Abstract
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 (FGlu), lithium chloride (FLiCl), or saline (control treatment, FNaCl). 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 18FDG 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 FLiCl than the FNaCl or FGlu meals. During the two-choice tests performed one and five weeks later, the FNaCl and FGlu foods were significantly preferred over the FLICl food even in the absence of i.d. infusions. Surprisingly, the FNaCl food was also preferred over the FGlu 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.
Highlights
Flavours are perceived during food consumption [1] and result from the combination of a taste and odour, through the stimulation of the gustatory system as well as the orthonasal and retronasal olfactory systems respectively [2,3]
Athough Touzani et al [10,11] reported that the AMY and the lateral hypothalamus were not involved in the expression of conditioned flavour preferences in rats, less is known about the brain structures that are involved in the last steps of those processes, i.e., retrieval of the learning when the conditioned stimulus (CS) is further encountered and expression of the appropriate behaviour
One out of the 12 animals was excluded from the study because it showed a generalized aversion for food, regardless of the flavour or the treatment associated with the meal, after only one pairing between the meal and the lithium chloride (LiCl) injection
Summary
Flavours are perceived during food consumption [1] and result from the combination of a taste and odour, through the stimulation of the gustatory system as well as the orthonasal and retronasal olfactory systems respectively [2,3]. Numerous brain lesion studies investigated the brain structures involved in the acquisition of flavour preference and aversion, such as the amygdala (AMY) [10,17,18,19] and the insular cortex (IC) [20,21,22,23] Those studies often focused on the only first steps of preference and aversion learning processes, i.e., detection of the conditioned stimulus (CS), detection of the visceral unconditioned stimulus (US), association between the US and the CS (e.g., role of the parabrachial nucleus [24,25]). There is a need for studies to investigate the brain structures that are involved in the recall of conditioned learning during subsequent exposure to the conditioned flavour (CS)
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