The multiple faces of footshock punishment in animal research on addiction.

Continued drug use despite negative consequences is a hallmark of addiction commonly modelled in rodents using punished drug intake. Over the years, addiction research highlighted two subpopulations of punishment sensitive and resistant animals. While helpful to interrogate the neurobiology of drug-related behaviors, these procedures carry some weaknesses that need to be recognized and eventually defused. Mainly focusing on footshock-related work, we will first discuss the criteria used to define punishment-resistant animals and how their relative arbitrariness may impact our findings. With the overarching goal of improving our interpretation of the punishment-resistant phenotype, we will evaluate how tailored punishment protocols may better apprehend resistance to punishment, and how testing the robustness of punishment resistance could yield new results and strengthen interpretations. Second, we will question whether and to what extent punishment sensitivity, as currently defined, is reflective of abstinence and suggest that punishment resistance is, in fact, a prerequisite to model abstinence from addiction. Again, we will examine how challenging the robustness of the punishment-sensitive phenotype may help to better characterize it. Finally, we will evaluate whether diminished relapse-like behavior after repeated punishment-induced abstinence could not only contribute to better understand the mechanisms of abstinence, but also uniquely model progressive recovery (i.e., after repeated failed attempts at recovery) which is the norm in people with addiction. Altogether, by questioning the strengths and weaknesses of our models, we would like to open discussions on the different ways we interpret punishment sensitivity and resistance and the aspects that remain to be explored.

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).

The importance of choice and agency in animal models of addiction.

The aim of this short commentary is twofold. First, it uncovers and outlines in broad strokes a historical, albeit oft-overlooked, trend toward a growing place for choice and agency in the design of animal models of addiction. Next, it tries to draw from this historical trend some perspectives and implications for future research.

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.

Sugar intake and craving during alcohol withdrawal in alcohol use disorder inpatients.

To assess whether changes in sugar intake and craving occur during alcohol withdrawal in humans, we conducted a prospective, observational study in a university hospital addictions treatment center. Recruited patients had severe alcohol use disorder and were hospitalized for 7 days in the short-stay unit for alcohol withdrawal and then for 6 weeks in the rehabilitation unit. During the hospital stay, they had no access to alcohol but had full access to sweet products and beverages in a shop and vending machines located inside the hospital. Alcohol craving was assessed using a visual analogue scale on Days 1, 15, and 45. Sugar craving, sweet products stored by patients in their rooms, and weight were assessed on the same days. Thirty-five patients were included. Sugar craving increased in 14 patients during the hospital stay, whereas no change was observed in the remaining 21. Significant increases in both the amounts of sweet products stored in the patients' rooms (p < 0.02) and weight (p < 0.05) were observed only in the sugar craving group. During the same period, alcohol craving decreased significantly in all patients. Changes in tobacco smoking were not different according to the sugar craving status and therefore cannot explain the observed differences. In conclusion, increased intake and craving for sugar after alcohol withdrawal were observed in 40% of the patients included in our prospective study, and these results were similar to those of a study conducted in the alcohol post-dependent state model in rats.

Sugar, a powerful substitute for ethanol in ethanol postdependent rats: Relevance for clinical consideration?

Sugar has been shown to be a powerful substitute for drugs in preclinical studies on addiction. However, the link between sugar intake and alcohol use disorder (AUD) is poorly understood. We assessed the influence of sucrose on ethanol drinking in both nondependent (ND) and dependent (D) Long-Evans rats during acute withdrawal using the postdependent state model. Ethanol (10%-40%) and sucrose (1%-4%) solutions were offered in an operant paradigm either independently or concurrently under ratio schedules of reinforcement. We showed that D rats displayed an enhanced motivation for both 10% ethanol solution (10E) and 4% sucrose solution (4S) as compared with ND rats, and a clear preference for 4S was observed in both groups. During acute withdrawal, D rats showed a strong motivation for 30% ethanol (30E), even when adulterated with quinine, but still preferred 4S despite the fact that a high level of negative reinforcement could be expected. However, when a premix solution (30E4S) was offered concurrently with 4S, the preference for 4S was lost in D animals, which consumed as much premix as 4S, whereas ND animals displayed preference for 4S. Altogether, those results suggest that reinforcing properties of sucrose surpass those of ethanol in D rats under acute withdrawal, which indicates that sugar is a powerful substitute for ethanol. Our results suggest that craving for sugar may be increased in AUD patients during withdrawal and raise the issue of dependence transfer from alcohol to sugar.

