The research, published in the journal Translational Psychiatry by a team at Radboud University Medical Center in the Netherlands together with collaborators at the Icahn School of Medicine at Mount Sinai, tackled the problem with a controlled animal model of severe stress. Because the timing of the stressor is known in advance in such a model, the researchers could time-lock their brain measurements to the traumatic event itself, something that is essentially impossible in clinical settings. Male mice were exposed to the stress protocol and then classified according to their behavioral outcome: some animals proved susceptible to the effects of stress, while others were resilient. This individual-level sorting is crucial, because it turns the experiment from a simple before-and-after comparison into a genuine search for the neural signatures that separate those who break from those who bend.

Technically, the study relied on whole-brain mapping of neuronal activity across three time windows: before the stress exposure, during or immediately surrounding it, and afterward. On top of the activity maps, the team analyzed functional connectivity, the statistical synchrony of activity between brain regions, within and between three large-scale networks that are well characterized in both rodents and humans. These are the salience network, which flags behaviorally important stimuli and coordinates responses to threat; the default mode network, which is active during internally directed thought, memory consolidation and self-referential processing; and the executive control network, known in rodents as the lateral cortical network, which governs flexible, goal-directed regulation of behavior. Comparing how these networks behaved in susceptible versus resilient mice offered a systems-level view of stress vulnerability rather than a picture confined to any single brain region.

The first major finding concerns what was already different before the stress ever happened. Stress-susceptible mice showed pre-existing hyperactivity in a region called the lateral orbital area, part of the prefrontal cortex involved in evaluating rewards and guiding decisions. In other words, some aspect of these animals’ baseline brain function marked them out as vulnerable before any trauma occurred. This is a striking result, because it suggests that stress susceptibility is not purely a consequence of the traumatic experience itself; at least in this model, the seeds of vulnerability are visible in advance. If such a pre-existing signature could be detected in humans, it might open the door to identifying individuals at elevated risk for stress-related psychopathology before they encounter trauma, whether soldiers before deployment, emergency workers before a career of critical incidents, or patients facing frightening medical procedures.

During and after the stressor, a second region took center stage: the retrosplenial cortex, an area deep in the posterior brain that is heavily implicated in spatial memory, contextual processing and the integration of memory with environmental cues. Susceptible mice showed heightened retrosplenial activity both during and after stress, a pattern the researchers interpret as potentially contributing to a maladaptive memory of the stressful event. The idea is compelling: PTSD is fundamentally a disorder of traumatic memory, in which the past intrudes relentlessly on the present. An overactive memory-integration hub at the moment of trauma could plausibly help stamp the experience in with excessive strength and emotional charge, laying the groundwork for the intrusive recollections and flashbacks that define the disorder.

The two groups also diverged sharply in how they recruited the rest of the brain when stress hit. Susceptible mice mounted a strong activation of visual and memory-related areas, as if their brains were locked onto the sensory and mnemonic details of the threatening experience. Resilient mice told the opposite story. In these animals, the researchers observed marked reductions in retrosplenial cortex activity alongside increased activation of the agranular insula, a region involved in interoception and the integration of bodily states with salient events, and the ventral striatum, a core node of the brain’s reward and motivation circuitry. Resilience, in this reading, is not the absence of a brain response to stress but a different kind of response: less amplification of the traumatic memory trace, and more engagement of circuits that evaluate and regulate the emotional and motivational meaning of what is happening.

Perhaps the most clinically resonant results came from the network-level analyses. Susceptible mice displayed enhanced connectivity within the salience network itself, and strengthened coupling between the salience network and the default mode network. Both patterns have been observed before in human neuroimaging studies of people with PTSD, which lends the mouse findings immediate translational weight. The parallel suggests that the circuit rearrangements seen in the stressed animals are not artifacts of the animal model but recapitulate genuine features of the human disorder. Moreover, the susceptible mice showed aberrant connectivity between the default mode network and the lateral cortical network both before and during stress, hinting that faulty communication between internally directed and executive control systems may be another pre-existing ingredient of vulnerability, one that could undermine the brain’s ability to regulate its own rumination and threat processing when trauma arrives.

