Sunday, January 11, 2009

Sugar can be addictive

And you may well have noticed this addictive characteristic if you overindulged in sugar over the recent holidays. Turns out, the neurotransmitter dopamine plays a big role here. There's a lot of research on dopamine coming out these days. The research discussed here is just one example.

We last dealt with dopamine back in November: here. One could paraphrase the conclusions of that research as suggesting that genetic problems with a dopamine receptor lead to reduced sensitivity to dopamine, which leads to stronger than normal craving for food, leading to overeating and obesity.

The research in question here looks at a special case of this in an animal model, involving rats and sugar. In this case, reduced sensitivity to dopamine isn't a consequence of a variant allele for a dopamine receptor. Instead, it seems to result from excessive intake of the addictive substance (sugar), resulting in fewer brain receptors for dopamine.

Sugar Can Be Addictive: Animal Studies Show Sugar Dependence (12/10/08)
"We have the first set of comprehensive studies showing the strong suggestion of sugar addiction in rats and a mechanism that might underlie it," [principal investigator Bart] Hoebel said. The findings eventually could have implications for the treatment of humans with eating disorders, he said.

Lab animals, in Hoebel's experiments, that were denied sugar for a prolonged period after learning to binge worked harder to get it when it was reintroduced to them. They consumed more sugar than they ever had before, suggesting craving and relapse behavior. Their motivation for sugar had grown. "In this case, abstinence makes the heart grow fonder," Hoebel said.

The rats drank more alcohol than normal after their sugar supply was cut off, showing that the bingeing behavior had forged changes in brain function. These functions served as "gateways" to other paths of destructive behavior, such as increased alcohol intake. And, after receiving a dose of amphetamine normally so minimal it has no effect, they became significantly hyperactive. The increased sensitivity to the psychostimulant is a long-lasting brain effect that can be a component of addiction, Hoebel said.

The research investigated the physiological changes responsible for this addictive behavior:
Hoebel has shown that rats eating large amounts of sugar when hungry, a phenomenon he describes as sugar-bingeing, undergo neurochemical changes in the brain that appear to mimic those produced by substances of abuse, including cocaine, morphine and nicotine. Sugar induces behavioral changes, too. "In certain models, sugar-bingeing causes long-lasting effects in the brain and increases the inclination to take other drugs of abuse, such as alcohol," Hoebel said.

Hoebel and his team also have found that a chemical known as dopamine is released in a region of the brain known as the nucleus accumbens when hungry rats drink a sugar solution. This chemical signal is thought to trigger motivation and, eventually with repetition, addiction. ...

Hungry rats that binge on sugar provoke a surge of dopamine in their brains. After a month, the structure of the brains of these rats adapts to increased dopamine levels, showing fewer of a certain type of dopamine receptor than they used to have and more opioid receptors. These dopamine and opioid systems are involved in motivation and reward, systems that control wanting and liking something. Similar changes also are seen in the brains of rats on cocaine and heroin.

It's interesting that overstimulating with an addictive substance (sugar) not only reduces the number of dopamine receptors but also increases the number of opioid receptors. What's not so clear is the relationship between the dopamine and opioid systems. Having more opioid receptors would appear to increase cravings for substances that stimulate them, which is the opposite of what happens with dopamine receptors. Hypothetically, perhaps, opioid stimulation is able to substitute for reduced dopamine sensitivity.

Research in this area may have even wider implications than behavior linked to overconsumption, as there is other recent research that suggests addictive behavior related to other kinds of stimulation, such as gambling and risk-taking.

Watch for a lot more discussion of this line of inquiry.

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Anonymous Omar said...

Charles, I just don't understand why you receive few comments. I have hit your blog and I'm doing some effort to escape from it. Your posts are very interesting and your scope is as broad as it can be for a single person.
I'm a biochemist specialized in enzymology. Just 7 few month ago I have jumped into cellular biology. I'm currently kind of blocked and paralysed by the tremendous complexity of signalling pathways.
How do you feel about that?
By the way, do you do something else besides writting in your blog?
That reading surely takes lot of time.



1/13/2009 06:07:00 PM  
Blogger Charles Daney said...

Thanks, Omar.

Just 7 few month ago I have jumped into cellular biology. I'm currently kind of blocked and paralysed by the tremendous complexity of signalling pathways. How do you feel about that?

Some things in nature are just complex. Given that the pathways arise from the random interaction of proteins floating around in cells, one wouldn't expect things to be neat and tidy.

Perhaps we're lucky that most proteins don't interact strongly enough to form recognizable pathways, given that the number of possible interactions is astronomical. Only a tiny percentage are repeatable enough to matter - and that's still a large number from our perspective.

By the way, do you do something else besides writting in your blog?

I wish I had time to write more than I do. As it is, I spend more time just following the news flow to find interesting things to write about.

And even then, yes, I do have a lot of other things to do during the day.

1/15/2009 01:50:00 AM  
Anonymous Anonymous said...

very interesting, thanks!

best, Robin

3/23/2010 01:30:00 AM  

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