The large number of groups and correspondingly large number of possible comparisons provides a challenge for statistical analysis and presentation. To provide some protection from Type II errors, we used a criterion for statistical significance of p < 0.001 unless otherwise noted.
Each of the 14 strain groups initially comprised of 8 male and 8 female rats. However, one male FHH rat died 2 days after arrival and so was excluded from all data analyses. Of the remaining rats, the following died during the course of the experiment: 6 Dahl, 3 FHH, 2 SD, 1 LE, 1 BN, 1 DA, 1 LEW, 1 Noble, and 1 SHR. All WI, BUF, COP, F344, and PVG rats survived. The data of rats that died were included in analyses up to but excluding the test series in which they died. Most of the rats died toward the end of the experiment (3 died during tests with sucrose, 7 died during tests with Polycose). We did not attempt to determine the cause of death, although we note that the two strains with the highest fatality rates (Dahl and FHH) are models of hypertension.
Previous work with SD rats suggests that spillage and evaporation during two-bottle tests is <0.5 mL/d (but see below). We did not attempt to correct for this source of error. However, we kept records of visible spillage. Occasionally during a 48-h test, a bottle fell off a cage, a stopper became dislodged, or there was lots of fluid on the cardboard sheet under a drinking spout. This could be due to a technical error (i.e., the drinking bottle stopper not firmly inserted) or to vigorous activity of the rat (i.e., gnawing at or playing with the drinking spout). The overall incidence of spillage was 0.55% (i.e., 0.60 spills/rat in 109 48-h tests). However, the SHR strain had much higher rates of spillage (3.95%) than did the other groups (all <0.69%). The incidence of spillage was too low to distinguish statistically whether some solutions were spilled more than others but MgCl2, ethanol and citric acid appeared to be the most frequently spilled.
There were large differences between the strains in body weight with the pattern remaining more-or-less constant but the range diverging as the experiment progressed (). Because of the markedly higher body weights of males than females [e.g., at end of experiment, F(1,178) = 1129.2, p <0.00001], we analyzed the body weights of each sex separately. shows differences between the strains for each sex at the end of the experiment.
Body weights of male and female rats of 14 strains
Body weight (BW), food intake (FI) and water intake (WI) of male (left) and female (right) rats of 14 strains
Food intake was measured during three 48-h tests, and the results from each test were similar so, for simplicity, we averaged the three values obtained from each rat together for subsequent analyses. Males ate considerably more food than did females, F(1,195) = 133.9, p < 0.00001, and there was no Strain × Sex interaction, so we analyzed food intakes of each sex separately (see ).
Each of the 17 series of tests began with a 48-h choice between two bottles of water. Total intake from both bottles during these tests provided daily fluid intakes at approximately 2-wk intervals between age 8 – 46 wk. We conducted two analyses on water intakes. First, to allow comparison with food intakes and body weights, we analyzed water intakes during the three tests when food intake was measured (i.e., before the NaCl, saccharin, and capsaicin test series) using the same methods as for food intake (). Second, to analyze the complete set of data, we first conducted an omnibus ANOVA with factors of Strain, Sex, and Series. In this analysis, we omitted the SHR strain because only 3 SHR rats completed all 17 tests without spillage. This revealed strain differences, F(12,174) = 40.4, p < 0.00001, but no effect of Sex and no Strain × Sex interaction. Differences among the strains are shown in [based on averages for each strain, including SHR, F(13,209) = 29.6, p < 0.00001]. There was also a main effect of Test Time, F(16,2784) = 15.1, p<0.00001, and interactions of Test Time with Strain, F(192,2784) = 3.70, p<0.00001, and with Sex, F(16,2784) = 3.46, p < 0.00001. The most obvious source of these effects was low water intake on the first test relative to subsequent tests, but there were other more complex sources as well ().
Water intakes of rats during tests with two bottles of water to drink
Within-subject and within-strain variability in water intakes and preferences
Since each rat was tested with two bottles of water 17 times, we could estimate both within-subject and within-strain variability. We used the standard error of the 17 observations around the mean water intake of each rat to assess within-subject variability. We used the standard error of each rat’s mean around the strain mean to assess within-strain variability. These two sources of variability were generally congruent; that is, individuals that differed the most from test-to-test also tended to belong to the strains with the most between-subject variation (r = 0.78; ). Differences in within-subject variability were assessed using an ANOVA with factors of strain and sex. There were large and significant differences in variability between strains, F(13,195) = 16.0, p < 0.00001 (; ), but no sex differences. Regression analyses revealed strong positive associations between strain mean water intakes and variability (i.e., the more rats drank, the higher the variability; ) indicating that most of the difference in variability among strains could be accounted for by differences in the mean volume consumed. Regressing variability of each rat against mean water intakes eliminated differences among the strains except for the SHR strain, which had higher variability than did the other 13 strains, F(13,195) = 9.45, p < 0.00001. In particular, once differences in intakes were accounted for, there was no difference in variability between outbred and inbred strains.
