Dopamine and ghrelin receptor co-expression and interaction in the spinal defecation centers

Physiological roles of ghrelin in the regulation of gastrointestinal motility in vertebrates

Ghrelin is known to be a multifunctional peptide hormone that stimulates not only growth hormone secretion and feeding but also gastrointestinal (GI) functions, including motility, secretion and mucosa proliferation. The aim of this review is to provide a comprehensive overview on the physiological roles of ghrelin in the regulation of GI motility from a comparative perspective. The effects of ghrelin on GI motility differ depending on the species, and ghrelin is a possible regulator of gastric migrating motor complexes (MMCs) in rodents, dogs and house musk shrew (suncus). However, the role of ghrelin has not been clarified in detail in other mammals, including humans and rabbits. Ghrelin is also effective to cause contraction in the GI tract of some non-mammals, but its physiological role is also not clarified at present. Distribution of the growth hormone secretagogue receptor (GHSR, ghrelin receptor) in the GI tract might be connected with the regulatory role of ghrelin in vertebrates. Comparative studies of ghrelin among animals and identification of knowledge gaps must lead us to the functional transition and importance of ghrelin in the GI tract.

Optogenetic Control of Dopamine Receptor 2 Reveals a Novel Aspect of Dopaminergic Neurotransmission in Motor Function

Dopaminergic neurotransmission plays a crucial role in motor function through the coordination of dopamine receptor (DRD) subtypes, such as DRD1 and DRD2, thus the functional imbalance of these receptors can lead to Parkinson's disease. However, due to the complexity of dopaminergic circuits in the brain, it is limited to investigating the individual functions of each DRD subtype in specific brain regions. Here, we developed a light-responsive chimeric DRD2, OptoDRD2, which can selectively activate DRD2-like signaling pathways with spatiotemporal resolution. OptoDRD2 was designed to include the light-sensitive component of rhodopsin and the intracellular signaling domain of DRD2. Upon illumination with blue light, OptoDRD2 triggered DRD2-like signaling pathways, such as Gαi/o subtype recruitment, a decrease in cAMP levels, and ERK phosphorylation. To explore unknown roles of DRD2 in glutamatergic cell populations of basal ganglia circuitry, OptoDRD2 was genetically expressed in excitatory neurons in lateral globus pallidus (LGP) of the male mouse brain. The optogenetic stimulation of OptoDRD2 in the LGP region affected a wide range of locomotion-related parameters, such as increased frequency of movement and decreased immobility time, resulting in the facilitation of motor function of living male mice. Therefore, our findings indicate a potentially novel role for DRD2 in the excitatory neurons of the LGP region, suggesting that OptoDRD2 can be a valuable tool enabling the investigation of unknown roles of DRD2 at specific cell types or brain regions.

Lipopolysaccharide accelerates peristalsis by stimulating glucagon-like peptide-1 release from L cells in the rat proximal colon

Upon epithelial barrier dysfunction, lipopolysaccharide (LPS) stimulates glucagon-like peptide-1 (GLP-1) secretion from enteroendocrine L cells by activating Toll-like receptor 4 (TLR4). Because GLP-1 accelerates peristalsis in the proximal colon, the present study aimed to explore whether LPS facilitates colonic peristalsis by stimulating L cell-derived GLP-1 release. In isolated segments of rat proximal colon that were serosally perfused with physiological salt solution and luminally perfused with 0.9% saline, peristaltic wall motion was video recorded and converted into spatio-temporal maps. Fluorescence immunohistochemistry was also carried out. Intraluminal administration of LPS (100 or 1 µg mL-1 but not 100 ng mL-1) increased the frequency of oro-aboral propagating peristaltic contractions. The LPS-induced acceleration of colonic peristalsis was blocked by TAK-242 (the TLR4 antagonist), exendin-3 (the GLP-1 receptor antagonist) or BIBN4096 (the calcitonin gene-related peptide receptor antagonist). GLP-1-positive epithelial cells co-expressed TLR4 immunoreactivity. In aspirin-pretreated preparations where epithelial barrier function had been impaired, a lower dose of LPS (100 ng mL-1) became capable of accelerating peristalsis. By contrast, luminally applied dimethyl sulphoxide, a reactive oxygen species scavenger that protects epithelial integrity, attenuated the prokinetic effects of a higher dose of LPS (100 µg mL-1). In colonic segments of a stress rat model leading to a leaky gut, LPS induced more pronounced prokinetic effects. Colonic L cells may well sense luminal LPS via TLR4 triggering the release of GLP-1 that stimulates calcitonin gene-related peptide-containing neurons. The resultant acceleration of peristalsis would facilitate excretion of Gram-negative bacteria from the intestine, and thus L cells may have a protective role against intestinal bacterial infections. KEY POINTS: Colonic epithelial cells form a barrier against bacterial invasion but also may contribute more actively to the exclusion of luminal pathogen by stimulating colonic motility. Luminal lipopolysaccharide (LPS) accelerated colonic peristalsis by stimulating calcitonin gene-related peptide-containing neurons. The prokinetic effect of LPS was mediated by the secretion of glucagon-like peptide-1 from enteroendocrine L cells in which Toll-like receptor 4 was expressed. The LPS-mediated acceleration of peristalsis depended on epithelial barrier integrity. L cells have a defensive role against Gram-negative bacterial infections by facilitating faecal excretion, and could be a potential therapeutic target for gastrointestinal infections.

