Intracerebroventricular administration of apelin-13 inhibits distal colonic transit in mice

Regulatory role of Apelin-13-mediated PI3K/AKT signaling pathway in the glucose and lipid metabolism of mouse with gestational diabetes mellitus

OBJECTIVE

To investigate the regulatory mechanism of Apelin-13-mediated PI3K/AKT signaling pathway in the glucose and lipid metabolism of gestational diabetes mellitus (GDM) mouse.

METHODS

GDM mice models were established and treated with Apelin-13 and/or PI3K/AKT inhibitor LY294002. Then, the indicators of glucose and lipid metabolism and the levels of inflammatory factors were detected. Besides, the levels of indicators of oxidative stress in the placenta of mice were measured. Western blotting was also carried out to determine the expression of PI3K/AKT pathway-related proteins in all groups.

RESULTS

In comparison with the Control group, mice in the GDM group presented with the continuous increase in the level of FBG as the time went on, while FINS level decreased evidently. Besides, the fetus alive ratio in the GDM group was much lower with significant increased weight of fetal mouse and weight of placenta; the mice had significant decreased levels of IL-6, IL-1β, TNF-α and MCP-1, and in the placenta, the levels of SOD, GPx, GSH and CAT were also reduced evidently, with significant downregulation of p-PI3K/PI3K and p-AKT/AKT. However, indicators above in the GDM mice treated with Apelin-13 had significant improvement as compared to those in the GDM group, and the improvement was reversed by LY294002 treatment.

CONCLUSION

Apelin-13, possibly by activating the PI3K/AKT pathway, could improve the glucose and lipid metabolism, reduce the damage caused by oxidative stress and inflammatory reaction, and protect the pancreas islet, thereby improving the pregnancy outcome of GDM mice.

Molecular mechanism of apelin-13 regulation of colonic motility in rats

Apelin is a novel neuropeptide identified as the endogenous ligand for the apelin receptor. Apelin and its receptor are widely distributed in the gastrointestinal tract. Studies have reported that apelin-13 is involved in modulating gastrointestinal motility; however, the evidence is insufficient and the relevant mechanism is still not fully clear. Consequently, our study designed to explore the effect induced by exogenous apelin-13, to analyze the mechanism of action in isolated rat colons and colonic smooth muscle cells. The spontaneous contractions of colonic smooth muscle strips from rat were measured in an organ bath system. L-type calcium currents and large conductance Ca2+-activated K+ (BKCa) currents in rat colonic smooth muscle cells were investigated using the electrophysiological patch-clamp technique. Apelin-13 decreased the spontaneous contractile activity of colonic smooth muscle strips in a dose-dependent manner, and the inhibitory effect was not abolished by tetrodotoxin. The electrophysiological recordings revealed that apelin-13 reduced the crest currents of L-type calcium in a concentration-dependent manner in colonic smooth muscle cells at the test potential of 0 mV. Moreover, apelin-13 moved the current-voltage (I-V) curves of L-type calcium channels upward, but did not change their contour. Furthermore, the characteristics of L-type calcium channels with steady-state activation and steady-state inactivation were not significantly changed. Similarly, application of apelin-13 also significantly decreased BKCa currents in a concentration-dependent manner. In conclusion, apelin-13 inhibited the spontaneous contractile activity of isolated rat colons via the suppression of L-type calcium channels and BKCa channels in colonic smooth muscle cells.

The role of autonomic pathways in peripheral apelin-induced gastrointestinal dysmotility: involvement of the circumventricular organs

NEW FINDINGS

What is the central question of this study? Are central autonomic pathways and circumventricular organs involved in apelin-induced inhibition of gut motility? What is the main finding and its importance? Peripherally administered apelin-13 inhibits gastric and colonic motor functions through sympathetic and parasympathetic autonomic pathways, which seems to be partly mediated by the apelin receptor in circumventricular organs.

