Introduction

Cholecystokinin (CCK) is a peptide hormone linked to the gastrointestinal (GI) system. The receptors are expressed in the central nervous system[1] specifically in the hippocampus, cerebral cortex, and striatum [2]. It is present in the nucleus of tractus solitarius and area postrema of the lower portion of the brain stem. CCK is tissue-specific and developmentally regulated. The expression of CCK-endocrine producing cells is biphasic, declines just before birth and increases immediately after birth. Levels of CCK-producing neurons in the brain are low at birth but steadily increase into adulthood. Low levels of CCK are clear in the thyroid C cells, adrenal medulla, bronchial mucosa, pituitary corticotrophs, and spermatogenic cells. CCK-1 and 2 are part of class 1 G-protein-coupled receptor family. CCK1R are found in the gallbladder smooth muscles, chief and D cells of gastric mucosa, pancreatic acinar cells, and selected areas of central and peripheral nervous systems while CCK2R/GR are identified in the stomach (in the parietal, chief, and ECL cell of gastric mucosa), human pancreas and central nervous system (CNS).[2]

Cellular Level

In peripheral neurons such as those found in the intestinal mucosa, they express CCK in I-cells. Food intake, pituitary adenylate cyclase-activating polypeptide (PACAP), and glucocorticoids play a role in regulating CCK expression. Estrogen, dopamine, and injury situations activate CCK-mRNA in neuronal cells. Activation of transcription factors of these signaling pathways is unknown. Receptors in the CNS are G protein-coupled; CCK-2 receptor subtype (formerly known as CCK-B) are found mainly in the brain, and CCK-1 receptor subtype (formerly known as CCK-A) found mainly peripherally, for example, in the pancreas. Adenylyl cyclase is activated inducing a rise in intracellular cyclic adenosine 3', 5'-monophosphate (cAMP) and the activation of protein kinase A (PKA). They associate this rise in cAMP with CCK-receptor-mediated pathway with high concentrations of CCK. cAMP-dependent protein kinase activity is CCK concentration dependent. PKA activity peaks rapidly and maintains a high level of activity at high doses of CCK.[3] CCK receptors found on pancreatic acinar cells have two types of affinity to CCK which elicits different responses. At low concentrations, it stimulates zymogen secretion while at high concentrations CCK inhibits stimulation and secretion of intracellular zymogen proteolysis.[3] Binding sites of CCK-1 and CCK-2 receptors have different affinities for various CCK neuropeptide fragments. CCK-A binding has a higher affinity for sulfated, intermediate, neuropeptide CCK-8 than CCK-4; therefore, most of its action is peripheral. CCK-2R has a high affinity for all CCK  fragments with a higher affinity for CCK-4, alluding to the fact that most of CCK action is on the brain. Stimulation of neuronal pathways containing CCK can amount to a panic-like reaction in humans. Panic attacks could arise from the activation of CCK. CCK antagonists can provide anxiolytic properties via their action on CCK-2 receptors.[4]

Molecular Level

The specific affinity of membrane receptors on target cells determines the action of CCK and gastrin. CCK-1 and 2 are part of the class 1 G-protein-coupled receptor family. It is made up of seven transmembrane domains connected by intracellular and extracellular loops with an extracellular N-terminal and intracellular C-terminal tails. They divide these receptors into subtypes based on their affinity to CCK or gastrin. The CCK-1 receptor has an affinity 500-times higher for CCK than gastrin. The CCK2 receptor has the same affinity for CCK as it does for gastrin. CCK-1 receptor also has a very high affinity for sulfated CCK than non-sulfated CCK while CCK-2 receptor cannot differentiate between the 2. One of the extracellular loops of CCK-2 receptor contains 5 amino acids paramount to gastrin sensitivity. Loss of His207 from CCK-2 receptor leads to a loss of CCK binding. His207 is also found in the CCK-1 receptor binding site, and an exchange in this region for another amino acid and the Asp region of CCK leads to a loss or gain of affinity. Therefore, the binding sites of CCK-1 and CCK-2 receptors share some homologous regions and the Asp of CCK further demarcates the binding site of the CCK-2 receptor.[2]

Function

Roles

In the Intestine

• Mediates digestion by regulating the release of pancreatic exocrine enzymes which plays a role in the digestion of fats, proteins, and carbohydrates
• Causes contraction and relaxation of the gallbladder via the sphincter of Oddi in response to food; CCK regulates the release of bile acid to aid in further fat digestion in the small intestine
• Regulates overall GI movement, in other words, gut motility[5]
• [5]Regulates gastric emptying: It inhibits gastric emptying to regulate the flow of chyme into the duodenum[6]
• [6]Inhibits gastric acid secretion after a meal by regulating gastrin production via somatostatin[7]
• [7]It enhances the release of leptin which inhibits basal gastric H+ secretion after a meal. In the intestine, it promotes further absorption of proteins.[8]
• [8]Stimulation of cell growth
• Energy production
• Gene expression
• Protein synthesis

