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Carbohydrate taste preferences in rats: Glucose, sucrose, maltose, fructose and polycose compared
Physiology&Behavior,Vol. 40, pp. 563-568. Copyright©Pergamon Journals Ltd., 1987. Printed in the U.S.A.
0031-9384/87 $3.00 + .00
Carbohydrate Taste Preferences in Rats: Glucose, Sucrose, Maltose, Fructose and Polycose Compared ANTHONY
SCLAFANI ~ AND STEVEN
MANN
Department o f Psychology, Brooklyn College and the Graduate School City University o f New York, Brooklyn, N Y 11210 R e c e i v e d 9 M a r c h 1987 SCLAFANI, A. AND S. MANN. Carbohydrate taste preferences in rats: Glucose, sucrose, maltose, fructose and Polycose compared. PHYSIOL BEHAV 40(5)563-568, 1987.--The taste preferences of adult female rats for solutions of five different carbohydrates were evaluated using brief (3-rain) two-bottle preference tests. At the lowest concentration tested (0.03 molar) the order of preference was Polycose > maltose > sucrose > glucose = fructose. Whereas at the highest concentrations tested (0.5 or 1.0 molar) the preference order was sucrose > maltose ~> Polycose > glucose > fructose. Thus, at low concentrations starch-derived polysaceharides (Polycose) are more palatable to rats than are sugars. These f'mdings are consistent with the hypothesis that rats have separate taste receptors for sugars and for starch-derived polysaccharides. The fact that maltose is the most preferred sugar at low concentrations is attributed to its stimulation of "polysaccharide" taste receptors. Sugar
Polysaccharides
Sweet taste
Polysaccharide taste
have used the sham-feeding preparation, i.e., a rat with an esophageal or gastric fistula, to eliminate the influence of postingestive factors in short-term one-bottle tests [11,29]. While these are useful techniques in assessing the relative palatability of different sugars, they do not provide direct measures o f sugar preference. A limitation of both procedures is that at high concentrations rats may ingest sugar solutions at maximum rates such that the relative palatability of different sugars cannot be distinguished. F o r example, rats lick at similar rates for concentrated sucrose and glucose solutions during brief tests [3] although sucrose is preferred to glucose in two-solution tests [2]. Furthermore, recent findings indicate that even at low concentrations the rat's shamfeeding response in one-bottle tests does not always reflect its taste preference as measured in two-choice tests [12]. Based on the results o f experiments in which rats were given the choice of two or more sugar solutions, the only certain fact is that sucrose is preferred to glucose [2, 20, 26]. The relative palatability o f fructose and maltose is less clear. One study reported a nonsignificant preference for glucose over fructose in a 24-hr three choice test (water was the third choice), but yet the authors concluded, based on the results of short-term one-bottle tests, that rats find fructose slightly sweeter than glucose [2]. Soulairac [27] claimed that maltose was the most preferred sugar based on the results of a 24-hr
T H E hedonic response of rats to the sweet taste of sugars has been the subject of considerable research [18]. Yet, the rat's preference for different sugars (e.g., sucrose, glucose, maltose, and fructose) has not been clearly established. This is because relatively few studies have directly measured sugar preference using choice tests. Instead most experiments have inferred sugar preference using short-term (10-60 min) single-bottle tests or long-term (24 hr) sugar solution vs. water tests, e.g., [4, 14, 17]. The results o f these tests, though, do not necessarily reflect the rat's " t r u e " preference for different sugars, see [15]. F o r example, at some concentrations rats consume more glucose than sucrose in short-term one-bottle tests, and display a greater preference for glucose over water than for sucrose over water in 24-hr two-bottle tests [2]. These findings would appear to indicate that glucose is more palatable than sucrose but in two-solution choice tests rats prefer sucrose to glucose over a wide range of concentrations [2]. A major problem with short- and long-term tests as measures of taste preference is that the amount of sugar consumed in these tests is determined by the postingestive as well as the taste properties of the sugars. To minimize the influence of postingestive factors some studies have used one-bottle tests of brief duration (e.g., 3 min) to assess the relative palatability o f different sugars [3]. Other studies
IA prelimina~ report of this experiment was presented by S. Mann and A. Sclafani at the Eastern Psychological Association Meeting, 1987 in Arlington, VA. This research was supported by grants from the National Institutes of Health (DK-31135) and the Faculty Research Award Program of the City University of New York. Polycose was generously supplied by Ross Laboratories. ZRequests for reprints should be addressed to Dr. Anthony Sclafani, Department of Psychology, Brooklyn College, Brooklyn, NY 11210.
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four-choice test conducted with maltose, glucose, sucrose, and lactose. The results of Soularic's experiment are ambiguous, however, in that the mean intakes of the maltose, glucose and sucrose solutions were very similar; also sugar preference was determined only at one, non-equimolar concentration (10%). In a more recent study Hammer [6] reported that rats preferred sucrose to maltose at concentrations of 8% to 32% but showed no preference at 4%. Other indirect measures of preference (lick rates, preference thresholds, sugar vs. water preferences, sham-feeding intakes) indicate that maltose may be the most palatable sugar at low concentrations [9, 16, 17]. In addition to their well-known taste for sugars, recent experiments performed in our laboratory indicate that rats are very attracted to the taste of starch and starch-derived polysaccharides. In fact, rats were found to prefer polysaccharide solutions (Polycose) to maltose solutions at concentrations of 0.0125 to 0.2 M [19]. Polycose was also preferred to sucrose at low concentrations, but sucrose was preferred at higher concentrations [19]. These observations were quite surprising since it has long been assumed that polysaccharides are relatively tasteless to rats as they are to humans [7, 8, 10, 25]. Based on their behavioral response, Sclafani I18] hypothesized that rats have separate taste receptors for sugars and for starch-derived polysaccharides. However, much remains to be learned about the polysaccharide taste responsivity of rats. In particular, the relative preference of polysaccharides compared to sugars other than maltose and sucrose has not been explored in detail. In order to clarify the rat's preference for different sugars, and to further specify its preference for sugars and polysaccharides, the present study systematically compared the intake of sucrose, glucose, maltose, fructose and Polycose solutions using two-solution choice tests. To minimize the influence of postingestive factors the preference tests were of brief duration (3-min). METHOD
Subjects Twenty-four adult female rats (CD Strain, Charles River Breeding Laboratories, Wilmington, MA) were used. The animals were housed individually in wire mesh cages in a vivarium maintained at 21°C and under a 12:12 hr light-dark cycle. Purina Chow (No. 5001) and tap water were available ad lib.
Apparatus Preference tests were conducted in eight cages similar to the animals' home cages kept in a room adjacent to the vivarium. Test solutions were available through stainless steel drinking tubes attached to 50 ml burets graduated in 0.1 ml units. The burets were mounted on a device that automatically positioned the drinking tubes into the cages at the start of the test session and retracted them at the end of the session.
