Nonvagal tachypnea following alpha2-adrenoceptor stimulation in awake goats.

To assess the influence of vagal afferent feedback in the development of respiratory instabilities induced by alpha2-adrenoceptor (alpha2-AR) stimulation in the goat, we examined the ventilatory effects of clonidine, an alpha2-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 microg 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 microg kg(-1)), a selective serotonin type 3 (5-HT3) receptor agonist. However, respiratory disturbances associated with alpha2-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 alpha2-AR agonists in the goat are mediated by alpha2-ARs in the CNS.