Author:

Abstract:

Glutamate is the most important excitatory neurotransmitter in the mammalian central nervous system. After stimulation, glutamate is released in the synaptic cleft by the presynaptic neuron and it acts on glutamate receptors that are located in the postsynaptic membrane. Glutamate receptors are G-protein-coupled (metabotropic) or ion channels (ionotropic). Ionotropic glutamate receptors are further subdivided in three major classes: AMPA-, kainate- and NMDA-receptors, named after their agonist. NMDA-receptors are Na+, K+ and Ca2+ permeable ion channels. They are composed of several subunits of which six types are known. We studied the laminar and cellular distribution pattern of three most abundant subunits NR1, NR2A and NR2B in adult cat primary visual cortex. We determined a part of the 3'-end sequence of each subunit in order to construct cat-specific oligonucleotide probes for use in in situ hybridization experiments. The three subunits showed a different mRNA-expression pattern. While NR1 and NR2B were expressed mainly in cortical layer II/III and V/VI, NR2A-mRNA appeared dominantly in layer IV with the lowest expression in layer V. The distribution of the NMDA-receptor subunit proteins in cat visual cortex was studied by immunocytochemistry using subunit specific antibodies. These were tested in western blot experiments for their selectivity in cat. All antibodies detected one band, corresponding to a protein of the expected molecular weight. For each subunit, the protein distribution pattern was consistent with the in situ hybridization results. Intense NR1- and NR2B-immunoreactivity was seen in pyramidal neurons of cortical layer II/III and V/VI. Small round cells were present predominantly in layer IV and to a lesser degree in the other cortical layers except for layer I. Both subunits and especially NR2B were found in fibres in all layers. NR2A on the contrary was exclusively found in small cells in all layers with the least expression in layer I and V.