Intracisternal injection of TRH precursor, TRH-glycine, stimulates gastric acid secretion in rats

Intracisternal injection of thyrotropin-releasing hormone (TRH)-Gly (pGlu-His-Pro-Gly) produced a dose-dependent (1-100 micrograms) stimulation of gastric acid secretion in urethane-anesthetized rats implanted acutely with a gastric fistula. The peak response occurred 20-30 min after intracisternal injection and lasted for more than 2 h. Intravenous injection of TRH-Gly (100 micrograms) did not modify gastric acid secretion. Following intracisternal injection of TRH-Gly, a peak elevation of both TRH-Gly and TRH levels is observed in the cerebrospinal fluid (CSF) within 15 min. Thereafter, TRH values are returned to basal levels at 75 min after the injection, whereas TRH-Gly concentrations remain significantly elevated throughout the 2-h period of measurement. Compartmental analysis revealed that CSF conversion of TRH-Gly to TRH was only 0.0072%/min. Medullary coronal sections containing the dorsal vagal complex and the raphé nucleus revealed increased content of TRH-Gly, but not TRH, 40 min after administration of TRH-Gly at an intracisternal dose effective in stimulating gastric acid secretion (100 micrograms). In addition, TRH but not TRH-Gly (10(-7)-10(-5) M) displaced [3H]MeTRH binding from rat medullary blocks containing the dorsal vagal complex. These data suggest that the intracisternal TRH-Gly-induced stimulation of gastric acid secretion is not related to its conversion to TRH in the CSF, or direct activation of TRH receptors in the medulla. The acid secretory response of TRH-Gly may be due to the formation of TRH at the active brain sites, or alternatively to activation of its own specific receptors.     

The effects of centrally administered neuropeptides on the development of gastric lesions in the rat

Centrally administered neuropeptides were investigated for their effects on the development of gastric lesions in rats. Thyrotropin releasing hormone (TRH), vasoactive intestinal peptide (VIP) and gonadotropin releasing hormone (LHRH) produced gastric lesions acutely, with TRH demonstrating the most pronounced effect in terms of incidence and severity. Ten-fold higher doses of the same peptides administered intravenously produced none or very few gastric lesions. Moreover, pretreatment with atropine partially inhibited their production. Corticotropin releasing factor (CRF) exhibited only mild ulcerogenic effects, and the gastric lesions induced with this peptide developed more slowly than with TRH, VIP and LHRH. Although ulcerogenic in their own right, none of these four neuropeptides significantly potentiated the potent ulcerogenic effects of cold-restraint stress. Since other neuropeptides, including somatostatin, human pancreatic growth hormone releasing factor (hpGRF), substance P, bombesin, and neurotensin, had no demonstrable effects on gastric mucosa, we can conclude that the lesions were not a general effect of intracisternal administration of neuropeptides. The results suggest that within the central nervous system, there are several neuropeptides that play a significant role in the development of gastric lesions via, at least in part, vagal-dependent mechanisms.     

The effects of thyrotropin-releasing hormone on cyclic AMP accumulation in rabbit cerebral cortical tissue in the presence and absence of CNS depressants

The $\text{in vitro}$ effects of thyrotropin-releasing hormone on cAMP accumulation in cortical brain slices from rabbits is reported. Incubation of cortical tissue at three concentrations of thyrotropin-releasing hormone (1,2,5nM) had no discernible effects on baseline cAMP levels. When cortical tissue was incubated in the presence of pentobarbital (.5mM) or if cortical tissue was taken from animals pretreated with α-methyl-p-tyrosine (α-MPT), the baseline cAMP accumulation was depressed. This depression could be eliminated by the addition of TRH to the incubation media. Where cortical tissue from atropine-pretreated animals was used or when atropine was added to the incubation media, there was an increase in baseline cAMP accumulation which was unaffected by addition of TRH. These results show that TRH can modify cAMP accumulation in mammalian cortical brain tissue but this ability may only become evident in situations where normal cAMP concentration has been depressed.     

