The effects of hyposmolality on spinal cord activity

The consequences of decreases in osmotic pressure were investigated using the isolated, hemisected frog spinal cord. The lateral column evoked ventral root response (LC-VRR) was depressed by a decrease in osmolality secondary to a decrease in the Na⁺ concentration in the bathing medium. This depression resulted from neither a depression of motoneuronal excitability nor a depression of central axonal conduction but rather from an increased excitability of lateral column terminals. The depression of the LC-VRR and augmentation of the lateral column terminal excitability were reversed by picrotoxin (10-⁴ M). Results are discussed in terms of the possible roles of endogenous gamma-aminobutyric acid (GABA) and extracellular K⁺ in the effect produced by the hyposmotic (low Na⁺) media.