EVIDENCE AGAINST VOLUME REGULATION BY CORTICAL BRAIN-CELLS DURING ACUTE OSMOTIC-STRESS

The cell bodies of neurons and glia examined in culture respond to sev ere osmotic stress (100 to 200 mOsm) by passive volume change that is followed within several minutes by volume regulation, even in the face of maintained osmotic change. However, in clinical situations, the br ain does not experience such precipitous and severe changes in brain h ydration. In this study we examined if there is evidence from the hipp ocampal slice preparation supporting the type of volume regulation obs erved in cultured brain cells. Within the CA1 region we imaged changes in light transmittance (LT), recorded the evoked field potential, and monitored tissue resistance (all measures of cell volume change) duri ng the first hour of osmotic stress to search for evidence of volume r egulation. During superfusion of hypo-osmotic aCSF (-40 mOsm), LT incr eased 24 to 28% in the dendritic regions of CA1 neurons. The LT reache d a plateau which was maintained throughout a 45-min application inter val, more than enough time to reveal a regulatory volume decrease. Upo n return to control saline, LT immediately returned to baseline and se ttled there. Hypo-osmolality reversibly increased the relative tissue resistance (RREL) measured across the CA1 region with a time course id entical to the increase in LT. Conversely, hyperosmotic aCSF (mannitol , +40 mOsm) decreased both RREL by 8% and LT by 15.5% with no indicati on of a regulatory volume increase. The CA1 cell body layer showed onl y slight hypo-osmotic swelling whereas exposure to the glutamate agoni st quinolinic acid caused pronounced swelling in this region. Even whe n osmolality was decreased by 120 mOsm for 20 min, dendritic regions r esponded passively with no regulatory volume decrease. However, when a CSF Cl- was substituted, the CA1 dendritic regions displayed immediate swelling followed by a dramatic volume reduction under normosmotic co nditions, indicating that such behavior can be evoked by extreme aCSF dilution. We conclude that in the brain slice preparation, the cortica l cells do not exhibit classic volume regulation in response to sudden physiological changes in osmolality. Moreover it is the dendritic reg ion, not the cell body region, that displays dynamic volume change dur ing osmotic challenge. (C) 1997 Academic Press.