In the Brain

• Regulates feeding behavior: Leptin acts on the brain to inhibit food intake resulting in satiety[8]
• [8]Managing anxiety
• Pain perception
• Memory

Mechanism

Fatty acids and proteins stimulate the release of CCK via a direct action on the I-cells. GPR40 is a G-protein-coupled receptor expressed on I-cells that responds to long-chain fatty acids. Discharge of vagal efferent neurons is stimulated by the action of CCK on CCK-1 receptors and increase intracellular calcium. They find these neurons in both the stomach and small intestines, and CCK initially activates the afferent fibers in the small intestine via a paracrine mechanism. This inhibits the excitatory vagal efferent pathway to the distal stomach. Gastric vagal afferents are stimulated in response to the hormonal effect coupled with the inhibitory vagal efferent pathway to the proximal stomach.[9] Due to the mechanism mentioned above, CCK can inhibit gastric emptying by relaxing the proximal portion of the stomach, which increases tension in the pyloric sphincter. At high levels of CCK can increase the effect of how fast gastric emptying occurs, and it does this by increasing the excitatory effect it has on both the small and large intestine, which leads to movement in the bowels or by improving the tension of the pyloric sphincter.[5] Therefore, the reflex control of gastric emptying is regulated by CCK action on vasovagal reflexes and the hormonal activation of a variety of pathways that are coupled to vagal efferent pathways controlling gastric motility.

Testing

Gallbladder dysfunction is defined as an abnormally low gallbladder ejection fraction (GBEF).[10] Gallbladder disease can manifest as bladder dyskinesia, chronic acalculous cholecystitis, biliary dyskinesia, and functional gallbladder disorder, among others. Cholecystokinin scintigraphy (CCK-HIDA) assesses GBEF and is used to test patients presenting with chronic upper abdominal pain together with a normal upper abdominal ultrasonography.[11] Tc-99m-labeled HIDA collects in the gallbladder after it is absorbed by the liver and excreted by the biliary system. CCK is injected to stimulate gallbladder contraction to calculate the GBEF.[10]

Clinical Significance

Obesity blunts the effect of CCK, which means there is insensitivity of vagal afferent neurons to CCK. This reduced expression of CCK accounts for reduced effect on satiety and the fact that most obese people always complain about feeling hungry. Consumption of high-fat diets with diminished expression of the CCK-1 receptor increases the levels of ghrelin in plasma. This increases food intake, and it does this by suppressing the expression of satiety peptide cocaine and amphetamine-regulated transcript (CART) in vagal afferent neurons. CCK is also involved in metabolic regulation and lipid absorption. They link inactivation of the CCK signaling pathway to reduced weight gain. Inactivation increases energy expenditure and lowers energy extraction.[9]

CCK acts via the vasovagal pathway and is activated peripherally via gastric wall distension. They use intragastric balloons in clinical practice to treat weight loss by mimicking this pathway. By distending the stomach it activates the vagal nerve and the nucleus of tractus solitarius and the paraventricular nucleus, leading to a centrally mediated feeling of satiety.[12] These devices physically reduce food intake by obstructing the outlet, delaying gastric emptying and physically reducing the capacity of the stomach. Pancreatic peptide (PP) secretion is impaired due to decreased gastric emptying which leads to reduced gut wall interactions with nutrients such as fat and protein which elicit a PP response. PP secretion is biphasic, and food and secretion of CCK control the second phase.[9]

CCK plays a minor role in the release of incretin as compared to glucagon-like peptide (GLP-1) from islet cells. Decreased size of islet cells and mass of beta-cells correlates with upregulation of CCK expression and increased sensitivity to CCK for insulin release in obesity. It, therefore, can mediate compensatory mechanisms within the islets of Langerhans.[9] Fat and energy restricted meals stimulate CCK and PP secretion and together with delayed gastric emptying secondary to the intragastric balloon can cause extensive weight loss and improved glucose homeostasis.[12] Presence of CCK in various regions of the midbrain suggests it plays a role in behavioral processes such and anxiety. They describe panic disorder as the feeling of unprovoked fear and an overwhelming feeling of anxiety. People with panic disorder or those that experience panic attacks present to the emergency department feeling a sense of impending doom, chest pain, abdominal pain, sometimes even shortness of breath. We know that CCK is expressed in the nucleus of tractus solitarius and area postrema. They associate these areas with nociception and patients with PD are usually sensitive with bodily sensations. Noradrenaline and serotonin (5-HT) containing nuclei found in the brainstem interact with these areas as well implicating then to the pathogenesis of PD.[4]

Cancers of the GI tract including medullary thyroid cancer, small cell lung cancer all express gastrin and CCK-2 receptors. Gastrin and CCK2R/gastrin receptor (GR) play a role in regulating cellular proliferation, loss of cell-cell adhesion, differentiation and morphology and the enhanced motility/invasion of epithelial cells. The activation of the CCK2R/GR via the intracellular signaling pathway can lead to carcinogenesis. These receptors, therefore, play a crucial role in starting the events leading up to preneoplastic lesions and cancer development.[13]