I read that passive diffusion causes the amount of neurotransmitters to vary at the synaptic cleft because glial cytes absorb the neurotransmitters (at the synaptic cleft) (wikipedia)
Creating glial cytes with different thickness membranes could change the amount of neurotransmitters between synapses;
that is kind of like an SSRI.
Gene therapy (genetic engineering) that modifies the outer membrane of glial cytes could do this, as could drugs that cause the transport proteins as well as receptors at the surface of glial cytes to absorb more (or fewer) neurotransmitter molecules
Glial cytes migrate suggesting more rapid development of gene therapy, or even in vitro transport or receptor modifying drug development.
combining known technologies, migrating glial cytes could migrate preferentially with photonic cueing so they could migrate locations at the CNS with cofocalized laser light (flashlight fingers at the CNS)