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How antidepressant drugs work as roadblocks for brain chemicals

August 9, 2007 By Kerri Smith This article courtesy of Nature News.

Antidepressant drugs work as roadblocks for brain chemicals.

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The way in which antidepressants exert their effects on brain cells has been revealed by two separate teams of researchers working independently of each other.

Antidepressants work by preventing neurons in the brain from importing certain chemicals, such as dopamine and serotonin, which are used to pass messages from cell to cell. The route by which these chemicals are imported depends on passageways in the outer membrane of the cells called transporter proteins, and it is on these passageways that the antidepressants exert their influence. But how exactly they hold up the process has remained a mystery since the drugs were discovered 45 years ago, says Les Iversen, a pharmacologist at the University of Oxford, UK.

To resolve the mystery, both teams — one led by Eric Gouaux of Oregon Health and Science University in Portland and the other based at New York University and led by Da-Neng Wang — set out to understand what happens when antidepressants lock onto a transporter at the most fundamental level. They zoomed in on the transporters' molecular structures as revealed at atomic resolution through X-ray crystallography.

The researchers couldn't use human transporter proteins because they are difficult to isolate and fall apart easily. Instead they used an equivalent found in bacteria called LeuT. They then selected drugs from a class called tricyclic antidepressants and created crystals in which the drug and the transporter were bound together. Gouaux's team used a drug called clomipramine while Wang's used a similar compound called desipramine.

Their results provide the first glimpse of the mechanism by which the drugs block the transporters. Both groups agree on what's happening: the drug binds to the outside of the transporter, changing its shape. This traps the brain chemical inside the tunnel like a cork in a bottle, preventing it from passing through to the inside of the neuron. They published their results almost simultaneously in Nature1 and Science2 this week.

Drug traps

"We now have our mind around the problem," says Gouaux, whose team focused on working out exactly how the drug traps and holds the chemical inside the transporter. Now that we understand the basic movements, he says, "we're in a much more powerful position to design molecules to inhibit it".

Wang and his team wanted to look at how easy it would be to generalize the results they got in bacteria to humans. To design better drugs for depression, Wang says, "eventually we'll need to work on the human protein". So his group created mutations in the genetic sequence of some human versions of the protein, and then watched what happened when they added desipramine.

Wang and his colleagues knew from their studies of LeuT which part of the protein the drug bound to. And when they mutated the same part in human versions of the protein, they found that the drug didn't stop cells taking in dopamine or serotonin, suggesting that it could not block the transporters on which its binding site had been modified as efficiently as it does the normal transporters. "We basically proved that the inhibitory mechanism and binding site is conserved in humans," Wang says.

But although Iversen describes the studies as "elegant science", he is sceptical about how useful they will be when it comes to creating new drugs for depression. Information about structure hasn't yielded fruit so far, and drugs have been developed without it. "I don't think we really need to know this," he says. "Drug discovery people have been studying these binding sites without knowing where they are."

References

  1. Singh, S. K. et al. Nature doi:10.1038/nature06038 (2007).
  2. Zhou, Z. et al. Science doi:10.1126/science.1147614 (2007).

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