Flavour exposures after conditioned aversion or preference trigger different brain processes in anaesthetised pigs

Review: Impact of food, gut-brain signals and metabolic status on brain activity in the pig model: 10 years of nutrition research using in vivo brain imaging

The purpose of this review is to offer a panorama on 10 years of nutrition research using in vivo brain imaging in the pig model. First, we will review some work describing the brain responses to food signals, including basic tastants such as sweet and bitter at both oral and visceral levels, as well as conditioned preferred and aversive flavours. Second, we will have a look at the impact of weight gain and obesity on brain metabolism and functional responses, drawing the parallel with obese human patients. Third, we will evoke the concept of the developmental origins of health and diseases, and how the pig model can shed light on the importance of maternal nutrition during gestation and lactation for the development of the gut-brain axis and adaptation abilities of the progeny to nutritional environments. Finally, three examples of preventive or therapeutic strategies will be introduced: the use of sensory food ingredients or pre-, pro-, and postbiotics to improve metabolic and cognitive functions; the implementation of chronic vagus nerve stimulation to prevent weight gain and glucose metabolism alterations; and the development of bariatric surgery in the pig model for the understanding of its complex mechanisms at the gut-brain level. A critical conclusion will brush the limitations of neurocognitive studies in the pig model and put in perspective the rationale and ethical concerns underlying the use of pig experimentation in nutrition and neurosciences.

Familiarity to a Feed Additive Modulates Its Effects on Brain Responses in Reward and Memory Regions in the Pig Model

Brain responses to feed flavors with or without a feed additive (FA) were investigated in piglets familiarized or not with this FA. Sixteen piglets were allocated to 2 dietary treatments from weaning until d 37: the naive group (NAI) received a standard control feed and the familiarized group (FAM) received the same feed added with a FA mainly made of orange extracts. Animals were subjected to a feed transition at d 16 post-weaning, and to 2-choice feeding tests at d 16 and d 23. Production traits of the piglets were assessed up to d 28 post-weaning. From d 26 onwards, animals underwent 2 brain imaging sessions (positron emission tomography of 18FDG) under anesthesia to investigate the brain activity triggered by the exposure to the flavors of the feed with (FA) or without (C) the FA. Images were analyzed with SPM8 and a region of interest (ROI)-based small volume correction (p < 0.05, k ≥ 25 voxels per cluster). The brain ROI were selected upon their role in sensory evaluation, cognition and reward, and included the prefrontal cortex, insular cortex, fusiform gyrus, limbic system and corpus striatum. The FAM animals showed a moderate preference for the novel post-transition FA feed compared to the C feed on d 16, i.e., day of the feed transition (67% of total feed intake). The presence or absence of the FA in the diet from weaning had no impact on body weight, average daily gain, and feed efficiency of the animals over the whole experimental period (p ≥ 0.10). Familiar feed flavors activated the prefrontal cortex. The amygdala, insular cortex, and prepyriform area were only activated in familiarized animals exposed to the FA feed flavor. The perception of FA feed flavor in the familiarized animals activated the dorsal striatum differently than the perception of the C feed flavor in naive animals. Our data demonstrated that the perception of FA in familiarized individuals induced different brain responses in regions involved in reward anticipation and/or perception processes than the familiar control feed flavor in naive animals. Chronic exposure to the FA might be necessary for positive hedonic effects, but familiarity only cannot explain them.

