D-Aspartate Uptake and Release in the Guinea Pig Spinal Cord After Partial Ablation of the Cerebral Cortex

This study attempts to determine if L-glutamate and L-aspartate may be transmitters of the guinea pig corticospinal tract. Unilateral ablations were made of the frontal and parietal neocortex which destroyed most of the motor and somatosensory areas in the right cerebral hemisphere. In lesioned animals, transverse sections of the cervical enlargement of the spinal cord (segments C6--T1) were stained to reveal degenerating fibers. Degeneration of axons first appeared 4 days after surgery, reached a maximum on the seventh day, and began to wane by the ninth day. The most prominent loss of axons appeared deep in the dorsal funiculus and in laminae IV-IX of the gray matter contralateral to the cortical lesion. Ipsilaterally, there was very sparse degeneration of fibers in the dorsal and ventral funiculi and in the spinal gray matter. The uptake and release of D-3H]aspartate, a putative nonmetabolizable marker for L-glutamate and L-aspartate, were measured in dissected quadrants of the cervical enlargement taken from intact and lesioned animals. The uptake and the electrically evoked, Ca2+-dependent release of D-3H]aspartate were depressed by 29-35% at 4 and 7 days after surgery, but only in tissue that was contralateral to the cortical ablation. The lesion had no effect on the uptake and release of exogenous gamma-14C]aminobutyric acid, which were measured as indices of the postlesion integrity of neurons in the spinal gray matter. These findings suggest that the synaptic endings of corticospinal fibers probably mediate the uptake and release of D-3H]aspartate and, therefore, may use L-glutamate and/or L-aspartate as a transmitter.