Role of acetylcholine, histamine and gastrin in the acid response to intracisternal injection of TRH analog, RX 77368, in the rat

The role of gastrin, acetylcholine and histamine in the acid response to central vagal activation induced by intracisternal injection of the stable analog, RX 77368, was further investigated in urethane-anesthetized rats with gastric fistula. The gastrin monoclonal antibody 28-2 injected intravenously, at a dose previously shown to prevent gastrin-induced stimulation of acid secretion, did not alter the peak acid response to intracisternal injection of RX 77368 (15 ng). The TRH analog (30 ng) injected into the cisterna magna increased levels of histamine measured in the hepatic portal blood. Cimetidine administered at a dose which completely blocked the stimulation of gastric acid secretion produced by intravenous infusion of histamine, inhibited by 62% the stimulatory effect of intracisternal RX 77368 (30 ng). The M1 muscarinic antagonist, pirenzepine, completely prevented the acid secretion induced by intracisternal RX 77368 (30 ng). These results indicate that the acid response to central vagal activation by the TRH analog in rats involved M1 muscarinic receptors along with histamine release acting on H2 histaminergic receptors whereas gastrin does not appear to play an important role.     

Capsaicin-sensitive vagal afferents contribute to gastric acid and vascular responses to intracisternal TRH analog

Central injection of TRH or its stable analog, RX77368, produces a vagal cholinergic stimulation of gastric acid secretion, mucosal blood flow and motor function. In the present study, we have investigated the contribution of capsaicin-sensitive vagal afferent fibers to the gastric responses to intracisternal injection of RX77368. Gastric acid secretion, measured in acute gastric fistula rats anesthetized with urethane, in response to intracisternal injection of RX77368 (3-30 ng) was reduced by 21-65% by perineural pretreatment of the vagus nerves with capsaicin 10-20 days before experiments. The increase in gastric mucosal blood flow measured by hydrogen gas clearance induced by intracisternal injection of RX77368 (30 ng) was also reduced by 65% in capsaicin-pretreated rats. In contrast, increases in gastric motor function measured manometrically or release of gastric luminal serotonin in response to intracisternal injection of RX77368 (3-30 ng) were unaltered by capsaicin pretreatment. The mechanism by which vagal afferent fibers contribute to the secretory and blood flow responses to the stable TRH analog is unclear at present, but it is possible that the decrease in gastric mucosal blood flow by lesion of capsaicin-sensitive vagal afferents limits the secretory response.     

Gastric erosions induced by intracisternal thyrotropin-releasing hormone (TRH) in rats

Intracisternal injection of TRH (1 microgram) under light ether anesthesia induced within 4 hr gastric lesions in 24-hr fasted rats maintained unrestrained at room temperature. Saline, ovine corticotropin-releasing factor (oCRF, 10 micrograms), or human pancreatic growth hormone-releasing factor [hpGRF(1-40), 10 micrograms] tested under the same conditions did not modify the integrity of the gastric mucosa. TRH injected intravenously (100 micrograms/kg) proved to be ineffective. The production of gastric erosions elicited by intracisternal TRH (0.1-1 microgram) or by a stabilized TRH analog, RX 77368 [pGlu-His-(3,3'-dimethyl)-ProNH2, (0.01-0.1 microgram)] was dose-dependent. RX 77368 shows an enhanced potency over TRH. TRH action on gastric mucosa was reversed by atropine, omeprazole and cimetidine. These results demonstrate that TRH, unlike the other hypothalamic releasing factors CRF or GRF, is able to act within the brain to cause the formation of gastric erosions probably through mechanisms involving changes in gastric acid secretion. Intracisternal injection of TRH or its potent analog RX 77368 appears also as a new, simple method to produce centrally mediated experimental gastric erosions in 24 hr-fasted rats.     

Fluoxetine pretreatment potentiates intracisternal TRH analogue-stimulated gastric acid secretion in rats.

