Effects of changes in osmolality on spinal cord activity

Abstract

The effects of change in the osmotic pressure of the bathing medium on central neural activity in the isolated, hemisected frog spinal cord were studied. The bathing medium was made hyperosmotic by either increasing all constituents (except NaHCO 3) proportionally or by adding a nonelectrolyte, mannitol; and hyposmotic by decreasing all constituents (except NaHCO 3) proportionally. Changes in osmotic pressure depressed the monosynaptic lateral column-evoked ventral root response (LC-VRR). Depression occurred with changes of only 25 mosmol/kg H 2O in bathing medium osmolality. This osmotic effect was fully expressed within the first 5 min of exposure. The osmotic effect was probably not due to a change in the excitability of the motoneurons as there was no change in the antidromic field potential recorded in the ventral horn following stimulation of the ventral root after exposure to the nonisosmotic bathing media that had depressed the LC-VRR. Exposure to nonisosmotic bathing media produced an increase in the excitability of the lateral column terminals as shown by an increase in the lateral column field potentials antidromically evoked by stimulation in the ventral horn. This increase mirrored the depression in the LC-VRR. The depression in the LC-VRR and the augmentation of the MN-LC are reversibly blocked by picrotoxin, a gamma-aminobutyric acid (GABA) antagonist. This suggests that the osmotic effect on isolated frog spinal cord may be due either to the release of endogenous GABA or perhaps to a change in extracellular K + concentration.