Chemosensitivity of medullary neurons in explant tissue cultures

To determine whether cultured medulla contains chemosensitive neurons which are excited by CO₂ and fixed acid and whether this function is specific to the ventral medulla, tissue expiants of ventral and dorsal medulla were prepared from neonatal rats and incubated for two to three weeks. Cultures were superfused with artificial cerebrospinal fluid, maintained at 37°C, and pH of the superfusate was varied either with PCO₂ (14-71 Torr) at constant HCO₃^- (22 mM) or HCO₃^- (10-30 mM) at constant PCO₂ (35 Torr). Spontaneous action potentials were recorded extracellularly in 51 ventral and 23 dorsal medullary neurons. Ventral medullary neurons exhibited a steady baseline firing frequency of 4 ± 0.8 Hz. In contrast, dorsal medullary neurons exhibited two different patterns of spontaneous activity: 11 fired continuously (7.2 ± 1.4 Hz) while 12 fired with a bursting pattern. Burst duration was 0.80 ± 0.14 min and cycle time was 1.74 ± 0.43 min. Decreasing pH with CO₂ caused an increase in the activity of 10 of 27 ventral medullary neurons and two of six dorsal medullary neurons with a mean response of 7.5 Hz/pH unit. Varying pH by changing HCO₃^- had no effect on firing frequency. These results demonstrate that: (i) chemosensitive neurons are present in both ventral and dorsal medullary expiant cultures; (ii) these cells only respond to changes in pH induced with CO₂; and (iii) about half of the dorsal medullary neurons fire spontaneously with a regular bursting pattern of activity.