Test Solutions The test solutions were prepared using the following saccharides: sucrose (commercial grade), glucose (Bio Serv, Frenchtown, NJ), maltose (maltose hydrate, 98% pure; Sigma Chemciai Co., St. Louis, MO), fructose (Sigma Chemical), and Polycose (Ross Laboratories, Columbus, OH). Sugar solutions were tested at 0.03, 0.1, 0.31 and 1.0 molar
concentrations, which represent 0.5 log steps. In the sugar vs. Polycose tests 0.03, 0.1, 0.31, and 0.5 molar concentrations were used. A 0.5 M rather than 1.0 M concentration was used in these latter tests because of the high molecular weight of Polycose (1000). All saccharide solutions were prepared using tap water.
Procedure The animals were divided into four subgroups (n=6 each) equated for body weight (mean weight=321 g). Each subgroup was initially trained and tested with a specific pair of sugars, i.e., sucrose vs. maltose, maltose vs. fructose, glucose vs. fructose, or sucrose vs. fructose. The rats were first familarized with their respective sugars by giving them 10 ml of each sugar (0.31 M concentration), on alternate days, in their home cage for four days. During the next 7 days the rats were given access to the sugars in the test cages during training sessions that were initially 15-min, but then reduced to 10-min and 5-min. The type of sugar and the right-left position of the drinking tube were alternated daily. During the next training session the rats were presented with two solutions: a 0.03 and 0.31 M solution of the same sugar. One concentration was presented for 30 sec and then the other was presented for 30 sec after which both concentrations were presented for 5-min. This procedure was repeated the next day except that the rat's alternate training sugar was used, the position of the low and high concentrations were reversed, and the final access period was shortened to 3-min. By the end of the training period the rats rapidly commenced drinking when the drinking tubes were inserted into the test cages, and they displayed a preference for the 0.31 M concentration over the 0.03 M concentration. During the test phase of the experiment the rats were given simultaneous access to their two training sugars at concentrations of 0.03, 0.1, 0.31, and 1.0 M (e.g., 0.03 M sucrose vs. 0.03 M maltose, 0.1 M sucrose vs. 0.1 M maltose, etc). The two-choice tests were 3-min in duration and each concentration was tested for two days, with the rightleft position of the two saccharides alternating each day. The rats were then retested for another 8 sessions (2 sessions at each of 4 concentrations) using a different pair of sugars. Thus, two subgroups, i.e., 12 rats, were tested with each of the four original pairs of sugars. Following a similar procedure, the rats were next tested with the following pairs of saccharides: sucrose vs. glucose, maltose vs. glucose, Polycose vs. sucrose, Polycose vs. maltose, Polycose vs. fructose and Polycose vs. glucose. In the comparisons involving Polycose the highest concentration tested was 0.5 M (e.g., 0.5 M sucrose vs. 0.5 M Polycose).
Statistical Analysis Solution intakes during the two tests at each concentration were averaged. Using these data, the results of the individual two-choice preference tests were evaluated using two-way analysis of variance (saccharide type and concentration). Following a significant saccharide by concentration interaction saccharide intakes at each concentration were evaluated using simple main effects tests at the 0.05 level of significance. In addition to this statistical analysis, the results of the individual preference tests were summarized by expressing the intakes of each saccharide as a percentage of total solution intake at each concentration. RESULTS
Tables 1 and 2 present the saccharide solution intakes
CARBOHYDRATE TASTE PREFERENCES
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TABLE 1 MEAN (_+SE)SUGARSOLUTIONINTAKE (mi) DURING 3-MIN, TWO-BOTTLEPREFERENCETESTS WITH SUCROSE, MALTOSE, FRUCTOSEAND GLUCOSE Molar Concentration Saccharides Sucrose Glucose Sucrose Fructose Sucrose Maltose
Molar Concentration
0.03
0.1
0.31
1.0
Saccharides
1.8" _+0.2 0.4 _0.1
4.8* _+0.4 0.5 _+0.1
5.8* -+0.5 1.0 ±0.3
4.6* _+0.4 2.0 _+0.3
Glucose
1.4" _+0.3 0.2 _+0.04
3.2* _+0.2 0.3 _+0.1
5.2* _+0.4 0.8 ±0.2
3.6* _+0.5 2.1 _+0.4
1.I* _+0.3 2.7 _+0.5
2.3 _+0.3 2.7 _+0.4
3.7 _+0.5 2.8 ±0.5
4.1" _+0.5 2.2 _+0.4
Fructose Maltose Glucose Maltose Fructose
0.03
0.1
0.31
1.0
0.4 _+0.1 0.3 _+0.1
0.2 _+0.1 0.7 _+0.3
1.0 _+0.2 1.8 _+0.5
2.9* _+0.7 1.5 _+0.5
3.5* _+0.3 0.3 _+0.1
4.9* -+0.4 0.3 -+0.1
5.8* _+0.5 0.8 _+0.3
4.4* _+0.5 2.4 _+0.5
2.7* -+0.5 0.2 _+0.04
4.0* _+0.2 0.5 -+0.1
4.0* _+0.4 1.7 _+0.3
3.6* _+0.5 2.0 _+0.4
*Significant difference, p<0.05 or lower.
TABLE 2 MEAN (-+SE) SACCHARIDESOLUTIONINTAKE (ml) DURING 3-MIN. TWO-BOTTLEPREFERENCETESTS WITH POLYCOSE AND SUGARS Molar Concentration Saccharides Polycose Sucrose Polycose Glucose
Molar Concentration
0.03
0.1
0.31
0.5
Saccharides
0.03
0.1
0.31
0.5
3.9* _+0.6 1.2 _+0.3
3.7 _+0.7 3.1 _+0.6
3.8 ___0.4 2.9 _+0.4
1.9* _+0.4 4.1 _+0.6
Polycose
4.8* _+0.5 0.6 _+0.1
5.2* _+0.6 1.2 _+0.4
4.2 _+0.7 2.7 _+0.8
2.6 _+0.5 3.3 _+0.6
4.8* _+0.3 0.4 _+0.1
6.2* _+0.4 0.3 _+0.1
6.3* ±0.5 0.4 ±0.1
4.3* _+0.4 0.4 _+0.1
Polycose
4.9* _+0.4 0.5 _+0.1
6.0* _+0.4 0.3 _+0.1
5.6* _+0.4 0.7 _+0.4
4.2* _+0.6 0.8 _+0.2
Maltose
Fructose
*Significant difference, p<0.05 or lower.
