Thyrotropin-releasing hormone-immunoreactive nerve terminals synapse on the dendrites of gastric vagal motoneurons in the rat

Thyrotropin-Releasing Hormone and Food Intake in Mammals: An Update

Thyrotropin-releasing hormone (TRH; pGlu-His-Pro-NH2) is an intercellular signal produced mainly by neurons. Among the multiple pharmacological effects of TRH, that on food intake is not well understood. We review studies demonstrating that peripheral injection of TRH generally produces a transient anorexic effect, discuss the pathways that might initiate this effect, and explain its short half-life. In addition, central administration of TRH can produce anorexic or orexigenic effects, depending on the site of injection, that are likely due to interaction with TRH receptor 1. Anorexic effects are most notable when TRH is injected into the hypothalamus and the nucleus accumbens, while the orexigenic effect has only been detected by injection into the brain stem. Functional evidence points to TRH neurons that are prime candidate vectors for TRH action on food intake. These include the caudal raphe nuclei projecting to the dorsal motor nucleus of the vagus, and possibly TRH neurons from the tuberal lateral hypothalamus projecting to the tuberomammillary nuclei. For other TRH neurons, the anatomical or physiological context and impact of TRH in each synaptic domain are still poorly understood. The manipulation of TRH expression in well-defined neuron types will facilitate the discovery of its role in food intake control in each anatomical scene.

Intracerebroventricular administration of TRH Agonist, RX-77368 alleviates visceral pain induced by colorectal distension in rats

Thyrotropin-releasing hormone (TRH) acts centrally to exert pleiotropic actions independently from its endocrine function, including antinociceptive effects against somatic pain in rodents. Whether exogenous or endogenous activation of TRH signaling in the brain modulates visceral pain is unknown. Adult male Sprague-Dawley rats received an intracerebroventricular (ICV) injection of the stable TRH analog, RX-77368 (10, 30 and 100 ng/rat) or saline (5 µl) or were semi-restrained and exposed to cold (4°C) for 45 min. The visceromotor response (VMR) to graded phasic colorectal distensions (CRD) was monitored using non-invasive intracolonic pressure manometry. Naloxone (1 mg/kg) was injected subcutaneously 10 min before ICV RX-77368 or saline. Fecal pellet output was monitored for 1 h after ICV injection. RX-77368 ICV (10, 30 and 100 ng/rat) reduced significantly the VMR by 56.7%, 67.1% and 81.1% at 40 mmHg and by 30.3%, 58.9% and 87.4% at 60 mmHg respectively vs ICV saline. Naloxone reduced RX-77368 (30 and 100 ng, ICV) analgesic response by 51% and 28% at 40 mmHg and by 30% and 33% at 60 mmHg respectively, but had no effect per se. The visceral analgesia was mimicked by the acute exposure to cold. At the doses of 30 and 100 ng, ICV RX-77368 induced defecation within 30 min. These data established the antinociceptive action of RX-77368 injected ICV in a model of visceral pain induced by colonic distension through recruitment of both opioid and non-opioid dependent mechanisms.

Brainstem neuropeptides and vagal protection of the gastric mucosal against injury: role of prostaglandins, nitric oxide and calcitonin-gene related peptide in capsaicin afferents

Earlier experimental studies indicated that the integrity of vagal pathway was required to confer gastric protection against damaging agents. Several peptides located in the brainstem initially identified to influence vagal outflow to the stomach, as assessed by electrophysiological approach or by vagal dependent alterations of gastric secretory and motor function, were investigated for their influence in the vagal regulation of the resistance of the gastric mucosa to injury. Thyrotropin releasing hormone (TRH), or its stable TRH analog, RX-77368, injected at low doses into the cisterna magna or the dorsal motor nucleus (DMN) was the first peptide reported to protect the gastric mucosa against ethanol injury through stimulation of vagal cholinergic pathways, inducing the release of gastric prostaglandins/nitric oxide (NO) and the recruitment of efferent function of capsaicin sensitive afferent fibers containing calcitonin-gene related peptide (CGRP). Activation of endogenous TRH-TRH1 receptor signaling located in the brainstem plays a role in adaptive gastric protection against damaging agents. Since then, an expanding number of peptides, namely peptide YY, CGRP, adrenomedullin, amylin, glugacon-like peptide, opioid peptides acting on µ, δ1 or δ2 receptors, nocicpetin, nocistatin, ghrelin, leptin and TLQP-21, a peptide derived from VGF prohormone, have been reported to act in the brainstem to afford gastric protection against ethanol injury largely through similar peripheral effectors mechanisms than TRH. Therefore gastric prostaglandins and CGRP/NO pathways represent a common final mechanism through which brain peptides confer vagally mediated gastroprotection against injury. A better understanding of brain circuitries through which these peptides are released will provide new strategies to recruit integrated and multifaceted gastroprotective mechanisms.

The dorsal motor nucleus of the vagus and regulation of pancreatic secretory function

Recent investigation of the factors and pathways that are involved in regulation of pancreatic secretory function (PSF) has led to development of a pancreatic vagovagal reflex model. This model consists of three elements, including pancreatic vagal afferents, the dorsal motor nucleus of the vagus (DMV) and pancreatic vagal efferents. The DMV has been recognized as a major component of this model and so this review focuses on the role of this nucleus in regulation of PSF. Classically, the control of the PSF has been viewed as being dependent on gastrointestinal hormones and vagovagal reflex pathways. However, recent studies have suggested that these two mechanisms act synergistically to mediate pancreatic secretion. The DMV is the major source of vagal motor output to the pancreas, and this output is modulated by various neurotransmitters and synaptic inputs from other central autonomic regulatory circuits, including the nucleus of the solitary tract. Endogenously occurring excitatory (glutamate) and inhibitory amino acids (GABA) have a marked influence on DMV vagal output to the pancreas. In addition, a variety of neurotransmitters and receptors for gastrointestinal peptides and hormones have been localized in the DMV, emphasizing the direct and indirect involvement of this nucleus in control of PSF.

Role of brainstem TRH/TRH-R1 receptors in the vagal gastric cholinergic response to various stimuli including sham-feeding

Pavlov's pioneering work established that sham-feeding induced by sight or smell of food or feeding in dogs with permanent esophagostomy stimulates gastric acid secretion through vagal pathways. Brain circuitries and transmitters involved in the central vagal regulation of gastric function have recently been unraveled. Neurons in the dorsal vagal complex including the dorsal motor nucleus of the vagus (DMN) express thyrotropin-releasing hormone (TRH) receptor and are innervated by TRH fibers originating from TRH synthesizing neurons in the raphe pallidus, raphe obscurus and the parapyramidal regions. TRH injected into the DMN or cisterna magna increases the firing of DMN neurons and gastric vagal efferent discharge, activates cholinergic neurons in gastric submucosal and myenteric plexuses, and induces a vagal-dependent, atropine-sensitive stimulation of gastric secretory (acid, pepsin) and motor functions. TRH antibody or TRH-R1 receptor oligodeoxynucleotide antisense pretreatment in the cisterna magna or DMN abolished vagal-dependent gastric secretory and motor responses to sham-feeding, 2-deoxy-D-glucose, cold exposure and chemical activation of cell bodies in medullary raphe nuclei. TRH excitatory action in the DMN is potentiated by co-released prepro-TRH-(160-169) flanking peptide, Ps4 and 5-HT, and inhibited by a number of peptides involved in the stress/immune response and inhibition of food-intake. These neuroanatomical, electrophysiological and neuropharmacological data are consistent with a physiological role of brainstem TRH in the central vagal stimulation of gastric myenteric cholinergic neurons in response to several vagal dependent stimuli including sham-feeding.

