An endogenous ligand for the kainate-type binding sites from rat brain.

Extracts from the rat brain were screened to identify a putative endogenous ligand for the binding sites of the neuroexcitant kainic acid (KA). The extracted substances were separated by chromatographic techniques and tested for their ability to inhibit KA binding to fish synaptosomes and to membranes from rat brain. A substance isolated in this way (rat kainate-binding inhibitor, RKBI) display a competitive interaction with KA for the low-affinity binding sites in rat brain membranes. According to the separation behavior in the purification step, RKBI is distinct from an inhibitor formerly isolated from fish nervous tissue (KBI). The substance exhibits positive co-operativity with KA for a very-low-affinity site population, particularly concentrated in the cerebellum, and could play a physiological role in this area.