ABSTRACT

Peripheral administration of apelin-13 has been shown to inhibit gastrointestinal (GI) motility, but the relevant mechanisms are incompletely understood. This study aimed to investigate (i) whether the apelin receptor (APJ) is expressed in circumventricular structures involved in autonomic functions, (ii) whether they are activated by peripherally administered apelin, (iii) the role of autonomic pathways in peripheral exogenous apelin-induced GI dysmotility, and (iv) the changes in apelin levels in the extracellular environment of the brain following its peripheral application. Ninety minutes after apelin-13 administration (300 μg kg^-1 , i.p.), gastric emptying (GE) and colon transit (CT) were measured in rats that underwent parasympathectomy and/or sympathectomy. Plasma and cerebrospinal fluid (CSF) samples were also collected from another group of rats that received apelin-13 or vehicle injection. The immunoreactivities for APJ and c-Fos in circumventricular organs (CVOs) were evaluated by immunohistochemistry. Compared with vehicle-treated rats, GE and CT were inhibited significantly by apelin-13 treatment, and were completely restored in animals that underwent the combination of parasympathectomy and sympathectomy and sympathectomy alone, respectively. Apelin concentrations were elevated in both plasma and CSF following peripheral administration of apelin-13. APJ expression was detected in area postrema (AP), subfornical organ and organum vasculosum of lamina terminalis, and c-Fos expression was observed in response to apelin injection. Apelin-induced c-Fos expression in AP was partially attenuated by pretreatment with the cholecystokinin-1 receptor antagonist lorglumide, whereas it was completely abolished in vagotomized rats. The present data suggest that APJ in CVOs could indirectly contribute to the inhibitory action of peripheral apelin on GI motor functions.

Adipocytes-released Peptides Involved in the Control of Gastrointestinal Motility

The present review focuses on adipocytes-released peptides known to be involved in the control of gastrointestinal motility, acting both centrally and peripherally. Thus, four peptides have been taken into account: leptin, adiponectin, nesfatin-1, and apelin. The discussion of the related physiological or pathophysiological roles, based on the most recent findings, is intended to underlie the close interactions among adipose tissue, central nervous system, and gastrointestinal tract. The better understanding of this complex network, as gastrointestinal motor responses represent peripheral signals involved in the regulation of food intake through the gut-brain axis, may also furnish a cue for the development of either novel therapeutic approaches in the treatment of obesity and eating disorders or potential diagnostic tools.

Function and regulation of apelin/APJ system in digestive physiology and pathology

Apelin is an endogenous ligand of seven-transmembrane G-protein-coupled receptor APJ. Apelin and APJ are distributed in various tissues, including the heart, lung, liver, kidney, and gastrointestinal tract and even in tumor tissues. Studies show that apelin messenger RNA is widely expressed in gastrointestinal (GI) tissues, including stomach and small intestine, which is closely correlated with GI function. Thus, the apelin/APJ system may exert a broad range of activities in the digestive system. In this paper, we review the role of the apelin/APJ system in the digestive system in physiological conditions, such as gastric acid secretion, control of appetite and food intake, cell proliferation, cholecystokinin secretion and histamine release, gut-brain axis, GI motility, and others. In pathological conditions, the apelin/APJ system plays an important role in the healing process of stress gastric injury, the clinical features and prognosis of patients with gastric cancers, the reduction of inflammatory response to enteritis and pancreatitis, the mediation of liver fibrogenesis, the promotion of liver damage, the inhibition of liver regeneration, the contribution of splanchnic neovascularization in portal hypertension, the treatment of colon cancer, and GI oxidative damage. Overall, the apelin/APJ system plays diversified functions and regulatory roles in digestive physiology and pathology. Further exploration of the relationship between the apelin/APJ system and the digestive system will help to find new and effective drugs for treating and alleviating the pain of digestive diseases.

Novel transmitters in brain stem vagal neurocircuitry: new players on the pitch

The last few decades have seen a major increase in the number of neurotransmitters and neuropeptides recognized as playing a role in brain stem neurocircuits, including those involved in homeostatic functions such as stress responsiveness, gastrointestinal motility, feeding, and/or arousal/wakefulness. This minireview will focus on the known physiological role of three of these novel neuropeptides, i.e., apelin, nesfatin-1, and neuropeptide-S, with a special emphasis on their hypothetical roles in vagal signaling related to gastrointestinal motor functions.

