T1r3 taste receptor involvement in gustatory neural responses to ethanol and oral ethanol preference

Abstract

Elevated alcohol consumption is associated with enhanced preference for sweet substances across species and may be mediated by oral alcohol-induced activation of neurobiological substrates for sweet taste. Here, we directly examined the contribution of the T1r3 receptor protein, important for sweet taste detection in mammals, to ethanol intake and preference and the neural processing of ethanol taste by measuring behavioral and central neurophysiological responses to oral alcohol in T1r3 receptor-deficient mice and their C57BL/6J background strain. T1r3 knockout and wild-type mice were tested in behavioral preference assays for long-term voluntary intake of a broad concentration range of ethanol, sucrose, and quinine. For neurophysiological experiments, separate groups of mice of each genotype were anesthetized, and taste responses to ethanol and stimuli of different taste qualities were electrophysiologically recorded from gustatory neurons in the nucleus of the solitary tract. Mice lacking the T1r3 receptor were behaviorally indifferent to alcohol (i.e., ∼50% preference values) at concentrations typically preferred by wild-type mice (5-15%). Central neural taste responses to ethanol in T1r3-deficient mice were significantly lower compared with C57BL/6J controls, a strain for which oral ethanol stimulation produced a concentration-dependent activation of sweet-responsive NTS gustatory neurons. An attenuated difference in ethanol preference between knockouts and controls at concentrations >15% indicated that other sensory and/or postingestive effects of ethanol compete with sweet taste input at high concentrations. As expected, T1r3 knockouts exhibited strongly suppressed behavioral and neural taste responses to sweeteners but did not differ from wild-type mice in responses to prototypic salt, acid, or bitter stimuli. These data implicate the T1r3 receptor in the sensory detection and transduction of ethanol taste.

Keywords: alcohol preference; knockout mice.

Fig. 1.

Voluntary intake (ml/kg/day) and percent preference for ethanol in a 2-bottle choice assay. A: mean ± SE ethanol intake as a function of concentration in T1r3 knockout (KO) and C57BL/6J wild-type (WT) mice; n = 12/genotype. B: mean ± SE ethanol intake by concentration in male and female mice; n = 12/sex. C: mean ± SE ethanol preference at each concentration in KO and WT mice; n = 12/genotype. D: mean ± SE ethanol preference by concentration in males and females; n = 12/sex. *Significant difference between KO and WT or male and female (P < 0.05).

Fig. 2.

Voluntary intake (ml/kg/day) and percent preference for sucrose in a 2-bottle choice assay. A: mean ± SE sucrose intake as a function of concentration in T1r3 KO and C57BL/6J WT mice; n = 12/genotype. B: mean ± SE sucrose intake by concentration in male and female mice; n = 12/sex. C: mean ± SE sucrose preference at each concentration in KO and WT mice; n = 12/genotype. D: mean + SE sucrose preference in KO and WT males and females collapsed across concentration; n = 6/sex/genotype. *Significant difference between KO and WT or male and female (P < 0.05).

Fig. 3.

Voluntary intake (ml/kg/day) and percent preference for quinine in a 2-bottle choice assay. A: mean ± SE quinine intake as a function of concentration in T1r3 KO and C57BL/6J WT mice; n = 12/genotype. B: mean ± SE quinine intake by concentration in male and female mice; n = 12/sex. C: mean ± SE quinine preference at each concentration in KO and WT mice; n = 12/genotype. D: mean + SE quinine intake in KO and WT males and females collapsed across concentration. n = 6/sex/genotype. *Significant difference between KO and WT or male and female (P < 0.05).

Fig. 4.

Digital oscilloscope records showing taste activity measured from a single nucleus of the solitary tract (NTS) neuron recorded from a C57BL/6J WT mouse. Taste-evoked trains of action potentials were elicited by oral application of various concentrations of ethanol and sweet (sucrose, glycine), salty (NaCl), acidic (HCl), and bitter (quinine) taste stimuli. ↑Stimulus onset.

Fig. 5.

Responses to ethanol and taste stimuli in sucrose responsive (S₁, n = 15) and sucrose nonresponsive (S₀, n = 11) NTS neurons recorded from C57BL/6J WT mice. A: mean ± SE responses in S₀ and S₁ cells to an ascending concentration (%, vol/vol) series of ethanol. B: mean + SE responses in S₀ and S₁ cells to glycine (G), sucrose (S), NaCl (N), HCl (H), and quinine (Q). Concentration ([M]) follows abbreviation. *Significant difference between S₀ and S₁ (P < 0.05).

Fig. 6.

Responses to ethanol and taste stimuli in all NTS neurons (n = 28) sampled from T1r3 KO mice and all NTS neurons (n = 26) recorded from C57BL/6J WT mice. A: mean ± SE responses in KO and WT cells to an ascending concentration (%, vol/vol) series of ethanol. B: for each genotype, percentage of all sampled neural responses to each concentration of ethanol that were significantly elevated above baseline (prestimulus) activity. C: mean + SE responses in KO and WT cells to glycine (G), sucrose (S), NaCl (N), HCl (H), and quinine (Q). Concentration ([M]) follows abbreviation. *Significant difference between KO and WT (P < 0.05).

Fig. 7.

Across-neuron patterns of response to 40% ethanol and sweet (glycine, sucrose), salty (NaCl), acidic (HCl), and bitter (quinine) stimuli measured across NTS neurons (n = 26) recorded from C57BL/6J WT mice and NTS cells (n = 28) sampled from T1r3 KO mice. The Pearson coefficient of correlation (r) calculated between the response pattern to ethanol and each stimulus is given. P < 0.05 indicates that a correlation was significant.