Acidosis of rat dorsal vagal neurons in situ during spontaneous and evoked activity.

1. Rat brainstem slices were taken for simultaneous measurements of intracellular pH (pHi) and membrane currents or potentials in dorsal vagal neurons, dialysed with the pH-sensitive dye BCECF. 2. Intrinsic intracellular buffering power was 18 mM per pH unit, as determined by exposure to trimethylamine in CO2/HCO3(-)-free, Hepes-buffered saline. 3. Tonic spike activity led to a stable fall in pHi of 0.05-0.2 pH units from a baseline of 7.19 in current-clamp mode, whereas depolarization from -60 to 0 mV for 1 min in voltage-clamp mode produced an intracellular acidification of 0.3 pH units. The depolarization-evoked fall in pHi was suppressed by 1 mM Ni2+ or 0.2 mM Cd2+, but not by 0.5 microM TTX or CO2/HCO3(-)-free saline. 4. Kainate (100 microM) led to an an inward current of -620 pA and a threefold increase in membrane conductance, accompanied by a fall in pHi of 0.33 pH units. 5. GABA (1 mM) evoked a bicuculline-blockable conductance increase and fall in pHi of up to 0.5 pH units. The GABA-induced pHi decrease, but not the conductance increase, was suppressed in Hepes solution. 6. Neither tonic spike activity, nor resting current or conductance were markedly changed upon Hepes-induced intracellular alkalinizations of up to 0.35 pH units, or by an anoxia-induced fall in pHi of a maximum of 0.36 pH units. 7. The data show that neuronal activity produces profound changes in pHi. It appears that spontaneous spike discharge of dorsal vagal neurons is rather tolerant of major perturbations in pHi.