Hypothyroidism increases Fos immunoreactivity in cholinergic neurons of brain medullary dorsal vagal complex in rats

Hypo- or hyperthyroidism is associated with autonomic disorders. We studied Fos expression in the medullary dorsal motor nucleus of the vagus (DMV), nucleus tractus solitarii (NTS), and area postrema (AP) in four groups of rats with different thyroid states induced by a combination of drinking water and daily intraperitoneal injection for 1-4 wk: 1) tap water and vehicle; 2) 0.1% propylthiouracil (PTU) and vehicle; 3) PTU and thyroxine (T4; 2 microg/100 g); and 4) tap water and T4 (10 microg/100 g). The numbers of Fos immunoreactive (IR) positive neurons in the DMV, NTS, and AP were low in euthyroid rats but significantly higher in the 4-wk duration in hypothyroid rats, which were prevented by simultaneous T4 replacement. Hyperthyroidism had no effect on Fos expression in these areas. There were significant negative correlations between T4 levels and the numbers of Fos-IR-positive neurons in the DMV (r = -0.6388, P < 0.008), NTS (r = -0.6741, P < 0.003), and AP (r = -0.5622, P < 0.004). Double staining showed that Fos immunoreactivity in the DMV of hypothyroid rats was mostly localized in choline acetyltransferase-containing neurons. Thyroid hormone receptors alpha1 and beta2 were localized in the observed nuclei. These results indicate that thyroid hormone influences the DMV/NTS/AP neuronal activity, which may contribute to the vagal-related visceral disorders observed in hypothyroidism.

Thyrotropin-releasing hormone in the dorsal vagal complex stimulates pancreatic blood flow in rats

Central administration of thyrotropin-releasing hormone (TRH) enhanced pancreatic blood flow in animal models. TRH nerve fibers and receptors are localized in the dorsal vagal complex (DVC), and retrograde tracing techniques have shown that pancreatic vagal nerves arise from the DVC. However, nothing is known about the central sites of action for TRH to elicit the stimulation of pancreatic blood flow. Effect of microinjection of a TRH analog into the DVC on pancreatic blood flow was investigated in urethane-anesthetized rats. After measuring basal flow, a stable TRH analog (RX-77368) was microinjected into the DVC and pancreatic blood flow response was observed for 120 min by laser Doppler flowmetry. Vagotomy of the several portions, or pretreatment with atoropine methyl nitrate or N(G)-nitro-l-arginine-methyl ester was performed. Microinjection of RX-77368 (0.1-10 ng) into the left or right DVC dose-dependently increased pancreatic blood flow. The stimulation of pancreatic blood flow by RX-77368 microinjection was eliminated by the same side of cervical vagotomy as the microinjection site or subdiaphragmatic vagotomy, but not by the other side of cervical vagotomy. The TRH-induced stimulation of pancreatic blood flow was abolished by atropine or N(G)-nitro-l-arginine-methyl ester. These results suggest that TRH acts in the DVC to stimulate pancreatic blood flow through vagal-cholinergic and nitric oxide dependent pathways, indicating that neuropeptides may act in the specific brain nuclei to regulate pancreatic function.

Characterization of pancreas-projecting rat dorsal motor nucleus of vagus neurons

The electrophysiological and morphological properties of rat dorsal motor nucleus of the vagus (DMV) neurons innervating the pancreas were examined by using whole cell patch clamp recordings from brain stem slices and postfixation morphological reconstructions of Neurobiotin-filled neurons. Recordings were made from 178 DMV neurons whose projections had been identified by previous apposition of the fluorescent neuronal tracer DiI to the body of the pancreas. DMV neurons projecting to the pancreas had an input resistance of 434 +/- 14 M omega, an action potential duration of 3 +/- 0.1 ms, and an afterhyperpolarization of 18 +/- 0.4 mV amplitude and 108 +/- 7 ms time constant of decay; these electrophysiological properties resembled those of gastric-projecting neurons but were significantly different from those of intestinal-projecting neurons. Interestingly, 14 of 178 pancreas-projecting neurons showed the presence of a slowly developing afterhyperpolarization whose presence was not reported in DMV neurons projecting to any other gastrointestinal area. The morphological characteristics of pancreas-projecting neurons (soma area 274 +/- 12 microm2; soma diameter of 25 +/- 0.7 microm; soma form factor 0.74 +/- 0.01; segments 9.7 +/- 0.41), however, were similar to those of intestinal- but differed from those of gastric-projecting neurons. In summary, these results suggest that pancreas-projecting rat DMV neurons are heterogeneous with respect to some electrophysiological and morphological properties. These differences might underlie functional differences in the vagal modulation of pancreatic functions.

Protective effect of central thyrotropin-releasing hormone analog on cerulein-induced acute pancreatitis in rats

Central neuropeptides play a role in many physiological functions through the autonomic nervous system. We have recently demonstrated that central injection of a thyrotropin-releasing hormone (TRH) analog increases pancreatic blood flow through vagal and nitric oxide-dependent pathways. In this study, the central effect of a TRH analog on experimental acute pancreatitis was investigated in rats. Acute pancreatitis was induced by two intraperitoneal injections of cerulein (40 microg/kg) at 1-h interval. Either stable TRH analog, RX 77368 (5-100 ng), or saline was injected intracisternally 15 min before the first cerulein injection under ether anesthesia. Serum amylase level was measured before and 5 h after the first cerulein injection. Pancreatic wet/dry weight ratio and histological changes were also evaluated. Intracisternal TRH analog inhibited cerulean-induced elevation of serum amylase level, increase in pancreatic wet/dry weight ratio and pancreatic histological changes, such as interstitial edema, inflammation and vacuolization. The pancreatic cytoprotection induced by central TRH analog was abolished by subdiaphragmatic vagotomy and N(G)-nitro-L-arginine-methyl ester (L-NAME), but not by 6-hydroxydopamine (6-OHDA). Intravenous administration of the TRH analog did not influence cerulein-induced acute pancreatitis. These results indicate that the TRH analog acts in the central nervous system to protect against acute pancreatitis through vagal and nitric oxide-dependent pathways.

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.

Thyrotropin-releasing hormone in the dorsal vagal complex stimulates hepatic blood flow in rats

Central administration of thyrotropin-releasing hormone (TRH) enhances hepatic blood flow in animal models. TRH nerve fibers and receptors are localized in the dorsal vagal complex (DVC), and retrograde tracing techniques have shown that hepatic vagal nerves arise mainly from the left DVC. However, nothing is known about the central sites of action for TRH to elicit the stimulation of hepatic blood flow. The effect of microinjection of a TRH analogue into the DVC on hepatic blood flow was investigated in urethane-anesthetized rats. After measuring basal flow, a stable TRH analogue (RX-77368) was microinjected into the DVC and hepatic blood flow response was observed for 120 minutes by laser Doppler flowmetry. Either left or right cervical vagotomy or hepatic branch vagotomy was performed 2 hours before the peptide. Microinjection of RX-77368 (0.5-5 ng) into the left DVC dose-dependently increased hepatic blood flow. The stimulation of hepatic blood flow by RX-77368 microinjection into the left DVC was eliminated by left cervical and hepatic branch vagotomy but not by right cervical vagotomy. By contrast, microinjection of RX-77368 into the right DVC did not significantly alter hepatic blood flow. These results suggest that TRH acts in the left DVC to stimulate hepatic blood flow through the left cervical and hepatic vagus, indicating that neuropeptides may act in the specific brain nuclei to regulate hepatic function.

Protective effect of central thyrotropin-releasing hormone on carbon tetrachloride-induced acute hepatocellular necrosis in rats

BACKGROUND/AIMS

Thyrotropin-releasing hormone (TRH) acts in the brain to stimulate hepatic proliferation and blood flow through vagal-muscarinic and prostaglandin-mediated pathways. Hepatic blood flow and prostaglandins are well recognized as cytoprotective factors for liver damage, and central TRH is known to play a role in gastric cytoprotection. The effect of central TRH on carbon tetrachloride (CCl(4))-induced acute hepatocellular necrosis was investigated in rats.

METHODS

Male fasted rats were injected with either TRH analog, RX 77368 (1-10 ng), or vehicle intracisternally, and CCl(4) (2.0 ml/kg) was injected subcutaneously 60 min later. Acute hepatocellular necrosis was assessed by serum hepatic enzymes and histological changes 24 h after CCl(4).

