Salty taste deficits in CALHM1 knockout mice

Abstract

Genetic ablation of calcium homeostasis modulator 1 (CALHM1), which releases adenosine triphosphate from Type 2 taste cells, severely compromises the behavioral and electrophysiological responses to tastes detected by G protein-coupled receptors, such as sweet and bitter. However, the contribution of CALHM1 to salty taste perception is less clear. Here, we evaluated several salty taste-related phenotypes of CALHM1 knockout (KO) mice and their wild-type (WT) controls: 1) In a conditioned aversion test, CALHM1 WT and KO mice had similar NaCl avoidance thresholds. 2) In two-bottle choice tests, CALHM1 WT mice showed the classic inverted U-shaped NaCl concentration-preference function but CALHM1 KO mice had a blunted peak response. 3) In brief-access tests, CALHM1 KO mice showed less avoidance than did WT mice of high concentrations of NaCl, KCl, NH(4)Cl, and sodium lactate (NaLac). Amiloride further ameliorated the NaCl avoidance of CALHM1 KO mice, so that lick rates to a mixture of 1000 mM NaCl + 10 µM amiloride were statistically indistinguishable from those to water. 4) Relative to WT mice, CALHM1 KO mice had reduced chorda tympani nerve activity elicited by oral application of NaCl, NaLac, and sucrose but normal responses to HCl and NH(4)Cl. Chorda tympani responses to NaCl and NaLac were amiloride sensitive in WT but not KO mice. These results reinforce others demonstrating that multiple transduction pathways make complex, concentration-dependent contributions to salty taste perception. One of these pathways depends on CALHM1 to detect hypertonic NaCl in the mouth and signal the aversive taste of concentrated salt.

Keywords: NaCl; gustatory electrophysiology; salt preference; salt taste.

Figure 1

NaCl avoidance in 48-h choice tests by CALHM1 WT and KO mice that had been poisoned with LiCl. There were no significant differences between the 2 groups.

Figure 2

Two-bottle choice preferences for concentration series of saccharin, QHCl, NaCl, and HCl by CALHM1 KO mice (n = 10) and their WT littermates (n = 11). *P < 0.05 relative to WT group. ⁺ P < 0.05 above indifference.

Figure 3

Two-bottle choice preferences for an ascending concentration series of NaCl and 3.2mM saccharin of CALHM1 KO mice (n = 8) and their WT controls (n = 8). *P < 0.05 relative to WT group. ⁺ P < 0.05 above indifference.

Figure 4

Lick rates of CALHM1 WT and KO mice presented with water and 4 concentrations of NaCl, KCl, NH₄Cl, or sodium lactate. NaCl and KCl were tested dissolved either in water or in 10 µM amiloride. *P < 0.05 relative to WT group(s). ⁺ P < 0.05 relative to tests without amiloride.

Figure 5

Representative neural recordings showing changes in the integrated voltage over time for a CALHM1 WT (top) and KO (bottom) mouse. For each animal, 1 concentration series of applications of NaCl mixed in water is shown, along with preceding and following reference KCl presentations for each series (on the left and right, respectively). Ten seconds before and after stimulus onset are shown.

Figure 6

Mean (±SEM) chorda tympani responses to applications of NaCl in CALHM1 WT and KO mice. Responses are shown for NaCl mixed in water (A) or in 10 μM amiloride (B). (C) The amiloride-sensitive NaCl component for each strain was calculated by subtracting values for NaCl mixed in amiloride from those for NaCl mixed in water. *P < 0.05, WT versus KO; ⁺ P < 0.05, effect of amiloride within WT mice.

Figure 7

Mean (±SEM) chorda tympani responses of CALHM1 WT and KO mice to 178mM sodium lactate (NaLac) mixed in water or in 10 μM amiloride. *P < 0.05, WT versus KO; ⁺ P < 0.05, effect of amiloride within WT mice.

Figure 8

Mean (±SEM) chorda tympani responses to nonsodium stimuli in CALHM1 WT and KO mice. QHCl = 2mM quinine hydrochloride; HCl = 10mM hydrochloric acid; NH₄Cl = 100mM ammonium chloride; Sucrose = 500mM sucrose. *P < 0.05, WT versus KO.