The Sweet-Blind Mice

Abstract

Understanding the brain’s reward system, mediated by the neurotransmitter dopamine, has the potential to help address the enormous health risk to humans posed by obesity. Like addictive drugs, good-tasting food increases the accumulation of dopamine in the nucleus accumbens (NAcc) of the ventral striatum. In fact, some mice with defects in the dopamine reward system fail to eat and die of starvation. But besides taste-induced or hedonic responses, animals appear also to sense the nutritional value of ingested food. De Araujo et al. present a series of experiments that implicate the same brain mesolimbic dopamine reward system in both processes. They distinguished the two responses by making knockout mice that lacked a functional TRPM5 (transient receptor potential channel M5) ion channel, which mediates taste sensation in taste receptor cells. Unlike wild-type animals, after being deprived of water, these mutant animals showed no preference for a sucrose solution over plain water. However, over a period of conditioning of 6 days in which mice were presented with bottles containing sucrose solution or water that could be distinguished by their sipper, the starved mutant animals had a distinct preference for the more nutritive solution. The authors implanted microdialysis probes in the NAcc of mutant or wild-type animals and showed that the mutant’s response to the sucrose solution was associated with the release of dopamine similar to that in wild-type mice drinking sucrose solution. Electrophysiological recordings from neurons in the NAcc and orbitofrontal cortex indicated that the increased dopamine release was correlated with activation of the brain reward circuitry. The authors, and Andrews and Horvath in commentary, conclude that both hedonic and nutritional or homeostatic rewards of ingested food are processed similarly, not separately as sometimes thought, in the higher brain centers that process reward responses. I. E. de Araujo, A. J. Oliveira-Maia, T. D. Sotnikova, R. R. Gainetdinov, M. G. Caron, M. A. L. Nicolelis, S. A. Simon, Food reward in the absence of taste receptor signaling. Neuron 57 , 930-941 (2008). [PubMed] Z. B. Andrews, T. L. Horvath, Tasteless food reward. Neuron 57 , 806-808 (2008). [PubMed]