Fig. 1.

Changes in pH_o associated with repetitive surface stimulation in the rat cerebellar cortex. The records show that heterogeneous levels of [H⁺] can be produced in the brain interstitial space which depend on the magnitude of brain excitation. (A) Local stimulation (loc) of the cerebellar surface excites a beam of parallel fibers (pf), which are axons of granule cells (gc), that in turn induce synaptic depolarization of Purkinje cell dendrites (pc) and interneurons. The cerebellar cortex is a highly ordered and well-defined laminar brain structure. Therefore, synchronous activation of a local population of parallel fibers and Purkinje cell dendrites produces well-defined changes in interstitial electrical potentials which can be used to accurately monitor microelectrode recording positions. Field potential analyses were used to determinepH_o recording depth in (B), (C), and (D). (B) Repetitive surface stimulation produced an alkaline then acid going response. Acidification of the interstitial space was proportional to the stimulus train rate. Records showpH_o transients 100 μm down in the cerebellar molecular layer in response to 5 Hz (lower record), 10 Hz (middle record), and 20 Hz (upper record) bipolar surface stimulation for 30 seconds. The stimulus began at the upward arrow and stopped at the downward arrow. The effect of increasing the duration of the stimulus train at 20 Hz is shown in (C). pH_o records again show an initial alkaline shift with initiation of the stimulus train (upward arrow). The subsequent acid shift increased in magnitude with the stimulus duration (4, 16, 32, and 64 seconds) untilpH_o began to reach a steady-state after 64 seconds. (D) Acidification of the interstitial space was also directly proportional to the density of activated neural tissue. pH_o changes diminished in depth away from the beam of parallel fibers activated for 30 seconds at 20 Hz beginning at the upward arrow and stopping at the downward arrow. The H⁺ selective microelectrode was advanced in increments of 150 μm vertically through a folium before each stimulus train (B), (C), and (D) are modified from Kraig et al., 1983.