Pathophysiology of Gastroesophageal Reflux Disease.

The pathogenesis of gastroesophageal reflux disease (GERD) is complex and involves changes in reflux exposure, epithelial resistance, and visceral sensitivity. The gastric refluxate is a noxious material that injures the esophagus and elicits symptoms. Esophageal exposure to gastric refluxate is the primary determinant of disease severity. This exposure arises via compromise of the anti-reflux barrier and reduced ability of the esophagus to clear and buffer the refluxate, leading to reflux disease. However, complications and symptoms also occur in the context of normal reflux burden, when there is either poor epithelial resistance or increased visceral sensitivity. Reflux therefore develops via alterations in the balance of aggressive and defensive forces.

Management of Gastroesophageal Reflux Disease.

Management of gastroesophageal reflux disease (GERD) commonly starts with an empiric trial of proton pump inhibitor (PPI) therapy and complementary lifestyle measures, for patients without alarm symptoms. Optimization of therapy (improving compliance and timing of PPI doses), or increasing PPI dosage to twice daily in select circumstances, can reduce persistent symptoms. Patients with continued symptoms can be evaluated with endoscopy and tests of esophageal physiology, to better determine their disease phenotype and optimize treatment. Laparoscopic fundoplication, magnetic sphincter augmentation, and endoscopic therapies can benefit patients with well-characterized GERD. Patients with functional diseases that overlap with or mimic GERD can also be treated with neuromodulators (primarily antidepressants), or psychological interventions (psychotherapy, hypnotherapy, cognitive and behavioral therapy). Future approaches to treatment of GERD include potassium-competitive acid blockers, reflux-reducing agents, bile acid binders, injection of inert substances into the esophagogastric junction, and electrical stimulation of the lower esophageal sphincter.

Opioid-Induced Foregut Dysfunction.

The impact of opioid use on the lower gastrointestinal tract is well described, but recent opioid crisis has caused increased awareness of the detrimental effects of these drugs on esophageal and gastroduodenal motility. Opioid use has been associated with increased incidence of spastic esophageal motility disorders and gastroduodenal dysfunction. Opioid receptors are present with high abundance in the myenteric and submucosal plexus of the enteric nervous system. Activation of these receptors leads to suppressed excitability of the inhibitory musculomotor neurons and unchecked tonic contraction of the autogenic musculature (such as the lower esophageal sphincter and the pylorus).

Ameliorating Effects of Transcutaneous Electrical Acustimulation Combined With Deep Breathing Training on Refractory Gastroesophageal Reflux Disease Mediated via the Autonomic Pathway.

AIMS: To investigate the effects and possible mechanisms of transcutaneous electrical acustimulation (TEA) combined with deep breathing training (DBT) on refractory gastroesophageal reflux disease (rGERD). METHODS: Twenty-one patients with rGERD were recruited and randomly assigned to receive either only esomeprazole (ESO, 20 mg bid) (group A, n = 7), TEA + DBT + ESO (group B, n = 7), or sham-TEA + DBT + ESO (group C, n = 7) in a four-week study. The reflux diagnostic questionnaire (RDQ) score and heart rate variability (HRV) were recorded and evaluated at baseline and at the end of each treatment. Blood samples were collected for the measurement of serum acetylcholine (Ach) and nitric oxide (NO). Esophageal manometry and 24-hour pH monitoring were performed before and after the treatment. RESULTS: After treatment, 1) the participants in group B had significantly lower scores of RDQ and DeMeester and increased lower esophageal sphincter pressure (LESP) than those in group C (all p < 0.05), suggesting the role of TEA; 2) low frequency band (LF)/(LF + HF) ratio in groups B and C was decreased, compared with group A (p = 0.010, p = 0.042, respectively); high frequency band (HF)/(LF + HF) ratio in B and C groups was significantly increased, compared with group A (p = 0.010, p = 0.042, respectively); 3) The serum Ach in groups B and C was significantly higher than group A (p = 0.022, p = 0.046, respectively); the serum NO in groups B and C was significantly lower than group A (p = 0.010, p = 0.027, respectively). CONCLUSIONS: TEA combined with the DBT can effectively improve the reflux symptoms in rGERD patients by increasing LESP and reducing gastroesophageal reflux, which may be mediated via the autonomic and enteric mechanisms.

Circular and longitudinal muscles shortening indicates sliding patterns during peristalsis and transient lower esophageal sphincter relaxation.

