The beta3-adrenoceptor agonist GW427353 (Solabegron) decreases excitability of human enteric neurons via release of somatostatin

Drugs of the future for diarrhea-predominant irritable bowel syndrome: an overview of current investigational drugs

INTRODUCTION

Irritable bowel syndrome (IBS) has a significant impact on society and quality of life. Current treatments are ineffective, and new investigational drugs are necessary.

AREAS COVERED

Numerous potential therapies are developing, targeting different areas such as cannabinoid signaling, opioid receptors, tachykinin (NK2) receptors, β3-adrenergic receptors, intestinal microbiota, inflammation, and 5HT receptors. Clinical trial evidence has shown that loperamide, eluxadoline, alosetron, ramosetron, bile acid sequestrants, and rifaximin can modulate GI alterations and benefit patients with IBS-D. Among the potential therapies, ibodutant, ibudilast, blautix, BOS-589, solabegron, vibegron, olorinab, ebastine, and ORP-101 have demonstrated possible effects but remain confirmed.

EXPERT OPINION

Individuals with IBS-D require cost-effective treatment options that do not impede their productivity or that of their caregivers. This is necessary for consistent healthcare and improved quality of life. Therefore, we should focus on developing new, efficient, and affordable medications for IBS-D. The government, insurers, and society must recognize this need and collaborate to ensure its fulfillment.

Beta 3-adrenoceptor agonism ameliorates early-life stress-induced visceral hypersensitivity in male rats

Visceral hypersensitivity, a hallmark of disorders of the gut-brain axis, is associated with exposure to early-life stress (ELS). Activation of neuronal β3-adrenoceptors (AR) has been shown to alter central and peripheral levels of tryptophan and reduce visceral hypersensitivity. In this study, we aimed to determine the potential of a β3-AR agonist in reducing ELS-induced visceral hypersensitivity and possible underlying mechanisms. Here, ELS was induced using the maternal separation (MS) model, where Sprague Dawley rat pups were separated from their mother in early life (postnatal day 2-12). Visceral hypersensitivity was confirmed in adult offspring using colorectal distension (CRD). CL-316243, a β3-AR agonist, was administered to determine anti-nociceptive effects against CRD. Distension-induced enteric neuronal activation as well as colonic secretomotor function were assessed. Tryptophan metabolism was determined both centrally and peripherally. For the first time, we showed that CL-316243 significantly ameliorated MS-induced visceral hypersensitivity. Furthermore, MS altered plasma tryptophan metabolism and colonic adrenergic tone, while CL-316243 reduced both central and peripheral levels of tryptophan and affected secretomotor activity in the presence of tetrodotoxin. This study supports the beneficial role of CL-316243 in reducing ELS-induced visceral hypersensitivity, and suggests that targeting the β3-AR can significantly influence gut-brain axis activity through modulation of enteric neuronal activation, tryptophan metabolism, and colonic secretomotor activity which may synergistically contribute to offsetting the effects of ELS.

The treatment of neurogenic lower urinary tract dysfunction in persons with spinal cord injury: An open label, pilot study of anticholinergic agent vs. mirabegron to evaluate cognitive impact and efficacy

STUDY DESIGN

Pre-post intervention.

OBJECTIVES

1. To test whether replacement of oral anticholinergic (AC) agents with mirabegron for neurogenic lower urinary tract dysfunction (NLUTD) yields improved cognitive function in older persons with spinal cord injury (SCI). 2. To test whether mirabegron is safe and as efficacious as AC.

SETTING

USA.

METHODS

Pilot study: Twenty older (>60 y/o) persons with SCI taking chronic (>6 months) AC medication for NLUTD were enrolled. All participants were first studied on AC at baseline then switched to mirabegron for 6 months. Primary outcomes were cognitive tests of (1) executive function (TEXAS, SDMT); (2) attention (SCWT); and (3) memory (SLUMS and WMS-IV Story A/B). Secondary outcomes assessed efficacy and safety including Neurogenic Bladder Symptom Score (NBSS), bladder diary, neurogenic bowel dysfunction (NBD) survey, heart rate (HR), electrocardiogram (EKG), and mean arterial pressure (MAP).