Subthalamic nucleus high frequency stimulation prevents and reverses escalated cocaine use.

One of the key features of addiction is the escalated drug intake. The neural mechanisms involved in the transition to addiction remain to be elucidated. Since abnormal neuronal activity within the subthalamic nucleus (STN) stands as potential general neuromarker common to impulse control spectrum deficits, as observed in obsessive-compulsive disorders, the present study recorded and manipulated STN neuronal activity during the initial transition to addiction (i.e., escalation) and post-abstinence relapse (i.e., re-escalation) in rats with extended drug access. We found that low-frequency (theta and beta bands) neuronal oscillations in the STN increase with escalation of cocaine intake and that either lesion or high-frequency stimulation prevents the escalation of cocaine intake. STN-HFS also reduces re-escalation after prolonged, but not short, protracted abstinence, suggesting that STN-HFS is an effective prevention for relapse when baseline rates of self-administration have been re-established. Thus, STN dysfunctions may represent an underlying mechanism for cocaine addiction and therefore a promising target for the treatment of addiction.

Preference for Cocaine is Represented in the Orbitofrontal Cortex by an Increased Proportion of Cocaine Use-Coding Neurons.

Cocaine addiction is a harmful preference for drug use over and at the expense of other nondrug-related activities. Here we identify in the rat orbitofrontal cortex (OFC) a mechanism that explains individual preferences between cocaine use and an alternative, nondrug action. OFC neuronal activity was recorded while rats performed each of these 2 actions separately or while they chose between them. First, we found that these actions are encoded by 2 nonoverlapping neuronal populations and that the relative size of the cocaine population represented individual preferences. A larger relative size was only observed in cocaine-preferring individuals. Second, OFC neurons encoding a given individual's preferred action progressively fired more than other action-coding neurons few seconds before the preferred action was actually chosen, suggesting a prechoice neuronal competition for action selection. In cocaine-preferring rats, this manifested by a prechoice ramping-up activity in favor of the cocaine population. Finally, pharmacological manipulation of prechoice activity in favor of the cocaine population caused nondrug-preferring rats to shift their choice to cocaine. Overall, this study suggests that an individual preference for cocaine is represented in the OFC by a population size bias that systematically advantages cocaine use-coding neurons during prechoice competition for action selection.

Neuronal representation of individual heroin choices in the orbitofrontal cortex.

Drug addiction is a harmful preference for drug use over and at the expense of other non-drug-related activities. We previously identified in the rat orbitofrontal cortex (OFC) a mechanism that influences individual preferences between cocaine use and an alternative action rewarded by a non-drug reward (i.e. sweet water). Here, we sought to test the generality of this mechanism to a different addictive drug, heroin. OFC neuronal activity was recorded while rats responded for heroin or the alternative non-drug reward separately or while they chose between the two. First, we found that heroin-rewarded and sweet water-rewarded actions were encoded by two non-overlapping OFC neuronal populations and that the relative size of the heroin population represented individual drug choices. Second, OFC neurons encoding the preferred action-which was the non-drug action in the large majority of individuals-progressively fired more than non-preferred action-coding neurons 1 second after the onset of choice trials and around 1 second before the preferred action was actually chosen, suggesting a pre-choice neuronal competition for action selection. Together with a previous study on cocaine choice, the present study on heroin choice reveals important commonalities in how OFC neurons encode individual drug choices and preferences across different classes of drugs. It also reveals some drug-specific differences in OFC encoding activity. Notably, the proportion of neurons that non-selectively encode both the drug and the non-drug reward was higher when the drug was heroin (present study) than when it was cocaine (previous study). We will discuss the potential functional significance of these commonalities and differences in OFC neuronal activity across different drugs for understanding drug choice.

Rats quit nicotine for a sweet reward following an extensive history of nicotine use.

