in brief.......
GABAa receptor complex appears to have distinct binding sites for furosemide, neuroactive steroids, picrotoxin, barbiturates, ethanol, kavalactones, GHB, and inhalational anesthetics.
GABAb- beavioral actions of ethanol, gamma-Hydroxybutyric acid (GHB), and possibly in pain. Baclofen is a GABA analogue which acts as a selective agonist of GABAb receptors, and is used as a muscle relaxant. However, it can aggravate absence seizures, and so is not used in [many forms of] epilepsy. [maybe the same reason Phenobarbital is used so little as well?]
GABAc - Don't know much about this one, not modulated by many GABAa receptor modulators such as barbiturates, benzodiazepines, and neuroactive steroids
(adapted from wikipedia)
just incase anyone cares..... i learned something new today anyway...