The rat's acceptance and preference for sucrose, maltodextrin, and saccharin solutions and mixtures

Effects of early-life stress followed by access to stevia or sucralose during adolescence on weight gain, glycemia, and anxiety-related behaviors in male and female rats

Early-life stress and subsequent high-calorie diets during adolescence are known to be risk factors for developing metabolic and psychological disorders. Although non-nutritive sweeteners such as stevia and sucralose have been a useful alternative to reduce sugar consumption, the effects of prolonged consumption of these sweeteners on metabolism and behavior in adolescents remain unclear. Here, we evaluated the effects of early-stress followed by access to stevia or sucralose during adolescence on weight gain, glycemia, and anxiety-related behaviors in male and female rats. During postnatal days (PNDs) 1-21, pups were separated twice a day, for 180 min each time, from their dam nest while non-separated pups served as controls. The pups were weaned, separated by sex and randomly distributed into the stevia, sucralose and water conditions. During PNDs 26-50, two bottles containing water and stevia or sucralose were placed in the animal home-cages, and body weight and blood glucose measures were scored. On PNDs 50 and 51, behavioral measures were obtained in the open-field test. Results showed that male rats consuming stevia reduced body weight gain, blood glucose and increased locomotion. Early-stress led to low blood glucose and alterations in anxiety and locomotion-related behaviors in a sex-dependent manner. Moreover, sucralose access during adolescence reversed the effects of early-stress on anxiety-related behaviors in female rats. The results suggest that the consumption of stevia and sucralose could be an alternative for the replacement of sugar-sweetened beverages, especially in adolescents who have had adverse early-life experiences.

Lipopolysaccharide (LPS) attenuates the primary conditioning of lithium chloride (LiCl)-induced context aversion but not the secondary conditioning of context aversion or taste avoidance

A first-order association can be formed between toxin-induced nausea and a context, as well as nausea and a taste cue. However, comparatively little is understood about second-order associations. The present study examined if the bacterial endotoxin, LPS, could impair the first- and second-order conditioning of context aversion (anticipatory nausea paradigm) and subsequent conditioned taste avoidance (two-bottle task). Adult male Long Evans rats were treated with LiCl (127 mg/kg, intraperitoneal [i.p.]) or vehicle control (NaCl) and then exposed to a distinct context for 4 first-order conditioning trials. LPS (200 μg/kg, i.p.) or NaCl were administered 24 h after each trial. Seventy-two h after the final first-order conditioning trial, rats underwent 2 second-order conditioning trials where they were treated with 2% saccharin (i.p.) and then exposed to the same context. Twenty-four h after the final second-order conditioning trial, rats were tested in a two-bottle task (2 trials), where they were given a choice between water and a palatable 0.2% saccharin solution. LiCl-treated rats demonstrated a context aversion by the 3rd conditioning trial in the anticipatory nausea paradigm. Rats previously exposed to LiCl also displayed a conditioned taste avoidance of saccharin within the two-bottle task. LPS attenuated first-order context aversion but did not alter either second-order context aversion or conditioned taste avoidance in the two-bottle task. This study demonstrated that a secondary association formed within an aversive context could result in a conditioned taste avoidance. Further, LPS may be able to attenuate primary conditioning, but not secondary conditioning.

Reducing sugar intake through chronic swimming training: Exploring palatability changes and central vasopressin mechanisms

Excessive sugar intake has been associated with the onset of several non-communicable chronic diseases seen in humans. Physical activity could affect sweet taste perception which may affect sugar intake. Therefore, it was investigated the chronic effects of swimming training on sucrose intake/preference, reactivity to sucrose taste, self-care in neurobehavioral stress, and the possible involvement of the vasopressin type V1 receptor in sucrose solution intake. Male Wistar rats, of from different cohorts were used, subjected to a sedentary lifestyle (SED) or swimming training (TR - 1 h/day, 5×/week, for 8 weeks, with no added load). Weekly intake was verified in SED and TR rats after access to a sucrose solution 1×/week, 2 h/day, for eight weeks. Chronic effects of swimming and/or a sedentary lifestyle were carried out three days after the end of the physical exercise protocol. Swimming training reduced the intake of sucrose solution from the third week onwards in the two-bottle test measured once a week for 8 weeks. After the ending of the swimming protocol, sucrose intake was also reduced as per its preference. This reduced intake is probably correlated with the carbohydrate aspect of sucrose since saccharin intake was not affected. In addition, chronic swimming training was shown to reduce ingestive responses, increase neutral responses, without interfering with aversive, in the sucrose solution taste reactivity test. In addition, these results are not related to a depressive-like behavior, nor to neurobehavioral stress. Furthermore, treatment with vasopressin V1 receptor antagonist abolished the reduced sucrose intake in trained rats. The results suggest that swimming performed chronically is capable of reducing intake and preference for sucrose by decreasing the palatability of sucrose without causing depressive-type behavior or stress. In addition, the results also suggest that central V1 vasopressin receptors are part of the mechanisms activated to reduce sucrose intake in trained rats.

