γ-aminobutyric acid receptors at the ventral surface of the medulla inhibit respiratory motor outflow to the laryngeal musculature

Abstract

The effect of activating γ-aminobutyric acid (GABA) receptors at the ventral surface of the medulla on the activity of the recurrent laryngeal nerve and phrenic nerve was assessed in the cat. Characteristics of the effects of GABA on the activity of the recurrent laryngeal nerve were compared with those on that of the phrenic nerve which has previously been shown to be inhibited by the application of GABA to the ventral surface of the medulla. Application of GABA (0.017–4.05 mg) to the intermediate area produced a dose-related inhibition of respiratory activity in the recurrent laryngeal nerve, as well as the phrenic nerve, that culminated in apnea. The inhibition in each nerve was seen as a decrease in amplitude of nerve activity with no change in respiratory rate. The onset time, peak time and recovery time from GABA-induced inhibition of activity in the recurrent laryngeal and phrenic nerves were not significantly different. The ED 50 value for GABA and its 95% confidence interval for inhibition of the activities of the recurrent laryngeal and phrenic nerves were 0.26 mg (0.19–0.36 mg) and 0.27 mg (0.20–0.37 mg), respectively. Therefore, the potency of GABA for the inhibition of the activity of these nerves was not significantly different. The GABA receptor antagonist, bicuculline (10 μg), reversed the inhibition of the activities of both the recurrent laryngeal and the phrenic nerves. The time for return of phasic activity in each nerve after bicuculline was not significantly different. No significant changes in the activity of either the recurrent laryngeal or the phrenic nerves occurred following application of GABA to the rostral or caudal areas on the ventral surface of the medulla. These findings indicate that GABA receptors, at the intermediate area on the ventral surface of the medulla, are involved in the regulation of respiratory motor outflow to the larynx as well as to the diaphragm.