Hypercapnia-induced long-term depression of respiratory activity requires α₂-adrenergic receptors

We investigated the effects of repeated hypercapnic episodes (inspired CO₂fraction = 0.10) on posthypercapnic respiratory nerve discharge. Anesthetized (urethan), vagotomized, and artificially ventilated rats were presented with three consecutive 5-min episodes of hyperoxic hypercapnia, separated by 5 min of hyperoxic normocapnia (inspired O₂ fraction = 0.5). Respiratory nerve discharge and blood gases were recorded before and 30 and 60 min after the final hypercapnic episode. Posthypercapnia, arterial$P{\text{CO}}_2$ was maintained within 1 Torr of initial baseline values. Integrated phrenic and hypoglossal burst amplitudes decreased posthypercapnia by up to 46 ± 17 and 55 ± 13% of baseline values, respectively, and remained reduced for at least 1 h [long-term depression (LTD)]. The protocol was repeated in rats pretreated with the α₂-adrenergic antagonists yohimbine HCl (0.5 mg/kg; n = 7) or 2-[2-(2-methoxy-1,4-benzodioanyl)]imidazoline (RX-821002) HCl (0.25 mg/kg; n = 3). Both drugs attenuated LTD in the phrenic and hypoglossal neurograms. Results indicate that episodic hypercapnia elicits a yohimbine- and RX-821002-sensitive LTD of respiratory nerve activity in rats, suggesting that LTD requires α₂-receptor activation.