Central injection of TRH or its metabolically stable analogue RX 77368 has been demonstrated to produce a vagal-dependent stimulation in gastric acid secretion. Accumulating evidence exists regarding the interaction of serotonin (5HT) with TRH containing neuronal systems. This study was performed to assess the effect of pretreatment with the 5HT uptake inhibitor fluoxetine on the TRH analogue-induced gastric acid secretory response. Systemic fluoxetine (30 mumol/kg, i.v.) produced a 43-85% increase in the intracisternal RX 77368 (78-780 pmol)-induced gastric acid output, while not affecting the basal acid response. The acid response to a lower dose of RX 77368 (26 pmol) was not altered. In addition, intracisternal fluoxetine (180 nmol) produced a 71% augmentation of the acid secretory response of i.c. RX 77368 (260 pmol). Intracisternal injection of lower doses (60, 120 nmol), or intravenous injection of 180 nmol of fluoxetine was ineffective in altering the intracisternal RX 77368-induced acid response. Pretreatment with the noradrenergic or dopaminergic uptake inhibitor desipramine or GBR 12909 did not alter the RX 77368-stimulated gastric acid secretory response. The results show that fluoxetine pretreatment potentiates the effect of intracisternal RX 77368 on acid secretion. The effect appears to be impulse dependent, and central sites of action are involved. The data suggest an interaction of synaptic serotonin with a RX 77368-elicited event (activation of TRH receptors, second messenger systems and/or firing of the motor vagus) results in potentiation of the RX 77368-induced gastric response.     

Central nervous system action of TRH to stimulate gastric emptying in rats

The effects of intracisternal injection of TRH on gastric emptying of a liquid meal was investigated in 24 h fasted rats using the phenol red method. Intracisternal injection of TRH, RX 77368, or [N-Val2]-TRH, an analog devoid of TSH-releasing activity, 5 min prior to a meal, stimulated gastric emptying measured 20 min later. TRH action was dose dependent (1-100 ng), and rapid in onset. The calculated time for emptying half of the meal was decreased from 16 +/- 3 min (control group) to 4 +/- 1 min (TRH 30 ng). The stable analog, RX 77368, unlike TRH, stimulated gastric emptying when the meal was given 60 min after peptide injection. Intravenous injection of atropine (2.5 micrograms) inhibited and that of carbachol (1 microgram) stimulated gastric emptying whereas i.v. injection of TRH (0.1-1 microgram) had no effect. Vagotomy but not adrenalectomy reversed the increase in gastric emptying induced by intracisternal TRH. Atropine blocked the stimulatory effect of TRH and carbachol. These results demonstrate that TRH acts within the brain to stimulate gastric emptying through vagus-dependent and cholinergic pathways whereas alterations of adrenal and pituitary-thyroid secretion do not play an important role.     

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.     

Bombesin microinjected into the dorsal vagal complex inhibits vagally stimulated gastric acid secretion in the rat

Medullary sites of action for bombesin-induced inhibition of gastric acid secretion were investigated in urethane-anesthetized rats with gastric fistula. Unilateral microinjection of bombesin or vehicle into the dorsal vagal complex was performed using a glass micropipet and pressure ejection of 100 nl volume; gastric acid output was measured every 10 min by flushing the stomach. Microinjection of vehicle into the dorsal vagal complex did not alter gastric acid secretion (1.9 +/- mumol/10) from preinjection levels (2.9 +/- 0.8 mumol/10 min). Microinjection of the stable thyrotropin-releasing hormone (TRH) analog, RX 77368, at a 77 pmol dose into the dorsal vagal complex stimulated gastric acid secretion for 100 min with a peak response at 40 min (24.1 +/- 3.2 mumol/10 min). Concomitant microinjection of RX 77368 (77 pmol) with bombesin (0.6-6.2 pmol) into the dorsal vagal complex dose dependently inhibited by 35-86% the gastric acid response to the TRH analog. Bombesin (6.2 pmol) microinjected into the dorsal vagal complex inhibited by 17% pentagastrin infusion-induced stimulation of gastric acid secretion (13.2 +/- 0.8 mumol/10 min) whereas intracisternal injection induced a 69% inhibition of the pentagastrin response. These results demonstrate that the dorsal motor complex is a sensitive site of action for bombesin-induced inhibition of vagally stimulated gastric secretion. However, other medullary sites must be involved in mediating the inhibitory effect of intracisternal bombesin on pentagastrin-stimulated gastric acid secretion.     

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.     

