Operant licking for intragastric sugar infusions: Differential reinforcing actions of glucose, sucrose and fructose in mice

Conditioned preference and avoidance induced in mice by the rare sugars isomaltulose and allulose

Isomaltulose, a slowly digested isocaloric analog of sucrose, and allulose, a noncaloric fructose analog, are promoted as "healthful" sugar alternatives in human food products. Here we investigated the appetite and preference conditioning actions of these sugar analogs in inbred mouse strains. In brief-access lick tests (Experiment 1), C57BL/6 (B6) mice showed similar concentration dependent increases in licking for allulose and fructose, but less pronounced concentration-dependent increases in licking for isomaltulose than sucrose. In Experiment 2, B6 male were given one-bottle training with a CS+ flavor (e.g., grape) mixed with 8% isomaltulose or allulose and a CS- flavor (e.g., cherry) mixed in water followed by two-bottle CS flavor tests. The isomaltulose mice showed only a weak CS+ flavor preference but a strong preference for the sugar over water. The allulose mice strongly preferred the CS- flavor and water over the sugar. The allulose avoidance may be due to gut discomfort as reported in humans consuming high amounts of the sugar. Experiment 3 found that the preference for 8% sucrose over 8% isomaltulose could be reversed or blocked by adding different concentrations of a noncaloric sweetener mixture (sucralose + saccharin, SS) to the isomaltulose. Experiment 4 revealed that the preference of B6 or FVB/N mice for isomaltulose+0.01%SS or sucrose over 0.1%SS increased after separate experience with the sugars and SS. This indicates that isomaltulose, like sucrose, has postoral appetition effects that enhances the appetite for the sugar. In Experiments 5 and 6, the appetition actions of the two sugars were directly compared by giving mice isomaltulose+0.05%SS vs. sucrose choice tests before and after separate experience with the two sugars. In general, the initial preference the mice displayed for isomaltulose+0.05%SS was reduced or reversed after separate experience with the two sugars although some strain and sex differences were obtained. This indicates that isomaltulose has weaker postoral appetition effects than sucrose.

The neural basis of sugar preference

When it comes to food, one tempting substance is sugar. Although sweetness is detected by the tongue, the desire to consume sugar arises from the gut. Even when sweet taste is impaired, animals can distinguish sugars from non-nutritive sweeteners guided by sensory cues arising from the gut epithelium. Here, we review the molecular receptors, cells, circuits and behavioural consequences associated with sugar sensing in the gut. Recent work demonstrates that some duodenal cells, termed neuropod cells, can detect glucose using sodium-glucose co-transporter 1 and release glutamate onto vagal afferent neurons. Based on these and other data, we propose a model in which specific populations of vagal neurons relay these sensory cues to distinct sets of neurons in the brain, including neurons in the caudal nucleus of the solitary tract, dopaminergic reward circuits in the basal ganglia and homeostatic feeding circuits in the hypothalamus, that alter current and future sugar consumption. This emerging model highlights the critical role of the gut in sensing the chemical properties of ingested nutrients to guide appetitive decisions.

A Glucokinase-linked Sensor in the Taste System Contributes to Glucose Appetite

Objectives

Dietary glucose is a robust elicitor of central reward responses and ingestion, but the key peripheral sensors triggering these orexigenic mechanisms are not entirely known. The objective of this study was to determine whether glucokinase, a phosphorylating enzyme with known glucosensory roles, is also expressed in taste bud cells and contributes to the immediate hedonic appeal of glucose-containing substances.

Methods and results

Glucokinase (GCK) gene transcripts were localized in murine taste bud cells with RNAScope®, and GCK mRNA was found to be upregulated in the circumvallate taste papillae in response to fasting and after a period of dietary access to added simple sugars in mice, as determined with real time-qPCR. Pharmacological activation of glucokinase with Compound A increased primary taste nerve and licking responses for glucose but did not impact responsivity to fructose in naïve mice. Virogenetic silencing of glucokinase in the major taste fields attenuated glucose-stimulated licking, especially in mice that also lacked sweet receptors, but did not disrupt consummatory behaviors for fructose or the low-calorie sweetener, sucralose in sugar naïve mice. Knockdown of lingual glucokinase weakened the acquired preference for glucose over fructose in sugar-experienced mice in brief access taste tests.