Mean intakes and variability during tests with two bottles of water to drink
We conducted informal analyses on the within-strain variability of taste preferences for several compounds and in each case found more-or-less the same pattern of results as was obtained with water intakes. A complicating factor was that, in general, the closer the mean preference of a group was to indifference (50%) the larger was the within-strain variation (see on-line material). If this source of variation was taken into account then the SHR strain usually had more variable preferences than did the other 13 strains, which did not differ in variability from each other. Because of space constraints we do not present formal analyses that lead to these generalizations but variability in preferences can be assessed by inspection of the standard errors displayed in figures or directly from supplemental data available on-line.
Relationship between body weight, food intake, and water intake
There are inconsistencies in prior reports due to food and fluid intakes being expressed as absolute values or in relation to body weight [see e.g., (6
)]. To determine whether adjusting for body weight influenced strain differences, the analyses of food and water intake (during the three food intake tests) were repeated using intakes divided by body weights (i.e., grams eaten per kilogram body weight, or milliliters drunk per kilogram body weight). These adjustments appeared to reduce the strain variation associated with food intake but not water intake (). To provide a more direct test, we compared the body weights of each strain and sex with corresponding average food and water intakes during the three tests when food intake was measured. There were moderately strong correlations between food intake and body weight (r = 0.75, p < 0.00001), and between food intake and water intake (r = 0.63, p = 0.0003; ). However, the correlation between body weight and water intake was much weaker and not statistically significant (r = 0.26, p = 0.181; ). This implies that body weight can account for less than 7% of the variation in water intake, so we judged it inappropriate to adjust fluid intakes for body weight.
Scatter plots showing the relationship between body weight, food intake, and water intake in male and female rats of 14 strains
Number of fungiform taste papillae
There were no concerted differences in the number of papillae on the left and right side of the tongue so analyses were conducted based on 4 segments from the base (Segment 1) to tip (Segment 4), with the left and right sides combined. There was a significant three-way interaction between Strain, Sex, and Segment, F(39,504) = 2.201, p = 00007. Analyses of each of the four segments separately revealed that the effect involving sex was confined to the anterior segment and this did not reach the criterion for significance used throughout the rest of this study [strain × sex interaction, F(13,168) = 2.63, p = 0.0022]. LSD post hoc tests showed that in the anterior segment, there were more papillae in SD male than SD female rats (52 ± 6 vs. 35 ± 2, p = 0.0077), and less papillae in Dahl male than Dahl female rats (29 ± 3 vs. 48 ± 5, p = 0.0121). There were no other sex differences in papillae number.
Based on the total number of papillae over all four segments of the tongue, there were significant differences among the strains, F(13,168) = 7.30, p < 0.00001, and no significant differences among the sexes, F(1,168) = 0.90, p = 0.3447, or Strain × Sex interaction, F(13,168) = 1.92, p = 0.0307. Nine strains belonged to the group with the most papillae and eight strains belonged to the group with the least papillae (3 strains belonged to both groups; ). The distribution of high- and low-counts of papillae was similar across the four segments, although minor differences were present (; individual data are available on-line).
Number of fungiform taste papillae on tongue of 14 strains of rats
We generated a correlation matrix to identify relationships between the number of fungiform papillae and preferences for each concentration of each of the 17 compounds tested. There were no correlations significant at the p<0.001 level, although three approached this level of significance: There were negative correlations between the total number of fungiform papillae and preferences for 3.16 mM HCl (r = −0.73), 0.0316 mM QHCl (r = −0.71) and 0.1 mM QHCl (r = −0.70).
Taste solution intakes and preferences
We conducted parallel analyses on each of the four dependent variables involved in two-bottle choice tests: taste solution intake, water intake, total fluid intake, and taste solution preference. As a first step, we used omnibus three-way analyses of variance with factors of Strain, Sex, and Concentration (i.e., 14 strains × 2 sexes × 5 – 8 concentrations) for each taste solution series. All analyses revealed highly significant main effects and interactions involving the Concentration factor but these proved uninteresting except to verify that, for all 17 taste solution series, we chose ranges of concentrations that influenced intakes and preferences. These analyses were complicated by two factors. First, there was sometimes heterogeneity of variance due to basement or ceiling effects at the highest concentrations. Second, missing data were difficult to deal with. The within-subject design necessitated dropping from the entire test series a subject that spilled even once. This led to very few SHR rats in some analyses (see Spillage, above). To avoid these problems and to simplify, we conducted discrete two-way ANOVAs with factors of Strain and Sex for each of the 109 tests of each concentration of each taste solution.
