Why some people wake up under anesthesia and others don’t (Hint: It’s your Hormones)
Dr. Andrzej (Andi) Wasilczuk was the lead author on this study. A recent graduate from Penn Bioengineering, Andi is captivated by the brain’s intricate ability to shift between states of consciousness. His research uses general anesthetics to uncover the neuronal circuits responsible for these shifts, aiming to understand how the brain sustains or disrupts consciousness. By identifying these critical networks, Andi is paving the way for personalized anesthesia and offering new insights into arousal state transitions.
or technically,
Hormonal basis of sex differences in anesthetic sensitivity
[See Original Abstract on Pubmed]
Authors of the study: Andrzej Z Wasilczuk, Cole Rinehart, Adeeti Aggarwal, Martha E Stone, George A Mashour, Michael S Avidan, Max B Kelz, Alex Proekt, ReCCognition Study Group
Waking up during surgery sounds like a nightmare, and did you know that females might be at higher risk for this than males? Through medication, general anesthesia makes a patient unconscious, which allows doctors to perform surgical procedures without the patient’s awareness or discomfort. General anesthesia puts the patient in a sleep-like state, and doing this is an involved process. Anesthesiologists must be highly trained to determine the best course of treatment. When creating a safe treatment plan, anesthesiologists take into account many factors, such as the patient’s body weight or pre-existing conditions. The sex of the patient, however, hasn’t been historically considered as an equally important factor in delivering a safe course of anesthesia.
Previous research about the link between sex and response to anesthesia was ambiguous and conflicting. Some early clinical trials suggested that females were more likely to wake up under anesthesia, while others found no significant difference between males and females. These clinical trials, however, had diverse patient populations and non-standardized anesthetic protocols, which would make it hard to directly compare anesthetic conditions between patients. Nevertheless, more recently, an analysis done on many of these studies has provided clear cut evidence that females are more resistant to the anesthetic state (Braithwaite et al., 2023). The question of how this sex difference arose, however, remained unanswered.
Dr. Andi Wasilczuk, a former Penn Bioengineering PhD student, and his team wanted to understand why females and males responded differently to anesthesia. To do this, they decided to focus on the hypothalamus, a structure in the brain heavily involved in both sleep-wake and anesthetic induced unconsciousness. The hypothalamus is regulated by hormones, which are the body’s chemical messengers—and the researchers knew that the levels of hormones typically differ between males and females. For example, males typically have a much higher level of the hormone testosterone, whereas females typically have higher levels of the hormone estrogen.
With these differences in mind, Dr. Wasilczuk wanted to know: Could hormonal differences across sexes alter the effectiveness of general anesthesia? He framed "effectiveness of general anesthesia" by using the idea of “anesthetic sensitivity.” Individuals who are more sensitive to anesthesia need less of the drug to fall and stay unconscious, and wake up smoothly after surgery. On the contrary, individuals with less anesthetic sensitivity, or have anesthetic resistance, require more anesthetic to fall and stay asleep, and wake up sooner once the anesthetic is removed.
Recognizing this gap in the research, Dr. Wasilczuk’s research group sought to test the influence of sex and sex hormones on anesthetic sensitivity in mice. First, the researchers compared the dosage of anesthetic required for the mice to be initially anesthetized (induction), and to wake up from anesthesia (emergence). They found that, across all four anesthetics the group tested, female mice required a much higher dose on induction, and were more likely to emerge at higher doses than males. Next, the researchers compared the time, given the same dosage, for female versus male mice to be induced and emerge from anesthesia. Female mice took significantly longer to be induced than males, and also were much quicker to emerge. These experiments indicated that female mice were indeed more resistant to anesthesia, compared to male mice.
Yet, the reason for these results remained unclear: Were these effects due to sex hormone differences? To find out, the researchers changed the mice's hormone levels by surgical removal of the testicles (castration) in male mice or ovaries (oophorectomy) in female mice post puberty. They repeated the experiments, this time using castrated males and oophorectomy females, then compared these mice to the untreated males and females tested before.
The results were striking. In both experiments, castrated males and oophorectomized females showed a similar resistance to anesthesia as untreated females. Oophorectomy did not change a female mouse’s anesthetic sensitivity. Castration, however, produced a female-like anesthetic sensitivity in males. Eliminating male sex hormones, therefore, seemed to remove the sex differences in response to anesthesia!
The researchers also directly measured the effect of testosterone. Under a steady dose of anesthetic, untreated males and castrated males were injected with testosterone, and continually tested for responsiveness using the righting reflex. Testosterone administration increased anesthetic sensitivity for both groups of mice in a dose-dependent manner. This finding could explain why males, who typically have higher testosterone, are more sensitive to general anesthetics, and therefore are at lower risk of waking up under anesthesia than females.
Intrigued, the researchers wondered: Can these sex differences be seen in brain activity? The conventional measure of anesthetic depth (how unconscious someone is in response to anesthesia) during surgery is the Electroencephalogram (EEG). EEG measures electrical brain activity through electrodes attached to the scalp. The researchers found that sex differences were not reflected in the EEG of the mice they tested. Similar conclusions were made when re-analyzing human data from another study. In this study, female volunteers displayed resistance to general anesthesia based on assessments of behavior and cognitive function, but not based on information gathered from the EEG.
Looking at the activity of individual neurons, however, clearly revealed sex differences. They looked for elevated levels of the protein c-Fos, an indicator of neuronal activity, throughout the whole brain. Compared to anesthetized male mice, anesthetized female mice had fewer neurons expressing c-Fos in sleep-promoting hypothalamic cells. In other words, anesthesia activates fewer sleep-promoting circuits in females than males, correlating with females’ greater resistance to anesthetics.
Compared to untreated male mice, castrated male mice also had reduced c-Fos expression in similar hypothalamic structures. Fewer sleep-promoting circuits were activated in castrated males (which displayed a similar aesthetic sensitivity to females) than untreated males. Thus, sex-dependent activity patterns, seen in hypothalamic structures, reflected anesthetic sensitivity trends!
Dr. Wasilczuk’s groundbreaking paper reveals why researching sex-dependence is incredibly important: females may need different anesthetic management than males due to their higher resistance to anesthesia. After years of standard general anesthesia administration to millions of patients, and using EEGs to measure anesthetic depth, Dr. Wasilczuk’s findings have huge clinical implications supporting personalized anesthetic care.
About the brief writer: Sydney Liu
Sydney is a guest writer for Brains in Briefs! She is a Penn undergraduate in Dr. Shinjae Chung’s lab researching what makes us sleep and the brain transitions between sleep states. She is a Junior majoring in neuroscience, and is interested in teaching. In her free time, she likes to draw!
Citations:
E. Braithwaite et al., Impact of female sex on anaesthetic awareness, depth, and emergence: A systematic review and meta-analysis. Br. J. Anaesth. 131, 510–522 (2023).