Presynaptic kainate receptors in the monkey striatum

Although kainate has long been known as a powerful axon-sparing neurotoxin, the localization and functions of kainate receptors in the CNS are largely unknown. In the present study we examined the distribution of kainate receptor subunits in the monkey striatum using kainate receptor subunits GluR6/7 and kainate receptor subunit KA2 subunit antibodies at the electron microscope level. We found that kainate receptor subunits GluR6/7 immunoreactivity is expressed not only in neuronal perikarya and dendritic processes, but also in a large population of terminals which form axospinous and axodendritic asymmetric synapses. The ultrastructural features of these terminals resembled those of glutamatergic corticostriatal boutons. In contrast, very few kainate receptor subunit KA2-containing terminals were encountered. Although the functions of these presynaptic kainate receptors remain to be established, the present data suggest the possibility that they are located to modulate the release of glutamate from cortical afferents in the monkey striatum, and that an abnormal regulation of these presynaptic receptors might be involved in the death of striatal neurons in Huntington's disease. Accordingly, recent findings demonstrated that the variance in the age of onset of Huntington's disease could be attributed to the genotype variation of kainate receptor subunit GluR6 in humans.