Postingestive Effects Research Articles

Both Gas Chromatography and an Electronic Nose Reflect Chemical Polymorphism of Juniper Shrubs Browsed or Avoided by Sheep

Chemical polymorphism may contribute to variation in browsing damage by mammalian herbivores. Earlier, we demonstrated that essential oil concentration in juniper, Juniperus communis, was negatively associated with herbivore browsing. The aim of the present study was to characterize the volatile chemical composition of browsed and non-browsed J. communis. By using either gas chromatography with flame ionization detection (GC-FID) or an electronic nose device, we could separate sheep-browsed or non-browsed juniper shrubs by their essential oil pattern and complex odor matrix. The main components of the essential oil from J. communis were monoterpenes. We distinguished three chemotypes, dominated either by α-pinene, sabinene, or δ-3-carene. Shrubs belonging to the α-pinene- or sabinene-dominated groups were browsed, whereas all individuals with the δ-3-carene chemotype were unused by the local herbivores. The electronic nose also separated the browsed and non-browsed shrubs indicating that their odor matrix could guide sheep browsing. Responses of sheep could integrate the post-ingestive effects of plant secondary metabolites with sensory experience that stems from odor-phytotoxin interactions. Chemotype diversity could increase the survival rate in the present population of J. communis as certain shrubs could benefit from relatively better chemical protection against the herbivores.

Transmission of self-medicative behaviour from mother to offspring in sheep

Herbivores challenged by diets with high concentrations of tannins learn by individual experience to self-select medicinal compounds such as polyethylene glycol (PEG), which neutralizes the negative postingestive effects of tannins. We investigated the transmission of this acquired self-medicative behaviour from mother to offspring. One group of ewes (experienced, N = 8) was conditioned to associate the beneficial effects of PEG after consuming a tannin-rich diet. Ewes ingested a meal of high-tannin food and were then offered PEG. Subsequently, ewes ingested the same tannin-rich meal and were then offered a food (grape pomace; control) that did not have the medicinal effects of PEG. After conditioning, the experienced group and a naïve group of ewes (N = 8) were given a choice between the high-tannin food, PEG and grape pomace. Experienced ewes showed higher intake and preference for PEG than did naïve ewes (P < 0.05). Subsequently, experienced and naïve ewes with their naïve lambs, as well as a group of naïve lambs without their mothers (N = 8), were exposed to the tannin-rich diet, PEG and grape pomace. Lambs were then tested for their ability to self-medicate with PEG by offering them a choice between the tannin-rich diet, PEG and grape pomace. Lambs from experienced and naïve mothers showed a higher preference for PEG than did lambs exposed without their mothers (P = 0.05). Thus, the presence of the mother (experienced or naïve) was important for naïve lambs to learn about the medicinal benefits of PEG. We conclude that the mother’s presence per se may increase the efficiency of creating new knowledge, such as preference for a medicine, within a group, beyond transmitting and maintaining this knowledge across generations.

Remembering Nutrient Quality of Sugar in Drosophila

Taste is an early stage in food and drink selection for most animals [1, 2]. Detecting sweetness indicates the presence of sugar and possible caloric content. However, sweet taste can be an unreliable predictor of nutrient value because some sugars cannot be metabolized. In addition, discrete sugars are detected by the same sensory neurons in the mammalian [3] and insect [4, 5] gustatory systems, making it difficult for animals to readily distinguish the identity of different sugars using taste alone [6-8]. Here we used an appetitive memory assay in Drosophila [9-11] to investigate the contribution of palatability and relative nutritional value of sugars to memory formation. We show that palatability and nutrient value both contribute to reinforcement of appetitive memory. Nonnutritious sugars formed less robust memory that could be augmented by supplementing with a tasteless but nutritious substance. Nutrient information is conveyed to the brain within minutes of training, when it can be used to guide expression of a sugar-preference memory. Therefore, flies can rapidly learn to discriminate between sugars using a postingestive reward evaluation system, and they preferentially remember nutritious sugars.

