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Aided by the most powerful microscope in the world, a team of researchers from the University of Copenhagen have succeeded in delving deep inside the brain’s reward system. Their discoveries represent a whole new understanding of the biology behind cocaine use.
 

Millions of addicts know that it is as hard to quit cocaine as it was easy to become hooked in the first place.

But why exactly is it so difficult to get out of the vice-like grip of cocaine addiction?

Science has been trying and generally failing to answer that question for decades, but that may be about to change:

A team of researchers from the University of Copenhagen (UCPH) have succeeded in imaging the brain’s dopamine transporter, and demonstrating at near-atomic level how cocaine binds to this transporter. Their discovery has just been published in Nature, one of the world’s most important scientific journals.

“This novel finding increases our understanding of the biochemistry behind cocaine addiction,” explains research team leader Professor Claus Juul Løland from the Department of Neuroscience at UCPH:

“The dopamine transporter, which is actually a protein, is so small that it was not possible until now to investigate its appearance and physical characteristics under a microscope. We decided to make an attempt using cryogenic electron microscopy (Cryo EM) in the world’s most powerful microscope, one of which is installed at UCPH. We’re proud to have finally pulled that off! And because we were able to demonstrate how cocaine affects the dopamine transporter, this discovery may be a key to developing a medication for treating cocaine addiction. 

This type of treatment does not yet exist, but is badly needed, and the more we can learn about the cascade of biochemical events triggered by cocaine intoxication in the brain, the greater the potential for developing an efficacious addiction therapy,” says Løland. The professor’s field is the biochemical mechanisms of action behind narcotics, and his research is funded by an Ascending Investigator grant from the Lundbeck Foundation.

Blocks the mops

Dopamine is one of the brain’s key signalling substances, and in that it controls our sense of pleasure, it is part of the brain’s reward centre. And the dopamine transporters, for there are many of them dotted around in the brain, serve in this biochemical circuit as tiny molecular ‘mops’,” Løland explains.

“Doing something pleasurable, like eating chocolate, causes dopamine release in the brain’s reward centre. But for this system to work, the released dopamine must be quickly removed again, i.e., it must be reabsorbed into the nervous system so as to stop the reward, and this is handled by the dopamine transporters.”

Over the course of a day, in a healthy circuit, a number of situations are typically associated with pleasure, ranging from food to sex and everything else in between.

But cocaine can sabotage this system:

“Cocaine has a very specific harmful effect on the dopamine transporters,” Løland explains. “We saw that in detail at near-atomic resolution when we analysed our microscope recordings. These revealed how cocaine blocks the dopamine transporters’ ‘mopping function’ so they are unable to soak up the released dopamine back into the nervous system.”

As a result of this blocking, the dopamine is not removed. Instead, the amount of dopamine increases and continues to transmit a strong reward signal, which is then independent of the sensory perceptions of the individual. You could say that during a ‘cocaine high’ the system is short-circuited, and that all sensory experiences are perceived as reward. And it is this intense sense of happiness or motivation that makes cocaine so attractive,” explains Løland.

“For someone on cocaine, even porridge tastes amazing. But the effect also has a downside because cocaine’s blockage of the dopamine transporter drains the nervous system of dopamine because it can no longer be recirculated. When the effect of cocaine subsides, there isn’t enough dopamine left to trigger the usual sense of reward. So now the chocolate is going to taste like porridge.

 “To get the reward back, the system now has to be boosted by more cocaine, which is how the addiction starts.”

A vast number of puzzle pieces

To create a picture of the brain’s dopamine transporters but also see how cocaine binds to these structures, the research team hit the laboratory. Here, with the aid of special cells in test tubes, they were able to produce the human dopamine transporter protein (DAT), to which they then added cocaine. It was samples of this mixture that were examined using a cryo EM microscope operating at an extremely low temperature of -180 degrees C.

The cryo-EM technique – which was launched in 2014 and resulted in a Nobel Prize only three years later – makes it possible to generate 3D images of structures in formerly unknown detail. However, the technique is highly complex, and requires expert familiarity with the hypersensitive equipment which has become far more sophisticated in terms of both its software and hardware, since it was invented.

Associate Professor Azadeh Shahsavar from the Department of Drug Design and Pharmacology at UCPH was involved in this part of the project. A 2022 Lundbeck Foundation Fellow, she is also one of the seven co-authors of the scientific article in Nature.

Shahsavar compares the process of creating the images of the dopamine transporter – and of the cocaine bonds to this structure – to doing a jigsaw puzzle. But before they could piece the puzzle together, the scientists had a few steps to work through first, as she explains:

“The microscope recorded around 25,000 video sequences of the biological material. From this, we were able to identify about 7.5 million potentially significant particles of the dopamine transporter. We then sifted through the particles several times over until we were down to some 200,000 particles that may be seen as the pieces of the puzzle”.

“We now had to assemble them into the biological structure of the dopamine transporter, and for that we had special software with special algorithms to help us. Once we had the structure in place, we could start to determine how the dopamine transporter works, and see how it is affected by cocaine. That’s the amount of effort that goes into answering questions like this,” Shahsavar explains.

Link to article in Nature: