GABA_A receptors activity is modulated by benzodiazepines and barbiturates. These drugs act as allosteric modulators, and do not activate the receptors independently.
Benzodiazepines are a positive allosteric modulator of the GABA_A channel, binding to the allosteric site and subsequently leading to easier opening of the channel. This allows for more frequent opening and thus greater excitability.
Barbiturates are also positive allosteric modulators, but instead cause longer openings of the GABA_A channel. This allows for greater chloride transport through the channel, providing greater excitability.