Opioid μ-receptors in the rostral medullary raphe modulate hypoxia-induced hyperpnea in unanesthetized rats

Aim:  It has been suggested that the medullary raphe (MR) plays a key role in the physiological responses to hypoxia. As opioid μ-receptors have been found in the MR, we studied the putative role of opioid μ-receptors in the rostral MR (rMR) region on ventilation in normal and 7% hypoxic conditions. Methods:  We measured pulmonary ventilation () and the body temperatures (Tb) of male Wistar rats before and after the selective opioid μ-receptor antagonist CTAP (d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2, cyclic, 0.1 μg per 0.1 μL) was microinjected into the rMR during normoxia or after 60 min of hypoxia. Results:  The animals treated with intra-rMR CTAP exhibited an attenuation of the ventilatory response to hypoxia (430 ± 86 mL kg−1 min−1) compared with the control group (790 ± 82 mL kg−1 min−1) (P < 0.05). No differences in the Tb were observed between groups during hypoxia. Conclusion:  These data suggest that opioids acting on μ-receptors in the rMR exert an excitatory modulation of hyperventilation induced by hypoxia.