Identification and regional distribution of the dopamine D(1A) receptor in the gastrointestinal tract

Understanding of Polydopamine Formulation for Oral Therapeutic Delivery in Ulcerative Colitis Treatment

Oral therapeutic delivery remains challenged by gastrointestinal tract (GI) barriers, which hinders the successful transition of therapeutic candidates into clinical treatments. Polydopamine (PDA), with its versatile ability to overcome GI barriers, offers a promising drug formulation technology to address the challenge. Nevertheless, many critical questions remain unanswered regarding the practicality of PDA‐based formulations. Building on the previous research, which tackled multiple physicochemical aspects, the current study aims to address another three outstanding issues, including the quantification of residual dopamine (DA) in PDA‐based formulations, the examination of these formulations stimulatory effects on colon tissue, and the potential anti‐inflammatory properties. To facilitate this investigation, a curcumin‐containing nanomedicine (CP@CCS) is prepared as a representative PDA‐based formulation. The results reveal a marked decrease of residual DA within the formulation. In the treatment of ulcerative colitis (UC), the formulation do not provoke the substantial contractions in colon tissue typically induced by DA. Furthermore, in vivo evaluation verified the supplementary anti‐UC benefits of PDA. These outcomes add evidence for the practicality of PDA‐based formulations in terms of safety and therapeutic efficacy. Finally, a conceptual framework is proposed for understanding the role of PDA in oral therapeutic delivery, thereby providing insightful directions for subsequent research.

Dopamine in the regulation of glucose and lipid metabolism: a narrative review

OBJECTIVE

Owing to the global obesity epidemic, understanding the regulatory mechanisms of glucose and lipid metabolism has become increasingly important. The dopaminergic system, including dopamine, dopamine receptors, dopamine transporters, and other components, is involved in numerous physiological and pathological processes. However, the mechanism of action of the dopaminergic system in glucose and lipid metabolism is poorly understood. In this review, we examine the role of the dopaminergic system in glucose and lipid metabolism.

RESULTS

The dopaminergic system regulates glucose and lipid metabolism through several mechanisms. It regulates various activities at the central level, including appetite control and decision-making, which contribute to regulating body weight and energy metabolism. In the pituitary gland, dopamine inhibits prolactin production and promotes insulin secretion through dopamine receptor 2. Furthermore, it can influence various physiological components in the peripheral system, such as pancreatic β cells, glucagon-like peptide-1, adipocytes, hepatocytes, and muscle, by regulating insulin and glucagon secretion, glucose uptake and use, and fatty acid metabolism.

CONCLUSIONS

The role of dopamine in regulating glucose and lipid metabolism has significant implications for the physiology and pathogenesis of disease. The potential therapeutic value of dopamine lies in its effects on metabolic disorders.

Systematic-Narrative Hybrid Literature Review: Crosstalk between Gastrointestinal Renin-Angiotensin and Dopaminergic Systems in the Regulation of Intestinal Permeability by Tight Junctions

In the first part of this article, the role of intestinal epithelial tight junctions (TJs), together with gastrointestinal dopaminergic and renin-angiotensin systems, are narratively reviewed to provide sufficient background. In the second part, the current experimental data on the interplay between gastrointestinal (GI) dopaminergic and renin-angiotensin systems in the regulation of intestinal epithelial permeability are reviewed in a systematic manner using the PRISMA methodology. Experimental data confirmed the copresence of DOPA decarboxylase (DDC) and angiotensin converting enzyme 2 (ACE2) in human and rodent enterocytes. The intestinal barrier structure and integrity can be altered by angiotensin (1-7) and dopamine (DA). Both renin-angiotensin and dopaminergic systems influence intestinal Na+/K+-ATPase activity, thus maintaining electrolyte and nutritional homeostasis. The colocalization of B0AT1 and ACE2 indicates the direct role of the renin-angiotensin system in amino acid absorption. Yet, more studies are needed to thoroughly define the structural and functional interaction between TJ-associated proteins and GI renin-angiotensin and dopaminergic systems.

Research progress on the roles of dopamine and dopamine receptors in digestive system diseases

Dopamine (DA) is a neurotransmitter synthesized in the human body that acts on multiple organs throughout the body, reaching them through the blood circulation. Neurotransmitters are special molecules that act as messengers by binding to receptors at chemical synapses between neurons. As ligands, they mainly bind to corresponding receptors on central or peripheral tissue cells. Signalling through chemical synapses is involved in regulating the activities of various body systems. Lack of DA or a decrease in DA levels in the brain can lead to serious diseases such as Parkinson's disease, schizophrenia, addiction and attention deficit disorder. It is widely recognized that DA is closely related to neurological diseases. As research on the roles of brain-gut peptides in human physiology and pathology has deepened in recent years, the regulatory role of neurotransmitters in digestive system diseases has gradually attracted researchers' attention, and research on DA has expanded to the field of digestive system diseases. This review mainly elaborates on the research progress on the roles of DA and DRs related to digestive system diseases. Starting from the biochemical and pharmacological properties of DA and DRs, it discusses the therapeutic value of DA- and DR-related drugs for digestive system diseases.

Bridging the Mind and Gut: Uncovering the Intricacies of Neurotransmitters, Neuropeptides, and their Influence on Neuropsychiatric Disorders

BACKGROUND

The gut-brain axis (GBA) is a bidirectional signaling channel that facilitates communication between the gastrointestinal tract and the brain. Recent research on the gut-brain axis demonstrates that this connection enables the brain to influence gut function, which in turn influences the brain and its cognitive functioning. It is well established that malfunctioning of this axis adversely affects both systems' ability to operate effectively.

OBJECTIVE

Dysfunctions in the GBA have been associated with disorders of gut motility and permeability, intestinal inflammation, indigestion, constipation, diarrhea, IBS, and IBD, as well as neuropsychiatric and neurodegenerative disorders like depression, anxiety, schizophrenia, autism, Alzheimer's, and Parkinson's disease. Multiple research initiatives have shown that the gut microbiota, in particular, plays a crucial role in the GBA by participating in the regulation of a number of key neurochemicals that are known to have significant effects on the mental and physical well-being of an individual.

METHODS

Several studies have investigated the relationship between neuropsychiatric disorders and imbalances or disturbances in the metabolism of neurochemicals, often leading to concomitant gastrointestinal issues and modifications in gut flora composition. The interaction between neurological diseases and gut microbiota has been a focal point within this research. The novel therapeutic interventions in neuropsychiatric conditions involving interventions such as probiotics, prebiotics, and dietary modifications are outlined in this review.

