclinical paradigms in which drug-induced behaviors are ranked by various criteria of drug use in the presence of negative consequences. Separation of more vulnerable animals according to these criteria, along with other innate predispositions including goal- or sign-tracking, sensation-seeking behavior or impulsivity, has established individual genetic susceptibilities to developing drug addiction and relapse vulnerability. We further examine current models of behavioral addictions such as gambling, a disorder included in the DSM-5, and exercise, mentioned in the DSM-5 but not included yet due to insufficient peer-reviewed evidence. Finally, after reviewing the face validity of the aforementioned models, we consider the most common standardized tests used by pharmaceutical companies to assess the addictive potential of a drug during clinical trials.

The effect of nicotine pre-exposure on demand for cocaine and sucrose in male rats

The aim of the present study was to determine how nicotine pre-exposure affects the elasticity of demand for intravenous cocaine and for sucrose pellets in adult male rats. In Experiment 1, demand for cocaine was assessed in rats that had nicotine in their drinking water. Nicotine pre-exposure significantly decreased rats' willingness to defend cocaine consumption as the price (measured as the number of responses per cocaine infusion) increased compared with a control group with no nicotine pre-exposure. That is, nicotine increased the elasticity of demand for cocaine infusions. Experiment 2 repeated the first experiment, but with rats working for sucrose pellets instead of cocaine. Nicotine pre-exposure had no effect on the elasticity of demand for sucrose. This pattern of results suggests that nicotine pre-exposure can reduce the reinforcing effects of cocaine, but not sucrose, in adult male rats.

Discriminative stimuli are sufficient for incubation of cocaine craving

In abstinent drug addicts, cues formerly associated with drug-taking experiences gain relapse-inducing potency ('incubate') over time. Animal models of incubation may help develop treatments to prevent relapse, but these models have ubiquitously focused on the role of conditioned stimuli (CSs) signaling drug delivery. Discriminative stimuli (DSs) are unique in that they exert stimulus-control over both drug taking and drug seeking behavior and are difficult to extinguish. For this reason, incubation of the excitatory effects of DSs that signal drug availability, not yet examined in preclinical studies, could be relevant to relapse prevention. We trained rats to self-administer cocaine (or palatable food) under DS control, then investigated DS-controlled incubation of craving, in the absence of drug-paired CSs. DS-controlled cocaine (but not palatable food) seeking incubated over 60 days of abstinence and persisted up to 300 days. Understanding the neural mechanisms of this DS-controlled incubation holds promise for drug relapse treatments.

Cocaine, nicotine, and their conditioned contexts enhance consolidation of object memory in rats

To test the hypothesis that drugs of abuse and their conditioned stimuli (CSs) enhance memory consolidation, the effects of post-training exposure to cocaine and nicotine were compared to the effects of post-training exposure to contextual stimuli that were paired with the effects of these drugs. Using the object recognition (OR) task, it was first demonstrated that both 10 and 20 mg/kg cocaine, and 0.2 and 0.4 mg/kg nicotine, enhanced recognition memory when administered immediately after, but not 6 h after the sample phase. To establish the drug CSs, rats were confined for 2 h in a chamber (the CS+) after injections of 20 mg/kg cocaine, or 0.4 mg/kg nicotine, and in another chamber (the CS−) after injections of vehicle. This was repeated over 10 d (5 drug/CS+ and 5 vehicle/CS− pairings in total). At the end of this conditioning period, when tested in a drug-free state, rats displayed conditioned hyperactivity in the CS+ relative to the CS−. More important, immediate, but not delayed, post-sample exposure to the cocaine CS+, or nicotine CS+, enhanced OR memory. Therefore, this study reports for the first time that contextual stimuli paired with cocaine and nicotine, like the drugs themselves, have the ability to enhance memory consolidation.

Trying to make sense of rodents' drug choice behavior

Since the first experimental hint for the existence of "an actual desire or striving for the drug" in nonhuman animals by Sidney Spragg in the late 1930s, much effort has been expended by lab researchers to try to model in a valid manner the key behavioral aspects and signs of addiction in animals, typically in rodents (i.e., mainly rats and, to a lesser extent, mice). Despite much advances, there still remains a lingering doubt about the disordered status of drug use in rodents. This is mainly because drug use occurs in a particular setting where animals have access to a drug for self-administration but without access to other valuable behavioral options that could compete with and divert from drug use. Here I review evidence showing that enriching the drug setting with other behavioral options can dramatically influence the pattern of drug choices in rodents. Overall, access to other options during drug access can divert the vast majority of rats from continued drug use. Only few individuals continue to engage in drug use despite access to and at the expense of other options. However, there exist certain high-risk settings in which virtually all animals are vulnerable to develop a harmful pattern of exclusive drug use that can even become fatal in the long run if not discontinued by an outside intervention. Paradoxically, it appears that the behavioral trait that is hypothesized to uniquely render rodents vulnerable to the latter settings (i.e., a narrow focus on the local, current choice, with no consideration of the global pattern of choice) would also protect most of them from using drugs in other choice settings. I conclude with an attempt to make sense of this peculiar setting-specific behavior and with some general propositions for future research.