The results of the analyses involving taste solution intakes, water intakes, and total intakes are given in supplemental material
on-line but can be summarized easily. Of the 109 analyses of each type conducted, nearly all had significant strain differences (106 involving taste solution intake, 97 involving water intake, and all 109 involving total intake), there were very few Strain × Sex interactions (6 involving taste solution intake, none involving water intake, and 8 involving total intake), and very few had significant main effects of sex (10 involving taste solution intake, 5 involving water intake, and 20 involving total intake).
The analyses of taste solution preference scores were a little more complex. As expected, there were no significant preferences displayed in any of the 17 tests involving a choice between two bottles of water. There were significant strain differences for all concentrations of NaCl, citric acid, MSG, corn oil and capsaicin, and most concentrations of CaCl2, NH4Cl, KCl, saccharin, sucrose, ethanol, HCl, QHCl, and caffeine. There were also strain differences at 31.6 mM MgCl2 and 32% Polycose but not at other concentrations of these compounds. The only compound for which preferences were not affected by strain at any concentration was denatonium although the effect of strain on preferences for 0.0316 mM denatonium was close to significance; F(13,193) = 2.73, p = 0.0015. None of the 109 analyses involved significant Strain × Sex interactions and only two involved significant Sex differences (for 100 mM saccharin and 316 mM MSG; in each case males had higher preference scores than did females).
Given the almost complete lack of sex differences in preference scores, to determine which strains differed from each other, we conducted post hoc LSD tests based on strain means with both sexes combined. Like the ANOVAs, these tests used a criterion for significance of p < 0.001. The results of these ANOVAs and differences between strains for each concentration of each taste compound are given in – . Note that in these figures, most tables and subsequent descriptions of the results, compounds believed to have similar taste qualities are grouped together rather than in the order they were tested, which is given in . This considerably simplifies description.
Two-bottle intake and preference for 7 concentrations of NaCl by 14 strains of rats
Two-bottle intake and preference for 4 concentrations of capsaicin by 14 strains of rats
Behavioral preference and avoidance thresholds
Behavioral preference and avoidance thresholds, originally called “taste thresholds” by Richter and Campbell (67
), are the lowest concentration at which rats demonstrate a discrimination between water and a taste compound in a choice test. To assess this, we used one-sample t
-tests to determine whether strain mean preferences at each concentration of each taste solution differed significantly from indifference (i.e., 50% preference). shows concentrations that were significantly preferred and shows the lowest concentrations that were avoided by each strain.
Ranges of preferred concentrations of taste solutions by strain
Lowest concentration of taste solutions avoided by each strain
Correlation among preferences for various taste compounds
Correlations among preferences for different taste compounds can provide information about common mechanisms, including those underlying similar taste qualities. To examine this, we generated the complete 109 × 109 correlation matrix based on strain mean preferences for each concentration of each taste compound (see on-line supplemental material
). We caution that this analysis is based on only 14 strain means, so it lacks statistical power; all correlations given below are significant at p<0.01, but few meet the p<0.001 (rcritical
= 0.78) criterion used throughout the rest of the paper. We consider them as descriptive rather than hypothesis tests. We attempted several descriptive methods including cluster analysis and multidimensional scaling but the resulting graphical representations were too complex to provide much insight (see online materials). The greatest problem was that strong correlations between taste compounds did not occur at all concentrations, and that different concentrations correlated highly for different pairs of taste compounds. Nevertheless, we believe useful insights can be gained by judicious cherry-picking of the highest correlations between various concentrations of different taste compounds.
