Why are only 20% of synapses active during neurotransmission?

Using a fluorescence microscope, researchers were able to view the release and re-uptake of dopamine, a neurotransmitter, in individual synapses. When all the neurons were electrically stimulated in a sample of brain tissue, the researchers expected all the synapses to release dopamine.
Instead, they found that less than 20 percent of dopaminergic synapses were active following a pulse of electricity. One possibility: these silent synapses hint at a mechanism of information coding in the brain that’s yet to be revealed, the researchers hypothesize.
The study’s authors plan to pursue that hypothesis in future experiments and examine how other neurotransmitters behave. “If we can work this out, we may learn a lot more about how alterations in dopamine levels are involved in brain disorders such as Parkinson’s disease, addiction, and schizophrenia,” Sulzer said.
The authors note in the paper that “the state of silent vesicle clusters may be important in disorders such as schizophrenia, which show striatal hyperdopaminergia [excessive release of dopamine in the brain’s reward center] and cortical hypodopaminergia [low amounts of dopamine in the cortex] and processes of  ‘unsilencing’ may have clinical applications for diseases such as Parkinson’s disease.”