Distinguishing characteristics between glutamate and kainic acid binding sites in brain synaptic membranes

Abstract

The demonstration of strong neuroexcitatory activity produced by extracellular application of L-glutamic and L-aspartic acid [l-4] has led to biochemical studies aimed at defining the membrane sites with which these putative neurotransmitters interact [S-lo]. The pursuit of the biochemical characterization of the excitatory amino acid receptor sites has been conducted along two lines: (1) Determination of membrane binding activity through the use of radioactively labeled glutamate or aspartate [5-81; or (2) Measurement of the receptor recognition function through the use of radioactively labeled kainic acid (2-carboxy-4-isopropenyl-3-pyrrolidine acetic acid) [9,10], a potent neuroexcitatory agent which is presumed to act on the excitatory amino acid receptors [ 11,121. We have shown that brain synaptic membranes are enriched in high affinity L-[3H]glutamic acid binding sites which have a number of the pharmacologic characteristics of the receptor for this excitatory amino acid [5,13,14]. It was also shown that this high affmity glutamate binding activity of synaptic membranes is associated with a small molecular weight glycoprotein [ 151. Glutamate binding to this protein exhibits a similar pattern of sensitivity to various glutamate analogs as does the binding of L-glutamate to the synaptic membranes [5,14,15]. However, the binding of L-[3H]glutamic acid either to the synaptic membranes [6,14] or to the purified binding protein [14] is not affected by the presence of a 103-fold greater concentration of kainic acid. Evidence is provided here which is strongly suggestive of the distinct nature of the membrane macromolecules which function as the binding sites for L-glutamic acid and those for kainic acid.