Identification of CO₂Chemoreceptors inHelix pomatia

Abstract

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 CO 2 . Respiratory CO 2 chemosensitivity resides in a discrete region of the subesophageal ganglia. The discharge pattern of many neurons in the chemoreceptor area changes during stimulation with CO 2 . However, the electrophysiological response to CO, stimulation alone does not discriminate between CO 2 chemoreceptor cells and CO 2 -insensitive neurons active in the pneumostomal response to CO 2 . We identified a subset of CO 2 -sensitive neurons from the larger population of neurons active during CO 2 stimulation. The action potential discharge frequency of CO 2 chemosensory neurons increased in response to CO 2 stimulation. An increased discharge frequency of CO 2 -sensitive neurons was associated with increased pneumostomal opening, and both the size and the frequency of pneumostomal opening increased during CO 2 stimulation. Injecting depolarizing current into individual chemosensory neurons elicited opening of the pneumostome in the absence of CO 2 . Action potential generation in response to CO 2 was independent of synaptic transmission. Removal of individual CO 2 -sensitive cells or inhibition of action potential generation in CO 2 -sensitive cells reduced or eliminated pneumostomal responses to CO 2 . CO 2 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 CO 2 sensitive in the absence of synaptic input.

CO2 chemosensitivity in Helix aspersa: three potassium currents mediate pH-sensitive neuronal spike timing

“…The AP upstroke was predominantly calcium dependent (n ϭ 5), which is consistent with results from a previous study (25). The reduction in spike amplitude suggests that acidification inhibited the calcium channels expressed in H. aspersa neurons.…”

“…We found respiratory chemosensory neurons. We also found neurons, however, that were CO 2 sensitive, but not respiratory chemoreceptors; their activity did not modulate pneumostomal activity (25). In studies of isolated cells, we require only that CO 2 -sensitive cells be activated or inhibited by exposure to CO 2 in the absence of synaptic activity, and we consider any significant change in membrane potential (even without AP formation) as evidence of CO 2 sensitivity.…”

“…We previously identified respiratory-related CO 2 -excited neurons within the CO 2 -chemosensitive area of the subesophageal ganglia (25), but the distribution of CO 2 -sensitive neurons outside the chemosensitive region was not evaluated systematically. To determine whether CO 2 -excited neurons existed only in the anatomically restricted chemosensitive region, we mapped the location of CO 2 -excited, CO 2 -inhibited, and nonchemosensitive neurons in the subesophageal ganglia by recording spontaneous or evoked activity under normocapnic and hypercapnic conditions.…”

CO2 chemosensitivity in Helix aspersa: three potassium currents mediate pH-sensitive neuronal spike timing

“…The AP upstroke was predominantly calcium dependent (n ϭ 5), which is consistent with results from a previous study (25). The reduction in spike amplitude suggests that acidification inhibited the calcium channels expressed in H. aspersa neurons.…”

“…We found respiratory chemosensory neurons. We also found neurons, however, that were CO 2 sensitive, but not respiratory chemoreceptors; their activity did not modulate pneumostomal activity (25). In studies of isolated cells, we require only that CO 2 -sensitive cells be activated or inhibited by exposure to CO 2 in the absence of synaptic activity, and we consider any significant change in membrane potential (even without AP formation) as evidence of CO 2 sensitivity.…”

“…We previously identified respiratory-related CO 2 -excited neurons within the CO 2 -chemosensitive area of the subesophageal ganglia (25), but the distribution of CO 2 -sensitive neurons outside the chemosensitive region was not evaluated systematically. To determine whether CO 2 -excited neurons existed only in the anatomically restricted chemosensitive region, we mapped the location of CO 2 -excited, CO 2 -inhibited, and nonchemosensitive neurons in the subesophageal ganglia by recording spontaneous or evoked activity under normocapnic and hypercapnic conditions.…”

Intracellular pH regulation in neurons from chemosensitive and nonchemosensitive regions of Helix aspersa

“…Focal hypercapnic stimulation of this CO 2 -sensitive region increased pneumostomal opening and mimicked the response that we observed in intact snails exposed to ambient hypercapnic gases (11). We also identified intrinsically CO 2 -sensitive neurons within the CO 2 -sensitive region (14).…”

“…In our previous electrophysiological studies of CO 2 -chemosensitive neurons, we found very few intrinsically CO 2 -sensitive neurons in each chemosensitive area, perhaps 8-12 neurons (14). It was our expectation that poor pH i regulation might be present only in a few cells in the chemosensitive area of H. aspersa, and we did find that poor pH i regulation among neurons in the subesophageal ganglia was significantly (P ϭ 0.007) segregated and much more likely in neurons confined to the CO 2sensitive area.…”