RESULTS

The findings of multiple studies carried out on mice show that modulating and monitoring gut microbiota can help treat symptoms of such diseases, which raises the possibility of the use of probiotics, prebiotics, and even dietary changes as part of a new treatment strategy for neuropsychiatric disorders and their symptoms.

CONCLUSION

The bidirectional communication between the gut and the brain through the gut-brain axis has revealed profound implications for both gastrointestinal and neurological health. Malfunctions in this axis have been connected to a range of disorders affecting gut function as well as cognitive and neuropsychiatric well-being. The emerging understanding of the role of gut microbiota in regulating key neurochemicals opens up possibilities for novel treatment approaches for conditions like depression, anxiety, and neurodegenerative diseases.

The critical role of gut-brain axis microbiome in mental disorders

The Gut-brain axis is a bidirectional neural and humoral signaling that plays an important role in mental disorders and intestinal health and connects them as well. Over the past decades, the gut microbiota has been explored as an important part of the gastrointestinal tract that plays a crucial role in the regulation of most functions of various human organs. The evidence shows several mediators such as short-chain fatty acids, peptides, and neurotransmitters that are produced by the gut may affect the brain's function directly or indirectly. Thus, dysregulation in this microbiome community can give rise to several diseases such as Parkinson's disease, depression, irritable bowel syndrome, and Alzheimer's disease. So, the interactions between the gut and the brain are significantly considered, and also it provides a prominent subject to investigate the causes of some diseases. In this article, we reviewed and focused on the role of the largest and most repetitive bacterial community and their relevance with some diseases that they have mentioned previously.

The multiple roles of dopamine receptor activation in the modulation of gastrointestinal motility and mucosal function

Dopamine (DA) is a catecholamine regulatory molecule with potential role in physiology and physiopathology of the intestinal tract. Various cellular sources of DA have been indicated as enteric neurons, immune cells, intestinal flora and gastrointestinal epithelium. Moreover, DA is produced by nutritional tyrosine. All the five DA receptors, actually described, are present throughout the gut. Current knowledge of DA in this area is reviewed, focusing on gastrointestinal function in health and during inflammation. Research on animal models and humans are reported. A major obstacle to understanding the physiologic and/or pharmacological roles of enteric DA is represented by the multiplicity of receptors involved in the responses together with many signalling pathways related to each receptor subtype. It is mandatory to map precisely the distributions of DA receptors, to determine the relevance of a receptor in a specific location in order to explore novel therapies directed to dopaminergic targets that may be useful in the control of intestinal inflammation.

Epigenetics in depression and gut-brain axis: A molecular crosstalk

Gut-brain axis is a dynamic, complex, and bidirectional communication network between the gut and brain. Changes in the microbiota-gut-brain axis are responsible for developing various metabolic, neurodegenerative, and neuropsychiatric disorders. According to clinical and preclinical findings, the gut microbiota is a significant regulator of the gut-brain axis. In addition to interacting with intestinal cells and the enteric nervous system, it has been discovered that microbes in the gut can modify the central nervous system through metabolic and neuroendocrine pathways. The metabolites of the gut microbiome can modulate a number of diseases by inducing epigenetic alteration through DNA methylation, histone modification, and non-coding RNA-associated gene silencing. Short-chain fatty acids, especially butyrate, are well-known histone deacetylases inhibitors. Similarly, other microbial metabolites such as folate, choline, and trimethylamine-N-oxide also regulate epigenetics mechanisms. Furthermore, various studies have revealed the potential role of microbiome dysbiosis and epigenetics in the pathophysiology of depression. Hence, in this review, we have highlighted the role of gut dysbiosis in epigenetic regulation, causal interaction between host epigenetic modification and the gut microbiome in depression and suggest microbiome and epigenome as a possible target for diagnosis, prevention, and treatment of depression.

Gut Inflammation Induced by Finasteride Withdrawal: Therapeutic Effect of Allopregnanolone in Adult Male Rats

The treatment with finasteride (i.e., an inhibitor of 5α-reductase) may be associated with different side effects (i.e., depression, anxiety, cognitive impairment and sexual dysfunction) inducing the so-called post finasteride syndrome (PFS). Moreover, previous observations in PFS patients and an experimental model showed alterations in gut microbiota populations, suggesting an inflammatory environment. To confirm this hypothesis, we have explored the effect of chronic treatment with finasteride (i.e., for 20 days) and its withdrawal (i.e., for 1 month) on the levels of steroids, neurotransmitters, pro-inflammatory cytokines and gut permeability markers in the colon of adult male rat. The obtained data demonstrate that the levels of allopregnanolone (ALLO) decreased after finasteride treatment and after its withdrawal. Following the drug suspension, the decrease in ALLO levels correlates with an increase in IL-1β and TNF-α, serotonin and a decrease in dopamine. Importantly, ALLO treatment is able to counteract some of these alterations. The relation between ALLO and GABA-A receptors and/or pregnenolone (ALLO precursor) could be crucial in their mode of action. These observations provide an important background to explore further the protective effect of ALLO in the PFS experimental model and the possibility of its translation into clinical therapy.

Small intestine neuromuscular dysfunction in a mouse model of dextran sulfate sodium-induced ileitis: Involvement of dopaminergic neurotransmission

AIMS

Anomalies in dopaminergic machinery have been shown in inflammatory bowel disease (IBD) patients and preclinical models of IBD. Thus, we aimed to evaluate the impact of dextran sodium sulfate (DSS)-induced ileitis on enteric dopaminergic pathways.

MATERIALS AND METHODS

Male C57/Bl6 mice (10 ± 2 weeks old) received 2% DSS in drinking water for 5 days and were then switched to regular drinking water for 3 days. To measure ileitis severity inflammatory cytokines (IL-1β, TNFα, IL-6) levels were assessed. Changes in ileal muscle tension were isometrically recorded following: 1) cumulative addition of dopamine on basal tone (0.1-1000 μM); ii) 4-Hz electric field stimulation (EFS) in the presence of 30 μM dopamine with/without 10 μM SCH-23390 (dopamine D1 receptor (D1R) antagonist) or 10 μM sulpiride (D2R antagonist). Immunofluorescence distribution of the neuronal HuC/D protein, glial S100β marker, D1R, and dopamine transporter (DAT) were determined in longitudinal-muscle-myenteric plexus whole-mounts (LMMPs) by confocal microscopy. D1R and D2R mRNA transcripts were evaluated by qRT-PCR.

KEY FINDINGS

DSS caused an inflammatory process in the small intestine associated to dysmotility and altered barrier permeability, as suggested by decreased fecal output and enhanced stool water content. DSS treatment caused a significant increase of DAT and D1R myenteric immunoreactivity as well as of D1R and D2R mRNA levels, accompanied by a significant reduction of dopamine-mediated relaxation, involving primarily D1-like receptors.

SIGNIFICANCE

Mouse ileitis affects enteric dopaminergic neurotransmission mainly involving D1R-mediated responses. These findings provide novel information on the participation of dopaminergic pathways in IBD-mediated neuromuscular dysfunction.

Activation of dopamine D2 receptor promotes pepsinogen secretion by suppressing somatostatin release from the mouse gastric mucosa

In vivo administration of dopamine (DA) receptor (DR)-related drugs modulate gastric pepsinogen secretion. However, DRs on gastric pepsinogen-secreting chief cells and DA D2 receptor (D2R) on somatostatin-secreting D cells were subsequently acquired. In this study, we aimed to further investigate the local effect of DA on gastric pepsinogen secretion through DRs expressed on chief cells or potential D2Rs expressed on D cells. To elucidate the modulation of DRs in gastric pepsinogen secretion, immunofluorescence staining, ex vivo incubation of gastric mucosa isolated from normal and D2R-/- mice were conducted, accompanied by measurements of pepsinogen or somatostatin levels using biochemical assays or enzyme-linked immunosorbent assays. D1R, D2R, and D5R-immunoreactivity (IR) were observed on chief cells in mouse gastric mucosa. D2R-IR was widely distributed on D cells from the corpus to the antrum. Ex vivo incubation results showed that DA and the D1-like receptor agonist SKF38393 increased pepsinogen secretion, which was blocked by the D1-like receptor antagonist SCH23390. However, D2-like receptor agonist quinpirole also significantly increased pepsinogen secretion, and D2-like receptor antagonist sulpiride blocked the promotion of DA. Besides, D2-like receptors exerted an inhibitory effect on somatostatin secretion, in contrast to their effect on pepsinogen secretion. Furthermore, D2R-/- mice showed much lower basal pepsinogen secretion but significantly increased somatostatin release and an increased number of D cells in gastric mucosa. Only SKF38393, not quinpirole, increased pepsinogen secretion in D2R-/- mice. DA promotes gastric pepsinogen secretion directly through D1-like receptors on chief cells and indirectly through D2R-mediated suppression of somatostatin release.

Gastrointestinal Dopamine in Inflammatory Bowel Diseases: A Systematic Review

BACKGROUND

an increased prevalence of gastro-duodenal ulceration was described almost sixty years ago as prodromal to idiopathic Parkinson's disease, while duodenal ulcers have been rarely diagnosed in patients with schizophrenia. The cytoprotective role of dopamine in animal models of gastrointestinal ulcerations has also been described. Interestingly, Parkinson's disease (PD) might share common pathophysiological links with inflammatory bowel disease (IBD) as epidemiological and genetic links already suggest. Thus, the aim of our study was to review the existing literature on the role of the gastrointestinal dopaminergic system in IBD pathogenesis and progression.

METHODS

a systematic search was conducted according to the PRISMA methodology.

RESULTS

twenty-four studies satisfied the predetermined criteria and were included in our qualitative analysis. Due to different observations (cross-sectional studies) as well as experimental setups and applied methodologies (in vivo and in vitro studies) a meta-analysis could not be performed. No ongoing clinical trials with dopaminergic compounds in IBD patients were found.

CONCLUSIONS

the impairment of the dopaminergic system seems to be a significant, yet underestimated, feature of IBD, and more in-depth observational studies are needed to further support the existing preclinical data.

Regulation of Neurotransmitters by the Gut Microbiota and Effects on Cognition in Neurological Disorders

Emerging evidence indicates that gut microbiota is important in the regulation of brain activity and cognitive functions. Microbes mediate communication among the metabolic, peripheral immune, and central nervous systems via the microbiota–gut–brain axis. However, it is not well understood how the gut microbiome and neurons in the brain mutually interact or how these interactions affect normal brain functioning and cognition. We summarize the mechanisms whereby the gut microbiota regulate the production, transportation, and functioning of neurotransmitters. We also discuss how microbiome dysbiosis affects cognitive function, especially in neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease.

A link between gastrointestinal disorders and migraine: Insights into the gut-brain connection

Background

Migraine is a complex, multifaceted, and disabling headache disease that is often complicated by gastrointestinal (GI) conditions, such as gastroparesis, functional dyspepsia, and cyclic vomiting syndrome (CVS). Functional dyspepsia and CVS are part of a spectrum of disorders newly classified as disorders of gut–brain interaction (DGBI). Gastroparesis and functional dyspepsia are both associated with delayed gastric emptying, while nausea and vomiting are prominent in CVS, which are also symptoms that commonly occur with migraine attacks. Furthermore, these gastric disorders are comorbidities frequently reported by patients with migraine. While very few studies assessing GI disorders in patients with migraine have been performed, they do demonstrate a physiological link between these conditions.

Objective

To summarize the available studies supporting a link between GI comorbidities and migraine, including historical and current scientific evidence, as well as provide evidence that symptoms of GI disorders are also observed outside of migraine attacks during the interictal period. Additionally, the importance of route of administration and formulation of migraine therapies for patients with GI symptoms will be discussed.

Methods

A literature search of PubMed for articles relating to the relationship between the gut and the brain with no restriction on the publication year was performed. Studies providing scientific support for associations of gastroparesis, functional dyspepsia, and CVS with migraine and the impact these associations may have on migraine treatment were the primary focus. This is a narrative review of identified studies.

Results

Although the association between migraine and GI disorders has received very little attention in the literature, the existing evidence suggests that they may share a common etiology. In particular, the relationship between migraine, gastric motility, and vomiting has important clinical implications in the treatment of migraine, as delayed gastric emptying and vomiting may affect oral dosing compliance, and thus, the absorption and efficacy of oral migraine treatments.

Conclusions

There is evidence of a link between migraine and GI comorbidities, including those under the DGBI classification. Many patients do not find adequate relief with oral migraine therapies, which further necessitates increased recognition of GI disorders in patients with migraine by the headache community.

Biogenic amines in the colon

The gastrointestinal (GI) tract contains the highest concentration of biogenic amines in the human body. Neurons located in the GI tract, modulated by biogenic amines and various peptide and non-peptide transmitters, are called Enteric Nervous System (ENS). That explains why many medications used in neurology and psychiatry present side effects from the gut. Serotonin (5-hyroxytrypatamine, 5-HT), 95% of which is synthesized in the gut, is the most important amine (beside epinephrine and norepinephrine) colon functionality but another substances such as histamine, dopamine and melatonin are also potent in modulating intestine’s actions. Over 30 receptors for 5-HT were described in the human body, and 5-HT3, 5-HT4 and 5-HT7 are known to have the highest influence on motility and are a potent target for the drugs for treatment GI disorders, such as Irritable Bowel Syndrome (IBS) and Inflammatory Bowel Diseases (IBD). Histamine is a key biogenic amine for pathogenesis of allergy also in the colon. Alteration in histaminergic system is found in patients with diarrhea and allergic enteropathy. Dopamine affects functions of the large intestine but its modulating actions are more presented in the upper part of GI tract. Melatonin is best known for regulating circadian circle, but may also be a potent anti-inflammatory agent within the gut. Despite many years of research, it seems that more studies are needed to fully understand human colon neurochemistry.

Dopamine receptors in emesis: Molecular mechanisms and potential therapeutic function

Dopamine is a member of the catecholamine family and is associated with multiple physiological functions. Together with its five receptor subtypes, dopamine is closely linked to neurological disorders such as schizophrenia, Parkinson's disease, depression, attention deficit-hyperactivity, and restless leg syndrome. Unfortunately, several dopamine receptor-based agonists used to treat some of these diseases cause nausea and vomiting as impending side-effects. The high degree of cross interactions of dopamine receptor ligands with many other targets including G-protein coupled receptors, transporters, enzymes, and ion-channels, add to the complexity of discovering new targets for the treatment of nausea and vomiting. Using activation status of signaling cascades as mechanism-based biomarkers to foresee drug sensitivity combined with the development of dopamine receptor-based biased agonists may hold great promise and seems as the next step in drug development for the treatment of such multifactorial diseases. In this review, we update the present knowledge on dopamine and dopamine receptors and their potential roles in nausea and vomiting. The pre- and clinical evidence provided in this review supports the implication of both dopamine and dopamine receptor agonists in the incidence of emesis. Besides the conventional dopaminergic antiemetic drugs, potential novel antiemetic targeting emetic protein signaling cascades may offer superior selectivity profile and potency.

Stomach gastrin is regulated by sodium via PPAR-α and dopamine D1 receptor

Gastrin, secreted by stomach G cells in response to ingested sodium, stimulates the renal cholecystokinin B receptor (CCKBR) to increase renal sodium excretion. It is not known how dietary sodium, independent of food, can increase gastrin secretion in human G cells. However, fenofibrate (FFB), a peroxisome proliferator-activated receptor-α (PPAR-α) agonist, increases gastrin secretion in rodents and several human gastrin-secreting cells, via a gastrin transcriptional promoter. We tested the following hypotheses: (1.) the sodium sensor in G cells plays a critical role in the sodium-mediated increase in gastrin expression/secretion, and (2.) dopamine, via the D1R and PPAR-α, is involved. Intact human stomach antrum and G cells were compared with human gastrin-secreting gastric and ovarian adenocarcinoma cells. When extra- or intracellular sodium was increased in human antrum, human G cells, and adenocarcinoma cells, gastrin mRNA and protein expression/secretion were increased. In human G cells, the PPAR-α agonist FFB increased gastrin protein expression that was blocked by GW6471, a PPAR-α antagonist, and LE300, a D1-like receptor antagonist. LE300 prevented the ability of FFB to increase gastrin protein expression in human G cells via the D1R, because the D5R, the other D1-like receptor, is not expressed in human G cells. Human G cells also express tyrosine hydroxylase and DOPA decarboxylase, enzymes needed to synthesize dopamine. G cells in the stomach may be the sodium sensor that stimulates gastrin secretion, which enables the kidney to eliminate acutely an oral sodium load. Dopamine, via the D1R, by interacting with PPAR-α, is involved in this process.

Systems Pharmacology Dissection of Multi-Scale Mechanisms of Action of Huo-Xiang-Zheng-Qi Formula for the Treatment of Gastrointestinal Diseases

Multi-components Traditional Chinese Medicine (TCM) treats various complex diseases (multi-etiologies and multi-symptoms) via herbs interactions to exert curative efficacy with less adverse effects. However, the ancient Chinese compatibility theory of herbs formula still remains ambiguous. Presently, this combination principle is dissected through a systems pharmacology study on the mechanism of action of a representative TCM formula, Huo-xiang-zheng-qi (HXZQ) prescription, on the treatment of functional dyspepsia (FD), a chronic or recurrent clinical disorder of digestive system, as typical gastrointestinal (GI) diseases which burden human physical and mental health heavily and widely. In approach, a systems pharmacology platform which incorporates the pharmacokinetic and pharmaco-dynamics evaluation, target fishing and network pharmacological analyses is employed. As a result, 132 chemicals and 48 proteins are identified as active compounds and FD-related targets, and the mechanism of HXZQ formula for the treatment of GI diseases is based on its three function modules of anti-inflammation, immune protection and gastrointestinal motility regulation mainly through four, i.e., PIK-AKT, JAK-STAT, Toll-like as well as Calcium signaling pathways. In addition, HXZQ formula conforms to the ancient compatibility rule of "Jun-Chen-Zuo-Shi" due to the different, while cooperative roles that herbs possess, specifically, the direct FD curative effects of GHX (serving as Jun drug), the anti-bacterial efficacy and major accompanying symptoms-reliving bioactivities of ZS and BZ (as Chen), the detoxication and ADME regulation capacities of GC (as Shi), as well as the minor symptoms-treating efficacy of the rest 7 herbs (as Zuo). This work not only provides an insight of the therapeutic mechanism of TCMs on treating GI diseases from a multi-scale perspective, but also may offer an efficient way for drug discovery and development from herbal medicine as complementary drugs.

Biogenic Amines: Signals Between Commensal Microbiota and Gut Physiology

There is increasing interest in the interactions among the gut microbiota, gut, and brain, which is often referred to as the "microbiota-gut-brain" axis. Biogenic amines including dopamine, norepinephrine, serotonin, and histamines are all generated by commensal gut microorganisms and are suggested to play roles as signaling molecules mediating the function of the "microbiota-gut-brain" axis. In addition, such amines generated in the gut have attracted attention in terms of possible clues into the etiologies of depression, anxiety, and even psychosis. This review covers the latest research related to the potential role of microbe-derived amines such as catecholamine, serotonin, histamine, as well as other trace amines, in modulating not only gut physiology but also brain function of the host. Further attention in this field can offer not only insight into expanding the fundamental roles and impacts of the human microbiome, but also further offer new therapeutic strategies for psychological disorders based on regulating the balance of resident bacteria.

Degree of SGLT1 phosphorylation is associated with but does not determine segment-specific glucose transport features in the porcine small intestines

Glucose‐induced electrogenic ion transport is higher in the porcine ileum compared with the jejunum despite equal apical abundance of SGLT1. The objective of this study was a detailed determination of SGLT1 and GLUT2 expressions at mRNA and protein levels along the porcine small intestinal axis. Phosphorylation of SGLT1 at serine 418 was assessed as a potential modulator of activity. Porcine intestinal tissues taken along the intestinal axis 1 h or 3 h after feeding were analyzed for relative mRNA (RT‐PCR) and protein levels (immunoblot) of SGLT1, pSGLT1, GLUT2, (p)AMPK, β₂‐receptor, and PKA substrates. Functional studies on electrogenic glucose transport were done (Ussing chambers: short circuit currents (I_sc)). Additionally, effects of epinephrine (Epi) administration on segment‐specific glucose transport and pSGLT1 content were examined. SGLT1 and GLUT2 expression was similar throughout the small intestines but lower in the duodenum and distal ileum. pSGLT1 abundance was significantly lower in the ileum compared with the jejunum associated with significantly higher glucose‐induced I_sc. SGLT1 phosphorylation was not inducible by Epi. Epi treatment decreased glucose‐induced I_sc and glucose flux rates in the jejunum but increased basal I_sc in the ileum. Epi‐induced PKA activation was detectable in jejunal tissue. These results may indicate that SGLT1 phosphorylation at Ser418 represents a structural change to compensate for certain conditions that may decrease glucose transport (unfavorable driving forces/changed apical membrane potential) rather than being the cause for the overall differences in glucose transport characteristics between the jejunum and ileum.

Dopamine induces inhibitory effects on the circular muscle contractility of mouse distal colon via D1- and D2-like receptors

Dopamine (DA) acts as gut motility modulator, via D1- and D2-like receptors, but its effective role is far from being clear. Since alterations of the dopaminergic system could lead to gastrointestinal dysfunctions, a characterization of the enteric dopaminergic system is mandatory. In this study, we investigated the role of DA and D1- and D2-like receptors in the contractility of the circular muscle of mouse distal colon by organ-bath technique. DA caused relaxation in carbachol-precontracted circular muscle strips, sensitive to domperidone, D2-like receptor antagonist, and mimicked by bromocriptine, D2-like receptor agonist. 7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390), D1-like receptor antagonist, neural toxins, L-NAME (nitric oxide (NO) synthase inhibitor), 2'-deoxy-N₆-methyl adenosine 3',5'-diphosphate diammonium salt (MRS 2179), purinergic P2Y1 antagonist, or adrenergic antagonists were ineffective. DA also reduced the amplitude of neurally evoked cholinergic contractions. The effect was mimicked by (±)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrobromide (SKF-38393), D1-like receptor agonist and antagonized by SCH-23390, MRS 2179, or L-NAME. Western blotting analysis determined the expression of DA receptor proteins in mouse distal colon. Notably, SCH-23390 per se induced an increase in amplitude of spontaneous and neurally evoked cholinergic contractions, unaffected by neural blockers, L-NAME, MRS 2179, muscarinic, adrenergic, or D2-like receptor antagonists. Indeed, SCH-23390-induced effects were antagonized by an adenylyl cyclase blocker. In conclusion, DA inhibits colonic motility in mice via D2- and D1-like receptors, the latter reducing acetylcholine release from enteric neurons, involving nitrergic and purinergic systems. Whether constitutively active D1-like receptors, linked to adenylyl cyclase pathway, are involved in a tonic inhibitory control of colonic contractility is questioned.

Dopamine enhances duodenal epithelial permeability via the dopamine D5 receptor in rodent

AIM

The intestinal barrier is made up of epithelial cells and intercellular junctional complexes to regulate epithelial ion transport and permeability. Dopamine (DA) is able to promote duodenal epithelial ion transport through D1-like receptors, which includes subtypes of D₁ (D₁ R) and D₅ (D₅ R), but whether D1-like receptors influence the duodenal permeability is unclear.

METHODS

FITC-dextran permeability, short-circuit current (I_SC ), Western blot, immunohistochemistry and ELISA were used in human D₅ R transgenic mice and hyperendogenous enteric DA (HEnD) rats in this study.

RESULTS

Dopamine induced a downward deflection in I_SC and an increase in FITC-dextran permeability of control rat duodenum, which were inhibited by the D1-like receptor antagonist, SCH-23390. However, DA decreased duodenal transepithelial resistance (TER), an effect also reversed by SCH-23390. A strong immunofluorescence signal for D₅ R, but not D₁ R, was observed in the duodenum of control rat. In human D₅ R knock-in transgenic mice, duodenal mucosa displayed an increased basal I_SC with high FITC-dextran permeability and decreased TER with a lowered expression of tight junction proteins, suggesting attenuated duodenal barrier function in these transgenic mice. D₅ R knock-down transgenic mice manifested a decreased basal I_SC with lowered FITC-dextran permeability. Moreover, an increased FITC-dextran permeability combined with decreased TER and tight junction protein expression in duodenal mucosa were also observed in HEnD rats.

CONCLUSION

This study demonstrates, for the first time, that DA enhances duodenal permeability of control rat via D₅ R, which provides new experimental and theoretical evidence for the influence of DA on duodenal epithelial barrier function.

Microbiome, HPA axis and production of endocrine hormones in the gut

Recent accumulating evidence indicates that the gut microbiome can affect the development and regulation of the hypothalamic-pituitary-adrenal axis and behavior, with central integrative systems being crucial in the successful physiological adaptation of the organism to external stressor. In contrast, host-derived hormones increase the bacterial proliferative capacity and pathogenicity. In the gut lumen, this type of cross-talk between microorganisms and the host is presumed to be performed continually through various kinds of luminal molecules, as numerous types of bacteria and host cells are in close proximity in the gastrointestinal tract of mammals.We herein focus on bidirectional signaling between the gut microbiome and the host in terms of commensal microbiota affecting the hypothalamic-pituitary-adrenal HPA axis response and behaviors and further discuss the role of gut luminal catecholamines and γ-aminobutyric acid, both of which are presumed to be involved in this signaling.

