User:Weissblair/sandbox/Food-Pleasure and Reward

=Cycles of Food and Pleasure= Three main stages have been proposed as a cycle for food and pleasure: “wanting,” “liking” and “learning.” The stage of wanting begins the cycle of food and pleasure by providing desire for food consumption or motivation for food-reward. This stage combines a reward-related sensory cue, such as the sight or smell of food, with a given state of hunger (mesolimbic state) that provides motivating power for the cue. Pleasure in this stage begins at a low level, and increases over time as the possibility of reward is anticipated. This stage is additionally referred to as “incentive salience”. The liking phase follows the wanting phase and consists of a hedonic or pleasurable reaction, commonly referred to as the hedonic impact. During this phase, pleasure continues to rise over time, during food consumption, until it peaks. This phase ends when an individual experiences satiation, a process that terminates eating. During the final learning phase, pleasure subsides back to a base level as the individual experiences satiety, or the feeling of being full, which persists after eating and suppresses further eating. Neural coding, or evaluation, occurs for the pleasurable reaction to the sensory stimuli from the liking phase. This evaluation, also known as the hedonic impact, is stored for future use in decision-making processes, and is subsequently important for liking and wanting in the future.

=Brain Systems Involved in Food and Reward= Various neural systems and regions involved in the rewarding aspects of food are discussed below.

Hedonic Neural Networks
Hedonic neural networks are networks involved in the pleasurable aspects of food-cues, including the evaluation of pleasure associated with food stimuli and subsequently, the associated value of reward. Hedonic networks are believed to be involved with cycles of food and pleasure.

Hedonic Hotspots
Recent rodent studies on the reactions and mechanisms of liking have revealed neural networks related to hedonics or pleasure that are used in food reward. This circuitry consists of a connected subcortical neural network in limbic forebrain structures formed by “hedonic hotspots”  Hedonic hotspots are specific sites in the brain that result in an amplification of liking or wanting reactions for food when stimulated. These hotspots have been located in several brain regions, including the nucleus accumbens, ventral pallidum, and some brain-stem regions like the parabrachial nucleus.

Nucleus Accumbens and Ventral Pallidum
The nucleus accumbens and ventral pallidum work together to enhance the pleasure of food (liking). These regions are believed to be part of a liking circuit, as a connected network of hedonic hotspots. Activations in both regions are required to increase the evaluated level of pleasure (hedonic impact).

The nucleus accumbens contains hotspots that are sensitive to opioid and endocannabinoid neural transmission signals (neurotransmitters). Liking or pleasurable reactions have been shown to double in rodents populations when hotspots in this region were manipulated to increase in neural transmission activity; Hotspots were injected with endocannabinoids or opioid agonists, which increased the opioid receptor affinity. The nucleus accumbens has also demonstrated separate regions for liking and wanting (desire to consume food). For example, stimulation directly within the hotspot caused both liking and wanting reactions at the same time, while stimulation outside of the hotspot caused only wanting, without liking. These results imply that stimulation outside of the hotspot in this region is not pleasurable or hedonic.

The ventral pallidum, a main output of the nucleus accumbens, also contains a hedonic hotspot that is sensitive to opioid neural transmissions. This region is thought to play a major role in food pleasure in terms of generating the desire to consume food, also known as incentive motivation. The ventral pallidal hotspot is needed to maintain normal and enhanced levels of pleasure (liking); neural death in this region is shown to obliterate all pleasurable liking reactions and replace them with disliking or aversive reactions.

Several other brain regions have been shown to interact with the pleasure or hedonic circuit involving the nucleus accumbens and ventral pallidum. Such regions include the orbitofrontal cortex (OFC), anterior cingulate cortex (ACC), insular cortex (INS), ventromedial prefrontal cortex (vmPFC), [Parabrachial_area| parabrachial]] nucleus, and the ventral tegmental area (VTA).

Orbitofrontal Cortex and Anterior Cingulate Cortex
While sensory systems are thought to establish the identity and intensity of food stimuli, hedonic impact, or the evaluation of the pleasure, is not assigned to stimuli until the sensory information reaches upper-level cognitive areas, such as the orbitofrontal cortex. Recent literature reviews suggest that pleasurable evaluation or hedonic coding of food-stimuli is realized through a convergence of all sensory systems within the orbitofrontal cortex. The orbitofrontal cortex, in conjunction with the anterior cingulate cortex, has been implicated in a variety of hedonic (pleasure) processing roles in food reward through human neuroimaging studies. The medial orbitofrontal cortex plays a key role in storing valence, a positive or negative assigned value, for learning and memory. The lateral orbitofrontal cortex is implicated in food desire or expectation, and is linked to anterior cingulate cortex activity in influencing future behavior. The anterior orbitofrontal cortex may represent stored values of pleasure (coded hedonic values) for food stimuli, while mid-anterior aspects play a role in subjective pleasant experience, and are linked to anterior cingulate cortex activity in the subjective pleasantness of taste.

Decision-Making and Reward
Decision-selection processes, such as choosing whether or not to consume pleasant or unpleasant food, follow hedonic coding. The nucleus accumbens and ventral pallidum, which are players in the hedonic hotspot circuit, are posited to be involved in these decision-making processes. The medial prefrontal cortex is hypothesized to function as an attractor network, which selects the single decision to consume or to abstain from food stimuli based on pleasantness, or if the food is perceived as a reward.

Control of Food-intake and the Hypothalamus
The hypothalamus is believed to work with the orbitofrontal cortex to control the intake of food. A recent literature review suggests these regions share a role in satiety, or the feeling of fullness, by implementing reward only when and individual is hungry. The hypothalamus is also linked to basic homeostatic regulation, such as the control of energy balance; a recent fMRI neuroimaging (meta-analysis) study, examining the results of several previous studies, found increased activity in this region when participants viewed images high-energy versus low energy foods. The hypothalamus is controlled by a system of dopamine neurotransmitters, and is thought to interact with the mesolimbic dopamine system, which plays a key role in neurotransmission for the hedonic network involving the ventral tegmental area, parabrachial nucleus and nucleus accumbens. Many researchers suggest that the failure of hypothalamic systems to modulate food intake can lead to overeating or the increased consumption of pleasurable foods.

=References=