Perception of sweet taste is important for voluntary alcohol consumption in mice

Abstract

To directly evaluate the association between taste perception and alcohol intake, we used three different mutant mice, each lacking a gene expressed in taste buds and critical to taste transduction: alpha-gustducin (Gnat3), Tas1r3 or Trpm5. Null mutant mice lacking any of these three genes showed lower preference score for alcohol and consumed less alcohol in a two-bottle choice test, as compared with wild-type littermates. These null mice also showed lower preference score for saccharin solutions than did wild-type littermates. In contrast, avoidance of quinine solutions was less in Gnat3 or Trpm5 knockout mice than in wild-type mice, whereas Tas1r3 null mice were not different from wild type in their response to quinine solutions. There were no differences in null vs. wild-type mice in their consumption of sodium chloride solutions. To determine the cause for reduction of ethanol intake, we studied other ethanol-induced behaviors known to be related to alcohol consumption. There were no differences between null and wild-type mice in ethanol-induced loss of righting reflex, severity of acute ethanol withdrawal or conditioned place preference for ethanol. Weaker conditioned taste aversion (CTA) to alcohol in null mice may have been caused by weaker rewarding value of the conditioned stimulus (saccharin). When saccharin was replaced by sodium chloride, no differences in CTA to alcohol between knockout and wild-type mice were seen. Thus, deletion of any one of three different genes involved in detection of sweet taste leads to a substantial reduction of alcohol intake without any changes in pharmacological actions of ethanol.

FIG.1. Mice lacking Gnat3, Tas1r3 or Trpm5 show markedly reduced preference for and consumption of alcohol

A. Gnat3 knockout mice. Pure ethanol (EtOH) consumption (g/kg/day); n=7 for wild-type and n=10 for knockout mice. B. Tas1r3 knockout mice. Pure ethanol consumption (g/kg/day); n=8 for each genotype. C. Trpm5 knockout mice. Pure ethanol consumption (g/kg/day); n=7 for wild-type. n=10 for knockout mice. D. Gnat3 knockout mice. Preference score for ethanol. E. Tas1r3 knockout mice. Preference score for ethanol. F. Trpm5 knockout mice. Preference score for ethanol. G. Gnat3 knockout mice. Total fluid intake (g/kg/day). H. Tas1r3 knockout mice. Total fluid intake. I. Trpm5 knockout mice. Total fluid intake. * - p<0.05; ** - p<0.01; *** - p<0.001 – significant differences relative to wild-type mice for the same concentration of ethanol (post-hoc Bonferroni Test).

FIG.2. Gnat3, Tas1r3 and Trpm5 null mice show markedly reduced preference for saccharin

A. Gnat3 knockout mice. Preference score for saccharin. n=7 for wild-type. n=10 for knockout mice. B. Tas1r3 knockout mice. Preference score for saccharin; n=8 for each genotype. C. Trpm5 knockout mice. Preference score for saccharin. n=7 for wild-type. n=10 for knockout mice. D. Gnat3 knockout mice. Preference score for quinine. E. Tas1r3 knockout mice. Preference score for quinine. F. Trpm5 knockout mice. Preference score for quinine. G. Gnat3 knockout mice. Preference score for sodium chloride (NaCl). E. Tas1r3 knockout mice. Preference score for NaCl. F. Trpm5 knockout mice. Preference score for NaCl. * - p<0.05; *** - p<0.001 – significant differences relative to wild-type mice for the same concentration of tastants (post-hoc Bonferroni Test).

FIG.3. Gnat3, Tas1r3 and Trpm5 null mice displayed markedly reduced conditioned taste aversion (CTA) to ethanol when saccharin was used as the conditioned stimulus

A. Gnat3 mouse colony. Saccharin as the conditioned stimulus. n=5 - for saline-treated groups. n=10 – for ethanol-treated groups. B. Tas1r3 mouse colony. Saccharin as the conditioned stimulus. n=5 - for saline-treated groups. n=7 – for ethanol-treated groups. C. Trpm5 mouse colony. Saccharin as the conditioned stimulus. n=5 - for saline-treated groups. n=9 – for ethanol-treated groups. D. Gnat3 mouse colony. NaCl as the conditioned stimulus. n=10 – for ethanol-treated groups. E. Tas1r3 mouse colony. NaCl as the conditioned stimulus. n=6 - for saline-treated groups. n=9 – for ethanol-treated groups. F. Trpm5 mouse colony. NaCl as the conditioned stimulus. n=6 - for saline-treated groups. n=8-9 – for ethanol-treated groups.

FIG.4. No difference in ethanol-induced conditioned place preference (CPP) was observed between wild-type and the null mice

A. Gnat3 mouse colony. Mean time (% of total) spent on the grid floor during a 30-min test by mice from GRID+ and GRID- groups. n=10 for all groups. B. Gnat3 mouse colony. Motor activity during each 5-min ethanol conditioned trials (CS+) and saline conditioned trials (CS-). C. Tas1r3 mouse colony. Mean time (% of total) spent on the grid floor during a 30-min test by mice from GRID+ and GRID- groups. n=10 for all groups. D. Tas1r3 mouse colony. Motor activity during each 5-min ethanol conditioned trials (CS+) and saline conditioned trials (CS-). E. Trpm5 mouse colony. Mean time (% of total) spent on the grid floor during a 30-min test by mice from GRID+ and GRID- groups. n=10 for all groups. F. Trpm5 mouse colony. Motor activity during each 5-min ethanol conditioned trials (CS+) and saline conditioned trials (CS-). * - p<0.05; ** - p<0.01; *** - p<0.001, between [GRID+] and [GRID-] groups of the same genotype (post-hoc Bonferroni test).

FIG.5. There were no differences in severity of ethanol-induced withdrawal between Gnat3, Tas1r3 and Trpm5 knockout and wild-type mice

The severity of ethanol induced acute withdrawal was assessed based on monitoring of handling induced convulsions (HIC). A. HIC scores after acute ethanol administration for Gnat3 mouse colony. n= 7-8 per genotype. B. Area below HIC curve and above the basal level for Gnat3 mouse colony. C. HIC scores for Tas1r3 mouse colony. n= 6-7 per genotype. D. Area below HIC curve and above the basal level for Tas1r3 mouse colony. E. HIC scores for Trpm5 mouse colony. n= 7 per genotype. F. Area below HIC curve and above the basal level for Trpm5 mouse colony.

FIG.6. There were no differences in hypnotic effect of ethanol between Gnat3, Tas1r3 and Trpm5 knockout and wild-type mice

The hypnotic effect of ethanol was assessed based on loss of righting reflex (LORR) procedure. A. Duration of LORR for Gnat3 mouse colony. n= 5 per dose and genotype. B. Duration of LORR for Tas1r3 mouse colony. n= 7-8 per dose and genotype. C. Duration of LORR for Trpm5 mouse colony. n= 8-10 per dose and genotype.

FIG.7. There were no differences in ataxic effect of ethanol between Gnat3, Tas1r3 and Trpm5 knockout and wild-type mice

The ataxia induced by ethanol was assessed based on motor coordination on the rotarod. A. Gnat3 mouse colony. Time on the rotarod. n= 6 per genotype. B. Tas1r3 mouse colony. Time on the rotarod. n= 6 per genotype. C. Trpm5 mouse colony. Time on the rotarod. n= 6 per genotype.