A pharmacological study of some relaxation processes in the rat oesophageal tunica muscularis mucosae

This thesis deals with pharmacological properties of rat oesophageal smooth muscle, viz. the tunica muscularis mucosae (TMM). Although the TMM contains the bulk of smooth muscle of the rodent oesophagus, its function is not fully understood. However, recent evidence supports an active role for the TMM in oesophageal motility. Present concepts of the processes involved in oesophageal peristalsis have emphasised the importance of neural inhibitory mechanisms in the smooth muscle layer of the oesophagus. The present study was thus chiefly concerned with relaxation responses of the TMM. -- The presence of a submucous ganglionated plexus in the isolated TMM was demonstrated by histological examination. The nerve plexus was found to contain acetyl- and butryl-cholinesterases and various peptide immunoreactive nerve fibres such as vasoactive intestinal peptide and calcitoningene related peptide. Adrenergic innervation was sparse in the proximal and supradiaphramatic portion of the TMM; serotonin was observed to be localised to mast cells but not to nervous elements. -- In the presence of cholinoceptor-induced tone relaxation responses were elicited in the TMM by field-stimulation and by stimulation of the vagus provided excitatory responses to electrical stimulation were blocked. Relaxations were also examined to pharmacological agents. -- Field-stimulation of the isolated TMM produced both a tetrodotoxin (TTX) -sensitive and TTX-insensitive relaxation. The former was sensitive to guanethidine in the distal but not in the proximal segment. The TTX-insensitive response was unaffected by a variety of pharmacological agents and not abolished by cold storage. It was however diminished by cooling and by calcium antagonists. This form of relaxation was characterised as purely myogenic and a consequence of K⁺ efflux activated by calcium entry through potential-operated channels following field-stimulation. Vagally-stimulated relaxations in the whole oesophagus preparation were insensitive to hexamethonium. Relaxations to pharmacological agents such as 5HT and A23187 were found to have a regional gradient with the proximal TMM segments being more responsive than the distal segments. -- This study demonstrates that the isolated TMM with its attached submucous plexus is capable of relaxing to both electrical and pharmacological stimuli

Morphine Decreases Enteric Neuron Excitability via Inhibition of Sodium Channels

Gastrointestinal peristalsis is significantly dependent on the enteric nervous system. Constipation due to reduced peristalsis is a major side-effect of morphine, which limits the chronic usefulness of this excellent pain reliever in man. The ionic basis for the inhibition of enteric neuron excitability by morphine is not well characterized as previous studies have mainly utilized microelectrode recordings from whole mount myenteric plexus preparations in guinea pigs. Here we have developed a Swiss-Webster mouse myenteric neuron culture and examined their electrophysiological properties by patch-clamp techniques and determined the mechanism for morphine-induced decrease in neuronal excitability. Isolated neurons in culture were confirmed by immunostaining with pan-neuronal marker, β-III tubulin and two populations were identified by calbindin and calretinin staining. Distinct neuronal populations were further identified based on the presence and absence of an afterhyperpolarization (AHP). Cells with AHP expressed greater density of sodium currents. Morphine (3 µM) significantly reduced the amplitude of the action potential, increased the threshold for spike generation but did not alter the resting membrane potential. The decrease in excitability resulted from inhibition of sodium currents. In the presence of morphine, the steady-state voltage dependence of Na channels was shifted to the left with almost 50% of channels unavailable for activation from hyperpolarized potentials. During prolonged exposure to morphine (two hours), action potentials recovered, indicative of the development of tolerance in single enteric neurons. These results demonstrate the feasibility of isolating mouse myenteric neurons and establish sodium channel inhibition as a mechanism for morphine-induced decrease in neuronal excitability

HIV-1 Tat exacerbates lipopolysaccharide-induced cytokine release via TLR4 signaling in the enteric nervous system

The loss of gut epithelium integrity leads to translocation of microbes and microbial products resulting in immune activation and drives systemic inflammation in acquired immunodeficiency syndrome (AIDS) patients. Although viral loads in HIV patients are significantly reduced in the post-cART era, inflammation and immune activation persist and can lead to morbidity. Here, we determined the interactive effects of the viral protein HIV-1 Tat and lipopolysaccharide (LPS) on enteric neurons and glia. Bacterial translocation was significantly enhanced in Tat-expressing (Tat+) mice. Exposure to HIV-1 Tat in combination with LPS enhanced the expression and release of the pro-inflammatory cytokines IL-6, IL-1β and TNF-α in the ilea of Tat+ mice and by enteric glia. This coincided with enhanced NF-κB activation in enteric glia that was abrogated in glia from TLR4 knockout mice and by knockdown (siRNA) of MyD88 siRNA in wild type glia. The synergistic effects of Tat and LPS resulted in a reduced rate of colonic propulsion in Tat+ mice treated with LPS. These results show that HIV-1 Tat interacts with the TLR4 receptor to enhance the pro-inflammatory effects of LPS leading to gastrointestinal dysmotility and enhanced immune activation

Specific Localization of β-Arrestin2 in Myenteric Plexus of Mouse Gastrointestinal Tract