Thyrotropin-Releasing Hormone Release Is Enhanced in Hippocampal Slices After Electroconvulsive Shock

Abstract: Hippocampal thyrotropin-releasing hormone (TRH) release was examined after seizures were induced by electroconvulsive shock (ECS). Rat hippocampal slices taken 12, 24, or 48 h after 3 days of alternate-day ECS treatment or sham-ECS treatment were stimulated with potassium with or without calcium in a superfusion system containing in-line charcoal adsorbent to concentrate TRH. Released TRH and tissue TRH were measured by radioimmunoassay. The TRH content of hippocampal slices was increased fivefold over sham-ECS levels 12, 24, and 48 h after ECS, but this was not associated with an increase in basal TRH release. Potassium-stimulated TRH release was significantly elevated over basal release 12, 24, and 48 h after ECS. Potassium-stimulated calcium-dependent TRH release increased linearly after ECS, reaching its highest level 48 h after seizure. Thus, although enhanced calcium-dependent TRH release was associated with elevated tissue levels, this relationship was not proportional in that tissue TRH was elevated to the same extent at all times after ECS, whereas potassium-evoked calcium-dependent TRH release increased gradually over time after seizure. These results suggest that postictal elevations in TRH are associated with an enhanced capacity for release that develops as a result of a time-dependent shift of TRH from a storage compartment to a readily releasable pool. The observed elevation in stimulated TRH release may be relevant to seizure-induced modulation of TRH receptors in vivo.     

The effects of thyrotropin releasing hormone (TRH) on the gastrointestinal tract.

Thyrotropin-releasing hormone (TRH) immunoreactivity is distributed throughout the gastrointestinal tract and the pancreas. We have studied the effect of TRH on several gastrointestinal functions in intact, unanesthetized dogs. Intravenous TRH stimulated gastric action potentials (p<0.01) and transiently inhibited tetragastrin-stimulated gastric acid secretion (p<0.05). TRH had no effect on basal or secretin-stimulated pancreatic exocrine secretion. TRH did not alter water absorption in dogs with Thiry-Vella loops constructed from proximal jejunum.     

Endogenous opiates inhibit gastric acid secretion induced by central administration of Thyrotropin-Releasing Hormone (TRH)

Thyrotropin-releasing hormone (TRH) administered intraventricularly (ICV) to rats causes a dose-dependent increase in gastric acid secretion over a range of 0.01 μg to 10 μg in the pyloris ligated rat. The maximum increase in gastric acid secretion occurs in the first hour. This effect of TRH is not mediated by its metabolites, histidyl-proline diketopiperazine or pyroglutamyl-histidyl-proline (acid TRH). β-endorphin, D-alanine-methionine-enkephalin and the leucine-enkephalin precursor, dynorphin, all inhibit TRH-induced gastric acid secretion. Bombesin, which reduces basal gastric acid secretion had no effect on TRH-induced secretion.     

Effect of thyrotropin-releasing hormone on gastro-intestinal secretions in dogs

Thyrotropin-releasing hormone (TRH) immunoreactivity has been shown previously to be distributed throughout the gastrointestinal tract and the pancreas. This study demonstrated that TRH given intravenously suppresses, in a dose-related manner, sham-feeding induced food intake and inhibits gastric secretion provoked by infusion of pentagastrin or instillation of 10% liver extract into the stomach. TRH also reduces pancreatic response to secretin, caerulein, feeding a meat meal or duodenal acidification. The findings that TRH inhibits gastric and pancreatic secretions induced by exogenous and endogenous stimulants, and that the inhibition by TRH of post-prandial secretion is not accompanied by any change in serum gastrin, indicate that TRH probably acts directly on the exocrine stomach and pancreas.     