from the 10 preference tests. Analysis of these data revealed that for each test there was a significant saccharide by concentration interaction (p <0.05 or lower). In most of the tests the main effects o f saccharide and/or concentration were also significant but these main effects are not further discussed. With respect to the sugars, the rats reliably preferred sucrose to glucose and sucrose to fructose at all concentrations tested. They also consumed (p<0.05) more sucrose than maltose at the highest concentration (1 M), but at the lowest concentration maltose was preferred (/9<0.05) to sucrose. The intakes at the two intermediate concentrations did not differ significantly, although the rats tended to prefer maltose at the 0.1 M, and sucrose at the 0.31 M concentrations. The results of the other sugar tests indicated that the rats significantly (p<0.05) preferred maltose to glucose and to fructose at all concentrations, but that they showed an inconsistent preference for the latter two sugars. That is, the rats displayed no preference at the lowest concentration, a
nonsignificant preference for fructose over glucose at the two intermediate concentrations, and reliable (p<0.05) glucose preference at the highest concentration. With respect to the Polycose versus sugar preference tests (Table 2), the rats reliably preferred Polycose to sucrose at the 0.03 M concentration and displayed nonreliable Polycose preferences at the 0.1 and 0,31 M concentrations. At the 0.5 M concentration, however, sucrose was significantly preferred to Polycose. Given the choice o f Polycose vs. maltose, the rats reliably preferred Polycose at the 0.03 and 0.1 M concentrations, displayed a nonreliable Polycose preference at the 0.31 M concentration, and a nonreliable maltose preference at the 0.5 M concentration. In contrast, Polycose was preferred (p<0.05) to glucose and to fructose at all concentrations tested. The results of the preference tests are graphically depicted in Fig. 1 in terms o f percent o f total intake. The five panels o f the figure show the preference o f each saccharide relative to the other four saccharides; values greater than
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S C L A F A N I A N D MANN
GLUCOSE PREFERENCE
FRUCTOSE PREFERENCE
LEGEND [J--E] FRUCTOSE (F)
tO0
[
80
I..d "'¢"
/? To
60
4.0
~7
20
~Z U.I
o
G o/
T
[3F
L
I"X .-
/1
t'
I
0--0
GLUCOSE
(G)
V-v
SUCROSE
(S)
A--A
MALTOSE
(M)
f
,@-41, POLYCOSE 0.03
0.1
0,31
0.03
1.0
SUCROSE PREFERENCE rY
0.1
0,31
(P)
1.0
MALTOSE PREFERENCE
POLYCOSE PREFERENCE
1oo
+/t
20 0 ~
~ 0.03
0.1
0.31
1.0
MOLAR
0.03
~ 0.1
0.31
1.0
0.03
0.1
0.31
1.0
CONCENTRATIONS
FIG. 1. Mean (_+BE) percent saccharide intake as a function of molar concentration. Each panel presents the intake of a specific saccharide expressed as a percent of total saccharide intake for each of the four preference test series involving that saccharide. Data points greater than 50% indicate that the saccharide specified at the top of the panel was preferred in the two-bottle test, whereas data points less than 50% indicate that the alternate saccharide was preferred.
50% indicate that the saccharide specified at the top of the panel was preferred, while values less than 50% indicates that the alternate saccharide was preferred. The figure clearly shows that glucose and fructose were less preferred than sucrose, maltose, and Polycose at all concentrations tested. With the latter three saccharides, the figure shows that the preference was concentration dependent. Thus, realtose and Polycose were preferred to sucrose at low concentrations but sucrose was preferred at high concentrations. In addition, Polycose was preferred to maltose at low but not at high concentrations. DISCUSSION
This experiment assessed the rat's preference for five different saccharides: four sugars and one polysa¢charide mixture. The results revealed that the preference profile for these sa¢charides varies as a function of concentration. That is, as summarized below, at low molar concentrations the rats preferred Polycose, maltose, and sucrose, in that order, whereas at high molar concentrations they preferred sucrose, maltose, and Polycose, in that order. The rats' preference for glucose and fructose also varied somewhat as a function of concentrations, but these two sugars were always less preferred than sucrose, maltose and Polycose.
Concentration 0.03 M 0.10 M 0.31 M 0.5 or 1 M
Saccharide Preference Profile Polycose > Maltose > Sucrose > Glucose = Fructose Polycose ~> Maltose ~> Sucrose > Fructose t> Glucose Polycose ~> Sucrose ~> Maltose > Fructose ~> Glucose Sucrose > Maltose ~> Polycose > Glucose > Fructose
The present f'mdings confirm and extend the results obtained in previous studies. With respect to the sugars, it is well documented that sucrose is preferred to glucose over a wide range of concentrations [2, 20, 26]. Apparently, only one study has compared the rat's preference for glucose and fructose [2], and although a 24 hr rather than a 3-rain test period was used in this earlier study, the results are similar to those obtained in the present experiment. That is, the rats showed an inconsistent, and nonreliable preference for glucose over fructose, particularly at the higher molar concentrations tested [2]. The relative preference for sucrose and fructose does not seem to have been previously examined, but the present f'mding that rats strongly preferred sucrose to
CARBOHYDRATE TASTE P R E F E R E N C E S fructose is consistent with previous observations that rats prefer sucrose to glucose [2, 20, 26], and show no reliable preference between glucose and fructose [2]. The maltose preference of rats has been the subject of considerable interest [16], but only one study has directly compared the rat's preference for maltose and sucrose over a range of concentrations [6]. Using a two-choice 30-min preference test as well as a two-choice bar-pressing task, Hammer [6] reported that rats showed no preference with a 4% concentration but preferred sucrose to maltose at concentrations of 8%, 16%, 24% and 32%. Consistent with these findings, the present experiment found no preference for sucrose or maltose at 0.1 M (~3.5%), a nonreliable sucrose preference at 0.31 M (~ 11%), and a significant sucrose preference at 1 M (~35%). However, at the lowest concentration tested (0.03 M ~ l % ) the rats reliably preferred maltose to sucrose during the 3-rain two-choice test. This latter f'mding is compatible with previous results obtained with one-bottle tests, sham-feeding tests, and sugar vs. water preference tests which indicated that maltose is more palatable than sucrose at low concentrations [9, 16, 17]. In addition, the present results show that rats prefer maltose to glucose and fructose over a wide range of concentrations. The present results provide additional information concerning the rat's recently discovered taste preference for starch-derived polysaccharides [19]. Consistent with our previous observations [12, 19, 21], the rats preferred Polycose to sucrose at low concentrations, but preferred sucrose to Polycose at high concentrations. The preference for Polycose over maltose at low to moderate concentrations also confirms previous findings as does the lack of a reliable preference at high concentrations [ 19]. The present study further revealed that rats given the choice between Polycose and glucose or fructose display a robust Polycose preference over a wide range of concentrations. It is