Drosophila Evaluates and Learns the Nutritional Value of Sugars

Living organisms need to search for and ingest nutritional chemicals, and gustation plays a major role in detecting and discriminating between chemicals present in the environment. Using Drosophila as a model organism, we asked whether animals have the ability to evaluate the nutritional value of sugars. In flies, chemosensilla on the tarsi and labellum are the gustatory organs used to discriminate between edible and nonedible compounds [1, 2]. We noticed that Drosophila do not assign nutritional values to all sweet chemicals. D-arabinose is sweet to flies, but it provides them with no nutrition. By contrast, the sugar alcohol D-sorbitol is not sensed as sweet, but flies can live on it. We performed behavioral and electrophysiological measurements to confirm these gustatory and feeding responses. We found that Drosophila can learn the nutritional value of nonsweet D-sorbitol when it is associated with an odor cue. The learning process involved the synapsin molecule, suggesting that a neuronal mechanism is involved. We propose that Drosophila uses neural machinery to detect, evaluate, and learn the nutritional value of foods after ingestion.

Chemosensory processing in the taste - reward pathway.

Although the act of eating is voluntary, its initiation depends on several factors including its taste and the animal's internal state as related to hunger or satiety. These factors together with the food's hedonic value will determine whether food will be ingested. The taste of food will depend on the activation of receptors located on taste cells but also on the expectation of what it will taste like. For these reasons, it is important to investigate, in behaving animals, the neural correlates of feeding behavior in the taste-reward pathway. Here we review particular coding strategies, present experiments using freely licking rodents with chronically implanted arrays of electrodes throughout the taste-reward pathway to investigate the changes that occur when animals learn to discriminate among tastants and after they are ingested. In summary, we found that gustatory processing does not only depend on the input from the oral cavity but on expectation, learning, and post-ingestive effects.

Bioactive saponins from Microsechium helleri and Sicyos bulbosus

Eleven oleanane-type saponins ( 1– 11) have been isolated from Microsechium helleri and Sicyos bulbosus roots and were evaluated for their antifeedant, nematicidal and phytotoxic activities. Saponins {3- O-β- d-glucopyranosyl (1 → 3)-β- d-glucopyranosyl-2β,3β,16α,23-tetrahydroxyolean-12-en-28-oic acid 28- O-α- l-rhamnopyranosyl-(1 → 3)-β- d-xylopyranosyl-(1 → 4)-[β- d-xylopyranosyl-(1 → 3)]-α- l-rhamnopyranosyl-(1 → 2)-α- l-arabinopyranoside} ( 1), and {3- O-β- d-glucopyranosyl-2β,3β,16α,23-tetrahydroxyolean-12-en-28-oic acid 28- O-α- l-rhamnopyranosyl-(1 → 3)-β- d-xylopyranosyl-(1 → 4)-[β- d-xylopyranosyl-(1 → 3)]-α- l-rhamnopyranosyl-(1 → 2)-α- l-arabinopyranoside} ( 2) were also isolated from M. helleri roots together with the two known compounds 3 and 4. Seven known structurally related saponins ( 5– 11) were isolated from S. bulbosus roots. The structures of these compounds were established as bayogenin and polygalacic glycosides using one- and two-dimensional NMR spectroscopy and mass spectrometry. Compounds 7, 10, bayogenin ( 12) and polygalacic acid ( 13) showed significant ( p < 0.05) postingestive effects on Spodoptera littoralis larvae, compounds 5– 11 and 12 showed variable nematicidal effects on Meloydogyne javanica and all tested saponins had variable phytotoxic effects on several plant species ( Lycopersicum esculentum, Lolium perenne and Lactuca sativa). These are promising results in the search for natural pesticides from the Cucurbitaceae family.

