Changes in phase timing and neuronal activity in the preBötzinger complex at the apneic threshold in adult decerebrate rabbits

Background: We have shown that a decrease in PCO2 affects the discharge frequency of respiratory neurons ( Fn ) in the preBötzinger Complex (preBötC) in a differential fashion. In a prior study where animals were hyperventilated to a PCO2 of 30-35mmHg that was 2-3 mmHg above the individual apneic threshold we observed an increase in inspiratory and expiratory duration and significant drop in peak phrenic activity (PPA). A subgroup of neurons of every discharge type did not have a change in Fn with hypocapnia, whereas there was a significant decrease in Fn of inspiratory augmenting (I-aug), decrementing (I-dec), parabolic (I-para), as well as expiratory parabolic (E-para) neurons. The current study investigated how neuronal activity and respiratory parameters changed when animals were hyperventilated to just below the apneic threshold. Methods: The study was approved by the local Animal Care Committee and conformed to NIH standards. Adult New Zealand White rabbits (3-4 kg) of either sex were anesthetized, tracheotomized, ventilated, decerebrated and vagotomized. Phrenic nerve activity was recorded from the C5 rootlet, time averaged and used to calculate inspiratory (TI) and expiratory duration (TE) and PPA. We extracellularly recorded individual neurons in the functionally identified preBötC using multichannel electrodes (Neuronexus, Ann Arbor, MI). Fn was displayed and recorded using Spike2 (CED, Cambridge, MA). Neurons were identified at PCO2 50 mmHg and continuously recorded while the animal was hyperventilated to its apneic threshold, i.e., complete cessation of phrenic activity. Statistical analysis was 1-way ANOVA, values are mean±SD, p<0.05. Results: In six animals, apnea was achieved at PCO2 34.2±1.1 mmHg. Phrenic apnea always resulted from a decrease in PPA to 0, while rhythmic respiratory neuronal discharge continued. With decreasing PCO2, TI initially increased from 0.9±0.2 to 1.0±0.4 sec (p=0.024) and then decreased to 0.6±0.3sec (p<0.001) shortly before apnea. TE increased from 1.1±0.4 to 1.9±0.7 sec (p=0.002) before apnea. The breakpoint between increase to decrease in TI was at 35.1±1.0 mmHg, i.e., on average 0.8±0.9 mmHg above the apneic threshold. Below the breakpoint, respiratory cycle time remained constant (p>0.05). We recorded 108 phasic respiratory neurons (68 I-aug, 25 I-para, 3 I-dec, 3 pre-I, 4 E-dec, and 5 IE neurons). During phrenic apnea Fn changed in one of three ways: 9 neurons stopped discharging (5 I-aug, 2 I-para, 1 I-dec, 1 pre-I). 36 neurons continued to discharge but were not phasic (24 I-aug. 5 I-para, 1 pre-I, 4 E-dec, 2 IE). 63 neurons showed ongoing phasic activity (39 I-aug, 18 I-para, 2 I-dec, 1 pre-I, 3 IE), although many also showed activity between phases. All changes reversed with subsequent increase in PCO2. Conclusion: We identified a breakpoint at ~1mmHg above the individual apneic threshold where inspiratory duration changed dramatically. Below this point, respiratory cycle time was constant. This rhythm with a cycle time of 1.5-3 sec may reflect the basic rhythm of the core inspiratory network in the preBötC. We also found that rhythmic inspiratory neuronal activity persisted during phrenic apnea, highlighting that motor output is regulated separately from rhythm. Together with our previous study, these data suggest that a core inspiratory rhythm exists at low levels of chemodrive, and that additional chemodrive leads to the recruitment of neurons that modulate inspiratory and expiratory duration and the magnitude of the motor output. NIH R01-HL159546 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.