Abstracts

Rationale: We have demonstrated that 532 nm low-power laser irradiation (LLI) increased the paroxysmal discharge threshold in the rabbit hippocampus, and almost the same effect was obtained by application of hyperpolarization-activated current (Ih) blocker (S. Kogure et al. Lasers Med Sci, 2010). Since the underlying mechanism of such LLI effect has not been well known at cellular level, the present study was designed to reveal LLI effect using whole-cell patch-clamp recording technique. Methods: The experiments were performed under appropriate conditions in accordance with the Declaration of Helsinki and the Bioethical Standards of Animal Experiments at Soka University. Four week old mice (FVB mouse) were used and their hippocampal slices were prepared according to the conventional methods. Pulsed LLI (532 nm, 1 ns pulse, 15 kHz, Average power: 90 mW) was introduced to the pyramidal cell layer of hippocampal CA1 region for 200 s. With a microelectrode of 4-5 MΩ, whole-cell current clampings at -400, -300, -200, and +200 pA with an interval of 5 s were performed for 500 ms each. Electrophysiological characteristics including amplitude, width and onset time of the action potential (AP) were measured before, during and after LLI. Results: Any differences between data before and during LLI, and between data during and after LLI were not observed in the resting membrane potential, AP amplitude, AP onset time, after-hyperpolarization and hyperpolarization-induced rebound depolarization ("sag"). During LLI, AP width measured at 50% of the AP peak decreased 1.6±0.9% (mean±SEM) in the pyramidal neurons (p<0.01), but it increased 2.0±1.0% after LLI (p<0.01), compared with data before LLI. After LLI, inter-spike intervals of 1st-2nd, 2nd-3rd, and 3rd-4th increased 8.7±3.9% (p<0.01), 6.5±2.9%, and 7.1±3.2%, respectively. Conclusions: These data suggest that 532 nm LLI directly activates potassium channels during the irradiation, but indirectly induces a delayed recovery of those channel after the irradiation. Since the temperature of the medium showed almost no changes, such effects are seemed to be based on the photochemical influence, not on the thermal effect.