CO2-sensitive neurons in organotypic cultures of the fetal rat medulla

Medullary slices of the fetal rat at gestational day 16 were cultivated (organotypic culture) for up to 20 days and current clamp experiments were performed on outgrowing neurons. CO₂-sensitivity was tested by changing the P_CO2 in the bath solution (equilibrating CO₂ fraction from 0.02 to 0.09). Two groups of CO₂-sensitive neurons were found; one with and the other without intrinsic CO₂-chemosensitivity. Neurons with intrinsic CO₂-sensitivity maintained their spontaneous activity and chemosensitivity after blockade of synaptic transmission. These neurons exhibited action potentials that were preceeded by a spontaneous interspike depolarization and followed by an afterhyperpolarization (beating neurons). Increasing P_CO2 either decreased (inhibited neurons, n=55) or increased the spike frequency of these neurons (stimulated neurons, n=31). The reduced activity of CO₂-inhibited neurons was associated with membrane hyperpolarization and/or decreases in the slope of interspike depolarization. In contrast CO₂-stimulated neurons were depolarized and the slope of their interspike depolarization was augmented during acidosis. In addition, we demonstrated a strong voltage dependence of CO₂-induced effects on membrane potential and spike frequency. Neurons with non-beating activity did not show a spontaneous interspike depolarization and their spike generation and CO₂-sensitivity appeared to be entirely produced through synaptic inputs. The CO₂-mediated changes in electrical properties of these neurons closely resemble those of various CNS neurons, including respiratory neurons, in whole animal or neonatal brainstem-spinal cord preparations.