#CAN2014 Does a junk food pigout trigger more binges?
Notes from the 2014 Canadian Association for Neuroscience annual meeting in Montreal, Canada. Image credit: mtlfoodpics.blogspot.com
I confess: during my stay in Montreal I stuffed myself everyday with smoked-meat sandwiches, poutine avec bacon and the sinful, sugary baked goods of Premiere Moison. My insulin levels must’ve been through the roof.
According to Dr. Stephanie Borgland at the University of Calgary, those shots of insulin may have prevented me from further food binges. In response to sweetened high-fat foods, insulin released in the gut directly activates their receptors in the VTA (ventrategmental area), a critical part of the brain’s reward system that promotes reward-seeking behaviours. Interestingly, when insulin receptors activate, they cause to neuron to secrete chemicals called endocannabinoids that dull the synapses and inhibit overall VTA neurotransmission in well-fed rats. This causes the animals – and presumably, us - to loose interest in seeking out more edible delights.
However, warned Dr. Borgland, this self-inhibiting neural circuit can break down. In animal studies, when researchers allowed satiated rats to pig out for just one hour on delicious treats, insulin promptly inhibited the VTA to regulate energy intake. However, when the animals were given 24 hours of non-stop access to fatty, sugary junk chow, the inhibitory network broke down.
Rather than decreasing food-seeking behaviours, a day of pigging out made the rats more motivated to hunt down food. After a 24-hour binge, researchers waited for the rats’ insulin levels to return to normal before placing them in a light-dark box, pictured above. In the middle of the well-lit area sat a plate full of enticing foods. Being nocturnal, rats perceive light as a threat to survival and generally avoid bright areas; yet these rats threw caution to the wind, barely hesitating before leaving the safety of darkness. They spent more time wandering around the well-lit side of the box, seemingly unaware of their natural aversion to light. On the other hand, rats that were allowed only one hour of access to junk food behaved as expected: fully satiated, they stayed put in the darkness, unwilling to risk danger just for a bite to eat.
Why did an extended binge override insulin-driven inhibitory circuits in the VTA? The answer seems to be an increase in the density of excitatory synapses on dopamine neurons in the VTA. In animals fed a sweetened high-fat diet for 24 hours, this increase in excitatory neurotransmission completely overwhelmed insulin-induced inhibition, causing stronger VTA activation. This, in turn, urges the animal to seek out and consume more palatable foods even in the face of danger.
The shift to VTA excitation lasted for over a week after the initial binge. Unsurprisingly, at the end of the experiment when animals were once again exposed to food for an hour, the “binger” mice ate much more than those that had only briefly tasted the high-fat/sugar food.
Taken together, it seems that a short-term food binge can override insulin-dependent inhibitory mechanisms in the VTA and sharpen synaptic transmission in the brain’s reward system. This, in turn, makes food (as well as food porn) seem much more appealing, motivating us to seek out and consume more tasty rewards. The take-home message? Food binges will probably sabotage your diet. Eat what you want – but in moderation.
Event: Does a junk food binge promote future feeding? Short-term access to palatable food causes a long lasting increase in synaptic efficacy in the VTA. Dr. Stephanie Borgland. University of Calgary. Young Investigator Award and Lecture.