High CO2 chemosensitivity versus wide sensing spectrum: a paradoxical problem and its solutions in cultured brainstem neurons.

CO2 central chemoreceptors play an important role in cardiorespiratory control. They are highly sensitive to P(CO2) in a broad range. These two sensing properties seem paradoxical as none of the known pH-sensing molecules can achieve both. Here we show that cultured neuronal networks are likely to solve the sensitivity versus spectrum problem with parallel and serial processes. Studies were performed on dissociated brainstem neurons cultured on microelectrode arrays. Recordings started after a 3 week initial period of culture. A group of neurons were dose-dependently stimulated by elevated CO2 with a linear response ranging from 20 to 70 Torr. The firing rate of some neurons increased by up to 30% in response to a 1 Torr P(CO2) change, indicating that cultured brainstem neuronal networks retain high CO2 sensitivity in a broad range. Inhibition of Kir channels selectively suppressed neuronal responses to hypocapnia and mild hypercapnia. Blockade of TASK channels affected neuronal response to more severe hypercapnia. These were consistent with the pKa values measured for these K+ channels in a heterologous expression system. The CO2 chemosensitivity was reduced but not eliminated by blockade of presynaptic input from serotonin, substance P or glutamate neurons, indicating that both pre and postsynaptic neurons contribute to the CO2 chemosensitivity. These results therefore strongly suggest that the physiological P(CO2) range appears to be covered by multiple sensing molecules, and that the high sensitivity may be achieved by cellular mechanisms via synaptic amplification in cultured brainstem neurons.