Scientists have long known that hunger causes increases in some brain chemicals while lowering others. However, the root cause of hunger's effects -- the initial chemical trigger of appetite -- has been elusive.
In experiments with mice, the Johns Hopkins researchers showed that appetite is immediately and directly tied to amounts of a chemical called malonyl-CoA. In hungry mice, malonyl-CoA was almost undetectable in the brain. Once fasting mice were given food, however, amounts of the chemical increased to high levels within two hours. Furthermore, chemically reducing appetite by injecting a compound called C75 into the brain brought levels of malonyl-CoA up to those of mice given food, helping to explain C75's effects.
"From this work, it appears as though malonyl-CoA levels control appetite and levels of other brain chemicals that we know go up and down with hunger and feeding," says Dan Lane, Ph.D., professor of biological chemistry in Hopkins' Institute for Basic Biomedical Sciences. "There may be other contributors, but this is the first direct evidence that malonyl-CoA could be the body's primary appetite controller."
In previous work, Lane and his colleagues had shown that giving mice C75, which blocks conversion of malonyl-CoA into fatty acids, dramatically reduced animals' appetites. Subsequently, they found that C75 triggers levels of several known appetite signals (NPY, AgRP, POMC and others) to register "full" even when animals should have been hungry.
However, the new experiments, during which C75 was injected directly into the animals' brains, suggest that increasing levels of malonyl-CoA, caused by "blocking the dam" with C75, is the first step in the process that alters levels of those appetite signals.
"Fully understanding how appetite is regulated by the brain should reveal ways to control appetite," says Lane, who was studying how fat cells develop when he and colleagues discovered the appetite-suppressing effects of C75 a few years ago. "Because C75 was injected into the brain, rather than into the abdomen as in earlier experiments, we also now know that the compound's effects on appetite stem primarily from its effects on chemicals in the brain, not from effects it might have elsewhere in the body."
The scientists also discovered that preventing formation of malonyl-CoA by injecting a different substance (TOFA) into the brain partially reversed the appetite-suppressing effect of C75. Lane suggests that a better blocker of malonyl-CoA formation should more completely counteract C75's effects.
The research was supported by the Yamanouchi Pharmaceutical Co., LTD., in Tokyo. Authors on the paper are Lane, Zhiyuan Hu and Seung Hun Cha of Johns Hopkins; and Shigeru Chohnan of Ibraki University, Japan. The authors have no conflict of interest to report.
PNAS Oct 28 2003; 100(22):12624-12629.
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