Responses of respiratory modulated and tonic units in the retrotrapezoid nucleus to CO 2

We hypothesized that the retrotrapezoid nucleus (RTN) contains both respiratory modulated (RM) and non-respiratory modulated (NRM) neurons which participate in the ventilatory response to increased CO 2. We made extracellular recordings of the activity of 46 single units in the RTN of 9 decerebrate, paralyzed, ventilated cats (5 intact; 4 with carotid body and sinus ablation) under eucapnic (P(CO 2) = 34.2 ± 3.5 mmHg; mean ± SD) and hypercapnic (P(CO 2) = 47.4 ± 3.4 mmHg) conditions. To define a RM unit, we used the η 2 statistic which is the ratio of the variance of the unit firing rate within respiratory cycles to that across respiratory cycles. We classified the units as RM (N = 17) if the η 2 values in eucapnia or hypercapnia were ≥0.25 and as NRM (N = 29) if the values were <0.25. Overall, 19/46 units (41%) increased their firing rate with increased CO 2, 5 decreased their firing rate, and 22 had no significant change in firing rate. Of 17 RM units, 8 (47%) increased their mean firing rate with hypercapnia from 7.6 ± 3.9 to 23.2 ± 6.8 spikes/sec. These included 5 inspiratory units, 2 inspiratory units that had an onset of firing in late expiration (Pre-I/I), and 1 expiratory unit. Seven of these also changed their discharge pattern (eucapnic η 2 = 0.02 to 0.12; hypercapnic η 2 = 0.34 to 0.79). Of 29 NRM units, 11 (38%) showed a significant increase in mean firing rate with CO 2 stimulation from 19.8 ± 7.2 to 31.3 ± 8.2 spikes/sec. The RTN has RM units which change their discharge pattern and firing rate in response to increased CO 2, as do units within the medulla and pons, and it has NRM units which are also responsive to increased CO 2. These data indicate that some neurons of the RTN are involved in the central chemoreceptor response but they provide no direct evidence that chemoreception resides within the RTN.link_to_subscribed_fulltex

Phrenic response to hypercapnia in the unanesthetized, decerebrate, newborn rat

We developed a decerebrate, vagotomized, newborn rat preparation to investigate brainstem respiratory control mechanisms without the influence of anesthesia, supra-pontine structures, or vagally mediated feedback mechanisms. We measured the changes in phrenic nerve electrical activity in response to breathing 3% and 5% CO 2 in unanesthetized, vagotomized, decerebrate newborn rats from 0 to 10 days of age and compared them with the changes in anesthetized, vagotomized, newborn rats and adult, vagotomized, decerebrate or anesthetized animals. Phrenic nerve activity was irregular in the young newborn rats and became more regular between 7 and 10 days of age. TI and TI/Ttot increased with age but increasing age had no influence on the response to CO 2. The response to CO 2, was dominated by increases in phrenic amplitude, minute activity, and inspiratory slope with no change in timing variables. These responses are similar to those that have been reported previously in vagally intact animals, suggesting that vagal feedback contributes little to the response to hypercapnia in the newborn rat. In summary, decerebrate newborn rats consistently respond to hypercapnia by increasing inspiratory drive similar to conscious animals.link_to_subscribed_fulltex