Apelin-13 inhibits gastric motility through vagal cholinergic pathway in rats

The expression of apelin and its receptors (APJ) in central autonomic networks suggests that apelin may regulate gastrointestinal motor functions. In rodents, central administration of apelin-13 has been shown to inhibit gastric emptying; however, the mechanisms involved remain to be determined. Using male adult Sprague-Dawley rats, the aims of the present study were 1) to determine the expression of APJ receptor in the dorsal vagal complex (DVC), 2) to assess the effects of central application of apelin-13 into the DVC on gastric tone and motility, and 3) to investigate the neuronal pathways responsible for apelin-induced alterations. APJ receptor immunoreactivity was detected in gastric-projecting and choline acetyltransferase-positive neurons of the DVC. Microinjection of apelin-13 into the DVC significantly decreased gastric tone and motility in both corpus and antrum. The apelin-induced reduction in gastric tone and motility was prevented by surgical vagotomy or fourth ventricular application of the APJ receptor antagonist, [Ala13]apelin-13 (F13A). Systemic administration of the muscarinic receptor antagonist atropine, but not the nitric oxide synthase inhibitor nitro-l-arginine methyl ester (l-NAME), abolished the apelin-induced inhibitory responses. The present results indicate a central modulatory role of apelin in the vagal neurocircuitry that controls gastric motor functions via withdrawal of the tonically active cholinergic pathway. NEW & NOTEWORTHY This is the first study investigating the effects induced by brain stem application of apelin-13 while monitoring gastric tone and motility in rats. We have found that gastric-projecting neurons of the dorsal vagal complex express apelin receptors (APJ), which mediate the inhibitory actions of apelin-13. The inhibitory effects of apelin were abolished by systemic preadministration of atropine, but not nitro-l-arginine methyl ester (l-NAME). Apelin seems to modulate gastric motility via withdrawal of the tonically active vagal cholinergic pathway.

The evidence of apelin has the bidirectional effects on feeding regulation in Siberian sturgeon (Acipenser baerii)

Apelin is a peptide, mainly produced in the brain, which participates in several physiologic effects. However, knowledge about the mechanism of appetite regulation in teleosts, including the role of apelin is not well understood. The aim of this study is to explore the effect of feeding status on the expression of apelin mRNA in the whole brain and the effects of injection of apelin on food intake in Siberian sturgeon (Acipenser baerii). In this study, we first cloned the apelin cDNA sequence of the Siberian sturgeon. We obtained a 1046-bp cDNA fragment, including a 237-bp open reading frame (ORF) that encoded 78 amino acids. Apelin was widely distributed in 11 tissues related to feeding regulation, with the highest expression in thewhole brain, followed by the spleen and trunk kidney. In addition, we measured the effects of periprandial (preprandial and postprandial) change, fasting and re-feeding on apelin mRNA expression in whole brain. The level of apelin mRNA was significantly decreased 1h after feeding. The results of the fasting experiment showed that the expression of apelin mRNA in the brain was significantly reduced after 1day of fasting but consistently increased throughout the 15-day food deprivation period. When the 15-day fasted fish were re-fed, apelin mRNA expression in the brain was significantly increased as compared to that of the control. These results suggest that apelin may play a bidirectional role in the regulation of food intake in the Siberian sturgeon. In order to further examine the effect of apelin on feeding regulation in Siberian sturgeons, acute and chronic intraperitoneal (i.p.) injection experiments were performed and food intakes were recorded. Results showed that acute i.p. injection of apelin-13 reduced food intake, however, chronic i.p. injection apelin-13 increased the food intake for 7days in Siberian sturgeons. In conclusion, our results show that apelin has a bidirectional effect on feeding regulation in Siberian sturgeons by acting as a satiety factor in short-term feeding regulation and a starvation factor in long-term feeding regulation.

Effect of the spinal apelin‑APJ system on the pathogenesis of chronic constriction injury‑induced neuropathic pain in rats

Apelin is hypothesized to serve a dual function in pain processing. Spinal administration of apelin induces hyperalgesia, while opioid receptors are implicated in the antinociceptive effects of apelin in acute nociceptive models. However, whether the apelin-apelin receptor (APJ) system is involved in neuropathic pain remains to be elucidated. The present study aimed to evaluate the impact and mechanism of the spinal apelin-APJ system in neuropathic pain. Chronic constriction injury (CCI) of the sciatic nerve produced sustained spinal apelin and APJ upregulation, which was associated with mechanical allodynia and heat hyperalgesia development in the hind-paw plantar surface. Immunofluorescence demonstrated that apelin and APJ were localized to the superficial dorsal horns. In order to further clarify the function of the apelin-APJ system, a single intrathecal administration of ML221, an APJ antagonist, was used; this transiently reduced CCI-induced pain hypersensitivity. However, apelin-13 (the isoform which binds most strongly to APJ) exhibited no effect on the nociceptive response, suggesting an essential role for the spinal apelin-APJ system in neuropathic pain sensitization. The present study demonstrated that a single application of ML221 alleviated mechanical allodynia and heat hyperalgesia 7 days following CCI, in a dose-dependent manner. Intraspinal delivery of ML221, at the onset of and in fully-established neuropathic pain, persistently attenuated CCI-induced pain hypersensitivity, indicating that the apelin-APJ system was involved in initiating and maintaining pain. It was demonstrated, using immunoblotting, that intrathecal ML221 downregulated phosphorylated extracellular signal-related kinase (ERK) in the rat spinal cord dorsal horn, suggesting that the effect of apelin on neuropathic pain may be mediated via ERK signaling. The results of the present study suggested that the spinal apelin-APJ system may drive neuropathic pain. Inhibition of APJ may provide novel pharmacological interventions for neuropathic pain.