Effect of masking organoleptic properties of fat on diet self-selection by the sparid Diplodus puntazzo

Fish are able to select a balanced diet according to their nutritional needs by choosing among incomplete feeds or even pure macronutrients. However, the relevance of both the organoleptic properties of diet and the postingestive signals that they produce remains unclear. Thus, sharpsnout seabream were allowed to select between diets containing different edible oils with their organoleptic properties masked by using gelatine capsules. Fish were fed capsules of two different colours so that they could associate the capsule colour with its corresponding postingestive effect. The longitudinal experiment included a first phase during which the fish were adapted to consuming the gelatine capsules. In a second phase, the fish were challenged with two different encapsulated diets: one comprising a complete diet containing fish oil and the other a fat-free diet. Sharpsnout seabream showed a preference for the fish oil capsules (3.8 ± 1.1 g kg−1 body weight (BW), 66.8% of total intake) over the fat-free capsules, showing that they were able to associate the colour of the capsule with their nutritional content through postingestive signals. After that, the fish were challenged to select between the capsules containing the fish oil diet and capsules containing a vegetable oil (linseed or soybean), in which case they showed no preference between diets (2.4 ± 0.3: 2.1 ± 0.5 g kg−1 BW of fish oil versus linseed oil capsules and 2.2 ± 0.2: 1.8 ± 0.6 g kg−1 BW of fish oil versus soybean oil capsules), indicating that the fatty acid composition of the different oils was not sufficient to affect dietary selection through postingestive signals. So, in conclusion, when orosensorial information from food is absent, the fish are able to select between diets at a macronutrient level by using postingestive information. However, this information is not sufficient for distinguishing between diets that differ in the type of oil used.

Decreased Preference for Sucrose in Rats Fed a Low Protein Diet and Its Intensification by Zinc-Deficiency

This study examined whether protein concentrations in the diet of rats fed adequate Zn or deficient Zn affect their preference for disaccharides of sucrose and maltose. Sucrose and maltose were used as a source of carbohydrate and the selection patterns of rats were analyzed for 28 d by a two-choice selection method. Diets provided as a set of two either Zn-adequate or Zn-deficient diets containing 10, 20 and 40% protein each were changed in position daily. For the first 24 h, both the control Zn-adequate and Zn-deficient rats preferred sucrose to maltose and then gradually selected the maltose diet. Protein in the diet affected the selection of the disaccharides for both the control and Zn-deficient rats. The decrease order of the ratio of consumed sucrose to maltose over 28 d was 10% > 20% > 40% protein diet group, and Zn-deficiency emphasized the decrease. These results suggest that sucrose has a stronger taste effect than maltose in rats, but that the selection of sucrose is decreased by the post-ingestive effect which is stimulated in low-protein and Zn-deficient diets.

Ethanol induces second-order aversive conditioning in adolescent and adult rats

Alcohol abuse and dependence are considered public health problems, with an etiological onset often occurring during late childhood and adolescence, and understanding age-related differences in ethanol sensitivity is important. Low to moderate ethanol doses (0.5 and 2.0 g/kg, intragastrically [i.g.]) induce single-trial, appetitive second-order place conditioning (SOC) in adolescent, but not adult, rats. Recent studies have demonstrated that adolescents may be less sensitive than adults to the aversive properties of ethanol, reflected by conditioned taste aversion. The present study assessed the aversive motivational effects of high-dose ethanol (3.0 and 3.25 g/kg, i.g., for adolescents and adults, respectively) using SOC. Experiment 1 revealed similar blood and brain ethanol levels in adolescent and adult rats given 3.0 and 3.25 g/kg ethanol, respectively. In Experiment 2, animals received ethanol or vehicle paired with intraoral pulses of sucrose (conditioned stimulus 1 [CS1]). After one, two, or three conditioning trials, the rats were presented with the CS1 while in a distinctive chamber (CS2). When tested for CS2 preference, ethanol-treated animals exhibited reduced preference for the CS2 compared with controls. This result, indicative of ethanol-mediated aversive place conditioning, was similar for adolescents and adults; for females and males; and after one, two, or three training trials. In conjunction with previous results, the present study showed that, in adolescent rats subjected to SOC, ethanol’s hedonic effects vary from appetitive to aversive as the ethanol dose increases. Adolescent and adult animals appear to perceive the postingestive effects of high-dose ethanol as similarly aversive when assessed by SOC.

