Rats and Taste
Originally published in the February 1999 issue of CLASnotes.
Most
of us take the sense of taste for granted. But if we were suddenly thrust from
the shelter of civilization into the wilderness, where foods are not labeled
with their ingredients as they are in the supermarket, the value of our chemical
senses would perhaps become more apparent.
Animals in the wild use their chemical senses in a variety of ways, one of which is to help guide their ingestive behavior. Taste provides an animal with information about the chemical composition of food and fluid, especially regarding compounds that are not particularly volatile and thus do not stimulate the olfactory system. If an animal ingests a relatively novel tasting food followed by an episode of gastrointestinal distress, it will subsequently avoid eating that same food again. This is true even if the onset of malaise was delayed by as much as 12 hours!
This phenomenon is referred to as taste aversion learning and it serves the very adaptive purpose of preventing animals from ingesting potentially toxic substances a second time; humans are not immune to such processes and reports of learned taste aversions to novel tasting alcoholic beverages are not uncommon.
In addition to using tastes as learned signals, animals—including humans—are born with some innate taste preferences and aversions. Human infants, just hours after birth, will display characteristic oral acceptance and rejection reflexes depending on whether a taste stimulus placed on the tongue is sweet or bitter. Many toxic plants are bitter tasting and animals naturally avoid them, whereas many energy rich foods, such as fruits, are sweet tasting and actively ingested.
Of course, there is no way to really know whether animals experience bitterness or sweetness as humans do (in fact, there is no way to know whether the experience of sweetness or bitterness, etc., is exactly the same in two human observers). This doesn't mean, however, that the taste sensations of animals cannot be studied scientifically. A research area referred to as animal psychophysics was developed by psychologists interested in studying sensory processes in nonhuman (and thus nonverbal) animals. These inventive testing procedures have allowed researchers to ask animals questions about their sensory experiences. Indeed, these techniques have proven invaluable in efforts to understand the neural basis of sensation and perception.
Spector uses rats in his research to model nerve regeneration in the oral
cavity. The arrow (above left) points to one of many magnified taste buds of
a rat. When the taste nerve is severd, the taste buds disappear (above, right).
Taste nerves, however, have an unusual prolivity to regenerate and reinnervate
their appropriate oral locations, causing taste buds to reappear.
In my laboratory, we apply animal psychophysical procedures in experiments aimed at understanding how the nervous system represents information about taste stimuli. Our behavioral microscope, so to speak, is an apparatus we refer to as a gustometer. This device is designed for use with rodents and allows us to deliver small volumes of taste stimuli and measure immediate responses. Using the gustometer, we have trained rats to press one lever in response to sampling sodium chloride (NaCl) and the other lever in response to potassium chloride (KCl). The fact that rats can perform reliably in such a task implies that they can discriminate between the respective tastes of these two physiologically significant salt stimuli. If we surgically transect the chorda tympani nerve, which transmits gustatory signals from the taste buds on the front of the tongue (rats are anesthetized during this procedure and carefully monitored postsurgically), the performance of the rats on this salt discrimination task is severely impaired.
In contrast, if the glossopharyngeal nerve, which innervates four times as many taste buds located on the posterior tongue, is transected, discrimination performance is entirely unaffected. Both nerves and their associated taste receptor cells respond to NaCl and KCl, but it appears that the way the nervous system uses information from the two nerves differs.
My laboratory is funded by the National Institute of Deafness and Other Communication Disorders (chemical senses research is a component of this institute's mission) to study the functional consequences of gustatory nerve injury and regeneration. Taste buds, each of which consist of about 50 taste receptor cells forming a bud-like shape, are distributed in distinct fields in the oral cavity (front and back of tongue, palate, and near the larynx). These fields of taste buds are innervated by four different nerve branches. We have developed a battery of behavioral tasks, each focusing on a different aspect of taste function that helps us assess the role of gustatory input from the different fields of taste buds in the mouth.
Interestingly, when taste nerves are severed they have a great proclivity to regenerate and reinnervate their appropriate receptor field. When the chorda tympani regenerates, only 70% of the taste buds reappear (they disappear upon nerve transection). Nevertheless, salt discrimination performance returns completely to normal. Actually, nerve regeneration is not as novel an event as it may seem for the gustatory system. Taste receptor cells have a life cycle of about 14 days, so that every couple of weeks you have a completely different complement of taste buds. It remains somewhat of a mystery how the system maintains perceptual stability in light of the ever-changing connections that are being formed between the nerve fibers and the taste receptor cells. In any event, animal models of nerve regeneration will ultimately be critical in the development and evaluation of treatments that promote recovery of function in humans sustaining sensory nerve damage in general.
I am privileged to serve as the Assistant Director of the newly formed University of Florida Smell and Taste Center (UFSTC); Dr. Barry Ache, an internationally renowned olfactory scientist from the Whitney Laboratory, is the Director. Our charter members are from various campus departments in the Colleges of Liberal Arts and Sciences, Medicine, Engineering and from IFAS. The primary mission of the Center is to stimulate interdisciplinary discussion and collaboration on chemical senses research.
I am excited by the prospects that the Center offers, and I encourage interested faculty in the College to please contact me for more information.
Credits
Writer
Alan Spector, Department of Psychology
