ROLE OF GLIA IN K+ AND PH HOMEOSTASIS IN THE NEONATAL RAT SPINAL-CORD

Stimulation-evoked transient changes in extracellular potassium ([K+]e) and pH (pH(e)) were studied in the neonatal rat spinal cords isolated from 3-13-day-old pups. In unstimulated pups the [K+]e baseline was elevated and pH(e) was more acid than that in Ringer's solution (3.5 mM K+, pH 7.3-7.35). The [K+]e and pH(e) in 3-6-day-old pups was 3.91 +/- 0.12 mM and pH(e) 7.19 +/- 0.01, respectively, while in 10-13-day-old pups it was 4.35 +/- 0.15 mM and 7.11 +/- 0.01, respectively. The [K+]e changes evoked in the dorsal horn by a single electrical stimulus were as large as 1.5-2.5 mM. Such changes in [K+]e are evoked in the adult rat spinal cord with stimulation at a frequency of 10-30 Hz. The maximal changes of 2.1-6.5 mM were found at a stimulation frequency of 10 Hz in 3-6-day-old animals. In older animals the [K+]e changes progressively decreased. The poststimulation K+-undershoot was found after a single stimulus as well as after repetitive stimulation. In 3-8-day-old pups, the stimulation evoked an alkaline shift, which was followed by a smaller poststimulation acid shift when the stimulation was discontinued. In pups 3-4-days-old the stimulation evoked the greatest alkaline shifts, i.e., by as much as 0.05 pH units after a single pulse and by about 0.1 pH units during stimulation at a frequency of 10 Hz. In 5-8-day-old pups, the alkaline shift became smaller and the poststimulation acid shift increased. Stimulation in 10-13-day-old pups produced an acid shift of 0.03-0.07 pH units, which was preceded by a scarely discernible alkaline shift. MgCl2 (20 mM) reversibly reduced the alkaline but not the acid shifts by 50-60%. Bath application of the carbonic anhydrase inhibitor acetazolamide had no effect on the alkaline shift, while the acid shift decreased by 70-80%. The superfusion of the cord with 10 mM KCl resulted in acid shifts of 0.10-0.14 pH units. We conclude that the [K+]e ceiling level and the character of pH(e) transients in the spinal cord are closely related to gliogenesis. Our results suggest that glial cells buffer the activity-related [K+]e increase and alkaline pH(e) shifts in the extracellular space.