Episodic breathing in alligators: role of sensory feedback

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 PO2 or PCO2 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, CO2-sensitive intrapulmonary chemoreceptors and pulmonary stretch receptors (n = 6); and (2) during unidirectional ventilation (UDV) at high flow rates (greater than 2 L/min), thereby minimizing oscillations in lung and blood PO2 and PCO2 (n = 6). Measurements on sedated alligators were made at 30 and 20 degrees 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 CO2; at 5% CO2 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 CO2, 2-4 breaths/cluster interspersed with occasional single breaths were evident in four alligators, while two had 6-8 breaths/cluster. Increasing CO2 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 CO2 condition (greater than 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 PCO2 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.