Neural Circuits Underlying Positive and Negative Reinforcement of Fentanyl Addiction
Fentanyl addiction is a major public health crisis marked by high rates of overdose and dependence, driven by the drug’s extreme potency and rapid pharmacokinetics that produce strong euphoria and robust behavioral reinforcement. Abrupt cessation of fentanyl induces a profound aversive withdrawal syndrome, motivating continued drug use to avoid negative affective states and thereby engaging powerful negative reinforcement mechanisms. Thus, fentanyl addiction reflects the convergence of positive and negative reinforcement acting on neural circuits, as supported by early rat studies.1
Opioid-induced positive reinforcement is thought to originate in the ventral tegmental area (VTA), where VTA γ-aminobutyric acid (GABA) neurons express µ-opioid receptors (µORs) that are acutely inhibited via Gi/o signaling, a pathway used by G-protein receptors to inhibit cell activity.2 Upon termination of opioid exposure, these long-lasting adaptations cause prolonged activation, including cAMP supersensitization, which can increase neuronal excitability. As such, fentanyl withdrawal is thought to arise from overactivity of VTA GABA neurons that suppress dopamine signaling and induce dysphoria.
In a prospective 2024 study, mice were made fentanyl-dependent through escalating intraperitoneal doses. Additionally, withdrawal was precipitated with naloxone, while neuronal activity was monitored using fiber photometry, dopamine sensors, and optogenetics. Targeted deletion of µ-opioid receptors in specific brain regions allowed the researchers to causally link VTA circuits to positive reinforcement of fentanyl addiction and other neural circuits to withdrawal-induced negative reinforcement.3
In the study, fentanyl-dependent mice exhibited clear withdrawal symptoms. Brain-wide mapping of neuronal activation showed that while fentanyl activated classic reward regions such as the VTA and nucleus accumbens, withdrawal selectively activated the central amygdala (CeA). Targeted deletion of µORs confirmed this dissociation: removing µORs in the VTA disrupted fentanyl-induced reward, whereas deletion of these receptors in the CeA reduced withdrawal-related behaviors. These findings demonstrate that positive and negative reinforcement of fentanyl addiction arise from anatomically distinct µOR-expressing circuits rather than a single shared pathway.3
To define the cellular mechanisms underlying these effects, the authors combined in vivo calcium imaging, dopamine sensing, and optogenetics. Fentanyl inhibited VTA GABA neurons, thereby disinhibiting dopamine neurons and increasing dopamine release in the nucleus accumbens.3 Other µOR-expressing neurons in the VTA and nucleus accumbens may have also contributed to this effect,4 and fentanyl can indirectly increase dopamine release through cholinergic pathways. Withdrawal produced the opposite effect: µOR-expressing neurons in the CeA became hyperactive specifically during withdrawal and encoded aversive behavioral events3,5 through downstream targets like the bed nucleus of the stria terminalis (BNST, a.k.a. “the extended amygdala”),6 parabrachial nucleus, and nucleus accumbens.
These CeA µOR neurons are distinct from stress-related corticotropin-releasing neurons and likely triggered dysphoria by influencing circuits that inhibit VTA dopamine neurons. Optogenetic inhibition of VTA GABA neurons was reinforcing and mimicked fentanyl’s rewarding effects, while optogenetic activation of CeA µOR neurons was aversive and drove escape-like behavior. Together, these results establish a dual-circuit model of opioid addiction in which fentanyl engages VTA circuits to produce reward and CeA circuits to generate negative reinforcement during withdrawal, helping to explain the drug’s high addictive potential and identifying distinct circuit-level targets for intervention.3
Fentanyl’s addictive power arises from two distinct neural circuits. VTA µ-opioid receptor–expressing GABA neurons mediate positive reinforcement through dopamine disinhibition, while CeA µOR neurons drive negative reinforcement during withdrawal by signaling aversion. These dual pathways operate independently but converge to promote compulsive fentanyl use, explaining the drug’s extreme addictive potential. Understanding this circuit-level separation offers targeted opportunities for interventions that could selectively reduce either reward- or withdrawal-driven behaviors in opioid addiction.
References
1. Stinus L, Le Moal M, Koob GF. Nucleus accumbens and amygdala are possible substrates for the aversive stimulus effects of opiate withdrawal. Neuroscience. 1990;37(3):767-773. https://doi.org/10.1016/0306-4522(90)90106-e
2. Corre J, van Zessen R, Loureiro M, et al. Dopamine neurons projecting to medial shell of the nucleus accumbens drive heroin reinforcement. eLife. 2018;7. https://doi.org/10.7554/elife.39945
3. Chaudun F, Python L, Liu Y, et al. Distinct µ-opioid ensembles trigger positive and negative fentanyl reinforcement. Nature. Published online May 22, 2024:1-8. https://doi.org/10.1038/s41586-024-07440-x
4. Darcq E, Kieffer BL. Opioid receptors: drivers to addiction? Nature Reviews Neuroscience. 2018;19(8):499-514. https://doi.org/10.1038/s41583-018-0028-x
5. Jiang C, Yang X, He G, et al. CRHCeA→VTA inputs inhibit the positive ensembles to induce negative effect of opiate withdrawal. Molecular Psychiatry. 2021;26(11):6170-6186. https://doi.org/10.1038/s41380-021-01321-9
6. Zhu Y, Wienecke CFR, Nachtrab G, Chen X. A thalamic input to the nucleus accumbens mediates opiate dependence. Nature. 2016;530(7589):219-222. https://doi.org/10.1038/nature16954