Conclusions

Collectively, our data establish that glucokinase contributes to glucose appetition at the very first site of nutrient detection, in the oral cavity. The findings expand our understanding of orosensory inputs underlying nutrition, metabolism, and food reward.

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Glucokinase is expressed in the taste bud cells.

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Gustatory glucokinase is upregulated by energy deficit and regular consumption of simple sugars.

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Gustatory glucokinase is required for normal glucose taste detection and contributes to the hedonic appeal of this nutrient.

The preference for sugar over sweetener depends on a gut sensor cell

Guided by gut sensory cues, humans and animals prefer nutritive sugars over non-caloric sweeteners, but how the gut steers such preferences remains unknown. In the intestine, neuropod cells synapse with vagal neurons to convey sugar stimuli to the brain within seconds. Here, we found that cholecystokinin (CCK)-labeled duodenal neuropod cells differentiate and transduce luminal stimuli from sweeteners and sugars to the vagus nerve using sweet taste receptors and sodium glucose transporters. The two stimulus types elicited distinct neural pathways: while sweetener stimulated purinergic neurotransmission, sugar stimulated glutamatergic neurotransmission. To probe the contribution of these cells to behavior, we developed optogenetics for the gut lumen by engineering a flexible fiberoptic. We showed that preference for sugar over sweetener in mice depends on neuropod cell glutamatergic signaling. By swiftly discerning the precise identity of nutrient stimuli, gut neuropod cells serve as the entry point to guide nutritive choices.

Buchanan, Rupprecht, Kaelberer and colleagues show that the preference for sugar over sweetener in mice depends on gut neuropod cells. Akin to other sensor cells, neuropod cells swiftly communicate the precise identity of stimuli to drive food choices.

Learning of food preferences: mechanisms and implications for obesity & metabolic diseases

Omnivores, including rodents and humans, compose their diets from a wide variety of potential foods. Beyond the guidance of a few basic orosensory biases such as attraction to sweet and avoidance of bitter, they have limited innate dietary knowledge and must learn to prefer foods based on their flavors and postoral effects. This review focuses on postoral nutrient sensing and signaling as an essential part of the reward system that shapes preferences for the associated flavors of foods. We discuss the extensive array of sensors in the gastrointestinal system and the vagal pathways conveying information about ingested nutrients to the brain. Earlier studies of vagal contributions were limited by nonselective methods that could not easily distinguish the contributions of subsets of vagal afferents. Recent advances in technique have generated substantial new details on sugar- and fat-responsive signaling pathways. We explain methods for conditioning flavor preferences and their use in evaluating gut-brain communication. The SGLT1 intestinal sugar sensor is important in sugar conditioning; the critical sensors for fat are less certain, though GPR40 and 120 fatty acid sensors have been implicated. Ongoing work points to particular vagal pathways to brain reward areas. An implication for obesity treatment is that bariatric surgery may alter vagal function.

Neural Circuits of Interoception

The present paper considers recent progress in our understanding of the afferent/ascending neural pathways and neural circuits of interoception. Of particular note is the extensive role of rostral neural systems, including cortical systems, in the recognition of internal body states, and the reciprocal role of efferent/descending systems in the regulation of those states. Together these reciprocal interacting networks entail interoceptive circuits that play an important role in a broad range of functions beyond the homeostatic maintenance of physiological steady-states. These include the regulation of behavioral, cognitive, and affective processes across conscious and nonconscious levels of processing. We highlight recent advances and knowledge gaps that are important for accelerating progress in the study of interoception.

