Rat taste behaviors for bitter and sour aversive stimuli

The gustatory and olfactory systems are the only two systems in which stimuli act directly on the receptor. There are five taste categories which have been identified as gustatory stimuli: sweet, salty, bitter, sour, and umami (MSG). Bitter and sour are known as aversive stimuli and help mammals avoid toxic and spoiled foods. Although both are similar in that they are aversive, they work through different transduction pathways. Research has shown that receptor (type II) cells are responsible for bitter taste transduction while presynaptic (type III) cells assist in the transduction of sour stimuli. Bitter stimuli act through metabotropic receptors and G-proteins to increase intracellular Ca2+. Sour stimuli are mediated by acids and work through ionotropic receptors. There are two major ion channels (ASICs and HCNs) that help in the transduction of bitter taste stimuli. Behavioral research has supported that rats are able to differentiate between the five taste categories. This is critical for animals in the wild in order to differentiate between foods that are full of nutrients and foods that will cause sickness or death. In addition to differentiating between different taste categories, researchers have shown that mammals are able to behaviorally differentiate between bitter and sour stimuli (Ekman and Rosenburg, 2005). Developmental research has suggested the idea that early experiences to bitter and sour stimuli can cause a decrease in their innately aversive tastes (London et. al, 1979). However, a critical period in age of development has still not been identified to support this claim. This critical review article examines the current research regarding the receptors, transduction mechanisms, discrimination between stimuli, and development for both sour and bitter taste categories.