Microinjection of TRH analogs into the raphe pallidus stimulates gastric acid secretion in the rat

Patterns of fos activation in rat raphe nuclei during feeding behavior

To analyze the differential recruitment of the raphe nuclei during different phases of feeding behavior, rats were subjected to a food restriction schedule (food for 2 h/day, during 15 days). The animals were submitted to different feeding conditions, constituting the experimental groups: search for food (MFS), food ingestion (MFI), satiety (MFSa) and food restriction control (MFC). A baseline condition (BC) group was included as further control. The MFI and MFC groups, which presented greater autonomic and somatic activation, had more FOS-immunoreactive (FOS-IR) neurons. The MFI group presented more labeled cells in the linear (LRN) and dorsal (DRN) nuclei; the MFC group showed more labeling in the median (MRN), pontine (PRN), magnus (NRM) and obscurus (NRO) nuclei; and the MFSa group had more labeled cells in the pallidus (NRP). The BC exhibited the lowest number of reactive cells. The PRN presented the highest percentage of activation in the raphe while the DRN the lowest. Additional experiments revealed few double-labeled (FOS-IR+5-HT-IR) cells within the raphe nuclei in the MFI group, suggesting little serotonergic activation in the raphe during food ingestion. These findings suggest a differential recruitment of raphe nuclei during various phases of feeding behavior. Such findings may reflect changes in behavioral state (e.g., food-induced arousal versus sleep) that lead to greater motor activation, and consequently increased FOS expression. While these data are consistent with the idea that the raphe system acts as gain setter for autonomic and somatic activities, the functional complexity of the raphe is not completely understood.

Somatic and visceral afferents to the 'vasodepressor region' of the caudal midline medulla in the rat

Previous research has found that the integrity of a restricted region of the caudal midline medulla (including caudal portions of nucleus raphé obscurus and nucleus raphé pallidus) was critical for vasodepression (hypotension, bradycardia, decreased cardiac contractility) evoked either by haemorrhage or deep pain. In this anatomical tracing study we found that the vasodepressor part of the caudal midline medulla (CMM) receives inputs arising from spinal cord, spinal trigeminal nucleus (SpV) and nucleus of the solitary tract (NTS). Specifically: (i) a spinal-CMM projection arises from neurons of the deep dorsal horn, medial ventral horn and lamina X at all spinal segmental levels, with approximately 60% of the projection originating from the upper cervical spinal cord (C1-C4); (ii) a SpV-CMM projection arises primarily from neurons at the transition between subnucleus caudalis and subnucleus interpolaris; (iii) a NTS-CMM projection arises primarily from neurons in ventrolateral and medial subnuclei. In combination, the specific spinal, SpV and NTS regions which project to the CMM receive the complete range of somatic and visceral afferents known to trigger vasodepression. The role(s) of each specific projection is discussed.

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.

Afferent connections of the caudal raphe pallidus nucleus in rats: a study using the fluorescent retrograde tracers fluorogold and true-blue

Afferent connections to the caudal region of the nucleus raphe pallidus (RPa) in rats were studied using fluorogold and true-blue as tracers. Due to its ability to produce limited injection sites, true-blue proved to be more appropriate than fluorogold for studying long distance connections in a narrow structure such as the RPa. Fluorescent, retrogradely-labeled perikarya were found in the preoptic area (median, medial and lateral nuclei), hypothalamus (anterior, dorsal, lateral and posterior areas, and the peri- and paraventricular nuclei), zona incerta, central gray (dorsal, ventral and ventro-lateral), reticular formation of the brainstem, trigeminal spinal nuclei and in the spinal cord (laminae V-X at thoracic, lumbar and sacral levels). This connection pattern suggests the involvement of the RPa in autonomic, somatic and endocrine functions.

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.

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.

CNS effects on gastric functions: from clinical observations to peptidergic brain-gut interactions

Clinical observations as early as the last century pointed to the stomach's link to the brain. Animal studies in this century have given us detailed information about the neuroanatomical and neurophysiological basis of brain-gut interactions. Psychological stress models and stereotaxic brain procedures have been important tools in gaining this information. During the last 10 years, there has been much focus on the effects of neuropeptides on gastric functions. Several CNS-peptides have indeed been shown to influence multiple gastric functions such as: acid secretion, bicarbonate secretion, mucus secretion, motility, blood flow and prostaglandin synthesis. Accordingly, direct CNS-application of these peptides also influences the development of gastric erosions during experimental stress procedures.

Thyrotropin-releasing hormone analog injected into the raphe pallidus and obscurus increases gastric contractility in rats

The present study was performed to investigate the influence of the chemical stimulation of medullary raphe nuclei by the stable TRH (thyrotropin-releasing hormone) analog, RX 77368, on gastric contractility. Urethane-anesthetized rats were acutely implanted with miniature strain gauge force transducers on the corpus of the stomach for continuous recording of gastric contractility. Traces were analyzed by computer. Microinjections of vehicle or RX 77368 into the raphe pallidus or raphe obscurus were performed using pressure injection of 50 nl through glass micropipettes 30 min following basal recording of gastric contractility. RX 77368 (0.7-77 pmol) dose dependently stimulated gastric contractility when microinjected into the raphe pallidus and raphe obscurus. The stimulation of gastric contractions induced by microinjection of RX 77368 (77 pmol) into these raphe nuclei was completely blocked by vagotomy and prevented (raphe obscurus) or reduced (raphe pallidus) by atropine. RX 77368 (7.7-77 pmol) microinjected into the inferior olive, pyramidal tract, medial lemiscus was ineffective. These results demonstrate that chemical stimulation of the raphe pallidus and obscurus by RX 77368 stimulates gastric contractility through vagal and muscarinic pathways. These data suggest a role for medullary raphe nuclei in the central vagal regulation of gastric contractility.

CRF microinjected into the dorsal vagal complex inhibits TRH analog- and kainic acid-stimulated gastric contractility in rats

The effect of CRF microinjected into the dorsal vagal complex (DVC) on centrally-stimulated gastric contractility was investigated in fasted, urethane-anesthetized rats. Miniature strain gauge force transducers were acutely implanted on the corpus of the stomach and contractility was analyzed by computer. Microinjection of the stable thyrotropin-releasing hormone (TRH) analog, RX 77368, (26 pmol) into the DVC induced a 12.2-fold stimulation of gastric contractility within 30 min. Corticotropin-releasing factor (CRF) (63-210 pmol) microinjected into the DVC concomitantly with RX 77368 (26 pmol) induced a dose-related inhibition of stimulated gastric contractility. Neither CRF alone (210 pmol) nor vehicle modified basal gastric contractility. Microinjection of kainic acid (141 pmol) into the raphe pallidus nucleus induced a 3.6-fold stimulation of gastric contractility after 45 min. This stimulation was suppressed by bilateral microinjection of CRF (105 pmol/site) into the DVC. These results demonstrate that CRF acts in the DVC to inhibit centrally-stimulated gastric contractility and suggest that TRH and CRF may interact in the DVC to regulate gastric motor function.