Involvement of the central amygdaloid nucleus in the regulation of sex differences in the stress relief response in mice.

backgroundSex differences in brain function critically influence vulnerability to stress-related disorders such as anxiety and depression. Stress relief, defined as a positive emotional state following the termination of a threat, has been proposed as a natural reward promoting resilience. However, little is known about sex differences in stress relief behavior and the underlying neural mechanisms involved.

methodsAdult male and female C57BL/6J mice were subjected to a conditioned place preference (CPP) paradigm to evaluate stress relief responses following acute restraint stress. Estrous cycle stages in females were monitored during the test day to exclude hormonal effects. Whole-brain neuronal activity was assessed using large-scale c-Fos mapping to identify sex-specific neural correlates of stress relief. To establish causality, chemogenetic manipulations were performed by bilaterally expressing hM3Dq or hM4Di DREADDs in the central amygdala (CeA), followed by clozapine-N-oxide administration to selectively activate or inhibit CeA neurons during the behavioral test.

resultsWe found that male mice exhibited a robust stress relief response, whereas female mice failed to display stress relief, independent of stress intensity or the estrous cycle. c-Fos mapping revealed CeA neuronal inactivation in males but not females during stress relief. Chemogenetic activation of CeA neurons abolished stress relief in males, whereas inhibition of CeA neurons facilitated stress relief in females.

conclusionsThese data highlight the sex-specific role of CeA neurons in regulating stress relief, with inactivation promoting relief in males and inhibition enabling relief in females. These findings may provide a neural basis for understanding sex-specific mechanisms of stress relief and offer insights into the circuit-level origins of sex-biased vulnerability to stress-related psychiatric disorders. HIGHLIGHTS: Male mice exhibited robust stress relief responses, whereas females failed to display stress relief. The absence of stress relief in females was independent of stress intensity and estrous cycle stage. Whole-brain c-Fos mapping revealed sex-specific neural activation patterns, with CeA neurons inactivated in males but not in females during stress relief. Chemogenetic activation of CeA neurons abolished stress relief in males, whereas inhibition of CeA neurons enabled stress relief in females. Exploring sex differences in the brain is important for understanding the effects of such differences in stress-related disorders characterized by sex bias, as well as their therapeutic implications. In this manuscript, we examined sex differences in stress relief, a positive emotion triggered by the absence of an expected threat or the termination of an ongoing threat, as well as its potential mechanism. Our findings revealed sex variation in the stress relief response, as female mice did not show a stress relief response, which was independent of stress intensity or estrous cycle fluctuations. By determining neuronal activation in the mouse brain following a stress relief assay, we found that CeA neurons are inactivated during this paradigm in male mice but not in female mice. Moreover, chemogenetic activation of CeA neurons abolished the stress relief responses in male mice, whereas chemogenetic inactivation of CeA neurons facilitated the stress relief responses in female mice. We speculated that CeA neurons might play a key role in the sexually dimorphic stress relief response in mice. Hence, it is important to consider sex differences in both preclinical and clinical research studies that attempt to understand the mechanism related to stress relief.