LIGHT-SCATTERING CHANGES FOLLOW EVOKED-POTENTIALS FROM HIPPOCAMPAL SCHAEFFER COLLATERAL STIMULATION

We assessed relationships of evoked electrical and light scattering ch anges from cat dorsal hippocampus following Schaeffer collateral stimu lation. Under anesthesia, eight stimulating electrodes were placed in the left hippocampal CA1 field and an optic probe, coupled to a photod iode or a charge-coupled device camera to detect scattered Light chang es, was lowered to the contralateral dorsal hippocampal surface. Light at 660 +/- 10 (SE) nm illuminated the tissue through optic fibers sur rounding the optic probe. An attached bipolar electrode recorded evoke d right hippocampal commissural potentials. Electrode recordings and p hotodiode output were simultaneously acquired at 2.4 kHz during single biphasic pulse stimuli 0.5 ms in duration with 0.1-Hz intervals. Came ra images were digitized at 100 Hz. An average of 150 responses was ca lculated for each of six stimulating current levels. Stimuli elicited a complex population synaptic potential that lasted 100-200 ms dependi ng on stimulus intensity and electrode position. Light scattering chan ges peaked 20 ms after stimuli and occurred simultaneously with popula tion spikes. A long-lasting light scattering component peaked 100-500 ms after the stimulus, concurrently with larger population postsynapti c potentials. Optical signals occurred over a time course similar to t hat for electrical signals and increased with larger stimulation ampli tude to a maximum, then decreased with further increases in stimulatio n current. Camera images revealed a topographic response pattern that paralleled the photodiode measurements and depended on stimulation ele ctrode position. Light scattering changes accompanied fast electrical responses, occurred too rapidly for perfusion, and showed a stimulus i ntensity relationship not consistent with glial changes.