Sites and mechanisms of action of colokinetics at dopamine, ghrelin and serotonin receptors in the rodent lumbosacral defecation centre

Agonists of dopamine D2 receptors (D2R), 5-hydroxytryptamine (5-HT, serotonin) receptors (5-HTR) and ghrelin receptors (GHSR) activate neurons in the lumbosacral defecation centre, and act as 'colokinetics', leading to increased propulsive colonic motility, in vivo. In the present study, we investigated which neurons in the lumbosacral defecation centre express the receptors and whether dopamine, serotonin and ghrelin receptor agonists act on the same lumbosacral preganglionic neurons (PGNs). We used whole cell electrophysiology to record responses from neurons in the lumbosacral defecation centre, following colokinetic application, and investigated their expression profiles and the chemistries of their neural inputs. Fluorescence in situ hybridisation revealed Drd2, Ghsr and Htr2C transcripts were colocalised in lumbosacral PGNs of mice, and immunohistochemistry showed that these neurons have closely associated tyrosine hydroxylase and 5-HT boutons. Previous studies showed that they do not receive ghrelin inputs. Whole cell electrophysiology in adult mice spinal cord revealed that dopamine, serotonin, α-methylserotonin and capromorelin each caused inward, excitatory currents in overlapping populations of lumbosacral PGNs. Furthermore, dopamine caused increased frequency of both IPSCs and EPSCs in a cohort of D2R neurons. Tetrodotoxin blocked the IPSCs and EPSCs, revealing a post-synaptic excitatory action of dopamine. In lumbosacral PGNs of postnatal day 7-14 rats, only dopamine's postsynaptic effects were observed. Furthermore, inward, excitatory currents evoked by dopamine were reduced by the GHSR antagonist, YIL781. We conclude that lumbosacral PGNs are the site where the action of endogenous ligands of D2R and 5-HT2R converge, and that GHSR act as a cis-modulator of D2R expressed by the same neurons. KEY POINTS: Dopamine, 5-hydroxytryptamine (5-HT, serotonin) and ghrelin (GHSR) receptor agonists increase colorectal motility and have been postulated to act at receptors on parasympathetic preganglionic neurons (PGNs) in the lumbosacral spinal cord. We aimed to determine which neurons in the lumbosacral spinal cord express dopamine, serotonin and GHSR receptors, their neural inputs, and whether agonists at these receptors excite them. We show that dopamine, serotonin and ghrelin receptor transcripts are contained in the same PGNs and that these neurons have closely associated tyrosine hydroxylase and serotonin boutons. Whole cell electrophysiology revealed that dopamine, serotonin and GHSR receptor agonists induce an inward excitatory current in overlapping populations of lumbosacral PGNs. Dopamine-induced excitation was reversed by GHSR antagonism. The present study demonstrates that lumbosacral PGNs are the site at which actions of endogenous ligands of dopamine D2 receptors and 5-HT type 2 receptors converge. Ghrelin receptors are functional, but their role appears to be as modulators of dopamine effects at D2 receptors.

G protein-coupled receptor interactions and modification of signalling involving the ghrelin receptor, GHSR1a

The growth hormone secretagogue receptor 1a (GHSR1a) is intriguing because of its potential as a therapeutic target and its diverse molecular interactions. Initial studies of the receptor focused on the potential therapeutic ability for growth hormone (GH) release to reduce wasting in aging individuals, as well as food intake regulation for treatment of cachexia. Known roles of GHSR1a now extend to regulation of neurogenesis, learning and memory, gastrointestinal motility, glucose/lipid metabolism, the cardiovascular system, neuronal protection, motivational salience, and hedonic feeding. Ghrelin, the endogenous agonist of GHSR1a, is primarily located in the stomach and is absent from the central nervous system (CNS), including the spinal cord. However, ghrelin in the circulation does have access to a small number of CNS sites, including the arcuate nucleus, which is important in feeding control. At some sites, such as at somatotrophs, GHSR1a has high constitutive activity. Typically, ghrelin-dependent and constitutive GHSR1a activation occurs via Gα_q/11 pathways. In vitro and in vivo data suggest that GHSR1a heterodimerises with multiple G protein-coupled receptors (GPCRs), including dopamine D1 and D2, serotonin 2C, orexin, oxytocin and melanocortin 3 receptors (MCR3), as well as the MCR3 accessory protein, MRAP2, providing possible mechanisms for its many physiological effects. In all cases, the receptor interaction changes downstream signalling and the responses to receptor agonists. This review discusses the signalling mechanisms of GHSR1a alone and in combination with other GPCRs, and explores the physiological consequences of GHSR1a coupling with other GPCRs.

