RESPONSES OF NEURONS TO EXTREME OSMOMECHANICAL STRESS

Neurons are often regarded as fragile cells, easily destroyed by mecha nical and osmotic insult. The results presented here demonstrate that this perception needs revision. Using extreme osmotic swelling, we sho w that molluscan neurons are astonishingly robust. In distilled water, a heterogeneous population of Lymnaea stagnalis CNS neurons swelled t o several times their initial volume, yet had a ST50 (survival time fo r 50% of cells) >60 min. Cells that were initially bigger survived lon ger. On return to normal medium, survivors were able, over the next 24 hr, to rearborize. Reversible membrane capacitance changes correspond ing to about 0.7 mu F/cm(2) of apparent surface area accompanied neuro nal swelling and shrinking in hypo- . and hyperosmotic solutions; reve rsible changes in cell surface area evidently contributed to the neuro ns' ability to accommodate hydrostatic pressures then recover. The rev ersible membrane area/capacitance changes were not dependent on extrac ellular Ca2+. Neurons were monitored for potassium currents during dir ect mechanical inflation and during osmotically driven inflation. The latter but not the former stimulus routinely elicited small potassium currents, suggesting that tension increases activate the currents only if additional disruption of the cortex has occurred. Under stress in distilled water, a third of the neurons displayed a quite unexpected b ehavior: prolonged writhing of peripheral regions of the soma. This su ggested that a plasma membrane-linked contractile machinery (presumabl y actomyosin) might contribute to the neurons' mechano-osmotic robustn ess by restricting water influx. Consistent with this possibility, 1 m M N-ethyl-maleimide, which inhibits myosin ATPase, decreased the ST50 to 18 min, rendered the survival time independent of initial size, and abolished writhing activity. For neurons, active mechanical resistanc e of the submembranous cortex, along with the mechanical compliance su pplied by insertion or eversion of membrane stores may account for the ability to withstand diverse mechanical stresses. Mechanical robustne ss such as that displayed here could be an asset during neuronal outgr owth or regeneration.