Impact of the estrous cycle on brain monoamines and behavioral and respiratory responses to CO₂ in mice

The prevalence of panic disorder is two to four times higher in women compared to that in men, and hormonal changes during the menstrual cycle play a role in the occurrence of panic attacks. Here, we investigated the effect of the estrous cycle on the ventilatory and behavioral responses to CO₂ in mice. Female mice in proestrus, estrus, metestrus, or diestrus were exposed to 20% CO₂, and their escape behaviors, brain monoamines, and plasma levels of 17β-estradiol (E₂) and progesterone (P₄) were measured. Pulmonary ventilation (V̇_E), oxygen consumption (V̇O₂), and body core temperature (T_B) were also measured during normocapnia followed by CO₂. Females exposed to 20% CO₂ exhibited an escape behavior, but the estrous cycle did not affect this response. Females in all phases of the estrous cycle showed higher V̇_E and lower T_B during hypercapnia. In diestrus, there was an attenuation of CO₂-induced hyperventilation with no change in V̇O₂, whereas in estrus, this response was accompanied by a reduction in V̇O₂. Hypercapnia also increased the concentration of plasma P₄ and central DOPAC, the main dopamine metabolite, in all females. There was an estrous cycle effect on brainstem serotonin, with females in estrus showing a higher concentration than females in the metestrus and diestrus phases. Therefore, our data suggest that hypercapnia induces panic-related behaviors and ventilatory changes that lead to an increase in P₄ secretion in female mice, likely originating from the adrenals. The estrous cycle does not affect the behavioral response but interferes in the ventilatory and metabolic responses to CO₂ in mice.