Chemosensitive medullary neurones in the brainstem--spinal cord preparation of the neonatal rat.

1. Using the isolated medulla and spinal cord of the neonatal rat, the response to CO2-induced changes in superfusate pH was examined in whole cell and perforated patch recordings from ventral medullary neurones which were identified by injection of Lucifer Yellow. The respiratory response to changing the CO2 concentration (from 2 to 8%) consisted of an increase in phrenic burst frequency, which could be accompanied by an increase, decrease or no change in burst amplitude. 2. Five classes of neurone - inspiratory, post-inspiratory, expiratory, respiration-modulated and ionic - were distinguished on the basis of their membrane potential and discharge patterns. Almost all (112 of 123) responded rapidly to 8% CO2 with a sustained change in membrane potential. Depolarizing responses (3-18 mV) occurred in inspiratory, respiration-modulated and 45% of tonic neurones. Hyperpolarizing responses (2-19 mV) occurred in expiratory and post-inspiratory neurones. The remaining tonic neurones were inhibited or showed no response. 3. In representatives of each class of neurone, membrane potential responses to 8% CO2 were retained when tested in the presence of tetrodotoxin (n = 7), low (0.2 mM) Ca(2+)-high (5 mM) Mg2+ (n = 23) or Cd2+ (0.2 mM) (n = 3)-containing superfusate, implying that they are mediated by intrinsic membrane or cellular mechanisms. 4. Neurones were distributed between 1200 microns rostral and 400 microns caudal to obex, and their cell bodies were located between 50 and 700 microns below the ventral surface (n = 104). Almost all responsive neurones (n = 78) showed dendritic projections to within 50 microns of the surface. 6. These experiments indicate that significant numbers of ventral medullary neurones, including respiratory neurones, are intrinsically chemosensitive. The consistency with which these neurones show surface dendritic projections suggests that this sensitivity may arise in part at this level.