Effect of Anoxia on Intracellular pH in Neurons from Chemosensitive and Non-Chemosensitive Regions of Medullary Brainstem Slices

We studied whether intracellular pH (pHi) of neurons in 2 chemosensitive areas (nucleus tractus solitarius, NTS, and ventrolateral medulla, VLM) responds to anoxia differently than neurons in 2 non-chemosensitive areas of the medulla oblongata (inferior olive, IO, and hypoglossai, Hyp). Medullary brain slices (∼300 μm; thick) from preweanling rats were loaded with BCECF and pHi was followed in individual neurons using fluorescence imaging microscopy. The initial pHi under control conditions (5% CO2/95% O2) was similar in VLM, IO and Hyp neurons (∼7.30) and was slightly higher in NTS neurons (∼7.49). When exposed to anoxic conditions (5% CO2/95% N2) for about 10 minutes, both the fall in pH (0.1-0.3 pH unit) and the initial rate of acidification (0.04-0.11 pH unit/min) varied among the 4 regions, but were not different in chemosensitive vs. non-chemosensitive areas. With the addition of the Na/H exchange blocker amiloride, a significant increase in both the magnitude (by 30-400%) and the rate of anoxia-induced acidification (by 8-30%) was seen in all 4 areas. These data indicate that anoxia-induced acidification: 1) is small in medullary neurons from neonatal rats; 2) is not different in neurons from chemosensitive vs. non-chemosensitive regions; and 3) is markedly blunted by the activity of the Na/H exchanger. These data are consistent with the high buffering power and with the role of Na/H exchange in pH recovery from acid loads in these neurons and may be a part of the mechanism that renders juveniles more anoxia tolerant than adult animals.