Rats' learned preferences for flavors encountered early or late in a meal paired with the postingestive effects of glucose

Rats learn to prefer flavors that are followed by postingestive effects of nutrients. This experiment investigated whether the timing of a flavor (specifically, in the first or second half of the meal) influences learning about that flavor. Stronger learning about earlier or later flavors would indicate when the rewarding postingestive effects of nutrients are sensed. Rats with intragastric (IG) catheters drank saccharin-sweetened, calorically-dilute solutions with distinct flavors added, accompanied by IG infusion of glucose (+sessions) or water (-sessions). In both types of sessions, an "Early" flavor was provided for the first 8 min and a "Late" flavor for the last 8 min. Thus, rats were trained with Early(+) and Late(+) in high-calorie meals, and Early(-) and Late(-) in low-calorie meals. Strength of the learned preference for Early(+) and Late(+) was then assessed in a series of two-bottle choice tests between Early(+) vs. Early(-), Late(+) vs. Late(-), Early(+) vs. Late(+), and Early(-) vs. Late(-). Rats preferred both Early(+) and Late(+) over the respective (-) flavors. But Early(+) was only preferred when rats were tested hungry. Late(+) was preferred when rats were tested hungry or recently satiated. This indicates qualitatively different associations learned about flavors at different points in the meal. While not supporting the idea that postingestive effects become most strongly associated with later-occurring ("dessert") flavors, it does suggest a reason dessert flavors may remain attractive in the absence of hunger.

Parallel Reinforcement Pathways for Conditioned Food Aversions in the Honeybee

SummaryAvoiding toxins in food is as important as obtaining nutrition. Conditioned food aversions have been studied in animals as diverse as nematodes and humans [1, 2], but the neural signaling mechanisms underlying this form of learning have been difficult to pinpoint. Honeybees quickly learn to associate floral cues with food [3], a trait that makes them an excellent model organism for studying the neural mechanisms of learning and memory. Here we show that honeybees not only detect toxins but can also learn to associate odors with both the taste of toxins and the postingestive consequences of consuming them. We found that two distinct monoaminergic pathways mediate learned food aversions in the honeybee. As for other insect species conditioned with salt or electric shock reinforcers [4–7], learned avoidances of odors paired with bad-tasting toxins are mediated by dopamine. Our experiments are the first to identify a second, postingestive pathway for learned olfactory aversions that involves serotonin. This second pathway may represent an ancient mechanism for food aversion learning conserved across animal lineages.

The Brain Renin-Angiotensin System Controls Divergent Efferent Mechanisms to Regulate Fluid and Energy Balance

The renin-angiotensin system (RAS), in addition to its endocrine functions, plays a role within individual tissues such as the brain. The brain RAS is thought to control blood pressure through effects on fluid intake, vasopressin release, and sympathetic nerve activity (SNA), and may regulate metabolism through mechanisms which remain undefined. We used a double-transgenic mouse model that exhibits brain-specific RAS activity to examine mechanisms contributing to fluid and energy homeostasis. The mice exhibit high fluid turnover through increased adrenal steroids, which is corrected by adrenalectomy and attenuated by mineralocorticoid receptor blockade. They are also hyperphagic but lean because of a marked increase in body temperature and metabolic rate, mediated by increased SNA and suppression of the circulating RAS. β-adrenergic blockade or restoration of circulating angiotensin-II, but not adrenalectomy, normalized metabolic rate. Our data point to contrasting mechanisms by which the brain RAS regulates fluid intake and energy expenditure.

High-fat diet-induced obesity in animal models

Epidemiological studies have shown a positive relationship between dietary fat intake and obesity. Since rats and mice show a similar relationship, they are considered an appropriate model for studying dietary obesity. The present paper describes the history of using high-fat diets to induce obesity in animals, aims to clarify the consequences of changing the amount and type of dietary fats on weight gain, body composition and adipose tissue cellularity, and explores the contribution of genetics and sex, as well as the biochemical basis and the roles of hormones such as leptin, insulin and ghrelin in animal models of dietary obesity. The major factors that contribute to dietary obesity - hyperphagia, energy density and post-ingestive effects of the dietary fat - are discussed. Other factors that affect dietary obesity including feeding rhythmicity, social factors and stress are highlighted. Finally, we comment on the reversibility of high-fat diet-induced obesity.

