Episodic breathing in alligators: Role of sensory feedback

Abstract

The episodic breathing pattern in many reptiles consists of two or more clustered breaths separated by variable non-ventilatory periods. This pattern is commonly postulated to result from oscillations in lung and/or blood P O 2 or P CO 2 via chemoreceptor feedback. We tested this hypothesis by monitoring breathing pattern in: (1) awake, undisturbed alligators and (2) sedated alligators (approx. 25 mg/kg pentobarbital, i.p.; 3 days prior to data collection). In sedated alligators, measurements were made: (1) before and after bilateral cervical vagotomy, a procedure that removes peripheral arterial chemoreceptors, CO 2-sensitive intrapulmonary chemoreceptors and pulmonary stretch receptors ( n = 6); and (2) during unidirectional ventilation (UDV) at high flow rates (> 2 L/min), thereby minimizing oscillations in lung and blood P O 2 and P CO 2 ( n = 6). Measurements on sedated alligators were made at 30 and 20 °C in each of these conditions. In awake, undisturbed alligators, breathing was typically episodic with 2–7 breaths/cluster, although the pattern was easily altered (increased breaths/cluster) by even seemingly minor disturbances. In sedated alligators, episodic breathing was still evident after vagotomy, but only at increased inspired CO 2; at 5% CO 2 four of six alligators exhibited episodic breathing consisting of 2–3 breaths/cluster interspersed with occasional single breaths. An episodic breathing pattern was also evident during UDV; at low levels of CO 2, 2–4 breaths/cluster interspersed with occasional single breaths were evident in four alligators, while two had 6–8 breaths/cluster. Increasing CO 2 in the UDV gas stream generally increased the number of breaths/cluster. After vagotomy, all six alligators could manifest an episodic breathing pattern during UDV in at least one CO 2 condition (> 2 breaths/cluster interspersed with occasional single breaths). The episodic breathing pattern was very labile, sometimes changing to single breaths without apparent cause. The results suggest: (1) episodic breathing requires neither feedback from vagal sensory receptors nor oscillations in respiratory gases; and (2) changes in arterial P CO 2 modulate, but do not initiate episodic breathing. Episodic breathing in alligators may be due to complex interactions of higher brain structures with the central rhythm generator.