important to note that the saccharides are ordered here based on taste preference and not on sweetness. In fact, recent evidence indicates that there are qualitative differences in the tastes of the saccharides tested in this experiment [13]. That is, the results of conditioned taste aversion experiments suggest that Polycose produces a taste sensation in rats that differs from the taste of sucrose and fructose. Maltose, at lower concentrations at least, seems to taste more like Polycose than like sucrose, whereas glucose appears to have both Polycose-like and sucrose-like tastes. Based on these and other findings, Sclafani and Nissenbaum [13,18] hypothesized that rats have two different carbohydrate taste receptors, one being a sugar receptor with glucose and fructose subsites that is most responsive to sucrose (a glucose-fructose disaccharide), the other being a polysaccharide receptor with multiple glucose subsites that is most responsive to starch-derived polysaccharides (oligosaccharides in particular, see [22]). According to this hypothesis, maltose (a glucose-glucose disaccharide), because it has a chemical structure similar to that of starch-derived polysaccharides, stimulates the polysaccharide receptor which accounts for its taste similarity to Polycose (see below). The two-carbohydrate taste model provides a framework to explain the present results, in particular, why the rat's preference for Polycose and sucrose varies as function of concentration. This latter finding would not be predicted if Polycose and sucrose stimulated the same taste receptors, but can be accounted for by a two-taste model assuming that
567 the response properties of the two taste systems differ. Evidence that the two carbohydrate taste systems do differ in important respects is provided by the findings that the rat's preference threshold for Polycose is much lower than their threshold for sucrose, that rats "satiate" to the taste of Polycose at a slower rate than they satiate to the taste of sucrose, and that Polycose and sucrose intakes are differentially affected by selective gustatory nerve transections [12, 23, 28]. The present findings along with other recent results [19,21] further indicate that the palatability-concentration functions of the polysaccharide and sucrose taste systems differ. That is, at low concentrations polysaccharides are more palatable than sucrose, but as concentration increases sucrose palatability increases at a faster rate than does polysaccharide palatability such that at high concentrations sucrose is preferred to polysaccharides. In addition to displaying a concentration-dependent reversal in preference from Polycose to sucrose, the rats in this experiment displayed a concentration-dependent preference reversal from maltose to sucrose (see Fig. 1). The similarity in the rat's response to Polycose and maltose vis-a-vis sucrose provides additional support for the hypothesis [ 13] that maltose activates the polysaccharide taste system. However, a preference shift was also observed in the Polycose versus maltose test, i.e., the rats preferred Polycose at low concentrations, but not at high concentrations. It is possible that the relative effectiveness of Polycose as a polysaccharide taste stimulus vis-a-vis maltose is reduced at high concentrations by the viscosity of the Polycose solution; at equimolar concentrations Polycose is more viscous than maltose, and solution viscosity is known to affect taste perception in humans [1]. Another possibility is that with increasing concentrations, maltose, in addition to stimulating polysaccharide receptors, also stimulates sucrose receptors, and the sweet taste component of maltose may improve its palatability relative to Polycose. Consistent with this interpretation, previous findings demonstrate that rats prefer saccharin-sweetened Polycose to unsweetened Polycose [24,25]. Much remains to be learned about the rat's gustatory response to carbohydrates, but the evidence reviewed above and other recent findings strongly indicate that rats have two different carbohydrate taste systems, see [18]. The available data suggests that the order of effectiveness of the different saccharides in stimulating the sweet taste system is sucrose > fructose /> glucose > maltose, and in stimulating the polysaccharide taste system is Polycose > maltose > glucose [18]. These response profiles, combined with the different thresholds and concentration-palatability functions of the polysaccharide and sucrose taste systems can account for the taste preferences observed in the present experiment. Furthermore, the rat's taste preference for polysaccharides, which humans lack [5], explains why the rat's order of preference for sucrose, fructose, glucose, maltose, and Polycose differs substantially from that of humans. Finally, the limitations of the present findings should be noted. The two-choice tests were brief in duration and therefore the observed preferences represent the rat's initial hedonic response to the saccharides. With longer test periods (i.e., 24-hr/day) different results may be obtained since postingestive factors can modulate the rat's preferences for different saccharides (see [21]). Also, only female rats were tested in the present study, and male rats may differ somewhat from females in their saccharide preferences [21].
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REFERENCES 1. Arabic, P. and H. R. Moskowitz. The effects of viscosity upon perceived sweetness. Percept Psychophysics 9: 410--412, 1971. 2. Cagan, R. H. and O. Maller. Taste of sugars: Brief exposure single-stimulus behavioral method. J Comp Physiol Psyehol 87: 47-55, 1974. 3. Davis, J, D. The effectiveness of some sugars in stimulating licking behavior in the rat. Physiol Behav 11: 3%45, 1973. 4. Emits, T. and J. D. Corbit. Taste as a dipsogenic stimulus. J Comp Physiol Psychol 83: 27-31, 1973. 5. Feigin, M. B., A. Sclafani and S. R. Sunday. Species differences in polysaccharide and sugar taste preferences. Neurosci Biobehav Rev 11: 231-240, 1987. 6. Hammer, L. R. Relationship of reinforcement value to consummatory behavior. J Comp Physiol Psychol 66: 667-672, 1968. 7. Hill, W., T. W. Castonguay and G. H. Collier. Taste or diet balancing? Physiol Behav 24: 765-767, 1980. 8. Jacobs, H. L. Some physical, metabolic, and sensory components in the appetite for glucose. Am J Physiol 203: 1043-1054, 1962. 9. Joyner, K., G. P. Smith, R. Shindledecker and C. Pfaffmann. Stimulation of sham feeding in the rat by sucrose, maltose, glucose and fructose. Soc Neurosci Abstr 11: 1223, 1985. 10. Mehiel, R. and R. C. Bolles. Learned flavor preferences based on caloric outcome. Anim Learn Behav 12: 421-427, 1984. 11. Mook, D. G. Oral and postingestional determinants of the intake of various solutions in rats with esophageal fistulas. J Comp Physiol Psychol 56: 645-659, 1%3. 12. Nissenbaum, J. W. and A. Sclafani. Sham-feeding response of rats to Polycose and sucrose. Neurosci Biobehav Rev 11: 215222, 1987. 13. Nissenbaum, J. W. and A. Sclafani. Qualitative differences in polysaccharide and sugar tastes in the rat: A two-carbohydrate taste model. Neurosci Biobehav Rev 11: 187-196, 1987. 14. Noma, A., J. Goto and M. Sato. The relative taste effectiveness of various sugars and sugar alcohols for the rat. Kumamoto Med J 24: 1-9, 1971. 15. Pfaffmann, C. Taste preference and reinforcement. In: ReinJbrcement and Behavior, edited by J. Tapp. New York: Academic Press, 1969, pp. 215-240.