Unequal rewarding of three metabolizable sugars - sucrose, fructose and glucose - in olfactory learning and memory in Bactrocera dorsalis

Learning and memory are the most characterized advanced neurological activities of insects, which can associate information with food. Our previous studies on Bactrocera dorsalis have shown that this fly can learn to evaluate the nutritional value of sugar rewards, although whether all metabolizable sugars are equally rewarding to flies is still unclear. To address this question, we used three sweet and metabolizable sugars - sucrose, fructose and glucose - as rewards for conditioning. The flies showed differences in learning and memory in response to the three sugar rewards. The level of learning performance in sucrose-rewarded flies was higher than that in fructose-rewarded and glucose-rewarded flies, and, strikingly, only sucrose and glucose stimulation led to the formation of robust 24-h memory. Furthermore, the unequal rewarding of three sugars was observed in two distinct processes of memory formation: preingestive and postingestive processes. When flies received the positive tastes (preingestive signal) by touching their tarsi and proboscis (mouthparts) to three sugars, they showed differences in learning for the three sugar rewards. The formation of a robust 24-h memory was dependent on the postingestive signal triggered by feeding on a sugar. A deficit of 24-h memory was observed only in fructose-feeding flies no matter what sugar was used to stimulate the tarsi. Taken together, our results suggest that three sweet and metabolizable sugars unequally rewarded B. dorsalis, which might be a strategy for flies to discriminate the nature of sugars.

Nutrient-conditioned intake stimulation does not require a distinctive flavor cue in rats

The postoral actions of nutrients in rodents can stimulate intake and condition flavor preferences through an appetition process. Appetition is revealed in rodents by their increased intake of and preference for a flavored solution paired with intragastric (IG) nutrient infusions. Here we determined if IG 16% maltodextrin (MD) infusions can stimulate intake and preference in the absence of a distinctive flavor cue. Rats implanted with IG catheters were given chow and water 2 h/day followed, 2 h later, by 20-h oral access to water paired with IG MD infusions. Other rats were given bitter sucrose octaacetate solution (SOA) paired with IG MD infusions 20 h/day. Over 8 test days, the SOA rats increased their total 20-h fluid intake (oral + IG) from 26 to 119 g/20 h and Water rats increased their intake from 31 to 96 g/20 h. When infused IG with water instead of MD in a 4-day extinction test, the SOA and Water groups reduced their fluid intakes to 45-48 g/20 h. When oral fluids were again paired with IG MD infusions, the SOA and Water groups increased their intakes to 115 and 109 g/20 h, respectively. In two-bottle tests, the SOA rats drank more SOA paired with IG MD than water paired with IG water. Water rats given the choice of a water bottle paired with IG MD and water bottle paired with IG water did not consistently prefer the H2O/ID MD bottle. Instead they displayed side or sipper tube preferences although neither cue was consistently paired with IG MD during one-bottle training.

Olfaction contributes to the learned avidity for glucose relative to fructose in mice

When offered glucose and fructose solutions, rodents consume more glucose solution because it produces stronger postoral reinforcement. Intake of these sugars also conditions a higher avidity for glucose relative to fructose. We asked which chemosensory cue mediates the learned avidity for glucose. We subjected mice to 18 days of sugar training, offering them 0.3, 0.6, and 1 M glucose and fructose solutions. Before and after training, we measured avidity for 0.3 and 0.6 M glucose and fructose in brief-access lick tests. First, we replicated prior work in C57BL/6 mice. Before training, the mice licked at a slightly higher rate for 0.6 M fructose; after training, they licked at a higher rate for 0.6 M glucose. Second, we assessed the necessity of the glucose-specific ATP-sensitive K+(KATP) taste pathway for the learned avidity for glucose, using mice with a nonfunctional KATPchannel [regulatory sulfonylurea receptor (SUR1) knockout (KO) mice]. Before training, SUR1 KO and wild-type mice licked at similar rates for 0.6 M glucose and fructose; after training, both strains licked at a higher rate for 0.6 M glucose, indicating that the KATPpathway is not necessary for the learned discrimination. Third, we investigated the necessity of olfaction by comparing sham-treated and anosmic mice. The mice were made anosmic by olfactory bulbectomy or ZnSO4treatment. Before training, sham-treated and anosmic mice licked at similar rates for 0.6 M glucose and fructose; after training, sham-treated mice licked at a higher rate for 0.6 M glucose, whereas anosmic mice licked at similar rates for both sugars. This demonstrates that olfaction contributes significantly to the learned avidity for glucose.

