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.
Tags: dopamine, addiction
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