The effect of economy type on demand and preference for cocaine and saccharin in rats

BACKGROUND

Economy type is an important determinant of reinforcer value. This study investigated the effect of open and closed economies on demand and preference for cocaine and saccharin in rats.

METHODS

In the first phase, rats were trained to lever press for cocaine infusions or saccharin. The number of presses required for each reinforcer increased across sessions. Cocaine and saccharin economy type was manipulated over groups by varying post-session availability of these reinforcers. One group of rats had three hours' post-session access to unlimited cocaine (open economy). A second group had three hours' post-session access to unlimited saccharin. A third group had no post-session access to either reinforcer (closed economy). In a second phase, rats in the three conditions could make mutually exclusive choices for cocaine or saccharin.

RESULTS

Post-session access to saccharin caused saccharin demand to become more elastic. Post-session access to cocaine had no effect on demand for cocaine but made demand for saccharin more elastic. Results from the choice phase generally paralleled those from the demand phase, the main finding being that post-session saccharin access caused an increase in cocaine preference.

CONCLUSIONS

These results show that manipulating economy type can affect cocaine and non-drug reinforcers differently. Opening the saccharin economy decreased saccharin's value. Opening the cocaine economy did not decrease cocaine's value, but instead led to a devaluation of saccharin. These results suggest that cocaine choice may be determined not only by the reinforcers immediately available, but also by those reinforcers' broader contexts of availability.

Restoration of Kv7 Channel-Mediated Inhibition Reduces Cued-Reinstatement of Cocaine Seeking

Cocaine addicts display increased sensitivity to drug-associated cues, due in part to changes in the prelimbic prefrontal cortex (PL-PFC). The cellular mechanisms underlying cue-induced reinstatement of cocaine seeking remain unknown. Reinforcement learning for addictive drugs may produce persistent maladaptations in intrinsic excitability within sparse subsets of PFC pyramidal neurons. Using a model of relapse in male rats, we sampled >600 neurons to examine spike frequency adaptation (SFA) and afterhyperpolarizations (AHPs), two systems that attenuate low-frequency inputs to regulate neuronal synchronization. We observed that training to self-administer cocaine or nondrug (sucrose) reinforcers decreased SFA and AHPs in a subpopulation of PL-PFC neurons. Only with cocaine did the resulting hyperexcitability persist through extinction training and increase during reinstatement. In neurons with intact SFA, dopamine enhanced excitability by inhibiting Kv7 potassium channels that mediate SFA. However, dopamine effects were occluded in neurons from cocaine-experienced rats, where SFA and AHPs were reduced. Pharmacological stabilization of Kv7 channels with retigabine restored SFA and Kv7 channel function in neuroadapted cells. When microinjected bilaterally into the PL-PFC 10 min before reinstatement testing, retigabine reduced cue-induced reinstatement of cocaine seeking. Last, using cFos-GFP transgenic rats, we found that the loss of SFA correlated with the expression of cFos-GFP following both extinction and re-exposure to drug-associated cues. Together, these data suggest that cocaine self-administration desensitizes inhibitory Kv7 channels in a subpopulation of PL-PFC neurons. This subpopulation of neurons may represent a persistent neural ensemble responsible for driving drug seeking in response to cues.SIGNIFICANCE STATEMENT Long after the cessation of drug use, cues associated with cocaine still elicit drug-seeking behavior, in part by activation of the prelimbic prefrontal cortex (PL-PFC). The underlying cellular mechanisms governing these activated neurons remain unclear. Using a rat model of relapse to cocaine seeking, we identified a population of PL-PFC neurons that become hyperexcitable following chronic cocaine self-administration. These neurons show persistent loss of spike frequency adaptation, reduced afterhyperpolarizations, decreased sensitivity to dopamine, and reduced Kv7 channel-mediated inhibition. Stabilization of Kv7 channel function with retigabine normalized neuronal excitability, restored Kv7 channel currents, and reduced drug-seeking behavior when administered into the PL-PFC before reinstatement. These data highlight a persistent adaptation in a subset of PL-PFC neurons that may contribute to relapse vulnerability.

