Stability Of Individual Differences In Sucralose Taste Preference - PLOS

Discussion

Here, we sought to determine whether outbread rats show individual differences in homecage sucralose preference that are 1) stable over time, and 2) consistent with preference of a known bittersweet compound solution of sucrose + quinine. We found that individual rats’ preferences for each solution remained stable over time and with intervening experience with the other bittersweet solution. Similar to previous studies, we observed a range of preference scores at the isohedonic concentrations, demonstrating that individual rats vary along the bittersweet preference/avoidance continuum [4, 8, 9]. We found that at isohedonic concentrations, sucralose preference correlated with preference of the sucrose/quinine solution. Despite classifying individual preference based on consumption at the isohedoinc concentrations, concentration-dependent choice of bittersweet solutions relative to water was significantly different between preferring and avoiding groups at every concentration tested. We observed overlapping population distributions for sucralose and sucrose/quinine preference scores, which were significantly right-shifted towards preferring for both solutions. Across the population, a majority of rats showed stability in preferences between solutions. While some rats displayed inconsistencies in preference within or between taste solutions, the number of rats shifting in either direction along the continuum was below what would be expected by chance, however order of tastant exposure may have contributed to the observed inconsistencies. Together, our results suggest that sucralose preference generalizes to preference of a known bittersweet compound solution of sucrose and quinine and those preferences for both bittersweet solutions remain stable over time and experience.

Prior work has shown that the concentration-dependent preference for sucrose/quinine solutions is different for sucralose preferring and avoiding rats [4, 10]. Similar to the present study, rats in a previous study [4] were tested with ascending concentrations of quinine in a fixed sucrose concentration. Compared to sucralose preferring rats, sucralose avoiding rats showed an accelerated reduction in preference of sucrose/quinine solutions as a function of increasing quinine concentration. This group difference was interpreted as stable preference across bittersweet solutions. Our study extends these findings showing that individual rats’ SUCRA and SQ preference scores are correlated, and that the population distributions at the isohedonic concentrations for these two solutions are overlapping. Furthermore, our study demonstrates that overwhelmingly rats display consistent preferences for each solution across repeated testing. This additional information provides much needed assessment of trait stability that would be necessary for using the sucralose preference as a model to investigate individual differences in other motivated behaviors.

The proportion of sucralose preferring rats observed here is similar to the first study exploring individual variability in sucralose preference [9]. However, the proportion of sucralose avoiding rats in the present study contrasts with subsequent studies that observed a majority of rats avoid sucralose [4, 8]. There are several methodological factors that differ between our study and prior reports that may account for this difference. First, prior studies used 24 hour homecage access 2 bottle choice testing, measuring fluid consumption over a greater time period than in the present study using 45-minute access to two bottle choice. In the 24 hour access conditions of prior sucralose preference studies, rats were not fluid restricted to determine preference groups [4, 8, 10]. Notably, these studies reported that 24 h homecage preference under ad libitum fluid conditions consistently predicted brief access licking and consumption of bittersweet solutions under acutely fluid restricted conditions [10]. In contrast, we chronically fluid restricted rats so that we could test preference in limited access sessions. Thus, in the present study, rats’ consistently high motivational state to consume fluids may have biased the population towards bittersweet preference. Other factors that are less likely to play a role in the different sucralose preference distributions observed across studies include rat strain, sex and estrous cycle phase. Each of these factors have been systematically evaluated and shown to have little influence over sucralose preference distributions [8, 13].

We observed some inconsistencies between SUCRA and SQ preferences. Most notably, we observed that order of tastant exposure affected SQ preference scores. This likely contributed to the inconsistency between tastant solutions. Other considerations with regard to the influence of taste on inconsistent preferences, the pattern of preference scores across concentrations differs between taste stimuli. Across the population, low concentrations of SQ were consistently preferred over water with decreasing consumption as quinine concentrations increased in all groups. However, the lowest concentration of SUCRA was similarly preferred to water, with SP rats increasing and SA rats decreasing consumption of SUCRA as concentration increased. These patterns suggest sweet perception dominance for low concentrations of SQ, but indifference at low concentrations of SUCRA. Thus, inconsistencies in preferences between taste stimuli could be attributable to negative/positive contrast within a specific concentration series. Notably, differences in caloric value and osmolality between SQ and SUCRA at the isohedonic concentrations may also affect preference in two-bottle choice. Rats may consume more sucrose-quinine relative to water for the caloric value of sucrose, while sucralose has no caloric value and thus consumption would not be influenced by this factor. Rats may also consume more water during choice tests with higher osmolality solutions, which may drive homeostatic water consumption to neutralize potential electrolyte imbalance. In limited access 2-bottle choice, this may influence preference scores. Both caloric value and osmolatity differences between taste solutions could contribute to the inconsistencies in preference that we observe in the population at isohedonic concentrations.

