Effect of thyrotropin-releasing hormone on gastric acid secretion in man

The effects of histamine on the concentrations of immunoreactive thyrotropin-releasing hormone in the stomach and hypothalamus in rats

The effects of histamine and its related compounds on the concentrations of immunoreactive thyrotropin-releasing hormone (ir-TRH) in the stomach, gastric juice and hypothalamus in rats were studied. Histamine, ranitidine or ethanolamine was injected intraperitoneally, and the rats were decapitated at various times after the injection. Ir-TRH concentrations in the stomach, gastric juice and hypothalamus were measured by a radioimmunoassay. Ir-TRH concentrations in the stomach decreased significantly after histamine injection and increased significantly after ranitidine injection in a dose-dependent manner, but did not change with ethanolamine. Ir-TRH concentrations in the gastric juice increased in a dose-dependent manner, peaking at 30 min after histamine injection, and its effect was blocked with ranitidine. Ir-TRH concentrations in the hypothalamus elevated significantly after histamine injection and reduced significantly after ranitidine injection, but did not change with ethanolamine. The effects of histamine on ir-TRH concentrations in the stomach and hypothalamus were significantly blocked with ranitidine, but not with ethanolamine. These findings suggest that histamine stimulates ir-TRH release from the stomach and inhibits ir-TRH release from the hypothalamus, and that these effects of histamine on ir-TRH release are mediated via an H2-receptor.

Central nervous system action of TRH to influence gastrointestinal function and ulceration

There is clear evidence in rats that TRH acts in the brain to stimulate gastric acid, pepsin, and serotonin secretion, mucosal blood flow, contractility, emptying, and ulceration through activation of parasympathetic outflow to the stomach (TABLE 3). A number of TRH analogues, including some devoid of TSH-releasing activity, mimic the effects of TRH. The most sensitive TRH sites of action to elicit gastric acid secretion and motility are located in the dorsal vagal complex and include the dorsal vagal, nucleus tractus solitarius, and nucleus ambiguus. The gastrointestinal tract is one of the most responsive visceral systems to the central effects of TRH, because doses in the range of 1-10 pmol in the dorsal vagal complex stimulate gastric function, whereas stimulation of cardiovascular and respiratory function on microinjection of the brainstem nuclei requires higher doses. Although fewer investigations have been carried out in other species, evidence from the available data clearly indicates that TRH acts in the brain to increase gastric secretion and motility in the rabbit, sheep, and cat. Lack of stimulation of gastric acid secretion after third ventricle injection in the dog may be related to species difference or to rapid degradation of the peptide before it reaches its site of action. TRH acts centrally to stimulate gastric function and also intestinal secretion, motility, and transit as reported mostly in rabbits (TABLE 3). TRH produces enteropooling and release of serotonin in portal blood, increases duodenal and intestinal contractility and colonic transit, and elicits diarrhea. All these effects were shown to be vagally mediated. Stimulation of intestinal motility and transit by central injection of TRH has been observed in rats and sheep. The biological activity of centrally injected TRH is well correlated with the presence of TRH immunoreactivity and receptors in the dorsal vagal complex containing afferent and efferent connections to the stomach. Moreover, endogenous release of brain TRH in rats mimics the stimulatory effect of centrally injected TRH on gastric function. Although the lack of a specific TRH antagonist has hampered assessment of the physiological role of TRH, converging neuropharmacological, neuroanatomical, and physiological findings support the concept that TRH in the dorsal vagal complex may play a physiological role in the vagal regulation of gastrointestinal function.(ABSTRACT TRUNCATED AT 400 WORDS)

Thyrotropin-releasing hormone (TRH) evokes chloride secretion via cholinergic pathways in the guinea-pig ileum

The effect of thyrotropin-releasing hormone (TRH) on ion transport in the guinea-pig ileum was investigated in flux chambers. TRH evoked a concentration-dependent increase in short-circuit current that was due to active anion secretion. The response to TRH was prevented by tetrodotoxin and nearly abolished by atropine, but was not affected by mecamylamine. The results suggest that TRH plays a role as a neuromodulator or neurotransmitter that influences secretory processes by directly or indirectly activating cholinergic motor neurons.

Thyrotropin-releasing hormone effects on guinea pig antrum

Thyrotropin-releasing hormone transiently increased resting tension and spontaneous contraction amplitude more potently in longitudinal muscle than in circular muscle. The frequency of the contraction did not change. Excitation was enhanced by eserine, and blocked by atropine and tetrodotoxin; this excitation was, therefore, probably mediated by acetylcholine release from cholinergic neurons in the myenteric plexus. In the presence of atropine, thyrotropin-releasing hormone (10(-10) M) inhibited spontaneous contraction, particularly in longitudinal muscle. This was accompanied by decreased resting tension at and above 10(-8) M. Because contraction was not blocked by adrenergic blockade, indomethacin, methysergide, or hexamethonium, but was abolished by tetrodotoxin, it was postulated that thyrotropin-releasing hormone might stimulate nonadrenergic, noncholinergic inhibitory neurons. There was no cross-tachyphylaxis between 5-hydroxytryptamine and thyrotropin-releasing hormone. Results show that 5-hydroxytryptamine does not interact with thyrotropin-releasing hormone.

Intraventricular calcitonin gene-related peptide inhibits gastric acid secretion

Calcitonin gene-related peptide (CGRP) is a 37 amino acid peptide recently demonstrated to be a peptide expressed by the calcitonin gene in the rat central nervous system. Intracerebroventricular administration of CGRP in pylorus ligated rats resulted in a dose dependent suppression of gastric acid secretion. This effect was also present in acutely vagotomized rats. In addition, CGRP inhibited the stimulation of gastric acid secretion by thyrotropin releasing hormone. CGRP was considerably less potent in its effect on gastric acid than calcitonin, a well known central inhibitor of gastric acid secretion in the rat. This study suggests that CGRP may be a factor in the central regulation of gastric acid secretion in the rat.

Potent central nervous system action of p-Glu-His-(3,3'-dimethyl)-Pro NH2, a stabilized analog of TRH, to stimulate gastric secretion in rats

Intracisternal injection of the TRH analog RX 77368 (p-Glu-His-(3,3'-dimethyl)-Pro NH2) increased gastric acid and pepsin output in conscious pylorus-ligated rats. In urethane-anesthetized, gastric fistula rats, intracisternal RX 77368 or TRH induced stimulation of gastric acid output which was rapid in onset, long lasting, and dose-dependent, in doses ranging from 3 to 100 ng/rat for RX 77368, and 0.1 to 1 micrograms/rat for TRH. Vagotomy or atropine pretreatment reversed RX 77368 gastric secretory response. The analog was less effective when infused intravenously (1-10 micrograms X kg-1 X h-1) and 22 times more potent than TRH when given intracisternally. These results demonstrated the ability of RX 77368 to act within the rat brain to enhance gastric secretion (acid and pepsin) through vagus cholinergic dependent mechanisms. The enhanced potency and extended duration of action of RX 77368 over TRH, could make intracisternal injection of this peptide a useful test to induce centrally mediated vagal dependent stimulation of gastric secretion in rats.