A team of scientists at Harvard Medical School (HMS) has identified a new mechanism of dopamine release in the brain in a study conducted in mice that was published in the journal Science (“An action potential initiation mechanism in distal axons for the control of dopamine release“). The researchers found the neurotransmitter acetylcholine triggers the firing of action potentials in dopamine-releasing neurons by binding to distal regions of the axon to regulate dopamine signaling.

“The most important insight that comes from this work is that a local signaling system can initiate an action potential in the axon, which is an output structure,” said Pascal Kaeser, PhD, professor of neurobiology in the Blavatnik Institute at HMS and the senior author of the paper. “This goes at a very old, core principle of how neurons work.”

In suggesting that axons can initiate action potential firing, the new findings challenge an established concept in neurobiology: that action potentials start in the cell body or “soma” of neurons and flow to the axonal end to release chemical neurotransmitters that convey the message through the synaptic gap. If confirmed through further research, this discovery could lead to new treatments for Parkinson’s disease that restore dopamine levels by targeting acetylcholine neurons.

“Defining the interactions of dopamine and acetylcholine is fundamental to understanding how the actions we perform in our daily lives are generated and modulated,” said Kaeser. “If we can define how the dopamine and acetylcholine systems interact, we will definitely better understand what happens when you take out dopamine neurons.”

It is generally understood that a neuron receives chemical signals through its dendrites, the cell body then integrates the chemical messages to initiate an electric impulse or action potential that travels to the far reaches of the long, thin axon where action potentials prompt the release of neurotransmitters to relay the message to other neurons.

Dopamine and acetylcholine are neurotransmitters important in regulating voluntary and involuntary movement, pain processing, emotions, smooth muscle contraction, and blood vessel dilation. Kaeser’s team investigates how dopamine neurons with cell bodies located in the midbrain and axons that project into the corpus striatum—a cluster of neurons in the forebrain that integrates inputs from other regions of the brain—communicate with the striatum to modulate its output.

Although it’s generally believed that cell bodies of dopamine neurons in the midbrain receive chemical signals at their dendrites and integrate these chemical messages to initiate action potentials that trigger dopamine release at their axonal ends in the striatum, past studies show this is not always true. At times, acetylcholine can trigger dopamine release directly in the striatum when all ties with the midbrain have been severed.

“We were fascinated by this because it’s a really strong mechanism, but how it actually works—how acetylcholine triggers the release of dopamine, was unknown,” Kaeser said.
Kaeser’s team used a microscope to analyze mouse brains where the striatum was separated from other brain regions. They saw sparks of dopamine in the tissue, even though the dendrites and cell bodies of dopamine neurons in the midbrain were no longer connected to the axons in the striatum.

“This was really striking because it happens without cell bodies, so the neurons don’t have their command center, and it happens without stimulation; it just happens on its own,” Kaeser said. “This is spontaneous local triggering of dopamine release.”

The researchers observed that dopamine signals were fewer than acetylcholine signals in the striatum but spread over a larger area—indicating a propagating, amplified signal when acetylcholine triggers local dopamine release.

Earlier studies showed axons of dopamine neurons have few sites for dopamine release, which are used when the cell body initiates an action potential. Kaeser’s team showed that those same sites are responsible for local dopamine release prompted by acetylcholine.
The researchers activated acetylcholine neurons or puffed a drug that acts like acetylcholine directly onto the dopamine axons. This, they found, induced action potentials in dopamine neurons and prompted dopamine release. The team also found acetylcholine initiated these action potentials by binding to acetylcholine receptors on the axons of dopamine neurons.

“Providing acetylcholine is sufficient to trigger an action potential out of the axon, so you don’t need the dendrites of the neuron,” Kaeser said.

In freely moving mice, the team found dopamine and acetylcholine signals correlated with the direction in which the mouse moved its head, with acetylcholine signals occurring just before dopamine signals. When the researchers disabled acetylcholine receptors on dopamine neurons, dopamine levels in the mouse striatum dropped.

Together, these experiments show the new mechanism may be physiologically relevant but further studies will be needed to determine how it affects striatal function and behavior.
Changliang Liu, PhD, a research fellow in neurobiology at HMS and first author of the paper suspects this might just be the tip of the iceberg and intends to better understand the need and benefits of this localized dopamine release mechanism.