Nutrient selection in the absence of taste receptor signaling

Abstract

When allowed to choose between different macronutrients, most animals display a strong attraction toward carbohydrates compared with proteins. It remains uncertain, however, whether this food selection pattern depends primarily on the sensory properties intrinsic to each nutrient or, alternatively, metabolic signals can act independently of the hedonic value of sweetness to stimulate elevated sugar intake. Here we show that Trpm5(-/-) mice, which lack the cellular mechanisms required for sweet and several forms of l-amino acid taste transduction, develop a robust preference for d-glucose compared with isocaloric l-serine independently of the perception of sweetness. Moreover, a close relationship was found between glucose oxidation and taste-independent nutrient intake levels, with animals increasing intake as a function of glucose oxidation rates. Furthermore, microdialysis measurements revealed nutrient-specific dopaminergic responses in accumbens and dorsal striatum during intragastric infusions of glucose or serine. Specifically, intragastric infusions of glucose induced significantly higher levels of dopamine release compared with isocaloric serine in both ventral and dorsal striatum. Intragastric stimulation of dopamine release seemed to depend on glucose utilization, because administration of an anti-metabolic glucose analog resulted in lower dopamine levels in striatum, an effect that was reversed by intravenous glucose infusions. Together, our findings suggest that carbohydrate-specific preferences can develop independently of taste quality or caloric load, an effect associated with the ability of a given nutrient to regulate glucose metabolism and stimulate brain dopamine centers.

Figure 1.

Trpm5 KO mice can develop a preference for glucose solutions through a conditioning protocol. Experimental data are presented as mean ± SEM across animals throughout Results. A, In short-term (10 min) two-bottle preference tests, whereas WT animals displayed strong attraction to 14.5% glucose against l-amino acids serine and arginine, with preference ratios significantly higher than 0.5, KO animals were indifferent between the two choices (*p < 0.001, independent t test against 0.5). B, These findings are further confirmed by analyzing the mean total consumption data for these short-term two-bottle sessions: whereas WT animals consumed significantly more glucose than serine (**p < 0.0005, paired two-sample t test) and arginine (data not shown), KO mice consumed approximately the same amounts from both solutions (not significant, p > 0.4). C, During 30-min-long conditioning sessions, animals were given alternated access to either glucose only (arbitrarily assigned to one sipper in the behavioral cage) or isocaloric serine only (assigned to the opposite sipper) for 6 consecutive days. During these conditioning sessions, which were designed to allow KO animals to associate a sipper side with its postingestive effects, both WT (C) and KO (D) animals consumed significantly more glucose than serine (paired two-sample t test, ***p < 0.000006; ****p < 0.002). E, During 10-min-long two-bottle postconditioning test sessions in which water was accessible from both sippers, a significant higher preference toward the sipper previously associated with glucose availability were observed for both WT and KO animals (*p < 0.02, independent t test against 0.5).

Figure 2.

Indirect calorimetry measurements during nutrient intake monitoring in Trpm5 KO and WT mice. A, In agreement with the conditioning sessions, long-term (21 h) nutrient availability resulted in significantly higher levels of consumption of glucose versus serine solutions in both WT and KO animals (*p < 0.0009; see Results). B, Both WT and KO animals displayed higher respiratory quotients during glucose versus serine sessions (*p < 0.0003; see Results). However, WT mice displayed higher quotients during glucose sessions compared with KO mice (**p < 0.01; see Results). C, D, Mean respiratory quotient value across mice of time courses for WT (C) and KO (D) mice. Vertical dotted lines represent the time point at which nutrients became available for the first of the four cages. E, Both WT and KO animals displayed higher heat production during glucose versus serine sessions (*p < 0.0005; see Results). F, No nutrient- or genotype-specific effects were detected on ambulatory activity.

Figure 3.

Glucose intake is more closely associated with glucose oxidation than with blood glucose levels. A, Blood glucose levels (in milligrams per deciliters) measured in WT and Trpm5 KO animals after food and water deprivation (Fasted), 1-h-long glucose intake sessions (Glucose), or 1-h-long serine intake sessions (l-serine). Blood glucose levels were significantly higher after glucose intake compared with both serine intake sessions and fasting, in both KO and WT animals (*p < 0.05 with respect to all other conditions, two-sample t test). B, Liver glycogen levels (expressed in glucosyl units micromoles per grams wet weight) were measured in WT and Trpm5 KO animals after food and water deprivation (Fasted), 1-h-long glucose intake sessions (Glucose), or 1-h-long serine intake sessions (l-serine). Liver glycogen levels were significantly higher after glucose intake compared with both serine intake sessions and fasting, in both KO and WT animals (*p < 0.05 with respect to all other conditions, two-sample t test). C–H, Scatter plots showing the association between glucose metabolism parameters and nutrient intake. The corresponding regression lines are shown in each case along with the computed values pairwise (Pearson's) correlation coefficients and associated p values. Correlations are shown between number of licks for glucose and the corresponding RQ values (C), number of licks for serine and the corresponding respiratory quotient values (D), licks for glucose and the corresponding blood glucose levels (E), licks for serine and the corresponding blood glucose levels (F), licks for glucose and the corresponding liver glycogen levels (G), and licks for serine and the corresponding liver glycogen levels (H). Note that the strongest association found was between glucose intake and glucose oxidation (C).

Figure 4.

Trpm5 KO, but not WT, mice fail to display increased preferences toward glucose solution during post-calorimetry two-bottle preference tests. WT and KO animals previously presented for 21 h with glucose or serine in different indirect calorimetry sessions were exposed to post-calorimetry 10 min glucose versus serine preference tests. A, Preference ratios, significantly above 0.5, were associated with WT but not KO mice (*p < 0.004). B, Glucose intake levels during preference tests were significantly higher compared with serine in WT but not KO mice (*p < 0.0002). C, WT mice display higher levels of glucose intake compared with serine within 30 min of onset of nutrient availability (*p < 0.04 at least). D, Conversely, such a pattern emerged, on average, only after 6 h in KO mice (*p < 0.003). For details, see Results.

Figure 5.

Intragastric (IG) infusions of glucose and serine induce differential brain dopamine responses. A, B, An infusion pump holding a syringe containing a given nutrient was connected to the gastric catheter and placed under control of the behavioral chamber, so that every lick for water detected by the lickometers immediately triggered a 3-s-long infusion of the nutrient into the stomach (at a rate of 10 μl/min). The positions of water bottles in the cage were associated with infusion of one specific nutrient. A, Across conditioning sessions, animals produced significantly more licks for water during glucose infusion sessions compared with serine infusion sessions (*p < 0.04). B, The effect depended on conditioning day, because the difference in water intake between glucose and serine infusion sessions was significant for sessions 2 and 3 combined but not for session 1 (*p < 0.02). C, Overall percentage changes produced by glucose and serine intragastric infusions. In the nucleus accumbens, glucose infusions were associated with significantly higher levels of extracellular dopamine when directly compared with serine infusions (*p < 0.03, two-sample t test). In addition, significant decreases in dopamine levels were observed after serine infusions (**p < 0.02, one-sample t test against 100%). D, Whereas glucose infusions were consistently associated with increased dopamine levels across samples, relative decreases produced by serine infusions were marked in particular at the third sample (***p < 0.04). E, In dorsal striatum, significant increases in dopamine levels were associated with glucose (*p = 0.009, one-sample t test against 100%), but not serine (p < 0.09), infusions. F, Across samples, significant increases in dopamine levels were observed only during glucose infusions at 30 min after infusion onset (**p < 0.04, one-sample t test against 100%).

Figure 6.

Intravenous infusions of glucose and serine induce differential behavioral responses in the absence of taste signaling, and inhibition of glucose oxidation decreases dopamine concentration levels in striatum. A, An infusion pump holding a syringe containing a given nutrient was connected to a jugular catheter and placed under control of the behavioral chamber, so that every lick for water detected by the lickometers immediately triggered a 3-s-long intravenous infusion of the nutrient (at a rate of 10 μl/min). Animals produced significantly more licks for water during glucose infusion sessions compared with serine infusion sessions (*p < 0.03 for within-subject glucose–serine difference, paired one-tailed two-sample t test). B, Infusing the antimetabolic glucose analog 2-DG via a jugular catheter resulted in a significant decrease in dopamine concentration levels in dorsal striatum (*p < 0.007, one-sample t test against 100% preinfusion levels). This suppressive effect was then reversed within 30 min via a 10 min intravenous glucose infusion. Whereas glucose infusions promoted a reinstatement of dopamine concentration to levels similar to those of baseline samples, it represented a robust increase in dopamine levels with respect to the samples collected immediately after 2-DG infusion (109.9 ± 37.6% increase in overall dopamine concentration; **p = 0.03 compared with 0% increase). C, Consistent with the dopamine measurements shown in B, presentation of glucose after an intraperitoneal injection of 2-DG resulted in significantly higher levels of glucose intake compared with after a vehicle injection in KO animals during 30-min-long sessions (*p < 0.03 for within-subject glucose–serine difference, paired two-sample t test). Therefore, glucose solutions were attributed a higher reward value when contributing to restore glucose oxidation in sweet-insensitive animals, as would be predicted from the equivalent changes in dopamine levels shown in B.