Light scattering in brain slices measured with a photon counting fiber optic system.

Measurements of intrinsic optical signals (IOSs) from neural tissue, commonly with a reflection-type or transmittance-type set-up, have been used increasingly to study physiological events. Even for the same event, however, such as spreading depression (SD) or osmotic challenge, signals of opposite polarities (increase or decrease) have been obtained by different investigators using similar set-ups under similar conditions. The origin of the inconsistencies is still unknown. It is suggested here that the inconsistencies may be caused by artifacts associated with tissue surface scattering. The main goal of this paper was to present a photon counting fiber optic (PCFO) system designed to exclude surface artifacts and predominantly measure the light scattering (LS) within the tissue. Experiments on rat neocortical slices under osmotic challenges demonstrated the consistency of the PCFO data: hypertonic challenge always increased LS signal while hypotonic challenge decreased it, as long as the challenge did not induce SD. Under strong osmotic challenge (-100 mOsm), the signal suddenly reversed the polarity at the onset of SD induced by the challenge and continued to increase until the challenge was removed. When SD was blocked by high Mg(2+), the LS signal remained decreased during the -100 mOsm challenge. A spectroscopic study with the PCFO system showed that the spectrum of tissue scattering was almost a flat function in most of the visible range (650-470 nm) with a maximum at 500 nm and a rapid drop at 450 nm.