Large lesions in the pre-B?tzinger complex area eliminate eupneic respiratory rhythm in awake goats.

In awake goats, 29% bilateral destruction of neurokinin-1 receptor-expressing neurons in the pre-B?tzinger complex (pre-B?tzC) area with saporin conjugated to substance P results in transient disruptions of the normal pattern of eupneic respiratory muscle activation (Wenninger JM, Pan LG, Klum L, Leekley T, Bastastic J, Hodges MR, Feroah T, Davis S, and Forster HV. J Appl Physiol 97: 1620-1628, 2004). Therefore, the purpose of these studies was to determine whether large or total lesioning in the pre-B?tzC area of goats would eliminate phasic diaphragm activity and the eupneic breathing pattern. In awake goats that already had 29% bilateral destruction of neurokinin-1 receptor-expressing neurons in the pre-B?tzC area, bilateral ibotenic acid (10 microl, 50 mM) injection into the pre-B?tzC area resulted in a tachypneic hyperpnea that reached a maximum (132 +/- 10.1 breaths/min) approximately 30-90 min after bilateral injection. Thereafter, breathing frequency declined, central apneas resulted in arterial hypoxemia (arterial Po2 approximately 40 Torr) and hypercapnia (arterial Pco2 approximately 60 Torr), and, 11 +/- 3 min after the peak tachypnea, respiratory failure was followed by cardiac arrest in three airway-intact goats. However, after the peak tachypnea in four tracheostomized goats, mechanical ventilation was initiated to maintain arterial blood gases at control levels, during which there was no phasic diaphragm or abdominal muscle activity. When briefly removed from the ventilator (approximately 90 s), these goats became hypoxemic and hypercapnic. During this time, minimal, passive inspiratory flow resulted from phasic abdominal muscle activity. We estimate that 70% of the neurons within the pre-B?tzC area were lesioned in these goats. We conclude that, in the awake state, the pre-B?tzC is critical for generating a diaphragm, eupneic respiratory rhythm, and that, in the absence of the pre-B?tzC, spontaneous breathing reflects the activity of an expiratory rhythm generator.