Thyrotropin-releasing hormone activates KCa channels in gastric smooth muscle cells via intracellular Ca2+ release

Thyrotropin-releasing hormone (TRH) is released in high concentrations into gastric juice, but its direct effect on gastric smooth muscles has not been studied yet. We undertook studies on TRH effect on gastric smooth muscle using contraction and patch clamp methods. TRH was found to inhibit both acetylcholine- and BaCl2-induced contractions of gastric strips. TRH, applied to single cells, inhibited the voltage-dependent Ca2+ currents and activated the whole-cell K+ currents. The TRH-induced changes in K+ currents and membrane potential were effectively abolished by inhibitors of either intracellular Ca2+ release channels or phospholipase C. Neither activators, nor blockers of protein kinase C could affect the action of TRH on K+ currents. In conclusion, TRH activates K+ channels via inositol-1,4,5-trisphosphate-induced release of Ca2+ in the direction to the plasma membrane, which in turn leads to stimulation of the Ca2+-sensitive K+ conductance, membrane hyperpolarization and relaxation. The data imply that TRH may act physiologically as a local modulator of gastric smooth muscle tone.     

Dexamethasone stimulates thyrotropin-releasing hormone production in a C cell line

The hypothalamic peptide hormone TRH is also found in other tissues, including the thyroid. While TRH may be regulated by T3 in the hypothalamus, other regulators of TRH have not been identified and the regulation of TRH in nonhypothalamic tissues is unknown. We recently demonstrated the biosynthesis of TRH in the CA77 neoplastic thyroidal C cell line. We studied the regulation of TRH by dexamethasone in this cell line because glucocorticoids have been postulated to inhibit TSH secretion by decreasing TRH in the hypothalamus. Furthermore, TRH in the thyroid inhibits thyroid hormone release. Thus by regulating thyroidal TRH, glucocorticoids could also directly affect thyroid hormone secretion. Treatment of CA77 cells for 4 days with dexamethasone produced dose-dependent increases in both TRH mRNA and cellular and secreted TRH. Increases in TRH mRNA and peptide levels could be seen with 10(-9) M dexamethasone. A 4.8-fold increase in TRH mRNA and a 4-fold increase in secreted peptide were seen with 10(-7) M dexamethasone. Dexamethasone treatment did not increase beta-actin mRNA levels or cell growth. These results suggest that glucocorticoids may be physiological regulators of TRH in normal C cells. In addition to their inhibitory effects on TSH, glucocorticoids may decrease thyroid hormone levels by increasing thyroidal TRH. Since the glucocorticoid effects on C cell TRH are the converse of what is expected for hypothalamic TRH, glucocorticoid effects in these two tissues may be mediated by different regulators.     

Morphine inhibits TRH-induced gastric contractile activity.

This study investigated the effect of centrally and peripherally administered thyrotropin releasing hormone (TRH) on gastric contractile activity of rats 14, 21, 28 and adult (greater than or equal to 50) days (D) of age, and the effect of morphine pretreatment on that response. Rats were anesthetized with urethane, then a tension transducer was implanted on the anterior gastric corpus. Following baseline recording, rats were pretreated with intraperitoneal morphine (2 mg/kg). TRH (5 micrograms) in saline or saline alone (0.6 microliters) was then injected into the cisternum magnum. Additionally, dose response to TRH was examined in 14- and 50-day-old rats. Intracisternal TRH induced a dose-related increase in gastric contractile activity in both 14- and 50-day-old rats. Higher doses of TRH (10 and 30 micrograms) prolonged the response as compared to low doses. Peripheral morphine pretreatment blocked the TRH-induced increase in gastric contractile activity in all age groups although a higher morphine dose (10 mg/kg) was needed to block the effect in 28D rats. Intravenous TRH (5, 10, 30 micrograms) produced an increase in gastric contractile activity in 14D rats which was blocked by vagotomy.     

Gastric effects of thyrotropin-releasing hormone microinjected into the dorsal vagal nucleus in cats

We investigated the gastric acid secretory and motility responses to microinjection of thyrotropin-releasing hormone (TRH) into the dorsal motor nucleus of the vagus (DMV) in anesthetized cats. Gastric acid output was collected every 15 min through a gastric cannula after saline flush and titrated to pH 7.0. Antral and corpus contractions were continuously recorded by extraluminal force transducers. TRH dissolved in 200 nl of saline and microinjected unilaterally into the DMV induced a dose-dependent (50-200 ng) increase in gastric acid secretion. The acid secretory response began in the first 15 min collection and lasted 45 min. TRH frequently increased the force of contractions of the antrum and corpus within one minute of microinjection. The minimal effective dose for eliciting increased motility was lower than for inducing acid secretion. These results demonstrate that TRH acts in the DMV of cats to stimulate gastric acid secretion and contractions.     

