A substance that does not act as agonist or antagonist but does affect the gamma-aminobutyric acid receptor-ionophore complex. GABA-A receptors appear to have at least three allosteric sites at which modulators act: a site at which benzodiazepines act by increasing the opening frequency of gamma-aminobutyric acid-activated chloride channels; a site at which barbiturates act to prolong the duration of channel opening; and a site at which some steroids may act. (Chemical Entities of Biological Interest Ontology, CHEBI_50268)
