Taste testing
Previous efforts to map QTL for bitter taste have utilized consumption tests that may be confounded by the contributions of post-ingestive effects [29]. We used a modified brief-access lick test, which minimizes the contribution of such effects [29,37] to determine whether B6 and D2 mice display differences in taste sensitivity to the taste stimuli QHCl and DB. After initially screening B6 and D2 mice to determine stimulus concentrations that were aversive but not saturating [47], we selected two ligand concentrations for each compound that best differentiated the two strains. Subsequent taste testing of BXD RI lines was restricted to these two concentrations (1 and 3 mM for both QHCl and DB). Avoidance by male and female B6 and D2 mice increased (as indicated by the decreased lick ratio) in a concentration-dependent manner for both compounds (Figure 1A; Table 1). There was a significant strain difference for both 1 and 3 mM QHCl (F[1,25] > 24.6; p < 0.0001). D2 mice displayed decreased aversion relative to B6 mice at both concentrations. On the other hand, the strains did not significantly differ in taste sensitivity to DB (Figure 1A). There were no significant effects of gender.
Table 1  Mean lick ratios for B6, D2 and BXD mice.
Strain   n   Water licks/5s   1 mM DB   3 mM DB   3 mM PR   10 mM PR   1 mM QH   3 mM QH
B6  16  29.02 ± 1.4  0.606  0.318  0.311  0.164  0.241  0.144
D2  12  33.91 ± 2.4  0.465  0.286  0.616  0.429  0.758  0.305
BXD1  5  32.48 ± 1.5  0.372  0.202  0.314  0.252  0.746  0.422
BXD2  4  36.50 ± 0.9  1.025  0.605  0.208  0.230  0.320  0.215
BXD5  5  37.23 ± 2.7  0.628  0.354  0.502  0.636  0.836  0.452
BXD6  5  22.43 ± 3.5  0.442  0.132  0.200  0.124  0.130  0.080
BXD11  7  34.48 ± 3.9  0.310  0.223  0.387  0.260  0.479  0.260
BXD13  5  27.74 ± 3.4  0.360  0.290  0.472  0.168  0.692  0.330
BXD14  7  31.09 ± 3.5  0.741  0.304  0.293  0.174  0.206  0.126
BXD15  5  37.36 ± 1.5  0.178  0.120  0.468  0.198  0.362  0.222
BXD20  5  31.26 ± 2.7  0.252  0.200  0.162  0.128  0.150  0.116
BXD21  6  19.77 ± 1.5  0.390  0.262  0.313  0.133  0.192  0.217
BXD24  5  33.03 ± 2.4  0.318  0.148  0.490  0.238  0.518  0.370
BXD27  5  33.46 ± 2.5  0.364  0.434  0.216  0.126  0.146  0.142
BXD29  3  40.19 ± 1.5  0.257  0.187  0.657  0.173  0.633  0.333
BXD31  5  19.45 ± 0.8  0.302  0.238  0.152  0.158  0.218  0.194
BXD32  6  27.22 ± 2.3  0.105  0.107  0.385  0.203  0.405  0.245
BXD33  6  29.03 ± 2.2  0.757  0.387  0.377  0.237  0.342  0.188
BXD34  6  23.01 ± 3.4  0.310  0.217  0.843  0.257  0.693  0.355
The number of individual mice tested for each strain (n) is listed in the second column. Subsequent columns show the mean lick rate to water during testing (± SEM), mean lick ratio for denatonium benzoate (DB), PROP (6-n-propylthiouracil; PR) and quinine hydrochloride (QH) at each of two concentrations (see Methods for details).
Figure 1  Lick ratios (mean ± SE) for B6, D2 and BXD strains. (A) Mean lick ratios for B6 (filled circles) and D2 (open circles) mice at two concentrations of QHCl and DB. In all panels, a lower mean lick ratio indicates a greater aversion, and therefore greater taste sensitivity, to the stimulus. For panels B, C, and D, each BXD strain is represented by a different color, and listed in order from least sensitive to most sensitive to 1 mM QHCL. (B) Mean lick ratios for the six BXD strains that are most sensitive to QHCl in this assay. (C) Mean lick ratios for the five BXD strains that are least sensitive to QHCl in this assay. (D) Mean lick ratios for the six BXD strains intermediate in QHCl taste sensitivity to those in (B) and (C). Cutoffs for the three QHCl taster groups were arbitrarily set, as there was a continuity of the phenotype at 1 mM QHCl: sensitive strains exhibited a lick ratio for 1 mM QHCl of ≤ 0.3, intermediate strains from 0.31-0.6, and insensitive strains > 0.6. The absence of two distinct phenotypic classes suggests that sensitivity to QHCL is under polygenic control. We next tested mice from 17 BXD lines in the same manner. BXD mice also typically avoided both stimuli in a concentration dependent manner (Figures 1B–1D; Table 1). However, QHCl and DB taste sensitivity vary independently across these RI strains: some strains highly sensitive to QHCl are relatively insensitive to DB, and vice versa (Figures 1B–1D).