A team at the Chinese Academy of Sciences say they have uncovered the conformations of the human dopamine transporter and how it can cause an excessive build-up of the “feel-good hormone” in the brain.
The research could lead to new treatments for disorders and addiction related to dopamine, according to the scientists.
They obtained images of the dopamine transporter – a protein known as DAT – in three different forms for the study. This was done using an advanced imaging technique called cryogenic electron microscopy.
“This study establishes a framework for understanding the functioning of the human dopamine transporter and developing therapeutic interventions for dopamine transporter-related disorders and cocaine addiction,” the team wrote in a paper published in Nature on August 7.
Dopamine – the neurotransmitter in our brains related to pleasure – is involved in motor function, memory, learning and reward.
It is synthesised in the midbrain and released into the synaptic cleft – or the space between neurons – where it can activate dopamine receptors.
DAT in the presynaptic membrane is responsible for terminating dopamine transmission by transporting the hormone back into neurons. This process helps maintain balance in the central nervous system.
“Despite decades of study, the structure, substrate binding, conformational transitions and drug-binding poses of human dopamine transporter remain unknown,” the researchers wrote.
Abnormalities in dopamine levels have been linked to conditions like depression, bipolar disorder, attention deficit hyperactivity disorder (ADHD) and Parkinson’s disease.
Inhibition of DAT – leading to increased dopamine concentrations – is also what allows psychostimulant drugs such as cocaine to trigger physical effects and become addictive.
“[DAT] belongs to the family of sodium/solute isotropic transporter proteins and undergoes three states of outward opening, closed and inward opening driven by sodium and chloride ions,” said co-first author Li Yue, from the CAS Institute of Biophysics in Beijing.
“By understanding how different drugs bind to different conformational states of DAT, the design of drugs can be more precisely guided to improve therapeutic efficacy and minimise unwanted side effects,” Li said.
The team was able to obtain images of DAT in its unbound state, as well as bound with dopamine, an ADHD medication called MPH, and two dopamine-uptake inhibitors.
MPH, or methylphenidate, is the most commonly prescribed drug for ADHD, however “despite its widespread clinical use and extensive pharmacological studies, the mechanism by which MPH inhibits DAT remains unclear”, the researchers wrote.
They said MPH stabilised DAT in an outward-facing conformation, while previous studies have found that cocaine also binds fruit fly DAT in the same conformation – which could explain why MPH has the potential for abuse.
Dopamine-uptake inhibitors like GBR12909 and benztropine have been found to block DAT without causing cocaine-like physical effects or addictive use.
“GBR12909, a prototypical noncompetitive inhibitor, shows promise for treating cocaine abuse in humans by antagonising the ability of cocaine to elevate extracellular dopamine concentration by 50 per cent,” the team wrote.
However, previous clinical studies using GBR12909 in humans were terminated due to uncertainty over how it binds to the dopamine transporter.
While cocaine stabilises the transporter in an outward opening state, the team found that GBR12909 stabilises the dopamine transporter in an inward opening state.
“This finding implies that the inhibitory mechanism of GBR12909 operates by preventing the transporter from transitioning from the inward-facing to the occluded state,” the paper said.
According to Li, “the conformational preference may have differential impacts on the downstream signalling events associated with DAT … which further affects their different addictive properties”.
She said the difference in addictive properties was also due to GBR12909 having a higher binding affinity and slower dissociation from DAT, allowing it to be “functionally antagonistic” to cocaine’s ability to increase extracellular dopamine.
This suggests that increased cocaine use is not likely to offer binding competition for GBR12909, which could make it an effective treatment to prevent addiction.
Li said it had been hard to study DAT due to its different structural states and small molecular weight, as well as the long experimental optimisation to get high-quality protein samples.
She said the team’s findings offered more insight into DAT and how it works, “suggesting potential therapeutic strategies for [human DAT-related] disorders and cocaine addiction”.