Operant conditioning paradigm for juxtacellular recordings in functionally identified cortical neurons during motor execution in head-fixed rats

BACKGROUND

Understanding the configuration of neural circuits and the specific role of distinct cortical neuron types involved in behavior, requires the study of structure-function and connectivity relationships with single cell resolution in awake behaving animals. Despite head-fixed behaving rats have been used for in vivo measuring of neuronal activity, it is a concern that head fixation could change the performance of behavioral task.

NEW METHOD

We describe the procedures for efficiently training Wistar rats to develop a behavioral task, involving planning and execution of a qualified movement in response to a visual cue under head-fixed conditions. The behavioral and movement performance in freely moving vs head-fixed conditions was analyzed.

RESULTS

The best behavioral performance was obtained in the rats that were trained first in freely moving conditions and then placed in a head-restrained condition compared with the animals which first were habituated to head-restriction and then learned the task. Moreover, head restriction did not alter the movement performance. Stable juxtacellular recordings from sensorimotor cortex neurons were obtained while the rats were performing forelimb movements. Biocytin electroporation and retrograde tracer injections, permits identify the hodology of individual long-range projecting neurons.

COMPARISON WITH EXISTING METHODS

Our method shows no difference in the behavioral performance of head fixed and freely moving conditions. Also includes a computer aided design of a discrete and ergonomic head-post allowing enough stability to perform juxtacellular recording and labeling of cortical neurons.

CONCLUSIONS

Our method is suitable for the in vivo characterization of neuronal circuits and their long-range connectivity.

Capsaicin-induced visceral deafferentation does not attenuate flavor conditioning by intragastric fat infusions in mice

The postoral actions of sugar and fat can rapidly stimulate the intake of and preference for flavors associated with these nutrients via a process known as appetition. Prior findings revealed that postoral glucose appetition is not attenuated following capsaicin-induced visceral deafferentation. The present experiment determined if capsaicin treatment altered fat appetition in C57BL/6 mice. Following capsaicin (Cap) or control (Con) treatment, mice were fitted with chronic intragastric (IG) catheters. They were then given 1-h sessions with a flavored saccharin solution (CS-) paired with IG water infusion or a different flavor (CS+) paired with IG 6.4% fat infusion. IG fat stimulated CS+ intakes in both Cap and Con mice, and the groups displayed similar preferences for CS+ over CS- in two-choice tests. These results confirm prior reports of normal fat conditioning in rats exposed to capsaicin or vagal deafferentation surgery. In contrast, other recent findings indicate that total or selective vagotomy alters the preference of mice for dilute vs. concentrated fat sources.

Parabrachial Interleukin-6 Reduces Body Weight and Food Intake and Increases Thermogenesis to Regulate Energy Metabolism

Chronic low-grade inflammation and increased serum levels of the cytokine IL-6 accompany obesity. For brain-produced IL-6, the mechanisms by which it controls energy balance and its role in obesity remain unclear. Here, we show that brain-produced IL-6 is decreased in obese mice and rats in a neuroanatomically and sex-specific manner. Reduced IL-6 mRNA localized to lateral parabrachial nucleus (lPBN) astrocytes, microglia, and neurons, including paraventricular hypothalamus-innervating lPBN neurons. IL-6 microinjection into lPBN reduced food intake and increased brown adipose tissue (BAT) thermogenesis in male lean and obese rats by increasing thyroid and sympathetic outflow to BAT. Parabrachial IL-6 interacted with leptin to reduce feeding. siRNA-mediated reduction of lPBN IL-6 leads to increased weight gain and adiposity, reduced BAT thermogenesis, and increased food intake. Ambient cold exposure partly normalizes the obesity-induced suppression of lPBN IL-6. These results indicate that lPBN-produced IL-6 regulates feeding and metabolism and pinpoints (patho)physiological contexts interacting with lPBN IL-6.

