Distribution of c-Fos immunoreactive neurons in the brain after intraperitoneal injection of apelin-12 in Wistar rats

The investigation of apelin and apelin receptor expressions in mouse endometrium during peri-implantation period

BACKGROUND

Fertilization, pre-implantation embryo development, implantation, and decidualization are critical for a healthy pregnancy. Successful implantation requires a competent blastocyst and a receptive uterus. Apelin was purified from the bovine stomach in 1998. Apelin receptor (APJ) is a member of G protein-coupled receptors. Apelin/APJ system's physiological role was shown in cardiovascular system, immune response, stress response, fluid regulation, nutrient uptake, angiogenesis, and adipoinsular axis; however, whether apelin/APJ system plays a role in implantation is unknown. In our study, we aimed to evaluate the localization and expressions of the apelin/APJ system in the peri-implantation period mouse uterus.

METHODS

Uteri and implantation sites were collected from mice on the estrous phase and the 1st, 4th, 5th, 6th, and 8th days of pregnancy. Also, inter-implantation sites were collected from the 5th day of the pregnancy group. Localization and expressions of apelin and APJ were determined by immunohistochemistry and Western blot, respectively.

RESULTS

Apelin and APJ were expressed in the luminal and gland epithelium, the stroma of all experimental groups. Two isoforms of apelin-8 and 16 kDa were detected by Western blot. While apelin expression increased from the estrous to the 8th day of pregnancy, APJ expression increased from the estrous to the 4th day of pregnancy, reached the highest expression level, then decreased.

CONCLUSIONS

Our findings suggest that the apelin/APJ system might be involved in implantation and decidualization. Our findings will guide further studies and may help elucidate the underlying causes of implantation failure and pregnancy loss.

Neuropeptide apelin presented in the dopaminergic neurons modulates the neuronal excitability in the substantia nigra pars compacta

The dopaminergic neurons in the substantia nigra pars compacta are characterized by autonomous pacemaking activity. The spontaneous firing activity of nigral dopaminergic neurons plays an important role in physiological function and is essential for their survival. Importantly, the spontaneous firing activity may also be involved in the preferential vulnerability of the nigral dopaminergic neurons in Parkinson's disease (PD). The neuropeptide apelin was reported to exert neuroprotective effects in neurodegenerative diseases, including PD. And it was noticed that apelin modulates neuronal activity in some brain regions. The present study investigated the electrophysiological and behavioral effects of apelin in the substantia nigra. Double-labeling immunofluorescence showed that apelin was present in nigral dopaminergic neurons and that these neurons expressed apelin receptor APJ. Further single unit in vivo electrophysiological recordings revealed that endogenous apelin tonically increased the firing rate of nigral dopaminergic neurons in both normal and parkinsonian animals. Exogenous apelin-13 exerted excitatory effects on the majority of nigral dopaminergic neurons, yet reduced excitability in a subset of neurons. In addition, nigral application of apelin-13 increased motor activity in normal rats and blocking endogenous apelin reduced motor activity. Considering the involvement of the spontaneous firing activity of nigral dopaminergic neurons in the development of PD and the possibility that apelin acts in an autocrine manner on apelin receptors expressed by nigral dopaminergic neurons, the modulation of the spontaneous firing activity of nigral dopaminergic neurons by apelin may serve as a neuroprotective factor in PD.

Correlation Between Apelin and Collateral Circulation in Patients with Middle Cerebral Artery Occlusion and Moyamoya Disease

Background and Objectives

Moyamoya disease (MMD) is a unique cerebrovascular occlusive disease with abnormal vascular hyperplasia, which causes cerebrovascular accidents like intracranial arteriosclerosis. This study aimed to explore whether plasma apelin levels are related to good collateral circulation in ischemic diseases, which may be higher in patients with MMD than middle cerebral artery (MCA) occlusion or healthy controls, and may have a connection with the MMD grades.

Methods

We recruited 68 MMD patients and 25 MCA occlusion patients diagnosed by angiography, including 29 patients without cerebrovascular problems as controls. We examined the plasma apelin, serum nitric oxide (NO), and vascular endothelial growth factor (VEGF) levels of all subjects by ELISA kit. We compared the relationship between apelin, NO, and VEGF in the blood of three groups, to explore the relationship. We also investigated whether the plasma apelin-13, apelin-17, and apelin-36 levels correlate with the MMD classification.

Results

Univariate analyses indicated that the MMD group had the higher plasma apelin-13, apelin-17, apelin-36, and serum NO levels than the MCA occlusion and healthy control groups. Binary logistic regression analyses further showed that the apelin-13 level was substantially higher in MMD patients than in MCA occlusion patients. Patients with MMD were significantly younger than patients with MCA occlusion by their mean ages. Linear regression analyses were performed to compare apelin levels between different grades of the patients with MMD. Apelin-13, apelin-17, and apelin-36 levels increased with the gradual increase of compensation grades level independent of NO and VEGF. Apelin-13 and apelin-36 showed a positive effect on the compensation scores in MMD.

