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.

Further disentangling the motivational processes underlying benzodiazepine hyperphagia.

In addition to their well-known anxiolytic functions, benzodiazepines produce hyperphagia. Previously, we reported that the benzodiazepine, chlordiazepoxide (CDP), increased consumption of both normally-preferred and normally-avoided taste stimuli during long-term (1 h) tests, primarily through changes in licking microstructure patterns associated with hedonic taste evaluation, whereas there was little effect on licking microstructure measures associated with post-ingestive feedback. In this study, we further examined the hedonic and motivational specificity of CDP effects on ingestive behavior. We tested brief access (15 s) licking responses for tastants spanning all taste qualities after treatment with either CDP (5 or 10 mg/kg) or the non-benzodiazepine anxiolytic, buspirone (1.5 or 3 mg/kg). A between-subjects, counterbalanced design compared the CDP or buspirone effects on licking responses for water and a range of weak to strong concentrations of NaCl, Q-HCl, citric acid, MSG, saccharin, and capsaicin under water-restricted (23 h) conditions; and sucrose, saccharin, and MSG under water-replete conditions. In a dose dependent manner, CDP increased licking for taste stimuli that were normally-avoided after saline treatment, with a notable exception observed for the trigeminal stimulus, capsaicin, which was not affected at any concentration or drug dose, suggesting a taste-specific effect of CDP on orosensory processing. Under water-replete conditions, CDP dose-dependently increased licking to normally-accepted concentrations of sucrose, saccharin, and MSG. There was no effect of either drug on licks for water under either water-restricted or water-replete conditions. Buspirone slowed oromotor coordination by increasing brief interlick intervals, but it did not affect licking for any concentrations of the tastants. Overall, these results indicate that benzodiazepines selectively enhance the hedonic acceptance of gustatory orosensory stimuli, independent of general anxiolytic or oromotor coordination effects, or physiological states such as thirst.

Behavioral and neurophysiological taste responses to sweet and salt are diminished in a model of subclinical intestinal inflammation.

There is strong evidence for gut-taste bud interactions that influence taste function, behavior and feeding. However, the effect of gut inflammation on this axis is unknown despite reports of taste changes in gastrointestinal (GI) inflammatory conditions. Lipopolysaccharide (LPS), an inflammatory stimulus derived from gram-negative bacteria, is present in the normal GI tract and levels increase during high-fat feeding and gut infection and inflammation. Recordings from the chorda tympani nerve (CT), which transmits taste information from taste buds on the anterior tongue to the brain, previously revealed a transient decrease in sucrose responses in mice that ingest LPS during a single overnight period. Here we test the effect of acute or chronic, weekly LPS gavage on licking behavior and CT responses. Using brief-access testing, rats treated with acute LPS and mice receiving acute or chronic LPS decreased licking responses to sucrose and saccharin and to NaCl in mice. In long-term (23 h) tests chronic LPS also reduced licking responses to saccharin, sucrose, and NaCl in mice. Neurophysiological recordings from the CT supported behavioral changes, demonstrating reduced responses to sucrose, saccharin, acesulfame potassium, glucose and NaCl in acute and chronic LPS groups compared to controls. Chronic LPS significantly elevated neutrophils in the small intestine and colon, but LPS was not detected in serum and mice did not display sickness behavior or lose weight. These results indicate that sweet and salt taste sensitivity could be reduced even in asymptomatic or mild localized gut inflammatory conditions such as inflammatory bowel disease.

Selective stimulation of central GABAAα2,3,5 receptors increases intake and motivation to consume sucrose solution in rats.

Benzodiazepines are one of the most commonly prescribed anxiolytic drugs in America, and between 2006 and 2015 prescription rates increased by an estimated 27.1%. Weight gain is a common side effect of these drugs and it may result from increased feeding caused by drug-enhanced food palatability. We investigated the role of specific GABAA receptor subtypes involved with benzodiazepine-induced food consumption through third ventricle injections of L-838,417, a partial agonist of GABAA α2, α3, and α5 subunits, and a full antagonist of the α1 receptor subunit. A microanalysis of the licking behavior of adult male rats to a sucrose solution was used to isolate drug effects on specific consummatory behaviors that include: hedonic taste evaluation, food approach behavior, and oromotor function. L-838,417 dose-dependently increased intake through increases in the motivation to approach the solution (shorter pause intervals between bouts of licking) and through enhancement of measures associated with hedonic taste evaluation. Oromotor depressant effects previously associated with broad-spectrum benzodiazepine receptor agonists were not observed. These results indicate that nuclei in proximity to the ventricles respond to GABAA α2, α3, or α5 activation to induce motivation to feed, absent of α1 receptor subunit activation. Furthermore, activation of the α1 subunit is not necessary for benzodiazepine hyperphagia and may instead contribute to the oromotor depressant and sedative properties of classic benzodiazepine agonists. Hypothalamic nuclei such as the paraventricular nucleus may be involved in the benzodiazepine-increased motivation to feed, while the parabrachial nucleus of the hindbrain could contribute to benzodiazepine-induced enhancement of taste palatability.

