Chairman: Harry R. Kissileff, Ph.D.
Rapporteur: Janet L. Guss
Date: Thursday, May 8th, 2003
Title: "Pleasure of Taste and Phylogeny"
Speaker: Michel Cabanac, Universitè Laval, Quebec, CA
Presentation (Supplied by Speaker):
The palatability of foods can be studied in humans through a psychophysical paradigm: subjects report on category, magnitude-estimation, or analog scales, whether a stimulus arouses them in a pleasurable or displeasurable way. The main characteristics of palatability, related to food intake, include:
Postingestive alliesthesia: During satiation, palatable foods turn unpleasant (negative alliesthesia). Then when hunger returns these same stimuli turn palatable again (positive alliesthesia).
Ponderostat and palatability: These are the long-term signals that modulate palatability, and are mainly related to body-weight set point and nutritional deficiencies. It is known that a less palatable diet lowers body-weight set point, and a highly palatable (e.g., cafeteria) one raises it.
The hedonic dimension of alimentary sensations allows for a tradeoff with other pleasures and pains. Thus, the most pleasurable sensation takes ‘first rank’ for access to behavior. One example of such a conflict would be comparing a tradeoff between palatability vs. money.
Various experiments have compared characteristics of palatability, as related to food intake, in mammals (e.g. the rat). Indeed, rats show signs of postingestive alliesthesia (Grill & Norgren’s method). In fact, all aspects of palatability reported by humans can be found in rats as well, including decision making in conflicts of motivation, palatability vs. cost. Various experiments have also examined these dimensions of palatability, as related to food intake, in lower vertebrates. A threshold of consciousness is believed to ‘sit’ between amphibians and reptiles, as the former shows no sign of emotion when handled, while the latter shows ‘emotional fever’ and tachycardia when handled. If consciousness exists in reptiles, then it should be possible to find evidence that they possess all the dimensions of sensation- especially pleasure/displeasure.
To verify whether lizards experience sensory pleasure, an experiment was conducted in which lizards were placed in a situation of conflict between two motivations: a thermoregulatory drive (to avoid cold) and an attraction to palatable bait. To reach the bait (lettuce), they had to leave a warm refuge, including standard food, and venture out into a cold environment. The lizards sought out the lettuce, even though they did not need the food. This finding demonstrates that, as with higher organisms, reptiles also engage in behaviors that are not merely compensatory; the pleasure derived from palatable food can also motivate behavior.
Indeed, reptiles also acquire taste aversion for new baits after a single IP injection of LiCl, but amphibians (toads, salamanders) do not. When palatability is yoked to an environmental cost, reptiles take into account both sensations in a way that is identical to mammals. Thus Reptiles, but not amphibians, likely know palatability. In them also, sensory pleasure seems to be a common currency for the behavioral tradeoffs leading to decision making in situations of conflict.
Altogether, these results suggest the following conclusions:
Discussion:
Q: Would bi-directional alliesthesia also be observed if a non-nutritive solution were used?
A: With mannitol we see the same results, so I suspect that a non-nutritive substance would yield similar outcomes.
Q: Since weight-loss abolished alliesthesia, wouldn’t you expect to see increased desire for sweet solutions in starved subjects?
A: Yes; we have considered the possibility that our scale (for measuring desire) may have had a ceiling effect.
Q: What is the mechanism responsible for changing alliesthesia?
A: The vagus is involved, and we believe there are neural receptors in the intestinal lumen that mediate post-ingestive alliesthesia in rats.
Q: How is alliesthesia changed, if at all, by over-feeding an animal?
A: The animals become sick.
Q: Have you tested whether or not alliesthesia differs if a two-bottle preference test is used?
A: No, but the results might be similar to those observed when an organism exhibits sensory-specific satiety.
Q: Since hoarding is inversely proportional to body weight, will hoarding continue until an individual achieves a new set point?
A: Yes.
Q: In your studies with cold exposure, how long does it typically take before the cold temperature extinguishes a food-pellet’s palatability?
A: the food pellet keeps its palatability, but the animal does not judge it pleasant enough to override the unpleasantness of the ambient cold. Such a judgment is instantaneous.
Q: Does alliesthesia also work with proteins and fats?
A: Yes, but with these macronutrients the alliesthesia is not as ‘clean’ as those observed with glucose.
Q: Do you think your findings would be similar if you studied foods having an acquired palatability (e.g., wine or cheese), as opposed to those that are intrinsically pleasurable (such as sweet solutions) to most organisms?
A: A correct answer is ‘I don't know’, but we may speculate from our own sensations during satiation…
Supported in part by Merck Research, Glaxosmithkline and
The New York Obesity Research Center, St. Luke’s-Roosevelt Hospital.