Critical review evaluating the pig as a model for human nutritional physiology

The present review examines the pig as a model for physiological studies in human subjects related to nutrient sensing, appetite regulation, gut barrier function, intestinal microbiota and nutritional neuroscience. The nutrient-sensing mechanisms regarding acids (sour), carbohydrates (sweet), glutamic acid (umami) and fatty acids are conserved between humans and pigs. In contrast, pigs show limited perception of high-intensity sweeteners and NaCl and sense a wider array of amino acids than humans. Differences on bitter taste may reflect the adaptation to ecosystems. In relation to appetite regulation, plasma concentrations of cholecystokinin and glucagon-like peptide-1 are similar in pigs and humans, while peptide YY in pigs is ten to twenty times higher and ghrelin two to five times lower than in humans. Pigs are an excellent model for human studies for vagal nerve function related to the hormonal regulation of food intake. Similarly, the study of gut barrier functions reveals conserved defence mechanisms between the two species particularly in functional permeability. However, human data are scant for some of the defence systems and nutritional programming. The pig model has been valuable for studying the changes in human microbiota following nutritional interventions. In particular, the use of human flora-associated pigs is a useful model for infants, but the long-term stability of the implanted human microbiota in pigs remains to be investigated. The similarity of the pig and human brain anatomy and development is paradigmatic. Brain explorations and therapies described in pig, when compared with available human data, highlight their value in nutritional neuroscience, particularly regarding functional neuroimaging techniques.

Combined compared to dissociated oral and intestinal sucrose stimuli induce different brain hedonic processes

The characterization of brain networks contributing to the processing of oral and/or intestinal sugar signals in a relevant animal model might help to understand the neural mechanisms related to the control of food intake in humans and suggest potential causes for impaired eating behaviors. This study aimed at comparing the brain responses triggered by oral and/or intestinal sucrose sensing in pigs. Seven animals underwent brain single photon emission computed tomography (^99mTc-HMPAO) further to oral stimulation with neutral or sucrose artificial saliva paired with saline or sucrose infusion in the duodenum, the proximal part of the intestine. Oral and/or duodenal sucrose sensing induced differential cerebral blood flow changes in brain regions known to be involved in memory, reward processes and hedonic (i.e., pleasure) evaluation of sensory stimuli, including the dorsal striatum, prefrontal cortex, cingulate cortex, insular cortex, hippocampus, and parahippocampal cortex. Sucrose duodenal infusion only and combined sucrose stimulation induced similar activity patterns in the putamen, ventral anterior cingulate cortex and hippocampus. Some brain deactivations in the prefrontal and insular cortices were only detected in the presence of oral sucrose stimulation. Finally, activation of the right insular cortex was only induced by combined oral and duodenal sucrose stimulation, while specific activity patterns were detected in the hippocampus and parahippocampal cortex with oral sucrose dissociated from caloric load. This study sheds new light on the brain hedonic responses to sugar and has potential implications to unravel the neuropsychological mechanisms underlying food pleasure and motivation.

An attempt to condition flavour preference induced by oral and/or postoral administration of 16% sucrose in pigs

The present study investigated the acquisition of conditioned flavour preferences in pigs using the caloric value and/or sweet taste of sucrose. Nine water-deprived juvenile pigs were given four three-day conditioning sessions during which they received flavoured solutions as conditioned stimuli (CS). The CS solutions were paired with three treatments that generated a gustatory and/or a caloric reinforcement (US). The CS++ solution was added with 16% sucrose and paired with an intraduodenal (ID) infusion of water, the CS+ solution was paired with an ID infusion of 16% sucrose and the CS- solution was paired with an ID infusion of water. One and two weeks after conditioning, the water-deprived pigs were subjected to two-choice preference tests with the unreinforced CS solutions. Solutions intake, behavioural activity and some drinking parameters were measured. Despite no difference in CS intake during conditioning, the animals spent less time inactive and more time standing during CS++ than CS+ conditioning. When receiving CS++, the pigs explored the drinking trough more than when receiving CS-. Compared to the CS- condition, the numbers of drinking episodes and intra-drinking episode (IDE) pauses were also 36% and 49% lesser in the CS++ condition, but these differences were not significant. During the two-choice tests, the pigs did not show significant preferences. Nevertheless, during the first session, the pigs seemed to show a slight preference for the CS++ (57% of total intake) compared to CS+. The duration of CS++ drinking episodes represented 64% of the total duration compared to CS+ and CS- . The total time spent drinking the CS++ also represented 57% of the total time in the CS++ vs. CS- test. To conclude, although no clear-cut preferences were found during two-choice tests, the oral perception of 16% sucrose during conditioning induced changes in behavioural activities, motivational responses and microstructure of CS intake, suggesting the importance of oral food perception for food selection processes in pigs. Further studies are needed to investigate the impact of water deprivation on the expression of flavour preferences in pigs.

