Opioid drugs acting at the mu opioid receptor produce analgesia, respiratory depression, constipation, and reward that can lead to substance use disorder. Although mu receptors are Gi/o-coupled and classically inhibitory, mu-mediated disinhibition of GABAergic neurons can produce downstream neuronal activation that varies with prior drug exposure and across receptor subtypes. The investigators sought to map, at whole-brain resolution, the patterns of neuronal activation produced by acute mu agonism, opioid dependence, and activation of the nociceptin/orphanin FQ peptide (NOP) receptor to reveal regional and network-level consequences.
The study used TRAP2/Ai9 reporter mice, which label recently active neurons by driving permanent tdTomato expression through the c-Fos locus. After pharmacologic challenge, whole brains were cleared and imaged using light-sheet microscopy to identify and quantify activated neuronal populations across the entire brain. This approach enables spatially comprehensive and anatomically resolved maps of activation and renders identified regions genetically accessible for follow-up manipulations.
Three treatment conditions were compared to vehicle: an acute morphine injection (10 mg/kg), an escalating twice-daily morphine regimen that produced dependence, and the NOP receptor agonist Ro 64-6198. The escalating regimen was designed to model dependent exposure; details of the exact escalation schedule were reported by the authors in the source but are not reproduced verbatim here. All conditions were evaluated with the same TRAP-based readout of neuronal activation.
Despite the Gi/o coupling of both mu and NOP receptors, each pharmacologic condition produced an overall increase in labeled neuronal activation relative to vehicle. The whole-brain increase reached statistical significance only in male mice. Among treatments, acute morphine produced the largest number of engaged regions: every region assessed showed increased activation after acute morphine, and none showed decreased activation in that comparison.
Sex differences were a prominent feature of the results. Males were more sensitive than females across treatments. The spatial distribution of activation diverged by sex: in males, morphine engagement localized primarily to pain- and reward-related midbrain circuitry, whereas in females the principal engagement was within hypothalamic circuitry. The authors emphasize that these sex-specific patterns may reflect differential circuit-level responses to opioid receptor agonism and could underlie sex-dependent behavioral or physiological consequences.
Acute morphine produced widespread increases across regions and the largest regional footprint of activation. In contrast, mice rendered dependent by the escalating morphine regimen exhibited lower activation across several brain regions compared with acute morphine. Notably, a cluster of cerebellar regions displayed a trend toward being turned off relative to the acute morphine condition. Activation following the NOP receptor agonist Ro 64-6198 was far weaker overall than the activation produced by mu agonism.
Although chronic (dependent) morphine produced less absolute regional activation than acute morphine, it was associated with more widespread co-activation across brain regions. Network-level organization at a global scale was preserved across conditions, but the composition of network hubs—regions with high integrative connectivity—differed between acute morphine, dependent morphine, and NOP receptor activation. These findings indicate that dependence modifies the pattern of regional interactions without abolishing global network topology.
By establishing whole-brain activation maps in the TRAP2/Ai9 system, the study creates a resource in which pharmacologically activated regions are permanently labeled and therefore genetically tractable. This enables future site-directed or cell-type–specific manipulations to test causal roles of identified regions in opioid-related behaviors and physiologic responses.
The authors report that data and analysis code are available via referenced repository links in the source. Funders declared in the source include the National Institute on Drug Abuse, the National Institute of Neurological Disorders and Stroke, and the U.S. Department of Defense. The authors declared no competing interests. The source provides additional supplementary material and code repositories for replication and further analysis.