Intracisternal TRH and RX 77368 Potently Activate Gastric Vagal Efferent Discharge in Rats

O-Lee, T. J., J. Y. Wei and Y. TachÉ. Intracisternal Trh and Rx 77368 potently activate gastric vagal efferent discharge in rats. Peptides 18(2) 213–219, 1997.—The influence of intracisternal (ic) TRH and the stable TRH analog, RX 77368, on gastric vagal efferent discharge (GVED) was investigated in urethane-anesthetized rats. Consecutive IC injections of TRH (3, 30, and 300 ng) at 60 min intervals stimulated dose dependently multi-unit GVED with a peak increase of 90 ± 21%, 127 ± 18% and 145 ± 16% respectively. In two separate studies, IC injection of RX 77368 at 1.5 or 15 ng stimulated multi-unit GVED by 142 ± 24% and 244 ± 95% respectively. Saline injection IC had no effect on GVED. RX 77368 (1.5 ng, ic) action was long lasting (84 ± 13 min) compared with TRH (3 ng: 44 ± 7 min). Single-unit analysis also showed that 13 of 13 units responded to ic RX 77368 (1.5 ng) by an increase in activity. These data indicate that low doses of TRH injected ic stimulate vagal efferent outflow to the rat stomach and that RX 77368 action is more potent than TRH.     

Vagus-mediated activation of mucosal mast cells in the stomach: effect of ketotifen on gastric mucosal lesion formation and acid secretion induced by a high dose of intracisternal TRH analogue.

TRH analogue, RX 77368, injected intracisternally (i.c.) at high dose (3 microg/rat) produces gastric mucosal lesion formation through vagal-dependent pathway. The gastric mucosal hyperemia induced by i.c. RX 77368 was shown to be mediated by muscarinic vagal efferent fibres and mast cells. Furthermore, electrical vagal stimulation was observed to induce gastric mucosal mast cell degranulation. The aim of the study was to assess the influence of ketotifen, a mast cell stabilizer, on RX 77368-induced gastric lesion formation and gastric acid secretion. RX 77368 (3 microg, i.c.) or vehicle (10 microL, i.c.) was delivered 240 min prior to the sacrifice of the animals. Ketotifen or vehicle (0.9% NaCl, 0.5 mL) was injected intraperitoneally (i.p.) at a dose of 10 mg x kg(-1) 30 min before RX 77368 injection. The extent of mucosal damage was planimetrically measured by a video image analyzer (ASK Ltd., Budapest) device. In the gastric acid secretion studies, the rats were pretreated with ketotifen (10 mg x kg(-1), i.p.) or vehicle (0.9% NaCl, 0.5 mL, i.p.), 30 min later pylorus-ligation was performed and RX 77368 (3 microg, i.c.) or vehicle (0.9% NaCl, 10 microL, i.c.) was injected. The rats were killed 240 min after i.c. injection, and the gastric acid secretion was measured through the titration of gastric contents with 0.1 N NaOH to pH 7.0. RX 77368 (3 microg, i.c.) resulted in a gastric mucosal lesion formation involving 8.2% of the corpus mucosa (n = 7). Ketotifen elicited an 85% inhibition on the development of mucosal lesions (n = 7, P < 0.001) whereas ketotifen alone had no effect on the lesion formation in the mucosa (n = 7). The RX 77368 induced increase of gastric acid secretion was not influenced by ketotifen pretreatment in 4-h pylorus-ligated animals. Central vagal activation induced mucosal lesion formation is mediated by the activation of mucosal mast cells in the stomach. Mast cell inhibition by ketotifen does not influence gastric acid secretion induced by i.c. TRH analogue in 4-h pylorus-ligated rats.     

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.     

