Histogranin (HN) was first isolated from bovine adrenal medulla and shown to be a pentadecapeptide displaying N-methyl-D-aspartate (NMDA) receptor antagonist activity. To determine the active pharmacophore of HN, fragments of the peptide were synthesized and their structure-activity relationships studied by measuring their ability to displace the binding of [125I][Ser1]HN to rat brain membrane preparations and to block NMDA-induced convulsions in mice. In the binding assay, only the full length peptide HN and HN(1-10) displayed a high affinity (Ki of 72 and 162 nM, respectively). All other tested fragments with deletions at the N- and (or) C-terminals of the molecule showed large (16- to 2500-fold) decreases in potency. The least active peptide fragment tested was HN(6-10) (Ki of 164 microM). In vivo, HN and HN(2-15) (100 nmol; i.c.v.) produced 94 and 40% protection against NMDA-induced convulsions in mice, respectively. None of the other peptide fragments displayed significant anticonvulsant activity. The protective activity of HN (60 and 100 nmol) was markedly antagonized by coadministration of HN(1-10) (100 nmol). The results indicate that the in vivo anti-NMDA and in vitro binding activities of HN and related peptides, with the exception of HN(1-10), depend upon the integrity of the molecule. On the other hand, the high affinity of HN(1-10) for HN binding sites correlates well with its antagonist effects towards the activity of the parent peptide.