Thyrotropin-releasing hormone (TRH) acts centrally to stimulate gastric contractility in rats

Changes in gastric contractility induced by intracisternal (ic) injection of thyrotropin-releasing hormone (TRH) or a stable TRH analog, RX77368 [p-Glu-His-(3,3'-dimethyl)-Pro NH2] were investigated in 24 h fasted-conscious rats. Gastric contractility was monitored using chronically implanted extraluminal force transducers sutured to the corpus. Response elicited by a standard meal was used as a physiologic standard. Intracisternal injection of TRH (1 microgram) or RX77368 (100 ng), unlike saline, stimulated high amplitude gastric contractions. The stimulation of gastric contractions induced by ic RX77368 was dose dependent (3-100 ng), rapid in onset, long lasting and not mimicked by the intravenous route of administration. Atropine (0.1 mg/kg) partially antagonized and vagotomy totally blocked the RX77368 (100 ng, ic)-induced stimulation of gastric contractility. These results demonstrated that TRH or RX77368 acts within the brain to elicit potent contractions of the stomach; TRH action appears vagally mediated probably through cholinergic mechanism.     

TRH-enkephalin interactions in the amygdaloid complex during gastric stress ulcer formation in rats

The formation of gastric stress ulcers was studied as a function of interactions between thyrotropin releasing hormone (TRH) and endogenous opioids in the central amygdalar nucleus (CEA) in rats. Bilateral microinjections of TRH (1 or 10 micrograms) into the CEA produced dose-related aggravations in cold restraint stress (CRS, 3 h at 4 degrees C)-induced gastric ulcer formation. Similar stress ulcer facilitating effects were also seen with intra-CEA injections of the opioid antagonists, naloxone (1 or 10 micrograms). On the other hand, the enkephalin analog, D-Ala2-metenkephalinamide (DAMEA, 1, 10 or 20 micrograms) produced dose-dependent attenuations in gastric stress pathology, the effects being most marked with the latter two doses. Pretreatment of rats with intra-CEA naloxone (1 microgram) (a) antagonized the gastric cytoprotective effects of DAMEA (20 micrograms) and (b) further aggravated the ulcerogenic response of TRH (1 microgram), without influencing significantly the TRH (10 micrograms) effect. Further, when DAMEA (20 micrograms) was administered intra-CEA just after TRH (10 micrograms), the stress ulcer facilitating effects of the latter was neutralized. The results indicate that TRH-enkephalin interactions are possible at the level of the CEA during CRS-induced gastric ulcer formation.     

Biosynthesis of thyrotropin-releasing hormone by a rat medullary thyroid carcinoma cell line

Prepro-thyrotropin-releasing hormone (TRH) messenger RNA was detected in the rat medullary thyroid carcinoma cell line CA77. The RNA of 1.6 kilobases comigrated with that found in rat hypothalamus. Using three radioimmunoassays specific for pro-TRH-derived peptides, we demonstrated that CA77 cells synthesize high levels of immunoreactive TRH and all of the other pro-TRH-derived peptides identified in hypothalamic tissue. The relative levels of the pro-TRH-derived peptides also indicate that CA77 cells process the TRH precursor in a manner similar to hypothalamic tissue. CA77 cells provide a promising model system for further studies of prepro-TRH gene regulation and post-translational maturation.     