Peripheral PYY inhibits intracisternal TRH-induced gastric acid secretion by acting in the brain

The site of action of peripheral peptide YY (PYY)-induced inhibition of vagally stimulated gastric acid secretion was studied using immunoneutralization with PYY antibody in urethan-anesthetized rats. Gastric acid secretion (59+/-7 micromol/90 min) stimulated by intracisternal injection of the stable thyrotropin-releasing hormone (TRH) analog RX-77368 (14 pmol/rat) was dose-dependently inhibited by 52%, 69%, and 83% by intravenous infusion of 0.25, 0.5, and 1.0 nmol. kg(-1) x h(-1) PYY, respectively. PYY or PYY(3-36) (2.4 pmol/rat) injected intracisternally also inhibited the acid response to intracisternal RX-77368 by 73% and 80%, respectively. Intravenous pretreatment with PYY antibody (4.5 mg/rat), which shows a 35% cross-reaction with PYY(3-36) by RIA, completely prevented the inhibitory effect of intravenously infused PYY (1 nmol x kg(-1) x h(-1)). When injected intracisternally, the PYY antibody (280 microg/rat) reversed intracisternal PYY (2.4 pmol)- and intravenous PYY (1 nmol x kg(-1) x h(-1))-induced inhibition of acid response to intracisternal RX-77368 by 64% and 93.5%, respectively. These results provide supporting evidence that peripheral PYY inhibits central vagal stimulation of gastric acid secretion through an action in the brain.     

Intracisternal TRH analog induces Fos expression in gastric myenteric neurons and glia in conscious rats

Activation of gastric myenteric cells by intracisternal injection of the stable thyrotropin-releasing hormone (TRH) analog RX-77368, at a dose inducing near maximal vagal cholinergic stimulation of gastric functions, was investigated in conscious rats. Fos immunoreactivity was assessed in gastric longitudinal muscle-myenteric plexus whole mount preparations 90 min after intracisternal injection. Fos-immunoreactive cells were rare in controls (~1 cell/ganglion), whereas intracisternal RX-77368 (50 ng) increased the number to 24.8 +/- 1.8 and 26.8 +/- 2.2 cells/ganglion in the corpus and antrum, respectively. Hexamethonium (20 mg/kg sc) prevented Fos expression by 90%, whereas atropine (2 mg/kg sc) had no effect. The neuronal marker protein gene product 9.5 and the glial markers S-100 and glial fibrillary acidic proteins showed that RX-77368 induced Fos in both myenteric neurons and glia. Vesicular ACh transporter and calretinin were detected around the activated myenteric neurons. These results indicated that central vagal efferent stimulation by intracisternal RX-77368 activates gastric myenteric neurons as well as glial cells mainly through nicotinic ACh receptors in conscious rats.     

Corticotropin-releasing factor acts centrally to suppress stimulated gastric contractility in the rat

The effects of intracisternal (i.c.) and intravenous (i.v.) administration of corticotropin-releasing factor (CRF) on gastric contractility stimulated by i.c. injection of the TRH analog RX77368 [p-Glu-His-(3,3'-dimethyl)-Pro-NH2], 2-deoxy-D-glucose (2DG) and i.v. infusion of carbachol were evaluated in rats under urethane anesthesia. Gastric contractility was monitored using acutely implanted extraluminal force transducers sutured to the corpus of the stomach. I.c. injection of CRF (6.3-210 pmol) resulted in a dose dependent suppression of gastric contractility stimulated by RX77368 (260 pmol) and 2DG (6 mg). Gastric inhibitory response to i.c. CRF was rapid in onset and lasted at least 45 min. Carbachol (200 mg/kg/h)-induced stimulation of gastric contractility was not modified by i.c. injection of CRF. The stimulation of contractility caused by both i.v. carbachol and i.c. 2DG were completely inhibited by atropine (1 mg/kg, i.v.). CRF (210 pmol) given i.v. suppressed RX77368-stimulated gastric contractions, but was less than 1/10 as potent as administered i.c. I.v. CRF (210 pmol) did not alter 2DG- or carbachol-induced gastric contractions. These results demonstrate that the i.c. administration of CRF acts within the brain to inhibit gastric contractility elicited by vagus-dependent mechanisms.     