Not surprisingly, there were many strong correlations between adjacent concentrations of the same compound (e.g., the correlation between 31.6 mM vs. 100 mM NaCl preference was r = 0.83). Generally, the greater the difference between the concentrations, the weaker the correlation. There were also the expected high correlations between compounds with similar taste qualities to humans. Examples include salty (e.g., 316 mM NaCl vs. 316 mM MSG, r = 0.89; 316 mM NaCl vs. 100 mM KCl, r = 0.72), sour (e.g., 0.316 mM HCl vs. 0.1 mM citric acid, r = 0.76), bitter (e.g., 1 mM QHCl vs. 10 mM caffeine, r = 0.75; 0.0316 mM QHCl vs. 0.1 mM denatonium, r = 0.80), and sweet (e.g., 0.316 mM saccharin vs. 31.6 mM sucrose, r = 0.72). In contrast, correlations between exemplars of the four primary taste qualities were generally very low, although an exception was between sour and bitter (e.g., 3.16 mM HCl vs. 0.01 mM QHCl, r = 0.75; 31.6 mM citric acid vs. 1 mM QHCl, r = 0.82). Preferences for the only umami taste compound tested, MSG, correlated highly with preferences for several compounds. In addition to the correlation with NaCl preference (see above) there were correlations with sweet compounds (10 mM MSG vs. 0.1 mM saccharin, r = 0.83; 31.6 mM MSG vs. 31.6 mM sucrose, r = 0.80), which is consistent with taste generalization data (39
). There were also correlations of MSG preference with compounds that had no obvious relationship to MSG or each other (e.g., 3.16 mM MSG vs. 31.6 mM CaCl2
, r = 0.70; 31.6 mM MSG vs. 100 mM NH4
Cl, r = 0.75; 10 mM MSG vs. 4% ethanol, r = 0.84; 31.6 mM MSG vs. 0.1 mM HCl, r = 0.84; 31.6 mM MSG vs. 0.01 mM denatonium, r = 0.79).
Consistent with human psychophysics and mouse studies, there were correlations between the non-sodium monovalent chlorides (e.g., 100 mM NH4Cl and 100 mM KCl, r = 0.80), between the divalent chlorides (e.g., 31.6 mM CaCl2 vs. 10 mM MgCl2, r = 0.67), and between exemplars of these two categories (e.g., 31.6 mM CaCl2 vs. 100 mM NH4Cl, r = 0.75).
There was a strong relationship between preferences for some concentrations of ethanol and some concentrations of saccharin (e.g., 0.1 mM saccharin and 4% ethanol, r = 0.79; 1 mM saccharin and 4% ethanol, r = 0.74, 100 mM saccharin and 8% ethanol, r = 0.77). Correlations between preferences for sucrose and ethanol were lower, presumably because sucrose preferences were constrained by ceiling effects. The relationship between preferences for ethanol and sweetness is consistent with other work [see (21
Preferences for sweet compounds were related to preferences for Polycose (e.g., 1% Polycose vs. 3.16 mM saccharin, r = 0.73; 1% Polycose vs. 56.2 mM sucrose, r = 0.70) and ethanol (e.g., 0.1 mM saccharin vs. 4% ethanol, r = 0.79). There were high negative correlations between preferences for ethanol and bitter compounds (e.g., 2% ethanol vs. 0.1 mM QHCl, r = −0.84; 2% ethanol vs. 10 mM caffeine, r = −0.81), and concentration-dependent correlations between preferences for ethanol and citric acid (8% ethanol vs. 3.16 mM citric acid, r = 0.66; 2% ethanol and 31.6 mM citric acid, r = −0.73). There were negative correlations between preferences for sweeteners and both CaCl2 (178 mM CaCl2 vs. 31.6 mM sucrose, r = −0.69) and capsaicin (e.g., 31.6 µM capsaicin and 1 mM saccharin, r = −0.71).
Results and discussion of preferences for specific taste compounds
This section describes the results found with individual taste compounds. It would be tedious to describe every strain difference for all 109 tests so these are depicted in – . In the text, we report (a) which concentrations of each taste compound are most informative with respect to revealing strain differences, and (b) which strains are likely to capture the most genetic variability responsible for strain differences. To this end, we list strains with the lowest and highest preferences at the taste solution concentration that produced the largest strain differences (based on the F-statistic of the ANOVA). provides the results of an alternative approach. The preference of each strain for each concentration of each compound was ranked from 1 to 14. These ranks were then summed, and the resulting sums ranked to get an overall ranking of preference for each compound. In general but not always, the conclusions drawn by the two approaches are complimentary.
Rankings of taste compound preferences for each strain
Each of the following subsections includes literature citations to previous studies in which rat strain preferences were compared, and where possible these are integrated with the present results. The literature involving rat preferences for most compounds is sparse so each section begins with a citation to pertinent mouse strain surveys, which provide an introduction to this literature.
There have been two large surveys of sodium preferences in mice (5
) but this is the first large survey in rats. The largest strain differences in preference were present at the 178 mM and 316 mM NaCl concentrations. With access to 316 mM NaCl, the DA, LEW and SHR rats had the highest preferences and 10 strains formed a group with the lowest preferences. The high NaCl preference of the SHR strain is well documented [e.g., (11
)]. The DA strain has not been tested with hypertonic NaCl previously but has been categorized as having normal NaCl preference based on tests with 150 mM NaCl (22
). The observation of high NaCl preferences in the LEW strain is apparently a novel finding.
