A novel specific binding site for anxiolytic homophthalazines in the rat brain

Radioligand binding studies were performed in order to elucidate the mechanism of action of anxiolytic-neuroleptic homophthalazines. Rat striatal membrane preparations were found to bind 3H-EGIS 6775 [3H-GYKI-52 322, 3H-(1-(4-aminophenyl)-4-methyl-7,8-dimethoxy-5H-homophthalazine)] in a specific and displaceable manner. Several other brain regions tested were devoid of similar binding activity. Saturation analysis revealed that binding affinity was in the 10(-8)-10(-7) M range. Binding was enhanced by Mg2+ ions and, to a smaller extent by Ca2+ ions. The binding principle was sensitive to heat or trypsin treatment. This specific binding site appears, according to competition studies, different from the receptors whose presence in the rat striatum has been reported earlier.