Researchers at Penn State College of Medicine (Hershey, PA, USA) tested the specific differences in the sense of taste and the pleasure derived from sweet foods by investigating taste responses of two strains of rats; lean and healthy Long-Evans Tokushima Otsuka (LETO) rats, and Otsuka Long-Evans Tokushima Fatty (OLETF) rats, which mirror obese humans. OLETF rats have normal body weight at first, but they tend to chronically overeat due to a missing satiety signal, become obese, and develop diabetes. The obese rats also show an increased preference for sweet foods and are willing to work harder to obtain sweet solutions as a reward for their learning. The researchers implanted electrodes in the rodents' brains to record the firing of nerve cells when the rats' tongues were exposed to various tastes -- salt, citric acid, plain water, and six different concentrations of sucrose. The researchers specifically examined the differences in processing taste in the pontine parabrachial nucleus (PBN), a part of the brain that uses nerve cells to relay information from the surface of the tongue to the brain.

The researchers found that when compared to the LETO rats, the OLETF rats had about 50% fewer neurons firing when their tongues were exposed to sucrose, suggesting that obese rats are overall less sensitive to sucrose. However, when the obese rats were fed a stronger concentration of sucrose, their nerve cells fired more vigorously than in the lean rats. In other words, obese rats have a weaker response to weak concentrations and a stronger response to strong concentrations. The response to salt, on the other hand, was the same for both strains. The Penn State researchers believe that the increased consumption of sweet foods over time could be influencing the brain's reward center by relaying progressively weaker nerve signals, which affects the perception of taste of the meals through the PBN. The study was published in the October 2008 issue of the Journal of Neurophysiology.

"When you have excess body weight, the brain is supposed to tell you not to eat more, or not choose high caloric meals," said Andras Hajnal, M.D., Ph.D., an associate professor of neural and behavioral sciences at Penn State College of Medicine. "But this control apparently fails and thus the obesity epidemic is rising, and we want to find out how the sense of taste drives up food intake."

In obese humans, an increase in the weight-height ratio is usually accompanied by a decrease in dopamine, which is a neurotransmitter associated with the brain's pleasure system. The researchers found that in the obese rats, like in humans, the dopamine system is suppressed and it is very possible that the obese rats are seeking a hedonistic experience or reward by eating larger meals, as well as eating more sweets when they can.

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