Mechanisms underlying intracisternal TRH-induced stimulation of gastric acid secretion in rats

The neurohumoral pathways mediating intracisternal TRH-induced stimulation of gastric acid secretion were investigated. In urethane-anesthetized rats, with gastric and intrajugular cannulas, TRH or the analog [N-Val2]-TRH (1 microgram) injected intracisternally increased gastric acid output for 90 min. Serum gastrin levels were not elevated significantly. Under these conditions the TRH analog, unlike TRH, was devoid of thyrotropin-releasing activity as measured by serum TSH levels. In pylorus-ligated rats, gastrin values were not modified 2 h after peptide injection whereas gastric acid output was enhanced. TRH (0.1-1 micrograms) stimulated vagal efferent discharge, recorded from a multifiber preparation of the cervical vagus in urethane-anesthetized rats and the response was dose-dependent. The time course of vagal activation was well correlated with the time profile of gastric stimulation measured every 2 min. These results demonstrated that gastric acid secretory stimulation elicited by intracisternal TRH is not related to changes in circulating levels of gastrin or TSH but is mediated by the activation of efferent vagal pathways that stimulated parietal cell secretion.     

Effect of repeated stimulation by thyrotropin-releasing hormone (TRH) on thyrotropin and prolactin secretion in perfused euthyroid and hypothyroid rat pituitary fragments

The previously reported refractoriness of pituitary response to thyrotropin-releasing hormone (TRH) stimuli was investigated here in an in vitro perfusion system using pituitary tissue from euthyroid and hypothyroid rats. Thyroid-stimulating hormone (TSH) and prolactin (PRL) responses to TRH (28 pmol) were significantly greater in hypothyroid tissue compared with euthyroid. Hypothyroid tissue showed a reduction in response to two consecutive stimuli in both TSH and PRL, however the TSH decline in response was more marked than PRL. Euthyroid tissue showed no significant decline in response to TRH. An increase in the dose of TRH (112 pmol), administered to euthyroid tissue, resulted in increased TSH and PRL response, but no decline in response to sequential stimuli was observed. Three consecutive stimuli by TRH (28 pmol) of hypothyroid tissue resulted in a consistent decline in TSH response. The decline in PRL response only reached statistical significance by the third stimulation. Euthyroid and hypothyroid pituitary tissue was subjected to sequential depolarising stimulation with KCl (50 mumol). Euthyroid tissue showed no decline in response in either TSH or PRL. In hypothyroid tissue only, the decline in TSH response reached statistical significance. This decline in TSH response was significantly smaller than the decline in response observed in hypothyroid tissue stimulated with TRH. Refractoriness of hypothyroid pituitary tissue to repeated TRH stimuli is reported here. Our data suggest that the decline in hormonal response cannot be explained solely on the basis of tissue depletion.     

Effects of orexin A on thyrotropin-releasing hormone and thyrotropin secretion in rats.

Effects of orexin A on secretion of thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) in rats were studied. Orexin A (50 microg/kg) was injected iv, and the rats were serially decapitated. The effects of orexin A on TRH release from the rat hypothalamus in vitro and on TSH release from the anterior pituitary in vitro were also investigated. TRH and thyroid hormone were measured by individual radioimmunoassays. TSH was determined by the enzyme-immunoassay method. The hypothalamic TRH contents increased significantly after orexin A injection, whereas its plasma concentrations tended to decrease, but not significantly. The plasma TSH levels decreased significantly in a dose-related manner with a nadir at 15 min after injection. The plasma thyroid hormone levels showed no changes. TRH release from the rat hypothalamus in vitro was inhibited significantly in a dose-related manner with the addition of orexin A. TSH release from the anterior pituitary in vitro was not affected with the addition of orexin A. The findings suggest that orexin A acts on the hypothalamus to inhibit TRH release.     

Oxytocin, oxytocin antagonist, TRH, and hypothalamic paraventricular nucleus stimulation effects on gastric motility

The roles of thyrotropin releasing hormone (TRH) and oxytocin as central regulators of gastric motility were investigated. Picomolar (4 picomoles) quantities of TRH injected into the dorsal motor nucleus of the vagus (DMN) elicited a significant increase in gastric motility while the same quantity of oxytocin elicited a reduction in phasic contractile activity and tone. The action of these peptides mimics the excitatory and inhibitory effects of stimulating the paraventricular nucleus of the hypothalamus (PVN); it is likely that this hypothalamic structure regulates gastric function through its peptidergic connections with medullary vagal structures. This hypothesis is supported by our observations that injections of an oxytocin antagonist into the DMN produced a disinhibition of gastric motility and an increase in the motility evoked by subsequent PVN stimulation. Vagotomy eliminated all subsequent central effects on motility of these peptides.     

