Autoradiographic evidence that NMDA receptor-coupled channels are located postsynaptically and not presynaptically in the perforant path-dentate granule cell system of the rat hippocampal formation

Abstract

In vitro quantitative autoradiography with [ 3H]MK-801 was used to determine K d and B max values for the NMDA receptor-coupled channel in subregions of the rat hippocampal formation. A single form of the channel with an apparent K d in the 15–20 nM range was found for [ 3H]MK-801 binding in the presence of both 1μM glutamate and 1μM glycine. Specific binding was highest in the molecular layer of the dentate gyrus, followed by CA1 stratum radiatum and CA1 stratum oriens. Fewer binding sites were observed in the hilus of the dentate gyrus, cerebral cortex, CA1 stratum pyramidale. CA3 subregion (stratum oriens, stratum pyramidale, stratum radiatum), and thalamus. Selective destruction of dentate granule cells by colchicine microinjections reduced the amount of specific [ 3H]MK-801 binding by half in the molecular layer of the dentate, compared to intact tissue. [ 3H]MK-801 binding did not change in other hippocampal subregions as a consequence of colchicine injection. Electrolytic entorhinal cortical lesions produced no changes in regional MK-801 binding site density in any of the regions under study. To address the tissue shrinkage following entorhinal cortex lesions, detailed analysis of the binding site density per fixed (16 μm) length of granule cell dendrite, and of the aggregate density across the entire molecular layer revealed no change in the number of MK-801 binding sites per unit length of dendrite in the molecular layer of the dentate gyrus. These findings indicate that NMDA receptor-coupled channels are confined to a postsynaptic location in the perforant path-dentate granule cell system of the adult rat.