Chronic intermittent hypoxia sensitizes acute hypothalamic-pituitary-adrenal stress reactivity and Fos induction in the rat locus coeruleus in response to subsequent immobilization stress

Abstract

Obstructive sleep apnea (OSA) is associated with several pathophysiological conditions, including hypertension, obesity, insulin resistance, hypothalamic-pituitary-adrenal (HPA) dysregulation, and other endocrine and metabolic disturbances comprising the "metabolic syndrome." Repeated episodes of hypoxia in OSA may represent a chronic intermittent stress, leading to HPA dysregulation. Alterations in HPA reactivity could then contribute to or exacerbate other pathophysiological processes. We showed previously that another metabolic stressor, chronic intermittent cold stress, enhanced noradrenergic facilitation of acute HPA stress reactivity. In this study, we investigated whether chronic intermittent hypoxia (CIH), a rat model for the arterial hypoxemia that accompanies OSA, similarly sensitizes the HPA response to novel acute stress. Rats were exposed to CIH (alternating cycles of normoxia [3 min at 21% O(2)] and hypoxia [3 min at 10% O(2)], repeated continuously for 8 h/day during the light portion of the cycle for 7 days). On the day after the final CIH exposure, there were no differences in baseline plasma adrenocorticotropic hormone (ACTH), but the peak ACTH response to 30 min acute immobilization stress was greater in CIH-stressed rats than in controls. Induction of Fos expression by acute immobilization stress was comparable following CIH in several HPA-modulatory brain regions, including the paraventricular nucleus, bed nucleus of the stria terminalis, and amygdala. Fos induction was attenuated in lateral hypothalamus, an HPA-inhibitory region. By contrast, acute Fos induction was enhanced in noradrenergic neurons in the locus coeruleus following CIH exposure. Thus, similar to chronic cold stress, CIH sensitized acute HPA and noradrenergic stress reactivity. Plasticity in the acute stress response is important for long-term adaptation, but may also contribute to pathophysiological conditions associated with states of chronic or repeated stress, such as OSA. Determining the neural mechanisms underlying these adaptations may help us better understand the etiology of such disorders, and inform the development of more effective treatments.

Figure 1

Sensitizing effect of chronic intermittent hypoxia on subsequent activation of the HPA axis in response to a novel heterotypic challenge, acute immobilization stress. There were no baseline differences in plasma ACTH between groups. Acute immobilization stress (bar) significantly increased plasma ACTH concentrations in both groups. However, the response at the 30 min time point was significantly elevated in CIH-exposed rats compared to controls, indicating sensitization. Data expressed as mean ± SEM; n = 7/group; *p < 0.05 for each group compared to their respective baseline; +p < 0.05 for CIH-exposed rats compared to controls at the same time point.

Figure 2

Acute immobilization stress-induced Fos expression in the locus coeruleus (LC) was sensitized following chronic intermittent hypoxia. Left: Representative examples of dual-immunohistochemical staining for Fos immunoreactivity (DAB immunoperoxidase labeling, top panels) and DβH (Cy3 immunofluorescence, bottom panels) in a single section cut through the LC of a rat given no chronic pretreatment (non-CIH) and in a rat exposed to 1 week chronic intermittent hypoxia (CIH). Both rats were sacrificed after 60 min of recovery following 60 min acute immobilization stress (i.e., 2 hours after acute stress onset). Right: Summary data showing the effects of CIH on the induction of Fos immunoreactvity in the LC by acute immobilization stress. Black bars represent rats with no chronic treatment prior to immobilization stress, and white bars represent rats exposed to chronic intermittent hypoxia for 7 day before receiving acute immobilization stress. Control indicates rats with no exposure to immobilization stress; 1 h indicates rats perfused immediately after one hour of immobilization stress; 2 h indicates rats exposed to 1 hour of immobilization stress then 1 hour of recovery prior to perfusion. *p < 0.05 compared to the group with no CIH treatment. n = 5–9 rats per group. Data expressed as mean ± SEM.

Figure 3

Acute immobilization stress-induced Fos expression in the lateral hypothalamus (LH) was attenuated following chronic intermittent hypoxia. Left: Representative examples of Fos immunoreactivity in the perifornical region of the LH of a rat given no chronic pretreatment (non-CIH) and in a rat exposed to 1 week chronic intermittent hypoxia (CIH). Both rats were sacrificed immediately after 60 min acute immobilization stress. fx, fornix; medial is to the right. Right: Summary data showing the effects of CIH on the induction of Fos immunoreactvity in the LH by acute immobilization stress. Black bars represent rats with no chronic treatment prior to immobilization stress, and white bars represent rats exposed to chronic intermittent hypoxia for 7 day before receiving acute immobilization stress. Control indicates rats with no exposure to immobilization stress; 1 h indicates rats perfused immediately after one hour of immobilization stress; 2 h indicates rats exposed to 1 hour of immobilization stress then 1 hour of recovery prior to perfusion. *p < 0.05 compared to the group with no CIH treatment. n = 5–9 rats per group. Data expressed as mean ± SEM.