The Boss' Healthy Buddies Nutrition Resource Is Effective for Elementary School Students.

Previously we have shown that our Healthy Eating Decisions school-based intervention can influence students' selections of the healthiest foods available in their elementary school cafeterias through positive reinforcement techniques. Although effective, we recognized that students were missing fundamental nutrition knowledge necessary to understand why the Healthy Eating Decisions program identified particular beverages and foods as the healthiest in the cafeteria. Therefore, we developed the Boss' Healthy Buddies nutrition education resource as a freely available curriculum matched with South Carolina education standards and designed for elementary school students from kindergarten through fourth grade. The current study implemented Boss' Healthy Buddies and compared its efficacy to a commercially available nutrition program, CATCH. Elementary school students in Spartanburg, South Carolina, received weekly twenty-minute Boss' Healthy Buddies lessons for eight weeks. Results from preassessment and postassessment surveys were compared with a positive control elementary school using the CATCH program and a negative control school receiving no nutrition education. Results show that Boss' Healthy Buddies was equally effective as the CATCH program in improving the nutrition attitudes regarding healthiest beverages and food selections with the advantage of being freely available and minimizing the impact on classroom instruction time. In order to reduce most effectively the high prevalence of childhood overweight and obesity, it is crucial that children are taught nutrition education to support healthy eating habits at an early age. Both the Healthy Eating Decisions school-based intervention and the Boss' Healthy Buddies nutrition education program are available online for use as free resources to aid in reducing childhood overweight and obesity within elementary schools.

The Palatability of Lopinavir and Ritonavir Delivered by an Innovative Freeze-Dried Fast-Dissolving Tablet Formulation.

Negative hedonic sensory qualities of HIV antiretroviral drugs often reduce patient adherence particularly in pediatric populations requiring oral consumption. This study examines the palatability of an innovative delivery mechanism utilizing a freeze-drying-in-blister approach to create fast-dissolving tablets (FDTs) containing a fixed-dose combination of lopinavir and ritonavir (LPV/r). Consumption patterns of solutions during brief-access and long-term testing and baby foodstuff consumption were analyzed to evaluate the orosensory detection and avoidance of placebo FDTs containing no LPV/r (FDT-) and FDTs containing LPV/r (FDT+). Rats showed no change in consumption patterns for the placebo FDT- compared with control solutions. Rats can detect but do not avoid FDT+ at body-weight-adjusted dosages in both brief-access (30-s) and long-term (23 h) consumption tests. There is an aversive response to concentrated doses of FDT+ during brief-access tests that cannot be masked by 25% sucrose. However, the strongest FDT+ concentration was not rejected when mixed with 50 g of applesauce, banana sauce, or rice cereal baby foodstuffs. The averseness of the FDT+ was associated with the presence of LPV/r and not the FDT- formulation itself. The novel FDT formulation appears to be a palatable delivery mechanism for oral antiretroviral pharmaceuticals especially when mixed with baby foodstuffs.

High-Fat Diet Alters the Orosensory Sensitivity to Fatty Acids in Obesity-Resistant but not Obesity-Prone Rats.

Gene-environment interactions play a role in the development of obesity but specific effects of diet on the orosensory detection of fatty acids have yet to be clarified. The objective of this study is to characterize the effect of prolonged (5-week) exposure to a high-fat (60%) diet on the behavioral sensitivity to the fatty acid linoleate following a conditioned taste aversion in obesity-prone and obesity-resistant rats. Exposure to the high-fat diet significantly enhanced the sensitivity of obesity-resistant (S5B/Pl) rats to linoleate while producing no effect on the fatty acid sensitivity for obesity-prone rats. Specifically, high-fat diet fed S5B/Pl rats showed stronger initial avoidance of linoleate and slower extinction rates than their normal diet cohorts. Our study suggests that prolonged dietary fat consumption may alter the behavioral sensitivity to fatty acids particularly in obesity-resistant animals.

Orosensory detection of fatty acids by obesity-prone and obesity-resistant rats: strain and sex differences.