Targeting appetite and satiety in diabetes and obesity, via G protein-coupled receptors

Type 2 diabetes and obesity have reached pandemic proportions throughout the world, so much so that the World Health Organisation coined the term "Globesity" to help encapsulate the magnitude of the problem. G protein-coupled receptors (GPCRs) are highly tractable drug targets due to their wide involvement in all aspects of physiology and pathophysiology, indeed, GPCRs are the targets of approximately 30% of the currently approved drugs. GPCRs are also broadly involved in key physiologies that underlie type 2 diabetes and obesity including feeding reward, appetite and satiety, regulation of blood glucose levels, energy homeostasis and adipose function. Despite this, only two GPCRs are the target of approved pharmaceuticals for treatment of type 2 diabetes and obesity. In this review we discuss the role of these, and select other candidate GPCRs, involved in various facets of type 2 diabetic or obese pathophysiology, how they might be targeted and the potential reasons why pharmaceuticals against these targets have not progressed to clinical use. Finally, we provide a perspective on the current development pipeline of anti-obesity drugs that target GPCRs.

Interstitial cells of Cajal and human colon motility in health and disease

Our understanding of human colonic motility, and autonomic reflexes that generate motor patterns, has increased markedly through high-resolution manometry. Details of the motor patterns are emerging related to frequency and propagation characteristics that allow linkage to interstitial cells of Cajal (ICC) networks. In studies on colonic motor dysfunction requiring surgery, ICC are almost always abnormal or significantly reduced. However, there are still gaps in our knowledge about the role of ICC in the control of colonic motility and there is little understanding of a mechanistic link between ICC abnormalities and colonic motor dysfunction. This review will outline the various ICC networks in the human colon and their proven and likely associations with the enteric and extrinsic autonomic nervous systems. Based on our extensive knowledge of the role of ICC in the control of gastrointestinal motility of animal models and the human stomach and small intestine, we propose how ICC networks are underlying the motor patterns of the human colon. The role of ICC will be reviewed in the autonomic neural reflexes that evoke essential motor patterns for transit and defecation. Mechanisms underlying ICC injury, maintenance, and repair will be discussed. Hypotheses are formulated as to how ICC dysfunction can lead to motor abnormalities in slow transit constipation, chronic idiopathic pseudo-obstruction, Hirschsprung's disease, fecal incontinence, diverticular disease, and inflammatory conditions. Recent studies on ICC repair after injury hold promise for future therapies.

LEAP2 Impairs the Capability of the Growth Hormone Secretagogue Receptor to Regulate the Dopamine 2 Receptor Signaling

The growth hormone secretagogue receptor (GHSR) signals in response to ghrelin, but also acts via ligand-independent mechanisms that include either constitutive activation or interaction with other G protein-coupled receptors, such as the dopamine 2 receptor (D2R). A key target of GHSR in neurons is voltage-gated calcium channels type 2.2 (Ca_V2.2). Recently, the liver-expressed antimicrobial peptide 2 (LEAP2) was recognized as a novel GHSR ligand, but the mechanism of action of LEAP2 on GHSR is not well understood. Here, we investigated the role of LEAP2 on the canonical and non-canonical modes of action of GHSR on Ca_V2.2 function. Using a heterologous expression system and patch-clamp recordings, we found that LEAP2 impairs the reduction of Ca_V2.2 currents induced by ghrelin-evoked and constitutive GHSR activities, acting as a GHSR antagonist and inverse agonist, respectively. We also found that LEAP2 prevents GHSR from modulating the effects of D2R signaling on Ca_V2.2 currents, and that the GHSR-binding N-terminal region LEAP2 underlies these effects. Using purified labeled receptors assembled into lipid nanodiscs and Forster Resonance Energy Transfer (FRET) assessments, we found that the N-terminal region of LEAP2 stabilizes an inactive conformation of GHSR that is dissociated from Gq protein and, consequently, reverses the effect of GHSR on D2R-dependent Gi activation. Thus, our results provide critical molecular insights into the mechanism mediating LEAP2 modulation of GHSR.

Biased Ghrelin Receptor Signaling and the Dopaminergic System as Potential Targets for Metabolic and Psychological Symptoms of Anorexia Nervosa

Anorexia Nervosa (AN) is a complex disease that impairs the metabolic, mental and physiological health of affected individuals in a severe and sometimes lethal way. Many of the common symptoms in AN patients, such as reduced food intake, anxiety, impaired gut motility or overexercising are connected to both the orexigenic gut hormone ghrelin and the dopaminergic system. Targeting the ghrelin receptor (GhrR) to treat AN seems a promising possibility in current research. However, GhrR signaling is highly complex. First, the GhrR can activate four known intracellular pathways Gαq, Gαi/o, Gα12/13 and the recruitment of β-arrestin. Biased signaling provides the possibility to activate or inhibit only one or a subset of the intracellular pathways of a pleiotropic receptor. This allows specific targeting of physiological functions without adverse effects. Currently little is known on how biased signaling could specifically modulate GhrR effects. Second, GhrR signaling has been shown to be interconnected with the dopaminergic system, particularly in the context of AN symptoms. This review highlights that a biased agonist for the GhrR may be a promising target for the treatment of AN, however extensive and systematic translational studies are still needed and the connection to the dopaminergic system has to be taken into account.