Central action of somatostatin analog, SMS 201-995, to stimulate gastric acid secretion in rats

The effects of intracisternal and intravenous injections of the somatostatin analog, SMS 201-995, on gastric acid secretion were investigated in rats with pylorus ligation or gastric cannula. Intracisternal injection of SMS 201-995 induced a dose-related (0.1-0.3 microgram) and long-lasting stimulation of gastric acid output with a peak response at 3 h postinjection in conscious, pylorus-ligated rats. Intracisternal SMS 201-995 increased histamine levels in the portal blood, whereas plasma gastrin levels were not modified. Atropine, cimetidine and adrenalectomy abolished the stimulatory effect of intracisternal SMS 201-995 (0.3 microgram). SMS 201-995 (0.03 microgram), microinjected unilaterally into the dorsal vagal complex, increased gastric acid output in urethane anesthetized rats. SMS 201-995, injected intravenously at 0.5 microgram, did not alter gastric secretion, whereas higher doses (5-20 micrograms) resulted in a dose-related inhibition of gastric acid secretion in conscious pylorus-ligated rats. These data indicate that SMS 201-995, a selective ligand for somatostatin-1 receptor subtype, induces a centrally mediated stimulatory effect on gastric acid secretion in rats. The central action involves the parasympathetic system, muscarinic and H2 receptors as well as adrenal-dependent pathways.     

Intracisternal injection of CRF antagonist blocks surgical stress-induced inhibition of gastric secretion in the rat

The effects of intracisternal injection of CRF antagonist, alpha-CRF 9-41, on the inhibition of gastric acid secretion elicited by intracisternal injection of corticotropin-releasing factor (CRF) and stress were investigated in conscious pylorus-ligated rats. Intracisternal injection of the alpha-helical CRF 9-41 (50 micrograms) did not influence basal gastric secretion, but injected concomitantly with intracisternal CRF (5 micrograms), completely blocked CRF (5 micrograms)-induced inhibition of gastric secretory volume, acid concentration and output. Intracisternal injection of alpha-helical CRF 9-41 (3, 10, 50 micrograms) produced a dose-related reversal (0, 52 and 100%) of brain surgery-induced inhibition of gastric acid output. By contrast intravenous injection of CRF antagonist (50 micrograms) did not inhibit gastric hyposecretory response to brain surgery. These data suggest that endogenous CRF in the brain may mediate stress-induced gastric hyposecretion in the rat.     

Brain-gut interactions between central vagal activation and abdominal surgery to influence gastric myenteric ganglia Fos expression in rats

We previously showed that medullary thyrotropin-releasing hormone (TRH) or the stable TRH agonist, RX-77368 administered intracisternally induces vagal-dependent activation of gastric myenteric neurons and prevents post surgery-induced delayed gastric emptying in rats. We investigated whether abdominal surgery alters intracisternal (ic) RX-77368 (50 ng)-induced gastric myenteric neuron activation. Under 10 min enflurane anesthesia, rats underwent an ic injection of saline or RX-77368 followed by a laparotomy and a 1-min cecal palpation, or no surgery and were euthanized 90 min later. Longitudinal muscle/myenteric plexus whole-mount preparations of gastric corpus and antrum were processed for immunohistochemical detection of Fos alone or double labeled with protein gene-product 9.5 (PGP 9.5) and vesicular acetylcholine transporter (VAChT). In the non surgery groups, ic RX-77368 induced a 17 fold increase in Fos-expression in both gastric antrum and corpus myenteric neurons compared to saline injected rats. PGP 9.5 ascertained the neuronal identity of myenteric cells expressing Fos. In the abdominal surgery groups, ic RX-77368 induced a significant increase in Fos-expression in both the corpus and antrum myenteric ganglia compared with ic saline injected rats which has no Fos in the gastric myenteric ganglia. However, the response was reduced by 73-78% compared with that induced by ic RX 77368 without surgery. Abundant VAChT positive nerve fibers were present around Fos positive neurons. These results indicate a bidirectional interaction between central vagal stimulation of gastric myenteric neurons and abdominal surgery. The modulation of gastric vagus-myenteric neuron activity could play an important role in the recovery phase of postoperative gastric ileus.     