The gut microbiome as a driver of individual variation in cognition and functional behaviour

Research into proximate and ultimate mechanisms of individual cognitive variation in animal populations is a rapidly growing field that incorporates physiological, behavioural and evolutionary investigations. Recent studies in humans and laboratory animals have shown that the enteric microbial community plays a central role in brain function and development. The 'gut-brain axis' represents a multi-directional signalling system that encompasses neurological, immunological and hormonal pathways. In particular it is tightly linked with the hypothalamic-pituitary-adrenal axis (HPA), a system that regulates stress hormone release and influences brain development and function. Experimental examination of the microbiome through manipulation of diet, infection, stress and exercise, suggests direct effects on cognition, including learning and memory. However, our understanding of these processes in natural populations is extremely limited. Here, we outline how recent advances in predominantly laboratory-based microbiome research can be applied to understanding individual differences in cognition. Experimental manipulation of the microbiome across natal and adult environments will help to unravel the interplay between cognitive variation and the gut microbial community. Focus on individual variation in the gut microbiome and cognition in natural populations will reveal new insight into the environmental and evolutionary constraints that drive individual cognitive variation.This article is part of the theme issue 'Causes and consequences of individual differences in cognitive abilities'.

From appetite setpoint to appetition: 50years of ingestive behavior research

I review the main themes of my 50-year research career in ingestive behavior as a graduate student at the University of Chicago and a professor at the City University of New York. A seminar course with my Ph.D. mentor, S. P. Grossman, sparked my interest in the hypothalamic obesity syndrome. I developed a wire knife to dissect the neuropathways and the functional disorder responsible for the syndrome. An elevated appetite setpoint that permitted the overconsumption of palatable foods appeared central to the hypothalamic syndrome. In brain-intact rats, providing an assortment of highly palatable foods (the cafeteria diet) stimulated diet-induced obesity that mimicked elements of hypothalamic obesity. Studies of the determinants of food palatability led to the discovery of a "new" carbohydrate taste (maltodextrin taste) and the confirmation of a fatty taste. In addition to oral taste receptors, gut nutrient sensors stimulated the intake/preference for carbohydrate- and fat-rich foods via an appetition process that stimulates brain reward systems. My research career greatly benefited from many diligent and creative students, collaborators and technicians and research support from my university and the National Institutes of Health.

Appetitive olfactory learning and memory in the honeybee depend on sugar reward identity

One of the most important tasks of the brain is to learn and remember information associated with food. Studies in mice and Drosophila have shown that sugar rewards must be metabolisable to form lasting memories, but few other animals have been studied. Here, we trained adult, worker honeybees (Apis mellifera) in two olfactory tasks (massed and spaced conditioning) known to affect memory formation to test how the schedule of reinforcement and the nature of a sugar reward affected learning and memory. The antennae and mouthparts of honeybees were most sensitive to sucrose but glucose and fructose were equally phagostimulatory. Whether or not bees could learn the tasks depended on sugar identity and concentration. However, only bees rewarded with glucose or sucrose formed robust long-term memory. This was true for bees trained in both the massed and spaced conditioning tasks. Honeybees fed with glucose or fructose exhibited a surge in haemolymph sugar of greater than 120mM within 30s that remained elevated for as long as 20min after a single feeding event. For bees fed with sucrose, this change in haemolymph glucose and fructose occurred with a 30s delay. Our data showed that olfactory learning in honeybees was affected by sugar identity and concentration, but that olfactory memory was most strongly affected by sugar identity. Taken together, these data suggest that the neural mechanisms involved in memory formation sense rapid changes in haemolymph glucose that occur during and after conditioning.