Conclusion

Our study demonstrated that apelin-13 was significantly increased in patients with MMD than patients with MCA occlusion independent of NO and VEGF. Moreover, plasma apelin-13, apelin-17, and apelin-36 levels increase with the grades of MMD.

Circumventricular Organ Apelin Receptor Knockdown Decreases Blood Pressure and Sympathetic Drive Responses in the Spontaneously Hypertensive Rat

The central site(s) mediating the cardiovascular actions of the apelin-apelin receptor (APJ) system remains a major question. We hypothesized that the sensory circumventricular organs (CVOs), interfacing between the circulation and deeper brain structures, are sites where circulating apelin acts as a signal in the central nervous system to decrease blood pressure (BP). We show that APJ gene (aplnr) expression was elevated in the CVOs of spontaneously hypertensive rats (SHRs) compared to normotensive Wistar Kyoto (WKY) controls, and that there was a greater mean arterial BP (MABP) decrease following microinjection of [Pyr¹]apelin-13 to the CVOs of SHRs compared to WKY rats. Lentiviral APJ-specific-shRNA (LV-APJ-shRNA) was used to knockdown aplnr expression, both collectively in three CVOs and discretely in individual CVOs, of rats implanted with radiotelemeters to measure arterial pressure. LV-APJ-shRNA-injection decreased aplnr expression in the CVOs and abolished MABP responses to microinjection of [Pyr¹]apelin-13. Chronic knockdown of aplnr in any of the CVOs, collectively or individually, did not affect basal MABP in SHR or WKY rats. Moreover, knockdown of aplnr in any of the CVOs individually did not affect the depressor response to systemic [Pyr¹]apelin-13. By contrast, multiple knockdown of aplnr in the three CVOs reduced acute cardiovascular responses to peripheral [Pyr¹]apelin-13 administration in SHR but not WKY rats. These results suggest that endogenous APJ activity in the CVOs has no effect on basal BP but that functional APJ in the CVOs is required for an intact cardiovascular response to peripherally administered apelin in the SHR.

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.

The effect of apelin-13 on gastric ischemia/reperfusion injury: the roles of sensory nerves and vagus nerve

Apelin is a peptide that plays a role in physiological processes such as angiogenesis, apoptosis, and proliferation. The aim of this study was to investigate the role of capsaicin-sensitive afferent neurons and vagus in the effect of apelin against ischemia/reperfusion (I/R) injury. The experimental groups were (1) control, (2) I/R, (3) apelin + I/R, (4) vagotomy + I/R, (5) vagotomy + apelin + I/R, (6) capsaicin + I/R, (7) capsaicin + apelin + I/R, (8) lorglumide + I/R, and (9) lorglumide + apelin + I/R. To test the potential gastroprotective effect of apelin-13, apelin-13 (2 mg/kg i.v.) was administered just before both ischemia and reperfusion. A vagotomy was performed 1 week before I/R in the vagotomized groups; capsaicin (125 mg/kg s.c.) was administrated 2 weeks before I/R in the capsaicin-treated groups and lorglumide (5 mg/kg i.p.) was administered 30 min before I/R in the lorglumide-treated groups. After I/R, a variety parameters in gastric tissue were analyzed. cfos expression was determined in brainstem samples. In the I/R group, the lesion index, myeloperoxidase activity, lipid peroxidation, nitric oxide, and tumor necrosis factor-α increased, and mucosal blood flow, prostaglandin-E₂, and calcitonin gene related peptide decreased. Apelin prevented the damaging effects of I/R and increased cfos expression in brainstem areas. Vagotomy, capsaicin, and lorglumide largely eliminated the gastroprotective effects of apelin-13. This study showed that sensory nerves and the vagus play regulatory roles in apelin-induced gastroprotection. Cholecystokinin may play a role in the effect of apelin through sensory neurons.

Prospective evaluation of the diagnostic value of plasma apelin 12 levels for differentiating patients with moyamoya and intracranial atherosclerotic diseases

Patients with moyamoya disease (MMD) or intracranial atherosclerotic disease (ICAD) experience similar cerebral ischaemic events. However, MMD patients show greater angiogenesis and arteriogenesis, which play crucial roles in collateral circulation development to enhance clinical prognosis and outcome. Apelins have been associated with angiogenesis and arteriogenesis. Therefore, the aim of the present study was to determine whether apelin levels were higher in patients with MMD than in patients with ICAD or in healthy controls. We compared plasma apelin levels in 29 patients with MMD, 82 patients with ICAD, and 25 healthy participants. Twelve-hour fasting blood samples were collected and analysed using commercially available kits. Univariate analyses indicated that compared with the ICAD and healthy control groups, the MMD group had higher apelin-12, apelin-13, apelin-36, and nitric oxide levels. Binary logistic regression analyses further showed that the plasma apelin-12 level was substantially higher in MMD patients than in ICAD patients. Patients with MMD were also differentiated from patients with ICAD by their mean ages, with the former being younger. Therefore, the plasma apelin-12 level is a potential diagnostic marker for differentiating MMD and ICAD and may provide a treatment strategy for enhancing collateral circulation development and clinical prognosis and outcome.

