Abstract

Inhibitory pathways in the spinal cord play an important role in establishing the pattern of motor discharge. In the wallaby spinal cord preparation, disruption of glycinergic and gamma-amino butyric acid (GABA)ergic neurotransmission abolished the alternation between antagonistic motor pools during fictive locomotion. A new pattern of motor discharge also appeared when both glycine and GABAA receptors were blocked simultaneously. This discharge pattern was biphasic, characterized by a distinct pause between two bursts of motoneurone firing during each cycle of motor activity. Whole cell patch recordings showed that the second burst of motor discharge was not caused by a separate inward current at a delayed time course. Furthermore, local injection of an N -methyl-D-aspartic acid (NMDA) specific antagonist converted the biphasic discharge to a continuous burst pattern. The result suggests an NMDA-mediated mechanism, which causes a suppression of motoneurone firing when glutamate release from interneurones is enhanced in the absence of glycinergic and GABAergic inhibition.

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