Metoclopramide does not increase gastric muscle contractility in newborn rats

Feeding intolerance resulting from delayed gastric emptying is common in premature neonates. Metoclopramide (MCP), the most frequently used prokinetic drug in neonates, enhances gastric muscle contractility through inhibition of dopamine receptors. Although its therapeutic benefit is established in adults, limited data are available to support its clinical use in infants. Hypothesizing that developmentally dependent differences are present, we comparatively evaluated the effect of MCP on fundus muscle contractility in newborn, juvenile, and adult rats. The muscle strips were either contracted with electrical field stimulation (EFS) to induce cholinergic nerve-mediated acetylcholine release or carbachol, a cholinergic agonist acting directly on the muscarinic receptor. Although in adult rats MCP increased EFS-induced contraction by 294 ± 122% of control (P < 0.01), no significant effect was observed in newborn fundic muscle. MCP had no effect on the magnitude of the carbachol-induced and/or bethanechol-induced gastric muscle contraction at any age. In response to dopamine, an 80.7 ± 5.3% relaxation of adult fundic muscle was observed, compared with only a 8.4 ± 8.7% response in newborn tissue (P < 0.01). Dopamine D2 receptor expression was scant in neonates and significantly increased in adult gastric tissue (P < 0.01). In conclusion, the lack of MCP effect on the newborn fundic muscle contraction potential relates to developmental differences in dopamine D2 receptor expression. To the extent that these novel data can be extrapolated to neonates, the therapeutic value of MCP as a prokinetic agent early in life requires further evaluation.

Critical role of gut microbiota in the production of biologically active, free catecholamines in the gut lumen of mice

There is increasing interest in the bidirectional communication between the mammalian host and prokaryotic cells. Catecholamines (CA), candidate molecules for such communication, are presumed to play an important role in the gut lumen; however, available evidence is limited because of the lack of actual data about luminal CA. This study evaluated luminal CA levels in the gastrointestinal tract and elucidated the involvement of gut microbiota in the generation of luminal CA by comparing the findings among specific pathogen-free mice (SPF-M), germ-free mice (GF-M), and gnotobiotic mice. Substantial levels of free dopamine and norepinephrine were identified in the gut lumen of SPF-M. The free CA levels in the gut lumen were lower in GF-M than in SPF-M. The majority of CA was a biologically active, free form in SPF-M, whereas it was a biologically inactive, conjugated form in GF-M. The association of GF-M with either Clostridium species or SPF fecal flora, both of which have abundant β-glucuronidase activity, resulted in the drastic elevation of free CA. The inoculation of E. coli strain into GF-M induced a substantial amount of free CA, but the inoculation of its mutant strain deficient in the β-glucuronidase gene did not. The intraluminal administration of DA increased colonic water absorption in an in vivo ligated loop model of SPF-M, thus suggesting that luminal DA plays a role as a proabsorptive modulator of water transport in the colon. These results indicate that gut microbiota play a critical role in the generation of free CA in the gut lumen.

Dopamine receptor D1 mediates the inhibition of dopamine on the distal colonic motility

The motility of distal colon could be inhibited by dopamine (DA), yet, the involved receptor is controversial according to the published reports. The goal of present study was to investigate DA receptor(s) mediated inhibition of DA on the colonic motility in rat. The contraction of isolated colon strips was assessed through isometric force transducer. The expressions of DA receptors in distal colon were detected through immunofluorescence and Western blot. DA concentration in colonic smooth muscle was measured by liquid chromatography/mass spectrometry. The results showed that DA inhibited the spontaneous motility of distal colon in a dose-dependent manner with EC50 8.3 μM. Tetrodotoxin increased colonic contractive frequency, but failed to affect the inhibition of DA on the colonic motility. Pretreatment with SCH-23390, an antagonist of dopaminergic receptor D1, shifted the dose-response curve to the right with EC50 of DA 37 μM. However, blocking dopaminergic receptor D2 with sulpiride, had no effect. The immunoreactivity of D1 and D2 were detected in the distal colon including myenteric plexus and smooth muscle. Acute cold-restraint stress (CRS) enhanced spontaneous contraction of rat distal colon, which was more sensitive to DA compared with control. Moreover, DA content and D1 expression in smooth muscle layer were increased under CRS condition. In conclusion, D1 in the smooth muscle is mediated DA inhibition on the spontaneous contraction of rat distal colon. The increased DA content and D1 receptor expression in the smooth muscle layer could be a compensatory effect under CRS condition to balance the enhanced colonic motility.

Expression of dopamine receptors in human lower esophageal sphincter

BACKGROUND AND AIM

Dopamine (DA) is considered to be an important modulator of enteric function. Recent experiments have suggested that DA receptors are widely expressed in animal gastrointestinal tract. The aim of this study was to explore the expression of DA receptors (D(1) R, D(2) R, D(3) R, D(4) R, D(5) R) in sling fibers and clasp fibers from the human lower esophageal sphincter (LES).

METHODS

Muscle strips of sling and clasp fibers from the LES were obtained from patients undergoing esophago-gastrectomy, and circular muscle strips from the esophagus and stomach were used as controls. Reverse transcription-polymerase chain reaction and western blotting were used to determine the expression of the five subtypes of DA receptors.

RESULTS

Messenger RNA and protein for three of five DA receptors were identified in the sling and clasp fibers of the LES. Expression was highest for D(1) R, then D(5) R and D(2) R in decreasing levels. D(3) R and D(4) R mRNA and protein were not identified in the muscle strips.

CONCLUSION

D(1) R, D(2) R, D(5) R can be detected in the human LES, and probably contribute to LES function. D(3) R and D(4) R are not expressed, and probably do not contribute to LES function in humans.

Cellular localization of dopamine receptors in the gastric mucosa of rats

Dopamine (DA) plays a critical role in the protection of gastric mucosa and is mediated through corresponding receptors. However, the details of the expression of DA receptors (D1-D5) in the gastric mucosa are lacking. The present study investigated the expression and cellular localization of DA receptors in rat gastric mucosa by means of real-time PCR and immunofluorescent techniques. The results indicated that the mRNA expressions of all five subtypes of DA receptors were found in the gastric mucosa, among which the D2 level was the highest. The immunopositive cells of D1-D3 and D5 were primarily localized to the basilar gland of the epithelial layer in gastric corpus, but D4 immunoreactivity (IR) was only observed in the enteric nerve plexus. The D1, D2, and D5 IR were found in pepsin C-IR cells except D3. No IR of any DA receptor was detected in the H(+)/K(+)-ATPase- or mucin 6-IR cells. In conclusion, for the first time, this study demonstrates the predominant distribution of DA receptors in the chief cells, not the parietal and mucous neck cells, in rat gastric mucosa, thus suggesting that DA may not directly regulate the function of parietal cells or mucous neck cells, but it may modulate the function of chief cells through the D1, D2, and D5 receptors.