16. Pfaffmann, C. Taste: A model of incentive motivation. In: 7~it Physiological Mechanisms o f Motivation. edited by D. W. Pfaff. New York: Springer-Verlag, 1982, pp. 61-97. 17. Richter, C. P. and K. H. Campbell. Taste thresholds and taste preferences of rats for five common sugars. ,I Nutr 20: 31-46, 1940. 18. Sclafani, A. Carbohydrate taste, appetite, and obesity: An overview. Neurosci Biobehav Rev 11: 131-153, 1987. 19. Sclafani, A. and A, E. Clyne. Hedonic response of rats to polysaccharide and sugar solutions. Neurosci Biobehav Rev 11: 173-180, 1987. 20. Sclafani, A., L. T. Einberg and J. W. Nissenbaum. Influence of saccharin on Polycose, sucrose, and glucose intake and preference in rats. Neurosci Biobehav Rev 11: 197-200, 1987. 21. Sclafani, A., H. Hertwig, M. Vigorito and M. B. Feigin. Sex differences in polysaccharide and sugar preferences in rats. Neurosei Biobehav Rev 11: 241-251, 1987. 22. Sclafani, A., H. Hertwig, M. Vigorito, H. Sloan and B. Kerzner. Influence of saccharide length on polysaccharide appetite in the rat. Nearosci Biobehav Rev 11: 197-200, 1987. 23. Sclafani, A. and J. W. Nissenbaum. Taste preference thresholds for Polycose, maltose and sucrose in rats. Neurosei Biobehav Rev 11: 181-185, 1987. 24. Sclafani, A. and M. Vigorito. Effects of SOA and saccharin adulteration on Polycose preference in rats. Neurosci Biobehav Rev 11: 163-168, 1987. 25. Sclafani, A. and S. Xenakis. Sucrose and polysaccharide induced obesity in the rat. Physiol Behav 32: 16%174, 1984. 26. Shuford, E. H., Jr. Palatability and osmotic pressure of glucose and sucrose solutions as determinants of intake. J Comp Physiol Psychol 52: 150--153, 1959. 27. Soulairac, A. La physiologic d'un comportement: L'appetit glucidique et sa r6gulation neuro-endocrinienne chez les rongeurs. Bull Biol France Belg 81: 247-432, 1947. 28. Vigorito, M., A. Sclafani and M. F. Jacquin. Effects of gustatory deafferentation on Polycose and sucrose appetite in the rat. Neurosci Biobehav Rev 11: 201-209, 1987. 29. Weingarten, H. P. and S. D. Watson. Sham feeding as a procedure for assessing the influence of diet palatability on food intake. Physiol Behav 28: 401-40% 1982.
0031-9384/87 $3.00 + .00
Carbohydrate Taste Preferences in Rats: Glucose, Sucrose, Maltose, Fructose and Polycose Compared ANTHONY
SCLAFANI ~ AND STEVEN
MANN
Department o f Psychology, Brooklyn College and the Graduate School City University o f New York, Brooklyn, N Y 11210 R e c e i v e d 9 M a r c h 1987 SCLAFANI, A. AND S. MANN. Carbohydrate taste preferences in rats: Glucose, sucrose, maltose, fructose and Polycose compared. PHYSIOL BEHAV 40(5)563-568, 1987.--The taste preferences of adult female rats for solutions of five different carbohydrates were evaluated using brief (3-rain) two-bottle preference tests. At the lowest concentration tested (0.03 molar) the order of preference was Polycose > maltose > sucrose > glucose = fructose. Whereas at the highest concentrations tested (0.5 or 1.0 molar) the preference order was sucrose > maltose ~> Polycose > glucose > fructose. Thus, at low concentrations starch-derived polysaceharides (Polycose) are more palatable to rats than are sugars. These f'mdings are consistent with the hypothesis that rats have separate taste receptors for sugars and for starch-derived polysaccharides. The fact that maltose is the most preferred sugar at low concentrations is attributed to its stimulation of "polysaccharide" taste receptors. Sugar
Polysaccharides
Sweet taste
Polysaccharide taste
have used the sham-feeding preparation, i.e., a rat with an esophageal or gastric fistula, to eliminate the influence of postingestive factors in short-term one-bottle tests [11,29]. While these are useful techniques in assessing the relative palatability of different sugars, they do not provide direct measures o f sugar preference. A limitation of both procedures is that at high concentrations rats may ingest sugar solutions at maximum rates such that the relative palatability of different sugars cannot be distinguished. F o r example, rats lick at similar rates for concentrated sucrose and glucose solutions during brief tests [3] although sucrose is preferred to glucose in two-solution tests [2]. Furthermore, recent findings indicate that even at low concentrations the rat's shamfeeding response in one-bottle tests does not always reflect its taste preference as measured in two-choice tests [12]. Based on the results o f experiments in which rats were given the choice of two or more sugar solutions, the only certain fact is that sucrose is preferred to glucose [2, 20, 26]. The relative palatability o f fructose and maltose is less clear. One study reported a nonsignificant preference for glucose over fructose in a 24-hr three choice test (water was the third choice), but yet the authors concluded, based on the results of short-term one-bottle tests, that rats find fructose slightly sweeter than glucose [2]. Soulairac [27] claimed that maltose was the most preferred sugar based on the results of a 24-hr
T H E hedonic response of rats to the sweet taste of sugars has been the subject of considerable research [18]. Yet, the rat's preference for different sugars (e.g., sucrose, glucose, maltose, and fructose) has not been clearly established. This is because relatively few studies have directly measured sugar preference using choice tests. Instead most experiments have inferred sugar preference using short-term (10-60 min) single-bottle tests or long-term (24 hr) sugar solution vs. water tests, e.g., [4, 14, 17]. The results o f these tests, though, do not necessarily reflect the rat's " t r u e " preference for different sugars, see [15]. F o r example, at some concentrations rats consume more glucose than sucrose in short-term one-bottle tests, and display a greater preference for glucose over water than for sucrose over water in 24-hr two-bottle tests [2]. These findings would appear to indicate that glucose is more palatable than sucrose but in two-solution choice tests rats prefer sucrose to glucose over a wide range of concentrations [2]. A major problem with short- and long-term tests as measures of taste preference is that the amount of sugar consumed in these tests is determined by the postingestive as well as the taste properties of the sugars. To minimize the influence of postingestive factors some studies have used one-bottle tests of brief duration (e.g., 3 min) to assess the relative palatability o f different sugars [3]. Other studies
IA prelimina~ report of this experiment was presented by S. Mann and A. Sclafani at the Eastern Psychological Association Meeting, 1987 in Arlington, VA. This research was supported by grants from the National Institutes of Health (DK-31135) and the Faculty Research Award Program of the City University of New York. Polycose was generously supplied by Ross Laboratories. ZRequests for reprints should be addressed to Dr. Anthony Sclafani, Department of Psychology, Brooklyn College, Brooklyn, NY 11210.
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S C L A F A N I AND MANN
four-choice test conducted with maltose, glucose, sucrose, and lactose. The results of Soularic's experiment are ambiguous, however, in that the mean intakes of the maltose, glucose and sucrose solutions were very similar; also sugar preference was determined only at one, non-equimolar concentration (10%). In a more recent study Hammer [6] reported that rats preferred sucrose to maltose at concentrations of 8% to 32% but showed no preference at 4%. Other indirect measures of preference (lick rates, preference thresholds, sugar vs. water preferences, sham-feeding intakes) indicate that maltose may be the most palatable sugar at low concentrations [9, 16, 17]. In addition to their well-known taste for sugars, recent experiments performed in our laboratory indicate that rats are very attracted to the taste of starch and starch-derived polysaccharides. In fact, rats were found to prefer polysaccharide solutions (Polycose) to maltose solutions at concentrations of 0.0125 to 0.2 M [19]. Polycose was also preferred to sucrose at low concentrations, but sucrose was preferred at higher concentrations [19]. These observations were quite surprising since it has long been assumed that polysaccharides are relatively tasteless to rats as they are to humans [7, 8, 10, 25]. Based on their behavioral response, Sclafani I18] hypothesized that rats have separate taste receptors for sugars and for starch-derived polysaccharides. However, much remains to be learned about the polysaccharide taste responsivity of rats. In particular, the relative preference of polysaccharides compared to sugars other than maltose and sucrose has not been explored in detail. In order to clarify the rat's preference for different sugars, and to further specify its preference for sugars and polysaccharides, the present study systematically compared the intake of sucrose, glucose, maltose, fructose and Polycose solutions using two-solution choice tests. To minimize the influence of postingestive factors the preference tests were of brief duration (3-min). METHOD
Subjects Twenty-four adult female rats (CD Strain, Charles River Breeding Laboratories, Wilmington, MA) were used. The animals were housed individually in wire mesh cages in a vivarium maintained at 21°C and under a 12:12 hr light-dark cycle. Purina Chow (No. 5001) and tap water were available ad lib.