There is a well-known inverted U-shaped function relating sodium concentration to preference, with a peak at about 150 mM [e.g., (66
)]. In our study, the function was shaped more like an inverted J, with most strains showing strong preferences for all hypotonic NaCl concentrations tested (i.e., 3.16, 10, 31.6, and 100 mM NaCl; ). Ten strains showed preferences relative to water for 3.16 mM NaCl (the lowest concentration tested) and all 14 strains showed preferences for 10 mM NaCl. This behavioral preference threshold is very close to the concentration observed in early-domesticated rats by Richter [~9 mM; (66
)] but lower than those observed by Fregly and Rowland in SD rats (30 mM NaCl) or in LE or Dahl rats, which did not have a preference threshold (31
). Differences in the maintenance diet the animals were fed may be responsible for the discrepancies (Richter used a McCallum diet, Fregly and Rowland used Purina Chow, we used AIN-76A diet) although the preference threshold did not track diet sodium content [3.9, ~3.9 g, and 1.02 g Na+
/kg diet respectively; see also (30
The F344 strain is considered to be a low-NaCl preferring strain relative to the SD or WI strains (51
). Consistent with this, in the present study, the F344 strain had lower intakes of 100 – 316 mM NaCl relative to the SD and WI strains. However, it also tended to have lower water intakes and consequently its NaCl preferences did not differ from the SD and WI strains at any concentration. It was ranked 8th
overall in NaCl preference. In our study, the WI and SD strains, which have been used for their “normal” NaCl preferences, were always in the lowest group of strains (except for the SD strain at 178 mM NaCl). Once again, we suspect that the type of maintenance diet may be responsible for the discrepancy with previous literature.
Consistent with our results, the BN strain was considered a low-sodium drinking strain based on the results of short-term tests (78
). Other rat strains that have been used in comparisons of NaCl preference include the Brattleboro (22
), BUF (51
), DA (22
), Dahl-R and Dahl-S (1
), Genetically Hypertensive [GHR (22
)], Holtzman (13
), Low- and High-saccharin intake [LoS and HiS (20
)], Munich-Wistar (51
), Smirk (22
), and Wistar-Furth (69
NH4Cl and KCl ( and )
There is survey of NH4
Cl and KCl preferences in 28 mouse strains (5
) but to our knowledge, there have been no previous rat strain comparisons of preferences for NH4
Cl or KCl. Confirming previous work with SD rats (81
), some strains showed an inverted U-shaped concentration-preference function in response to NH4
Cl (SD, Dahl, PVG, and SHR) and KCl (SD, PVG, and SHR; ). The largest strain differences were observed at the 100 mM concentration of both NH4
Cl and KCl, and the pattern of responses was similar (r = 0.80, see above). For both 100 mM NH4
Cl and 100 mM KCl, the group with the highest preferences included the SD, LE, DA, Dahl, FHH, LEW, PVG and SHR strains. The group with the lowest preferences included the WI, BN, BUF, COP, and Noble strains with the F344 also having low NH4
Cl preferences and the SD and FHH strains also having low KCl preferences).
Two-bottle intake and preference for 6 concentrations of NH4Cl by 14 strains of rats
Two-bottle intake and preference for 5 concentrations of KCl by 14 strains of rats
CaCl2 and MgCl2 ( and )
There are 28- and 40-strain surveys of calcium preferences in mice (5
), but the only previous rat strain comparisons of calcium preferences are confined to the WKY and SHR strains (24
), and there is no work in either species on magnesium preferences. As was the case with the monovalent chlorides, some strains showed an inverted U-shaped concentration-preference function in response to CaCl2
. Four strains preferred low concentrations of CaCl2
to water (SD, LE, COP, DA) and the PVG strain preferred 1 mM MgCl2
to water (). Peak preferences were at lower concentrations than for the monovalent chlorides [confirming (81
)]. There was a large range in the rejection threshold for both divalent chlorides. For CaCl2
, the FHH strain significantly avoided concentrations as low as 3.16 mM CaCl2
whereas the majority of strains were indifferent to concentrations up to 100 mM CaCl2
. For MgCl2
, the rejection threshold was 10 mM for the WI and F344 strains but 31.6 or 100 mM for the other 12 strains (). For both divalent chlorides, the largest strain differences were captured at the 31.6 mM concentration, and there was a fairly similar pattern of response (r = 0.57). The COP, DA, and PVG strains had the highest CaCl2
preferences although for MgCl2
the SD, LE, BN, and F344 strains were also included in this group. Ranked lowest were the WI, BN, FHH, LEW, Noble PVG and SHR strains with the BUF, COP, Dahl and F344 strains also joining the lowest MgCl2
The finding that the SHR strain had relatively low CaCl2
preferences is surprising because two reports suggest this strain has higher preferences for a range of concentrations of CaCl2
and calcium lactate than does the Wistar Kyoto (WKY) strain (24
). It may be that the WKY strain has very low CaCl2
preferences rather than the SHR strain that has high preferences.