Drug use may be exacerbated in environments which lack alternative means of engaging in rewarding behaviour. When alternative rewards are available, drug use may decrease-an effect that can be harnessed for therapeutic benefit. This idea is particularly well-supported by recent preclinical evidence demonstrating that a majority of rats will readily choose a potent non-drug reward over cocaine or heroin. Here we examine whether the same holds true for nicotine, a drug considered to have one of the highest addiction liabilities amongst drugs of abuse. Rats were trained to nose-poke separately for saccharin or nicotine on alternate days. Using a discrete-trial, forced-choice procedure, rats were then allowed to choose between nicotine and saccharin. This was followed by choice testing after a decrease in saccharin concentration (0.2-0%), omission of the fluid reward, an increase in nicotine concentration and following an extended nicotine self-administration history. All rats demonstrated a clear and immediate preference for saccharin at all times. This was despite variations in reward concentrations, or after an extensive nicotine history. Notably, rats preferred to nose-poke for water over nicotine and would omit responses when no fluid was delivered, rather than resume responding for nicotine. Overall, this study confirms and extends to nicotine previous research on other drugs of abuse, including cocaine and heroin. The ease with which rats quit nicotine in the present study contrasts with the well-known difficulty of humans to quit tobacco smoking. Possible factors that could explain this apparent discrepancy are discussed.

Coordinated Recruitment of Cortical-Subcortical Circuits and Ascending Dopamine and Serotonin Neurons During Inhibitory Control of Cocaine Seeking in Rats.

People with cocaine addiction retain some degree of prefrontal cortex (PFC) inhibitory control of cocaine craving, a brain capacity that may underlie the efficacy of cognitive behavioral therapy for addiction. Similar findings were recently found in rats after extended access to and escalation of cocaine self-administration. Rats' inhibitory control of cocaine seeking was flexible, sufficiently strong to suppress cocaine-primed reinstatement and depended, at least in part, on neuronal activity within the prelimbic (PL) PFC. Here, we used a large-scale and high-resolution Fos mapping approach to identify, beyond the PL PFC, how top-down and/or bottom-up PFC-subcortical circuits are recruited during inhibition of cocaine seeking. Overall, we found that effective inhibitory control of cocaine seeking is associated with the coordinated recruitment of different top-down cortical-striatal circuits originating from different PFC territories, and of different bottom-up dopamine (DA) and serotonin (5-HT) midbrain subsystems that normally modulate activity in these circuits. This integrated brain response suggests that rats concomitantly engage and experience intricate cognitive and affective processes when they have to inhibit intense cocaine seeking. Thus, even after extended drug use, rats can be successfully trained to engage whole-brain inhibitory control mechanisms to suppress cocaine seeking.

Drug versus sweet reward: greater attraction to and preference for sweet versus drug cues.

Despite the unique ability of addictive drugs to directly activate brain reward circuits, recent evidence suggests that drugs induce reinforcing and incentive effects that are comparable to, or even lower than some nondrug rewards. In particular, when rats have a choice between pressing a lever associated with intravenous cocaine or heroin delivery and another lever associated with sweet water delivery, most respond on the latter. This outcome suggests that sweet water is more reinforcing and attractive than either drug. However, this outcome may also be due to the differential ability of sweet versus drug levers to elicit Pavlovian feeding-like conditioned responses that can cause involuntary lever pressing, such as pawing and biting the lever. To test this hypothesis, rats first underwent Pavlovian conditioning to associate one lever with sweet water (0.2% saccharin) and a different lever with intravenous cocaine (0.25 mg) or heroin (0.01 mg). Choice between these two levers was then assessed under two operant choice procedures: one that permitted the expression of Pavlovian-conditioned lever press responses during choice, the other not. During conditioning, Pavlovian-conditioned lever press responses were considerably higher on the sweet lever than on either drug lever, and slightly greater on the heroin lever than on the cocaine lever. Importantly, though these differences in Pavlovian-conditioned behavior predicted subsequent preference for sweet water during choice, they were not required for its expression. Overall, this study confirms that rats prefer the sweet lever because sweet water is more reinforcing and attractive than cocaine or heroin.

Levodopa gains psychostimulant-like properties after nigral dopaminergic loss.

Dopamine dysregulation syndrome shares some core behavioral features with psychostimulant addiction, suggesting that dopamine replacement therapy can acquire psychostimulantlike properties in some patients with Parkinson disease (PD). We here report strong experimental evidence supporting this hypothesis in an α-synuclein rat model of PD. Although levodopa had no effect in controls, it acquired 2 prominent psychostimulantlike properties in Parkinsonian rats: (1) it produced intense reward on its own and in parallel (2) decreased interest in other nondrug reward. These 2 effects may combine to explain the addictive use of levodopa after loss of midbrain dopamine neurons in some PD patients.

Bidirectional relationship between attentional deficits and escalation of nicotine intake in male rats.