A series of brief-access (15s) behavioral assays following the formation of a conditioned taste aversion (CTA) to linoleic acid were performed in order to follow up on observations showing differences in the chemosensory responses to dietary fat in obesity-prone (Osborne-Mendel [O-M]) and obesity-resistant (S5B/Pl) rat strains. Strong aversions to linoleic acid (conditioned stimulus 100 microM) were generated in both O-M and S5B/Pl rats to concentrations as low as 2.5 microM. Observed strain differences were in contrast to expectations based upon electrophysiological studies previously showing greater fatty acid-induced inhibition of delayed rectifying K+ channels in S5B/Pl rats. In the CTA assays, the O-M rats showed aversions at lower fatty acid concentrations with more resistance to extinction in brief-access orosensory tests, suggesting that the obesity-prone strain may be more sensitive in the detection and subsequent avoidance of linoleic acid than the obesity-resistant strain. The independent variable of sex produced even greater differences in the avoidance of linoleic acid following conditioning than the effects of strain. Female rats of both strains were significantly more sensitive to fatty acids, showed greater cross-generalization from linoleic to oleic acid, and showed greater avoidance of linoleic acid than male counterparts. These findings suggest genetic influences on yet to be identified mechanisms potentially within the gustatory system that affect the sensitivity to detect the fatty acid chemicals found in dietary fat during brief-access orosensory testing.

Linoleic and oleic acids alter the licking responses to sweet, salt, sour, and bitter tastants in rats.

The free fatty acids (FFAs), linoleic and oleic acids, commonly found in dietary fats can be detected by rats on the basis of gustatory cues following conditioned taste aversion pairings. FFAs depolarize the membrane potential of isolated rat taste receptor cells by inhibiting delayed rectifying potassium channels. This study examined the licking response of rats to sweet, salt, sour, and bitter taste solutions when 88 muM linoleic acid, 88 muM oleic acid, or an 88 muM linoleic-oleic acid mixture was added to the solutions. The presence of linoleic, oleic, and the linoleic-oleic acid mixture in sweet solutions produced increases in the licking responses, whereas adding linoleic, oleic, and the linoleic-oleic acid mixture to salt, sour, or bitter taste solutions produced decreases in licking responses when compared with the licking responses to the solutions in the absence of the FFAs. We conclude that FFAs may act in the oral cavity to depolarize taste receptor cells and therefore to increase the perceived intensity of concomitant tastants, thus contributing to the enhanced palatability associated with foods containing high dietary fat.

Detection of free fatty acids following a conditioned taste aversion in rats.

A gustatory transduction mechanism for free fatty acids (FFAs) has been described in isolated rat taste receptor cells; however, the ability of behaving rats to detect FFAs has not been characterized. Through conditioned taste aversion (CTA) methodology, this study defines the ability of rats to detect and avoid the two principal FFA components of corn oil, linoleic and oleic acid. Following taste aversion conditioning, rats avoided both linoleic and oleic acid at greater than or equal to 66 muM and failed to avoid either 44 muM linoleic or oleic acid. Rats demonstrated generalized avoidances between 88 muM linoleic and oleic acid irrespective of presenting the FFAs as either unesterified acids dissolved in 5 mM ethanol or aqueous sodium salts, sodium linoleate and sodium oleate. Following a CTA to linoleic acid, rats did not show generalized avoidance of NaCl or ethanol, two potentially concomitant tastants in the oral cavity. A CTA to linoleic or oleic acid did produce a generalized avoidance to the other FFA. These results support the ability of rats to detect linoleic and oleic acid (>44 muM) and suggest that the two FFAs share common orosensory properties. Furthermore, it is unlikely that the detection of the FFAs is due to an enhancement of other concomitant tastants such as saliva or the delivery solution.

Behavioral effects on rats of high strength magnetic fields generated by a resistive electromagnet.

