Surprise! Most synapses are silent

Scientists have taken a detailed look at how information is transmitted in the brain, and what they found surprised them: Only a fraction of the synapses that serve as connections seem to be active. They focused on synapses that respond to the neurotransmitter dopamine. Here’s what lead researcher and neuroscientist Dr. David Sulzer of Columbia said about the findings, published in Nature Neuroscience.

What do we know about how synapses in the brain work?

The way most synapses work is that a chemical like dopamine [accumulates] in very small compartments called synaptic vesicles. When the synaptic vesicle fuses with the neuron, it releases the neurotransmitter inside. It can then travel to the receptive part of the synapse and then onto the next neuron.

What are you doing to better look at those?

We’re making a new kind of chemical compound which is a fluorescent derivative of dopamine. We call these fluorescent, false neurotransmitters (FFNs), because they’re not real neurotransmitters. This new FFN is taken up as if it were dopamine into the synaptic vesicle. We can see it being taken up because it’s fluorescent — it’s essentially like spying on dopamine. And we saw lots of surprises, but the most shocking surprise is that it looks as if only about 10 to 20 percent of the synapses, at least in the way we’re doing it, are active. The rest are silent.

What are the limitations of that finding?

We have not done this yet in other animals, so we don’t know to the extent this is going to stand up. And this FFM works for dopamine, and also probably works for a couple of other neurotransmitters. But those are only [used by] a minority of synapses in the brain.

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