A computational analysis of mouse behavior in the sucrose preference test

The sucrose preference test (SPT) measures the relative preference of sucrose over water to assess hedonic behaviors in rodents. Yet, it remains uncertain to what extent the SPT reflects other behavioral components, such as learning, memory, motivation, and choice. Here, we conducted an experimental and computational decomposition of mouse behavior in the SPT and discovered previously unrecognized behavioral subcomponents associated with changes in sucrose preference. We show that acute and chronic stress have sex-dependent effects on sucrose preference, but anhedonia was observed only in response to chronic stress in male mice. Additionally, reduced sucrose preference induced by optogenetics is not always indicative of anhedonia but can also reflect learning deficits. Even small variations in experimental conditions influence behavior, task outcome and interpretation. Thus, an ostensibly simple behavioral task can entail high levels of complexity, demonstrating the need for careful dissection of behavior into its subcomponents when studying the underlying neurobiology.

Mixtures of Sweeteners and Maltodextrin Enhance Flavor and Intake of Alcohol in Adolescent Rats

Sweet flavorants enhance palatability and intake of alcohol in adolescent humans. We asked whether sweet flavorants have similar effects in adolescent rats. The inherent flavor of ethanol in adolescent rats is thought to consist of an aversive odor, bitter/sweet taste, and burning sensation. In Experiment 1, we compared ingestive responses of adolescent rats to 10% ethanol solutions with or without added flavorants using brief-access lick tests. We used 4 flavorants, which contained mixtures of saccharin and sucrose or saccharin, sucrose, and maltodextrin. The rats approached (and initiated licking from) the flavored ethanol solutions more quickly than they did unflavored ethanol, indicating that the flavorants attenuated the aversive odor of ethanol. The rats also licked at higher rates for the flavored than unflavored ethanol solutions, indicating that the flavorants increased the naso-oral acceptability of ethanol. In Experiment 2, we offered rats chow, water, and a flavored or unflavored ethanol solution every other day for 8 days. The rats consistently consumed substantially more of the flavored ethanol solutions than unflavored ethanol across the 8 days. When we switched the rats from the flavored to unflavored ethanol for 3 days, daily intake of ethanol plummeted. We conclude that sweet and sweet/maltodextrin flavorants promote high daily intake of ethanol in adolescent rats (i.e., 6-10 g/kg) and that they do so in large part by improving the naso-oral sensory attributes of ethanol.

Obesogenic diets may differentially alter dopamine control of sucrose and fructose intake in rats

Chronic overeating of obesogenic diets can lead to obesity, reduced dopamine signaling, and increased consumption of added sugars to compensate for blunted reward. However, the specific role of diet composition yet remains unknown. To study this, Sprague-Dawley male rats were fed a high-energy diet with high fat and low carbohydrate content (HFHE), a fat-sugar combination high-energy diet (FCHE), or standard chow for 24 weeks. We found that both high-energy diets produced substantial body weight gain compared to chow-fed controls. To investigate dopamine control of short (2-h) intake of palatable sucrose or fructose solutions, rats were pretreated peripherally (IP) with equimolar doses (0-600 nmol/kg) of the dopamine D1 (SCH23390) and D2 (raclopride) subtype-specific receptor antagonists. The results showed an overall increase in the efficacy of D1 and D2 receptor antagonists on suppression of intake in obese rats compared to lean rats, with effects differing based on diets and test solutions. Specifically, SCH23390 potently reduced both sucrose and fructose intake in all groups; however, lower doses were more effective in HFHE rats. In contrast, raclopride was most effective at reducing fructose intake in the obese FCHE rats. Thus, it appears that obesity due to the consumption of combinations of dietary fat and sugar rather than extra calories from dietary fat alone may result in reduced D2 receptor signaling. Furthermore, such deficits seem to preferentially affect the control of fructose intake. These findings demonstrate for the first time a plausible interaction between diet composition and dopamine control of carbohydrate intake in diet-induced obese rats. It also provides additional evidence that sucrose and fructose intake is regulated differentially by the dopamine system.

Rats' preferences for high fructose corn syrup vs. sucrose and sugar mixtures

High fructose corn syrup (HFCS) has replaced sucrose in many food products, which has prompted research comparing these two sweeteners in rodents. The present study examined the relative palatability of HFCS and sucrose for rats, offering 11% carbohydrate solutions to match the content of common beverages for human consumption. The animals initially preferred HFCS to sucrose but after separate experience with each solution they switched to sucrose preference. Approximating the composition of HFCS with a mixture of fructose and glucose (55:45) yielded a solution that was less attractive than sucrose or HFCS. However, HFCS contains a small amount of glucose polymers, which are very attractive to rats. A 55:42:3 mixture of fructose, glucose and glucose polymers (Polycose) was equally preferred to HFCS and was treated similarly to HFCS in comparisons vs. sucrose. Post-oral effects of sucrose, which is 50% fructose and 50% glucose, may be responsible for the shift in preference with experience. This shift, and the relatively small magnitude of differences in preference for HFCS and sucrose, suggest that palatability factors probably do not contribute to any possible difference in weight gain responses to these sweeteners.