It has been reported previously that exposure to static high magnetic fields of 7 T or above in superconducting magnets has behavioral effects on rats. In particular, magnetic field exposure acutely but transiently suppressed rearing and induced walking in tight circles; the direction of circular locomotion was dependent on the rats' orientation within the magnet. Furthermore, when magnet exposure was paired with consumption of a palatable, novel solution, rats acquired a persistent taste aversion. In order to confirm these results under more controlled conditions, we exposed rats to static magnetic fields of 4 to 19.4 T in a 189 mm bore, 20 T resistive magnet. By using a resistive magnet, field strengths could be arbitrary varied from -19.4 to 19.4 T within the same bore. Rearing was suppressed after exposure to 4 T and above; circling was observed after 7 T and above. Conditioned taste aversion was acquired after 14 T and above. The effects of the magnetic fields were dependent on orientation. Exposure to +14 T induced counter-clockwise circling, while exposure to -14 T induced clockwise circling. Exposure with the rostral-caudal axis of the rat perpendicular to the magnetic field produced an attenuated behavioral response compared to exposure with the rostral-caudal axis parallel to the field. These results in a single resistive magnet confirm and extend our earlier findings using multiple superconducting magnets. They demonstrate that the behavioral effects of exposure within large magnets are dependent on the magnetic field, and not on non-magnetic properties of the machinery. Finally, the effects of exposure to 4 T are clinically relevant, as 4 T magnetic fields are commonly used in functional MRI assays.

Behavioral effects of high-strength static magnetic fields on rats.

Advances in magnetic resonance imaging are driving the development of more powerful and higher-resolution machines with high-strength static magnetic fields. The behavioral effects of high-strength magnetic fields are largely uncharacterized, although restraint within a 9.4 T magnetic field is sufficient to induce a conditioned taste aversion (CTA) and induce brainstem expression of c-Fos in rats. To determine whether the behavioral effects of static magnetic fields are dependent on field strength, duration of exposure, and orientation with the field, rats were restrained within the bore of 7 or 14 T superconducting magnets for variable durations. Behavioral effects were assessed by scoring locomotor activity after release from the magnetic field and measuring CTA acquisition after pairing intake of a palatable glucose and saccharin (G+S) solution with magnetic field exposure. Magnetic field exposure at either 7 or 14 T suppressed rearing and induced tight circling. The direction of the circling was dependent on the rat's orientation within the magnetic field: if exposed head-up, rats circled counterclockwise; if exposed head-down, rats circled clockwise. CTA was induced after three pairings of taste and 30 min of 7 T exposure or after a single pairing of G+S and 1 min of 14 T exposure. These results suggest that magnetic field exposure has graded effects on rat behavior. We hypothesize that restraint with high-strength magnetic fields causes vestibular stimulation resulting in locomotor circling and CTA acquisition.

Rearing on basal or high dietary NaCl modifies chorda tympani nerve responses in rats.

Two experiments were conducted to determine the influence of dietary NaCl level on the integrated responses of the chorda tympani (CT) nerve to salt stimulation alone and mixed with the sodium-channel blocker, amiloride hydrochloride. Five groups of adult male rats were reared on regular chow containing either basal 0.1%, intermediate 1.0%, or high 3.0% NaCl from conception to postnatal day (PD) 30 or from conception to adulthood. Adult rats reared from conception to adulthood on basal dietary NaCl demonstrated a reduction in the CT nerve response to NaCl due to a decrease in the amiloride sensitive transduction mechanism. However, the CT nerve responses of adult rats reared on basal dietary NaCl to PD30 and then switched to intermediate dietary NaCl were similar to those of rats reared for a lifetime on intermediate dietary NaCl. Similarly, the CT nerve responses to NaCl in animals reared on high dietary NaCl from conception to PD30 and then switched to an intermediate NaCl diet were comparable to animals reared on intermediate and basal dietary NaCl. However, we found that exposure to high dietary NaCl led to a greater amiloride inhibition of NaCl responses. Thus, there is critical association between dietary NaCl level over two different exposure periods and CT nerve responsiveness to NaCl specifically regarding the degree of amiloride inhibition.

Dietary NaCl influences the organization of chorda tympani neurons projecting to the nucleus of the solitary tract in rats.

Prior research has shown that maintained exposure to either a low or high NaCl diet from conception to adulthood is associated with changes in NaCl solution intake and neural responses of the chorda tympani (CT) nerve. The present study examined the influence of maintained exposure to a low or high NaCl diet on the central organization of CT neurons projecting to the nucleus of the solitary tract (NST). Three groups of rats were reared and maintained on regular chow containing either basal 0.1%, intermediate 1.0% or high 6% NaCl from conception to adulthood. The fluorescent marker Dil was applied to the CT for characterization of afferent terminations and efferent cell body labeling in the brainstem. The total NST area occupied by CT afferent fibers was the same for all three dietary groups. However, the pattern of CT innervation differed such that there was an enlarged dorsal terminal field in the high group. There were no group differences in body and brain weight, or in efferent labeled neurons. Thus, Dil has been demonstrated to be an effective transport marker of the gustatory system and the parameters of dietary NaCl exposure that influence the pattern of the CT fibers projecting to the NST have been further clarified.