Effects of osmotic stress on dextran diffusion in rat neocortex studied with integrative optical imaging.

Effects of osmotic stress on dextran diffusion in rat neocortex studied with integrative optical imaging. This study investigated how dextran (Mr = 3,000) diffused in rat cortical slices when the osmolarity of the bathing artificial cerebrospinal fluid was altered by varying the NaCl content. The apparent diffusion coefficient, D*, was measured in the neocortex region using fluorescent molecules and the integrative optical imaging (IOI) method. The main results were: 1) the value of D* in rat neocortex in the isotonic (300 mOsm) artificial cerebrospinal fluid at 34 degrees C was D* = 0.68 +/- 0. 01 x 10(-6) cm2 s-1 (mean +/- SE, n = 78) and it could be changed within minutes by varying the extracellular osmolarity. 2) Hypotonic stresses up to -100 mOsm decreased D* by 35% and were fully reversible when the slices were returned to the isotonic medium. Further hypotonic stress to -150 mOsm caused further decrease in D* but after removal of the stress, D* overshot its control value. 3) Hypertonic stress of +50 mOsm increased D*, but the maximum reversible increase in D* was only 15%. Further hypertonic stress (to +200 mOsm) did not cause any further increase in D* and, after removal of the stress, D* undershot the control value. The changes in D* are thought to be related to volume changes of cells in tissue: hypotonic solutions caused cell swelling, resulting in reduced extracellular space and compressed extracellular matrix so that the dextran diffusion was more hindered. Hypertonic solutions had the opposite effect. Recordings of extracellular field potentials in the hippocampal CA1 region demonstrated that, on return to the isotonic solution after exposure to an extreme hypotonic or hypertonic stress, the neurons retained their ability to generate synaptic responses.