Esophageal axial shortening is caused by longitudinal muscle (LM) contraction, but circular muscle (CM) may also contribute to axial shortening because of its spiral morphology. The goal of our study was to show patterns of contraction of CM and LM layers during peristalsis and transient lower esophageal sphincter (LES) relaxation (TLESR). In rats, esophageal and LES morphology was assessed by histology and immunohistochemistry, and function with the use of piezo-electric crystals and manometry. Electrical stimulation of the vagus nerve was used to induce esophageal contractions. In 18 healthy subjects, manometry and high frequency intraluminal ultrasound imaging during swallow-induced esophageal contractions and TLESR were evaluated. CM and LM thicknesses were measured (40 swallows and 30 TLESRs) as markers of axial shortening, before and at peak contraction, as well as during TLESRs. Animal studies revealed muscular connections between the LM and CM layers of the LES but not in the esophagus. During vagal stimulated esophageal contraction there was relative movement between the LM and CM. Human studies show that LM-to-CM (LM/CM) thickness ratio at baseline was 1. At the peak of swallow-induced contraction LM/CM ratio decreased significantly (<1), whereas the reverse was the case during TLESR (>2). The pattern of contraction of CM and LM suggests sliding of the two muscles. Furthermore, the sliding patterns are in the opposite direction during peristalsis and TLESR.

Gestational and postnatal modulation of esophageal sphincter reflexes in human premature neonates.

BACKGROUND: Effects of gestational age (GA) and postnatal maturation on upper and lower esophageal sphincter (UES and LES) reflex development remain unclear. We hypothesized very-preterm (VPT) born neonates (< 32 wk GA) have delayed maturation of UES contractile reflex (UESCR) and LES relaxation reflex (LESRR) vs. preterm (PT) born (32-37 wk GA) neonates. METHODS: Using provocative manometry, effects of 1,263 graded mid-esophageal stimuli (air, liquid) on sensory-motor characteristics of UESCR and LESRR were investigated in 24 VPT-born and 12 PT-born neonates (37.8 ± 0.6 vs. 38.9 ± 0.4 wk postmenstrual age respectively, P = 0.14). RESULTS: In response to liquid stimuli (vs. air), VPT-born neonates displayed prolonged UESCR and LESRR response latencies (P < 0.001) and prolonged UESCR and LESRR durations (P < 0.01); unlike PT-born neonates, who exhibit prolonged LESRR response latency (P < 0.01), but similar UESCR and LESRR durations (P = 0.2). Differences were noted in LESRR duration in VPT vs. PT neonates for air stimuli (P = 0.04). With liquid stimuli, increasing GA was associated with decreasing response onset latencies to UESCR and LESRR (P < 0.05), and increasing LESRR duration (P = 0.02). CONCLUSION: Using GA as categorical or continuous variable, vagus-mediated mechano-sensitive and liquid-sensitive reflex characteristics of UESCR and LESRR are distinct; LESRR differs with varying intrauterine maturation suggesting inhibitory modulation progresses with advancing maturation.

Gastric and lower esophageal sphincter pressures during nausea: a study using visual motion-induced nausea and high-resolution manometry.

Nausea is the subjective unpleasant sensation that immediately precedes vomiting. Studies using barostats suggest that gastric fundus and lower esophageal sphincter (LES) relaxation precede vomiting. Unlike barostat, high-resolution manometry allows less invasive, detailed measurements of fundus pressure (FP) and axial movement of the gastroesophageal junction (GEJ). Nausea was induced in 12 healthy volunteers by a motion video and rated on a visual analog scale. FP was measured as the mean value of the five pressure channels that were clearly positioned below the LES. After intubation, a baseline (BL) recording of 15 min was obtained. This was followed by presentation of the motion video (at least 10 min, maximum 20 min) followed by 30 min recovery recording. Throughout the experiment we recorded autonomic nervous system (ANS) parameters [blood pressure, heart rate (HR), and cardiac vagal tone (CVT), which reflects efferent vagal activity]. Ten out of 12 subjects showed a drop in FP during peak nausea compared with BL (-4.0 ± 0.8 mmHg; P = 0.005), and 8/10 subjects showed a drop in LES pressure (-8.8 ± 2.5 mmHg; P = 0.04). Peak nausea preceded peak fundus and LES pressure drop. Nausea was associated with configuration changes at the GEJ such as LES shortening and esophageal lengthening. During nausea we observed a significantly increased HR and decreased CVT. In conclusion, nausea is associated with a drop in fundus and LES pressure, configuration changes at the GEJ as well as changes in the ANS activity such as an increased sympathetic tone (increased HR) and decreased parasympathetic tone (decreased CVT).

