Figure 6. CO₂/pH sensitive neurons from a chemoreceptive nucleus, the locus coeruleus (LC), have reduced firing responses to hypercapnia after submerged overwintering .

A, and B, representative integrated firing rate traces (top) and whole cell current‐clamp recordings of action potentials (bottom) in LC neurons from semi‐terrestrial and aquatic overwintered (air access only) bullfrogs. The dashed grey line indicates the interspike V _m (i.e. V _m between action potentials). A, upon transition from normocapnia (1.3% CO₂) to hypercapnia (5% CO₂), LC neurons from semi‐terrestrial bullfrogs increase action potential firing frequency and undergo slight membrane depolarization. This increase is reversed upon returning to normocapnia. B, in contrast, transition to hypercapnia in an LC neuron from an aquatic overwintered bullfrog results in a less robust increase in firing frequency with minimal membrane depolarization. C, mean firing frequencies before, during and after hypercapnia. Samples sizes are included in the figure. There is a significant interaction between CO₂ and acclimation group indicating that aquatic overwintered bullfrogs have reduced firing responses to hypercapnia (temperature acclimation × CO₂ interaction; P = 0.0062, two‐way ANOVA), even though LC neurons from both groups of bullfrogs undergo slight increases in firing frequency during hypercapnia (P < 0.05; Holm–Sidak's multiple comparisons test). D, firing rate in LC neurons from semi‐terrestrial bullfrogs expressed as percentage increase from baseline firing rate. Firing rate increases by ∼215% in semi‐terrestrial bullfrogs, while aquatic overwintered bullfrogs increase firing frequency by ∼90% (P = 0.01; two‐tailed unpaired t test). ^* P < 0.05, ^**** P < 0.0001 for within group comparisons. ^† P < 0.05 for between group comparisons. Error bars represent SEM.