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 CO ₂ 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 % CO ₂ , 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 % CO ₂ , 77 mm HCO ₃ ⁻ , 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 HCO ₃ ⁻ -buffered medium (5 % CO ₂ , 26 mm HCO ₃ ⁻ , 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 % CO ₂ , 7 mm HCO ₃ ⁻ , 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.