RATIONALE: People with tobacco addiction have deficits in cognition, in particular deficits in attention. It is not clear however, whether deficits are a cause or a consequence, or both, of chronic nicotine use. Here we set out a series of experiments in rats to address this question and, more specifically, to assess the effects of exposure to and withdrawal from chronic nicotine self-administration on attentional performance. METHODS: Animals were trained in a 5-choice serial reaction time task to probe individual attentional performance and, then, were given access to a fixed versus increasing dose of intravenous nicotine for self-administration, a differential dose procedure known to induce two between-session patterns of nicotine intake: a stable versus escalation pattern. Attentional performance was measured daily before, during and also 24-h after chronic access to the differential dose procedure of nicotine self-administration. CONCLUSIONS: We found that pre-existing individual variation in attentional performance predicts individual vulnerability to develop escalation of nicotine intake. Moreover, while chronic nicotine self-administration increases attention, withdrawal from nicotine intake escalation induces attentional deficits, a withdrawal effect that is dose-dependently reversed by acute nicotine. Together, these results suggest that pre-existing individual variation in attentional performance predicts individual vulnerability to develop escalation of nicotine intake, and that part of the motivation for using nicotine during escalation might be to alleviate withdrawal-induced attentional deficits.

A large-scale c-Fos brain mapping study on extinction of cocaine-primed reinstatement.

Individuals with cocaine addiction can experience many craving episodes and subsequent relapses, which represents the main obstacle to recovery. Craving is often favored when abstinent individuals ingest a small dose of cocaine, encounter cues associated with drug use or are exposed to stressors. Using a cocaine-primed reinstatement model in rat, we recently showed that cocaine-conditioned interoceptive cues can be extinguished with repeated cocaine priming in the absence of drug reinforcement, a phenomenon we called extinction of cocaine priming. Here, we applied a large-scale c-Fos brain mapping approach following extinction of cocaine priming in male rats to identify brain regions implicated in processing the conditioned interoceptive stimuli of cocaine priming. We found that cocaine-primed reinstatement is associated with increased c-Fos expression in key brain regions (e.g., dorsal and ventral striatum, several prefrontal areas and insular cortex), while its extinction mostly disengages them. Moreover, while reinstatement behavior was correlated with insular and accumbal activation, extinction of cocaine priming implicated parts of the ventral pallidum, the mediodorsal thalamus and the median raphe. These brain patterns of activation and inhibition suggest that after repeated priming, interoceptive signals lose their conditioned discriminative properties and that action-outcome associations systems are mobilized in search for new contingencies, a brain state that may predispose to rapid relapse.

Knowledge by omission: the significance of omissions in the 5-choice serial reaction time task.

RATIONALE: The 5-choice serial reaction time task (5-CSRTT) is commonly used to assess attention in rodents. Manipulation of this task by decreasing the light stimulus duration is often used to probe attentional capacity and causes a decrease in accuracy and an increase in omissions. However, although a decrease in response accuracy is commonly interpreted as a decrease in attention, it is more difficult to interpret an increase in omissions in terms of attentional performance. METHODS: Here we present a series of experiments in rats that seeks to investigate the origins of these key behavioral measures of attention in the 5-CSRTT. After an initial training in the 5-CSRTT, rats were tested in a variable stimulus duration procedure to increase task difficulty and probe visual attentional capacity under several specific controlled conditions. CONCLUSIONS: We found that response accuracy reflects visuospatial sustained attentional processing, as commonly interpreted, while response omission reflects rats' ignorance about the stimulus location, presumably due to failure to pay attention to the curved wall during its presentation. Moreover, when rats lack of relevant information, they choose not to respond instead of responding randomly. Finally, pretreatment with nicotine selectively decreased response omissions, without affecting response accuracy, particularly when the attentional demand was high. Overall, our results indicate that response accuracy and response omission thus correspond to two distinct attentional states.

Reinforcing properties of Pramipexole in normal and parkinsonian rats.