Reconciling sensory cues and varied consequences of avian repellents

We learned previously that red-winged blackbirds (Agelaius phoeniceus) use affective processes to shift flavor preference, and cognitive associations (colors) to avoid food, subsequent to avoidance conditioning. We conducted three experiments with captive red-winged blackbirds to reconcile varied consequences of treated food with conditioned sensory cues. In Experiment 1, we compared food avoidance conditioned with lithium chloride (LiCl) or naloxone hydrochloride (NHCl) to evaluate cue-consequence specificity. All blackbirds conditioned with LiCl (gastrointestinal toxin) avoided the color (red) and flavor (NaCl) of food experienced during conditioning; birds conditioned with NHCl (opioid antagonist) avoided only the color (not the flavor) of food subsequent to conditioning. In Experiment 2, we conditioned experimentally naïve blackbirds using free choice of colored (red) and flavored (NaCl) food paired with an anthraquinone- (postingestive, cathartic purgative), methiocarb- (postingestive, cholinesterase inhibitor), or methyl anthranilate-based repellent (preingestive, trigeminal irritant). Birds conditioned with the postingestive repellents avoided the color and flavor of foods experienced during conditioning; methyl anthranilate conditioned only color (not flavor) avoidance. In Experiment 3, we used a third group of blackbirds to evaluate effects of novel comparison cues (blue, citric acid) subsequent to conditioning with red and NaCl paired with anthraquinone or methiocarb. Birds conditioned with the postingestive repellents did not avoid conditioned color or flavor cues when novel comparison cues were presented during the test. Thus, blackbirds cognitively associate pre- and postingestive consequences with visual cues, and reliably integrate visual and gustatory experience with postingestive consequences to procure nutrients and avoid toxins.

Cephalic phase responses and appetite

The current food supply in many parts of the world differs substantially from that which existed during most of human evolution. It is characterized by a high variety of palatable foods with high energy density and low fiber content. Many foods can be eaten very quickly, and there is not always congruency between the sensory properties of the food and the subsequent metabolic consequences of its ingestion, (e.g., as in the consumption of artificially sweetened foods). It is not presently known how the human body copes with this incongruent food environment in terms of short-term satiety responses and long(er)-term regulation of food intake. Cephalic phase responses (CPRs) are innate and learned physiological responses to sensory signals that prepare the gastrointestinal tract for the optimal processing of ingested foods. CPRs could be affected by inconsistencies in the associations between sensory signals and subsequent post-ingestive consequences. Reviewed here are the available data on how CPRs affect the control of food intake.

What Aspects of Human Alcohol Use Disorders Can Be Modeled Using Selectively Bred Rat Lines?

The use of selective breeding to produce animal models for the study of alcohol abuse and alcoholism represents one of the major advances in the field of alcohol research. Rats selectively bred for alcohol preference and alcohol nonpreference have been useful to both preclinical and clinical investigators in the alcohol research community for studying the behavioral, neurobiological, and molecular basis of alcohol drinking, for identifying the genes that may contribute to the development of alcohol abuse and alcoholism, and for evaluating the utility of drugs aimed at reducing alcohol intake and preventing alcohol relapse. Rats selectively bred for alcohol preference (alcohol preferring or “P” line) have enhanced responsiveness to the low dose reinforcing effects of alcohol, less aversion to moderate/high doses of alcohol, and are able to develop tolerance to the aversive effects of alcohol more rapidly and to maintain tolerance longer than rats selectively bred for alcohol nonpreference (alcohol nonpreferring or “NP” line). The increased potency of low-dose alcohol as a reinforcer for P rats might be expected to foster and maintain alcohol drinking. Weaker aversion to the pharmacological effects of moderate/high doses of alcohol in the P line would allow P rats to drink more alcohol than NP rats before the postingestional effects become aversive. Rapid induction of tolerance to the aversive effects of alcohol with repeated bouts of voluntary alcohol drinking, as well as persistence of alcohol tolerance in rats of the P line might serve to maintain alcohol drinking. These are powerful mechanisms that may serve to promote and maintain a high alcohol drinking behavior. Although these rat lines have been used to address several characteristics of excessive alcohol consumption in humans, they have not yet been used to model several aspects of human alcohol use disorders. New applications of these selectively bred rat lines are discussed which may further our understanding of the factors contributing to alcohol abuse and alcoholism.

