Phenibut is a derivative of GABA with anxiolytic properties that was discovered by the Russians in the 1960’s.
It remains controversial whether Phenibut binds the GABA(A) receptor in vivo. However, phenibut has been demonstrated to activate the metabotropic GABA(B) receptor. (Metabotropic receptors are those receptors which are not co-localized at the synpase; in contrast, synpatic receptors are “ionotropic.”)
One possibility is that phenibut only affects GABA(A)-mediated neurotransmission at higher doses, but activates GABA(B) receptors at low doses. The fact that Phenibut preferentially activates GABA(B) receptors likely gives it an advantage over other drugs that affect that GABAergic system (e.g., benzodiazepines).
Though Phenibut has never been approved or evaluated by the FDA, it is used in Russia for the pharmacological management of insomnia, PTSD, generalized anxiety and other disease states involving hyperarousal.