RESULTS

Intracisternal TRH dose-dependently inhibited elevation of serum alanine aminotransferase level induced by CCl(4). Intracisternal TRH reduced CCl(4)-induced hepatic histological changes. The cytoprotective effect of central TRH on CCl(4)-induced acute hepatocellular necrosis was abolished by hepatic branch vagotomy, atropine, indomethacin and N(G)-nitro-L-arginine methyl ester, but not by 6-hydroxydopamine. Intravenous TRH did not influence CCl(4)-induced acute hepatocellular necrosis.

CONCLUSIONS

These results suggest that the cytoprotective effect of central TRH on acute hepatocellular necrosis is mediated through vagal-muscarinic, and prostaglandin- and nitric oxide-dependent pathways.

Cephalic phase of acid secretion involves activation of medullary TRH receptor subtype 1 in rats

Mechanisms involved in the cephalic phase of gastric acid secretion were studied in awake fasted rats with chronic gastric fistula and exposed to the sight and smell of chow for 30 min. Acid secretion was monitored using constant intragastric perfusion and automatic titration. Sham feeding induced a peak acid response reaching 82 +/- 7 micromol/10 min within 20 min compared with the average 22 +/- 2 micromol/10 min in controls. The sham-feeding response was abolished by intracisternal pretreatment with the TRH(1)-receptor antisense oligodeoxynucleotides or subcutaneous injection of atropine, whereas TRH(1) mismatch oligodeoxynucleotides had no effect. Serum gastrin was not altered by the sham feeding and increased by refeeding. Gastrin antibody did not block the rise in acid during sham feeding, although the net acid response was reduced by 47% compared with the control group. Glycine-gastrin antibody, indomethacin and nitro-l-arginine methyl ester had no effect. Atropine and gastrin antibody decreased basal acid secretion by 98 and 75%, respectively, whereas all other pretreatments did not. These results indicate that the cholinergic-dependent acid response to sham feeding is mediated by brain medullary TRH(1) receptors in rats.

Methiothepin attenuates gastric secretion and motility effects of vagal stimulants at the dorsal vagal complex

Methiothepin, a nonselective 5-HT receptor antagonist was utilized to explore the 5-HT modulation of dorsal vagal complex-TRH (thyrotropin releasing hormone) analogue stimulated gastric functional parameters. Intracisternal methiothepin pretreatment (200, 0.1 nmol) produced significant inhibition (70%, 44%, respectively) of the TRH analogue [p-Glu-His-(3,3'-dimethyl)-Pro NH2; RX 77368 (12 pmol)]-induced gastric acid output compared to vehicle pretreatment. Intracisternal pretreatment with methysergide (nonspecific 5-HT receptor antagonist) or combined cyanopindolol (5-HT(1A and 1B) receptor antagonist)+ritanserin (receptor antagonist of the 5-HT(2) family) did not alter the dorsal vagal complex-RX 77368 response. Unilateral dorsal vagal complex pretreatment with methiothepin (50 nmol/50 nl) attenuated ipsilateral dorsal vagal complex-TRH analog (12 pmol) induced gastric secretory response by 57%. The gastric secretagogue response to stimulation of the raphe obscurus (mediated by TRH release into the dorsal vagal complex) was inhibited 50% by pretreatment with intracisternal dorsal medullary methiothepin (0.1 nmol/10 microl). Intracisternal methiothepin (200 nmol/20 microl) also attenuated (a) dorsal vagal complex-glutamate (60 nmol/30 nl) stimulated gastric acid secretion and (b) gastric motility stimulated by dorsal vagal complex-RX 77368 (12 pmol/30 nl). The data suggest that other properties of methiothepin, alone or in addition to its 5-HT receptor antagonist effect, mediate its inhibitory actions at the dorsal vagal complex.

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.

The peptide TRH uncovers the presence of presynaptic 5-HT1A receptors via activation of a second messenger pathway in the rat dorsal vagal complex

It is well recognized that brainstem microinjections of 5-hydroxytryptamine (serotonin, 5-HT) and thyrotropin-releasing hormone (TRH) act synergistically to stimulate gastric function in vivo. Previous in vitro experiments have shown that this synergism does not occur at the level of the dorsal motor nucleus of the vagus (DMV) motoneurone. In order to determine the mechanism of this action, whole cell patch clamp recordings were made from identified gastric-projecting rat DMV neurones to investigate the effects of 5-HT and TRH on GABAergic inhibitory postsynaptic currents (IPSCs) evoked by stimulation of the nucleus of the tractus solitarius (NTS). 5-HT (30 microM) decreased IPSC amplitude by 26 +/- 2.5% in approximately 43% of DMV neurones. In the remaining neurones in which 5-HT had no effect on IPSC amplitude, exposure to TRH (1 microM) uncovered the ability of subsequent applications of 5-HT to decrease IPSC amplitude by 28 +/- 3%. Such TRH-induced 5-HT responses were prevented by the 5-HT1A antagonist NAN-190 (1 microM) and mimicked by the 5-HT1A agonist 8-OH-DPAT (1 microM). Increasing cAMP levels using the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX; 10 microM), the non-hydrolysable cAMP analogue 8-bromo-cAMP (1 mM), or the adenylate cyclase activator forskolin (10 microM), like TRH, uncovered the ability of 5-HT to decrease evoked IPSC amplitude (17 +/- 2.2 %, 28.5 +/- 5.3 % and 30 +/- 4.8%, respectively), in neurones previously unresponsive to 5-HT. Conversely, the adenylate cyclase inhibitor, dideoxyadenosine (10 microM) and the protein kinase A inhibitor, Rp-cAMP (10 microM), blocked the ability of TRH to uncover the presynaptic inhibitory actions of 5-HT. These results suggest that activation of presynaptic TRH receptors initiates an intracellular signalling cascade that raises the levels of cAMP sufficient to uncover previously silent 5-HT1A receptors on presynaptic nerve terminals within the dorsal vagal complex.

Localization of thyroid hormone receptor beta2 in the ventral medullary neurons that synthesize thyrotropin-releasing hormone

Altered thyroid statuses are associated with autonomic disorders. Thyrotropin-releasing hormone (TRH) synthesized in medullary raphe pallidus (Rpa), raphe obscurus (Rob) and the parapyramidal regions (PPR) regulates vagal and sympathetic preganglionic motoneurons. Hypothyroidism increased TRH gene expression and c-Fos immunoreactivity (IR) in these nuclei. Whether these increases represent a direct action of thyroid hormone was studied by detecting the presence of thyroid hormone receptor beta2 (TRbeta2) in pro-TRH-synthesizing neurons in the Rpa, Rob and the PPR using immunohistochemistry with specific TRbeta2 antiserum and in situ hybridization with digoxigenin-labeled pro-TRH cRNA probe. TRbeta2 IR was widely distributed throughout the medulla and primarily localized within the cell nuclei. Particularly intense immunostaining was presented in the Rpa, Rob and the PPR neurons. The combination of immunohistochemistry with in situ hybridization revealed that all pro-TRH mRNA-positive neurons in these ventral medullary nuclei were also TRbeta2 IR positive. The numbers of TRbeta2 IR-positive neurons in each nucleus were identical in both euthyroid rats and hypothyroid rats induced by 6-n-propyl-2-thiouracil in drinking water for 4 weeks. The finding that TRbeta2 localized in pro-TRH-synthesizing neurons in the ventral medullary nuclei provides an anatomical substrate for a direct thyroid hormone action on these neurons in the regulation of TRH gene expression, which may contribute to the altered autonomic activity in different thyroid statuses.