Essential values of cocaine and non-drug alternatives predict the choice between them

This study investigated the relationship between reinforcer value and choice between cocaine and two non-drug alternative reinforcers in rats. The essential value (EV, a behavioral economic measure based on elasticity of demand) of intravenous cocaine and food (Experiment 1) or saccharin (Experiment 2) was determined in the first phase of each experiment. Food had higher EV than cocaine, whereas the EVs of cocaine and saccharin did not differ. In the second phase of each experiment, rats were allowed to make mutually exclusive choices between cocaine and the non-drug alternative reinforcer. The main findings were that the EV of cocaine was a positive predictor of cocaine preference and the EV of food or saccharin was a negative predictor of cocaine preference. An analysis of within-session patterns of choice behavior revealed sequential dependencies, whereby rats were more likely to choose cocaine on a particular trial after having chosen the non-drug alternative on previous trials. When the time between choices was increased, these sequential dependencies disappeared. The results of these experiments are consistent with the suggestion that addiction-like behavior involves both overvaluation of drug reinforcers and undervaluation of non-drug reinforcers.

Corticostriatal plasticity, neuronal ensembles, and regulation of drug-seeking behavior

The idea that interconnected neuronal ensembles code for specific behaviors has been around for decades; however, recent technical improvements allow studying these networks and their causal role in initiating and maintaining behavior. In particular, the role of ensembles in drug-seeking behaviors in the context of addiction is being actively investigated. Concurrent with breakthroughs in quantifying ensembles, research has identified a role for synaptic glutamate spillover during relapse. In particular, the transient relapse-associated changes in glutamatergic synapses on accumbens neurons, as well as in adjacent astroglia and extracellular matrix, are key elements of the synaptic plasticity encoded by drug use and the metaplasticity induced by drug-associated cues that precipitate drug-seeking behaviors. Here, we briefly review the recent discoveries related to ensembles in the addiction field and then endeavor to link these discoveries with drug-induced striatal plasticity and cue-induced metaplasticity toward deeper neurobiological understandings of drug seeking.

Heroin and saccharin demand and preference in rats

BACKGROUND

Several recent studies have investigated the choice between heroin and a non-drug alternative reinforcer in rats. A common finding in these studies is that there are large individual differences in preference, with some rats preferring heroin and some preferring the non-drug alternative. The primary goal of the present study was to determine whether individual differences in how heroin or saccharin is valued, based on demand analysis, predicts choice.

METHODS

Rats lever-pressed for heroin infusions and saccharin reinforcers on fixed-ratio schedules. The essential value of each reinforcer was obtained from resulting demand curves. Rats were then trained on a mutually exclusive choice procedure where pressing one lever resulted in heroin and pressing another resulted in saccharin. After seven sessions of increased access to heroin or saccharin, rats were reexposed to the demand and choice procedures.

RESULTS

Demand for heroin was more elastic than demand for saccharin (i.e., heroin had lower essential value than saccharin). When allowed to choose, most rats preferred saccharin. The essential value of heroin, but not saccharin, predicted preference. The essential value of both heroin and saccharin increased following a week of increased access to heroin, but similar saccharin exposure had no effect on essential value. Preference was unchanged after increased access to either reinforcer.

CONCLUSION

Heroin-preferring rats differed from saccharin-preferring rats in how they valued heroin, but not saccharin. To the extent that choice models addiction-related behavior, these results suggest that overvaluation of opioids specifically, rather than undervaluation of non-drug alternatives, could identify susceptible individuals.

Food and drug addictions: Similarities and differences

This review examines the merits of 'food addiction' as an explanation of excessive eating (i.e., eating in excess of what is required to maintain a healthy body weight). It describes various apparent similarities in appetites for foods and drugs. For example, conditioned environmental cues can arouse food and drug-seeking behaviour, 'craving' is an experience reported to precede eating and drug taking, 'bingeing' is associated with both eating and drug use, and conditioned and unconditioned tolerance occurs to food and drug ingestion. This is to be expected, as addictive drugs tap into the same processes and systems that evolved to motivate and control adaptive behaviours, including eating. The evidence, however, shows that drugs of abuse have more potent effects than foods, particularly in respect of their neuroadaptive effects that make them 'wanted.' While binge eating has been conceptualised as form of addictive behaviour, it is not a major cause of excessive eating, because binge eating has a far lower prevalence than obesity. Rather, it is proposed that obesity results from recurrent overconsumption of energy dense foods. Such foods are, relatedly, both attractive and (calorie for calorie) weakly satiating. Limiting their availability could partially decrease excessive eating and consequently decrease obesity. Arguably, persuading policy makers that these foods are addictive could support such action. However, blaming excessive eating on food addiction could be counterproductive, because it risks trivialising serious addictions, and because the attribution of excessive eating to food addiction implies an inability to control one's eating. Therefore, attributing everyday excessive eating to food addiction may neither explain nor significantly help reduce this problem.