Two-bottle intake and preference for CaCl2 by 14 strains of rats
Two-bottle intake and preference for 5 concentrations of MgCl2 by 14 strains of rats
Saccharin, sucrose and Polycose (, , and )
Large surveys of mouse strain preferences for saccharin and sucrose are available [(45
) see also (43
)], but there have been no concerted rat strain surveys of sweet or carbohydrate preferences. In our study, all strains showed strong preferences for nearly all concentrations of saccharin, sucrose and Polycose. Depending on the strain, the threshold at which preferences were significantly above indifference was between 0.316 – 3.16 mM saccharin, 10 – 17.8 mM sucrose, and 0.5 – 1% Polycose (for some strains, this was the lowest concentration tested). At concentrations above threshold, all strains strongly preferred sucrose and Polycose to water. Indeed, most strains drank virtually all their fluid from the sucrose or Polycose bottle. There were also very high preferences for moderate concentrations of saccharin, but some strains had less high preferences for the highest concentrations, and 7 strains avoided 100 mM saccharin, perhaps because of its bitter component [review; (19
Because all strains had very high preferences for moderate concentrations of saccharin and nearly all concentrations of sucrose and Polycose there was very little range in preference and so few strain differences (, , and ). Despite the restricted range, there were several significant correlations between strain mean preferences for the three compounds (0.316 mM saccharin vs. 17.8, 31.6, 56.2, and 100 mM sucrose, 1 mM saccharin vs. 17.8 mM sucrose, 3.16 mM saccharin vs. 0.5% and 1% Polycose, 56.2 mM sucrose vs. 1% Polycose]. Based on intakes, it was clear that the FHH strain was an outlying, avid consumer of saccharin, sucrose and Polycose [see also (36
)]. Consistent with this, the FHH was the only strain that preferred 100 mM saccharin to water. The SHR strain had unremarkable intakes of low concentrations of the sweeteners but notably high intakes of high concentrations of the sweeteners and all concentrations of Polycose.
Our results are consistent with other reports that LE and SD rats do not differ in 0.25% (~12 mM) saccharin preference (32
) and that WI, BUF or F344 rats do not differ in 0.25 – 32% sucrose preference (51
). Other strains that have been tested with sweeteners and/or Polycose include the HiS and LoS (20
), and Munich-Wistar (51
Two-bottle intake and preference for 7 concentrations of saccharin by 14 strains of rats
Two-bottle intake and preference for 5 concentrations of sucrose by 14 strains of rats
Two-bottle intake and preference for 7 concentrations of Polycose by 14 strains of rats
HCl and citric acid ( and )
We do not know of a comprehensive survey of mouse strain acid preferences although in one study, intake of 3 mM HCl was measured in 10 strains in 6-h one-bottle tests (43
). We are also unaware of any concerted attempts to compare rat strain responses to acids. In our study, there were significant associations between strain mean preferences for HCl and citric acid (1 mM HCl vs. 3.16 mM citric acid, r = 0.69, 10 mM HCl vs. 3.16 mM citric acid, r = 0.75; 10 mM HCl vs. 10 mM citric acid, r = 0.81) suggesting common mechanisms underlie the response to both acids. The most informative concentrations were 1 mM HCl and 3.16 mM citric acid ( and ). At these concentrations the PVG strain had notably high preferences and the WI, COP, LEW and Noble strains had notably low preferences for HCl; the FHH strain had the highest preferences and the WI strain the lowest preferences for citric acid.
Five strains preferred at least one concentration of HCl to water (SD, LE, DA, Dahl, PVG) but none found citric acid preferable to water (the FHH strain came close, i.e., p = 0.0084). The threshold for avoidance was 3.16 or 10 mM HCl and 3.16 or 10 mM citric acid for every strain, except for the Dahl and FHH strains which did not significantly avoid any concentration of HCl.
