RESPONSES OF RESPIRATORY MODULATED AND TONIC UNITS IN THE RETROTRAPEZOID NUCLEUS TO CO2

We hypothesized that the retrotrapezoid nucleus (RTN) contains both re spiratory modulated (RM) and non-respiratory modulated (NRM) neurons w hich participate in the ventilatory response to increased CO2. We made extracellular recordings of the activity of 46 single units in the RT N of 9 decerebrate, paralyzed, ventilated cats (5 intact; 4 with carot id body and sinus ablation) under eucapnic (P(CO2) = 34.2 +/- 3.5 mmHg ; mean +/- SD) and hypercapnic (P(CO2) = 47.4 +/- 3.4 mmHg) conditions . To define a RM unit, we used the eta2 statistic which is the ratio o f the variance of the unit firing rate within respiratory cycles to th at across respiratory cycles. We classified the units as RM (N = 17) i f the eta2 values in eucapnia or hypercapnia were greater-than-or-equa l-to 0.25 and as NRM (N = 29) if the values were < 0.25. Overall, 19/4 6 units (41 %) increased their firing rate with increased CO2, 5 decre ased their firing rate. and 22 had no significant change in firing rat e. Of 17 RM units, 8 (47%) increased their mean firing rate with hyper capnia from 7.6 +/- 3.9 to 23.2 +/- 6.8 spikes/sec. These included 5 i nspiratory units, 2 inspiratory units that had an onset of firing in l ate expiration (Pre-1/1), and 1 expiratory unit. Seven of these also c hanged their discharge pattern (eucapnic eta2 = 0.02 to 0.12; hypercap nic eta2 = 0.34 to 0.79) Of 29 NRM units, 11 (38 %) showed a significa nt increase in mean firing rate with CO2 stimulation from 19.8 +/- 7.2 to 31.3 +/- 8.2 spikes/sec. The RTN has RM units which change their d ischarge pattern and firing rate in response to increased CO2, as do u nits within the medulla and pons, and it has NRM units which are also responsive to increased CO2. These data indicate that some neurons of the RTN are involved in the central chemoreceptor response but they pr ovide no direct evidence that chemoreception resides within the RTN.