Apparatus Preference tests were conducted in eight cages similar to the animals' home cages kept in a room adjacent to the vivarium. Test solutions were available through stainless steel drinking tubes attached to 50 ml burets graduated in 0.1 ml units. The burets were mounted on a device that automatically positioned the drinking tubes into the cages at the start of the test session and retracted them at the end of the session.
Test Solutions The test solutions were prepared using the following saccharides: sucrose (commercial grade), glucose (Bio Serv, Frenchtown, NJ), maltose (maltose hydrate, 98% pure; Sigma Chemciai Co., St. Louis, MO), fructose (Sigma Chemical), and Polycose (Ross Laboratories, Columbus, OH). Sugar solutions were tested at 0.03, 0.1, 0.31 and 1.0 molar
concentrations, which represent 0.5 log steps. In the sugar vs. Polycose tests 0.03, 0.1, 0.31, and 0.5 molar concentrations were used. A 0.5 M rather than 1.0 M concentration was used in these latter tests because of the high molecular weight of Polycose (1000). All saccharide solutions were prepared using tap water.
Procedure The animals were divided into four subgroups (n=6 each) equated for body weight (mean weight=321 g). Each subgroup was initially trained and tested with a specific pair of sugars, i.e., sucrose vs. maltose, maltose vs. fructose, glucose vs. fructose, or sucrose vs. fructose. The rats were first familarized with their respective sugars by giving them 10 ml of each sugar (0.31 M concentration), on alternate days, in their home cage for four days. During the next 7 days the rats were given access to the sugars in the test cages during training sessions that were initially 15-min, but then reduced to 10-min and 5-min. The type of sugar and the right-left position of the drinking tube were alternated daily. During the next training session the rats were presented with two solutions: a 0.03 and 0.31 M solution of the same sugar. One concentration was presented for 30 sec and then the other was presented for 30 sec after which both concentrations were presented for 5-min. This procedure was repeated the next day except that the rat's alternate training sugar was used, the position of the low and high concentrations were reversed, and the final access period was shortened to 3-min. By the end of the training period the rats rapidly commenced drinking when the drinking tubes were inserted into the test cages, and they displayed a preference for the 0.31 M concentration over the 0.03 M concentration. During the test phase of the experiment the rats were given simultaneous access to their two training sugars at concentrations of 0.03, 0.1, 0.31, and 1.0 M (e.g., 0.03 M sucrose vs. 0.03 M maltose, 0.1 M sucrose vs. 0.1 M maltose, etc). The two-choice tests were 3-min in duration and each concentration was tested for two days, with the rightleft position of the two saccharides alternating each day. The rats were then retested for another 8 sessions (2 sessions at each of 4 concentrations) using a different pair of sugars. Thus, two subgroups, i.e., 12 rats, were tested with each of the four original pairs of sugars. Following a similar procedure, the rats were next tested with the following pairs of saccharides: sucrose vs. glucose, maltose vs. glucose, Polycose vs. sucrose, Polycose vs. maltose, Polycose vs. fructose and Polycose vs. glucose. In the comparisons involving Polycose the highest concentration tested was 0.5 M (e.g., 0.5 M sucrose vs. 0.5 M Polycose).
Statistical Analysis Solution intakes during the two tests at each concentration were averaged. Using these data, the results of the individual two-choice preference tests were evaluated using two-way analysis of variance (saccharide type and concentration). Following a significant saccharide by concentration interaction saccharide intakes at each concentration were evaluated using simple main effects tests at the 0.05 level of significance. In addition to this statistical analysis, the results of the individual preference tests were summarized by expressing the intakes of each saccharide as a percentage of total solution intake at each concentration. RESULTS
Tables 1 and 2 present the saccharide solution intakes
CARBOHYDRATE TASTE PREFERENCES
565
TABLE 1 MEAN (_+SE)SUGARSOLUTIONINTAKE (mi) DURING 3-MIN, TWO-BOTTLEPREFERENCETESTS WITH SUCROSE, MALTOSE, FRUCTOSEAND GLUCOSE Molar Concentration Saccharides Sucrose Glucose Sucrose Fructose Sucrose Maltose
Molar Concentration
0.03
0.1
0.31
1.0
Saccharides
1.8" _+0.2 0.4 _0.1
4.8* _+0.4 0.5 _+0.1
5.8* -+0.5 1.0 ±0.3
4.6* _+0.4 2.0 _+0.3
Glucose
1.4" _+0.3 0.2 _+0.04
3.2* _+0.2 0.3 _+0.1
5.2* _+0.4 0.8 ±0.2
3.6* _+0.5 2.1 _+0.4
1.I* _+0.3 2.7 _+0.5
2.3 _+0.3 2.7 _+0.4
3.7 _+0.5 2.8 ±0.5
4.1" _+0.5 2.2 _+0.4
Fructose Maltose Glucose Maltose Fructose
0.03
0.1
0.31
1.0
0.4 _+0.1 0.3 _+0.1
0.2 _+0.1 0.7 _+0.3
1.0 _+0.2 1.8 _+0.5
2.9* _+0.7 1.5 _+0.5
3.5* _+0.3 0.3 _+0.1
4.9* -+0.4 0.3 -+0.1
5.8* _+0.5 0.8 _+0.3
4.4* _+0.5 2.4 _+0.5
2.7* -+0.5 0.2 _+0.04
4.0* _+0.2 0.5 -+0.1
4.0* _+0.4 1.7 _+0.3
3.6* _+0.5 2.0 _+0.4
*Significant difference, p<0.05 or lower.