Do sheep use umami and bitter tastes as cues of post-ingestive consequences when selecting their diet?

Grazing herbivores use their senses to select their diet. As tastes could reveal certain plant properties, we investigated whether sheep use them as cues to anticipate post-ingestive consequences. We focused on umami and bitter tastes reported to be associated with protein and toxin contents, respectively. We aimed to determine (1) lambs’ preferences for umami and bitter tastes offered together or with a control, and (2) lambs’ efficiency in learning positive and negative post-ingestive consequences associated with tastes coherently (i.e. umami taste → positive, bitter → negative) or incoherently (i.e. bitter → positive, umami → negative). The umami taste was induced by either 15 ( MSG1) or 25 (MSG2) g monosodium glutamate per kg DM hay, whereas the bitter taste was induced by 2 (Quin1) or 3 (Quin2) g quinine per kg DM hay that were sprayed on an orchard grass hay. In the first experiment, 24 ewe lambs were tested on eight two-choice types: MSG1-Quin1, MSG1-Quin2, MSG2-Quin1, MSG2-Quin2, MSG1-Control, MSG2-Control, Quin1-Control, and Quin2-Control, in a Latin-square design. They always preferred the umami-tasting hay to the control and the bitter-tasting hay ( P < 0.01) while they were indifferent to the bitter-tasting hay compared to control hay. After having checked that the lambs perceived the bitter taste induced by quinine (experiment 2) and that none of the substances used to modify the tastes induced their own post-ingestive consequences (experiment 3), we conducted the fourth experiment. Then, both coherent and incoherent groups decreased their preference for the taste associated with negative consequences compared to the taste associated with positive consequences, and aversion was greater in the coherent than in the incoherent group (0.04 ± 0.02 and 0.19 ± 0.08, respectively, P < 0.05). This difference could however be a residual effect of the large difference between initial preferences. After conditionings, to assess changes in individual tastes values, we re-tested the lambs’ preferences between umami- or bitter-tasting hay and control hay, and compared them with the initial preference patterns recorded in experiment 1. Preference for umami-tasting hay logically varied according to whether post-ingestive consequences were positive or negative, contrary to preference for bitter tasting-hay which was unaffected and close to umami value once negatively conditioned. Umami taste thus appears to have a positive value that can decrease after aversive conditionings, while bitter taste appears to have a negative value that made sheep suspicious about it even when associated with positive consequences.

Feeding behaviour of sheep on shrubs in response to contrasting herbaceous cover in rangelands dominated by Cytisus scoparius L.

The foraging responses of ewes faced with a diversity of feed items and their effects on broom ( Cytisus scoparius L.) consumption were examined. The experiment was conducted on a farm in the autumn with ewes ( n = 33) grazing three small paddocks (0.44 ha on average, for at least 10 days each) located in broom shrubland. The effects of three different herbaceous covers on broom consumption were compared: 100% of paddock area previously grazed in summer; 50% of paddock area previously grazed in summer; and paddock area non-grazed during the year. The characteristics of herbaceous cover (availability and quality) and the ewes’ diet selection were encoded as bite categories. Flock activities were recorded through scan sampling. We used logistic regression to assess the relationship between feeding behaviour of sheep on herbaceous vegetation and on broom species, and calculated selectivity indices for this shrub. We showed that the presence of high-quality bite categories in the herbaceous cover affected the way ewes integrated broom into their diet. At the start of each paddock use period, ewes favoured high-quality larger and medium bites of the herbaceous cover. They gradually included larger bites of broom and reduced their bite size, but continued to seek out higher quality herbaceous plants, a pattern which suggested a stabilisation of their daily average digestibility and bite mass over time. A negative relation was observed between the percentage of ewes taking large and medium bites on highly digestible plant parts and the percentage of ewes browsing broom. A maximum of 26% of the flock browsing broom was observed on any given day. Hence, ewes have a threshold for this target shrubby species that they do not exceed during any paddock utilisation period. This finding was interpreted as a mechanism to deal with post-ingestive consequences and complementary interactions between nutrients and toxins. When comparing broom selection between paddocks in autumn, we found an earlier and thus longer broom selection in areas with herbaceous cover that had not been grazed during the year (possibly because of a lower palatability). Our results provide new insights into ways to manipulate diet selection in order to stimulate the use of broom by ewes. Bite categories are proposed as functional feed indicators that facilitate prediction of the herbaceous cover state preliminary to initial broom integration in the sheep's diet.

