Evidence against volume regulation by cortical brain cells during acute osmotic stress

The cell bodies of neurons and glia examined in culture respond to severe 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 brain does not experience such precipitous and severe changes in brain hydration. In this study we examined if there is evidence from the hippocampal slice preparation supporting the type of volume regulation observed 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) during the first hour of osmotic stress to search for evidence of volume regulation. During superfusion of hypo-osmotic aCSF (-40 mOsm), LT increased 24 to 28% in the dendritic regions of CA1 neurons. The LT reached a plateau which was maintained throughout a 45-min application interval, more than enough time to reveal a regulatory volume decrease. Upon return to control saline, LT immediately returned to baseline and settled there. Hypo-osmolality reversibly increased the relative tissue resistance (RREL) measured across the CA1 region with a time course identical to the increase in LT. Conversely, hyperosmotic aCSF (mannitol, +40 mOsm) decreased both RREL by 8% and LT by 15.5% with no indication of a regulatory volume increase. The CA1 cell body layer showed only slight hypo-osmotic swelling whereas exposure to the glutamate agonist quinolinic acid caused pronounced swelling in this region. Even when osmolality was decreased by 120 mOsm for 20 min, dendritic regions responded passively with no regulatory volume decrease. However, when aCSF Cl- was substituted, the CA1 dendritic regions displayed immediate swelling followed by a dramatic volume reduction under normosmotic conditions, indicating that such behavior can be evoked by extreme aCSF dilution. We conclude that in the brain slice preparation, the cortical cells do not exhibit classic volume regulation in response to sudden physiological changes in osmolality. Moreover it is the dendritic region, not the cell body region, that displays dynamic volume change during osmotic challenge.