Stress-induced attenuation of the hypercapnic ventilatory response in awake rats

To test the hypothesis that stress alters the performance of the respirator y control system, we compared the acute (20 min) responses to moderate hypo xia and hypercapnia of rats previously subjected to immobilization stress ( 90 min/day) with responses of control animals. Ventilatory measurements wer e performed on awake rats using whole body plethysmography. Under baseline conditions, there were no differences in minute ventilation between stresse d and unstressed groups. Rats previously exposed to immobilization stress h ad a 45% lower ventilatory response to hypercapnia (inspiratory CO2 fractio n = 0.05) than controls. In contrast, stress exposure had no statistically significant effect on the ventilatory response to hypoxia (inspiratory O-2 fraction = 0.12). Stress-induced attenuation of the hypercapnic response wa s associated with reduced tidal volume and inspiratory flow increases; the frequency and timing components of the response were not different between groups. We conclude that previous exposure to a stressful condition that do es not constitute a direct challenge to respiratory homeostasis can elicit persistent (greater than or equal to 24 h) functional plasticity in the ven tilatory control system.