Acquired sensory preference for protein in diabetic and ... - Springer Link

Physiological Psychology 1974, Vol. 2 (3A) , 344-348

Acquired sensory preference for protein in diabetic and normal rats D.A. BOOTH Department ofPsychology, University ofBirmingham, P. O. Box 363, Birmingham B15 21T, England

Rats made diabetic by injection of streptozotocin, either after adaptation to a cafeteria or some weeks beforehand, increased their intake of protein relative to that of other macronutrients. Preference for an arbitrary odor added to the protein was increased. Normal rats were repeatedly given brief access to either protein-free or protein-containing diet while maintenance chow was withheld for 10 h . They came to prefer the odor-taste combination included in the protein diet to the flavor included in the protein-free diet. It is suggested that, not only in diabetic and protein-deprived rats, but also in normal rats not long since the end of amino acid absorption, the supply of amino acids establishes a relative conditioned attraction for associated food flavors.

One mechanism by which food intake could contribute to the regulation of nitrogen exchange would be the development of a preference for protein-containing foodstuffs when the supply of amino acids to critical tissues was no longer abundant. There is no satisfactory evidence for the existence of an innate protein appetite-that is, a propensity to select those foodstuffs which have sensory qualities naturally allied to protein or free amino acids, independently of previous experience of similar sensory qualities . No such odors , tastes, or textures have been characterized experimentally-let alone a preference for them demonstrated not to arise from associative learning. An alternative is that whatever happen to be the distinctive sensory qualities of a protein-containing foodstuff become discriminative or conditioned stimuli for consumption of or attraction to that food. Rejection of a diet containing a disproportion of amino acids has been repeatedly observed (Harper , Benevenga, & Wohlheuter, 1970), although the behavioral mechanism of this rejection remains to be elucidated (Leung, Larson, & Rogers, 1972). Association of a diet flavor with administration of an amino acid-deficient mixture to protein-deprived rats establishes specific rejection of a protein-free diet having that flavor (Simson & Booth, 1973, 1974). Such learning, or conditioning, is not limited to rejection: administration of a balanced amino acid mixture to protein-deprived rats establishes an acquired preference for an associated flavor (Booth & Simson, 1971; Simson & Booth , 1973, 1974). The present experiments sought to determine whether acquired preferences might play a role in the protein appetite of diabetic rats, and also whether an acquired protein appetite could be established in the intact rat maintained on a complete diet.

EXPERIMENT I Richter, Schmidt, and Malone (1945) examined selection of protein in diabetic rats on the presumption that such animals had to obtain energy from

noncarbohydrate sources. The increased utilization of amino acids during diabetes could also givespecifically to protein some reinforcing power in the establishment of an acquired appetite. Richter et al indeed found increased intake of protein or fat, relative to control rats. However, their data did not distinguish between the appearance of an increased innate propensi ty to select protein-like food in diabetes and the capacity of protein-containing foods to establish an acquired preference for their sensory characteristics. The present experiment introduced rats to a cafeteria before or after induction of diabetes by streptozotocin injection (Rakieten, Rakieten, & Nadkarni , 1963; Booth, 1972), with the cafeteria items sometimes artificially flavored to determine whether protein appetite was established by discriminative conditioning or learning. Method

Animals and Environment. Male rats of a Sprague-Dawley-derived strain bred at the University of Sussex were maintained on Small Animals Diet (Spillers) until introduction to cafeteria feeding for the experiment. Water was always freely available. The rats were housed individually in mesh cages measuring 50 x 25 x 17 em in an air-conditioned room at 21°·23°C. The lighting was on a reverse cycle (dark from 0900 to 2100 h) to which they had been adapted for at least a month before the experiment. Experimental Diabetes. Streptozotocin was injected into the tail vein at a dose of 60 mg/kg, or vehicle was injected in control groups. All streptozotocin-injected animals became glucosuric and polydipsic from the second day after injection, and hyperphagic from tile fourth to seventh days, and remained so indefinitely. Cafeterias. Groups of six rats were placed on cafeteria feeding ad lib, either 1 week before or 3 weeks after tail injection. Each diet of a cafeteria was presented in a 10D-ml beaker clipped to the door of the home cage. A fresh sample was given each day in a changed position on the door. Diet, chow, and water intakes and body weights were measured once each day , at the start of the dark phase of the lighting cycle. The carbohydrate opt ion in the cafeterias was always 60% Jalan dextrine (Laing-National) and 40% granulated sugar (Tate and Lyle). As another option, one set of groups also had a protein-fat mixture, which was calcium caseinate (Casilan, Glaxo) and corn oil (Mazola: Brown and Polson) in equal proportions by weight. Other groups had protein and fat presented as separate options. The protein diet was 40%

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PROTEIN APPETITE technical casein (Sigma), 40% Casilan, and 20% low-glucose maltodextrin (MOOS, Manbre). The fat diet was 35% corn oil, 35% cellulose (CF 12, BOH Chemicals), 20% heavy kaolin (BOH) and 10% potato starch (BOH). Small Animals Diet was presented either simultaneously with the two or three options or alone on days alternating with days on the three options alone. For some groups, the three options were odorized: for each rat, any particular option was consistently given the same odor, and the diet-odor pairings were varied between rats, balancing across the group. The odors were oil of lemongrass (mainly citral), clove oil, and geraniol (Griffin), at a concentration of 10 microliters{100 g, supplemented daily . For odor-preference tests, the odors were incorporated in a mixture in equal proportions of the carbohydrate, protein, and fat options.

Results and Discussion One pair of groups was familiarized with a choice of: a carbohydrate diet, a protein- fat diet, maintenance chow, and water, all ad lib. One group was then made diabetic. Significant hyperphagia (p < .05, Mann-Whitney U test) appeared on the fifth day of diabetes (Fig. 1), confirming an earlier observation (Booth, 1972) . An elevated intake of protein- fat diet appeared earlier (p < .05 from Days 3-4), but a consistent depression of carbohydrate intake, like the hyperhpagia, took several days to develop (p < .05 from Days 15·16 ; Fig. 1). These results are similar to those seen in rats made diabetic by pancreatectomy (Richter et al, 1945) or by alloxan treatment (Vartiainen & Bastman-Heiskanen, 1950) . They differ from those seen I-INJECTION

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Fig. 1. Body weights and food intakes of rats adapted to a choice of water, chow, carbohydrate, and a protein-fat mixture, and then injected with streptozotocin (filled symbols) or with vehicle (open symbols). Total food intake is kcal/day. The intakes of carbohydrate and protein-fat diets are given as a percentage of each rat's total caloric intake per day.

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