I wonder if there are approaches to apply psychoactive drugs (more) locally, e.g. by drug carriers.
Usually psychoactive drugs are taken orally/enterally and thus flood the brain unspecifically: each neuron and synapse sensitive to it - whereever it is located - will react on it.
I assume that a lot of effort is taken to design the drugs such that they get effective at locations/neurons as specific as possible.
This can be done chemically, but another way of doing so would be to apply them more locally and make use of diffusion and possibly retardation effects.
I wonder which role such considerations play today for psychoactive drugs. Are there biological, technical, or even ethical reasons that prohibit local application of psychoactive drugs - or make it seem futile?
For more somatic and/or localized diseases, applying more localized therapies is good practice, e.g. (minimal) surgical operations, radiation therapy, any kind of parenteral drug administration, esp. drug carriers.
[Related to this question is this one: Density maps of neurotransmitter systems.]