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Presynaptic Neurons Fine-Tune Dopamine Signaling

Vesicles load more of the neurotransmitter in response to neuronal activity, researchers find.

By | November 1, 2017

PUMPING IT UP: Researchers have found that synaptic vesicles releasing dopamine across neuronal synapses in fruit flies and mice can dynamically adjust the neurotransmitter content in response to neuronal firing. And they propose a mechanism to explain how. When the axon terminal depolarizes, sodium ions flow into the cell (1). The increased sodium ion concentration activates sodium-hydrogen exchangers in the vesicle membrane that transport one sodium ion into the vesicle for every proton out (2). This action increases the difference in electrical charge across the vesicle membrane, activating a transporter protein, VGLUT, which pumps another neurotransmitter, negatively charged glutamate, into the vesicle (3). The resulting buildup of internal negative charge triggers the pumping of more protons into the vesicle, increasing the pH gradient across the membrane (4). Finally, this drop in pH inside the vesicle triggers the loading of more dopamine into the vesicles via the VMAT protein (5).© EVAN OTO/SCIENCE SOURCE

EDITOR'S CHOICE IN NEUROSCIENCE

The paper
J.I. Aguilar, “Neuronal depolarization drives increased dopamine synaptic vesicle loading via VGLUT,” Neuron, 95:1074-88.e7, 2017.

Psychiatrist Zachary Freyberg thought he knew the basics of dopamine signaling. When a dopamine neuron fires, vesicles containing the neurotransmitter migrate to the cell membrane, where they fuse and release their cargo into the synapse, all in the course of about a millisecond. But a chance observation by Freyberg a few years ago revealed a new dimension to this critical aspect of neural communication.

At Columbia University, beginning in 2009, Freyberg had helped develop a technique to observe dopamine signaling in living Drosophila brains. The method used molecules of FFN206, Freyberg says, which “behave like dopamine, but unlike dopamine, they’re fluorescent and therefore can be readily visualized.”

He and his colleagues expected that when neurons were artificially stimulated with potassium chloride, vesicles would transport FFN206 to the cell membrane and release it into the synapse. That did happen—but the fluorescence indicated something else was going on, too. “You’d expect the dopamine signal to go down,” Freyberg, now at the University of Pittsburgh, explains. “Instead, it was going up before going down.” The vesicles, the team realized, were loading extra cargo before fusing with the membrane—a contradiction of the textbook view that vesicles’ dopamine levels were fixed.

To investigate further, the researchers looked for signals associated with the boost in vesicle content. They found that before fusion but after cell membrane depolarization—a sign of neuronal activity—the pH inside vesicles dropped. “For dopamine, it’s the pH of the vesicles that creates the driving force for loading,” Freyberg explains, with more-acidic conditions promoting loading.

However, it was unclear what triggered the extra acidification. The researchers suspected chloride, a negatively charged ion often involved in establishing proton gradients. But experiments didn’t back that theory up. So the team turned to glutamate, a neurotransmitter that is also negatively charged. “When we blocked the entry of glutamate into these dopamine vesicles, they no longer acidified more, and no longer loaded more in response to activity,” Freyberg says.

The researchers observed similar processes in mice, and in a new paradigm, suggest how this unexpected role for glutamate links neuronal activity to dopamine vesicle content across species. “It’s showing a mechanism by which presynaptic neurons can be regulated,” says Thomas Hnasko, a neurobiologist at the University of California, San Diego. “Most people think about plasticity in the brain as a postsynaptic phenomenon. . . . This is all really quite novel.”

Freyberg is now investigating how these mechanisms fine-tune the amount of dopamine sent across the synapse, and their effects on neuronal communication in normal and diseased brains. “It’s as if we’ve been thinking all our lives that when you turn on a light, you just flip a switch, and it’s on or off,” he says. “But what this suggests is that neurons are capable of a great deal more subtlety.”