Presence of an Endogenous Factor in Avian CNS with Agonistic Action on Benzodiazepine Receptors

In the present paper we describe the presence in avian CNS of an endogenous inhibitor of [³H]flunitrazepam binding. This compound was extracted from a synaptic membrane fraction isolated from chick optic lobe and brain using an exhaustive aqueous washing procedure, then purified by means of solid-phase extraction with C₁₈ cartridges and several HPLC steps until an homogeneous peak was obtained. Its chemical structure was studied by size-exclusion chromatography of the purified material which indicated that it possesses a molecular weight below 1350. Although its inhibitory activity was lost by HCl treatment, its peptidic nature was ruled out by an amino acid and N-terminal sequence analyses. Ultraviolet absorption spectrum showed two main peaks at 230 and 280 nm. The endogenous compound was found to inhibit competitively [³H]flunitrazepam binding to its recognition site without affecting [³H]GABA binding to the same receptor complex. The behavior of the endogenous factor in an in vitro GABA shift test and GABA-dependent chloride flux experiments were similar to that of benzodiazepine receptor agonists. In conclusion, these results demonstrate the existence in avian CNS of a competitive endogenous inhibitor of benzodiazepine binding with agonistic action on benzodiazepine receptors.