FURTHER CHARACTERIZATION OF [H-3] IFENPRODIL BINDING IN RAT-BRAIN

The present study was undertaken to characterize [H-3]ifenprodil binding in rat brain. [H-3]ifenprodil showed saturable, high-affinity binding at 4 degrees C. Specific binding, defined with 10 mu M ifenprodil as a competitor, was inhibited biphasically by the s receptor ligands, GBR 12909, 1,3-di-o-tolylguanidine (DTG), and (+)-3-(3-hydroxyphenyl)-N-propylpiperidine ((+)3-PPP). At 4 degrees C, 3 mu M GBR 12909, which inhibited about 50% of specific binding of [H-3]ifenprodil, was used to mask a receptors. Under these conditions, specific binding of [H-3]ifenprodil was inhibited potently by ifenprodil, SL 82.0715, poly(L-arginine), poly(L-lysine), neomycin, ruthenium red, spermine, arcaine and spermidine. In the presence of 3 mu M GBR 12909, Zn2+ and Mg2+ partially inhibited specific binding of [H-3]ifenprodil at 4 degrees C. In contrast, in the absence of GBR 12909, at 37 degrees C specific binding of [H-3]ifenprodil was partially inhibited by Zn2+, but not by Mg2+. The anatomical distribution of [3H]ifenprodil binding at 4 degrees C (GBR 12909 included) in rat brain closely paralleled that of [3H]MK-801 (dizocilpine) binding (r=0.971, P<0.005). Without GBR 12909, specific [H-3]ifenprodil binding at 37 degrees C was inhibited potently by a ligands. In the presence of 3 mu M GBR 12909, [H-3]ifenprodil binding at 4 degrees C was highest in synaptosomal and myelin fractions; however, without GBR 12909, [H-3]ifenprodil binding at 37 degrees C was highest in microsomal and myelin fractions, consistent with the subcellular distribution of a receptors. The results suggest that, in the presence of 3 mu M GBR 12909, at 4 degrees C, [H-3]ifenprodil binds to sites that are sensitive to polyamines and related compounds; and that without GBR 12909, at 37 degrees C, [H-3]ifenprodil interacts with a receptors in rat brain.