Abstract β-arrestin2 is a key molecule involved in signaling and internalization of activated G protein-coupled receptors including µ-opioid receptors (MOR). Previously we have shown that decreased expression of β-arrestin2 upon chronic morphine is associated with the development of opioid tolerance in the gastrointestinal tract. However, the localization of β-arrestin2 within the gastrointestinal wall is not known. In this study we found that β-arrestin2 is localized in the soma of a select group of neurons in the myenteric ganglia but not in smooth muscle. The density of β-arestin2 was significantly higher in the ileum than the colon. We identified four variants of β-arrestin2 in the ileum, with ARRB-005 and ARRB-013 being the most abundant. Further, the current study utilized multiple-labeling immunofluorescence to characterize the chemical coding of neurons expressing β-arrestin2 in the murine myenteric plexus and the co-localization of MOR1 and β-arrestin2. β-arrestin2 co-localized with choline acetyltransferase and calretinin. In contrast, β-arrestin2 neither co-localized with substance P, nitric oxide synthase nor calbindin. Genetic deletion of β-arrestin2 did not affect cholinergic neuron activation by nicotine in the isolated ileum (-log M EC50: wild type = 5.8 vs. β-arrestin2 knockout = 5.9). Our findings suggest specificity in the localization of β-arrestin2 in the myenteric plexus within MOR1-expressing neurons and provide a relation for direct intracellular crosstalk between MOR1 receptor activation and β-arrestin2 signaling in the myenteric neurons. β-arrestin2 deletion does not directly alter basal enteric cholinergic neuronal function

Sex differences and drug dose influence the role of the alpha 7 nicotinic acetylcholine receptor in the mouse dextran sodium sulfate-induced colitis model

Introduction: alpha 7 nicotinic acetylcholine receptors (nAChRs) play an important role in vagus nerve-based cholinergic anti-inflammatory effects. This study was designed to assess the role of alpha 7 nAChRs in dextran sodium sulfate (DSS)-induced colitis in male and female mouse. We first compared disease activity and pathogenesis of colitis in alpha 7 knockout and wild-type mice. We then evaluated the effect of several alpha 7 direct and indirect agonists on the severity of disease in the DSS-induced colitis. Methods: Male and female adult mice were administered 2.5% DSS solution freely in the drinking water for 7 consecutive days and the colitis severity (disease activity index) was evaluated as well as colon length, colon histology, and levels of tumor necrosis factor-alpha colonic levels. Results: Male, but not female, alpha 7 knockout mice displayed a significantly increased colitis severity and higher tumor necrosis factor-alpha levels as compared with their littermate wild-type mice. Moreover, pretreatment with selective alpha 7 ligands PHA-543613, choline, and PNU-120596 decreased colitis severity in male but not female mice. The anti-colitis effects of these alpha 7 compounds dissipated when administered at higher doses. Conclusions: Our results suggest the presence of a alpha 7-dependent anti-colitis endogenous tone in male mice. Finally, our results show for the first time that female mice are less sensitive to the anticolitis activity of alpha 7 agonists. Ovarian hormones may play a key role in the sex difference effect of alpha 7 nAChRs modulation of colitis in the mouse. Implications: Our collective results suggest that targeting alpha 7 nAChRs could represent a viable therapeutic approach for intestinal inflammation diseases such as ulcerative colitis with the consideration of sex differences.United States Department of Health & Human Services National Institutes of Health (NIH) - USA - DA-019377 - DK046367United States Department of Health & Human Services National Institutes of Health (NIH) - USA NIH National Institute on Drug Abuse (NIDA) European Commission - R01DA03697

Up-regulation of brain-derived neurotrophic factor in primary afferent pathway regulates colon-to-bladder cross-sensitization in rat

Background In humans, inflammation of either the urinary bladder or the distal colon often results in sensory cross-sensitization between these organs. Limited information is known about the mechanisms underlying this clinical syndrome. Studies with animal models have demonstrated that activation of primary afferent pathways may have a role in mediating viscero-visceral cross-organ sensitization. Methods Colonic inflammation was induced by a single dose of tri-nitrobenzene sulfonic acid (TNBS) instilled intracolonically. The histology of the colon and the urinary bladder was examined by hematoxylin and eosin (H&E) stain. The protein expression of transient receptor potential (TRP) ion channel of the vanilloid type 1 (TRPV1) and brain-derived neurotrophic factor (BDNF) were examined by immunohistochemistry and/or western blot. The inter-micturition intervals and the quantity of urine voided were obtained from analysis of cystometrograms. Results At 3 days post TNBS treatment, the protein level of TRPV1 was increased by 2-fold (p \u3c 0.05) in the inflamed distal colon when examined with western blot. TRPV1 was mainly expressed in the axonal terminals in submucosal area of the distal colon, and was co-localized with the neural marker PGP9.5. In sensory neurons in the dorsal root ganglia (DRG), BDNF expression was augmented by colonic inflammation examined in the L1 DRG, and was expressed in TRPV1 positive neurons. The elevated level of BDNF in L1 DRG by colonic inflammation was blunted by prolonged pre-treatment of the animals with the neurotoxin resiniferatoxin (RTX). Colonic inflammation did not alter either the morphology of the urinary bladder or the expression level of TRPV1 in this viscus. However, colonic inflammation decreased the inter-micturition intervals and decreased the quantities of urine voided. The increased bladder activity by colonic inflammation was attenuated by prolonged intraluminal treatment with RTX or treatment with intrathecal BDNF neutralizing antibody. Conclusion Acute colonic inflammation increases bladder activity without affecting bladder morphology. Primary afferent-mediated BDNF up-regulation in the sensory neurons regulates, at least in part, the bladder activity during colonic inflammation