Traditionally it was thought that exogenous GABA did not penetrate the blood–brain barrier, however more current research indicates that it may be possible, or that exogenous GABA (i.e. in the form of nutritional supplements) could exert GABAergic effects on the enteric nervous system which in turn stimulate endogenous GABA production. The direct involvement of GABA in the glutamate–glutamine cycle makes the question of whether GABA can penetrate the blood-brain barrier somewhat misleading, because both glutamate and glutamine can freely cross the barrier and convert to GABA within the brain.