Twenty-four hour rhythms of hypothalamic corticotropin-releasing hormone, thyrotropin-releasing hormone, growth hormone-releasing hormone and somatostatin in rats injected with Freund's adjuvant

The effect of Freund's adjuvant injection on 24-hour variation of hypothalamic corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH), GH-releasing hormone (GRH) and somatostatin levels was examined in adult rats kept under light between 0800 and 2000 h daily. Groups of rats receiving Freund's complete adjuvant or its vehicle 3 days before sacrifice were killed at six different time intervals throughout a 24-hour cycle. In the median eminence, adjuvant vehicle-injected rats exhibited significant 24-hour variations for the four hormones examined, with maxima at noon. These 24-hour rhythms were inhibited or suppressed by Freund's adjuvant injection. In the anterior hypothalamus of adjuvant vehicle-treated rats, CRH content peaked at 1600 h, while two peaks were found for TRH and GRH levels, i.e., at 2400-0400 h and 1600 h. Freund's adjuvant injection suppressed 24-hour rhythm of anterior hypothalamic CRH, TRH and GRH content and uncovered a peak in anterior hypothalamic somatostatin levels at 0400 h. In the medial hypothalamus of adjuvant vehicle-treated rats, significant 24-hour variations were detectable for TRH (peaks at 1600 and 2400 h) and somatostatin (peak at 2400 h) which disappeared after Freund's adjuvant injection. In the posterior hypothalamus of adjuvant vehicle-treated rats, two peaks were apparent for CRH, TRH and somatostatin levels, i.e. at 1600 h and 2400-0400 h, this hormonal profile remaining unmodified after Freund's adjuvant administration. The administration of the immunosuppressant drug cyclosporine (5 mg/kg, 5 days) impaired the depressing effect of Freund's adjuvant injection on CRH, TRH and somatostatin content in median eminence, but not that on GRH. In the anterior hypothalamus, cyclosporine generally prevented the effect of immunization on hormone levels an revealed a second maximum in TRH at 0400 h. Cyclosporine also restored 24-hour variations in TRH and somatostatin levels of medial hypothalamus of Freund's adjuvant-injected rats but was unable to modify them in the posterior hypothalamus. The results further support the existence of a significant effect of immune-mediated inflammatory response at an early phase after Freund's adjuvant injection on hypothalamic levels which was partially sensitive to immunosuppression by cyclosporine.     

Regional Distribution of Immunoreactive-Thyrotrophin-Releasing Hormone and Substance P, and Indoleamines in Human Spinal Cord

The regional distributions of thyrotrophin-releasing hormone (TRH) and substance P in postmortem human spinal cord were determined by radioimmunoassay in fresh tissue taken from 22 patients who died without known neurological disease. Dorsal, ventral, and intermediolateral spinal cord regions were obtained from different segmental levels (lumbar L1, 2, 3, and 4; thoracic groups T1-3, T4-6, T7-9, and T10-12) together with selective regions of grey matter of lumbar spinal cord. The effects on peptide levels of the age of the patient, the postmortem time interval, and freezing the tissue samples prior to assay were assessed. Levels of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were determined in regional lumbar and thoracic tissue using HPLC with electrochemical detection. Substance P was found in the highest concentration in the dorsal spinal cord, with no significant segmental differences. In contrast, TRH was present in higher levels in the ventral rather than the dorsal spinal cord, with segmental differences. There was a significant difference in the 5-HT/5-HIAA ratio between dorsal and ventral spinal cord, with the highest ratio in the ventral spinal cord. There were no significant differences in substance P, TRH, or 5-HT levels in spinal cords between 5 and 20 h postmortem or from patients aged between 65 and 90 years. Freezing the tissue (-80 degrees C for 24 h) prior to assay significantly reduced TRH and substance P levels compared to samples assayed immediately without prior freezing.(ABSTRACT TRUNCATED AT 250 WORDS)