Substances that do not act as agonists or antagonists but do affect the GAMMA-AMINOBUTYRIC ACID receptor-ionophore complex. GABA-A receptors (RECEPTORS, GABA-A) 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. General anesthetics probably act at least partly by potentiating GABAergic responses, but they are not included here.