Nonvagal tachypnea following α 2-adrenoceptor stimulation in awake goats

Abstract

To assess the influence of vagal afferent feedback in the development of respiratory instabilities induced by α 2-adrenoceptor (α 2-AR) stimulation in the goat, we examined the ventilatory effects of clonidine, an α 2-AR agonist, in awake tracheostomized goats before and after bilateral mid-cervical vagotomy. Prior to vagal section, systemic administration of clonidine (0.5–3.0 μg kg −1) induced a highly dysrhythmic pattern of breathing in all animals that was characterized by alternating episodes of tachypnea and slow irregular breathing patterns including prolonged and variable expiratory time ( Te) intervals. Periods of apnea were commonly observed. Bilateral vagotomy resulted in a slower deeper breathing pattern and abolished the tachypnea evoked by intravenous administration of phenylbiguanide (PBG; 20–50 μg kg −1), a selective serotonin type 3 (5-HT 3) receptor agonist. However, respiratory disturbances associated with α 2-AR stimulation (including tachypnea) persisted after vagal section and were qualitatively and quantitatively similar to pre-vagotomy data demonstrating that vagal afferent feedback is not necessary for the development of respiratory disturbances induced by clonidine. The results suggest that respiratory dysrhythmias caused by α 2-AR agonists in the goat are mediated by α 2-ARs in the CNS.