Brain–gut communication via vagus nerve modulates conditioned flavor preference

It is well known that the postingestive effect modulates subsequent food preference. We previously showed that monosodium L-glutamate (MSG) can increase flavor preference by its postingestive effect. The neural pathway involved in mediating this effect, however, remains unknown. We show here the role of the vagus nerve in acquiring this learned flavor preference and in the brain's response to intragastric glutamate infusion. Adult rats with an intragastric cannula underwent total abdominal branch vagotomies (TVX), common hepatic branch vagotomies (HVX), total abdominal branch vagotomies with the common hepatic branch intact (TVXh), or sham operations (Sham). Following recovery, rats were subjected to a conditioned flavor preference paradigm, in which they drank a flavored solution (CS+) paired with intragastric MSG or another flavored solution (CS-) paired with intragastric distilled water. After conditioning, the Sham and HVX groups demonstrated significantly higher intake of CS+ than CS-, whereas the TVXh and TVX groups showed no significant differences. We then conducted an fMRI study to identify the brain areas that responded to the intragastric glutamate in each group. In the Sham, HVX and TVXh groups, intragastric MSG significantly increased the BOLD intensity in the nucleus of the solitary tract. The amygdala, hippocampus and lateral hypothalamus were also activated in the Sham and HVX groups but not in the TVXh and TVX groups. These results indicate that the abdominal vagus nerve is necessary for acquiring preference and that the lateral hypothalamus and limbic system could be key areas for integrating the information on gut glutamate and oronasal stimuli.

Selection of tannins by sheep in response to gastrointestinal nematode infection1

Herbivores learn to select compounds that attenuate the aversive effects of plant secondary metabolites (PSM), but can they increase intake of PSM they typically avoid when these PSM provide medicinal effects? We hypothesized that herbivores learn to increase intake of PSM-containing feeds when experiencing a gastrointestinal parasitic infection. Ten lambs with natural gastrointestinal parasitic burdens (PB) and 10 nonparasitized lambs (NP) were offered a choice of alfalfa (Medicago sativa) and alfalfa mixed with 10% quebracho tannin (Schinopsis quebracho-colorado; alfalfa:tannins) before and after they were conditioned with the postingestive effects of tannins. Preference for alfalfa:tannins did not differ between groups before experiencing the postingestive effects of tannins (P = 0.85) or when parasite loads were terminated due to the administration of ivermectin (P = 0.18). In contrast, when tested with a parasite burden, lambs in PB consumed more alfalfa:tannins (P = 0.08), showed greater preference for alfalfa:tannins (P = 0.07), and consumed less alfalfa than lambs in NP (P = 0.06). Ingestion of tannins by lambs in PB was followed by reduced fecal egg counts (FEC; P = 0.006), and there was a direct proportional relationship between preference for alfalfa:tannins and FEC (P = 0.07). In summary, parasitized lambs increased their intake of alfalfa:tannins when they experienced a parasite burden, which suggests they self-medicated with tannins against parasites. Self-selection of PSM has implications for the quest for alternatives to chemoprophylaxis in the treatment and well-being of parasitized wild and domestic animals grazing in pasturelands and in confinement.