Pantoprazole decreases gastroesophageal muscle tone in newborn rats via rho-kinase inhibition.

Proton pump inhibitors reduce gastric acid secretion and are commonly utilized in the management of gastroesophageal reflux disease across all ages. Yet a decrease in lower esophageal sphincter tone has been reported in vitro in rats through an unknown mechanism; however, their effect on the gastroesophageal muscle tone early in life was never studied. Hypothesizing that proton pump inhibitors also reduce gastroesophageal muscle contraction in newborn and juvenile rats, we evaluated the in vitro effect of pantoprazole on gastric and lower esophageal sphincter muscle tissue. Electrical field stimulation and carbachol-induced force were significantly (P < 0.01) reduced in the presence of pantoprazole, whereas the drug had no effect on the neuromuscular-dependent relaxation. When administered in vivo, pantoprazole (9 mg/kg) significantly (P < 0.01) reduced gastric emptying time at both ages. To ascertain the signal transduction pathway responsible for the reduction in muscle contraction, we evaluated the tissue ROCK-2 and CPI-17 activity. Pantoprazole reduced myosin light chain phosphatase MYPT-1, but not CPI-17 phosphorylation of gastric and lower esophageal sphincter tissue, strongly suggesting that it is a ROCK-2 inhibitor. To the extent that these findings can be extrapolated to human neonates, the use of pantoprazole may impair gastric and lower sphincter muscle tone and thus paradoxically exacerbate esophageal reflux. Further studies addressing the effect of proton pump inhibitors on gastroesophageal muscle contraction are warranted to justify its therapeutic use in gastroesophageal reflux disease.

Circulating antimyenteric autoantibodies in Tunisian patients with idiopathic achalasia.

The physiopathology of idiopathic achalasia is still unknown. The description of circulating antimyenteric autoantibodies (CAA), directed against enteric neurons in sera of patients, suggests an autoimmune process. Recent data showed controversies according to the existence and the significance of CAA. The aims of this study were to investigate whether CAA are detected in Tunisian patients with idiopathic achalasia and to look for associated clinical or manometrical factors with CAA positivity. Twenty-seven patients with idiopathic achalasia and 57 healthy controls were prospectively studied. CAA were assessed by indirect immunofluorescence on intestinal monkey tissue sections. Western blot on primate cerebellum protein extract and dot technique with highly purified recombinant neuronal antigens (Hu, Ri, and Yo) were further used to analyze target antigens of CAA. CAA were significantly increased in achalasia patients compared with controls when considering nuclear or cytoplasmic fluorescence patterns. (33% vs. 12%, P = 0.03 and 48% vs. 23%, P = 0.001 respectively). By immunoblot analysis, CAA did not target neuronal antigens, however 52/53 and 49 kDa bands were consistently detected. CAA positivity was not correlated to specific clinical features. The results are along with previous studies demonstrating high CAA prevalence in achalasia patients. When reviewing technical protocols and interpretation criteria, several discrepancies which could explain controversies between studies were noted.

Physiology of the LES.

The purpose of this review is to consider the neuromuscular mechanism of LES contractility both by itself and in relation to the esophagogastric junction (EGJ) complex in order to appreciate the intricacies of EGJ valvular function.

Variability in the muscle composition of rat esophagus and neural pathway of lower esophageal sphincter relaxation.

Several studies from our laboratory show that axial stretch of the lower esophageal sphincter (LES) in an oral direction causes neurally mediated LES relaxation. Under physiological conditions, axial stretch of the LES is caused by longitudinal muscle contraction (LMC) of the esophagus. Because longitudinal muscle is composed of skeletal muscle in mice, vagal-induced LMC and LES relaxation are both blocked by pancuronium. We conducted studies in rats (thought to have skeletal muscle esophagus) to determine if vagus nerve-mediated LES relaxation is also blocked by pancuronium. LMC-mediated axial stretch on the LES was monitored using piezoelectric crystals. LES and esophageal pressures were monitored with a 2.5-Fr solid-state pressure transducer catheter. Following bilateral cervical vagotomy, the vagus nerve was stimulated electrically. LES, along with the esophagus, was harvested after in vivo experiments and immunostained for smooth muscle (smooth muscle α-actin) and skeletal muscle (fast myosin heavy chain). Vagus nerve-stimulated LES relaxation and esophageal LMC were reduced in a dose-dependent fashion and completely abolished by pancuronium (96 µg/kg) in six rats (group 1). On the other hand, in seven rats, LES relaxation and LMC were only blocked completely by a combination of pancuronium (group 2) and hexamethonium. Immunostaining revealed that the longitudinal muscle layer was composed of predominantly skeletal muscle in the group 1 rats. On the other hand, the longitudinal muscle layer of group 2 rats contained a significant amount of smooth muscle (P < 0.05). Our study shows tight coupling between axial stretch on the LES and relaxation of the LES, which suggests a cause and effect relationship between the two. We propose that the vagus nerve fibers that cause LMC induce LES relaxation through the stretch-sensitive activation of inhibitory motor neurons.

