Dorsomedial medullary 5-HT2 receptors mediate immediate onset of initial hyperventilation, airway dilation, and ventilatory decline during hypoxia in mice.

The dorsomedial medulla oblongata (DMM) includes the solitary tract nucleus and the hypoglossal nucleus, to which 5-HT neurons project. Effects of 5-HT in the DMM on ventilatory augmentation and airway dilation are mediated via 5-HT2 receptors, which interact with the CO(2) drive. The interaction may elicit cycles between hyperventilation with airway dilation and hypoventilation with airway narrowing. In the present study, effects of 5-HT2 receptors in the DMM on hypoxic ventilatory and airway responses were investigated, while 5-HT release in the DMM was monitored. Adult male mice were anesthetized, and then a microdialysis probe was inserted into the DMM. The mice were placed in a double-chamber plethysmograph. After recovery from anesthesia, the mice were exposed to hypoxic gas (7% O(2) in N(2)) for 5 min with or without a 5-HT2 receptor antagonist (LY-53857) perfused in the DMM. 5-HT release in the DMM was increased by hypoxia regardless of the presence of LY-53857. Immediate onset and the peak of initial hypoxic hyperventilatory responses were delayed. Subsequent ventilatory decline and airway dilation during initial hypoxic hyperventilation were suppressed with LY-53857. These results suggest that 5-HT release increased by hypoxia acts on 5-HT2 receptors in the DMM, which contributes to the immediate onset of initial hypoxic hyperventilation, airway dilation, and subsequent ventilatory decline. Hypoxic ventilatory and airway responses mediated via 5-HT2 receptors in the DMM may play roles in immediate rescue and defensive adaptation for hypoxia and may be included in periodic breathing and the pathogenesis of obstructive sleep apnea.