Sex-Specific Effects of Neonatal Stress on Brainstem Expression of Orexin1-Receptors
Authors: Fournier Stephanie , Plamondon Julie , Richard Denis , Kinkead Richard 
Published: May 2024

Introduction and hypothesis: Located within the hypothalamus, orexin-producing neurons (ORX) regulate arousal, feeding, and cardiorespiratory function. Panic disorders (PD) affects ~5% of the North American population. Stress-related “hyperactivation” of the ORX system is a leading hypothesis to explain the pathophysiology of PD. One important sub-group of PD patients show significant respiratory symptoms, including an excessive ventilatory response to CO2 inhalation. Manifestations of PD are sex-specific: the prevalence of PD is ~2 times higher in women compared to men and the intensity of the ventilatory response to CO2 fluctuates with the hormonal cycle. Early life stress is a risk factor for PD and we recently demonstrated that in rats, neonatal maternal separation (NMS) augments the ventilatory response to CO2 (5%; 10 min) of adult females (but not males); this effect peaks with the rise of 17-b estradiol (E2) during proestrus. Because NMS-related enhancement of the hyperventilatory response to CO2 can be prevented by a selective ORX1-antagonist, we tested the hypothesis that sex, hormonal status and NMS interact to influence the expression of ORX-1 receptors in brainstem regions that regulate breathing. Methods: Following birth, pups were either exposed to NMS (3h/day from postnatal days 3 to 12) or raised under standard conditions. Rats were then raised until adulthood (8 weeks); experiments were performed on males, females in proestrus (high E2), and ovariectomised (low E2) females (2 weeks post-surgery). We used in situ hybridization histochemistry to quantify the expression of mRNA for ORX-1 receptors. The ORX-1 receptors signal was revealed by autoradiography emulsion and slides were examined under darkfield microscopy. Expression of ORX-1 receptors was quantified by measuring the pixel density of the hybridization signal in selected brainstem regions that regulate CO2 response. Results: The locus coeruleus (LC) contained the highest expression of ORX-1 receptors amongst the structures studied. In males, NMS was associated with significant decreases in ORX-1 expression in the LC and nucleus of the solitary tract (NTS). By comparison with OVX females, proestrus augmented ORX-1 expression in the LC of controls, but not NMS. By comparison with OVX, proestrus augmented ORX-1 expression in the raphe obscurus, especially in NMS females. Expression of ORX-1 was unchanged in the NTS of females. Conclusion and perspectives: We conclude that sex and the female’s hormonal status are important determinants of ORX-1 expression and that NMS can alter these relationships; however, those effects are structure-specific. In light of the evidence indicating that 5-HT dysfunction contributes to PD, the results observed in the raphe obscurus point to a plausible mechanism. This study was supported by ‘the Québec Heart and Lung Institute Research Foundation’. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.