Electrophysiological and morphological heterogeneity of rat dorsal vagal neurones which project to specific areas of the gastrointestinal tract

1. The electrophysiological properties of rat dorsal motor nucleus of the vagus (DMV) neurones (n = 162) were examined using whole cell patch clamp recordings from brainstem slices. Recordings were made from DMV neurones whose projections to the gastrointestinal tract had been identified by previously applying fluorescent retrograde tracers to the gastric fundus, corpus or antrum/pylorus, or to the duodenum or caecum. 2. The neuronal groups were markedly heterogeneous with respect to several electrophysiological properties. For example, neurones which projected to the fundus had a higher input resistance (400 +/- 25 Momega), a smaller and shorter after-hyperpolarization (16.7 +/- 0.49 mV and 63.5 +/- 3.9 ms) and a higher frequency of action potential firing (19.3 +/- 1.4 action potentials s-1) following injection of depolarizing current (270 pA) when compared with caecum-projecting neurones (302 +/- 22 Momega; 23. 5 +/- 0.87 mV and 81.1 +/- 5.3 ms; 9.7 +/- 1.1 action potentials s-1; P < 0.05 for each parameter). Differences between neuronal groups were also apparent with respect to the distribution of several voltage-dependent potassium currents. Inward rectification was present only in caecum-projecting neurones, for example. 3. Neurones (n = 82) were filled with the intracellular stain Neurobiotin allowing post-fixation morphological reconstruction. Neurones projecting to the caecum had the largest cell volume (5238 +/- 535 microm3), soma area (489 +/- 46 microm2) and soma diameter (24.6 +/- 1.24 microm) as well as the largest number of dendritic branch segments (23 +/- 2). 4. In summary, these results suggest that DMV neurones are heterogeneous with respect to some electrophysiological as well as some morphological properties and can be divided into subgroups according to their gastrointestinal projections.

Synthesis of nitric oxide in the dorsal motor nucleus of the vagus mediates the inhibition of gastric acid secretion by central bombesin

1. Central administration of bombesin inhibits gastric acid production independently of the centrally or peripherally-acting stimuli employed. This study evaluates the role and location of the cerebral nitric oxide (NO) implicated in the inhibitory effect of central bombesin on in vivo rat gastric acid secretion, as induced by distension with 15 cm H2O, insulin (0.75 u.i. kg-1 i.p.) TRH (1.2 microg kg-1, i.c.) or pentagastrin (100 microg kg-1, i.p.). 2. The acid-inhibitory effect of i.c. bombesin (40 ng kg-1) was prevented by prior administration of L-NAME (80 microg kg-1) in the dorsal motor nucleus of the vagus (DMN). This dose of L-NAME when administered into the nucleus of the tractus solitarious (NTS) did not influence the effects of bombesin. Administration of L-arginine (400 microg kg-1) into the DMN restored the acid-inhibitory effect of i.c. bombesin in animals treated with L-NAME. 3. Microinjection of bombesin (12 ng kg-1) into the paraventricular nucleus of the hypothalamus (PvN) inhibits acid secretion stimulated by pentagastrin. This inhibitory effect was prevented by a previous injection of L-NAME (80 microg kg-1) into the DMN. 4. The release of NO in the DMN following i.c. administration of bombesin was confirmed by in vivo electrochemical detection. 5. Administration by microdialysis in the DMN of the NO-donor SNAP (25 mM in 1.5 microl min-1) into the DMN inhibits pentagastrin-stimulated gastric acid secretion. 6. The present study suggests that nNOS-containing neurons in the DMN have an inhibitory role in the control of gastric acid responses.

Central autonomic activation by intracisternal TRH analogue excites gastric splanchnic afferent neurons

Intracisternal (ic) injection of thyrotropin-releasing hormone (TRH) or its stable analogue RX 77368 influences gastric function via stimulation of vagal muscarinic pathways. In rats, the increase in gastric mucosal blood flow evoked by a low ic dose of RX 77368 occurs via release of calcitonin gene-related peptide from capsaicin-sensitive afferent neurons, most probably of spinal origin. In this study, the effect of low ic doses of RX 77368 on afferent impulse activity in splanchnic single fibers was investigated. The cisterna magna of overnight-fasted, urethan-anesthetized Sprague-Dawley rats was acutely cannulated, and fine splanchnic nerve twigs containing at least one fiber responsive to mechanical probing of the stomach were isolated at a site immediately distal to the left suprarenal ganglion. Unit mechanoreceptive fields were encountered in all portions of the stomach, both superficially and in deeper layers. Splanchnic afferent unit impulse activity was recorded continuously during basal conditions and in response to consecutive ic injections of saline and RX 77368 (15-30 min later; 1.5 or 3 ng). Basal discharge rates ranged from 0 to 154 impulses/min (median = 10.2 impulses/min). A majority of splanchnic single units with ongoing activity increased their mean discharge rate by >/=20% after ic injection of RX 77368 at either 1.5 ng (6/10 units; median increase 63%) or 3 ng (19/24 units; median increase 175%). Five units lacking impulse activity in the 5-min before ic RX 77368 (3 ng) were also excited, with the onset of discharge occurring within 1.0-5.0 min postinjection. In units excited by ic RX 77368, peak discharge occurred 15.6 +/- 1.3 min after injection and was followed by a decline to stable activity levels </=20-40 min thereafter. In a few cases (4/24), ic RX 77368 (3 ng) inhibited the impulse activity of initially active units, with a time course comparable to that seen in units excited by the same treatment. The pattern of discharge in most units was not suggestive of mechanical modulation of activity by rhythmic gastric contractions. The data demonstrate that low ic doses of TRH analogue induce sustained increases in afferent discharge in a substantial proportion of splanchnic neurons innervating the rat stomach. These findings support the notion that splanchnic afferent excitation occurs concomitantly with vasodilatory peptide release from gastric splanchnic afferent nerve terminals after ic TRH-induced autonomic activation.

Central and peripheral vagal mechanisms involved in gastric protection against ethanol injury

Activation of medullary thyrotropin-releasing hormone (TRH), at a dose subthreshold to increase gastric acid secretion, protects the gastric mucosa against ethanol injury through vagal cholinergic pathways in urethane-anaesthetized rats. Peripheral mediators involve the efferent function of capsaicin-sensitive splanchnic afferents leading to calcitonin gene-related peptide (CGRP)- and nitric oxide (NO)-dependent gastric vasodilatory mechanisms. In addition, gastric prostaglandins participate in gastric protection through mechanisms independent of the stimulation of gastric mucosal blood flow and mucus secretion. Medullary TRH has physiological relevance in the vagal-dependent adaptive gastric protection induced by mild (acid or ethanol), followed by strong, irritants. Additional neuropeptides, namely peptide YY (PYY), somatostatin analogues, CGRP and adrenomedullin, also act in the brainstem to induce a vagal-dependent gastric protection against ethanol through interactions with their specific receptors in the medulla. Central PYY and adrenomedullin act through vagal cholinergic prostaglandins and NO pathways, while somatostatin analogue acts through vagal non-adrenergic, non-cholinergic vasoactive intestinal peptide and NO mechanisms. Although their biological relevance is still to be established, these peptides provide additional tools to investigate the multiple vagal-dependent mechanisms which increase the resistance of the gastric mucosa to injury.

TRH microinjection into DVC enhances motility of rabbits gallbladder via vagus nerve

AIM: To investigate the effects of TRH in DVC on motility of the gallbladder in rabbits.

METHODS: fter fasted for 15h-18h, rabbits were anesthetized with urethane (1.0 g/kg). Gall-bladder pressure (GP) was measured by a frog bladder perfused with normal saline.

RESULTS: After microinjection of TRH (8.8 nmol, 1 μL) into DVC, GP was raised and the frequency of phasic contraction of gallbladder (FPCGB) increased. All the doses of TRH (0.13, 0.25, 0.50, 0.80, 1.30 nmol, 1 μL) injected into DVC could excite the motility of gallblader. As the dose of TRH was enlarged, the amplitude and duration of the reaction increased. Effects of TRH in DVC on motility of the gallbladder could be completely abolished by atropine (0.2 mg/g, i.v.) or vagotomy, but could not be inhibited by phentolamine iv (1.5 mg/g) or propranolol iv (1.5 mg/g)or by transecting the spinal cord.

CONCLUSION: Thyrotropin-releasing hormone in DVC can excite motility of gallbladder. This effect was mediated by vagus nerves and peripheral M receptor. Its physiological significance may be related to maintaining the phasic contraction of gallbladder in interdigestive period.

