CHEMOSENSITIVE MEDULLARY NEURONS IN THE BRAIN-STEM-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 who le cell and perforated patch recordings from ventral medullary neurone s which were identified by injection of Lucifer Yellow. The respirator y 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 clas ses of neurone - inspiratory, post-inspiratory, expiratory, respiratio n-modulated and tonic - were distinguished on the basis of their membr ane potential and discharge patter ns. Almost all (112 of 123) respond ed rapidly to 8% CO2 with a sustained change in membrane potential. De polarizing responses (3-18 mV) occurred in inspiratory, respiration-mo dulated and 45% of tonic neurones. Hyperpolarizing responses (2-19 mV) occurred in expiratory and post-inspiratory neurones. The remaining t onic neurones were inhibited or showed no response. 3. In representati ves of each class of neurone, membrane potential responses to 8% CO2 w ere retained when tested in the presence of tetrodotoxin (n = 7), low (0.2 mM) Ca2+-high (5 mM) Mg2+ (n = 23) or Cd2+ (0.2 mM) (n = 3)-conta ining superfusate, implying that they are mediated by intrinsic membra ne or cellular mechanisms. 4. Neurones were distributed between 1200 m u m rostral and 400 mu m caudal to obex, and their cell bodies were lo cated between 50 and 700 mu m below the ventral surface (n = 104). Alm ost all responsive neurones (n = 78) showed dendritic projections to w ithin 50 mu m of the surface. 5. These experiments indicate that signi ficant numbers of ventral medullary neurones, including respiratory ne urones, are intrinsically chemosensitive. The consistency with which t hese neurones show surface dendritic projections suggests that this se nsitivity may arise in part at this level.