Endogenous opioids modulate ventilation in the obese Zucker rat.

This study evaluated the modulatory role of endogenous opioids on ventilation in young and mature, lean and obese male Zucker rats. Naloxone, an opioid receptor antagonist, and saline (control) were administered subcutaneously to awake rats, and ventilation in air and in response to an hypoxic and an hypercapnic gas challenge measured. In response to naloxone young, obese but not lean rats exhibited a marked increase of ventilation in all three conditions. Older obese Zucker rats that were morbidly obese breathed at a frequency of over 200 breaths per minute and showed only a modest increase of ventilation in response to naloxone. Older lean rats increased ventilation with naloxone only when exposed to hypercapnia. Unlike the stimulatory effects hypoxia and hypercapnia had on ventilation in older, lean rats, the ventilatory responses of the obese, older rats to hypoxia and to hypercapnia were blunted. We conclude that the obese Zucker rat may be a good animal model to assess how chest wall loading and endogenous opioids interact in the development of ventilatory control abnormalities.