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1. The isolated frog spinal cord was used to study the action of amino acids and their antagonists on primary afferent terminals and motoneurones. The direct effects of these substances were observed by bathing the cord in 20 mM magnesium sulphate (thus blocking synaptic transmission) and recording the polarization level of the dorsal and ventral roots.

2. -Aminobutyric acid (GABA) and glutamic acid depolarized the dorsal root and reduced dorsal-root potentials, while glycine produced only weak and variable effects. Glutamic acid also depolarized the ventral root; GABA usually produced either a hyperpolarization or had little effect, while glycine caused variable effects.

3. Bicuculline and picrotoxin antagonized all the synaptic potentials recorded on the dorsal root, as well as the GABA responses on both dorsal and ventral roots.

4. All the synaptic potentials examined remained and were markedly prolonged in the absence of external chloride except the ventral root-dorsal root potential. Replacement of the physiologic complement of chloride during chloride-free perfusion restored the potentials to their original time courses.

5. Depolarizing amino acid responses remained in the absence of external chloride, while hyperpolarizing responses were reversed into depolarizations. Return to normal Ringer solution re-established the hyperpolarizations.

6. Removal of external sodium reversibly abolished the amino acid depolarizations but had little effect on the depolarizations in response to applications of high external potassium concentrations.

7. The results support the hypotheses (a) that GABA mediates presynaptic inhibition by depolarizing primary afferent terminals and (b) that the GABA-mediated depolarization is sodium dependent.

8. The results also indicate that GABA utilizes different ionic mechanisms to mediate presynaptic inhibition (sodium) and post-synaptic inhibition (chloride) in the amphibian (and presumably in the mammal).