PACEMAKER BEHAVIOR OF RESPIRATORY NEURONS IN MEDULLARY SLICES FROM NEONATAL RAT

1. We have hypothesized that pacemaker neurons in the pre-Botzinger co mplex (pre-BotC) form the kernel for respiratory rhythm generation. A prediction of this hypothesis is that oscillatory behavior in some res piratory neurons could persist in the absence of synaptic transmission . In this study we used extracellular recording of neuronal activity i n slice preparations from neonatal rat medulla that generate respirato ry rhythm in vitro to determine 1) whether pacemaker properties are pr esent in pre-BotC and unique to respiratory neurons, 2) whether pacema ker properties are common to all respiratory neurons, and 3) the spati otemporal patterns of pacemaker neuron activity. 2. Whole cell recordi ngs from respiratory neurons verified that bathing the slices in a low -Ca2+/high-Mg2+ solution (low-Ca2+ solution) eliminated endogenous res piratory synaptic inputs and electrically evoked synaptic inputs. 3. S ixty-three neurons spontaneously generated rhythmic bursts of action p otentials in low-Ca2+ solution. After we switched to control solution to reactivate the respiratory network, these neurons were classified o n the basis of their spike discharge patterns relative to the respirat ory cycle as: 1) inspiratory (I) neurons (n = 41), 2) tonic expiratory (tonic E) neurons (n = 4), and 3) tonic neurons (n = 18). 4. In other experiments we tested I and tonic E neurons identified first in contr ol solution for bursting behavior in low-Ca2+ solution. Several I neur ons (n = 5 of 33), but none of the tonic E neurons (n = 0 of 13), cont inued to burst rhythmically. 5. Bursting and nonbursting respiratory n eurons were distributed throughout the ventrolateral reticular formati on within the pre-BotC as well as in the ventral respiratory group (VR G) immediately caudal to the pre-BotC. 6. We conclude that subpopulati ons of VRG neurons in vitro have rhythmic bursting properties when syn aptic transmission is abolished. Respiratory neurons, especially I neu rons, were the most prevalent class of bursting cells. Only a small pe rcentage of respiratory neurons, however, had pacemaker properties. Th ese findings are consistent with the hypothesis that the respiratory o scillator includes specialized neurons with intrinsic oscillatory prop erties.