ATLANTA, GEORGIA--You and your date just had a lovely dinner, but the popcorn at the movie theater snack counter still looks oh-so tempting. If you break down and buy that bucket of buttery goodness, it may be because your brain's reward centers have been sensitized by ghrelin, a peptide a new study implicates in anticipating food rewards.
Ghrelin is secreted by the gut, and its levels peak just before meal times. Injections of the peptide prompt feeding in lab animals, but exactly how isn't known. Previous studies found that ghrelin targets receptors in the hypothalamus, a brain region important for regulating food intake and maintaining a healthy physiological status quo. But ghrelin receptors have also been found in other brain regions, including the ventral tegmental area (VTA), part of the feel-good circuitry that's activated by food, sex, and some illicit drugs--or even just by the expectation of such rewards.
To investigate ghrelin's effects on the brain, a team of neuroscientists led by Tamas Horvath of Yale University School of Medicine in New Haven, Connecticut, and Alfonso Abizaid, now at Carleton University in Ottawa, Canada, exposed slices of rat and mouse brain tissue to the peptide. The treatment made dopamine-releasing neurons in the VTA more excitable, and it reorganized their synapses with other neurons to make this excitability last. These findings suggest that ghrelin stimulates the brain's reward pathways and may make them more sensitive to future rewards, Abizaid says.
Next, the researchers looked at the feeding behavior of mice missing the gene for ghrelin. When normal mice are given access to food for just 6 hours each day, they quickly learn to scarf down their daily allotment of chow. But the knockout mice never seemed to catch on and ate substantially less than their normal counterparts. And while normal mice grew hyperactive just before meal time rolled around each day, the knockout mice did not. Together the findings indicate that without ghrelin, the animals are less able to anticipate feeding opportunities, says Abizaid, who reported the results here 16 October at the annual meeting of the Society for Neuroscience. The research is also published today in the Journal of Clinical Investigation.
The study suggests that ghrelin may regulate the sensitivity to food rewards by altering connections in the brain's reward pathway, says William Colmers of the University of Alberta in Edmonton, Canada. The findings may have implications for understanding the current obesity epidemic, he adds, as some research suggests that the temptation to eat when we're not hungry is a major culprit. Now, how about that popcorn?
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