Identification of CO2 Chemoreceptors in Helix pomatia

SYNOPSIS. Gas exchange in pulmonate snails of the family Helicidaeoccurs through a highly vascularized diffusion lung known asthe mantle. The extent of ventilation of the mantle dependsupon the duration and size of opening of an occlusible poreknown as the pneumostome. In Helix aspersa and Helix pomatia,pneumostomal size and frequency of opening are exquisitely sensitiveto CO₂. Respiratory CO₂ chemosensitivity resides in a discreteregion of the subesophageal ganglia. The discharge pattern ofmany neurons in the chemoreceptor area changes during stimulationwith CO₂. However, the electrophysiological response to CO,stimulation alone does not discriminate between CO₂ chemoreceptorcells and CO₂-insensitive neurons active in the pneumostomalresponse to CO₂. We identified a subset of CO₂-sensitive neuronsfrom the larger population of neurons active during CO₂ stimulation.The action potential discharge frequency of CO₂ chemosensoryneurons increased in response to CO₂ stimulation. An increaseddischarge frequency of CO₂-sensitive neurons was associatedwith increased pneumostomal opening, and both the size and thefrequency of pneumostomal opening increased during CO₂ stimulation.Injecting depolarizing current into individual chemosensoryneurons elicited opening of the pneumostome in the absence ofCO₂. Action potential generation in response to CO₂ was independentof synaptic transmission. Removal of individual CO₂-sensitivecells or inhibition of action potential generation in CO₂-sensitivecells reduced or eliminated pneumostomal responses to CO₂. CO₂sensitivity in chemoreceptor cells required extracellular calcium,but not sodium. Substituting barium for calcium supported chemoreceptoractivity. In summary, we have identified respiratory related,chemosensory neurons that are CO₂ sensitive in the absence ofsynaptic input.