Evaluation of early gastric mucosal permeability induced by central thyrotropin-releasing hormone administration

Accumulating evidence suggests that central thyrotropin-releasing hormone (TRH) administration induces gastric erosion 4 h after administration through the vagal nerves. However, early changes in the gastric mucosa during these 4 h have not been described. To assess early changes in the gastric mucosa after intracisternal injection of a stable TRH analog, pGlu-His-(3,3'-dimethyl)-ProNH2 (RX-77368), we measured the blood-to-lumen 51Cr-labeled EDTA clearance and examined the effects of vagotomy, atropine, omeprazole, and hydrochloric acid (HCl) on RX-77368-induced mucosal permeability. A cytoprotective dose of RX-77368 (1.5 ng) did not increase mucosal permeability. However, higher doses significantly increased mucosal permeability. Permeability peaked within 20 min and gradually returned to control levels in response to a 15-ng dose (submaximal dose). Increased mucosal permeability was not recovered after a 150-ng dose (ulcerogenic dose). This increase in permeability was inhibited by vagotomy or atropine. Intragastric perfusion with HCl did not change the RX-77368 (15 ng)-induced increase in permeability, but completely inhibited the recovery of permeability after the peak. Pretreatment with omeprazole did not change the RX-77368 (15 ng)-induced increase in permeability, but quickened the recovery of permeability after the peak. These data indicate that the RX-77368-induced increase in permeability is mediated via the vagal-cholinergic pathway and is not a secondary change in RX-77368-induced acid secretion. Inhibited recovery of permeability on exposure to an ulcerogenic RX-77368 dose or on exposure to HCl plus a submaximal dose of RX-77368 may be crucial for the induction of gastric mucosal lesions by central RX-77368 administration.     

Central vagal stimulation activates enteric cholinergic neurons in the stomach and VIP neurons in the duodenum in conscious rats

The influence of central vagal stimulation induced by 2h cold exposure or intracisternal injection of thyrotropin-releasing hormone (TRH) analog, RX-77368, on gastro-duodenal enteric cholinergic neuronal activity was assessed in conscious rats with Fos and peripheral choline acetyltransferase (pChAT) immunoreactivity (IR). pChAT-IR was detected in 68%, 70% and 73% of corpus, antrum and duodenum submucosal neurons, respectively, and in 65% of gastric and 46% of duodenal myenteric neurons. Cold and RX-77368 induced Fos-IR in over 90% of gastric submucosal and myenteric neurons, while in duodenum only 25-27% of submucosal and 50-51% myenteric duodenal neurons were Fos positive. In the stomach, cold induced Fos-IR in 93% of submucosal and 97% of myenteric pChAT-IR neurons, while in the duodenum only 7% submucosal and 5% myenteric pChAT-IR neurons were Fos positive. In the duodenum, cold induced Fos in 91% of submucosal and 99% of myenteric VIP-IR neurons. RX-77368 induces similar percentages of Fos/pChAT-IR and Fos/VIP-IR neurons. These results indicate that increased central vagal outflow activates cholinergic neurons in the stomach while in the duodenum, VIP neurons are preferentially stimulated.     

Effect of central thyrotropin-releasing hormone on pancreatic blood flow in rats

Central neuropeptides play a role in physiological regulation through the autonomic nervous system. Thyrotropin-releasing hormone (TRH) is a neuropeptide distributed throughout the central nervous system and acts as a neurotransmitter to regulate gastric and hepatic functions through vagal-cholinergic pathways. In this study, the central effect of TRH on pancreatic blood flow was investigated in urethane-anesthetized rats. Pancreatic blood flow was determined by laser Doppler flowmetery. After measurement of basal blood flow, a stable TRH analog, RX 77368 (1-50 ng) or saline was injected intracisternally. Pancreatic blood flow was observed for 120 min thereafter. In some experiments, pretreatment with atropine methyl nitrate (0.15 mg/kg, i.p.), NG-nitro-L-arginine-methyl ester (10 mg/kg, i.v.), or 6-hydroxydopamine (6-OHDA;180 mg/kg, i.p.), or subdiaphragmatic vagotomy was performed. Intracisternal injection of TRH analog dose-dependently increased pancreatic blood flow with a peak response occurring 30 min after injection. The stimulatory effect of TRH analog on pancreatic blood flow was blocked by vagotomy, atropine, and NG-nitro-L-arginine-methyl ester, but not by 6-hydroxydopamine. Intravenous administration of the TRH analog did not influence pancreatic blood flow in the same animal model. These results indicate that TRH acts in the central nervous system to stimulate pancreatic blood flow through vagal-cholinergic and nitric oxide-dependent pathways.