Interrelated Levels of the System

Before we go on to consider the individual systems underpinning the expression of appetite it is useful to try and understand how neural, nutritional, and psychological events interact before, during, and after a meal. The biopsychological system underlying the expression of appetite can be conceptualized as having three domains (Figure 2):

1. Psychological events (e.g., subjective sensations of hunger, satiety, hedonics, and cravings) accompanying observable behavioral operations (meal intake, snacking behavior, food choice) and their measurable consequences (energy intake and macronutrient composition of food consumed).

2. Peripheral physiology and metabolic events related to the effect of absorbed nutrients and their utilization or subsequent conversion for storage (i.e., the changes in the body due to either energy intake and/or energy deficit). 3. Neurochemical (classic neurotransmitters, neuro-peptides, and hormones) and metabolic interactions within the CNS (i.e., how various signals of the body's energy status are detected in the brain).

The expression of appetite reflects the synchronous operation of events and processes in all three domains.

5-HT2c MC-4 CRF CART Leptin

Cephalic phase

5-HT2c MC-4 CRF CART Leptin

Galanin

GLP-1

AgRP

Orexin-A

HYPOTHALAMIC NUCLEI

Vagal Input

GI hormones Ghrelin

Taste Olfaction Chewing

Gastric factors

Gut peptides

CCK GLP-1

GRP Enterostatin Insulin Amylin

Liver

, Oxidative metabolism

Liver

, Oxidative metabolism

Glucose FFA AA (T:LNAA)

LEPTIN & Cytokines

Corticosteroids

LEPTIN & Cytokines

Corticosteroids

Taste Olfaction Chewing

CCK GLP-1

GRP Enterostatin Insulin Amylin

Glucose FFA AA (T:LNAA)

Preprandial motivation

Satiation

Figure 2 The psychobiological expression of appetite and the three levels of operation: (A) psychological and behavioral events; (B) physiological and metabolic operations; and (C) neurochemical and metabolic interactions within the CNS. Abbreviations: 5-HT, serotonin; AA, amino acids; AgRP, agouti-related peptide; CART, cocaine and amphetamine-regulated transcript; CCK, cholecysto-kinin; CRF, corticotropin releasing factor; FFA, free fatty acids; GI, gastrointestinal; GLP-1, glucagon-like peptide-1; GRP, gastric releasing peptide; MC, melanocortin; NPY, neuropeptide Y; T/LNAA, NTS, nucleus tractus solitarius; T/LNAA, tryptophan large neutral amino acids ratio.

Preprandial motivation

Satiation

Figure 2 The psychobiological expression of appetite and the three levels of operation: (A) psychological and behavioral events; (B) physiological and metabolic operations; and (C) neurochemical and metabolic interactions within the CNS. Abbreviations: 5-HT, serotonin; AA, amino acids; AgRP, agouti-related peptide; CART, cocaine and amphetamine-regulated transcript; CCK, cholecysto-kinin; CRF, corticotropin releasing factor; FFA, free fatty acids; GI, gastrointestinal; GLP-1, glucagon-like peptide-1; GRP, gastric releasing peptide; MC, melanocortin; NPY, neuropeptide Y; T/LNAA, NTS, nucleus tractus solitarius; T/LNAA, tryptophan large neutral amino acids ratio.

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