Consistent with our results, there was no difference between SD and LE rats in preference for 3 mM HCl (32
), and no differences among WI, BUF and F344 rats in preferences for 0.006 – 0.05% (0.31 – 2.6 mM) citric acid (51
). The WI, BUF and F344 rats had lower preferences than did Munich-Wistar rats (51
). Other strains tested with acids include the HiS and LoS strains, which did not differ in citric acid preference (20
Two-bottle intake and preference for 5 concentrations of HCl by 14 strains of rats
Two-bottle intake and preference for 4 concentrations of citric acid by 14 strains of rats
QHCl, caffeine, and denatonium (, and )
Several surveys of mouse strain preferences for bitter compounds have been conducted [e.g., (48
)] and a recent 10-strain survey involved 6-h one-bottle tests (43
). There have also been a smattering of previous studies investigating taste preferences for bitter compounds in rats (see below). One issue is whether bitter taste compounds have a unitary bitter taste. Based on correlations in preferences among the strains there was evidence for similar patterns of response to QHCl and caffeine (0.1 mM QHCl vs. 10 mM caffeine, r = 0.75; 1 mM QHCl vs. 10 mM caffeine, r = 0.75) and between QHCl and denatonium (0.0316 mM QHCl vs. 0.1 mM denatonium, r = 0.80, 0.0316 mM QHCl vs. 0.316 mM denatonium, r = 0.74; 0.316 mM QHCl vs. 0.316 mM denatonium, r = 0.69, 1 mM QHCl vs. 0.316 mM denatonium, r = 0.69) but only a weak, nonsignificant positive relationship between preferences for any concentration of caffeine and any concentration of denatonium (all r’s < 0.55). In other words, preferences for QHCl were related to preferences for caffeine and denatonium but preferences for caffeine and denatonium were unrelated to each other.
None of the 14 strains preferred any concentration of the three bitter compounds to water, which is surprising in view of reports that LE rats drink more sucrose octaacetate than water in one-bottle tests (34
) and that FHH rats prefer low concentrations of cycloheximide (79
) and quinine (36
) to water. There were large differences in avoidance thresholds among the strains. The FHH strain was outlying in that it had the highest avoidance threshold for caffeine and it was also a member of the groups with the highest avoidance thresholds for the other two bitter compounds. In contrast, the BN strain was outlying in that it had the lowest avoidance threshold of all strains for denatonium and was also a member of the groups with lowest avoidance threshold for QHCl and caffeine.
The largest strain differences in preference for bitter compounds were observed with 0.0316 mM QHCl and 1 mM caffeine. For 0.0316 mM QHCl, ten strains comprised the group with the lowest preferences and the BUF, COP, Dahl, and FHH strains comprised the group with the highest preferences (the SHR strain had preferences intermediate between the extremes and also high variability so they did not differ from either the highest or the lowest responders). For 1 mM caffeine, there were 10 strains in the group with highest preferences; the WI, BN and Noble stains had the lowest preferences. There were no significant differences among the strains in preference for any concentration of denatonium, although results with 0.0316 mM denatonium bordered on significance (p = 0.0015). The muted effect was not because the concentrations were undetectable or unduly disliked by all strains ().
Rats drank so little of the 0.316 mM QHCl, 1 mM QHCl and 31.6 mM caffeine solutions that these concentrations were not informative. We are not the first to use too high concentrations of bitter compounds in rat taste preference studies. Preference for 0.25% quinine solution did not differ among 7 strains of rats in one study (57
) or in a comparison of LEW, WIS and WKY rats in another (37
) but, in each case, intakes of this concentration of quinine were essentially zero (i.e., <1 mL/4 days).
Consistent with our results with 0.1 mM QHCl and 10 mM caffeine, FHH rats had lower avoidances than did WI or LEW rats for a wide range of quinine concentrations (36
), and lower avoidances of high concentrations of cycloheximide and phenylthiocarbamide than did LE or WI rats (79
). WI, BUF and F344 rats did not differ in quinine sulfate preference (0.0001% – 0.0009%) but these strains avoided some concentrations less than did Munich-Wistar rats (51
). Brattleboro rats avoided high concentrations of quinine sulfate (20 – 50 mg/L) more than did LE rats (34
Two-bottle intake and preference for 5 concentrations of quinine hydrochloride (QHCl) by 14 strains of rats
Two-bottle intake and preference for 4 concentrations of caffeine by 14 strains of rats
Two-bottle intake and preference for 4 concentrations of denatonium by 14 strains of rats
Monosodium glutamate ()
Monosodium glutamate (MSG) is the compound most commonly used as an exemplar of umami taste. The only published survey of mouse strain responses to umami involved 6-h one-bottle tests (43
). However, a previous survey using 12-h one-bottle tests to assess consumption of 13 concentrations of MSG involved 14 rat strains, of which 7 were the same or similar to those used here [SD, LE, WI, BN, F344, LEW and SHR; additional strains were Donryu, Hairless, LEA, LEC, WKY and SD×LEA F1
)]. Unfortunately, comparison with the current study is precluded because of differences in the methods and presentation of the results.
