Behavioral Regulation of the Milieu Interne in Man and Rat

Abstract

In regulating the internal homeostatic environment mammals, by necessity, employ behavioral strategies that differ from the tactics used in coping with contingencies in the external environment. When an animal consumes a meal, the palatability of that meal is automatically adjusted in accordance with the ultimate internal effects of that meal. If the meal causes toxicosis, the animal acquires an aversion for the taste of the meal; conversely, if recuperation follows ingestion of the meal, the taste of that meal is enhanced. Unlike the learning that occurs when externally referred visual and auditory signals are followed by punishment in the form of peripheral pain or reward in the form of food in the mouth, conditioning to the homeostatic effects of food can occur in a single trial and rarely requires more than three to five trials, even though the ultimate effects of the meal are delayed for hours. Paradoxically, the animal need not be aware of the ultimate internal effect in the same sense that it is aware of external contingencies. For example, an aversion can be acquired even if the animal is unconscious when the agent of illness is administered. Thus, the way in which food-effects are stored in memory may be fundamentally different from the way in which memories of specific time-space strategies devised for external contingencies are stored. This separation of function is indicated by limbic lesions which disrupt conditioning to a buzzer that is followed by shock and facilitate conditioning to a taste that is followed by illness. Operationally speaking, one can describe both aversion conditioning and buzzer-shock conditioning in the spacetime associationistic terms of classical conditioning. However, psychologically speaking, one must realize that in aversion conditioning the animal does not act as if it were acquiring an "if-then" strategy. It acts as if a hedonic shift, or a change in the incentive value of the flavor were taking place. Such hedonic shifts are critical in regulation of the internal milieu. When an animal is in need of calories, food tends to be more palatable; as the caloric deficit is restored, food becomes less palatable. If the animal's body temperature is below optimum, a warm stimulus applied to the skin is pleasant. When body temperature is too high, the converse is true. In this way, homeostatic states monitored by internal receptors produce changes in the incentive values of external stimuli sensed by the peripheral receptors, and guide feeding behavior. In mammals at least, the gustatory system, which provides sensory control of feeding, sends fibers to the nucleus solitarius. This brainstem relay station also receives fibers from the viscera and the internal monitors of the area postrema. Ascending fibers bifurcate at the level of the pons and project toward the feeding areas of the hypothalamus and the cortex. The olfactory system which primarily projects to the limbic system does not play a primary role in adjusting food incentives. Rather, it plays a secondary role in the acti...

Food preferences and aversions in human health and nutrition: how can pigs help the biomedical research?

“…A conditioned stimulus (CS) is associated with an unconditioned stimulus (US). In the case of conditioned food preference and aversion, animals come to consume or avoid a food (CS) that produces positive or negative post-ingestive symptoms (US), respectively (Pavlov, 1960;Garcia et al, 1974).…”

“…This ability to learn to avoid potentially toxic foods has been demonstrated in numerous animal species, from invertebrate to humans (for a review, see Bernstein, 1999;Paradis and Cabanac, 2004). Indeed, food aversion has been described in a variety of mammals, in addition to humans (Garcia et al, 1974; Bellisle, 1999;Ravasco, 2005; Bellisle, 2006) or rats (Yasoshima et al, 2000;Ferreira, 2004), and in livestock species such as horses, sheep and cattle (Houpt et al, 1990;Burritt and Provenza, 1996;Halaweish et al, 2002;Ginane and Dumont, 2006;Pfister et al, 2007), and also in birds (Skelhorn and Rowe, 2006;Halpin et al, 2008;Skelhorn et al, 2008) and reptile species (Terrick et al, 1995;Paradis and Cabanac, 2004). Experimentally induced food aversions are frequently conducted by an intragastric or an intraperitoneal injection of lithium chloride, an emetic substance known to induce visceral malaise.…”

“…Establishment of food selection implies that, during its first experience with food, the organism memorizes the sensorial characteristics of the food (e.g. taste, odour, texture and visual cues) and the postingestive consequences of its ingestion, and associates these food characteristics with these consequences (Garcia et al, 1974;Sclafani, 2001;Ferreira, 2004). This regulation of food choices requires learning and memory capacities (Bernstein, 1999;Houpt, 2000;Welzl et al, 2001), which enable the animal to adapt its feeding behaviour towards a novel food.…”

“…non-caloric sweet taste) to induce an oral-hedonic reinforcement, whereas flavour-nutrient association was achieved by pairing food ingestion (CS) with an intragastric or an intraperitoneal injection (US) of energy (e.g. glucose or fructose) to induce positive post-ingestive consequences.Considered as forms of classical conditioning, food preference and aversion learning have certain features in common: they are extremely robust and can be acquired in a single learning trial for a novel food, that is, after only one pairing of CS to US (Garcia et al, 1974; Bellisle, 2006;Myers, 2007). Significant aversions also develop to the CS despite long delays between exposure to the CS and US (Garcia et al, 1966).…”

Anticipatory physiological regulation in feeding biology: Cephalic phase responses

“…Conditioned taste preferences and conditioned taste aversions provide strong proof that responses to food cues can be learned and modified. For example, most animals readily learn to avoid foods that render them ill; a special visceral learning linked to food ingestion (Garcia et al, 1974;Rozin, 1976). Bait shyness linked to poison is well known and is an important adaptation (Richter, 1953).…”