Axonal accessibility and adaptation to osmotic stress in an extreme osmoconformer

Abstract

IT is generally assumed that neuronal function is critically dependent on a relatively stable chemical environment. In most animals investigated homeostasis of the neuronal microenvironment is achieved by regulation of the chemical composition of the body fluids and, in higher vertebrates^1–3 and insects⁴, by additional homeostatic control within the nervous system. The axons of an extreme invertebrate osmoconformer⁵ (the worm Mercierella enigmatica Fauvel) have been shown, however, to adapt and to continue to produce sodium-dependent action potentials when exposed to massive blood dilution^6,7. It has also been suggested that short-term protection is provided by a facultative blood-brain barrier which enables the axons to adapt relatively slowly to hyposmotic stress⁷. We now report that the effects isosmotic and hyposmotic dilution of the surrounding medium on the giant axons of Mercierella are direct, and that axons respond rapidly to changes in ionic concentration and adapt to extreme osmotic stress almost immediately.