The chloride-dependent depression by GABA in the frog spinal cord

Spontaneous activities from ventral and dorsal roots of the isolated perfused spinal cord of the bullfrog were inhibited by GABA. β-Alanine showed a strong and glycine a weaker inhibitory effect. The inhibitory effect of GABA was markedly reduced in a chloride-free medium, whereas glycine and β-alanine still showed an inhibitory effect similar to that seen in normal medium. Employing the sucrose-gap method, a marked depolarization in the dorsal root and a small but obvious hyperpolarization in the ventral root were found by the application of GABA. These results support the view that GABA is one of the transmitters involved in pre- and postsynaptic inhibition of the spinal cord. The postsynaptic hyperpolarizing effect of GABA on motoneurones would be caused directly through increased permeability of the membrane to chloride ion. The depolarizing effect of GABA on primary afferent terminals is discussed in connection with chloride dependency.