Levodopa ameliorated anorectal constipation in de novo Parkinson's disease: The QL-GAT study

BACKGROUND

Gastrointestinal tract (GIT) dysfunction is common in Parkinson's disease (PD) patients. However, it remains unclear whether levodopa affects GIT function in PD.

OBJECTIVE

To perform an open study of levodopa's effects on anorectal constipation in de novo PD patients by the quantitative lower-gastrointestinal autonomic test (QL-GAT).

METHODS

Nineteen unselected de novo PD patients (10 men, 9 women; mean age, 66 years; mean duration of the disease, 2.2 years) were recruited for the study. Eighteen of the patients reported constipation. These patients were treated with 200/20 mg b.i.d. of levodopa/carbidopa for 3 months. Pre- and post-treatment, objective parameters in the QL-GAT that comprised the colonic transit time (CTT) and rectoanal videomanometry were obtained.

RESULTS

Levodopa was well tolerated by all patients. There was a trend toward subjective improvements in bowel frequency and difficulty defecating. Levodopa did not significantly change CTT of the total colon or any segment of the colon. During rectal filling, levodopa significantly lessened the first sensation (p < 0.05). It also tended to augment the amplitude of spontaneous phasic rectal contraction (not statistically significant). During defecation, levodopa significantly lessened the amplitude in paradoxical sphincter contraction upon defecation (PSD) (p < 0.01). It also tended to augment the amplitude of rectal contraction and lessen the amplitude of abdominal strain (not statistically significant). Overall, levodopa significantly lessened post-defecation residuals (p < 0.05).

CONCLUSIONS

The QL-GAT in the present study showed for the first time that levodopa augmented rectal contraction, lessened PSD, and thereby ameliorated anorectal constipation in de novo PD patients.

Bladder, bowel, and sexual dysfunction in Parkinson's disease

Bladder dysfunction (urinary urgency/frequency), bowel dysfunction (constipation), and sexual dysfunction (erectile dysfunction) (also called “pelvic organ” dysfunctions) are common nonmotor disorders in Parkinson's disease (PD). In contrast to motor disorders, pelvic organ autonomic dysfunctions are often nonresponsive to levodopa treatment. The brain pathology causing the bladder dysfunction (appearance of overactivity) involves an altered dopamine-basal ganglia circuit, which normally suppresses the micturition reflex. By contrast, peripheral myenteric pathology causing slowed colonic transit (loss of rectal contractions) and central pathology causing weak strain and paradoxical anal sphincter contraction on defecation (PSD, also called as anismus) are responsible for the bowel dysfunction. In addition, hypothalamic dysfunction is mostly responsible for the sexual dysfunction (decrease in libido and erection) in PD, via altered dopamine-oxytocin pathways, which normally promote libido and erection. The pathophysiology of the pelvic organ dysfunction in PD differs from that in multiple system atrophy; therefore, it might aid in differential diagnosis. Anticholinergic agents are used to treat bladder dysfunction in PD, although these drugs should be used with caution particularly in elderly patients who have cognitive decline. Dietary fibers, laxatives, and “prokinetic” drugs such as serotonergic agonists are used to treat bowel dysfunction in PD. Phosphodiesterase inhibitors are used to treat sexual dysfunction in PD. These treatments might be beneficial in maximizing the patients' quality of life.

Role of CAMKII in reinforcement learning: a computational model of glutamate and dopamine signaling pathways

Timely release of dopamine (DA) at the striatum seems to be important for reinforcement learning (RL) mediated by the basal ganglia. Houk et al. (in: Houk et al (eds) Models of information processing in the basal ganglia, (1995) proposed a cellular signaling pathway model to characterize the interaction between DA and glutamate pathways that have a role in RL. The model simulation results, using GENESIS KINETIKIT simulator, point out that there is not only prolongation of duration as proposed by Houk et al. (1995), but also an enhancement in the amplitude of autophosphorylation of CaMKII. Further, the autophosphorylated form of CaMKII may form a basis for the "eligibility trace" condition required in RL. This simulation study is the first of its kind to support the comprehensive theoretical proposal of Houk et al. (1995).

The possible involvement of dopamine D3 receptors in the regulation of gastric emptying in rats

AIM

The inhibitory effect of dopamine on gastric motility is thought to be mediated via a decrease in acetylcholine release resulting from stimulation of enteric neuronal dopamine D(2) receptors. The aim of this study was to investigate the possible involvement of the dopamine D(3) receptor in the regulation of gastric motility in rats using selective dopamine D(3) receptor agonists or a dopamine D(3) receptor antagonist.

MAIN METHODS

Gastric emptying was assessed using the phenol red method after rats were treated with varying doses of dopamine D(3) receptor agonists or a dopamine D(3) receptor antagonist.

KEY FINDINGS

S(+)-PD 128,907 (0.01-1 mg/kg, s.c.), a selective dopamine D(3) receptor agonist, dose-dependently delayed gastric emptying in rats. Other dopamine D(3) receptor agonists (i.e., R(+)-7-OH-DPAT [0.03-1 mg/kg, s.c.] and quinpirole [0.01-1 mg/kg, s.c.]) also delayed gastric emptying in rats. Both the selective dopamine D(1) and D(5) receptor agonist SKF-38393 and the selective dopamine D(4) receptor agonist PD 168,077 failed to delay gastric emptying in rats. The selective dopamine D(3) receptor antagonist (+)-S 14297 (10mg/kg, s.c.) partially inhibited the S(+)-PD 128,907-induced delay in gastric emptying. Although an administration of S(+)-PD 128,907 (1-100 μg/kg) into the 4th cerebral ventricle partially and dose-dependently delayed gastric emptying in rats, its administration into the lateral cerebral ventricle did not affect gastric emptying.

SIGNIFICANCE

The results presented here suggest that peripheral dopamine D(2) receptors and, at least in part, dopamine D(3) and central dopamine D(2)/D(3) receptors play an important role in the regulation of gastric motility in rats.

Parkinson's disease and the gut: a well known clinical association in need of an effective cure and explanation

Parkinson's disease (PD) is a neurodegenerative disorder which leads to severe movement impairment; however, Parkinsonian patients frequently suffer from gastrointestinal (GI) problems which at present are poorly understood, scarcely investigated, and lack an effective cure. Traditionally, PD is attributed to the loss of mesencephalic dopamine-containing neurons; nonetheless, additional nuclei, such as the dorsal motor nucleus of the vagus nerve and specific central noradrenergic nuclei, are now identified as targets of PD. While the effects of PD on the somatic motor systems are well characterized, the influence on the digestive system still needs to be clarified. Recent findings demonstrate the occurrence of pathological alterations within peripheral neuronal networks in the GI tract of Parkinsonian patients. However, it remains unclear whether a real cell loss occurs, and whether this happens specifically for a subclass of autonomic neurons or if it reflects the sole loss of autonomic nerves. This review summarizes the neurochemical and morphological changes which might be responsible for impaired GI motility. Moreover, we focus on the experimental models to reproduce the altered digestive system of Parkinsonian patients since an experimental model able to mimic such features of PD is required. In the last part of the manuscript, we suggest potential therapeutic targets.