Glucose Intake Alters Expression of Neuropeptides Derived from Proopiomelanocortin in the Lateral Hypothalamus and the Nucleus Accumbens in Fructose Preference Rats

To study the neuroendocrine mechanism of sugar preference, we investigated the role of glucose feeding in the regulation of expression levels of neuropeptides derived from proopiomelanocortin (POMC) in the lateral hypothalamus (LH) and nucleus accumbens (NAc) in fructose preference rats. Fructose preference rats were induced by using the lithium chloride backward conditioning procedure. The fructose preference was confirmed by the two-bottle test. The drinking behavior of rats was assessed by the fructose concentration gradient test. The preference of 10% glucose or 0.1% saccharine was assessed, and the expression levels of neuropeptides derived from POMC in the LH and the NAc in fructose preference rats were measured by Western blot analysis. Fructose preference rats displayed a greater fructose preference than control rats. Furthermore, fructose preference rats preferred glucose solution rather than saccharine solution, while control rats preferred saccharine solution rather than glucose solution. The expression levels of neuropeptides derived from POMC in the LH and the NAc were changed by glucose but not saccharine intake. In summary, the data suggests that glucose intake increases the expression of neuropeptides derived from POMC in the LH and the NAc in fructose preference rats.

Rewarding Effects of Operant Dry-Licking Behavior on Neuronal Firing in the Nucleus Accumbens Core

Certain eating behaviors are characterized by a trend of elevated food consumption. However, neural mechanisms mediating the motivation for food consumption are not fully understood. Food impacts the brain-rewarding-system via both oral-sensory and post-ingestive information. Recent studies have reported an important role of visceral gut information in mediating dopamine (DA) release in the brain rewarding system. This is independent of oral sensation, suggesting a role of the gut-brain-DA-axis in feeding behavior. In this study, we investigated the effects of intra-gastric (IG) self-administration of glucose on neuronal firings in the nucleus accumbens (NA) of water-deprived rats. Rats were trained in an operant-licking paradigm. During training, when the light was on for 2 min (light-period), rats were required to lick a spout to acquire the water oral-intake learning, and either an IG self-infusion of 0.4 M glucose (GLU group) or water (H₂O group). Rats rested in the dark-period (3 min) following the light-period. Four cycles of the operant-licking paradigm consisting of the light–dark periods were performed per day, for 4 consecutive days. In the test session, the same rats licked the same spout to acquire the IG self-administration of the corresponding solutions, without oral water ingestion (dry licking). Behavioral results indicated IG self-administration of glucose elicits more dry-licking behavior than that of water. Neurophysiological results indicated in the dark period, coefficient of variance (CV) measuring the inter-spike interval variability of putative medial spiny neurons (pMSNs) in the NA was reduced in the H₂O group compared to the GLU group, while there was no significant difference in physical behaviors in the dark period between the two groups. Since previous studies reported that DA release increases CV of MSNs, the present results suggest that greater CV of pMSNs in the GLU group reflects greater DA release in the NA and elevated motivation in the GLU group, which might increase lickings in the test session in the GLU group compared to the H₂O group.

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

Animal models of decision-making rely on an animal's motivation to decide and its ability to detect differences among various alternatives. Food reinforcement, although commonly used, is associated with problematic confounds, especially satiety. Here, we examined the use of brain stimulation reward (BSR) as an alternative reinforcer in rodent models of decision-making and compared it with the effectiveness of sugar pellets. The discriminability of various BSR frequencies was compared to differing numbers of sugar pellets in separate free-choice tasks. We found that BSR was more discriminable and motivated greater task engagement and more consistent preference for the larger reward. We then investigated whether rats prefer BSR of varying frequencies over sugar pellets. We found that animals showed either a clear preference for sugar reward or no preference between reward modalities, depending on the frequency of the BSR alternative and the size of the sugar reward. Overall, these results suggest that BSR is an effective reinforcer in rodent decision-making tasks, removing food-related confounds and resulting in more accurate, consistent, and reliable metrics of choice.