Effect of intracisternal thyrotropin-releasing hormone on hepatic blood flow in rats

Central neuropeptides play a role in many physiological regulatory processes through the autonomic nervous system. Thyrotropin-releasing hormone (TRH) is distributed in the central nervous system and acts as a neurotransmitter to regulate gastric functions through vagal-muscarinic pathways. The central effect of the TRH analog on hepatic blood flow was investigated in urethan-anesthetized rats. Hepatic blood flow was determined by the hydrogen gas clearance technique. Intracisternal injection of the stable TRH analog RX-77368 (5-100 ng) dose dependently increased hepatic blood flow with peak response at 15 min after the peptide was administered (net change from basal for vehicle and 5, 10, 100, and 500 ng RX-77368 was 2.0 +/- 0.2, 8.9 +/- 0.8, 19.4 +/- 2.6, 32.6 +/- 3.3, and 28.5 +/- 6.8 ml.min-1.100 g-1, respectively), and this stimulatory effect returned to baseline at 90 min. The stimulation of hepatic blood flow by the intracisternally administered TRH analog was abolished by atropine methyl nitrate (0.15 mg/kg ip), indomethacin (5 mg/kg ip), NG-nitro-L-arginine methyl ester (10 mg/kg iv), and hepatic branch vagotomy but not by cervical spinal cord transection (C6 level). Intravenous injection of RX-77368 did not have any effect on hepatic blood flow. These results indicate that TRH acts in the central nervous system to stimulate hepatic blood flow through vagal-muscarinic and indomethacin- and nitric oxide-dependent pathways.

5-CT or DOI augments TRH analog-induced gastric acid secretion at the dorsal vagal complex

Serotonin (5-HT) interacts with thyrotropin-releasing hormone (TRH) at the dorsal vagal complex (DVC) to augment TRH-induced stimulation of gastric acid secretion. To investigate the 5-HT receptor family involved in the augmentation response, prototypical 5-HT receptor-selective agonists (146 pmol) were coinjected with the TRH analog RX-77368 (RX; 12 pmol) into the rat DVC in a 30-nl volume. The DVC coordinates were 0.2 mm anterior, 0.2 mm right, 0.6 mm ventral with respect to the calamus scriptorius. Coinjection of RX with the 5-HT agonists 5-carboxyamidotryptamine (5-CT) or (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI; 5-HT2 agonist) produced a 183 or 103% increase in gastric acid output compared with the RX injection alone. In contrast, coinjection of 2-methyl-5-HT (5-HT3 agonist) with RX produced no effect on RX-induced increase in gastric acid secretion. Moreover, coinjection of SC-53116 (5-HT4 agonist) decreased the gastric acid output by 45% compared with the RX response itself. Examination of the RX/5-HT agonist coinjection response in more rostral regions of the DVC using the same doses (5-CT/RX or DOI/RX) revealed that only 5-CT was effective in producing the augmented response to TRH analog. The results suggest that activation of 5-CT- or DOI-sensitive receptors augments, and of 5-HT4 receptors inhibits, the gastric acid response to TRH analog injected into the DVC. Thus the integrated response to several serotonin receptor subtypes may mediate changes to the TRH response induced by 5-HT at the DVC.

Ultrastructure of bombesin-like immunoreactive nerve terminals in the nucleus of the solitary tract and the dorsal motor nucleus

Bombesin (gastrin-releasing peptide 14-27) inhibits gastric function and feeding when microinjected into the nucleus of the solitary tract (NTS)/dorsal motor nucleus of the vagus (DMV) complex. We performed a preembedding immunoelectron microscopic study in rats to describe the bombesin containing nerve terminals and to characterize their postsynaptic structures. 228 bombesin-L1 nerve terminals which made synaptic contacts in the NTS/DMV complex were studied. Labeling was heaviest over dense core vesicles and lighter over small clear vesicles. The dense core vesicles were typically located along the plasmalemma away from the synaptic face, a finding that is typical of neuropeptide containing nerve terminals. The postsynaptic structures were most often medium sized dendrites (56%) and small sized dendrites (27%), with similar percentages in the NTS and DMV. In the DMV, synapses on cell bodies (8%) were more frequent than in the NTS (1%). In the NTS, synapses on dendritic spines (10%) were more frequent than in the DMV (4%). Only a single axo-axonal contact was identified. These findings add to the increasing body of evidence that bombesin is a neurotransmitter/neuromodulator in the NTS/DMV complex. Bombesin rarely makes presynaptic (axo-axonal) contacts that might inhibit the release of excitatory neurotransmitters, but rather makes postsynaptic contacts potentially effecting vagal motoneurons.

The protective effect of 5-HT3 receptor antagonist in thyrotropin releasing hormone-induced gastric lesions

The present study examined 1) oxidative stress and gastric lesions induced by thyrotropin releasing hormone (TRH) 2) The effect of a 5-hydroxytryptamine3 (5-HT3) receptor antagonist, ICS 205930 on protective effect of calcitonin on gastric lesions produced by TRH. Calcitonin (5 micrograms/10 microliter) was injected i.c.v. 10 min before TRH (10 micrograms/10 microliter, i.c.v.) injection or ICS 0.5 mg/kg, (i.p.) was given 60 min prior to calcitonin or TRH to rats. Ulcer index, lipid peroxidation (LP) and glutathione (GSH) levels were quantified 3 h after TRH injection in the stomach, liver and brain. TRH caused mucosal lesions (UI: 10.0 +/- 2.0 mm) without changing gastric GSH and LP. JCS did not alter the protective effect of calcitonin against TRH-induced lesions but attenuated. TRH-induced lesion formation. The oxidative effects of calcitonin or ICS were similar to TRH but both drugs attenuated gastric lesion formation. Hence, oxidative changes in tissues studied are not directly involved in TRH-induced lesions.

Distribution of bombesin-like immunoreactivity in the nucleus of the solitary tract and dorsal motor nucleus of the rat and human: colocalization with tyrosine hydroxylase

Bombesin is a peptide neurotransmitter/neuromodulator with important autonomic and behavioral effects that are mediated, at least in part, by bombesin-containing neurons and nerve terminals in the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus (DMV). The distribution of bombesin-like immunoreactive nerve terminals/fibers and cell bodies in relation to a viscerotopically relevant subnuclear map of this region was studied by using an immunoperoxidase technique. In the rat, bombesin fiber/terminal staining was heavy in an area that included the medial subnucleus of the NTS and the DMV over their full rostral-caudal extent. Distinctly void of staining were the gelatinous, central, and rostral commissural subnuclei and the periventricular area of the NTS, regions to which gastric, esophageal, cecal, and colonic primary afferents preferentially project. The caudal commissural and dorsal subnuclei had light bombesin fiber/terminal staining, as did the intermediate, interstitial, ventral, and ventrolateral subnuclei. With colchicine pretreatment, numerous cell bodies were stained in the medial and dorsal subnuclei, with fewer neurons in the caudal commissural, intermediate, interstitial, ventral, and ventrolateral subnuclei. Bombesin-like immunoreactive neurons were found in numerous other areas of the brain, including the ventrolateral medulla, the parabrachial nucleus, and the medial geniculate body. In the human NTS/DMV complex, the distribution of bombesin fiber/terminal staining was very similar to the rat. In addition, occasional bombesin-like immunoreactive neurons were labeled in a number of subnuclei, with clusters of neurons labeled in the dorsal and ventrolateral subnuclei. Double immunofluorescence studies in rat demonstrated that bombesin colocalizes with tyrosine hydroxylase in neurons in the dorsal subnucleus of the NTS. Bombesin does not colocalize with tyrosine hydroxylase in any other location in the brain. In conclusion, the distribution of bombesin in the NTS adheres to a viscerotopically relevant map. This is the anatomical substrate for the effects of bombesin on gastrointestinal function and satiety and its likely role in concluding a meal. The anatomic similarities between human and rat suggest that bombesin has similar functions in the visceral neuraxis of these two species. Bombesin coexists with catecholamines in neurons in the dorsal subnucleus, which likely mediate, in part, the cardiovascular effects of bombesin.