TABLE 2 MEAN (-+SE) SACCHARIDESOLUTIONINTAKE (ml) DURING 3-MIN. TWO-BOTTLEPREFERENCETESTS WITH POLYCOSE AND SUGARS Molar Concentration Saccharides Polycose Sucrose Polycose Glucose
Molar Concentration
0.03
0.1
0.31
0.5
Saccharides
0.03
0.1
0.31
0.5
3.9* _+0.6 1.2 _+0.3
3.7 _+0.7 3.1 _+0.6
3.8 ___0.4 2.9 _+0.4
1.9* _+0.4 4.1 _+0.6
Polycose
4.8* _+0.5 0.6 _+0.1
5.2* _+0.6 1.2 _+0.4
4.2 _+0.7 2.7 _+0.8
2.6 _+0.5 3.3 _+0.6
4.8* _+0.3 0.4 _+0.1
6.2* _+0.4 0.3 _+0.1
6.3* ±0.5 0.4 ±0.1
4.3* _+0.4 0.4 _+0.1
Polycose
4.9* _+0.4 0.5 _+0.1
6.0* _+0.4 0.3 _+0.1
5.6* _+0.4 0.7 _+0.4
4.2* _+0.6 0.8 _+0.2
Maltose
Fructose
*Significant difference, p<0.05 or lower.
from the 10 preference tests. Analysis of these data revealed that for each test there was a significant saccharide by concentration interaction (p <0.05 or lower). In most of the tests the main effects o f saccharide and/or concentration were also significant but these main effects are not further discussed. With respect to the sugars, the rats reliably preferred sucrose to glucose and sucrose to fructose at all concentrations tested. They also consumed (p<0.05) more sucrose than maltose at the highest concentration (1 M), but at the lowest concentration maltose was preferred (/9<0.05) to sucrose. The intakes at the two intermediate concentrations did not differ significantly, although the rats tended to prefer maltose at the 0.1 M, and sucrose at the 0.31 M concentrations. The results of the other sugar tests indicated that the rats significantly (p<0.05) preferred maltose to glucose and to fructose at all concentrations, but that they showed an inconsistent preference for the latter two sugars. That is, the rats displayed no preference at the lowest concentration, a
nonsignificant preference for fructose over glucose at the two intermediate concentrations, and reliable (p<0.05) glucose preference at the highest concentration. With respect to the Polycose versus sugar preference tests (Table 2), the rats reliably preferred Polycose to sucrose at the 0.03 M concentration and displayed nonreliable Polycose preferences at the 0.1 and 0,31 M concentrations. At the 0.5 M concentration, however, sucrose was significantly preferred to Polycose. Given the choice o f Polycose vs. maltose, the rats reliably preferred Polycose at the 0.03 and 0.1 M concentrations, displayed a nonreliable Polycose preference at the 0.31 M concentration, and a nonreliable maltose preference at the 0.5 M concentration. In contrast, Polycose was preferred (p<0.05) to glucose and to fructose at all concentrations tested. The results of the preference tests are graphically depicted in Fig. 1 in terms o f percent o f total intake. The five panels o f the figure show the preference o f each saccharide relative to the other four saccharides; values greater than
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S C L A F A N I A N D MANN
GLUCOSE PREFERENCE
FRUCTOSE PREFERENCE
LEGEND [J--E] FRUCTOSE (F)
tO0
[
80
I..d "'¢"
/? To
60
4.0
~7
20
~Z U.I
o
G o/
T
[3F
L
I"X .-
/1
t'
I
0--0
GLUCOSE
(G)
V-v
SUCROSE
(S)
A--A
MALTOSE
(M)
f
,@-41, POLYCOSE 0.03
0.1
0,31
0.03
1.0
SUCROSE PREFERENCE rY
0.1
0,31
(P)
1.0
MALTOSE PREFERENCE
POLYCOSE PREFERENCE
1oo
+/t
20 0 ~
~ 0.03
0.1
0.31
1.0
MOLAR
0.03
~ 0.1
0.31
1.0
0.03
0.1
0.31
1.0
CONCENTRATIONS
FIG. 1. Mean (_+BE) percent saccharide intake as a function of molar concentration. Each panel presents the intake of a specific saccharide expressed as a percent of total saccharide intake for each of the four preference test series involving that saccharide. Data points greater than 50% indicate that the saccharide specified at the top of the panel was preferred in the two-bottle test, whereas data points less than 50% indicate that the alternate saccharide was preferred.
50% indicate that the saccharide specified at the top of the panel was preferred, while values less than 50% indicates that the alternate saccharide was preferred. The figure clearly shows that glucose and fructose were less preferred than sucrose, maltose, and Polycose at all concentrations tested. With the latter three saccharides, the figure shows that the preference was concentration dependent. Thus, realtose and Polycose were preferred to sucrose at low concentrations but sucrose was preferred at high concentrations. In addition, Polycose was preferred to maltose at low but not at high concentrations. DISCUSSION
This experiment assessed the rat's preference for five different saccharides: four sugars and one polysa¢charide mixture. The results revealed that the preference profile for these sa¢charides varies as a function of concentration. That is, as summarized below, at low molar concentrations the rats preferred Polycose, maltose, and sucrose, in that order, whereas at high molar concentrations they preferred sucrose, maltose, and Polycose, in that order. The rats' preference for glucose and fructose also varied somewhat as a function of concentrations, but these two sugars were always less preferred than sucrose, maltose and Polycose.
Concentration 0.03 M 0.10 M 0.31 M 0.5 or 1 M
Saccharide Preference Profile Polycose > Maltose > Sucrose > Glucose = Fructose Polycose ~> Maltose ~> Sucrose > Fructose t> Glucose Polycose ~> Sucrose ~> Maltose > Fructose ~> Glucose Sucrose > Maltose ~> Polycose > Glucose > Fructose
The present f'mdings confirm and extend the results obtained in previous studies. With respect to the sugars, it is well documented that sucrose is preferred to glucose over a wide range of concentrations [2, 20, 26]. Apparently, only one study has compared the rat's preference for glucose and fructose [2], and although a 24 hr rather than a 3-rain test period was used in this earlier study, the results are similar to those obtained in the present experiment. That is, the rats showed an inconsistent, and nonreliable preference for glucose over fructose, particularly at the higher molar concentrations tested [2]. The relative preference for sucrose and fructose does not seem to have been previously examined, but the present f'mding that rats strongly preferred sucrose to
CARBOHYDRATE TASTE P R E F E R E N C E S fructose is consistent with previous observations that rats prefer sucrose to glucose [2, 20, 26], and show no reliable preference between glucose and fructose [2]. The maltose preference of rats has been the subject of considerable interest [16], but only one study has directly compared the rat's preference for maltose and sucrose over a range of concentrations [6]. Using a two-choice 30-min preference test as well as a two-choice bar-pressing task, Hammer [6] reported that rats showed no preference with a 4% concentration but preferred sucrose to maltose at concentrations of 8%, 16%, 24% and 32%. Consistent with these findings, the present experiment found no preference for sucrose or maltose at 0.1 M (~3.5%), a nonreliable sucrose preference at 0.31 M (~ 11%), and a significant sucrose preference at 1 M (~35%). However, at the lowest concentration tested (0.03 M ~ l % ) the rats reliably preferred maltose to sucrose during the 3-rain two-choice test. This latter f'mding is compatible with previous results obtained with one-bottle tests, sham-feeding tests, and sugar vs. water preference tests which indicated that maltose is more palatable than sucrose at low concentrations [9, 16, 17]. In addition, the present results show that rats prefer maltose to glucose and fructose over a wide range of concentrations. The present results provide additional information concerning the rat's recently discovered taste preference for starch-derived polysaccharides [19]. Consistent with our previous observations [12, 19, 21], the rats preferred Polycose to sucrose at low concentrations, but preferred sucrose to Polycose at high concentrations. The preference for Polycose over maltose at low to moderate concentrations