Pharmacological characterization of [H-3]-ifenprodil binding to polyamine binding sites on rabbit and rat retinal homogenates: Role in neuroprotection?

Polyamine binding sites (PBS) represent one of the modulatory sites on the N-methyl-D-aspartate (NMDA) receptor-channel complex. We have characterized [H-3]-ifenprodil binding to the PBS on washed homogenates of rabbit and ra t retinas. Specific binding of [H-3]-ifenprodil (2 nM) tin the presence of 3 mu M 1,3-Di [2-tolyl] guanidine HCl and 10 mu M GBR12909 to block sigma s ites) comprised 47-56% of the total binding. Scatchard analyses indicated i nteraction with apparent high- and low- affinity sites: dissociation consta nts (K(d)s)= 0.5-0.6 mu M and apparent density of sites (B-max) = 1.5-4.3 p mol/mg protein and K(d)s = 2.0-2.9 mu M, and B-max values = 15.8-17.8 pmol/ mg protein (n = 3). Ifenprodil (K-i = 0.4 -0.8 mu M), eliprodil (K-i = 0.7- 0.8 mu M), spermine (K-i = 72-79 mu M), spermidine (K-i = 283-330 mu M), pu trescine (K-i > 650 mu M) and MK-801 (K-i > 1 mM) (n = 3-5) differentially competed for [H-3]-ifenprodil binding. The biphasic competition curves for ifenprodil were resolved into two binding components: rat retinas, IC50high = 0.19 +/- 0.13 mu M and IC50low = 8.7 +/- 1.3 mu M; rabbit retinas, IC50h igh = 0.1 +/- 0.01 mu M and IC50low = 16.0 +/- 7.8 mu M. These studies have shown the presence of specific PBS labeled by [H-3]-ifenprodil in the rabb it and rat retinas which may, in part, be responsible for mediating the neu roprotective effects of eliprodil and ifenprodil.