Evidence for altered circular smooth muscle cell function in lower esophageal sphincter of W/Wv mutant mice.

Nitrergic neurotransmission to gut smooth muscle is impaired in W/W(v) mutant mice, which lack intramuscular interstitial cells of Cajal (ICC-IM). In addition, these mice have been reported to have smaller amplitude unitary potentials (UPs) and a more negative resting membrane potential (RMP) than control mice. These abnormalities have been attributed to absence of ICC-IM, but it remains possible that they are due to alterations at the level of the smooth muscle itself. Amphotericin-B-perforated patch-clamp recordings and Ca(2+) imaging (fura 2) were compared between freshly isolated single circular smooth muscle cells (CSM) from W/W(v) mutant and control mice lower esophageal sphincter (LES). There was no significant difference in seal resistance, capacitance, or input resistance in response to applied electrotonic current pulses between CSM cells from W/W(v) mutants and controls. Compared with control mice, RMP was more negative and UPs significantly smaller in CSM cells from mutant mice LES. Administration of caffeine induced an inward current in cells from both mutant and control mice, but the current density was significantly larger in cells from W/W(v) mutants. Membrane potential hyperpolarization induced by sodium nitroprusside was larger in cells from control mice vs. W/W(v) mutants. In addition, intracellular Ca(2+) transients induced by caffeine were significantly increased in cells from mutants. These findings indicate that LES CSM is abnormal in W/W(v) mutant mice. Thus some physiological functions attributed to ICC-IM based on experiments in smooth muscle of ICC deficient mice may need to be reconsidered.

Regional functional specialization and inhibitory nitrergic and nonnitrergic coneurotransmission in the human esophagus.

The aim of this study was to explore the myenteric mechanisms of control of human esophageal motility and the effect of nitrergic and nonnitrergic neurotransmitters. Human circular esophageal strips were studied in organ baths and with microelectrodes. Responses following electrical field stimulation (EFS) of enteric motoneurons (EMNs) or through nicotinic acetylcholine receptors were compared in the esophageal body (EB) and in clasp and sling regions in the lower esophageal sphincter (LES). In clasp LES strips: 1) sodium nitroprusside (1 nM to 100 µM), adenosine-5'-[ß-thio]diphosphate trilithium salt (1-100 µM), and vasoactive intestinal peptide (1 nM to 1 µM) caused a relaxation; 2) 1 mM N(ω)-nitro-L-arginine (L-NNA) shifted the EFS "on"-relaxation to an "off"-relaxation, partly antagonized by 10 µM 2'-deoxy-N(6)-methyladenosine 3',5'-bisphosphate tetrasodium salt (MRS2179) or 10 U/ml α-chymotrypsin; and 3) nicotine-relaxation (100 µM) was mainly antagonized by L-NNA, and only partly by MRS2179 or α-chymotrypsin. In sling LES fibers, EFS and nicotine relaxation was abolished by L-NNA. In the EB, L-NNA blocked the latency period, and MRS2179 reduced "off"-contraction. The amplitude of cholinergic contraction decreased from the EB to both LES sides. EFS induced a monophasic inhibitory junction potential in clasp, sling, and EB fibers abolished by L-NNA. Our study shows a regional specialization to stimulation of EMNs in the human esophagus, with stronger inhibitory responses in clasp LES fibers and stronger cholinergic excitatory responses in the EB. Inhibitory responses are mainly triggered by nitrergic EMNs mediating the inhibitory junction potentials in the LES and EB, EFS on-relaxation in clasp and sling LES sides, and latency in the EB. We also found a minor role for purines (through P2Y(1) receptors) and vasoactive intestinal peptide-mediating part of nonnitrergic clasp LES relaxation.

Sensory and motor innervation of the crural diaphragm by the vagus nerves.

