Capsaicin inhibits the pentagastrin-stimulated gastric acid secretion in anaesthetized rats with acute gastric fistula

Effects of capsaicin on gastric acid secretion and mechanisms involved

Although capsaicin has multiple pharmacological actions, its effects on gastric acid secretion attract the most attention. Most studies show that low-dose capsaicin can inhibit gastric acid secretion while high-dose may accelerate gastric acid secretion. However, some other studies show that capsaicin does not affect gastric acid secretion at all. The difference in the effects of capsaicin on gastric acid secretion may be related with the location of vanilloid receptor subtype 1, dose of capsaicin, route of administration, and the release of some substances such as calcitonin gene-related peptide, neurokinin A, vasoactive intestinal peptide and substance P. Capsaicin is a potentially promising drug used for modulation of gastric acid secretion.

The effect and mechanism of action of capsaicin on gastric acid output

BACKGROUND

Capsaicin has beneficial pharmacological properties, such as the ability to improve appetite and digestion. However, capsaicin has been reported to suppress gastric acid output, but to increase secretion; no consensus as to its effects on gastric acid output has been reached, and the underlying mechanisms remain to be elucidated.

METHODS

Rat gastric lumen was perfused with capsaicin. Basal acid output and gastric acid secretion stimulated by vagal nerve activation and bethanecol, a muscarinic receptor agonist, were measured. After intravenous infusion of calcitonin gene-related peptide (CGRP), the measurements were repeated. The secretion of gastrin, somatostatin, and histamine was measured in isolated vascularly perfused rat stomach after vagal nerve and bethanecol stimulation, and under the influence of capsaicin.

RESULTS

Capsaicin administration had no effect on basal gastric acid output, but inhibited acid secretion resulting from vagal stimulation. Capsaicin had no effect on acid secretion resulting from stimulation with bethanecol. Administration of high-dose CGRP inhibited basal acid output and gastric acid secretion from both vagal nerve and bethanecol stimulation. Low-dose CGRP inhibited gastric acid secretion because of vagal stimulation, but had no effect on basal secretion or acid secretion following stimulation with bethanecol. Capsaicin administration inhibited the stimulated gastrin and histamine secretion and reversed the suppression of somatostatin secretion mediated by vagal stimulation. However, capsaicin had no effect on stimulated gastrin secretion, suppression of somatostatin secretion, or stimulated histamine secretion because of bethanecol.

CONCLUSIONS

Capsaicin inhibited gastric acid output, and the mechanism underlying this effect appears to involve vagal nerve inactivation.

Capsaicin and gastric ulcers

In recent years, infection of the stomach with the organism Helicobacter Pylori has been found to be the main cause of gastric ulcers, one of the common ailments afflicting humans. Excessive acid secretion in the stomach, reduction in gastric mucosal blood flow, constant intake of non-steroid anti-inflammatory drugs (NSAIDS), ethanol, smoking, stress etc. are also considered responsible for ulcer formation. The prevalent notion among sections of population in this country and perhaps in others is that "red pepper" popularly known as "Chilli," a common spice consumed in excessive amounts leads to "gastric ulcers" in view of its irritant and likely acid secreting nature. Persons with ulcers are advised either to limit or avoid its use. However, investigations carried out in recent years have revealed that chilli or its active principle "capsaicin" is not the cause for ulcer formation but a "benefactor." Capsaicin does not stimulate but inhibits acid secretion, stimulates alkali, mucus secretions and particularly gastric mucosal blood flow which help in prevention and healing of ulcers. Capsaicin acts by stimulating afferent neurons in the stomach and signals for protection against injury causing agents. Epidemiologic surveys in Singapore have shown that gastric ulcers are three times more common in the "Chinese" than among Malaysians and Indians who are in the habit of consuming more chillis. Ulcers are common among people who are in the habit of taking NSAIDS and are infected with the organism "Helicobacter Pylori," responsible for excessive acid secretion and erosion of the mucosal layer. Eradication of the bacteria by antibiotic treatment and avoiding the NSAIDS eliminates ulcers and restores normal acid secretion.

