A Female Brain’s Immune Cells May Hold the Key to Recovery After Ketamine Anesthesia
When consciousness fades under ketamine, communication across the brain’s neural networks is dramatically altered. As the anesthetic takes effect, neurons reduce their coordinated activity, disrupting perception, memory, pain processing, and the integrated signaling required for conscious awareness. But waking is not simply the reversal of this shutdown. Researchers at the Institute of Science and Technology Austria (ISTA), working with colleagues at the Allen Institute in Seattle, have discovered that the brain’s immune cells may actively help rebuild neural connections as consciousness returns—and that this recovery process differs markedly between female and male mice.
The study, published in Science Advances, focuses on microglia, the brain’s resident immune cells. Often described as its surveillance and maintenance system, microglia constantly extend and retract fine cellular processes to monitor neurons, synapses, and the surrounding tissue. They remove damaged cellular material, regulate inflammatory responses, and influence the formation and remodeling of synapses—the junctions through which neurons communicate. The new findings suggest that microglia are not merely responding to the effects of anesthesia. In female mice, they appear to participate directly in restoring the brain’s flexibility after ketamine-induced unconsciousness.
To observe this process in living animals, Alessandro Venturino, Sandra Siegert, and their colleagues used a cranial window, a surgically implanted transparent opening that permits repeated high-resolution imaging of the brain. Fluorescent markers were used to distinguish microglia from neurons, allowing the researchers to track both cell types while mice recovered from ketamine anesthesia. This approach revealed a striking sequence of events: in female mice, microglial processes began establishing prolonged physical contacts with neurons as the animals regained consciousness. The timing of these contacts coincided with synaptic remodeling, a form of neuroplasticity in which neural connections are strengthened, weakened, formed, or eliminated.
The same phenomenon was not detected in male mice under the experimental conditions. The researchers also found that when microglia were absent, the synaptic remodeling associated with recovery did not occur. This result indicates that microglia are not passive observers of the awakening brain but critical mediators of the plastic changes that follow anesthesia. The findings do not establish that male brains lack all forms of recovery-related plasticity; instead, they raise the possibility that males may use a different mechanism or follow a different time course. Nevertheless, the sex-specific cellular response was sufficiently clear to challenge the assumption that anesthetic recovery is biologically identical in females and males.
The team then investigated what might trigger the female-specific microglial response. Their experiments pointed to corticosterone, the principal stress hormone in mice and the functional counterpart of cortisol in humans. Corticosterone levels rise during recovery from anesthesia, creating a temporary physiological stress signal. In female mice, this signal activated the stress-response gene Fkbp5 in microglia. The gene encodes FKBP51, a protein involved in regulating the cell’s sensitivity to glucocorticoid hormones and in managing intracellular stress signaling. According to the researchers, activation of this pathway appears to prepare microglia to engage with neurons and support remodeling of synaptic connections.
The hormone’s role was tested directly by removing the adrenal glands, the organs responsible for producing corticosterone. Without the adrenal source of the hormone, the prolonged microglia-neuron contacts observed during recovery disappeared, as did the associated synaptic remodeling. This result links the process to a specific neuroendocrine pathway: anesthesia and recovery alter systemic stress-hormone levels; corticosterone activates a microglial stress-response program; and microglia then interact with neurons in a manner associated with renewed plasticity. The work illustrates how immune, endocrine, and neural systems can converge during a brief but biologically important transition between unconsciousness and wakefulness.
The discovery may also help explain why the effects of ketamine are not uniform across individuals. Ketamine is widely used as an anesthetic and analgesic, but it is also prescribed or investigated as a rapid-acting treatment for depression. Its antidepressant effects are thought to involve changes in glutamatergic signaling, synaptic strength, and network plasticity, although the precise mechanisms remain under active study. If microglial and stress-hormone pathways contribute to the brain’s response to ketamine, sex-dependent differences in those pathways could influence recovery, side effects, or therapeutic outcomes. The researchers emphasize that their findings come from mice and cannot yet be translated directly into clinical recommendations.
The study also draws attention to a longstanding problem in biomedical research: the underrepresentation of females in experiments involving drugs and brain function. During their review of previous work, the ISTA researchers found relatively few studies specifically examining how female subjects respond to ketamine anesthesia. Existing clinical observations have suggested that women may experience nausea and sickness more frequently after ketamine, but the biological explanations remain poorly understood. By showing that female and male mice can recruit different cellular pathways during recovery, the new research strengthens the case for analyzing sex as an essential biological variable in pharmacology, anesthesiology, and neuroscience rather than treating male results as universally representative.
The evolutionary explanation for the observed difference remains speculative. Siegert proposes that female microglia may have become more responsive to certain stress signals because rapid adaptation could have offered advantages in complex social, emotional, and caregiving environments. Greater plasticity, however, may carry costs as well as benefits. A system that readily reshapes neural networks in response to stress may become vulnerable when stress signals are excessive, prolonged, or repeatedly triggered. This possibility is particularly relevant because several psychiatric disorders, including depression, are more prevalent in women. The researchers stress that their work does not prove a direct connection between the ketamine-response pathway and depression, but it identifies a cellular mechanism worthy of further investigation.
The findings ultimately present awakening from anesthesia as an active biological reconstruction rather than a simple return to a pre-existing state. In female mice, a temporary rise in corticosterone appears to engage microglia through Fkbp5 and FKBP51, bringing these immune cells into close contact with neurons at a moment when synaptic networks are beginning to reorganize. Future studies will need to determine whether similar mechanisms operate in humans, whether male brains rely on delayed or alternative pathways, and how these processes relate to ketamine’s anesthetic and antidepressant effects. For now, the research offers a vivid reminder that the immune cells embedded within the brain can help shape consciousness, recovery, and the capacity of neural circuits to change.
Subject of Research: Animals
Article Title: Corticosterone-linked microglial activity underpins sexually dimorphic neuroplasticity after ketamine anesthesia
News Publication Date: 31-Jul-2026
Web References: Institute of Science and Technology Austria; Allen Institute; Science Advances article: https://www.science.org/doi/10.1126/sciadv.adz6517
References: DOI: 10.1126/sciadv.adz6517
Image Credits: © ISTA
Keywords: ketamine anesthesia, microglia, neuroplasticity, corticosterone, FKBP51, Fkbp5, sex differences, neuroscience, synaptic remodeling, brain immune cells, mouse models, antidepressants