Striatal D2 and D3 dopamine receptors are involved in mediating the reinforcing properties of natural rewards and drugs. In Parkinson's disease, while D2/3 dopamine agonists alleviate motor symptoms, behavioral addictions and withdrawal syndrome are reported in up to 15% of patients. The origin of such adverse effects is poorly understood but suggests that D2/3 agonists could possess reinforcing properties. We evaluated the reinforcing properties of the widely used D2/3 agonist, Pramipexole (PPX), in normal and parkinsonian rats. Intracerebroventricular injections of 6-OHDA induced a bilateral loss of tyrosine hydroxylase-positive cells in the substantia nigra (-51%) and ventral tegmental area (-31%). The animals were then allowed to self-administer intravenous PPX under fixed ratio and progressive ratio (PR) reinforcement schedules before being tested for extinction of PPX seeking. While parkinsonian were slower than sham rats in acquiring self-administration behavior, they later reached the same level of intake. The reinforcing value of PPX, as assessed during PR and extinction, was moderate in both groups. PPX heightened ∆FosB expression in dorsal striatum of lesioned rats and similar PR results involved different striatal subregions between groups. Altogether, our results show that drug-naïve rats self-administer PPX and that the dopaminergic lesion does not affect its reinforcing effects. While PPX reinforcing value was moderate in most rats, a subset of animals displayed a high number of responses and appeared to be particularly sensitive to this drug. These data suggest that PPX may not be responsible for the reported side-effects but rather call for further investigating the differential vulnerability among individuals.

Sugar addiction: pushing the drug-sugar analogy to the limit.

PURPOSE OF REVIEW: To review research that tests the validity of the analogy between addictive drugs, like cocaine, and hyperpalatable foods, notably those high in added sugar (i.e., sucrose). RECENT FINDINGS: Available evidence in humans shows that sugar and sweetness can induce reward and craving that are comparable in magnitude to those induced by addictive drugs. Although this evidence is limited by the inherent difficulty of comparing different types of rewards and psychological experiences in humans, it is nevertheless supported by recent experimental research on sugar and sweet reward in laboratory rats. Overall, this research has revealed that sugar and sweet reward can not only substitute to addictive drugs, like cocaine, but can even be more rewarding and attractive. At the neurobiological level, the neural substrates of sugar and sweet reward appear to be more robust than those of cocaine (i.e., more resistant to functional failures), possibly reflecting past selective evolutionary pressures for seeking and taking foods high in sugar and calories. SUMMARY: The biological robustness in the neural substrates of sugar and sweet reward may be sufficient to explain why many people can have difficultly to control the consumption of foods high in sugar when continuously exposed to them.

Altered neuronal activity in the ventromedial prefrontal cortex drives nicotine intake escalation.

Nicotine addiction develops after prolonged drug use and escalation of drug intake. However, because of difficulties in demonstrating escalation of nicotine use in rats, its underlying neuroadaptations still remain poorly understood. Here we report that access to unusually high doses of nicotine (i.e., from 30 µg to 240 µg/kg/injection) for self-administration precipitated a rapid and robust escalation of nicotine intake and increased the motivation for the drug in rats. This nicotine intake escalation also induced long-lasting changes in vmPFC neuronal activity both before and during nicotine self-administration. Specifically, after escalation of nicotine intake, basal vmPFC neuronal activity increased above pre-escalation and control activity levels, while ongoing nicotine self-administration restored these neuronal changes. Finally, simulation of the restoring effects of nicotine with in vivo optogenetic inhibition of vmPFC neurons caused a selective de-escalation of nicotine self-administration.

Diazepam promotes choice of abstinence in cocaine self-administering rats.

When facing a choice between cocaine and a potent, albeit inessential, non-drug alternative (i.e. water sweetened with saccharin), most cocaine self-administering rats abstain from cocaine in favor of the non-drug pursuit, regardless of the dose available and even after extended drug use. Only a minority continues to take the drug despite the opportunity of making a different choice and increasing stakes. This pattern of individual variation could suggest that the majority of rats are resilient to addiction, taking cocaine by default of other options. Only a minority would be vulnerable to addiction. This study tested the hypothesis that rats choose to refrain from cocaine self-administration because cocaine would be conflictual, having both rewarding and anxiogenic properties. Contrary to this hypothesis, however, we report here that diazepam-a broad-spectrum benzodiazepine anxiolytic-did not decrease, but instead, further increased cocaine abstinence. Interestingly, although diazepam decreased locomotion, rats adapted to this effect by spending more time near the lever associated with the preferred reward, a behavior that minimized the need for locomotion at the moment of choice. When responding for cocaine or saccharin was analyzed separately, we found that diazepam decreased responding for cocaine without affecting responding for saccharin. Finally, the abstinence-promoting effects of diazepam were also induced in cocaine-preferring rats treated chronically with diazepam. Overall, this study demonstrates that abstinence from cocaine cannot be explained away by the anxiogenic effects of cocaine, thereby reinforcing the notion of resilience to addiction. It also supports the use of benzodiazepines in the treatment of cocaine addiction.