The l-theanine is believed to stimulate alpha brain waves, which contribute to states of relaxation and assist in the formation of GABA, which then influences the brain's "feel good" transmitters, serotonin and dopamine.
Drugs called GABA agonists, such as baclofen and carbamazepine, are useful in treatment of seesaw nystagmus and PAN, if the nystagmus is acquired and not congenital.
The University of Maryland Medical Center also notes that Passion Flower may increase levels of GABA like valerian root.
There's some evidence that valerian works with the neurotransmitter GABA in the central nervous system.
Taking a B-vitamin complex is often recommended, as are supplements such as GABA.