The new study explores the relationship between receptors in the gut and dopamine circuits in the brain.
Four types of bariatric surgery are currently available for the purposes of treating obesity, all of which have a strong track record.
In most reports, more than 80% of procedures are regarded as a success. However, the exact pathways that the procedures use to achieve their results are not yet fully understood.
There are a number of mechanisms that work together to create the positive outcomes in these surgeries. Malabsorption and restriction are the two most obvious contenders.
Malabsorption (a reduction in nutrient uptake) and a restriction in the total amount of food that can be consumed are both valid modes of action, but alone they cannot explain bariatric surgery's impressive achievements.
Anecdotally, patients have described changes in the types of foods that attract them after surgery, but, until now, the mechanisms that induce these changes in dietary desires have been up for debate.
New research, conducted by Ivan de Araujo of Yale University School of Medicine, has unveiled at least part of the solution.
Dopamine and attraction to sugar
Previous studies have shown that caloric intake is, in part, mediated by a dopamine reward system in the dorsal striatum. This dopamine system is sensitive to sugar in the gastrointestinal (GI) tract.
Because of the addictive nature of sweets, mice that have had their stomachs artificially filled with sweet solution will persistently lick at a spout that produces sugary water, despite being satiated.
This counterintuitive behavior is believed to be mediated by the activation of sugar receptors in the duodenum, creating an addictive loop by activating dopamine pathways in the dorsal striatum.
The dorsal striatum's role in motor coordination has been well described, but that is not the brain area's only function. Modern research has also uncovered more subtle roles. It appears that the dorsal striatum is also involved in decision making. Specifically, it selects and initiates actions by combining motivational and emotional information.
Experimental bariatric surgery
De Araujo's study, published in Cell Metabolism, involved intricate bariatric procedures in mice - a surgery similar to the commonly utilized Roux-en-Y intervention.
The experimental procedure consisted of bypassing the first section of gut (that exits the stomach) and attaching the stomach directly to a lower region of the GI tract. This operation only differed from the human version in that no gastric pouch was constructed to limit the rodent's food intake.
Mice who had undergone the surgery, and been artificially satiated with sweetened water, gave significantly less attention to the sugar fountain. The rodent's sweet-tooths had been markedly diminished by the intervention.
The researchers hypothesize that the GI reroute worked by reducing sugar-induced dopamine release in the dorsal striatum, essentially diminishing the rewarding effects of sugar.
To further demonstrate this relationship, the team solidified their results using optogenetics.
As an additional arm to the research, the team utilized optogenetics, an advanced neuroscience technique. Optogenetics involves modifying neurons to express light-sensitive ion channels.
This procedure allows scientists to turn a predetermined set of neurons off and on without the need for invasive electrode insertion.
Dopamine neural circuits in the modified mice were specifically stimulated. This re-activation of the reward centers effectively overpowered the changes induced by the surgery, seeing the mice return to their pre-op rates of fountain licking.
De Araujo commented:
"Our findings provide the first evidence for a causal link between striatal dopamine signaling and the outcomes of bariatric interventions.
However, we certainly do not want to give the impression that we have an answer for how and why bariatric surgery works. Much more research is needed in this field."
His team plans to extend this research by investigating similar effects in all four of the currently available bariatric surgeries. De Araujo also adds that the long-term goal is to find solutions to obesity and diabetes progression without the need for surgery.
Medical News Today recently covered research that showed a link between bariatric surgery, self-harm and suicide.