Thyrotropin-releasing hormone immunoreactive boutons form close appositions with medullary expiratory neurons in the rat

The aim of the present study was to assess the size of the input from TRH immunoreactive varicosities to medullary respiratory neurons in the Bötzinger complex and caudal ventral respiratory group. Neurobiotin was intracellularly injected into seven neurons in the Bötzinger complex, between 0.4 and 0.9 mm caudal to the facial nucleus. Five of the seven Bötzinger neurons had extensive local axonal projections, with bouton-like varicosities clustered predominantly between their somata and the nucleus ambiguus. Seven neurons in the caudal ventral respiratory group, located between 1.6 and 2.4 mm caudal to the facial nucleus, were also labelled. All but one caudal respiratory neurons had no, or very few, medullary collaterals. TRH immunoreactive fibres were seen in many medullary nuclei, including the ventral reticular formation. Bötzinger neurons were closely apposed by an average of 29 +/- 8 TRH immunoreactive boutons/neuron (mean +/- S.D., n = 7). In contrast, caudal ventral respiratory group neurons were apposed by only 5 +/- 3 TRH immunoreactive boutons/neuron (n = 7). Bötzinger neurons form many intramedullary and bulbospinal inhibitory connections with premotoneurons and motoneurons that are important in the timing, amplitude and shape, of respiratory activity. Our findings suggest a role for endogenous TRH-containing neurons in modulating the activity of inhibitory Bötzinger neurons and neurons in the caudal ventral respiratory group. The significance of the apparent difference in size of this input remains to be determined.

Cold-restraint- and TRH-induced ulcer models demonstrate different biochemical and morphological manifestations in gastric and hepatic tissues in rats. Role of calcitonin

In the present study, two ulcer models--central thyrotropin-releasing hormone (TRH) injection and cold-restraint stress (CRS) application--were compared. Animals were treated either with salmon calcitonin (sCT) or saline intracerebroventricularly (ICV) before CRS exposure or ICV TRH injection. In both models, besides ultrastructural properties, ulcer indexes and lipid peroxidation (LP) and glutathione (GSH) levels of liver and stomach were determined. While TRH treatment did not affect GSH and LP levels of the stomach and led to a slight decrease in hepatic GSH levels, CRS induced a marked reduction in gastric and hepatic GSH and an increase in LP levels of both tissues. sCT pretreatment prevented the reduction of gastric and hepatic GSH levels and morphological damage of both tissues in the CRS group. However, the same treatment did not prevent the TRH-induced reduction of hepatic GSH levels and, interestingly, it worsened the ultrastructural disturbances in the liver. Although sCT prevented macroscopic ulcer formation in both models, it did not totally reverse the microscopic effects of TRH.

TRH in the dorsal motor nucleus of vagus is involved in gastric erosion induced by excitation of raphe pallidus in rats

The influence of excitation of the raphe pallidus neurons on gastric mucosal lesions was examined in urethane-anesthetized rats pretreated with indomethacin (5 mg/kg, i.p., -60 min). Kainic acid (12 ng/30 nl), delivered into the raphe pallidus, increased gastric acid secretion (94.8 +/- 15.9 mumol/60 min) and produced gastric lesions in 2.7 +/- 0.3% of the corpus mucosa. No gastric erosions were observed when kainic acid was injected nearby, but outside, of the raphe pallidus. The gastric lesions induced by microinjection of kainic acid into the raphe pallidus neurons were completely prevented by atropine (0.3 mg/kg, -30 min, s.c.) and bilateral microinjection of TRH antibody # 8964 (1.3 microgram/site) into the dorsal motor nucleus of the vagus (DMN). Microinjection of the TRH antibody into the hypoglossal nucleus or control antibody into the DMN did not modify the mucosal lesions induced by kainic acid into the raphe pallidus. These data suggest that activation of raphe pallidus induced vagal cholinergic mediated gastric erosion through TRH release in the DMN.

Thyrotropin-releasing hormone-immunoreactive varicosities synapse on rat phrenic motoneurons

The relationship between retrogradely labelled or intracellularly filled phrenic motoneurons and varicosities containing thyrotropin-releasing hormone immunoreactivity was investigated in rats by light and electron microscopy. Phrenic motoneurons were identified via retrograde tracing from the diaphragm with cholera toxin B subunit, which was followed by immunocytochemistry to visualise retrogradely labelled motoneurons and thyrotropin-releasing hormone-immunoreactive nerve fibres in their vicinity. At the light microscopic level, varicose thyrotropin-releasing hormone-immunoreactive nerve fibres were distributed sparsely in the phrenic motor nucleus, with some axons surrounding retrogradely labelled motoneurons. In separate intracellular experiments, four phrenic motoneurons identified by antidromic activation from the C5 phrenic nerve root were subsequently filled with Neurobiotin, and nerve fibres that contained thyrotropin-releasing hormone immunoreactivity were identified by immunocytochemistry. The numbers and locations of thyrotropin-releasing hormone-immunoreactive varicosities that were closely appeared to the intracellularly labelled motoneurons were mapped using a camera lucida technique. Close appositions by thyrotropin-releasing hormone-immunoreactive varicosities were seen on somata as well as on proximal and distal dendrites. The closely apposed varicosities were usually present in tight clusters, which were formed by single varicose axons. However, the distribution was nonuniform, in that some dendrites did not receive any close appositions. Ultrastructural analysis of random ultrathin sections through retrogradely labelled neurons showed that varicosities with thyrotropin-releasing hormone immunoreactivity made 1.8% of all synapses and direct contacts on somata and 2.3% of synapses and contacts with dendrites of the retrogradely labelled phrenic motoneurons. The results of these experiments suggest that thyrotropin-releasing hormone-immunoreactive varicosities provide similar numbers of inputs to both the somata and dendrites of phrenic motoneurons. These thyrotropin-releasing hormone-containing inputs seen via light and electron microscopy could modulate the excitability of phrenic motoneurons.

Caudal raphe-dorsal vagal complex peptidergic projections: role in gastric vagal control

Subpopulations of raphe pallidus (Rpa) and raphe obscurus (Rob) neurons containing TRH, serotonin (5-HT), and substance P contribute projections to the dorsal vagal complex (DVC). Activation of Rpa and Rob neurons induces a vagal cholinergic-dependent stimulation of gastric secretory and motor function and modulates resistance of the gastric mucosa to gastric injury in rats and cats. The caudal raphe nuclei-DVC pathways containing TRH/5-HT are involved in mediating cold-induced vagal stimulation of gastric function and erosion formation. These results suggest that Rpa/Rob-DVC projections containing TRH/5-HT may be an important pathways in the medullary regulation of vagal activity to the viscera.

Neuropeptides in the human dorsal vagal complex: an immunohistochemical study

The distribution of twelve biologically active neuropeptides, i.e., thyrotropin-releasing hormone, corticotropin-releasing factor, pro-opiomelanocortin-derived peptides (adrenocorticotropic hormone, beta-endorphin, alpha-melanocyte-stimulating hormone), leucine-enkephalin, dynorphin A, dynorphin B, cholecystokinin, substance P, galanin and calcitonin gene-related peptide, was examined by immunohistochemistry in the human dorsal vagal complex including the nucleus of the solitary tract, the dorsal motor nucleus of the vagus and the area postrema. Immunoreactivity of all the twelve neuropeptides was found widely distributed in the various subdivisions of the nucleus of the solitary tract, showing a unique distribution for every peptide. Neuronal cell bodies immunostained with leucine-enkephalin, galanin and dynorphin B were found in this region. There were no immunopositive perikarya for any of the peptides in the other structures studied. Fibers containing galanin, corticotropin-releasing factor, substance P, dynorphin B, thyrotropin-releasing hormone and calcitonin gene-related peptide were observed at a relatively high density in the nucleus of the solitary tract. In the same structure, a moderately dense network of fibers immunostained with dynorphin A, cholecystokinin and leucine-enkephalin, but only solitary pro-opiomelanocortin-derived peptides-containing fiber fragments were observed. In the dorsal motor nucleus of the vagus the most prominent network of fibers was found to contain thyrotropin-releasing hormone, galanin and substance P. In contrast to these, no beta-endorphin immunoreactivity was detected. The area postrema contained only moderate to low densities of galanin-, substance P-, calcitonin gene-related peptide-, dynorphin B- and cholecystokinin-immunoreactive fibers.

