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Addiction-related compulsion-like behavior can be modeled in rodents with drug self-administration (SA) despite harmful consequences. Recent studies suggest that the potentiation of glutamatergic transmission at the orbitofrontal cortex (OFC) to dorsal striatum (DS) synapses drives the transition from controlled to compulsion-like SA. However, the timing of the induction of this synaptic plasticity remains elusive. Here, mice were first allowed to intravenously self-administer cocaine. When mice had to endure a risk of electrical foot shock, only a fraction persevered in cocaine SA. In these persevering mice, we recorded high A/N ratios (AMPA-R/NMDA-R: α-amino-3hydroxy-5-methyl-4-isoxazolepropionic acid receptor/N-methyl-D-aspartate receptor) in both types of spiny projection neurons (i.e., D1 and D2 dopamine receptor-expressing SPNs). By contrast, when we prepared slices at the end of the acquisition period, in all mice, the A/N was high in D1R- but not D2R-SPNs. These results indicate that the transition to compulsion-like cocaine SA emerges during the punishment sessions, where synapses onto D2R-SPNs are strengthened. In renouncing individuals, the cocaine-evoked strengthening in D1R-SPNs is lost. Our study thus reveals the cell-type specific sequence of the induction of plasticity that eventually may cause compulsion-like SA.

Ketamine is used clinically as an anaesthetic and a fast-acting antidepressant, and recreationally for its dissociative properties, raising concerns of addiction as a possible side effect. Addictive drugs such as cocaine increase the levels of dopamine in the nucleus accumbens. This facilitates synaptic plasticity in the mesolimbic system, which causes behavioural adaptations and eventually drives the transition to compulsion 1 – 4 . The addiction liability of ketamine is a matter of much debate, in part because of its complex pharmacology that among several targets includes N -methyl- d -aspartic acid (NMDA) receptor (NMDAR) antagonism 5 , 6 . Here we show that ketamine does not induce the synaptic plasticity that is typically observed with addictive drugs in mice, despite eliciting robust dopamine transients in the nucleus accumbens. Ketamine nevertheless supported reinforcement through the disinhibition of dopamine neurons in the ventral tegmental area (VTA). This effect was mediated by NMDAR antagonism in GABA (γ-aminobutyric acid) neurons of the VTA, but was quickly terminated by type-2 dopamine receptors on dopamine neurons. The rapid off-kinetics of the dopamine transients along with the NMDAR antagonism precluded the induction of synaptic plasticity in the VTA and the nucleus accumbens, and did not elicit locomotor sensitization or uncontrolled self-administration. In summary, the dual action of ketamine leads to a unique constellation of dopamine-driven positive reinforcement, but low addiction liability. Experiments in mice show that although ketamine has positive reinforcement properties, which are driven by its action on the dopamine system, it does not induce the synaptic plasticity that is typically observed with addiction.

Activation of the mesolimbic dopamine system reinforces goal-directed behaviours. With repetitive stimulation—for example, by chronic drug abuse—the reinforcement may become compulsive and intake continues even in the face of major negative consequences. Here we gave mice the opportunity to optogenetically self-stimulate dopaminergic neurons and observed that only a fraction of mice persevered if they had to endure an electric shock. Compulsive lever pressing was associated with an activity peak in the projection terminals from the orbitofrontal cortex (OFC) to the dorsal striatum. Although brief inhibition of OFC neurons temporarily relieved compulsive reinforcement, we found that transmission from the OFC to the striatum was permanently potentiated in persevering mice. To establish causality, we potentiated these synapses in vivo in mice that stopped optogenetic self-stimulation of dopamine neurons because of punishment; this led to compulsive lever pressing, whereas depotentiation in persevering mice had the converse effect. In summary, synaptic potentiation of transmission from the OFC to the dorsal striatum drives compulsive reinforcement, a defining symptom of addiction. In mice, synaptic potentiation of transmission from the orbitofrontal cortex to the dorsal striatum drives compulsive reinforcement, a defining symptom of addiction.

