EVIDENCE THAT GLIAL-CELLS MODULATE EXTRACELLULAR PH TRANSIENTS INDUCED BY NEURONAL-ACTIVITY IN THE LEECH CENTRAL-NERVOUS-SYSTEM

1. The role of the giant neuropile glial cells in the buffering of act ivity-related extracellular pH changes was studied in segmental gangli a of the leech Hirudo medicinalis L. using: pH-sensitive microelectrod es and a slow, two-electrode voltage-clamp system. Neuronal activity w as induced by electrical stimulation of a ganglionic side nerve (20 Hz , 1 min). 2. In CO2-HCO3--buffered saline the glial cells were depolar ized by 6.5 +/- 2.3 mV and alkalinized by 0.024 +/-0.006 pH units (mea n +/- S.D.) during the stimulation. The stimulation induced an acidifi cation of 0.032 +/- 0.006 pH units in the extracellular spaces (ECS). 3. Voltage clamping the glial cells suppressed the stimulus-induced gl ial depolarization and turned the intraglial alkalinization into an ac idification of 0.045 +/- 0.021 pH units (n = 6) that closely resembled the acidification observed in the presence of the anion transport blo cker DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid, 0.5 mM ), and in CO2-HCO3--free saline. 4. Voltage clamping the glial cell re sulted in the appearance of a distinct stimulus-induced extracellular alkalinization of 0.024 +/- 0.013 pH units at the onset of the stimula tion, as also observed during DIDS application and in the absence of C O2-HCO3-. 5. The results suggest that glial uptake of bicarbonate is m ediated by depolarization-induced activation of the electrogenic Na+-H CO3- cotransport, which suppresses the profound alkalinization of the ECS during neuronal activity. This is the first direct evidence that g lial cells actively modulate extracellular pH changes in a voltage-dep endent manner.