© 1997 by The Society for Integrative and Comparative Biology
Identification of CO2 Chemoreceptors in Helix pomatia1
Departments of Physiology and Medicine, Dartmouth Medical School Lebanon, New Hampshire 03756 Tel.: (603)-650-8533, FAX: (603)-650-6l30, email: james.c.leiter{at}dartmouth.EDU
Correspondence: 2Corresponding author
SYNOPSIS. Gas exchange in pulmonate snails of the family Helicidae occurs through a highly vascularized diffusion lung known as the mantle. The extent of ventilation of the mantle depends upon the duration and size of opening of an occlusible pore known as the pneumostome. In Helix aspersa and Helix pomatia, pneumostomal size and frequency of opening are exquisitely sensitive to CO2. Respiratory CO2 chemosensitivity resides in a discrete region of the subesophageal ganglia. The discharge pattern of many neurons in the chemoreceptor area changes during stimulation with CO2. However, the electrophysiological response to CO, stimulation alone does not discriminate between CO2 chemoreceptor cells and CO2-insensitive neurons active in the pneumostomal response to CO2. We identified a subset of CO2-sensitive neurons from the larger population of neurons active during CO2 stimulation. The action potential discharge frequency of CO2 chemosensory neurons increased in response to CO2 stimulation. An increased discharge frequency of CO2-sensitive neurons was associated with increased pneumostomal opening, and both the size and the frequency of pneumostomal opening increased during CO2 stimulation. Injecting depolarizing current into individual chemosensory neurons elicited opening of the pneumostome in the absence of CO2. Action potential generation in response to CO2 was independent of synaptic transmission. Removal of individual CO2-sensitive cells or inhibition of action potential generation in CO2-sensitive cells reduced or eliminated pneumostomal responses to CO2. CO2 sensitivity in chemoreceptor cells required extracellular calcium, but not sodium. Substituting barium for calcium supported chemoreceptor activity. In summary, we have identified respiratory related, chemosensory neurons that are CO2 sensitive in the absence of synaptic input.