Chemosensory responses to CO₂ in multiple brain stem nuclei determined using a voltage-sensitive dye in brain slices from rats

We used epifluorescence microscopy and a voltage-sensitive dye, di-8-ANEPPS, to study changes in membrane potential during hypercapnia with or without synaptic blockade in chemosensory brain stem nuclei: the locus coeruleus (LC), the nucleus of the solitary tract, lateral paragigantocellularis nucleus, raphé pallidus, and raphé obscurus and, in putative nonchemosensitive nuclei, the gigantocellularis reticular nucleus and the spinotrigeminal nucleus. We studied the response to hypercapnia in LC cells to evaluate the performance characteristics of the voltage-sensitive dye. Hypercapnia depolarized many LC cells and the voltage responses to hypercapnia were diminished, but not eradicated, by synaptic blockade (there were intrinsically CO ₂ -sensitive cells in the LC). The voltage response to hypercapnia was substantially diminished after inhibiting fast Na ⁺ channels with tetrodotoxin. Thus action potential–related activity was responsible for most of the optical signal that we detected. We systematically examined CO ₂ sensitivity among cells in brain stem nuclei to test the hypothesis that CO ₂ sensitivity is a ubiquitous phenomenon, not restricted to nominally CO ₂ chemosensory nuclei. We found intrinsically CO ₂ sensitive neurons in all the nuclei that we examined; even the nonchemosensory nuclei had small numbers of intrinsically CO ₂ sensitive neurons. However, synaptic blockade significantly altered the distribution of CO ₂ -sensitive cells in all of the nuclei so that the cellular response to CO ₂ in more intact preparations may be difficult to predict based on studies of intrinsic neuronal activity. Thus CO ₂ -sensitive neurons are widely distributed in chemosensory and nonchemosensory nuclei and CO ₂ sensitivity is dependent on inhibitory and excitatory synaptic activity even within brain slices. Neuronal CO ₂ sensitivity important for the behavioral response to CO ₂ in intact animals will thus be determined as much by synaptic mechanisms and patterns of connectivity throughout the brain as by intrinsic CO ₂ sensitivity.