Fentanyl is a powerful painkiller that elicits euphoria and positive reinforcement 1 . Fentanyl also leads to dependence, defined by the aversive withdrawal syndrome, which fuels negative reinforcement 2 , 3 (that is, individuals retake the drug to avoid withdrawal). Positive and negative reinforcement maintain opioid consumption, which leads to addiction in one-fourth of users, the largest fraction for all addictive drugs 4 . Among the opioid receptors, µ-opioid receptors have a key role 5 , yet the induction loci of circuit adaptations that eventually lead to addiction remain unknown. Here we injected mice with fentanyl to acutely inhibit γ-aminobutyric acid-expressing neurons in the ventral tegmental area (VTA), causing disinhibition of dopamine neurons, which eventually increased dopamine in the nucleus accumbens. Knockdown of µ-opioid receptors in VTA abolished dopamine transients and positive reinforcement, but withdrawal remained unchanged. We identified neurons expressing µ-opioid receptors in the central amygdala (CeA) whose activity was enhanced during withdrawal. Knockdown of µ-opioid receptors in CeA eliminated aversive symptoms, suggesting that they mediate negative reinforcement. Thus, optogenetic stimulation caused place aversion, and mice readily learned to press a lever to pause optogenetic stimulation of CeA neurons that express µ-opioid receptors. Our study parses the neuronal populations that trigger positive and negative reinforcement in VTA and CeA, respectively. We lay out the circuit organization to develop interventions for reducing fentanyl addiction and facilitating rehabilitation. Experiments using fentanyl treatment of mice show that µ-opioid receptors mediate positive reinforcement in the ventral tegmental area and negative reinforcement in central amygdala, thereby identifying the circuits that lead to opioid addiction.

Repeated exposure to psychostimulants produces locomotor sensitization, a durable behavioral adaptation thought to reflect enhanced incentive salience driven by mesolimbic dopamine. However, the causal contribution of dopamine transients themselves, independent of drug pharmacology, remains elusive. Here we show that repeated optogenetic activation of ventral tegmental area (VTA) dopamine neurons is sufficient to induce persistent locomotor sensitization. Across successive stimulation sessions, mice exhibited a progressive escalation of locomotor activity that persisted for at least ten days after the last stimulation. Sensitization generalized beyond laser-on epochs, elevating baseline locomotion throughout the session. Importantly, mice previously exposed to optogenetic dopamine neuron stimulation displayed an enhanced locomotor response to a subsequent cocaine challenge, demonstrating cross-sensitization between optogenetic and pharmacological reinforcement. These findings establish phasic dopamine neuron activation as a sufficient driver of locomotor sensitization and reveal shared neural substrates underlying dopamine-dependent behavioral plasticity induced by optogenetic and drug reinforcers.Competing Interest StatementThe authors have declared no competing interest.Funder Information DeclaredSwiss National Science Foundation, https://ror.org/00yjd3n13

This study presents a stereotactic microbiopsy technique for sampling defined brain regions in living mice, enabling transcriptomic and epigenomic analyses without sacrificing the animal. The method will allow pre-intervention tissue collection, making it possible to separate preexisting molecular differences from experience- or treatment-induced changes. We show that microbiopsies yield sufficient, high-quality RNA and chromatin for sequencing, with minimal tissue damage that largely resolves over time. The procedure uses standard stereotactic equipment and achieves reproducible spatial precision when the syringe is stabilised. This approach provides a practical framework for within-subject molecular comparisons, reducing animal use and enabling longitudinal profiling of the living mouse brain. It establishes a foundation for investigating how baseline molecular states influence later physiological or behavioural outcomes.Competing Interest StatementThe authors have declared no competing interest.Funder Information DeclaredSwiss National Science Foundation, 323630_214535Carigest SA

Abstract Addicted individuals compulsively seek drugs. Cortico-striatal projections have been implicated in persevering to seek rewards even when punished. The temporo-spatial determinants of the activity underlying the compulsive reward seeking however remains elusive. Here we trained mice in a seek-take chain, rewarded by optogenetic dopamine neuron self-stimulation (oDASS). Mice that persevered when seeking was punished, exhibited an increased AMPA/NMDA ratio selectively at orbitofrontal cortex (OFC) to dorsal striatum (DS) synapses. In addition, an activity peak of spiny projection neurons (SPNs) in the DS at the moment of signalled reward availability was detected. Chemogenetic inhibition of OFC neurons curbed the activity peak and reduced punished reward seeking, as did optogenetic hyperpolarization of SPNs time locked to the cue predicting reward availability, establishing a causal link. Taken together, we conclude that the strengthening of OFC-DS synapses drives SPNs activity when a reward predictive cue is delivered, thus encouraging reward seeking in subsequent trials. Competing Interest Statement The authors have declared no competing interest. Footnotes * ↵* e-mail: Christian.Luscher{at}unige.ch