RESULTS

When switching from AC to mirabegron for NLUTD, older persons with SCI exhibited statistically significant improvements in immediate Story A recall (p = 0.01), delayed story A and B recall (p = 0.01, 0.004), and in TEXAS (p = 0.04). Three subscores within NBSS significantly improved (p = 0.001) and the frequency of incontinence decreased (p = 0.03) on mirabegron. NBD, HR, MAP, and EKGs were unchanged.

CONCLUSIONS

Older persons with SCI on AC for NLUTD demonstrated improved short-term and delayed memory (WMS-IV Story A/B) as well as executive function (TEXAS) when switched to mirabegron. Efficacy of mirabegron for NLUTD symptoms was superior to AC with no adverse effects on bowel or cardiovascular function.

SPONSORSHIP

Claude D. Pepper Older Americans Independence Center.

Single-cell transcriptomics and functional target validation of brown adipocytes show their complex roles in metabolic homeostasis

Brown adipocytes (BAs) are specialized for adaptive thermogenesis and, upon sympathetic stimulation, activate mitochondrial uncoupling protein (UCP)-1 and oxidize fatty acids to generate heat. The capacity for brown adipose tissue (BAT) to protect against obesity and metabolic disease is recognized, yet information about which signals activate BA, besides β3-adrenergic receptor stimulation, is limited. Using single-cell transcriptomics, we confirmed the presence of mRNAs encoding traditional BAT markers (i.e., UCP1, expressed in 100% of BAs Adrb3, expressed in <50% of BAs) in mouse and have shown single-cell variability (>1000-fold) in their expression at both the mRNA and protein levels. We further identified mRNAs encoding novel markers, orphan GPCRs, and many receptors that bind the classic neurotransmitters, neuropeptides, chemokines, cytokines, and hormones. The transcriptome variability between BAs suggests a much larger range of responsiveness of BAT than previously recognized and that not all BAs function identically. We examined the in vivo functional expression of 12 selected receptors by microinjecting agonists into live mouse BAT and analyzing the metabolic response. In this manner, we expanded the number of known receptors on BAs at least 25-fold, while showing that the expression of classic BA markers is more complex and variable than previously thought.

Potential Causes and Present Pharmacotherapy of Irritable Bowel Syndrome: An Overview

BACKGROUND

Irritable bowel syndrome (IBS) is currently one of the most common disorders of the digestive system in the Western society. Almost 2 out of 10 people suffer from IBS with women being more affected than men. IBS is associated with abdominal pain, bloating and altered stool consistency and imposes a heavy burden for the affected patients.

SUMMARY

The pathophysiology of IBS remains elusive although potential causes have been suggested, such as a deranged brain-gut signaling, hypersensitivity of visceral sensory afferent fibers, bacterial gastroenteritis, small intestinal bacterial overgrowth (SIBO), genetic alterations and food sensitivity. Targets for the pharmacotherapy of IBS include the serotonergic and opioidergic system, and the microbial population of the gut. Alternative therapies like traditional Chinese medicine have shown some success in the combat against IBS. Key Messages: Many therapeutics for the treatment of IBS have emerged in the past; however, only a few have met up with the expectations in larger clinical trials. Additionally, the multifactorial etiology of IBS and its variety of cardinal symptoms requires an individual set of therapeutics. This review provides a short overview of potential causes and current pharmacological therapeutics and of additional and alternative therapies for IBS.

Neuropharmacology of purinergic receptors in human submucous plexus: Involvement of P2X₁, P2X₂, P2X₃ channels, P2Y and A₃ metabotropic receptors in neurotransmission

RATIONALE

The role of purinergic signaling in human ENS is not well understood. We sought to further characterize the neuropharmacology of purinergic receptors in human ENS and test the hypothesis that endogenous purines are critical regulators of neurotransmission.

EXPERIMENTAL APPROACH

LSCM-Fluo-4/(Ca(2+))-imaging of postsynaptic Ca(2+) transients (PSCaTs) was used as a reporter of synaptic transmission evoked by fiber tract electrical stimulation in human SMP surgical preparations. Pharmacological analysis of purinergic signaling was done in 1,556 neurons (identified by HuC/D-immunoreactivity) in 235 ganglia from 107 patients; P2XR-immunoreactivity was evaluated in 19 patients. Real-time MSORT (Di-8-ANEPPS) imaging tested effects of adenosine on fast excitatory synaptic potentials (fEPSPs).

