Abstracts

RATIONALE: After fluid percussion head injury, dentate granule cells demonstrate hyperexcitability (Toth et al., 1997; Santhakumar et al., 2000). One hypothesis is that impaired clearance of extracellular potassium contributes to post-traumatic hippocampal hyperexcitability. This study was conducted to compare the steady-state and activity-dependent changes in extracellular potassium concentration in the dentate gyrus of head-injured and control animals.
METHODS: Single-barrel ion-selective microelectrodes (ISME)(1-10G[Omega]) with valinomycin-based potassium-selective membrane solution (Fluka) were used with field reference electrodes. Temporal characteristics of the ISME were determined by fast application switch using a nanostepper motor. The potassium concentration was measured in the granule cell layer in response to perforant path (orthodromic) or hilar (antidromic) stimulation in acute hippocampal slices from fluid percussion head injured (FPI) and control animals. GABA and glutamate receptor antagonists were used in some experiments.
RESULTS: First, there was no difference in the baseline potassium concentration in the granule cell layer between control and FPI animals 2 days, 1 week and 1 month after injury. Second, the clearance of potassium increase evoked by orthodromic tetanic stimulation was not different between head injured and control animals. Third, since calibration of the temporal characteristics showed that ISMEs had a latency of about 7ms to detect a 0.05mM change potassium concentration, ISMEs were used to compare single-shock stimulation evoked rapid transient changes in extracellular potassium between FPI and control animals. Again, although the amplitude of the evoked extracellular potassium increase and field population response were significantly greater after FPI, the clearance of the evoked potassium increase was not statistically different between injured and control animals. Fourth, the clearance of exogenously applied potassium was not different between injured and control animals. Finally, the post-traumatic clearance of potassium increase, evoked by antidromic stimulation of the granule cells in the presence of ionotropic glutamate and GABA receptor antagonists, was not statistically different from controls. Interestingly, although previous studies have used antidromic stimulation in ionotropic glutamate and GABA antagonists as a method to normalize action potential firing between injured and control groups, both the evoked field response and extracellular potassium increase in the dentate were significantly greater in the injured animals compared to controls.
CONCLUSIONS: There is no increase in the steady-state extracellular potassium concentration in the dentate gyrus days or weeks after head injury. The clearance of both activity-dependent endogenously released and exogenously applied potassium is not different between the injured and control animals. Post-traumatic increase in amplitude of extracellular potassium in response to both orthodromic and antidromic stimulation is a consequence (and not a cause) of increased excitability after head injury.
[Supported by: NIH (NS35915) to I.S.]