Nonvagal tachypnea following α₂-adrenoceptor stimulation in awake goats

To assess the influence of vagal afferent feedback in the development of respiratory instabilities induced by α₂-adrenoceptor (α₂-AR) stimulation in the goat, we examined the ventilatory effects of clonidine, an α₂-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- HT₃) receptor agonist. However, respiratory disturbances associated with α₂-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 α₂-AR agonists in the goat are mediated by α₂-ARs in the CNS.