We found that MSG yielded the largest strain differences in preference of all 17 taste compounds tested. Eleven strains preferred at least one concentration of MSG to water but the range of preferred concentrations differed among the strains and the WI, BN and F344 strains did not prefer any concentration of MSG to water (). The largest strain differences in preference were obtained with 316 mM MSG, the highest concentration tested (). The DA, LEW, and SHR strains had the highest preferences and each member of this group drank significantly more 316 mM MSG than water. In contrast, the WI and Noble strains significantly avoided 316 mM MSG and along with 5 other strains were members of the group with the lowest preferences.
Two-bottle intake and preference for 6 concentrations of monosodium glutamate (MSG) by 14 strains of rats
Corn Oil ()
There is a survey of oil (intralipid) preferences in 11 strains of mice (46
) but no previous strain survey of rats. We found that 8 strains preferred at least one concentration of corn oil; the other 6 strains did not prefer any corn oil emulsion significantly more than the suspension agent mixture (). Three strains (LE, BUF and DA) significantly avoided the highest (16%) concentration of corn oil ().
The 16% corn oil emulsion supported the largest strain differences in preference. Even so, at this concentration there were no particularly outstanding strains with respect to corn oil preference. There were 9 strains in the group with the highest preferences and 8 strains in the group with the lowest preferences (the BN, COP, and Noble strains were in both the high-and low-preferring groups).
Differences in fat preference between Osborne-Mendel and S5B/Pl) rat (56
) have been exploited by Gilbertson and colleagues (35
) to compare fat taste transduction and the enhancement of saccharin preference by fat. Based on these two strains, Gilbertson et al have argued that there is an inverse relationship between sensitivity to fat taste and preference, which is related to body weight. In our results, there were nonsignificant negative correlations between preferences for corn oil and body weight (the strongest correlation was preference for 4% corn oil vs. body weight, r = −0.36, p = 0.2061) and no significant difference in body weight between those strains that preferred corn oil to its suspension agent mixture and those that did not. Thus, we saw no simple relationship between fat preference and body weight.
Two-bottle intake and preference for 5 concentrations of corn oil by 14 strains of rats
A recent study compares the ethanol preferences of 22 inbred mouse strains (94
). In contrast to most of the other taste compounds tested in this project, there has been considerable interest in rat strain differences in ethanol preference. Several strains have been bred to selectively prefer or avoid ethanol [e.g., High Alcohol Drinking (HAD) and Low Alcohol Drinking (LAD), Alcohol-Preferring (P) and Nonpreferring (NP), Alcohol Accepting (AA) and Nonaccepting (ANA) see (57
)]. Other strains have been selectively bred for other traits but differ in ethanol preferences [e.g., Flinders Sensitive and Resistant Lines (FSL and FRL), Maudsley Reactive and Nonreactive (59
), Floripa high (H) and low (L) (17
), Tryon maze-bright and maze-dull (3
)]. Some of the more “general use” strains including several of those used here have also been tested for ethanol preference [e.g., LEW (17
), Dahl, SHR, Wistar Kyoto Hyperactive (WKHA), and WKY (12
In our study, all strains except the F344 and SHR preferred at least one low (1 – 4%) ethanol concentration to water and all except the FHH and SHR avoided high (8 or 12%) concentrations. Strain differences appeared to be concentration-specific. For example, the BN strain had notably low preferences for 4% ethanol but not for other ethanol concentrations. Similarly, the SHR strain had notably high preferences for 12% ethanol but not for other concentrations.
The FHH strain has been intensively investigated as an ethanol-preferring strain, with the ACI or SD strains often used as a low-preferring controls [e.g., (3
)]. An important consideration is that we tested the FHH/Har substrain, which does not have as high ethanol preferences as the FHH/Wjd sub-strain (58
). Even so, the FHH strain was always in the highest ethanol-preferring group, had notably high intakes of 8% ethanol (), and was ranked 2nd
highest in ethanol preference overall (BUF was highest).
Two-bottle intake and preference for 5 concentrations of ethanol by 14 strains of rats
Capsaicin is often used as an exemplar of a compound that activates the trigeminal system. There is a report of the response of 10 strains of mice during one-bottle access to various concentrations of capsaicin (33
) but no previous rat strain comparisons of preferences for capsaicin or other trigeminal stimuli. In our study, no strain preferred any concentration of capsaicin to water. However, six strains significantly avoided the lowest (1 µM) concentration we tested so, in retrospect, it would have been useful to have tested lower concentrations of capsaicin. The LEW strain had a higher rejection threshold than did the other 13 strains. It was also ranked highest in capsaicin preference, and along with the BUF and SHR strains it had the highest preferences for 3.16 µM capsaicin, the concentration supporting the greatest strain differences. The remaining 11 strains all had equivalently low preferences at this concentration.