Complex structure of quisqualate-sensitive glutamate receptors in rat cortex.

Multiplicity of excitatory amino acid receptors has previously been demonstrated. The functional size of the quisqualate-type receptors was investigated by high energy radiation inactivation analysis of the binding of [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ([3H]AMPA) to rat cortical membranes. The AMPA binding site is coupled to a high molecular weight modulatory unit, which down-regulates the affinity of the binding site for quisqualate, glutamate and AMPA. The estimated molecular target size of the AMPA binding site is different from the reported molecular weight of the glutamate binding site isolated from bovine brain. Furthermore, enhanced [3H]AMPA binding induced by chaotropic ions seems not to be a consequence of removal of the modulatory unit.