The brain is a tightly regulated system. Levels of neurotransmitters, for example, are regulated by reuptake proteins, which move transmitters from outside the cell to inside, where they are inactive. This means that after cells release a neurotransmitter, such as dopamine, it is rapidly taken back up again.
Interestingly, however, the levels of the reuptake protiens themselves are variable and can change in response to various things. If dopamine levels rise, for example, nearby cells rapidly increase the number of dopamine transporters (DAT), thus helping to reduce dopamine levels again. This happens when DAT proteins waiting dormant within nerve cells are sent to the surface (the cell membrane) in response to increased dopamine levels.
This much is fairly well known, but a lovely experiment from a University of Michigan team has revealed just how fast the process is. (Dopamine and Amphetamine Rapidly Increase Dopamine Transporter Trafficking to the Surface: Live-Cell Imaging Using Total Internal Reflection Fluorescence Microscopy).
The authors used a form of light microscopy which allows the membrane of a single cell to be imaged. They created cells genetically engineered to have dopamine transporter protein (DAT) which glows, because it was linked to Green Fluorescent Protein. This allowed them to view changes in the level of DAT on the surface of the cells, in real time, in living cells.
They found that adding dopamine caused DAT levels to rise astonishly fast - within just a few seconds. Amphetamine, a drug which acts on the DAT, had the same effect. However, cocaine, a drug which blocks DAT, prevented this effect.They've even made a video so that you can see the dopamine transporters bubbling up on the surface of a single cell. Watch it (if you have academic access) - it beats 99% of YouTube.
This is a fascinating result, and it underlines the fact that nothing in the brain is ever straightforward. For example, most people will tell you that amphetamine and cocaine both have stimulant effects by "increasing dopamine levels" - cocaine by blocking dopamine reuptake and amphetamine by causing the dopamine transporter to actually go into reverse and start releasing dopamine. But this result suggests that amphetamine also increases membrane dopamine transporter levels. That could have any number of indirect effects. Then again over longer time-scales (minutes), amphetamine reduces the DAT levels. That could have indirect effects too...
It's also worth bearing in mind that although this experiment involved the dopamine transpoter, other reuptake proteins like the serotonin transporter might well be regulated in the same way, which could have big implications for antidepressant action...
Furman, C., Chen, R., Guptaroy, B., Zhang, M., Holz, R., & Gnegy, M. (2009). Dopamine and Amphetamine Rapidly Increase Dopamine Transporter Trafficking to the Surface: Live-Cell Imaging Using Total Internal Reflection Fluorescence Microscopy Journal of Neuroscience, 29 (10), 3328-3336 DOI: 10.1523/JNEUROSCI.5386-08.2009