Leptin and its receptor in normal human gastric mucosa and in Helicobacter pylori-associated gastritis

Role of leptin in the stomach and the pancreas

Leptin, a 16 kDa protein encoded by the ob gene, is known mainly for its role in the regulation of food intake, body composition and energy expenditure through a central feedback mechanism. Initially leptin was considered as an ob gene product of adipocytes but recently the presence of leptin and its receptors have been revealed in other organs including gastric mucosa and the pancreas and found to be released from these organs by cholecystokinin (CCK), gastrin and ordinary feeding. Furthermore, leptin was found to mimic the action of CCK on gastric and pancreatic integrity, while reducing the food intake and to affect gastric and pancreatic secretion. This report emphasizes the role of leptin originating from the gastrointestinal tract acting synergistically with CCK at the hypothalamus level on the mechanism of food intake and locally on the protection of gastric mucosa and the pancreas against noxious agents and to maintain tissue integrity.

Gastric secretion

This article summarizes data published during the past year that improve our understanding of the mechanisms by which various neurotransmitters, paracrine agents, and hormones regulate gastric acid secretion and are themselves regulated. The main stimulants of acid secretion are histamine, gastrin, and acetylcholine. The main inhibitor is somatostatin, which exerts a tonic restraint on parietal, enterochromaffin-like (ECL), and gastrin cells. Histamine, released from ECL cells, stimulates the parietal cell directly via H(2) receptors and indirectly via H(3) receptors coupled to inhibition of somatostatin secretion. Gastrin, acting via gastrin/cholecystokinin-B (CCK-B), now termed CCK(2), receptors on ECL cells activates histidine decarboxylase, releases histamine, and induces ECL hypertrophy and hyperplasia. The latter might be responsible for the rebound hyperacidity observed after withdrawal of long-term antisecretory therapy. The neurotransmitter pituitary adenylate cyclase-activating polypeptide stimulates histamine secretion from isolated ECL cells, but its physiologic role, if any, is not known. Acetylcholine, released from gastric postganglionic intramural neurons, stimulates the parietal cell directly via muscarinic M(3) receptors and indirectly by inhibiting somatostatin secretion. Although infection with H. pylori is associated with increased basal and stimulated acid outputs in patients with duodenal ulcer, most people infected with the organism are asymptomatic and have pangastritis with decreased acid output. In the latter, eradication of the bacterium leads to an increase in gastric acidity and is associated with a two-to threefold increase in gastroesophageal reflux.

Synergistic interaction between CCK and leptin to regulate food intake

Leptin administered (either intracerebroventricularly, icv, or intraperitoneally, ip) acts in synergy with CCK to suppress food intake and body weight in lean mice or rats. The potentiating effect induced by the co-injection of ip CCK and leptin to inhibit food consumption in mice is mediated by the CCK-A receptor and capsaicin sensitive afferents. In vitro, studies in rats showed that a subset of gastric vagal afferent fibers responded to leptin injected directly into the gastric artery only after a prior intra-arterial CCK injection. Moreover, the tonic activity of gastric-related neurons in the nucleus tractus solitarius (NTS) increased when leptin was delivered into the gastric chamber of an in vitro stomach-brainstem preparation. CCK co-injected with leptin potentiated Fos expression selectively in the area postrema, NTS and paraventricular nucleus of the hypothalamus (PVN), which points to the PVN as part of the afferent and efferent limbs of the circuitry involved in the synergistic interaction between leptin and CCK. The dampening of CCK or leptin inhibitory action on ingestive behavior when either factor is not present or their receptors are non functional supports the notion that such leptin-CCK interaction may have a physiological relevance. These observations provide a mean through which leptin and CCK integrate short- and mid-term meal-related input signals into long-term control of energy balance.