CHARACTERIZATION OF [I-125] [SER(1)]HISTOGRANIN BINDING-SITES IN RAT-BRAIN

The binding characteristics of histogranin (HN), an endogenous peptide first recognized for its antagonism of N-methyl-D-aspartate (NMDA) responses, were determined in membrane preparations of rat brain. [I-125][Ser1]HN, a stable bioactive analog of HN, bound specifically and reversibly to a homogenous population of high-affinity sites with a K(d) of 25 nM and a B(max) of 410 fmol/mg protein. The binding of [I-125][Ser1]HN increased linearly with membrane protein concentration and was destroyed upon membrane pretreatment with trypsin. The binding displayed rapid association and dissociation kinetics and was blocked by peptides possessing close homology with HN in the following order: [Ser1]HN-(1-15) > HN > [Ser1]HN-(1-14) > HN-(2-15) > [Ser1]HN-(1-10) > HN-(6-10). Unrelated peptides such as substance P, beta-endorphin, neuropeptide Y, [Met5]enkephalin, [Leu5]enkephalin, dynorphin A(1-13) and neuromedin C were inactive in competition binding assays against [I-125]Ser1]HN. Ligands of the binding domains of the NMDA receptor, such as (+3)-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid, (+) 5-methyl-10,11-dihydro 5H-dibenzo[a, d]cyclohepten-5,10-imine hydrogen maleate, 1-N-(2-thienyl)cyclohexylpiperidine, glycine and glutamate were also ineffective in competing for [I-125][Ser1]HN binding sites. Interestingly, specific ligands for the polyamine site on the NMDA receptor, as well as the cations Mg++ and Zn++ inhibited [I-125] [Ser1]HN binding. The polyamine antagonist diethylenetriamine produced a noncompetitive inhibition with an IC50 (175 nM) comparable to that of HN (75 nM). The cations Zn++ and Mg++ displaced [I-125][Ser1]HN binding with IC50 values of 18 and 240 muM, respectively. Elevated levels of [I-125][Ser1]HN binding were observed in brain regions that are known to possess a high density of NMDA receptors. The data demonstrate the presence of a [I-125][Ser1]HN binding site in rat brain that may mediate the modulatory effects of the endogenous peptide on NMDA receptor functions.