CO2 chemosensitivity in Helix aspersa: three potassium currents mediate pH-sensitive neuronal spike timing

“…The AP upstroke was predominantly calcium dependent (n ϭ 5), which is consistent with results from a previous study (25). The reduction in spike amplitude suggests that acidification inhibited the calcium channels expressed in H. aspersa neurons.…”

“…We found respiratory chemosensory neurons. We also found neurons, however, that were CO 2 sensitive, but not respiratory chemoreceptors; their activity did not modulate pneumostomal activity (25). In studies of isolated cells, we require only that CO 2 -sensitive cells be activated or inhibited by exposure to CO 2 in the absence of synaptic activity, and we consider any significant change in membrane potential (even without AP formation) as evidence of CO 2 sensitivity.…”

“…We previously identified respiratory-related CO 2 -excited neurons within the CO 2 -chemosensitive area of the subesophageal ganglia (25), but the distribution of CO 2 -sensitive neurons outside the chemosensitive region was not evaluated systematically. To determine whether CO 2 -excited neurons existed only in the anatomically restricted chemosensitive region, we mapped the location of CO 2 -excited, CO 2 -inhibited, and nonchemosensitive neurons in the subesophageal ganglia by recording spontaneous or evoked activity under normocapnic and hypercapnic conditions.…”

Intracellular pH regulation in neurons from chemosensitive and nonchemosensitive regions of Helix aspersa

“…Focal hypercapnic stimulation of this CO 2 -sensitive region increased pneumostomal opening and mimicked the response that we observed in intact snails exposed to ambient hypercapnic gases (11). We also identified intrinsically CO 2 -sensitive neurons within the CO 2 -sensitive region (14).…”

“…In our previous electrophysiological studies of CO 2 -chemosensitive neurons, we found very few intrinsically CO 2 -sensitive neurons in each chemosensitive area, perhaps 8-12 neurons (14). It was our expectation that poor pH i regulation might be present only in a few cells in the chemosensitive area of H. aspersa, and we did find that poor pH i regulation among neurons in the subesophageal ganglia was significantly (P ϭ 0.007) segregated and much more likely in neurons confined to the CO 2sensitive area.…”

CO2 chemosensitivity in Helix aspersa: three potassium currents mediate pH-sensitive neuronal spike timing

“…The AP upstroke was predominantly calcium dependent (n ϭ 5), which is consistent with results from a previous study (25). The reduction in spike amplitude suggests that acidification inhibited the calcium channels expressed in H. aspersa neurons.…”

“…We found respiratory chemosensory neurons. We also found neurons, however, that were CO 2 sensitive, but not respiratory chemoreceptors; their activity did not modulate pneumostomal activity (25). In studies of isolated cells, we require only that CO 2 -sensitive cells be activated or inhibited by exposure to CO 2 in the absence of synaptic activity, and we consider any significant change in membrane potential (even without AP formation) as evidence of CO 2 sensitivity.…”

“…We previously identified respiratory-related CO 2 -excited neurons within the CO 2 -chemosensitive area of the subesophageal ganglia (25), but the distribution of CO 2 -sensitive neurons outside the chemosensitive region was not evaluated systematically. To determine whether CO 2 -excited neurons existed only in the anatomically restricted chemosensitive region, we mapped the location of CO 2 -excited, CO 2 -inhibited, and nonchemosensitive neurons in the subesophageal ganglia by recording spontaneous or evoked activity under normocapnic and hypercapnic conditions.…”

Intracellular pH regulation in neurons from chemosensitive and nonchemosensitive regions of Helix aspersa

“…Focal hypercapnic stimulation of this CO 2 -sensitive region increased pneumostomal opening and mimicked the response that we observed in intact snails exposed to ambient hypercapnic gases (11). We also identified intrinsically CO 2 -sensitive neurons within the CO 2 -sensitive region (14).…”

“…In our previous electrophysiological studies of CO 2 -chemosensitive neurons, we found very few intrinsically CO 2 -sensitive neurons in each chemosensitive area, perhaps 8-12 neurons (14). It was our expectation that poor pH i regulation might be present only in a few cells in the chemosensitive area of H. aspersa, and we did find that poor pH i regulation among neurons in the subesophageal ganglia was significantly (P ϭ 0.007) segregated and much more likely in neurons confined to the CO 2sensitive area.…”