Pharmacological characterization of [3H]MK-801 binding in the rat spinal cord

Using a receptor binding assay for [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-10-imi ne (MK-801) the pharmacology of spinal cord NMDA receptors was compared to that of NMDA receptors in the cerebral cortex. The affinities of glutamate site agonists L-glutamate, L-aspartate, ibotenic acid, NMDA and quinolinic acid for stimulation of [3H]MK-801 binding were 6-10 times lower in the spinal cord and the efficacy of quinolinic acid was 50% of that of the other agonists in this region. Also the affinities of glycine site agonists glycine, D-serine, D-alanine and L-serine were lower in the spinal cord as were the affinities of the non-competitive antagonists phencyclidine, (+/-)-cyclazocine and dextromethorphan. The divalent cations Zn2+, Mg2+ and Ca2+ had 4-8 times lower affinity for spinal NMDA receptors while the affinity of Co2+ was 50 times lower. The affinity of [3H]MK-801 was 2.5-fold lower in the spinal cord. These data show that spinal cord NMDA receptors show qualitative and quantitative differences compared to those in the cerebral cortex.