Brain processing of duodenal and portal glucose sensing

Peripheral and central glucose sensing play a major role in the regulation of food intake. Peripheral sensing occurs at duodenal and portal levels, although the importance of these sensing sites is still controversial. The present study aimed to compare the respective influence of these sensing pathways on the eating patterns; plasma concentrations of glucose, insulin and glucagon-like peptide-1 (GLP-1); and brain activity in juvenile pigs. In Experiment 1, we characterised the changes in the microstructure as a result of a 30-min meal in eight conscious animals after duodenal or portal glucose infusion in comparison with saline infusion. In Experiment 2, glucose, insulin and GLP-1 plasma concentrations were measured during 2 h after duodenal or portal glucose infusions in four anaesthetised animals. In Experiment 3, single photon emission computed tomography brain imaging was performed in five anaesthetised animals receiving duodenal or portal glucose or saline infusions. Both duodenal and portal glucose decreased the amount of food consumed, as well as the ingestion speed, although this effect appeared earlier with the portal infusion. Significant differences of glucose and GLP-1 plasma concentrations between treatments were found at the moment of brain imaging. Both duodenal and portal glucose infusions activated the dorsolateral prefrontal cortex and primary somatosensory cortex. Only duodenal glucose infusion was able to induce activation of the prepyriform area, orbitofrontal cortex, caudate and putamen, as well as deactivation of the anterior prefrontal cortex and anterior entorhinal cortex, whereas only portal glucose infusion induced a significant activation of the insular cortex. We demonstrated that duodenal and portal glucose infusions led to the modulation of brain areas that are known to regulate eating behaviour, which probably explains the decrease of food intake after both stimulations. These stimulation pathways induced specific systemic and central responses, suggesting that different brain processing matrices are involved.

Exposures to conditioned flavours with different hedonic values induce contrasted behavioural and brain responses in pigs

This study investigated the behavioural and brain responses towards conditioned flavours with different hedonic values in juvenile pigs. Twelve 30-kg pigs were given four three-day conditioning sessions: they received three different flavoured meals paired with intraduodenal (i.d.) infusions of 15% glucose (F(Glu)), lithium chloride (F(LiCl)), or saline (control treatment, F(NaCl)). One and five weeks later, the animals were subjected to three two-choice feeding tests without reinforcement to check the acquisition of a conditioned flavour preference or aversion. In between, the anaesthetised pigs were subjected to three (18)FDG PET brain imaging coupled with an olfactogustatory stimulation with the conditioned flavours. During conditioning, the pigs spent more time lying inactive, and investigated their environment less after the F(LiCl) than the F(NaCl) or F(Glu) meals. During the two-choice tests performed one and five weeks later, the F(NaCl) and F(Glu) foods were significantly preferred over the F(LICl) food even in the absence of i.d. infusions. Surprisingly, the F(NaCl) food was also preferred over the F(Glu) food during the first test only, suggesting that, while LiCl i.d. infusions led to a strong flavour aversion, glucose infusions failed to induce flavour preference. As for brain imaging results, exposure to aversive or less preferred flavours triggered global deactivation of the prefrontal cortex, specific activation of the posterior cingulate cortex, as well as asymmetric brain responses in the basal nuclei and the temporal gyrus. In conclusion, postingestive visceral stimuli can modulate the flavour/food hedonism and further feeding choices. Exposure to flavours with different hedonic values induced metabolism differences in neural circuits known to be involved in humans in the characterization of food palatability, feeding motivation, reward expectation, and more generally in the regulation of food intake.