CHARACTERIZATION OF THE BINDING OF [H-3]NS-257, A NOVEL COMPETITIVE AMPA RECEPTOR ANTAGONIST, TO RAT-BRAIN MEMBRANES AND BRAIN SECTIONS

The binding of [H-3]NS 257 l-2-oxobenzo[2,1-b:3,4-c']dipyrrole-5-sulfo namide} to rat cortical membranes was characterized in the absence and presence of thiocyanate, Specific [H-3]NS 257 binding was saturable a nd reversible, and the stimulating effect of thiocyanate on binding wa s optimal at 100 mM. In the presence of thiocyanate [H-3]NS 257 bound to a single population of binding sites with an affinity of 225 +/- 8 nM and a binding site density of 0.61 +/- 0.04 pmol/mg of original tis sue. Thiocyanate increased the affinity of the binding site labeled by [H-3]NS 257 for both lpha-amino-3-hydroxy-5-methylisoxazole-4-propion ic acid (AMPA) and L-glutamate by a factor of 20 and 5, respectively. However, the affinity of the agonist domoate and the antagonists 6-cya no-7-nitroquinoxaline-2,3-dione (CNQX) and -dihydroxy-6-nitro-7-sulfam oylbenzo(f)-quinoxaline (NBQX) was decreased in the presence of thiocy anate. Apparently, the affinities of antagonists as well as agonists f or the AMPA receptor can be either increased or decreased by thiocyana te. The rank order of potency of the putative agonists quisqualate > A MPA > L-glutamate > domoate > kainate and of the antagonists NBQX > CN QX is consistent with the labeling of AMPA receptors. Autoradiographic studies showed that the distribution of [H-3]NS 257 binding sites in rat brain was similar to that of [H-3]AMPA binding sites. NS 257 is th e first AMPA antagonist to be described showing an increased affinity for the AMPA receptor in the presence of thiocyanate.