Role of intracellular and extracellular pH in the chemosensitive response of rat locus coeruleus neurones

The chemosensitive response of locus coeruleus (LC) neurones to changes in intracellular pH (pH_i), extracellular pH (pH_o) and molecular CO₂ were investigated using neonatal rat brainstem slices. A new technique was developed that involves the use of perforated patch recordings in combination with fluorescence imaging microscopy to simultaneously measure pH_i and membrane potential (V_m). Hypercapnic acidosis (15% CO₂1, pH_o 6.8) resulted in a maintained fall in pH_i of 0.31 pH units and a 93% increase in the firing rate of LC neurones. On the other hand, isohydric hypercapnia (15% CO₂, 77 mM HCO₃^-, pH_o 7.45) resulted in a smaller and transient fall in pH_i of about 0.17 pH units and an increase in firing rate of 76%. Acidified Hepes (N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid) -buffered medium (pH_o 6.8) resulted in a progressive fall in pH_i of over 0.43 pH units and an increase in firing rate of 126%. Isosmotic addition of 50 mM propionate to the standard HCO₃^--buffered medium (5% CO₂, 26 mM HCO₃^-, pH_o 7.45) resulted in a transient fall in pH_i of 0.18 pH units but little increase in firing rate. Isocapnic acidosis (5% CO₂, 7 mM HCO₃6-, pH_o 6.8) resulted in a slow intracellular acidification to a maximum fall of about 0.26 pH units and a 72% increase in firing rate. For all treatments, the changes in pH_i preceded or occurred simultaneously with the changes in firing rate and were considerably slower than the changes in pH_o. In conclusion, an increased firing rate of LC neurones in response to acid challenges was best correlated with the magnitude and the rate of fall in pH_i, indicating that a decrease in pH_i is a major part of the intracellular signalling pathway that transduces an acid challenge into an increased firing rate in LC neurones.