Sex as a biological variable in neurobiological research
Alexandra Nowlan, PhD
Historically, males have been the standard subjects studied in biomedical research. This practice was established due to the fear that the variability in hormone signaling across the female estrous cycle would complicate the interpretation of experimental results, though comparable fluctuations in male testosterone levels were of little concern. The exclusion of female subjects has resulted in an incomplete understanding of the effect that sex has on the pathophysiological processes of disease. This lack of understanding has limited our knowledge of how to diagnose and treat disease in women. This is particularly problematic in the context of conditions that disproportionately affect women, like anxiety, depression, and substance use disorders.
Since 2016, the NIH has mandated the examination of both sexes, which has resulted in discoveries that have encouraged researchers to rethink how we assess rodent behavior to infer neurological function. For example, in fear learning assays in which a neutral tone is presented with an unpleasant stimulus, the amount of time spent freezing in response to the tone is typically used to determine whether fear learning took place. However, in rats, females are more likely to dart in response to a fearful stimulus, suggesting that in certain contexts, females use different behavioral strategies than their male counterparts (Gruene et al., 2015). As beneficial as traditional assays have been in furthering our understanding of the brain thus far, it is important to remain open-minded and acknowledge our weaknesses when it comes to interpreting animal behavior. Otherwise, we risk neglecting important behaviors that have traditionally been overlooked in female populations and may be clinically relevant, especially in the context of psychiatric disorders.
Men and women experience differing sensitivities to pain, which likely contributes to the differences observed in the prescription and misuse of pain-relieving opioids. Opioid use disorder (OUD) is a chronically relapsing condition in which a patient has become dependent to the point where opioids serve as a negative reinforcer to alleviate the negative effects of withdrawal. Opioid withdrawal is characterized by acute physical symptoms and a prolonged state of unease. Recent studies suggest that although men exhibit more opioid-related overdoses compared to women, the rates of opioid prescription, use, and overdose are rapidly increasing in women (Serdarevic et al., 2017; VanHouten et al., 2019). Furthermore, women are more vulnerable to opioid use; progressing more quickly to dependency and experiencing more severe withdrawal symptoms (Final Report: Opioid Use, Misuse, and Overdose in Women, n.d.). Despite the accumulating evidence of sex differences in OUDs, few animal-based studies have examined the underlying neurobiology in both male and female subjects.
As a postdoctoral research associate in the McElligott Lab, I aim to remedy this issue in my work by studying both sexes in a preclinical (animal) model of OUD. The McElligott Lab has developed a model of rapid opioid dependence in mice that allows us to monitor and manipulate specific brain regions and their connections to determine their influence on withdrawal. While multiple brain areas and neurochemical signals are involved in substance use disorders, my work focuses particularly on noradrenaline (NE), a neurotransmitter involved in the fight-or-flight response. I am focused specifically on the activity of NE within two brain areas that exhibit distinct differences in male and female subjects; the locus coeruleus (LC) and the bed nucleus of the stria terminalis (BNST). The LC is a major source of NE input throughout the brain and is involved in a variety of brain functions including arousal, attention, emotion, and stress response. The BNST receives NE input and is involved in anxiety, reward, and pain processing. Previous work investigating the effects of morphine withdrawal has revealed sex differences in inhibitory signaling in the BNST as well as differences in approach/avoidance behavior and social interactions (Bravo et al., 2020; Luster et al., 2020). I am building upon these findings by examining whether LC activity and NE release in the BNST is correlated with specific withdrawal behaviors (like escape jumps or paw tremors) and whether the strength and/or frequency of that activity has any bearing on the affective state of males, females, or both sexes equally. The results of this research could reveal whether the sex differences in neural circuitry can explain the differences in withdrawal sensitivity that have been observed in the clinical literature, and provide insight regarding therapeutic targets.
As an advocate for greater diversity, equity, and inclusion in the scientific community, I am grateful to be working alongside like-minded researchers who are dedicated to examining the neurobiology underlying alcohol and substance use disorders in an unbiased manner. I’m looking forward to the discoveries on the horizon that will reveal further insight and help to course-correct sex and gender-based health disparities.