Endogenous inhibitors of [3H] Ro 5-4864 binding to "peripheral-type" binding sites for benzodiazepines are present in peripheral tissues and brain.

Recent observations have shown that "peripheral-type" binding sites for benzodiazepines (PBS) are under neural and/or hormonal control in the pineal gland, olfactory bulb, and kidney. These studies resulted in a search for endogenous substances which might physiologically subserve PBS. Acidified methanol or trichloroacetic acid extraction of both peripheral tissues and brain followed by ultrafiltration and/or gel filtration and high performance liquid chromatography revealed the presence of both high (Mr greater than 10,000) and low (Mr less than 500) molecular weight substances which inhibit the binding of [3H] Ro 5-4864 to PBS while only slightly inhibiting the binding of [3H] diazepam to classical "brain-type" benzodiazepine receptors.