Study of an octadecaneuropeptide derived from diazepam binding inhibitor (DBI): biological activity and presence in rat brain.

An endogenous brain neuropeptide with 104 amino acid residues that modulates gamma-aminobutyric acid receptor function was termed DBI because it displaces diazepam from its specific brain binding sites. Tryptic digestion of DBI generates an octadecaneuropeptide (ODN) that is more potent than the parent compound in the displacement of specifically bound beta-[3H]carboline-3-carboxylate methyl ester [( 3H]BCCM) and in proconflict action (Vogel test in thirsty rats). The proconflict action of ODN is antagonized by the imidobenzodiazepinone Ro 15-1788, which is a specific antagonist of beta-carboline and benzodiazepine recognition sites. The ODN amino acid sequence is Gln-Ala-Thr-Val-Gly-Asp-Val-Asn-Thr-Asp-Arg-Pro-Gly-Leu-Leu-Asp-Leu-Lys. The pharmacological properties associated with this sequence were confirmed by comparing the activity of ODN generated from tryptic digestion of DBI with that of ODN obtained by synthesis. Amidation of the terminal lysine of ODN produces a peptide (ODN-NH2) devoid of pharmacological activity. Three peptides containing the COOH-terminal segment of ODN were synthesized. All these peptides [Arg-Pro-Gly-Leu-Leu-Asp-Leu-Lys (octapeptide), Pro-Gly-Leu-Leu-Asp-Leu-Lys (heptapeptide), and Gly-Leu-Leu-Asp-Leu-Lys (hexapeptide)] express the displacing and proconflict actions of ODN. In primary cultures of cerebellar granule cells of rat, DBI, ODN, octapeptide, heptapeptide, and hexapeptide preferentially displace [3H]BCCM over [3H]flunitrazepam; moreover, they displace bound [3H]BCCM completely but [3H]flunitrazepam only by 50%. These data suggest that ODN includes a specific ligand for the gamma-aminobutyric acid receptor regulatory site occupied by beta-carbolines. Using rabbit antibodies directed against the NH2-terminal portion of ODN, we detected ODN-like material in rat brain homogenates. However, whether this material is identical to the ODN generated by tryptic digestion of DBI remains to be established.