also confirms previous findings as does the lack of a reliable preference at high concentrations [ 19]. The present study further revealed that rats given the choice between Polycose and glucose or fructose display a robust Polycose preference over a wide range of concentrations. It is important to note that the saccharides are ordered here based on taste preference and not on sweetness. In fact, recent evidence indicates that there are qualitative differences in the tastes of the saccharides tested in this experiment [13]. That is, the results of conditioned taste aversion experiments suggest that Polycose produces a taste sensation in rats that differs from the taste of sucrose and fructose. Maltose, at lower concentrations at least, seems to taste more like Polycose than like sucrose, whereas glucose appears to have both Polycose-like and sucrose-like tastes. Based on these and other findings, Sclafani and Nissenbaum [13,18] hypothesized that rats have two different carbohydrate taste receptors, one being a sugar receptor with glucose and fructose subsites that is most responsive to sucrose (a glucose-fructose disaccharide), the other being a polysaccharide receptor with multiple glucose subsites that is most responsive to starch-derived polysaccharides (oligosaccharides in particular, see [22]). According to this hypothesis, maltose (a glucose-glucose disaccharide), because it has a chemical structure similar to that of starch-derived polysaccharides, stimulates the polysaccharide receptor which accounts for its taste similarity to Polycose (see below). The two-carbohydrate taste model provides a framework to explain the present results, in particular, why the rat's preference for Polycose and sucrose varies as function of concentration. This latter finding would not be predicted if Polycose and sucrose stimulated the same taste receptors, but can be accounted for by a two-taste model assuming that
567 the response properties of the two taste systems differ. Evidence that the two carbohydrate taste systems do differ in important respects is provided by the findings that the rat's preference threshold for Polycose is much lower than their threshold for sucrose, that rats "satiate" to the taste of Polycose at a slower rate than they satiate to the taste of sucrose, and that Polycose and sucrose intakes are differentially affected by selective gustatory nerve transections [12, 23, 28]. The present findings along with other recent results [19,21] further indicate that the palatability-concentration functions of the polysaccharide and sucrose taste systems differ. That is, at low concentrations polysaccharides are more palatable than sucrose, but as concentration increases sucrose palatability increases at a faster rate than does polysaccharide palatability such that at high concentrations sucrose is preferred to polysaccharides. In addition to displaying a concentration-dependent reversal in preference from Polycose to sucrose, the rats in this experiment displayed a concentration-dependent preference reversal from maltose to sucrose (see Fig. 1). The similarity in the rat's response to Polycose and maltose vis-a-vis sucrose provides additional support for the hypothesis [ 13] that maltose activates the polysaccharide taste system. However, a preference shift was also observed in the Polycose versus maltose test, i.e., the rats preferred Polycose at low concentrations, but not at high concentrations. It is possible that the relative effectiveness of Polycose as a polysaccharide taste stimulus vis-a-vis maltose is reduced at high concentrations by the viscosity of the Polycose solution; at equimolar concentrations Polycose is more viscous than maltose, and solution viscosity is known to affect taste perception in humans [1]. Another possibility is that with increasing concentrations, maltose, in addition to stimulating polysaccharide receptors, also stimulates sucrose receptors, and the sweet taste component of maltose may improve its palatability relative to Polycose. Consistent with this interpretation, previous findings demonstrate that rats prefer saccharin-sweetened Polycose to unsweetened Polycose [24,25]. Much remains to be learned about the rat's gustatory response to carbohydrates, but the evidence reviewed above and other recent findings strongly indicate that rats have two different carbohydrate taste systems, see [18]. The available data suggests that the order of effectiveness of the different saccharides in stimulating the sweet taste system is sucrose > fructose /> glucose > maltose, and in stimulating the polysaccharide taste system is Polycose > maltose > glucose [18]. These response profiles, combined with the different thresholds and concentration-palatability functions of the polysaccharide and sucrose taste systems can account for the taste preferences observed in the present experiment. Furthermore, the rat's taste preference for polysaccharides, which humans lack [5], explains why the rat's order of preference for sucrose, fructose, glucose, maltose, and Polycose differs substantially from that of humans. Finally, the limitations of the present findings should be noted. The two-choice tests were brief in duration and therefore the observed preferences represent the rat's initial hedonic response to the saccharides. With longer test periods (i.e., 24-hr/day) different results may be obtained since postingestive factors can modulate the rat's preferences for different saccharides (see [21]). Also, only female rats were tested in the present study, and male rats may differ somewhat from females in their saccharide preferences [21].
568
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16. Pfaffmann, C. Taste: A model of incentive motivation. In: 7~it Physiological Mechanisms o f Motivation. edited by D. W. Pfaff. New York: Springer-Verlag, 1982, pp. 61-97. 17. Richter, C. P. and K. H. Campbell. Taste thresholds and taste preferences of rats for five common sugars. ,I Nutr 20: 31-46, 1940. 18. Sclafani, A. Carbohydrate taste, appetite, and obesity: An overview. Neurosci Biobehav Rev 11: 131-153, 1987. 19. Sclafani, A. and A, E. Clyne. Hedonic response of rats to polysaccharide and sugar solutions. Neurosci Biobehav Rev 11: 173-180, 1987. 20. Sclafani, A., L. T. Einberg and J. W. Nissenbaum. Influence of saccharin on Polycose, sucrose, and glucose intake and preference in rats. Neurosci Biobehav Rev 11: 197-200, 1987. 21. Sclafani, A., H. Hertwig, M. Vigorito and M. B. Feigin. Sex differences in polysaccharide and sugar preferences in rats. Neurosei Biobehav Rev 11: 241-251, 1987. 22. Sclafani, A., H. Hertwig, M. Vigorito, H. Sloan and B. Kerzner. Influence of saccharide length on polysaccharide appetite in the rat. Nearosci Biobehav Rev 11: 197-200, 1987. 23. Sclafani, A. and J. W. Nissenbaum. Taste preference thresholds for Polycose, maltose and sucrose in rats. Neurosei Biobehav Rev 11: 181-185, 1987. 24. Sclafani, A. and M. Vigorito. Effects of SOA and saccharin adulteration on Polycose preference in rats. Neurosci Biobehav Rev 11: 163-168, 1987. 25. Sclafani, A. and S. Xenakis. Sucrose and polysaccharide induced obesity in the rat. Physiol Behav 32: 16%174, 1984. 26. Shuford, E. H., Jr. Palatability and osmotic pressure of glucose and sucrose solutions as determinants of intake. J Comp Physiol Psychol 52: 150--153, 1959. 27. Soulairac, A. La physiologic d'un comportement: L'appetit glucidique et sa r6gulation neuro-endocrinienne chez les rongeurs. Bull Biol France Belg 81: 247-432, 1947. 28. Vigorito, M., A. Sclafani and M. F. Jacquin. Effects of gustatory deafferentation on Polycose and sucrose appetite in the rat. Neurosci Biobehav Rev 11: 201-209, 1987. 29. Weingarten, H. P. and S. D. Watson. Sham feeding as a procedure for assessing the influence of diet palatability on food intake. Physiol Behav 28: 401-40% 1982.