RESULTS

Synaptic transmission is sensitive to pharmacological manipulations that alter accumulation of extracellular purines: Apyrase blocks PSCaTs in a majority of neurons. An ecto-NTPDase-inhibitor 6-N,N-diethyl-D-β,γ-dibromomethyleneATP or adenosine deaminase augments PSCaTs. Blockade of reuptake/deamination of eADO inhibits PSCaTs. Adenosine inhibits fEPSPs and PSCaTs (IC50 = 25 µM), sensitive to MRS1220-antagonism (A3AR). A P2Y agonist ADPβS inhibits PSCaTs (IC50 = 111 nM) in neurons without stimulatory ADPbS responses (EC50 = 960 nM). ATP or a P2X1,2,2/3 (α,β-MeATP) agonist evokes fast, slow, biphasic Ca(2+) transients or Ca(2+) oscillations (ATP,EC50 = 400 mM). PSCaTs are sensitive to P2X1 antagonist NF279. Low (20 nM) or high (5 µM) concentrations of P2X antagonist TNP-ATP block PSCaTs in different neurons; proportions of neurons with P2XR-immunoreactivity follow the order P2X2 > P2X1 >> P2X3; P2X1 + P2X2 and P2X3 + P2X2 are co-localized. RT-PCR identified mRNA-transcripts for P2X1-7, P2Y1,2,12-14R.

CONCLUSIONS

Purines are critical regulators of neurotransmission in human ENS. Purinergic signaling involves P2X1, P2X2, P2X3 channels, P2X1 + P2X2 co-localization and inhibitory P2Y or A3 receptors. These are potential novel therapeutic targets for neurogastroenterology.

Nutrient-induced changes in the phenotype and function of the enteric nervous system

The enteric nervous system (ENS) integrates numerous sensory signals in order to control and maintain normal gut functions. Nutrients are one of the prominent factors which determine the chemical milieu in the lumen and, after absorption, also within the gut wall. This review summarizes current knowledge on the impact of key nutrients on ENS functions and phenotype, covering their acute and long-term effects. Enteric neurones contain the molecular machinery to respond specifically to nutrients. These transporters and receptors are not expressed exclusively in the ENS but are also present in other cells such as enteroendocrine cells (EECs) and extrinsic sensory nerves, signalling satiety or hunger. Glucose, amino acids and fatty acids all activate enteric neurones, as suggested by enhanced c-Fos expression or spike discharge. These excitatory effects are the result of a direct neuronal activation but also involve the activation of EECs which, upon activation by luminal nutrients, release mediators such as ghrelin, cholecystokinin or serotonin. The presence or absence of nutrients in the intestinal lumen induces long-term changes in neurotransmitter expression, excitability, neuronal survival and ultimately impact upon gut motility, secretion or intestinal permeability. Together with EECs and vagal nerves, the ENS must be recognized as an important player initiating concerted responses to nutrients. It remains to be studied how, for instance, nutrient-induced changes in the ENS may influence additional gut functions such as intestinal barrier repair, intestinal epithelial stem cell proliferation/differentiation and also the signalling of extrinsic nerves to brain regions which control food intake.

Translational neuropharmacology: the use of human isolated gastrointestinal tissues

Translational sciences increasingly emphasize the measurement of functions in native human tissues. However, such studies must confront variations in patient age, gender, genetic background and disease. Here, these are discussed with reference to neuromuscular and neurosecretory functions of the human gastrointestinal (GI) tract. Tissues are obtained after informed consent, in collaboration with surgeons (surgical techniques help minimize variables) and pathologists. Given the difficulties of directly recording from human myenteric neurones (embedded between muscle layers), enteric motor nerve functions are studied by measuring muscle contractions/relaxations evoked by electrical stimulation of intrinsic nerves; responses are regionally dependent, often involving cholinergic and nitrergic phenotypes. Enteric sensory functions can be studied by evoking the peristaltic reflex, involving enteric sensory and motor nerves, but this has rarely been achieved. As submucosal neurones are more accessible (after removing the mucosa), direct neuronal recordings are possible. Neurosecretory functions are studied by measuring changes in short-circuit current across the mucosa. For all experiments, basic questions must be addressed. Because tissues are from patients, what are the controls and the influence of disease? How long does it take before function fully recovers? What is the impact of age- and gender-related differences? What is the optimal sample size? Addressing these and other questions minimizes variability and raises the scientific credibility of human tissue research. Such studies also reduce animal use. Further, the many differences between animal and human GI functions also means that human tissue research must question the ethical validity of using strains of animals with unproved translational significance.