Cosegregation of gastrointestinal ulcers and schizophrenia in a large national inpatient discharge database: revisiting the "brain-gut axis" hypothesis in ulcer pathogenesis

The lifetime prevalence of duodenal ulcer in the United States is 8 to 10%, whereas another 1% of the population is affected by gastric ulcer. Both central and peripheral dopamine pathways may influence ulcer pathogenesis. Dopamine agonists prevent whereas antagonists augment stress- and chemically induced gastrointestinal ulcers in preclinical models. The dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,36-tetrahydropyridine (MPTP) depletes central dopamine and induces lesions in the substantia nigra, and, if given in high doses, MPTP induces a Parkinson disease-like syndrome and gastric ulcers. Because schizophrenia is attributed, in part, to an overactive dopaminergic system, persons with schizophrenia may display a reduced susceptibility toward gastrointestinal ulcers. A case-control study was conducted in patients represented in the 2002 National Inpatient Sample, the largest all-payer inpatient care database in the United States, consisting of 5 to 8 million inpatient hospital stays per year, which approximates a 20% sample of community hospitals. A significant association was observed between schizophrenia and diminished risk for duodenal (odds ratio [OR] 0.55; 95% confidence interval [CI] 0.45-0.67) and gastric (OR 0.54; 95% CI 0.46-0.63) (p < .01) ulcers but not for gastrojejunal ulcers (OR 0.44; 95% CI 0.16-1.20) (p = .11). Potential confounders such as age, gender, race, tobacco or alcohol dependence, and Helicobacter pylori infection were controlled in multivariate analyses. This observational study in a large sample of patients in community hospitals suggests that schizophrenia and attendant neurobiologic mechanisms (eg, variability in dopamine pathways) may act in concert to modify the composite risk for gastrointestinal ulcers. Dopamine pathways warrant further prospective research as new potential drug targets in ulcer disease.

Bladder and bowel dysfunction in Parkinson's disease

Bladder dysfunction (urinary urgency/frequency) and bowel dysfunction (constipation) are common non-motor disorders in Parkinson's disease (PD). In contrast to motor disorder, the pelvic autonomic dysfunction is often non-responsive to levodopa treatment. Brain pathology mostly accounts for the bladder dysfunction (appearance of overactivity) via altered dopamine-basal ganglia circuit, which normally suppresses the micturition reflex. In contrast, peripheral enteric pathology mostly accounts for the bowel dysfunction (slow transit and decreased phasic contraction) via altered dopamine-enteric nervous system circuit, which normally promotes the peristaltic reflex. In addition, weak strain and paradoxical anal contraction might be the results of brain pathology. Pathophysiology of the pelvic organ dysfunction in PD differs from that in multiple system atrophy; therefore it might aid the differential diagnosis. Drugs to treat bladder dysfunction in PD include anticholinergic agents. Drugs to treat bowel dysfunction in PD include dietary fibers, peripheral dopaminergic antagonists, and selective serotonergic agonists. These treatments might be beneficial not only in maximizing patients' quality of life, but also in promoting intestinal absorption of levodopa and avoiding gastrointestinal emergency.

The dopaminergic system in hypertension

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport, vascular smooth muscle contractility and production of reactive oxygen species and by interacting with the renin–angiotensin and sympathetic nervous systems. Dopamine receptors are classified into D1-like (D1 and D5) and D2-like (D2, D3 and D4) subtypes based on their structure and pharmacology. Each of the dopamine receptor subtypes participates in the regulation of blood pressure by mechanisms specific for the subtype. Some receptors regulate blood pressure by influencing the central and/or peripheral nervous system; others influence epithelial transport and regulate the secretion and receptors of several humoral agents. This review summarizes the physiology of the different dopamine receptors in the regulation of blood pressure, and the relationship between dopamine receptor subtypes and hypertension.

Simultaneous absence of dopamine D1 and D2 receptor-mediated signaling is lethal in mice

Dopamine (DA) controls a wide variety of physiological functions in the central nervous system as well as in the neuroendocrine and gastrointestinal systems. DA signaling is mediated by five cloned receptors named D1-D5. Knockout mouse models for the five receptors have been generated, and, albeit impaired for some important DA-mediated functions, they are viable and can reproduce. D1 and D2 receptors are the most abundant and widely expressed DA receptors. Cooperative/synergistic effects mediated by these receptors have been suggested, in particular, in the control of motor behaviors. To analyze the extent of such interrelationship, we have generated double D1/D2 receptor mutants. Interestingly, in contrast to single knockouts, we found that concurrent ablation of the D1 and D2 receptors is lethal during the second or third week after birth. This dramatic phenotype is likely to be related to altered feeding behavior and dysfunction of the gastrointestinal system, especially because major anatomical changes were not identified in the brain. Similarly, in the absence of functional D1, heterozygous D2 mutants (D1r(-/-);D2r(+/-)) showed severe growth retardation and did not survive their postweaning period. The analysis of motor behavior in D1r/D2r compound mutants showed that loss of D2-mediated functions reduces motor abilities, whereas the effect of D1r ablation on locomotion strongly depends on the experimental paradigms used. These studies highlight the interrelationship between D1 and D2 receptor-mediated control of motor activity, food intake, and gastrointestinal functions, which has been elusive in the single-gene ablation studies.

Characterization and mapping of bovine dopamine receptors 1 and 5

A cDNA encoding the bovine dopamine receptor 1 (DRD1) was isolated from a bovine cDNA library, cloned and completely sequenced. The coding region showed 93 and 91% sequence identity on DNA level and 96 and 94% on protein level with its respective porcine and human orthologs. The bovine DRD1 and dopamine receptor 5 (DRD5) were mapped, respectively, to BTA10 and 6 by radiation hybrid mapping. One SNP was found in DRD1 and four in DRD5. Using polymerase chain reaction-restriction fragment length polymorphism, 11 different European cattle breeds were screened for the presence of the DRD1 and DRD5 substitutions. Allele frequencies for DRD1 and DRD5 alleles were very similar across all the breeds examined. Allele frequency discrepancies were found between Belgian Blue beef breed and the other breeds.