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> A recent discovery as how they work:
> 
> https://phys.org/news/2020-03-neuroscientists-important-protein-brain.html
> 
> 
> "After five years of experimentation, researchers from the University of Copenhagen have succeeded in crystallising and mapping a novel conformation of LeuT, a bacterial protein that belongs to the same family of proteins as the brain's so-called neurotransmitter transporters. 
> 
> "[b]These transporters are special proteins that sit in the cell membrane. As a kind of vacuum cleaner, they reuptake some of the neurotransmitters that nerve cells release when sending a signal to one another. [/b] (my bold)
> ***
> 
> "'Transporters are extremely important for regulating the signalling between neurons in the brain and thus the balance of how the whole system works. You cannot do without them," says Kamil Gotfryd, first author 
> 
> ***
> 
> "Evolutionary, transporters derive from the most primitive bacteria, which have developed them to absorb nutrients, such as amino acids, from the environment in order to survive.
> Since then, specialised transporters have developed to perform a variety of functions. For example, to transport neurotransmitters into neurons in the human brain. Still, the basic principle is the same, namely that the transporter functions by alternately opening and closing to the interior and exterior of a cell, respectively.
> 
> "When a transporter is open outwardly, it may capture transmitter substances or amino acids. Thereafter, the protein uses sodium ions to change its structure so that it will close outwardly and instead open to the interior of the cell where the transported substance is released and absorbed.
> 
> "In recent years, X-ray crystallography has enabled researchers to map three stages of the transporter mechanism: Outwardly open, outwardly occluded and inwardly open.
> 
> "In order for the cycle to be complete, researchers have long concluded that there must also be an inwardly occluded stage of the protein. However, since this structure is unstable, it has long been difficult to freeze it and thus be able to map it.
> 
> "But now, after many trials, researchers at the University of Copenhagen have succeeded in retaining a transporter for the transmitter leucine—a LeuT—in precisely that stage."
> 
> Comment:  an obviously necessary clean-up control since the neurons must produce new transmitters to deliver new messages constantly. Old trash transmitters have got to be constantly cleaned out. Has to be designed. Cannot be worked out by chance attempts.