IDENTIFICATION OF A SUBSURFACE AREA IN THE VENTRAL MEDULLA SENSITIVE TO LOCAL CHANGES IN PCO2

The exact location of the central respiratory chemoreceptors sensitive to changes in PCO2 has not yet been determined. To avoid the confounding effects of the cerebral circulation, we used the in vitro brain stem-spinal cord of neonatal rats (1-5 days old) to identify areas within 500-mu-m of the ventral surface of the medulla where changes in PCO2 evoked a sudden increase in the rate of respiratory neural activity. The preparation was superfused with mock cerebrospinal fluid (CSF) while maintained at constant temperature (26 +/- 1-degrees-C) and pH (7.34). Respiratory frequency increased linearly with decreases in superfusate pH (r2 = 0.92, P < 0.001), indicating that the respiratory circuitry for the detection of CO2 and stimulation of breathing was intact in this preparation. The search for central chemoreceptors was performed with a specially designed micropipette that allowed microejection of 2-10 nl of mock CSF equilibrated with different CO2-O2 gas mixtures. The pipette was advanced in 50- to 100-mu-m steps by use of a microdrive to a maximum depth of 500-mu-m from the surface of the ventral medulla. Depending on the location of the micropipette, ejection of CO2-acidified mock CSF at depths of 100-350-mu-m below the ventral surface of the medulla stimulated neural respiratory output. Using this response as an indication of the location of central respiratory chemoreceptors, we found that chemoreceptive elements were located in a column in the ventromedial medulla extending from the hypoglossal rootlets caudally to an area 0.75 mm caudal to VI nerve in the rostral medulla. At its caudal end, the column was 250-mu-m wide and lay 0.50-0.75 mm from midline. Rostrally, it fanned laterally and lay 0.50-1.00 mm lateral to midline. In this column, ejections of CO2-enriched CSF elicited an increase in respiratory neural output in 50-67% of the trials. We conclude that chemoreception of CO2 occurs in part in an area located beneath the surface of the ventral medulla.