Consumption of SC45647 and sucralose by rats selectively bred for high and low saccharin intake

Mammals' affinity for sweet tastes exists alongside dramatic variation among species and individuals in responses to sweeteners. The present paper focused on consumption by Occidental High- (HiS) and Low-Saccharin (LoS)-consuming rats in 23-h 2-bottle tests of 2 sweeteners for which few data from rats are available: SC45647 and sucralose. Every HiS and LoS rat preferred SC45647 to water at every concentration, with HiS rats consuming it more avidly. Most HiS rats preferred sucralose to water at one or more concentrations; some HiS rats and most LoS rats avoided sucralose at every concentration. However, both HiS and LoS rats preferred a sucralose-maltodextrin mixture (Splenda) to water; thus, Splenda's "bulking" ingredient maltodextrin transforms highly variable responses to sucralose into a relatively homogeneous preference for the product. Implications for the study of variation in sweet taste are discussed.

Palatable solutions during paradoxical sleep deprivation: reduction of hypothalamic-pituitary-adrenal axis activity and lack of effect on energy imbalance

Paradoxical sleep deprivation (PSD) induces increased energy expenditure in rats, insofar as rats eat more but loose weight throughout the deprivation period. In the present study, rats were offered water, saccharin or sucrose to drink during the deprivation period, since it has been proposed that carbohydrates reduce the hypothalamic-pituitary-adrenal (HPA) axis response to stress. Rats were submitted to the flower pot technique for 96 h. During the PSD period, they were weighed daily and food and fluid intake was assessed twice a day. At the end of the PSD period, rats were killed and plasma concentrations of glucose, adrenocorticotropic hormone (ACTH) and corticosterone were assayed. Compared to their control counterparts, all paradoxical sleep-deprived rats consumed more food, but lost weight. Paradoxical sleep-deprived rats given sucrose drank more than their control counterparts (especially in the light phase of the light/dark cycle). Paradoxical sleep-deprived rats showed increased food intake during all periods throughout the experiment, with peak intake during the dark phase and nadir during the light phase of the light/dark cycle. All paradoxical sleep-deprived rats showed lower glucose plasma levels than control rats and increased relative adrenal weight. However, when given saccharin or sucrose, paradoxical sleep-deprived rats showed lower concentrations of ACTH and corticosterone than their water-provided counterparts, indicating that palatable fluids were capable of lowering HPA axis activation produced by PSD. The fact that PSD induced energy imbalance regardless of the relative attenuation of the HPA axis activity produced by saccharin or sucrose suggests that the HPA axis may play only a secondary role in this phenomenon, and that other mechanisms may account for this effect. The data also suggest that supply of palatable fluids can be an additional modification to reduce the stress of the flower pot method.

Dissociation of licking and volume intake controls in rats ingesting glucose and maltodextrin

The volume of fluid that rats acquire with each lick was systematically varied across short-term tests with 12.5% glucose (Experiment 1) or 12.5% maltodextrin (Experiment 2). For glucose, rats increased the number of licks emitted as lick volume was reduced such that meal size remained remarkably stable across all (8, 4, and 2 microl) but the smallest (1 microl) lick volume conditions tested. Rats similarly compensated for lick volume reduction (8 to 4 microl) with maltodextrin by approximately doubling the number of licks emitted. Meal duration and a number of lick-microstructural parameters (initial ingestion rate, mean burst duration, terminal lick and ingestion rates, and burst duration) were not correlated with the intake outcome insofar as they varied significantly across conditions over which intake remained stable. Thus, in response to lick volume manipulation, rats demonstrated an impressive degree of behavioral flexibility in what may be regarded as a defense of meal size.

Corticosterone facilitates saccharin intake in adrenalectomized rats: does corticosterone increase stimulus salience?

Unlike normal rats, adrenalectomized rats do not voluntarily drink sweet saccharin solutions. To test whether this is a function of corticosterone in the circulation, and if corticosterone also increases the impetus for drinking saccharin after a period of withdrawal, we performed the following experiments. Young male rats were sham adrenalectomized (sham) or adrenalectomized (ADX); the ADX rats were provided with subcutaneous pellets containing (percent replacement of corticosterone, %B) 0%B, 15%B, 30%B or 100%B. Sham and ADX rats were immediately provided with saline (0.5%) and saccharin (2 mM) bottles in their home cages. Saccharin was allowed for 4 days on, 3 days off, 4 days on, 3 days off and a final day on, over the 15 days experiment. The dose of corticosterone determined both how much saccharin was voluntarily drunk by the ADX rats and the degree of overshoot after days off. Corticosterone also determined energy balance of the groups of ADX rats. The 30%B pellets restored food intake, body weight gain, insulin and caloric efficiency to the normal levels observed in sham rats. White fat depot weights and uncoupling protein concentration in brown adipose tissue were restored to sham levels by 100%B, suggesting that these variables which depend on activity in the sympathetic nervous system require considerable glucocorticoid receptor occupancy. We conclude that corticosterone increases the willingness to ingest sweetened water in a unimodal, dose-related manner, while moderate doses of corticosterone restore energy balance.