Chemosensitive signaling of locus coeruleus neurons in the Bullfrog, Lithobates catesbeianus

Bullfrogs, Lithobates catesbeianus , experience broad changes in pH inversely proportional to temperature while retaining whole body CO 2 stores. Ventilatory patterns to maintain constant CO 2 are controlled through negative feedback with chemosensory neurons detecting CO 2 /pH. We hypothesize that neurons of the locus coeruleus (LC) will respond to hypercapnic acidosis (HA). Whole‐cell patch‐clamp electrophysiological techniques were used to measure firing rates in excitatory (chemosensitivity index (CI)=260±35%), inhibitory (CI=24±18%), and non‐chemosensitive (CI=105±3%) LC neurons in bullfrog brainstem slice preparations. LC neurons have resting membrane potentials of −42±0.6mV, and when stimulated by HA membrane potentials are depolarized to −39±0.7mV ( p <0.001). 81% of neurons increased and 11% decreased firing rates when exposed to HA, though increasing ΔCO 2 to acidify the superfusate by >2‐fold (0.13–0.33pH units) did not produce different firing rates ( p > 0.5). Greater than 80% of LC neurons are excited by HA which corroborate percentages typically reported for neonatal rat LC neurons; however, bullfrog excitatory output is doubled and appears to be maximally activated with lower CO 2 /H + changes compared with neonatal rats. Our data begin to provide insight into a neurological mechanism for α‐stat pH regulation in bullfrogs based on chemosensitivity to CO 2 /pH.