Evidence for glutamate as a neurotransmitter in the corticofugal fibres to the dorsal lateral geniculate body and the superior colliculus in rats

Abstract

The high-affinity uptake of L-glutamate, D-aspartate and GABA were examined in homogenates from the dorsal lateral geniculate body and the superior colliculus after removal of the right visual cortex of adult rats. The high-affinity uptake of D-aspartate and L-glutamate were reduced by 53 to 75% respectively in the dorsal lateral geniculate body and by 46 and 53% respectively in the superior colliculus ipsilateral to the lesion. The uptake on the contralateral side was unaffected. The reductions were detected 3 days after the lesion and were maximally developed after 7 days. Subcellular fractionation showed that the main part of the uptake was confined to the synaptosomal fraction of both regions and that the reductions were most prominent in this fraction. The lesion was not accompanied by significant changes in high-affinity uptake of GABA nor in changes of choline acetyltransferase and glutamate decarboxylase activities. The high-affinity L-glutamate uptake on the contralateral visual cortex was unchanged from control values. After ablation of the visual cortex the level of L-glutamate was reduced by 32 and 17% in the ipsilateral dorsal lateral geniculate body and superior colliculus, respectively. The levels of the other amino acids examined, including L-aspartate, were unchanged. Enucleation had no effect on the uptake of L-glutamate and of GABA in the dorsal lateral geniculate body or in the superior colliculus.