Induction of antinociceptive tolerance to the chronic intrathecal administration of apelin-13 in rat

Pain represents a major contributing factor to the individual's quality of life. Although pain killers as opioids, endogenous or exogenous peptides can decrease pain perception, the chronic use of them leads to antinociceptive tolerance. It has been demonstrated that neuropeptide apelin has potent antinoceptive effect. However, the possibility of the induction of its antinociceptive tolerance has not yet been clarified. The tail-flick test was used to assess the nociceptive threshold. All experiments were carried out on male Wistar rats which received intrathecal apelin for 7days. To determine the role of apelin and opioid receptors on the development of apelin analgesic tolerance, their receptor antagonists (F-13 A and naloxone, respectively) were injected simultaneously with apelin. The lumbar spinal cord was assayed to determine apelin receptor levels by the western blotting method. Plasma corticosterone levels were assayed using ELISA. Results showed that apelin (3μg/rat) induced strong thermal antinociception. In addition, chronic apelin produced tolerance to its antinociceptive effect and down regulated spinal apelin receptor. F-13 A and naloxone could inhibit apelin tolerance development. The corticosterone levels did not change following drug administration. Taken together, the data indicated that apelin like other analgesic drugs leads to the induction of side effects such as analgesic tolerance which is mediated partly via the apelin and opioid receptors activation.

Central apelin mediates stress-induced gastrointestinal motor dysfunction in rats

Apelin, an endogenous ligand for APJ receptor, has been reported to be upregulated in paraventricular nucleus (PVN) following stress. Central apelin is known to stimulate release of corticotropin-releasing factor (CRF) via APJ receptor. We tested the hypothesis that stress-induced gastrointestinal (GI) dysfunction is mediated by central apelin. We also assessed the effect of exogenous apelin on GI motility under nonstressed (NS) conditions in conscious rats. Prior to solid gastric emptying (GE) and colon transit (CT) measurements, APJ receptor antagonist F13A was centrally administered under NS conditions and following acute stress (AS), chronic homotypic stress (CHS), and chronic heterotypic stress (CHeS). Plasma corticosterone was assayed. Strain gage transducers were implanted on serosal surfaces of antrum and distal colon to record postprandial motility. Stress exposure induced coexpression of c-Fos and apelin in hypothalamic PVN. Enhanced hypothalamic apelin and CRF levels in microdialysates were detected following AS and CHeS, which were negatively and positively correlated with GE and CT, respectively. Central F13A administration abolished delayed GE and accelerated CT induced by AS and CHeS. Central apelin-13 administration increased the plasma corticosterone and inhibited GE and CT by attenuating antral and colonic contractions. The inhibitory effect elicited by apelin-13 was abolished by central pretreatment of CRF antagonist CRF9-41 in antrum, but not in distal colon. Central endogenous apelin mediates stress-induced changes in gastric and colonic motor functions through APJ receptor. The inhibitory effects of central exogenous apelin-13 on GI motility appear to be partly CRF dependent. Apelin-13 inhibits colon motor functions through a CRF-independent pathway.

Regulation of feeding behavior, gastrointestinal function and fluid homeostasis by apelin

Apelin was first identified and characterized from bovine stomach extracts as an endogenous ligand for the APJ receptor. Apelin/APJ system is abundantly present in peripheral tissues and central nervous system. Apelin plays a broad role in regulating physiological and pathological functions. Recently, many reports have showed the effects of apelin on feeding behavior, however the results are inconsistent, due to different administration routes, animal species, forms of apelin, etc. Apelin has been involved in stimulating gastric cell proliferation, cholecystokinin (CCK) secretion, histamine release, gastric acid and bicarbonate secretion, and regulation of gastrointestinal motility. In addition, apelin produced regulatory effects on drinking behavior, diuresis, arginine vasopressin (AVP) release and glucocorticoids secretion. This article reviews the role of apelin on feeding behavior, gastrointestinal function and fluid homeostasis.