Expression of β2 adrenoceptors within enteric neurons of the horse ileum

The activity of the gastrointestinal tract is regulated through the activation of adrenergic receptors (ARs). Since data concerning the distribution of ARs in the horse intestine is virtually absent, we investigated the distribution of β2-AR in the horse ileum using double-immunofluorescence. The β2-AR-immunoreactivity (IR) was observed in most (95%) neurons located in submucosal plexus (SMP) and in few (8%) neurons of the myenteric plexus (MP). Tyrosine hydroxylase (TH)-IR fibers were observed close to neurons expressing β2-AR-IR. Since β2-AR is virtually expressed in most neurons located in the horse SMP and in a lower percentage of neurons in the MP, it is reasonable to retain that this adrenergic receptor could regulate the activity of both secretomotor neurons and motor neurons innervating muscle layers and blood vessels. The high density of TH-IR fibers near β2-AR-IR enteric neurons indicates that the excitability of these cells could be directly modulated by the sympathetic system.

Leptin modulates enteric neurotransmission in the rat proximal colon: an in vitro study

Leptin has been shown to modulate gastrointestinal functions including nutrient absorption, growth, and inflammation and to display complex effects on gut motility. Leptin receptors have also been identified within the enteric nervous system (ENS), which plays a crucial role in digestive functions. Although leptin has recently been shown to activate neurons in the ENS, the precise mechanisms involved are so far unknown. Therefore, the aim of the present study was to determine the effects of leptin on rat proximal colon smooth muscle and enteric neuron activities. The effects of exogenous leptin on tone and on responses to transmural nerve stimulation (TNS) of isolated circular smooth muscle of proximal colon in rats were investigated using an organ bath technique. The effects of a physiological concentration (0.1 μM) of leptin were also studied on tone and TNS-induced relaxation in the presence of atropine, hexamethonium, L-N(G)-nitroarginine methyl ester (L-NAME) and capsazepine. Leptin caused a slight but significant decrease in tone, TNS-induced relaxation and contraction in a concentration-dependent manner in colonic preparations. Cholinergic antagonists abolished the effects of 0.1 μM leptin on TNS-induced relaxation. This concentration of leptin had no further effect on relaxation in the presence of L-NAME. In the presence of capsazepine, leptin had no further effect either on tone or relaxation compared to the drug alone. In conclusion, leptin modulates the activity of enteric inhibitory and excitatory neurons in proximal colon. These effects may be mediated through nitrergic neurons. Intrinsic primary afferent neurons may be involved.

Activity of protease-activated receptors in the human submucous plexus

BACKGROUND & AIMS

Protease-activated receptors (PARs) are expressed in the enteric nervous system. Excessive release of proteases has been reported in functional and inflammatory bowel diseases. Studies in several animal models indicate the involvement of neural PARs. We studied the actions of different PAR-activating peptides (AP) in the human submucous plexus and performed comparative studies in guinea pig submucous neurons.

METHODS

We used voltage- and calcium-sensitive dye recordings to study the effects of PAR1-AP, PAR2-AP, PAR4-AP, the PAR1 activator thrombin, and the PAR2 activator tryptase on neurons and glia in human and guinea pig submucous plexus. Human preparations were derived from surgical resections. Levels of mucosal secretion evoked by PAR-APs were measured in Ussing chambers.

