New study details how ketamine alters neural networks differently in males and females

A recent study published in Science Advances suggests that recovering from ketamine anesthesia initiates a distinct brain response in female mice that is absent in males. Specifically, the research indicates that the stress hormone corticosterone triggers specialized brain immune cells in females to promote new neural connections. These findings provide evidence that sex-specific hormonal and immune pathways shape how the brain rewires itself after anesthesia.

Ketamine is a medication commonly used to induce anesthesia, manage severe pain, and treat severe depression. It works in part by temporarily blocking specific receptors in the brain to reduce electrical signaling. The brain relies on a process called neuroplasticity to recover and adapt after this kind of widespread dampening. Neuroplasticity refers to the brain’s ability to reorganize its physical structure and function by forming new connections between neurons.

Microglia, the resident immune cells of the brain, play a role in this remodeling. These cells constantly survey their environment, clearing away cellular debris and physically interacting with neurons to shape synaptic connections. Prior research indicates that males and females can experience different immune and metabolic responses to drugs.

Sandra Siegert, a professor at the Institute of Science and Technology Austria, and a team of researchers from the Allen Institute conducted this study. They aimed to explore whether the dynamic relationship between microglia and neurons during ketamine recovery differs between sexes.

The researchers administered a single anesthetic dose of a ketamine mixture to male and female mice. Four hours later, they examined brain tissue from the primary visual cortex, focusing on a protein called CD68. CD68 is a marker that indicates how active and reactive microglia are within the brain tissue. The authors found that the volume of CD68 inside microglia increased only in the female mice, whereas male mice showed no such increase compared to those given a saline control.

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To observe this process in real time, the team used two-photon microscopy to look into the brains of living, genetically modified mice. They monitored ten mice, including five males and five females, before, during, and up to three hours after ketamine administration. About an hour into the recovery phase, female microglia began making much more frequent physical contact with dendritic spines, which are the small protrusions on neurons where synapses form. In contrast, male microglial activity was inconsistent and did not show an overall group increase in contacts.

The scientists then examined miniature excitatory postsynaptic currents, which are small electrical signals indicating active communication between neurons. They recorded these signals in brain slices from three mice per condition. The frequency of these electrical currents increased in the female mice, indicating newly formed, functional synaptic connections. When looking at the physical density of dendritic spines on neurons, the researchers observed a large relative effect size in females compared to controls, though the specific spine density numbers were not statistically significant.

To test the direct role of microglia, the researchers fed another group of female mice a drug called PLX5622 for one and a half weeks. This drug depleted about 80 percent of their microglia. In these microglia-depleted females, the ketamine-induced increase in electrical signaling frequency did not occur. This finding provides evidence that microglia mediate the observed neuroplasticity.

To identify the genetic drivers of this response, the team sequenced the genetic material of 36,701 individual cells from the visual cortices of four female mice. They looked for genes that were unusually active two hours after ketamine exposure. The analysis pointed to a gene called Fkbp5, which produces a stress-responsive protein known as Fkbp51. Subsequent tissue staining in five mice per condition confirmed that the Fkbp5 gene and its resulting protein were upregulated entirely within the microglia of female mice.

The scientists then blocked the Fkbp51 protein using a targeted drug, or by using genetically modified mice that lacked the Fkbp5 gene specifically in their microglia. In both scenarios, involving five animals per condition, the female-specific increase in microglial reactivity was neutralized. The typical spike in neuronal electrical signaling frequency in females also failed to materialize. This suggests that the Fkbp5 genetic pathway drives the female microglia response.

Because the Fkbp5 gene regulates cellular responses to stress hormones, the researchers measured blood levels of corticosterone, the primary stress hormone in mice. Blood samples from five mice per group showed that corticosterone levels remained stable 30 minutes after anesthesia. After 120 minutes, female mice experienced a near threefold absolute increase in blood corticosterone compared to control animals. Male mice did not exhibit this delayed hormone surge.

To establish a link between the hormone and the brain changes, the authors surgically removed the adrenal glands of a group of female mice. The adrenal glands are the body’s main source of corticosterone. These mice were given a steady, low-dose baseline of the hormone in their drinking water to maintain normal daily functions. When these adrenal-gland-free mice received ketamine, their microglia did not increase interactions with neurons.

However, when the researchers injected these mice with a dose of corticosterone, live imaging of three animals showed an immediate spike in microglia-neuron contacts. This represented a massive shift relative to their baseline state, showing a statistical effect size of 1.74. A similar immediate spike occurred when non-adrenalectomized males were given supplemental corticosterone. This indicates that circulating corticosterone dictates the microglial interactions across both sexes if the hormone is present in high enough quantities.

Interpreting these findings requires noting that this study relies entirely on a mouse model. Mice have different baseline metabolic rates, hormone cycles, and immune structures than humans. The biological time frames observed in rodents recovering from anesthesia might not directly map onto human recovery windows. The results suggest a mechanism linked to circulating stress hormones in rodents, but they do not guarantee that human females experience identical microglial activation after receiving ketamine.

Future studies will need to explore how varying dosages of ketamine interact with this pathway. This is especially relevant for the lower, subanesthetic doses of ketamine used in human depression treatments. Researchers also plan to investigate whether other brain cells, such as astrocytes, participate in this hormone-driven network remodeling. Understanding this biological pathway could guide tailored medical treatments that account for physiological differences between sexes.

The study, “Corticosterone-linked microglial activity underpins sexually dimorphic neuroplasticity after ketamine anesthesia,” was authored by Alessandro Venturino, MohammadAmin Alamalhoda, Thomas Negrello, Kelly Jin, Cindy T. J. van Velthoven, Ryan John A. Cubero, Jake Yeung, Peter Koppensteiner, Bosiljka Tasic, and Sandra Siegert.