Cocaine Self-Administration Elevates GluN2B within dmPFC Mediating Heightened Cue-Elicited Operant Responding

Cue-elicited drug-craving correlates with hyperactivity within prefrontal cortex (PFC), which is theorized to result from dysregulated excitatory neurotransmission. The NMDA glutamate receptor is highly implicated in addiction-related neuroplasticity. As NMDA receptor function is regulated critically by its GluN2 subunits, herein, we assayed the relation between incubated cue-elicited cocaine-seeking following extended access to intravenous cocaine (6 h/d; 0.25 mg/infusion for 10 d) and the expression of GluN2A/B receptor subunits within PFC sub regions during early versus late withdrawal (respectively, 3 vs. 30 days). Cocaine-seeking rats exhibited elevated GluN2B expression within the dorsomedial aspect of the PFC (dmPFC); this effect was apparent at both 3 and 30 days withdrawal and occurred in cocaine-experienced rats, regardless of experiencing an extinction test or not. Thus, elevated dmPFC GluN2B expression appears to reflect a pharmacodynamic response to excessive cocaine intake that is independent of the duration of drug withdrawal or re-exposure to drug-taking context. The functional relevance of elevated dmPFC GluN2B expression for drug-seeking was assessed by the local infusion of the prototypical GluN2B-selective antagonist ifenprodil (1.0 µg/side). Ifenprodil did not alter cue-elicited responding in animals with a history of saline self-administration. In contrast, ifenprodil lowered cue-elicited cocaine-seeking, while potentiating cue-elicited sucrose-seeking. Thus, the effects of an intra-dmPFC ifenprodil infusion upon cue reactivity are reinforcer-specific, arguing in favor of targeting GluN2B-containing NMDA receptors as a pharmacological strategy for reducing behavioral reactivity to drug-associated cues with the potential benefit of heightening the reinforcing properties of cues associated with non-drug primary rewards.

Incubation of cocaine-craving relates to glutamate over-flow within ventromedial prefrontal cortex

Craving elicited by drug-associated cues intensifies across protracted drug abstinence - a phenomenon termed "incubation of craving" - and drug-craving in human addicts correlates with frontal cortical hyperactivity. Herein, we employed a rat model of cue-elicited cocaine-craving to test the hypothesis that the time-dependent incubation of cue-elicited cocaine-craving is associated with adaptations in dopamine and glutamate neurotransmission within the ventromedial prefrontal cortex (vmPFC). Rats were trained to self-administer intravenous cocaine (6 h/day × 10 days) and underwent in vivo microdialysis procedures during 2 h-tests for cue-elicited cocaine-craving at either 3 or 30 days withdrawal. Controls rats were trained to either self-administer sucrose pellets or received no primary reinforcer. Cocaine-seeking rats exhibited a withdrawal-dependent increase and decrease, respectively, in cue-elicited glutamate and dopamine release. These patterns of neurochemical change were not observed in either control condition. Thus, cue-hypersensitivity of vmPFC glutamate terminals is a biochemical correlate of incubated cocaine-craving that may stem from dopamine dysregulation in this region.

A test of stress, cues, and re-exposure to large wins as potential reinstaters of suboptimal decision making in rats

The present experiment investigated potential reinstaters of suboptimal economic decision making in rats. Rats were first trained on a version of the rat Gambling Task under conditions designed to promote choice of a suboptimal option that occasionally resulted in large “wins” (four sucrose pellets). In a second phase, preference for this economically suboptimal option was reduced by substantially increasing the probability of punishment when this option was chosen. Then, three events were tested for their ability to reinstate choice of the suboptimal option. A brief period of re-exposure to a high frequency of large wins significantly increased choice of the suboptimal option. The pharmacological stressor yohimbine did not reinstate suboptimal choice, but did increase impulsive action as indexed by premature responding. Presentation of cues previously associated with large wins did not alter behavior. Results suggest reinstaters of suboptimal choice may differ from reinstaters of extinguished drug- and food-seeking behavior.