BACKGROUND & AIMS: During gastroesophageal reflux, transient lower esophageal sphincter relaxation and crural diaphragm (CD) inhibition occur concomitantly. Modifying vagus nerve control of transient lower esophageal sphincter relaxation is a major focus of development of therapeutics for gastroesophageal reflux disease, but neural mechanisms that coordinate the CD are poorly understood. METHODS: Nerve tracing and immunolabeling were used to assess innervation of the diaphragm and lower esophageal sphincter in ferrets. Mechanosensory responses of vagal afferents in the CD and electromyography responses of the CD were recorded in novel in vitro preparations and in vivo. RESULTS: Retrograde tracing revealed a unique population of vagal CD sensory neurons in nodose ganglia and CD motor neurons in brainstem vagal nuclei. Anterograde tracing revealed specialized vagal endings in the CD and phrenoesophageal ligament-sites of vagal afferent mechanosensitivity recorded in vitro. Spontaneous electromyography activity persisted in the CD following bilateral phrenicotomy in vivo, while vagus nerve stimulation evoked electromyography responses in the CD in vitro and in vivo. CONCLUSIONS: We conclude that vagal sensory and motor neurons functionally innervate the CD and phrenoesophageal ligament. CD vagal afferents show mechanosensitivity to distortion of the gastroesophageal junction, while vagal motor neurons innervate both CD and distal esophagus and may represent a common substrate for motor control of the reflux barrier.

Effects of acetylcholine on sling and clasp fibers of the human lower esophageal sphincter.

BACKGROUND AND AIM: Manometric studies on the human lower esophageal sphincter (LES) have shown radial asymmetry of the high-pressure zone (HPZ). The aim of this study was to compare the functional properties of human LES clasp and sling muscles, and to look at their relationship with the expression of muscarinic receptors and intracellular Ca(2+) concentration. METHODS: Muscle strips of sling and clasp fibers from the LES were obtained from patients undergoing subtotal esophagectomy. Isometric tension responses of the strips to acetylcholine were studied. Western blotting and reverse transcription-polymerase chain reaction (RT-PCR) were used to determine the expression of five subtypes of muscarinic receptors. Intracellular Ca(2+) ([Ca(2+) ]i) was measured using laser scanning confocal microscopy. RESULTS: Acetylcholine caused a concentration-dependent increase in the tension of sling and clasp strips, the sling strip being stronger than clasp (P=0.00). Messenger RNA and protein for the five muscarinic acetylcholine receptor (mAChRs) expressed in the sling and clasp muscles were highest for M2, and then in decreasing levels: M(3)>M(1)>M(4)>M(5) . Acetylcholine caused significant elevation of [Ca(2+) ]i in sling and clasp muscle cells in the presence of extracellular Ca2+ (1.5mmol/L), and Ach-induced [Ca(2+) ]i elevation was 1.6 times greater in sling cells than in clasp cells. CONCLUSION: Variation of intracellular concentrations of Ca(2+) may be the reason for differential responses to acetylcholine for sling versus clasp fibers. However, these differences are not associated with the distribution and the level of expression of the five mAChRs between the two muscle types. Further study should focus on the ligand affinity and signal transduction pathway.

[The efficiency of structural resonance electromagnetotherapy for the treatment of patients with gastroesophageal reflux disease].

The results of application of structural resonance electromagnetotherapy (SRT) for the treatment of patients presenting with gastroesophageal reflux disease (GERD) are presented. They show that SRT has positive influence on the neurohumoral regulation of the lower oesophageal sphincter, ensures psychocorrection, and optimizes the vegetative regulation. Taken together, these effects result in the marked clinical improvement in patients presenting with GERD. It is concluded that structural resonance electromagnetotherapy can be recommended as an efficacious and pathogenetically sound method for the treatment of patients with gastroesophageal reflux disease.

Neurotransmission in lower esophageal sphincter of W/Wv mutant mice.