Introducing Clicker Training as a Cognitive Enrichment for Laboratory Mice

Establishing new refinement strategies in laboratory animal science is a central goal in fulfilling the requirements of Directive 2010/63/EU. Previous research determined a profound impact of gentle handling protocols on the well-being of laboratory mice. By introducing clicker training to the keeping of mice, not only do we promote the amicable treatment of mice, but we also enable them to experience cognitive enrichment. Clicker training is a form of positive reinforcement training using a conditioned secondary reinforcer, the "click" sound of a clicker, which serves as a time bridge between the strengthened behavior and an upcoming reward. The effective implementation of the clicker training protocol with a cohort of 12 BALB/c inbred mice of each sex proved to be uncomplicated. The mice learned rather quickly when challenged with tasks of the clicker training protocol, and almost all trained mice overcame the challenges they were given (100% of female mice and 83% of male mice). This study has identified that clicker training for mice strongly correlates with reduced fear in the mice during human-mice interactions, as shown by reduced anxiety-related behaviors (e.g., defecation, vocalization, and urination) and fewer depression-like behaviors (e.g., floating). By developing a reliable protocol that can be easily integrated into the daily routine of the keeping of laboratory mice, the lifetime experience of welfare in the mice can be improved substantially.

Post-oral sugar detection rapidly and chemospecifically modulates taste-guided behavior

Several recent studies have shown that post-oral sugar sensing rapidly stimulates ingestion. Here, we explored the specificity with which early-phase post-oral sugar sensing influenced ingestive motivation. In experiment 1, rats were trained to associate the consumption of 0.3 M sucrose with injections of LiCl (3.0 meq/kg ip, conditioned taste aversion) or given equivalent exposures to the stimuli, but in an unpaired fashion. Then, all rats were subjected to two brief-access tests to assess appetitive and consummatory responses to the taste properties of sucrose (0.01-1.0 M), 0.12 M NaCl, and dH₂O (in 10-s trials in randomized blocks). Intraduodenal infusions of either 0.3 M sucrose or equiosmolar 0.15 M NaCl (3.0 ml) were administered, beginning just before each test. For unpaired rats, intraduodenal sucrose specifically enhanced licking for 0.03-1.0 M sucrose, with no effect on trial initiation, relative to intraduodenal NaCl. Rats with an aversion to sucrose suppressed licking responses to sucrose in a concentration-dependent manner, as expected, but the intraduodenal sucrose preload did not appear to further influence licking responses; instead, intraduodenal sucrose attenuated trial initiation. Using a serial taste reactivity (TR) paradigm, however, experiment 2 demonstrated that intraduodenal sucrose preloads suppressed ingestive oromotor responses to intraorally delivered sucrose in rats with a sucrose aversion. Finally, experiment 3 showed that intraduodenal sucrose preloads enhanced preferential licking to some representative tastants tested (sucrose, Polycose, and Intralipid), but not others (NaCl, quinine). Together, the results suggest that the early phase-reinforcing efficacy of post-oral sugar is dependent on the sensory and motivational properties of the ingesta.

MCH receptor deletion does not impair glucose-conditioned flavor preferences in mice

The post-oral actions of glucose stimulate intake and condition flavor preferences in rodents. Hypothalamic melanin-concentrating hormone (MCH) neurons are implicated in sugar reward, and this study investigated their involvement in glucose preference conditioning in mice. In Exp. 1 MCH receptor 1 knockout (KO) and C57BL/6 wildtype (WT) mice learned to prefer 8% glucose over an initially more-preferred non-nutritive 0.1% sucralose+saccharin (S+S) solution. In contrast, the KO and WT mice preferred S+S to 8% fructose, which is consistent with this sugar's weak post-oral reinforcing action. In Exp. 2 KO and WT mice were trained to drink a flavored solution (CS+) paired with intragastric (IG) infusion of 16% glucose and a different flavored solution (CS-) paired with IG water. Both groups drank more CS+ than CS- in training and preferred the CS+ to CS- in a 2-bottle test. These results indicate that MCH receptor signaling is not required for flavor preferences conditioned by the post-oral actions of glucose. This contrasts with other findings implicating MCH signaling in other types of sugar reward processing.