Behavioral Evidence for More than One Taste Signaling Pathway for Sugars in Rats

Abstract

By conventional behavioral measures, rodents respond to natural sugars, such as glucose and fructose, as though they elicit an identical perceptual taste quality. Beyond that, the metabolic and sensory effects of these two sugars are quite different. Considering the capacity to immediately respond to the more metabolically expedient sugar, glucose, would seem advantageous for energy intake, the present experiment assessed whether experience consuming these two sugars would modify taste-guided ingestive responses to their yet unknown distinguishing orosensory properties. One group (GvF) had randomized access to three concentrations of glucose and fructose (0.316, 0.56, 1.1 M) in separate 30-min single access training sessions, whereas control groups received equivalent exposure to the three glucose or fructose concentrations only, or remained sugar naive. Comparison of the microstructural licking patterns for the two sugars revealed that GvF responded more positively to glucose (increased total intake, increased burst size, decreased number of pauses), relative to fructose, across training. As training progressed, GvF rats began to respond more positively to glucose in the first minute of the session when intake is principally taste-driven. During post-training brief-access taste tests, GvF rats licked more for glucose than for fructose, whereas the other training groups did not respond differentially to the two sugars. Additional brief access testing showed that this did not generalize to Na-saccharin or galactose. Thus, in addition to eliciting a common taste signal, glucose and fructose produce distinct signals that are apparently rendered behaviorally relevant and hedonically distinct through experience. The taste pathway(s) underlying this remain to be identified.

The Role of Post-Ingestive Feedback in the Development of an Enhanced Appetite for the Orosensory Properties of Glucose over Fructose in Rats

“…The results of Experiment 1 demonstrated that rats given explicit experience consuming glucose and fructose faithfully paired with their respective and distinct post-ingestive effects (Matched group) subsequently displayed a different pattern of licking for the two sugars than did the rats that were given equivalent orosensory experience with glucose and fructose but uncoupled from their distinguishing post-ingestive effects (Mismatched). These results generally accorded with previous studies by Schier et al [9,10], though we noted a couple exceptions.…”

“…Second, Schier et al [9] found that naïve or single sugar-exposed rats treated glucose and fructose quite comparably in the post-conditioning brief access taste test. In contrast, here, rats in the Mismatched group licked more for fructose than glucose at the low to mid-range concentrations.…”

“…Simple sugars, artificial sweeteners, and some D-amino acids are thought to reinforce ingestion through signals generated mainly from a single common receptor in the peripheral gustatory system-the T1R2+T1R3 taste receptor [1][2][3][4][5][6][7][8]. However, a recent series of studies by Schier et al showed that ingestive exposure to two metabolically distinct sugars, glucose and fructose, produced a strong avidity for the orosensory properties of glucose over those of fructose in rats and mice through a T1R2+T1R3-independent receptor mechanism [9,10]. The sensory, physiological, and/or psychological processes that contribute to this exposure-dependent phenomenon are not well understood.…”

Brain Stimulation Reward Supports More Consistent and Accurate Rodent Decision-Making than Food Reward

“…Additionally, these alternatives may activate alternative sensory and reward systems (Yasoshima et al, 2015; Schier and Spector, 2016; Sclafani and Ackroff, 2016) and may still be susceptible to many of the confounds discussed above (Aoyama et al, 2014). Thus, a deeper appreciation for the confounds associated with food reward can help design more accurate experiments investigating the neurophysiological and cognitive elements of a behavior, and perhaps circumvent such confounds by using alternative approaches.…”

A gustocentric perspective to understanding primary sensory cortices

“…Gustatory stimuli have hedonic valence, they are either palatable or aversive. The reward value of taste can be easily measured relying either on consummatory behaviors [36,37] or orofacial reactions [38,39]. The ability to assess objectively different dimensions of reward with specific behavioral tests has allowed researchers in the field to explore the involvement of GC in processing reward.…”