HYPERCAPNIA AND MEDULLARY NEURONS IN THE ISOLATED BRAIN STEM-SPINAL CORD OF THE RAT

We have extracellularly recorded single neuron activity in the ventral medulla of the isolated brain stem-spinal cord preparation of the neo natal rat (37 preparations) in order to test their sensitivity to chan ges in CO2/H+. Search for neuronal activity was performed when the pre paration was superfused with control mock CSF (equilibrated with 2% CO 2, 90% O2 in N2; pH = 7.8 at 27-degrees-C). Neurons, found down to abo ut 500 mum from the surface, could be classified as R neurons when the y showed rhythmic discharge in phase with phrenic activity, recorded f rom C4 ventral roots; or as Non-R neurons when they did not exhibit su ch phasic discharge. Among the 89 Non-R neurons, 20 responded to rapid ly replacing the control CSF by hypercapnic CSF (8% CO2, 90% O2 in N2; pH = 7.2) with increased, 44 with reduced activity, while 25 did not respond to hypercapnia. Five Non-R neurons became phasic with respirat ion during hypercapnia. Of the 14 R neurons, 10 fired predominantly in expiration (R-E), 4 in inspiration (R-I). Only one R-E and two R-I ne urons were excited by hypercapnia, the remaining were either inhibited or did not respond. Excited Non-R and R neurons were mainly encounter ed in rostral parts of those areas in the ventral medulla that have be en reported as chemosensitive.