Estrous Cycle

Although both males and females become addicted to cocaine, females transition to addiction faster and experience greater difficulties remaining abstinent. We demonstrate an oestrous cycle-dependent mechanism controlling increased cocaine reward in females. During oestrus, ventral tegmental area (VTA) dopamine neuron activity is enhanced and drives post translational modifications at the dopamine transporter (DAT) to increase the ability of cocaine to inhibit its function, an effect mediated by estradiol. Female mice conditioned to associate cocaine with contextual cues during oestrus have enhanced mesolimbic responses to these cues in the absence of drug. Using chemogenetic approaches, we increase VTA activity to mechanistically link oestrous cycle-dependent enhancement of VTA firing to enhanced cocaine affinity at DAT and subsequent reward processing. These data have implications for sexual dimorphism in addiction vulnerability and define a mechanism by which cellular activity results in protein alterations that contribute to dysfunctional learning and reward processing.

BackgroundOvarian hormone fluctuations over the rodent estrous cycle and the human menstrual cycle are known to significantly impact brain physiology and disease risk, yet this variable is largely ignored in preclinical neuroscience research, clinical studies, and psychiatric practice.MethodsTo assess the importance of the estrous cycle information for the analysis of sex differences in neuroscience research, we re-analyzed our previously published data with or without the estrous cycle information, giving a side-by-side comparison of the analyses of behavior, brain structure, gene expression, and 3D genome organization in female and male mice. We also examined and compared the variance of female and male groups across all neurobehavioral measures.ResultsWe show that accounting for the estrous cycle significantly increases the resolution of the neuroscience studies and allows for: (a) identification of masked sex differences; (b) mechanistic insight(s) into the identified sex differences, across different neurobehavioral outcomes, from behavior to molecular phenotypes. We confirm previous findings that female data from either mixed- or staged-female groups are, on average, not more variable than that of males. However, we show that female variability is not, at all, predictive of whether the estrous cycle plays an important role in regulating the outcome of interest.ConclusionsWe argue that "bringing back" the estrous cycle variable to the main stage is important in order to enhance the resolution and quality of the data, to advance the health of women and other menstruators, and to make research more gender-inclusive. We strongly encourage the neuroscience community to incorporate the estrous cycle information in their study design and data analysis, whenever possible, and we debunk some myths that tend to de-emphasize the importance and discourage the inclusion of this critically important biological variable. Highlights Ovarian hormone fluctuation impacts brain physiology and is a major psychiatric risk factor, yet this variable has been overlooked in neuroscience research and psychiatric practice. From rodent behavior to gene regulation, accounting for the estrous cycle increases the resolution of the neuroscience data, allowing identification and mechanistic insight(s) into sex differences. Female variability does not equal (and is not predictive of) the estrous cycle effect and should not be used as a proxy for the effects of ovarian hormones on the outcome of interest. Neuroscience researchers are advised to incorporate the estrous cycle information in their studies to foster more equitable, female- and gender-inclusive research. Studies of the ovarian cycle are especially important for improving women's mental health.

Male and female brains differ significantly in both health and disease, and yet the female brain has been understudied. Sex-hormone fluctuations make the female brain particularly dynamic and are likely to confer female-specific risks for neuropsychiatric disorders. The molecular mechanisms underlying the dynamic nature of the female brain structure and function are unknown. Here we show that neuronal chromatin organization in the female ventral hippocampus of mouse fluctuates with the oestrous cycle. We find chromatin organizational changes associated with the transcriptional activity of genes important for neuronal function and behaviour. We link these chromatin dynamics to variation in anxiety-related behaviour and brain structure. Our findings implicate an immediate-early gene product, Egr1, as part of the mechanism mediating oestrous cycle-dependent chromatin and transcriptional changes. This study reveals extreme, sex-specific dynamism of the neuronal epigenome, and establishes a foundation for the development of sex-specific treatments for disorders such as anxiety and depression.

Clostridioides difficile infection (CDI) causes the majority of identifiable antibiotic-associated diarrhea. Epidemiological studies have shown that biological human females are more susceptible to CDI than males. In this study, we show that female mice developed more severe CDI than males under all conditions tested. We found time-delayed effects of the female estrus cycle on CDI. Indeed, animals in proestrus at any time during CDI progression developed severe signs 1-2 days later. In contrast, animals that were in the estrus stage were protected. Consistent with the delayed effect of the estrous cycle on CDI, we found that pre-infection levels of the sexual hormone prolactin (PRL), immunoglobulin IgG2b, cytokine IL-1β, cytokine G-CSF, and chemokine KC (CXCL1) were the primary nodes of a complex network that correlated with CDI symptomatology on the day after spore challenge. Similarly, we found that the pre-infection levels of sex hormone progesterone, sex hormone luteinizing hormone (LH), immunoglobulin IgG1, chemokine eotaxin, and chemokine IP-10 (CXCL10) were the primary network nodes that affected CDI severity, but with a 2-day delay. As expected, early post-infection levels of immunoglobulins, cytokines, and chemokines formed a hormone-independent network that concurrently correlated with CDI severity. Interestingly, early post-infection levels of FSH, together with cytokine IL-1β and chemokine KC (CXL1), were the main nodes of a network that affected CDI 2 days later, during the recovery phase of the infection. In summary, we show that murine female sexual hormones affect CDI progression, probably by affecting the immune system both before infection and during disease development.