Naloxone attenuates poststimulatory respiratory depression of laryngeal origin in the adult cat.

Poststimulatory depression in respiratory activity induced by superior laryngeal nerve (SLN) stimulation was quantitatively investigated in 20 adult cats. The role played in this phenomenon by endogenous opioids was studied using the opiate antagonist naloxone. The effects of hypercapnia on the same phenomenon were also investigated for comparison. Experiments were performed on cats anesthetized with pentobarbitone or a-chloralose, vagotomized, paralyzed, and artificially ventilated with 100% O2. Some animals were also carotid sinus denervated. Respiratory output was monitored as integrated phrenic nerve activity. SLN stimulation produced apnea, which outlasted the stimulation period; when respiration resumed, it was markedly depressed as revealed mainly by a decrease in phrenic minute output, respiratory frequency, and rate of rise of inspiratory activity. Phrenic output recovered gradually to control levels following an exponential time course. These effects varied as a function of the duration of SLN stimulation. Naloxone administration (0.8 mg/kg iv) significantly reduced the duration of poststimulatory apnea and attenuated the depression of phrenic minute output of the first recovery breath as a result of changes in peak phrenic activity; it also accelerated the time course of recovery. Hypercapnia did not affect the duration of poststimulatory apnea, but attenuated the initial poststimulatory depression because of changes in respiratory frequency; the rate of recovery was reduced. The results provide characterization of poststimulatory respiratory depression of laryngeal origin in the adult cat and suggest a role of endogenous opioids in its genesis or modulation.