Differential sensitivity of individual primary afferents to glutamic and gamma-aminobutyric acids in the amphibian spinal cord in vitro.

In the isolated amphibian spinal cord the responses to glutamate and GABA recorded intracellularly from individual primary afferents differed considerably according to the fibre type. Muscle afferents giving rise to direct sensory-motor synapses were strongly depolarized by glutamate, whereas GABA produced smaller and inconsistent depolarization. In contrast, fibres both of muscle and cutaneous origin establishing polysynaptic connections with motoneurones were relatively insensitive to glutamate but very sensitive to GABA. These observations suggest that the effects of glutamate on fibres synapsing with motoneurones are probably mediated via the depolarization of motoneurones electrically coupled with them and do not result from direct activation of presynaptic terminals by glutamate.