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Rodents of unusual appetites
How food cravings and eating affects the brain
By Jennifer Lu | MU Bond Life Sciences Center
When it comes to cookie dough, we’re not the only ones who can’t control our cravings. Kyle Parker’s rats couldn’t resist, either, thanks to a tweak in their brain chemistry.
Parker studies the neuroscience of food-based rewards.
Matthew Will, associate professor of psychological sciences at the Bond Life Sciences Center, studies the neuroscience of behaviors such as over-eating and addiction | photo by Jennifer Lu, Bond LSC
“It’s like when I eat dessert after I’ve eaten an entire meal,” said Parker, a former grad student from the lab of Bond LSC’s Matthew Will. “I know that I’m not hungry, but this stuff is so good so I’m going to eat it. We’re looking at what neural circuitry is involved in driving that behavior.”
Behavior scientists view non-homeostatic eating — that’s noshing when you’re not hungry — as a two-step process.
“I always think of the neon sign for Krispy Kreme donuts.” Will said, by way of example.
“The logo and the aroma of warm glazed donuts are the environmental cues that kick-start the craving, or appetitive, phase that gets you into the store. The consummatory phase is when you “have that donut in your hand and you eat it.”
Parker activated a “hotspot” in the brains of rats called the nucleus accumbens, which processes and reinforces messages related to reward and pleasure.
He then fed the rats a tasty diet similar to cookie dough, full of fat and sugar, to exaggerate their feeding behaviors. Rats with activated nucleus accumbens ate twice as much as usual.
But when he simultaneously inactivated another part of the brain called the basolateral amygdala, the rats stopped binge eating. They consumed a normal amount, but kept returning to their baskets in search for more food.
“It looked like they still craved it,” Will said. “I mean, why would a rat keep going back for food but not eat? We thought we found something interesting. We interrupted a circuit that’s specific to the feeding part — the actual eating — but not the craving. We’ve left that craving intact.”
To find out what was happening in the brain during cravings, Parker set up a spin-off experiment. Like before, he switched on the region of the brain associated with reward and pleasure and then inactivated the basolateral amygdala in one group of rats but not the other.
This time, however, he restricted the amount of the tasty, high-fat diet rats had access to so that both groups ate the same amount.
This way, both groups of rats outwardly displayed the same feeding behaviors. They ate similar portions and kept searching for more food.
But inside the brain, Parker saw clear differences. Rats with activated nucleus accumbens showed increased dopamine production in the brain, which is associated with reward, motivation and drug addiction. Whether the basolateral amygdala was on or off had no effect on dopamine levels.
However, in a region of the brain called the hypothalamus, Parker saw elevated levels of orexin-A, a molecule associated with appetite, only when the basolateral amygdala was activated.
“We showed that what could be blocking the consumption behavior is this block of the orexin behavior,” Parker said.
The results reinforced the idea that dopamine is involved in the approach — or the craving phase — and orexin-A in the consumption, Will said.
Their next steps are to see whether this dissociation in neural activity between cravings and consumption exists for other types of diets.
Will also plans to manipulate dopamine and orexin-A signaling in rats to see whether they have direct effects on feeding.
“Right now, we know these behaviors are just associated with these neural circuits, but not if they’re causal.”
Behavioral Neuroscience published the study, “Neural Activation Patterns Underlying Basolateral Amygdala Influence on Intra-Accumbens Opioid-Driven Consummatory Versus Appetitive High-Fat Feeding Behaviors in the Rat,” in the December 2015 issue. A grant from the National Institute of Drug Abuse supported this research.