Biochemical evidence for glutamate as neurotransmitter in corticostriatal and corticothalamic fibres in rat brain

The effects of ablation of frontal, occipital or entire hemicortex on several neurotransmitter parameters in the rostral and caudal neostriatum, thalamus and the contralateral anterior medial cortex were investigated. In particular the effects on the high affinity uptake of d-aspartate and on the endogenous level of amino acids, especially glutamate and aspartate, were studied in order to identify glutamate- or aspartate-containing nerve terminals in these regions. The results show a specific decrease in high affinity uptake of d-aspartate in both rostral and caudal neostriatum ipsilateral to the lesion after frontal or entire hemidecortication. There was also a small but significant decrease in d-aspartate uptake on the contralateral side. Only the level of endogenous glutamate decreased in the neostriatum after hemidecortication. There was a specific decrease in d-aspartate uptake in the thalamus only ipsilateral to the cortical lesions. In thalamus there was a significant decrease both in the level of glutamate and to a smaller extent in that of aspartate after hemidecortication. Anterior medial cortex showed a very active high affinity uptake of d-aspartate, which was slightly reduced after removal of the contralateral hemicortex. The high affinity uptake of d-aspartate was in all cases mainly due to uptake in synaptosomes. The results show that the neostriatum receives glutamate-containing fibres from the neocortex, particularly the frontal part. This projection is mainly ipsilateral with a small element derived from the contralateral side. The thalamus, both the rostral and caudal parts, receives glutamate-containing fibres from the whole extent of the ipsilateral neocortex. Some of the corticothalamic fibres may also contain aspartate. The anterior medial cortex probably contains a high proportion of glutamate- and/or aspartate-containing nerve terminals but only a low proportion of these are derived from the contralateral cortex.