AN IMMUNOCYTOCHEMICAL STUDY OF GLUTAMATE RECEPTORS AND GLUTAMINE-SYNTHETASE IN THE HIPPOCAMPUS OF RATS INJECTED WITH KAINATE

Immunocytochemistry was used to study the distribution of the kainate receptors GluR1, GluR2/3 and GluR4 and of the N-methyl-D-aspartate (NM DA) receptor NMDAR1 as well as the astrocyte markers glutamine synthet ase (GS) and glial fibrillary acidic protein (GFAP) in the hippocampus of normal and kainate-lesioned rats. Hippocampal pyramidal neurons an d dentate granule neurons were labelled heavily for GluR1 and GluR2/3, but only lightly for GluR4. Dense GluR4 immunopositivity was, however , observed in oligodendrocyte-like glial cells. Hippocampal pyramidal neurons and dentate granule neurons were moderately labelled for NMDAR 1. Intravenous kainate injections resulted in a decrease in GluR1 and GluR2/3 immunoreactivity on the apical dendrites of pyramidal neurons as early as 7 h postinjection. At 18 h, there was a marked reduction i n GluR1 and GluR2/3 receptors in the terminal tuft of dendrites of mos t hippocampal pyramidal neurons in the affected area, although some ce lls showed labelling in other portions of the apical dendrites and in basal dendrites. Immunostaining for GluR4 and NMDAR1 was also reduced at this time. At postinjection day 3, only the cell bodies and the bas al dendrites of a few scattered pyramidal cells were labelled. Taken t ogether, these results indicate a progressive loss of glutamate recept ors, which affects the apical dendritic tree before the basal dendriti c tree. The decrease in receptor immunoreactivity could be due to a do wnregulation of the receptors, since it occurred as early as 7 h postl esion, before cell death was evident in Nissl-stained sections. At lon g intervals after kainate injection, all pyramidal cells at the centre of the lesion showed a lack of glutamate receptor staining, and no pa rtially labelled pyramidal cells were observed. The periphery of the l esion, however, contained many partially labelled pyramidal neurons am ong the unlabelled cells and had features of early lesions. The presen t study also showed an early decrease in GS immunoreactivity in the af fected CA fields of the hippocampus (18 h to 3 days postinjection), fo llowed by a medium-term increase (5-68 days) and a late decrease in GS immunoreactivity (81 days). The decrease in GS immunoreactivity at 81 days is not due to an absence of astrocytes, since GFAP staining show ed many densely labelled astrocytes in the affected CA field.