Cold exposure elevates thyrotropin-releasing hormone gene expression in medullary raphe nuclei: relationship with vagally mediated gastric erosions

The stimulation of thyrotropin release by cold is associated with an increase in thyrotropin-releasing hormone gene expression in the paraventricular nucleus of the hypothalamus. Cold exposure also stimulates autonomic outflow to viscera. There is evidence that caudal raphe nuclei are involved in autonomic regulation through thyrotropin-releasing hormone projections to the dorsal vagal complex and spinal cord. To determine whether cold modulates thyrotropin-releasing hormone gene expression in the caudal raphe nuclei, the effect of cold exposure on thyrotropin-releasing hormone messenger RNA levels in the rat lower brainstem was examined by quantitative Northern blot analysis and thyrotropin-releasing hormone messenger RNA was localized by in situ hybridization. The gastric responses to cold exposure were also assessed in sham or vagotomized rats with pylorus ligation. Thyrotropin-releasing hormone messenger RNA signal was detected in the RNA extracted from the medulla and hypothalamus but not from the amygdala, periaqueductal gray or cerebellum. Cold exposure (4 degrees C) for 1 or 3 h increased thyrotropin-releasing hormone messenger RNA levels in the medulla by 77 +/- 37 and 142 +/- 39% respectively. In situ hybridization histochemistry showed that the increase in silver grain density occurred exclusively in the raphe pallidus and raphe obscurus. Exposure to cold stress for 2 h stimulated gastric acid secretion and resulted in gastric lesion formation in sham but not vagotomized rats. There are established thyrotropin-releasing hormone projections from the raphe pallidus and obscurus to the dorsal vagal complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of Fos immunoreactivity in the rat brain after cold-restraint induced gastric lesions and fecal excretion

Cold-restraint alters gastrointestinal function through vagal pathways. Immunohistochemical detection of the nuclear phosphoprotein Fos (Fos-IR) was used to map brain neuronal pathways activated by cold exposure for 3 h in fasted rats maintained individually in semi-cylindrical restraining cages. Gastric lesions and fecal pellet output were also monitored. In rats exposed to cold (4 degrees C) restraint for 3 h, numerous Fos-positive nuclei were observed in the dorsal motor nucleus of the vagus, raphe pallidus, locus coeruleus, and paraventricular nucleus of the hypothalamus, and, to a lesser extent, in the raphe obscurus, parapyramidal region, and medullary noradrenergic region, bed nucleus of the stria terminalis and septum. Fecal pellet output was increased by 8 fold and gastric lesions covered 19.5 +/- 1.1% of the corpus mucosa. Rats restrained at room temperature under otherwise same conditions had little or no Fos-positive cells in these brain nuclei, no gastric erosion and a low pellet output (1.3 +/- 0.5 nb/3 h). These data, in addition to previous functional studies, provide anatomic support for the involvement of neurons in the caudal raphe nuclei, dorsal motor nucleus of the vagus and paraventricular nucleus of the hypothalamus in the autonomic and endocrine responses to cold-restraint.

Raphe pallidus stimulation increases gastric contractility via TRH projections to the dorsal vagal complex in rats

The role of thyrotropin releasing hormone (TRH) in the dorsal vagal complex (DVC) in mediating the enhanced gastric contractility induced by glutamate (100 pmol) microinjected into the raphe pallidus (Rpa) was investigated in urethane-anesthetized rats acutely implanted with miniature strain gauge force transducers on the corpus of the stomach. Glutamate-induced stimulation of gastric contractility was dose-dependently inhibited by bilateral microinjection into the DVC of TRH antibody (0.17, 0.85 or 1.7 micrograms/100 nl/site) but not by vehicle. TRH antibody microinjected into the dorsal medullary reticular field had no effect. These data indicate that activation of Rpa neurons by glutamate increases gastric motor function through TRH release in the DVC.

Intracisternal thyrotropin-releasing hormone-induced vagally mediated gastric protection against ethanol lesions: central and peripheral mechanisms

The vagus is involved in mediating gastric cytoprotection and adaptive cytoprotection. However, the central and peripheral mechanisms through which the vagus expresses its action are still poorly known. Medullary thyrotropin-releasing hormone (TRH) plays an important role in the vagal regulation of gastric function. The stable TRH analogue, RX 77368, micro-injected into the cisterna magna or the dorsal motor nucleus (DMN) of the vagus at a dose that did not influence gastric acid secretion prevented gastric injury induced by intragastric administration of 60% ethanol in conscious or urethane-anaesthetized rats. The cytoprotective action of TRH is mediated through vagal cholinergic release of prostaglandin E2 (PGE2). Prostaglandin E2 action is unrelated to changes in gastric mucosal blood flow (GMBF). In addition, other peripheral mechanisms involve calcitonin gene-related peptide (CGRP) contained in capsaicin sensitive afferent fibres and nitric oxide, both of which mediate the associated increase in GMBF induced by intracisternal injection of RX 77368. These data indicate that medullary TRH induces vagally mediated gastric protection against ethanol lesions. Its action is expressed through the muscarinic dependent release of PGE2 and nitric oxide, and efferent function of capsaicin-sensitive afferent fibres releasing CGRP.

Neuroendocrine regulation of thyrotropin-releasing hormone (TRH) in the tuberoinfundibular system

[...] It is now required to list each part needed for mucous excretion. They are two ducts in the brain substance, then a thin portion of membrane shaped as the infundibulum, then the gland that receives the tip of this infundibulum and the ducts that drive the mucus (pituita) from this gland to the palate and nares. [...] and I said that one (duct) [...] from the middle of the common cavity (third ventricle) descends [...] into the brain substance, and the end of this duct is [...] the sinus of the gland where the brain mucus is collected [...].

Response of neurons in the dorsal motor nucleus of the vagus to thyrotropin-releasing hormone

Autonomic motoneurons in the dorsal motor nucleus of the vagus (DMX) were recorded intracellularly in an in vitro slice preparation of the guinea pig brainstem. Bath-applied thyrotropin releasing hormone (TRH) (1-10 microM) induced a reversible depolarization of neurons that was typically accompanied by an increase in the spontaneous firing of the cells. In some cells, TRH induced rhythmic bursting activity. The TRH-induced depolarization occurred also in the presence of reduced Ca2+ and TTX. The response was dose-dependent over TRH concentrations of 0.1-10 microM. The TRH-induced depolarization was accompanied by an increase in input resistance. The reversal potential of this effect corresponded to that of K+. Our results indicate that TRH increases the excitability of DMX neurons by reducing a resting K+ conductance.

Effects of vagotomy on neurotransmitter receptors in the rat dorsal vagal complex

The dorsal vagal complex contains many different neurotransmitter receptors. The cyto-architectural localizations of some of these receptors remain largely unknown. In rats, vagotomy was performed to destroy vagal afferents terminating in the nucleus of the solitary tract and to produce chromatolysis of preganglionic motoneurons in the dorsal motor nucleus of the vagus. Quantitative receptor autoradiography was then employed to determine the effect of vagotomy upon the distribution of receptors for thyrotropin-releasing hormone, substance P, and serotonin within individual regions and subnuclei of the entire dorsal vagal complex. Vagotomy reduced the concentrations of thyrotropin-releasing hormone and substance P, but not serotonin1A, or serotonin1B, receptors in the dorsal motor nucleus of the vagus. Within the nucleus of the solitary tract, substance P receptors were reduced in only the medial and central subnuclei after vagotomy. In contrast, no effect was observed upon the concentrations of thyrotropin-releasing hormone, serotonin1A, or serotonin1B receptors in any subnuclei of the solitary tract following vagotomy. These results suggest that in the dorsal motor nucleus of the vagus, thyrotropin-releasing hormone and substance P receptors are localized upon vagal preganglionic motoneurons, while serotonin1A and serotonin1B receptors are present upon interneurons or other neuronal elements. These results also suggest that thyrotropin-releasing hormone, substance P, serotonin1A, and serotonin1B receptors in the nucleus of the solitary tract are localized upon internuncial neurons in the nucleus of the solitary tract.

