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 endog enous inhibitor of [H-3]flunitrazepam binding. This compound was extra cted 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 C-18 cartridges and several H PLC steps until an homogeneous peak was obtained. Its chemical structu re was studied by size-exclusion chromatography of the purified materi al which indicated that it possesses a molecular weight below 1350. Al though its inhibitory activity was lost by HCl treatment, its peptidic nature was ruled out by an amino acid and N-terminal sequence analyse s. Ultraviolet absorption spectrum showed two main peaks at 230 and 28 0 nm. The endogenous compound was found to inhibit competitively [H-3] flunitrazepam binding to its recognition site without affecting [H-3]G ABA binding to the same receptor complex. The behavior of the endogeno us factor in an ''in vitro'' GABA ''shift'' test and GABA-dependent ch loride flux experiments were similar to that of benzodiazepine recepto r agonists. In conclusion, these results demonstrate the existence in avian CNS of a competitive endogenous inhibitor of benzodiazepine bind ing with agonistic action on benzodiazepine receptors.