A number of factors suggest SUCRA preference has greater utility than SQ preference for assessing risk for conflict processing and disorders of motivation. First, SUCRA preference shows stability without being influenced by order of tastant exposure, which is not the case for SQ preference. The more pronounced bimodality of SUCRA distributions relative to SQ distributions and the fewer number of inconsistent SUCRA avoiders relative to SQ avoiders also suggest that preference of non-nutritive, sucralose, may have more utility as a screening procedure than SQ preference. The extent to which bimodality of the SUCRA distribution reflects biological differences between SP and SA rats, this preference phenotype may be better suited to predict variation in conflict processing, which we postulate has common neurobiological underpinnings. However, the utility of SQ screening may be improved by examining the stability and consistency of taste preference for compound SQ mixtures when quinine concentrations remain fixed and sucrose concentrations vary.

Several studies have speculated and even designed studies to directly examine the nature of these individual differences in preference for bittersweet solutions [3, 4, 8–11]. Individual differences could arise from genetic differences in taste perception. Notably, the aversive bitter or metallic taste qualities of high concentrations of artificial sweeteners result from activation of bitter taste receptors (T2R) and/or transient receptor potential vanilloid-1 (TRPV1) receptors [14–16]. Genetic mutations in either one of these receptors may contribute to differences in sensory processing of sucralose and other bittersweet solutions. There is evidence that SP and SA rats show differences in sucralose perception, with SP rats classifying high concentrations of sucralose as having “sucrose-like” qualities, whereas SA rats don’t [10]. Having controlled for motivation in that study, individual differences in sucralose acceptance were explained by perceptual variation in taste quality, and likely reflect differences at the level of taste perception. However, in the same study, differences in the acceptance of sucrose-quinine mixtures were not predicted by perceptual variation in taste quality. Our study and others [4] have identified a relationship between sucralose acceptance and sucrose/quinine acceptance, suggesting consistency in preference and avoidance across solutions may arise from differences in higher-order taste processing in gustatory brain regions and downstream hedonic processing in reward circuitry. Consistent with this idea, while increased palatability of sweet solutions is typically associated with increased number of lick bouts and increased lick rates [17], a microstructural lick analysis study demonstrated that sucralose preferring rats have consistent lick bouts and reduced lick rates with increasing sucralose concentrations [11]. Reduced lick rates are associated with aversive taste qualities [17]. Thus, the bitter taste detection in SP and SA may not drive differences in consumption; instead preferring rats may be more motivated to consume high concentrations of sucralose than sucralose avoiding rats, perhaps due to enhanced motivational salience of sweet stimuli [10, 11].

Rats selectively bred for high and low consumption of sweet taste stimulus, saccharin, have had tremendous utility for predicting many facets of addiction-related behaviors across drug classes [3]. Rats that consume high quantities of saccharin (HiS) show consistently greater vulnerability across drug classes than rats that consume low quantities of saccharin (LoS) [3]. Studies using HiS and LoS inbred lines have also examined artificial sweetener consumption, demonstrating LoS rats consume less sucralose and other artificial sweeteners than HiS rats [13, 18–20]. While consumption of bitter tastes alone does not differ between LoS and HiS inbred rats, LoS rats avoid sucrose/quinine mixtures at lower quinine concentrations than HiS rats [19], similar to what has been found with outbred Sucralose Avoiding relative to Sucralose Preferring rats [4]. The Sucralose Preferring and Avoiding phenotypes may have similar utility for predicting addiction related behaviors in outbred rats.

The enhanced motivational salience of appetitive relative to aversive stimuli has relevance for substance use disorder (SUD). Several criteria for diagnosing SUD describe a conflict process, in which motivation for the appetitive drug reward overcomes its potentially aversive consequences [2]. The current study validates the reproducibility and the test/retest reliability of sucralose preference procedures. Our limited access procedures adequately capture behavioral variability for appetitive/aversive taste preference, which is stable over time, consistent across taste stimuli and is evident prior to drug-experience in outbred rats. Future studies are warranted to explore the potential utility of this model for predicting individual differences in natural and drug reward seeking under conflict conditions.