To address the controversy surrounding the role of interstitial cells of Cajal (ICC) in nitrergic neurotransmission to gastrointestinal smooth muscle, circular smooth muscle from the lower esophageal sphincter (LES) of W/W(v) wild-type and mutant (ICC-deficient) mice were studied by using intracellular and tension recordings in vitro. Resting membrane potential was more negative, and the spontaneous unitary potentials diminished in mutant mice. In wild-type mice, nerve stimulation induced a biphasic inhibitory junction potential (IJP) consisting of a fast initial IJP followed by a long-lasting slow IJP (LSIJP). The IJP was markedly impaired in a significant proportion of mutant mice, whereas in others it was normal. Pharmacological studies in the mice with markedly impaired IJPs revealed that cholinergic and purinergic components of the nerve-mediated responses appeared intact. In wild-type mice, caffeine hyperpolarized smooth muscle cells, inhibited the initial fast IJP, and completely abolished the LSIJP. In mutant mice, caffeine depolarized smooth muscle cells and abolished the impaired LSIJP but did not affect the initial fast IJP. Immunohistochemical staining for c-Kit confirmed deficiency of ICC in mutant mice with a normal nitrergic IJP. Rings of LES circular smooth muscle from W/W(v) mutant mice generated significantly less spontaneous tone than controls. When tone was restored with carbachol, normal nitrergic LES relaxation was recorded. These data suggest that 1) there is significant variability in the generation of nitrergic neurotransmission in the LES of W/W(v) mutant mice, whereas purinergic and cholinergic neurotransmission are intact; 2) the altered nitrergic responses appear to be associated with abnormal Ca2+-dependent signaling initiated by spontaneous Ca2+ release from sarcoplasmic reticulum in smooth muscle cells; and 3) c-Kit-positive ICC are not essential for nitrergic neurotransmission in mouse LES smooth muscle.

Mechanism of stretch-activated excitatory and inhibitory responses in the lower esophageal sphincter.

We recently found that an orally directed stretch of the esophagus activates a neurally mediated relaxation of the lower esophageal sphincter (LES). Goals of our study were to characterize the neural mechanisms responsible for axial and transverse stretch-activated responses in the LES. LES pressure was monitored in anesthetized and artificially ventilated mice. Sutures were placed in the esophagus to exert graded stretch in the longitudinal and transverse directions. Effects of bilateral vagotomy and pharmacological agents on the stretch-activated LES responses were investigated. The relationship between vagally stimulated axial stretch and LES relaxation was also studied. Stretch in the longitudinal and transverse directions caused a dose-dependent LES relaxation and contraction, respectively, that were not affected by bilateral vagotomy and sympathectomy but were blocked by tetrodotoxin. In bilateral vagotomized animals, hexamethonium, atropine, pyridoxalphosphate-6-azophenyl-2',4' disulfonic acid (PPADS), and ondansetron did not block the stretch-activated LES relaxation and contraction. Axial stretch-activated LES relaxation was blocked by nitric oxide inhibitor and transverse stretch-activated LES contraction was blocked by a combination of atropine and substance P antagonist. Electrical stimulation of the vagus nerve induced LES relaxation and axial stretch on the LES, both of which were blocked by rocuronium. Axial and transverse stretch-activated LES relaxation and contraction were present in the W/W(v) mice that lack interstitial cells of Cajal (ICC). Stretch-activated LES relaxation and contraction are mediated through mechanosensitive neurons located in the myenteric plexus, which involves neither synaptic transmission nor ICC.

(R)-(3-amino-2-fluoropropyl) phosphinic acid (AZD3355), a novel GABAB receptor agonist, inhibits transient lower esophageal sphincter relaxation through a peripheral mode of action.

Gastroesophageal reflux disease (GERD) affects >10% of the Western population. Conventionally, GERD is treated by reducing gastric acid secretion, which is effective in most patients but inadequate in a significant minority. We describe a new therapeutic approach for GERD, based on inhibition of transient lower esophageal sphincter relaxation (TLESR) with a proposed peripherally acting GABA(B) receptor agonist, (R)-(3-amino-2-fluoropropyl)phosphinic acid (AZD3355). AZD3355 potently stimulated recombinant human GABA(B) receptors and inhibited TLESR in dogs, with a biphasic dose-response curve. In mice, AZD3355 produced considerably less central side effects than the prototypical GABA(B) receptor agonist baclofen but evoked hypothermia at very high doses (blocked by a GABA(B) receptor antagonist and absent in GABA(B)-/- mice). AZD3355 and baclofen differed markedly in their distribution in rat brain; AZD3355, but not baclofen, was concentrated in circumventricular organs as a result of active uptake (shown by avid intracellular sequestration) and related to binding of AZD3355 to native GABA transporters in rat cerebrocortical membranes. AZD3355 was also shown to be transported by all four recombinant human GABA transporters. AR-H061719 [(R/S)-(3-amino-2-fluoropropyl)phosphinic acid], (the racemate of AZD3355) inhibited the response of ferret mechanoreceptors to gastric distension, further supporting its peripheral site of action on TLESR. In summary, AZD3355 probably inhibits TLESR through stimulation of peripheral GABA(B) receptors and may offer a potential new approach to treatment of GERD.