The main purpose of the rat's sense of taste is to perform a quality check on food items. Checking for quality is an important job for foraging rats because they are omnivores, capable of eating many different types of food, and some of the things they put in their mouths may be poisonous or inedible or unhealthy. Living all over the world like they do, there are no foods guaranteed to be in a rat's environment, so rats must test and try what's available and this is a dangerous business. It is critical that they be able to distinguish good foods from bad ones, and to remember the result!

How does taste work?

When a rat puts something in its mouth, tiny chemicals from it dissolve in saliva. These dissolved chemicals stimulate taste receptors which are located on taste-sensitive cells in the mouth. Thirty to a hundred of these taste cells are organized into clusters called taste buds. Taste buds are found all over the mouth: the tongue, the pharynx, the palate, the epiglottis and the entrance of the larynx. Taste buds all contain neurons that detect five basic tastes:

• Salt (sodum chloride)
• Sour (acids: protons H+)
• Sweet (glucose, sucrose and other carbohydrates)
• Bitter (Ca++ and IP3)
• Umami (amino acids like glutamate and aspartate, and MSG)

Sensitivity to all five tastes is distributed over the entire tongue and other regions of the mouth, but some areas of the tongue are more responsive to certain tastes than others.

Where does the message go in the brain?

Each taste receptor cell is connected, through a synapse, to a sensory neuron. Each sensory neuron is connected to several taste cells, and each sensory neuron responds best to one of the five tastes sensations.

Taste messages shoot up a pathway to the medulla. From there the information is relayed to the gustatory cortex for conscious perception of taste (located right under the somatosensory cortex), and to the hypothalamus, amygdala and insula, which are responsible for behavioral responses like aversion, stomach secretion, and feeding behavior.

Taste and behavior

Taste and feeding behavior interact, enabling the rat to gravitate toward safe foods, avoid noxious ones, and fulfill dietary needs:

• Forming food preferences: newborn rats taste their mother's diet through her milk, and prefer the foods she ate when they grow up (Galef 1996).
• Taste aversions: a kind of associative learning in which rats react negatively to the taste of a food that previously made them ill. A single experience is enough! Taste aversions have an obvious adaptive role: a food that makes a rat ill may be poisonous and should be avoided in the future (Garcia et al. 1974).
• Filling dietary needs: A rat deprived of a particular nutrient learns which tastes indicate foods that provide that nutrient, and prefers the taste of those foods until its need is filled (Markison 2001, Lucas & Schlafani 1996).
• Feeling full: When rats are satiated, they are no longer attracted to tastes (Bardos 2001, Thaw & Quinn 2003).

Further reading

• The physiology of taste
• ChemoReception Web
• Receptors and transduction in taste (pdf)
• Gustatory neural processing in the brain stem of the rat (Streefland 1998) (pdf)