RESULTS

PAR1-AP and thrombin evoked a prominent spike discharge and intracellular Ca(2+) concentration ([Ca](i)) transients in most human submucous neurons and glia. PAR2-AP, tryptase, and PAR4-AP caused significantly weaker responses in a minor population. In contrast, PAR2-AP evoked much stronger responses in enteric neurons and glia of guinea pigs than did PAR1-AP or PAR4-AP. PAR1-AP, but not PAR2-AP or PAR4-AP, evoked a nerve-mediated secretion in human epithelium. The PAR1 antagonist SCH79797 inhibited the PAR1-AP, and thrombin evoked responses on neurons, glia, and epithelial secretion. In the submucous layer of human intestine, but not guinea pig intestine, PAR2-AP evoked [Ca](i) signals in CD68(+) macrophages.

CONCLUSIONS

In the human submucous plexus, PAR1, rather than PAR2 or PAR4, activates nerves and glia. These findings indicate that PAR1 should be the focus of future studies on neural PAR-mediated actions in the human intestine; PAR1 might be developed as a therapeutic target for gastrointestinal disorders associated with increased levels of proteases.

Fast calcium and voltage-sensitive dye imaging in enteric neurones reveal calcium peaks associated with single action potential discharge

Slow changes in [Ca(2+)](i) reflect increased neuronal activity. Our study demonstrates that single-trial fast [Ca(2+)](i) imaging (≥200 Hz sampling rate) revealed peaks each of which are associated with single spike discharge recorded by consecutive voltage-sensitive dye (VSD) imaging in enteric neurones and nerve fibres. Fast [Ca(2+)](i) imaging also revealed subthreshold fast excitatory postsynaptic potentials. Nicotine-evoked [Ca(2+)](i) peaks were reduced by -conotoxin and blocked by ruthenium red or tetrodotoxin. Fast [Ca(2+)](i) imaging can be used to directly record single action potentials in enteric neurones. [Ca(2+)](i) peaks required opening of voltage-gated sodium and calcium channels as well as Ca(2+) release from intracellular stores.

Leptin excites enteric neurons of guinea-pig submucous and myenteric plexus

BACKGROUND

Leptin, one of the most prominent mediators released from adipocytes, influences neuronal activity in the central nervous system. The enteric nervous system (ENS) expresses leptin receptors but consequence of activation of these receptors on enteric neuron activity has not been systematically studied. An adipocyte-ENS axis is suggested by close apposition between enteric nerves and adipocytes. The aim of this study was to investigate the effects of leptin on guinea-pig submucous and myenteric neurons.

METHODS

Using voltage sensitive dye imaging, we recorded neural responses to application of leptin (0.0625 nmol L(-1)) in myenteric and submucous neurons, nicotine (10 μmol L(-1)) served as a reference for neuronal excitation. Mucosal ion secretion and muscle activity were measured in vitro with Ussing and organ bath techniques, respectively.

KEY RESULTS

Leptin induced spike discharge in 13.6% of submucous neurons and in 8.2% of myenteric neurons (1.1 ± 0.9 and 1.2 ± 1.0 Hz, respectively). Although there was an overlap of nicotine and leptin responses, 38.5% of submucous and 25% of myenteric neurons activated by leptin did not respond to nicotine. Leptin did not inhibit ongoing spike discharge or fast excitatory postsynaptic potentials. Leptin (0.0625 nmol L(-1)) did not affect mucosal secretion or muscle activity suggesting a subtle modulatory action of leptin at the level of the ENS.

CONCLUSIONS & INFERENCES

Leptin activates submucous and myenteric neurons indicating relevance for adipocyte-ENS signaling. These results set the basis for further studies to reveal the functional correlate of the neural action of leptin in the ENS.

Maternal separation as a model of brain-gut axis dysfunction

RATIONALE

Early life stress has been implicated in many psychiatric disorders ranging from depression to anxiety. Maternal separation in rodents is a well-studied model of early life stress. However, stress during this critical period also induces alterations in many systems throughout the body. Thus, a variety of other disorders that are associated with adverse early life events are often comorbid with psychiatric illnesses, suggesting a common underlying aetiology. Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder that is thought to involve a dysfunctional interaction between the brain and the gut. Essential aspects of the brain-gut axis include spinal pathways, the hypothalamic pituitary adrenal axis, the immune system, as well as the enteric microbiota. Accumulating evidence suggest that stress, especially in early life, is a predisposing factor to IBS.

