Effects of Glycine and GABA on Monosynaptic EPSP in Frog Motoneurons

We report here our studies of the effects of the inhibitory transmitters glycine and GABA on monosynaptic EPSP recorded in response to microstimulation of presynaptic fibers (PF PSP) close to the bodies of motoneurons in the isolated frog spinal cord. Monosynaptic PF PSP had two components, mediated by AMPA/KA and NMDA receptors. Blockade of NMDA receptors was used to analyze the characteristics of the AMPA/KA and NMDA components of PF PSP. The amplitude of the NMDA component was 20% of PF PSP amplitude. The areas under the curves of the AMPA/KA and NMDA components were 78% and 22% respectively of the area of the PF PSP. Glycine and GABA had little effect on PF PSP, but significantly decreased the decay time constant (τ_decay), by an average of 33.4 ± 4.0% (n = 18) on application of glycine and by 40.2 ± 3.6% (n = 18) on application of GABA. Blockade of NMDA receptors with DL-2-amino-5-phosophonovaleric acid (AP5) decreased the effects of glycine and GABA on the time constant. Effects were reversible and the amplitude and time characteristics of responses recovered in normal solution. Assessment of the inhibitory influences of glycine and GABA on each of the components of PF PSP showed that suppression of the NMDA component produced a greater proportionate effect than suppression of the AMPA/KA component. These results show that the inhibitory effects of glycine and GABA on the monosynaptic EPSP of motoneurons are mediated mainly by the NMDA component, with a small influence from the AMPA/KA component.

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