The Winding Road to Relapse: Forging a New Understanding of Cue-Induced Reinstatement Models and Their Associated Neural Mechanisms

In drug addiction, cues previously associated with drug use can produce craving and frequently trigger the resumption of drug taking in individuals vulnerable to relapse. Environmental stimuli associated with drugs or natural reinforcers can become reliably conditioned to increase behavior that was previously reinforced. In preclinical models of addiction, these cues enhance both drug self-administration and reinstatement of drug seeking. In this review, we will dissociate the roles of conditioned stimuli as reinforcers from their modulatory or discriminative functions in producing drug-seeking behavior. As well, we will examine possible differences in neurobiological encoding underlying these functional differences. Specifically, we will discuss how models of drug addiction and relapse should more systematically evaluate these different types of stimuli to better understand the neurobiology underlying craving and relapse. In this way, behavioral and pharmacotherapeutic interventions may be better tailored to promote drug use cessation outcomes and long-term abstinence

Interactions of Neuroimmune Signaling and Glutamate Plasticity in Addiction

Chronic use of drugs of abuse affects neuroimmune signaling; however, there are still many open questions regarding the interactions between neuroimmune mechanisms and substance use disorders (SUDs). Further, chronic use of drugs of abuse can induce glutamatergic changes in the brain, but the relationship between the glutamate system and neuroimmune signaling in addiction is not well understood. Therefore, the purpose of this review is to bring into focus the role of neuroimmune signaling and its interactions with the glutamate system following chronic drug use, and how this may guide pharmacotherapeutic treatment strategies for SUDs. In this review, we first describe neuroimmune mechanisms that may be linked to aberrant glutamate signaling in addiction. We focus specifically on the nuclear factor-kappa B (NF-κB) pathway, a potentially important neuroimmune mechanism that may be a key player in driving drug-seeking behavior. We highlight the importance of astroglial-microglial crosstalk, and how this interacts with known glutamatergic dysregulations in addiction. Then, we describe the importance of studying non-neuronal cells with unprecedented precision because understanding structure-function relationships in these cells is critical in understanding their role in addiction neurobiology. Here we propose a working model of neuroimmune-glutamate interactions that underlie drug use motivation, which we argue may aid strategies for small molecule drug development to treat substance use disorders. Together, the synthesis of this review shows that interactions between glutamate and neuroimmune signaling may play an important and understudied role in addiction processes and may be critical in developing more efficacious pharmacotherapies to treat SUDs

Using complex behavior to understand brain mechanisms in health and disease

At this point in the history of the science of behavior, a focus on neuroscience-based outcomes has become dominant in neuropsychiatric fields at the preclinical and clinical levels of analysis. The notion that behavior is caused by brain function, and that changing brain function can alter behavior, has fueled this push to understand these neurobiological mechanisms. Within this conceptual framework and the funding to incentivize its adoption, the neuroscience field grew rapidly with the goal to understand the relation between the brain and behavior. As such, a reductionist perspective emerged whereby neural manipulations of increasing sophistication became required for assessing the necessity and sufficiency of a particular brain mechanism’s role in behavior (Krakauer et al., 2017). Yet, despite the amazing advances in neuroscience, some, such as the former director of the National Institute of Mental Health, Dr. Thomas Insel, have noted the lack of progress in treatment outcomes for mental illness following the shift in funding from behavioral research to genetics and neuroscience research (Barry, 2022)

Accumbens Cholinergic Interneurons Mediate Cue-Induced Nicotine Seeking and Associated Glutamatergic Plasticity

Nicotine, the primary addictive substance in tobacco, is widely abused. Relapse to cues associated with nicotine results in increased glutamate release within nucleus accumbens core (NAcore), modifying synaptic plasticity of medium spiny neurons (MSNs), which contributes to reinstatement of nicotine seeking. However, the role of cholinergic interneurons (ChIs) within the NAcore in mediating these neurobehavioral processes is unknown. ChIs represent less than 1% of the accumbens neuronal population and are activated during drug seeking and reward-predicting events. Thus, we hypothesized that ChIs may play a significant role in mediating glutamatergic plasticity that underlies nicotine-seeking behavior. Using chemogenetics in transgenic rats expressing Cre under the control of the choline acetyltransferase (ChAT) promoter, ChIs were bidirectionally manipulated before cue-induced reinstatement. Following nicotine self-administration and extinction, ChIs were activated or inhibited before a cue reinstatement session. Following reinstatement, whole-cell electrophysiology from NAcore MSNs was used to assess changes in plasticity, measured via AMPA/NMDA (A/N) ratios. Chemogenetic inhibition of ChIs inhibited cued nicotine seeking and resulted in decreased A/N, relative to control animals, whereas activation of ChIs was unaltered, demonstrating that ChI inhibition may modulate plasticity underlying cue-induced nicotine seeking. These results demonstrate that ChI neurons play an important role in mediating cue-induced nicotine reinstatement and underlying synaptic plasticity within the NAcore

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

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

Preference for 50% reinforcement over 75% reinforcement by pigeons

PMID: 19815925International audienceWhen pigeons are given a choice between an initial-link alternative that results in either a terminal-link stimulus correlated with 100% reinforcement or a stimulus correlated with 0% reinforcement (overall 50% reinforcement) and another initial-link alternative that always results in a terminal-link stimulus correlated with 100% reinforcement, some pigeons show a preference for the initial-link alternative correlated with 50% reinforcement. Using this procedure, in Experiment 1, we found a relatively modest preference for 100% over 50% reinforcement. In Experiment 2, we decreased the reinforcement density for the second initial-link alternative to 75% and found a significant preference for the 50% reinforcement initial-link alternative. It may be that this "maladaptive" behavior results from a positive contrast between the expectation of reinforcement correlated with the 50% reinforcement initial-link alternative and the terminal-link stimulus correlated with 100% reinforcement. But apparently, the complementary negative contrast does not develop between the expectation of reinforcement correlated with the 50% reinforcement initial-link alternative and the terminal-link stimulus correlated with 0% reinforcement that often follow. Such paradoxical choice may account for certain human appetitive risk-taking behavior (e.g., gambling) as well

Within-trial contrast: The effect of probability of reinforcement in training

PMID: 19607889International audienceThere is evidence that pigeons prefer conditioned reinforcers that are preceded by greater effort over those that are preceded by less effort (an effect that has been attributed to within-trial contrast). In past research the probability of reinforcement for correct choice of the conditioned reinforcer has been 100%, however, the high level of reinforcement for both alternatives in training may result in a performance ceiling when choice between those alternatives is provided on test trials. In the present study we tested this hypothesis by including a group for which the probability of reinforcement in training was only 50%. Pigeons were trained on two simultaneous discriminations, one that was preceded by a 30 peck requirement the other by a single peck requirement. On test trials, we found a significant preference for the S+ that required the greater effort in training for pigeons trained with 100% and a small but nonsignificant effect for pigeons trained with 50% reinforcement. Although the hypothesis that the within-trial contrast effect was constrained by a performance ceiling was not confirmed, we did find a reliable within-trial contrast effect with 100% reinforcement