Taken together, the study sketches a two-part architecture of stress susceptibility. There are trait-like risk factors, such as lateral orbital hyperactivity and abnormal default mode-executive connectivity, that exist before trauma and presumably bias the system toward a maladaptive response. And there are state-like responses that unfold during and after the trauma, such as retrosplenial overactivation, excessive recruitment of visual and memory circuits, and salience network hyperconnectivity, which may actively construct the pathological memory. Resilience, by contrast, appears to involve a quieter memory system and stronger engagement of insular and ventral striatal circuits. The distinction matters for therapy: pre-existing markers could guide prevention and risk stratification, while the peri- and post-stress dynamics point to windows of opportunity for intervention, moments when the trajectory toward PTSD might still be redirected.

The authors emphasize that these findings reveal circuit-level signatures of stress susceptibility that mirror human PTSD and point toward novel potential biomarkers and targets for mechanistic intervention. Translating that promise into the clinic will take further work, including studies in female animals, validation of the candidate biomarkers, and tests of whether normalizing the identified circuit dysfunctions can prevent stress-related symptoms. But the conceptual advance is already significant. For the first time, researchers have watched, across the entire brain and in fine temporal alignment with the traumatic event itself, how vulnerability and resilience unfold as distinct neural trajectories. What was previously a retrospective mystery, pieced together from the brains of people already suffering, has become a process that can be observed, dissected and, ultimately, targeted. In the effort to understand and eventually prevent PTSD, that shift in perspective may prove to be the study’s most enduring contribution.

Subject of Research: Brain-wide neural activity and functional connectivity differences between stress-susceptible and resilient male mice before, during and after severe stress

Article Title: Brain-wide circuit dynamics underlying stress susceptibility and resilience in male mice

Article References: Dirven, B. C. J., Negwer, M., Botan, A., Maas, R., van Melis, L., Merjenburgh, S., van Rijn, R., Grandjean, J., Homberg, J. R., Kozicz, T., & Henckens, M. J. A. G. (2026). Brain-wide circuit dynamics underlying stress susceptibility and resilience in male mice. Translational Psychiatry. https://doi.org/10.1038/s41398-026-04477-6

Image Credits: AI Generated

DOI: 10.1038/s41398-026-04477-6

Keywords: PTSD, stress susceptibility, resilience, functional connectivity, salience network, default mode network, retrosplenial cortex, prefrontal cortex, mouse model, neuroimaging, traumatic memory, translational psychiatry

The scientific target of the new grants is mood state switching, the abrupt transition between opposing emotional states that defines bipolar disorder. For the vast majority of people living with the condition, these sudden shifts are not gradual mood changes but dramatic reversals that can upend sleep, energy, judgment, and behavior within days or even hours. Despite decades of clinical observation, the biological mechanisms that drive these switches remain largely unknown, as does the question of why certain external stressors—substance use, disrupted light exposure, seasonal change—reliably seem to precipitate episodes in vulnerable individuals. The three newly funded teams will each attack this problem from a distinct biological angle, and their combined results are intended to build a complementary picture of what tips the bipolar brain from one state into another.

At Stanford University, a team led by Laramie Duncan, PhD, will pursue a striking recent discovery: a rare neuron type located in the retrosplenial cortex, a brain region that has received comparatively little attention in psychiatric research, appears to be strongly linked to bipolar disorder. The retrosplenial cortex sits at the intersection of memory, spatial navigation, and internally directed thought, making it a plausible hub for the self-referential distortions that characterize both depressive and manic states. Duncan’s team will now investigate how this specific cell type contributes to mood switching, combining genetic analysis with neuroimaging to assess whether changes in retrosplenial cortex connectivity track the onset of episodes. The project will also draw on BD²’s longitudinal cohort of more than 600 people with bipolar disorder to test a clinically ambitious idea: whether an individual’s genetic risk profile can help predict which treatments are most likely to work, moving the field closer to genetically informed treatment matching.

The Harvard Medical School team, led by Susan Dymecki, MD, PhD, is focused on a circuit-level mechanism that connects the body’s master circadian clock to mood and arousal centers in the brain. Dymecki’s group recently identified a neural pathway that shifts its signaling in response to changes in light exposure, and this discovery is directly relevant to one of bipolar disorder’s most consistent clinical observations: patients are exquisitely sensitive to changes in day length, jet lag, and nighttime screen exposure, all of which are well-documented triggers for mood-switch episodes. The team will now test the hypothesis that dysfunction in this light-responsive circuit is what converts an ordinary change in illumination into a pathological mood episode, and whether the circuit can be safely targeted to stabilize mood. If successful, the work could open a fundamentally new therapeutic avenue grounded in the brain’s timing machinery rather than in traditional neurotransmitter systems.