Suppression by protease-activated receptor-2 activation of gastric acid secretion in rats

Activation of protease-activated receptor-2 (PAR-2), a receptor activated by trypsin/tryptase, induces neurally mediated gastric mucus secretion accompanied by mucosal cytoprotection. In the present study, we investigated whether PAR-2 could modulate gastric acid secretion in rats. Messenger RNAs for PAR-2 and PAR-1 were detected in the gastric mucosa and smooth muscle. The PAR-2-activating peptide SLIGRL-NH(2), but not the inactive control peptide, when administered i.v., strongly suppressed gastric acid secretion in response to carbachol, pentagastrin or 2-deoxy-D-glucose in the rats with a pylorus ligation. The PAR-2-mediated suppression of acid secretion was resistant to cyclooxygenase inhibition or ablation of sensory neurons by capsaicin. Our results provide novel evidence that in addition to stimulating neurally mediated mucus secretion, activation of PAR-2 suppresses gastric acid secretion independently of prostanoid production or sensory neurons. These dual actions of PAR-2 would result in gastric mucosal cytoprotection.

Four response stages of capsaicin-sensitive primary afferent neurons to capsaicin and its analog: gastric acid secretion, gastric mucosal damage and protection

Capsaicin is the active component of red hot peppers, which modifies specifically the capsaicin-sensitive sensory afferent nerves. The action of capsaicin is an initial short-lasting stimulation, which is followed by desensitization to capsaicin itself, and to other stimuli of afferent sensory nerves. Four response stages of capsaicin-sensitive primary afferents exist to capsaicin, depending on the dose and duration of exposure to the drug. These are excitation, a sensory blocking effect, long-term selective neurotoxic impairment, and irreversible cell destruction. The possible roles of four stages of capsaicin-sensitive primary afferents can be evaluated in relation to gastric acid secretion, and to the details of the defensive side of gastric mucosa against different chemicals, physical agents, drugs and other pathological stress. Capsaicin inhibited the gastric acid secretion in pylorus-ligated rats when it was given intragastrically at a dose of 0.4-1.8 microg/kg. Small doses of capsaicin (up to 800 microg, i.g.) produced a dose-dependent inhibition (ID50 = 400 microg), and its inhibitory effect was exerted for 1 h in healthy human subjects. While a small dose (5 microg/kg) of capsaicin caused inhibition, a high dose (50-100 mg/kg) enhanced the gastric mucosal lesions productivity by causing hyperacidity in pylorus-ligated animals. Capsaicin and its analog inhibited the development of different chemically induced gastric mucosal damage in various experimental models if they were given intragastric doses (microg/kg). The final effects of capsaicin depend on the dosage and timing. The different effects are excitation, a sensory-blocking effect, long-term selective neurotoxic impairment and irreversible cell destruction.

Small doses of capsaicin given intragastrically inhibit gastric basal acid secretion in healthy human subjects

Although the direct inhibitory effect of small dose of capsaicin on gastric secretory responses was proved in animal observations, the role of capsaicin-sensitive afferent nerves (CSAN) and the effect of capsaicin applied in small and high doses on gastric secretion in human has not been clarified yet. In this study we investigated the influence of different small doses (100-800 microg) of capsaicin given intragastrically through an orogastric tube on gastric basal secretory responses in 10 healthy human subjects. Gastric basal secretory responses (volume, H+-concentration, H+-output) were measured from the suctions of gastric juice for a 1-h period. It has been found that: a) capsaicin dose-dependently inhibited the volume and H+-output of gastric juice; b) ID50 was found to be about 400 microg for capsaicin on gastric acid secretion; c) the time interval for capsaicin-induced gastric inhibition existed for about 1 h indifferently from the higher dose (800 microg) of capsaicin given after. It has been concluded that the capsaicin (given in small doses) inhibits the gastric basal acid output via stimulation of the inhibition of capsaicin sensitive afferent nerves.