Supraspinal antinociceptive effect of apelin-13 in a mouse visceral pain model

Apelin, as the endogenous ligand of the APJ receptor, is a novel identified neuropeptide whose biological functions are not fully understood. APJ receptor mRNA was found in several brain regions related to descending control system of pain, such as amygdala, hypothalamus and dorsal raphe nucleus (DRN). The present study was designed to determine whether supraspinal apelin-13 may produce antinociceptive effect observed in the acetic acid-induced writhing test, a model of visceral pain. Apelin-13 not only significantly produced preemptive antinociception at the dose of 0.3, 0.5, 1 and 3 μg/mouse when injected intracerebroventricularly (i.c.v.) before acetic acid, but also significantly induced antinociception at a dose of 0.5, 1 and 3 μg/mouse when injected i.c.v. after acetic acid. And i.c.v. apelin-13 did not influence 30-min locomotor activity counts in mice. Intrathecal (i.t.) administration of apelin-13 (1 and 3 μg/mouse) significantly decreased the number of writhes, however, intraperitoneal (i.p.) injection of apelin-13 (10-100 μg/mouse) had no effect on the number of writhes in the writhing test. The specific APJ receptor antagonist apelin-13(F13A), no-specific opioid receptor antagonist naloxone and μ-opioid receptor antagonist β-funaltrexamine hydrochloride (β-FNA) could significantly antagonize the antinociceptive effect of i.c.v. apelin-13, suggesting APJ receptor and μ-opioid receptor are involved in this process. Central low dose of apelin-13 (0.3 μg/mouse, i.c.v.) could significantly potentiate the analgesic potencies of modest and even relatively ineffective doses of morphine administrated at supraspinal level. This enhanced antinociceptive effect was reversed by naloxone, suggesting that the potentiated analgesic response is mediated by opioid-responsive neurons.

Central apelin-13 inhibits food intake via the CRF receptor in mice

Apelin, the novel identified peptide, is the endogenous ligand for the APJ. Previous studies have reported the effect of apelin on food intake, however the action of acute central injected apelin on food intake in mice remains unknown. The present study was designed to investigate the mechanism as well as the effect of central apelin-13 on food intake in mice. During the dark period, the cumulative food intake was significantly decreased at 4h after the intracerebroventricular (i.c.v.) injection of 1 and 3μg/mouse apelin-13 and the period food intake was significantly reduced during 2-4h after treatment. In the fasted mice, the cumulative food intake was significantly decreased at 2 and 4h after injection of 3μg/mouse apelin-13. The cumulative water intake was significantly reduced by apelin-13 (3μg/mouse) at 4h after injection in freely feeding and fasted mice. However, during light period, apelin-13 had no influence on food and water intake in freely feeding mice. The APJ receptor antagonist apelin-13(F13A) (6μg/mouse) and the corticotrophin-releasing factor (CRF) receptor antagonist α-helical CRF(9-41) (3μg/mouse) could reverse the inhibitory effect on cumulative food intake/0-4h induced by apelin-13 (3μg/mouse) in freely feeding mice during the dark period, whereas the anorexic effect could not be antagonized by the arginie vasopressin (AVP) receptor antagonist deamino(CH(2))(5)Tyr(Me)AVP (0.5μg/mouse). Taken together, these results suggest that central apelin-13 inhibits food intake in mice and it seems that APJ receptor and CRF receptor, but not AVP receptor, might be involved in this process.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Effect of centrally administered apelin-13 on gastric emptying and gastrointestinal transit in mice

Apelin, as the endogenous ligand for the APJ, regulates many biological functions, including blood pressure, neuroendocrine, drinking behavior, food intake and colonic motility. The present study was designed to investigate the effect of central apelin-13 on gastric emptying and gastrointestinal transit in mice. Intracerebroventricular (i.c.v.) injection of apelin-13 (3 and 10 μg/mouse) decreased gastric emptying rate by 10.9% and 17.1%. This effect was significantly antagonized by the APJ receptor antagonist apelin-13(F13A) and the opioid receptor antagonist naloxone, respectively. However, intraperitoneal (i.p.) injection of apelin-13 (10-100 μg/mouse) did not affect gastric emptying. Apelin-13 (0.3, 1 and 3 μg/mouse, i.c.v.) inhibited gastrointestinal transit by 16.8%, 23.4% and 19.2%. Apelin-13(F13A) and naloxone could also reverse this antitransit effect induced by apelin-13. Taken together, these results suggest that i.c.v. injected apelin-13 inhibits gastric emptying and gastrointestinal transit and it seems that APJ receptor and opioid receptor might be involved in these processes.