LIGHT TRANSMITTANCE AS AN INDEX OF CELL-VOLUME IN HIPPOCAMPAL SLICES - OPTICAL-DIFFERENCES OF INTERFACED AND SUBMERGED POSITIONS

Light transmittance (T) in the CA1 region of hippocampal slices was me asured during exposure to media of various osmolarities to determine t he utility of optical measurements as an index of changes in cell volu me. In slices positioned at the gas-liquid interface, hypo-osmotic med ium consistently produced a decrease in T and hyperosmotic medium prod uced an increase in T. The magnitude of Delta T was graded as a functi on of the strength of osmotic change. All changes in T were reversible upon return to isosmotic medium. In contrast, osmotically induced cha nges in T in submerged slices were consistently opposite in direction to those observed in slices at the interface. The magnitude and direct ion of Delta T could be altered by systematic variation of the level o f the bathing medium within the same chamber, indicating that both ext rinsic optical properties of various interfaces, such as refraction an d reflection, and intrinsic optical properties of the tissue contribut e to the observed T. Spectral measurements eliminated the possibility that osmotically induced Delta T was the result of changes in light ab sorbance by intrinsic chromophores such as cytochromes or hemoglobin. The results show that measurements of Delta T can be a useful index of changes in cell volume in brain slices, provided that the level of th e bath remains constant.