TRH regulation of tracheal tension through vagal preganglionic motoneurons

TRH-immunoreactive nerve terminals innervate the ambiguous nucleus in the rabbit. Vagal preganglionic motoneurons that innervate the trachea, were revealed by HRP histochemistry in the rostral part of the ambiguous nucleus and the dorsal motor nucleus of the vagus. HRP histochemistry plus TRH immunocytochemistry revealed that TRH-immunoreactive axon terminals synapsed on ambiguous nucleus neurons retrogradely labeled by HRP injection into tracheal smooth muscle and the superior laryngeal nerve. Microinjection of 50 ng TRH into the rostral ambiguous nucleus caused slight dilation followed by constriction, which was inhibited by atropine and vagotomy. Results show that central TRH-containing neurons regulate tracheal tension through synapses on vagal preganglionic motoneurons that innervate tracheal smooth muscle.

Distribution of thyrotropin-releasing hormone receptor messenger RNA in the rat brain: an in situ hybridization study

Based on the recent cloning of the mouse thyrotropin-releasing hormone receptor, oligonucleotide probes complementary to the DNA sequence were constructed and used for in situ hybridization studies on the rat brain. Thyrotropin-releasing hormone receptor messenger RNA was found in many areas of the brain, mostly showing high degree of overlap with the distribution thyrotropin-releasing hormone binding sites as previously revealed in autoradiographic studies. Thus, a strong signal was observed in the accessory olfactory bulb, the perirhinal sulcus, the ventral aspects of the hippocampal formation, some amygdaloid nuclei, the diagonal band nucleus, parts of nucleus accumbens, the bed nucleus of the stria terminalis, dorsomedial, lateral and perifornical hypothalamic regions, the septohippocampal nucleus, parts of the vestibular complex, as well as many bulbar motoneurons including the facial, dorsal vagal, ambiguus and hypoglossal nuclei, the superficial layer of the spinal trigeminal nucleus, and motoneurons and dorsal horn neurons in the spinal cord. Cells within one and the same nucleus expressed varying levels of thyrotropin releasing hormone receptor messenger RNA suggesting marked differences in rate of receptor synthesis. Most of these areas receive an input by thyrotropin-releasing hormone-positive nerve endings. Taken together these results suggest that thyrotropin-releasing hormone receptors are mostly localized in the vicinity of the cell bodies which express thyrotropin-releasing hormone receptor messenger RNA and mediate the wide range of actions that have been recorded after administration of exogenous thyrotropin-releasing hormone.

An atlas of the rat subpostremal nucleus tractus solitarius

The nucleus tractus solitarius (NTS) in the dorsal medulla is the principal visceral sensory relay nucleus in the brain. In the rat, numerous lines of evidence indicate that the caudal NTS at the level of the area postrema serves as a major integrating site for coordinating cardiorespiratory reflexes and viscerobehavioral responses. This region of the caudal NTS not only exhibits high densities of binding sites for an impressive array of transmitters and modulators but microinjections of many of these same neuroactive substances into the rat subpostremal NTS elicit pronounced cardiorespiratory and visceral response patterns. This report provides an abbreviated atlas of the rat subpostremal NTS consisting of a series of transverse, sagittal, and horizontal plates. Photomicrographs, together with their corresponding schematic drawings, are provided for the serial sections generated from each reference plane.

Notes on a light and electron microscopic double-labeling method combining anterograde tracing with Phaseolus vulgaris leucoagglutinin and retrograde tracing with cholera toxin subunit B

Investigations of monosynaptic connections in the central nervous system have been hindered by the lack of compatible markers that can be used at both light and electron microscopic levels. In attempts to determine synaptic contacts between fibers originating in the substantia nigra and neurons projecting to the spinal cord, we have developed a double immunolabeling technique using anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and retrograde transport of unconjugated cholera toxin subunit B (CTB). In this report, we describe technical modifications which consistently produced superior labeling together with adequate ultrastructural preservation of the tissue and discuss the advantages of the two tracers.

Central thyrotropin-releasing factor analog prevents ethanol-induced gastric damage through prostaglandins in rats

The effects of intracisternal injection of the stable thyrotropin-releasing factor (TRH) analog RX 77368 on gastric lesions induced by 60% ethanol and gastric prostaglandin E2 (PGE2) release were studied in rats. RX 77368 (1.0 and 1.5 ng) injected intracisternally inhibited (by 58% and 78%, respectively) macroscopic gastric damage induced by ethanol. Higher doses (3 and 300 ng) inhibited ethanol-induced gastric injury only in rats pretreated with omeprazole (20 mg/kg SC). Gastric acid output measured in conscious rats 2 hours after pylorus ligation was not modified by intracisternal injection of RX 77368 at 1.5 ng but was significantly increased by 54% at the 3-ng dose. The protective effect of TRH analog (1.5 ng) was completely abolished by indomethacin (5 mg/kg IP) and atropine (2 mg/kg SC) pretreatment. In pylorus-ligated rats, intracisternal RX 77368 (1.5 ng) inhibited ethanol-induced gastric lesions by 64%. Intracisternal injection of RX 77368 (1.5 ng) increased PGE2 levels measured in the effluent of dialysis fibers implanted into the corpus submucosa of urethane-anesthetized rats. Peripheral administration of omeprazole, atropine, indomethacin, or RX 77368 (1.5 ng IV) did not influence gastric damage induced by ethanol. These data show that the stable TRH analog, RX 77368, injected intracisternally at low non-secretory doses acts in the brain to protect against ethanol lesions through prostaglandin and cholinergic pathways. These findings suggest that central vagal activation induced by TRH may play a role in the control of mucosal integrity against ethanol through cholinergic prostaglandin release.

Thyrotropin releasing hormone (TRH) and a degradation stabilized analogue (RX77368) stimulate phosphoinositide turnover in cultured astrocytes in a regionally specific manner

Whilst the CNS effects of thyrotropin releasing hormone (TRH) may result from a direct action on neurones it is also a possibility that another cell type may mediate an indirect action. A potential candidate for such a role is the astrocyte. The ability of TRH and a stable analogue RX77368 (di-methyl proline TRH) to stimulate phosphoinositide turnover has been investigated in cultured astrocytes from a number of CNS regions. Significant increases in turnover were noted in three of the four regions studied. Percentage values were: in spinal cord, 33% TRH, 31% RX77368 (n = 15); in brain stem, 33% TRH, 37% RX77368 (n = 6); in cerebellum, 72% TRH, 73% RX77368 (n = 6); in cortex, 4% TRH, 13% RX77368 (n = 6). EC50 values for both peptides were in the picomolar range. These results indicate that some astrocytes in vitro possess functional TRH receptors linked to the phosphoinositide second messenger system and hence this cell type would potentially be capable of mediating an indirect action of the peptide. Also, because the response was limited to certain regions, heterogeneity in receptor expression is implied. Furthermore, in the light of other evidence to support astrocyte heterogeneity within a region, we suggest that certain characteristics of the response seen in our experiments may be the result of TRH receptors being restricted to a subpopulation of astrocytes within a given culture. Thus, our data show that astrocytes prepared from some CNS regions possess functionally coupled TRH receptors.

Effects of diazepam and Ro 15-1788 on duodenal bicarbonate secretion in the rat

Bicarbonate secretion by duodenal mucosa just distal to the Brunner's glands area and devoid of pancreatic secretions was titrated in situ in anesthetized rats. Intravenous injection of diazepam (0.1 and 0.5 mg/kg) significantly increased the secretion; this stimulation was abolished by proximal bilateral vagotomy. Ro 15-1788, a benzodiazepine antagonist that also has well-known intrinsic activity, caused similar stimulation of the secretion when administered IV (0.01 and 0.1 mg/kg). Intracerebroventricular infusion of Ro 15-1788 (10 micrograms/h) resulted in a greater increase in secretion; again, this stimulation was prevented by vagotomy. Adrenoceptor blockade by phentolamine increased basal alkaline secretion but did not affect the stimulation by diazepam. The tricyclic antidepressant trimipramine (2.5 mg/kg IV) did not affect the duodenal bicarbonate secretion. For comparison, effects of diazepam and Ro 15-1788 (10(-6)-10(-4) mol/L) were also tested in isolated bullfrog duodenal mucosa. Neither drug effected the alkaline secretion in vitro. The combined results strongly suggest that benzodiazepines, as previously shown for certain brain peptides, influence the central nervous control of duodenal mucosal alkaline secretion and that their stimulation of secretion is vagally mediated. This action benzodiazepines might be used in modulating mucosal protection against acid.