OBJECTIVE

The objective of this review was to assess and compile the most relevant data on early life stress and alterations at all levels of the brain gut axis.

RESULTS

In this review, we describe the components of the brain-gut axis individually and how they are altered by maternal separation. The separated phenotype is characterised by alterations of the intestinal barrier function, altered balance in enteric microflora, exaggerated stress response and visceral hypersensitivity, which are all evident in IBS.

CONCLUSION

Thus, maternally separated animals are an excellent model of brain-gut axis dysfunction for the study of disorders such as IBS and for the development of novel therapeutic interventions.

Expression and activation of β-adrenoceptors in the colorectal mucosa of rat and human

BACKGROUND

The functions of the distal colon are regulated by local and extrinsic neural pathways. In previous studies, we have found that dopamine (DA) and norepinephrine (NE) could evoke colonic ion transport by activating β-adrenoceptors. The present study aims to investigate the segmental differences in expression and activation of β-adrenoceptors in the distal colon in physiological and pathophysiological conditions.

METHODS

Real-time PCR, immunofluorescence, and Western blotting were used to detect the expression of β-adrenoceptors in the rat and human distal colon. Short-circuit current measurements (Isc) were used to assess the role of β-adrenoceptors in ion transport.

KEY RESULTS

DA and NE caused greater suppression of baseline Isc in distal colon adjacent to the rectum than in segments further away from the anus. These responses were inhibited by selective antagonists of β₁- and β₂-adrenoceptors, but not β₃-adrenoceptor. The expression levels of β₁- and β₂-adrenoceptors in colonic mucosa were higher in colorectum than the regions away from the anus of rats and humans. In wrap-restraint stress (2 h), DA-, NE-induced ΔIsc and the expression of β-adrenoceptors in the colorectum were significantly reduced. However, when endogenous catecholamines were depleted by 6-hydroxydopamine (75 mg kg(-1), i.p., 3 days), DA-, NE-induced ΔIsc as well as the expression of β-adrenoceptors were significantly enhanced in the rat colorectum but not in more proximal regions of the distal colon.

CONCLUSIONS & INFERENCES

β₁- and β₂-adrenoceptors are predominantly expressed in the colorectal mucosa. Perturbation of endogenous catecholamine levels influences the expression and activation of β-adrenoceptors in the colorectal region.

Relaxation by beta 3-adrenoceptor agonists of the isolated human internal anal sphincter

AIMS

In this study, responses of beta(3)-adrenoceptor agonists were examined on human isolated internal anal sphincter (IAS) in order to explore their relaxant effects on hypertonicity of IAS.

MAIN METHODS

The relaxant efficacy (E(max)) and potency (-logIC(50)) of BRL37344 and SR58611A, beta(3)-adrenoceptor agonists, were examined in contracted IAS muscle strips. The presence of beta(3)-adrenoceptors, and changes in intracellular calcium and cyclic nucleotide levels in IAS muscle were tested by Western blotting, epifluorescence microscopy and enzyme immunoassay, respectively.

KEY FINDINGS

BRL37344 and SR58611A relaxed contracted IAS muscle (E(max)=27+/-3% and 35+/-3%; -logIC(50)=6.26+/-0.24 and 4.87+/-0.13; respectively). These relaxant responses were blocked by SR59230A, a selective beta(3)-antagonist but not by beta(1)/beta(2)-selective antagonists, neuronal inhibitor or inhibition of nitric oxide synthase. The E(max) of beta(3)-agonists was similar to that of beta(2)-selective agonists but smaller than that of isoprenaline (nonselective agonist) or beta(1)-selective agonists. BRL37344 (100 microM) increased cAMP (1.5-fold) without cGMP change, and depressed intracellular calcium signal. beta(3)-Adrenoceptor expression was smaller than that of beta(1)- and beta(2)-adrenoceptors.

SIGNIFICANCE

This is the first study demonstrating the presence of beta(3)-adrenoceptor in human IAS muscle and beta(3)-mediated relaxation of augmented sphincter tone. However, direct beta(3)-relaxation appears smaller than that obtained for nonselective agonists which may limit their potential use in the treatment of anorectal hypertonicity disorders.