At Johns Hopkins University, Akira Sawa, MD, will lead a team examining the problem at the level of individual neurons, focusing on potassium and calcium channels—the molecular gates that control electrical signaling in the brain. These channels are critical to neural activity, and the team will study how they interact and how abnormal changes in that interaction may disrupt signaling in ways that drive mood switching. The experimental strategy is unusually broad for a single grant, combining cell-based models, animal studies, and human primary neurons derived from nasal biopsy tissue, an approach that gives researchers access to living human neurons without invasive brain surgery. Crucially, the team will also use data from the first data release of BD²’s longitudinal cohort, testing whether the channel mechanism they identify in the laboratory holds up as both a potential treatment target and a biomarker usable for diagnosis in real patients.

Two of the three new teams will maintain formal collaborations with the BD² Integrated Network, the organization’s longitudinal cohort study designed to accelerate findings and connect laboratory discoveries to real-world clinical settings. This structure reflects a deliberate design philosophy: rather than allowing basic science and clinical research to proceed on separate tracks, BD² requires its grantees to build data-sharing pipelines so that molecular and circuit-level findings can be tested against the lived experience of hundreds of patients followed over time. For a condition as variable and episodic as bipolar disorder, this kind of longitudinal infrastructure is considered essential, because single snapshots of brain state cannot capture the dynamic transitions that are the disorder’s defining feature.

“Understanding why and how mood states switch is essential to developing more precise, effective treatments, and we’re proud to support this work as part of our long-term commitment to helping people with bipolar disorder lead thriving lives,” said Daniel Pham, PhD, Head of Science and Innovation at BD². The statement underscores the organization’s framing of the grants not as incremental funding but as part of a long-haul effort to move bipolar disorder research from descriptive psychiatry toward mechanistic biology, where interventions can be matched to the specific processes malfunctioning in each patient.

With the new awards, BD² has now committed more than $119 million to bipolar disorder research since its founding, spanning genetics platforms, brain omics, and clinical cohort infrastructure. The fourth round of Discovery Research builds on earlier grantee cohorts that have examined the genetic risk factors of the illness, the molecular and circuit-level mechanisms underlying sleep disruption, and the mechanisms of action of existing drug-based therapies. Taken together, these strands form an increasingly interconnected portfolio: genetic risk informs who is vulnerable, circadian and sleep biology explains some environmental triggers, and the new grants probe the switch mechanisms themselves. The open-science model, in which data, methods, and resources are shared across initiatives, is intended to shorten the time between scientific breakthrough and meaningful clinical impact for the tens of millions of people affected globally.

Alongside the new awards, BD² has opened a fifth round of Discovery Research funding, inviting teams to apply for grants of up to $5 million over four years—a notable expansion from previous cycles, which offered $4.5 million over three years. The extended duration and larger budget are designed to give successful teams more time and resources to increase their scientific impact, a response to what the organization describes as the community’s need for greater funding and long-term stability in a field that has historically been under-resourced relative to the global burden of the illness. The new request for applications also places a spotlight on women’s health and sex differences in bipolar disorder, an area of growing urgency given mounting evidence that disease course, episode patterns, and treatment responses can differ substantially between women and men, particularly across reproductive life stages. Applications must include connections into the Integrated Network, ensuring that the next generation of BD²-funded studies remains tethered to patient data and real-world outcomes.

For researchers and clinicians watching the field, the significance of the new grants lies less in any single experiment than in the convergence of scales: a rare cell type in an overlooked cortical region, a light-sensitive circuit linking circadian timing to arousal, and ion channels governing neuronal excitability are all being interrogated as candidate mechanisms for the same phenomenon. If even one of these lines of investigation identifies a druggable target or a usable biomarker, the path from laboratory insight to stabilized mood could shorten considerably. In the meantime, the expansion of funding and the explicit focus on sex differences signal that bipolar disorder research is entering a phase in which the field’s biggest questions—why moods flip, and how to predict and prevent the flip—are finally being addressed with the tools, cohorts, and resources their complexity demands.

Subject of Research: Biological mechanisms of mood state switching in bipolar disorder

Article Title: BD² awards over $13 million in grants to understand the mechanisms of mood state switching in bipolar disorder and announces new funding available

Article References: BD² awards over $13 million in grants to understand the mechanisms of mood state switching in bipolar disorder and announces new funding available. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: bipolar disorder, mood state switching, BD2 grants, discovery research, circadian rhythm, retrosplenial cortex, ion channels, women's health, longitudinal cohort, neural circuits, biomarkers, mental health funding