Motor cortex stimulation and neuropathic pain: how does motor cortex stimulation affect pain-signaling pathways?

OBJECTIVE

Neuropathic pain is often severe. Motor cortex stimulation (MCS) is used for alleviating neuropathic pain, but the mechanism of action is still unclear. This study aimed to understand the mechanism of action of MCS by investigating pain-signaling pathways, with the expectation that MCS would regulate both descending and ascending pathways.

METHODS

Neuropathic pain was induced in Sprague-Dawley rats. Surface electrodes for MCS were implanted in the rats. Tactile allodynia was measured by behavioral testing to determine the effect of MCS. For the pathway study, immunohistochemistry was performed to investigate changes in c-fos and serotonin expression; micro-positron emission tomography (mPET) scanning was performed to investigate changes of glucose uptake; and extracellular electrophysiological recordings were performed to demonstrate brain activity.

RESULTS

MCS was found to modulate c-fos and serotonin expression. In the mPET study, altered brain activity was observed in the striatum, thalamic area, and cerebellum. In the electrophysiological study, neuronal activity was increased by mechanical stimulation and suppressed by MCS. After elimination of artifacts, neuronal activity was demonstrated in the ventral posterolateral nucleus (VPL) during electrical stimulation. This neuronal activity was effectively suppressed by MCS.

CONCLUSIONS

This study demonstrated that MCS effectively attenuated neuropathic pain. MCS modulated ascending and descending pain pathways. It regulated neuropathic pain by affecting the striatum, periaqueductal gray, cerebellum, and thalamic area, which are thought to regulate the descending pathway. MCS also appeared to suppress activation of the VPL, which is part of the ascending pathway.

The effects of intracerebroventricular infusion of apelin-13 on reproductive function in male rats

Apelin is a novel bioactive peptide as the endogenous ligand for APJ. Apelin and APJ have also been identified in the testis, hypothalamic nuclei such as arcuate, supraoptic and paraventricular nuclei, implicating roles in the control of reproduction. Therefore, the present study was designed to investigate the effects of chronic central infusion of apelin-13 on LH, FSH and testosterone levels and testis morphology. 21 Wistar-Albino male rats received continuous intracerebroventricular infusion via Alzet osmotic mini pumps filled artificial cerebrospinal fluid (vehicle) or apelin-13 at concentrations of 1 or 10 nmol (10 μl/h) for seven days. At the last 90 min of the infusion period, the blood samples were collected at 15 min intervals (0-90 min) for LH and FSH analyses. At the last sampling point, the blood samples were analyzed for testosterone levels. Infusion of high dose apelin-13 significantly suppressed LH release compared with the vehicle values at 30, 60 and 75 min (p<0.05). However, FSH levels did not significantly differ among the groups. Serum testosterone levels in high dose apelin-13 group were statistically lower than the control group (p<0.05). In addition, histological examination showed that infusion of high dose apelin-13 significantly decreased the number of Leydig cells compared with the control and lower dose apelin-13 groups (p<0.05, p<0.01). Our results suggest that apelin-13 may play a role in the central regulation and decreases testosterone release by suppressing LH secretion. Thus, antagonists of the apelin receptor may, therefore, be useful for pharmaceuticals in the treatment of infertility.

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.

Apelin acts in the subfornical organ to influence neuronal excitability and cardiovascular function

Apelin is an adipocyte-derived hormone involved in the regulation of water balance, food intake and the cardiovascular system partially through actions in the CNS. The subfornical organ (SFO) is a circumventricular organ with identified roles in body fluid homeostasis, cardiovascular control and energy balance. The SFO lacks a normal blood-brain barrier, and is thus able to detect circulating signalling molecules such as angiotensin II and leptin. In this study, we investigated actions of apelin-13, the predominant apelin isoform in brain and circulatory system, on the excitability of dissociated SFO neurons using electrophysiological approaches, and determined the cardiovascular consequences of direct administration into the SFO of anaesthetized rats. Whole cell current clamp recording revealed that bath-applied 100 nm apelin-13 directly influences the excitability of the majority of SFO neurons by eliciting either depolarizing (31.8%, mean 7.0 ± 0.8 mV) or hyperpolarizing (28.6%, mean -10.4 ± 1.8 mV) responses. Using voltage-clamp techniques, we also identified modulatory actions of apelin-13 on specific ion channels, demonstrating that apelin-13 activates a non-selective cationic conductance to depolarize SFO neurons while activation of the delayed rectifier potassium conductance underlies hyperpolarizing effects. In anaesthetized rats, microinjection of apelin into SFO decreased both blood pressure (BP) (mean area under the curve -1492.3 ± 357.1 mmHg.s, n = 5) and heart rate (HR) (-32.4 ± 10.39 beats, n = 5). Our data suggest that circulating apelin can directly affect BP and HR as a consequence of the ability of this peptide to modulate the excitability of SFO neurons.

Study on the expression of c-Fos protein in the brain of rats after ingestion of food rich in lycopene

Lycopene, a reddish pigment contained in tomato, belongs to the carotenoid family along with beta-carotene and rutein. This study examined whether administration of lycopene to rats would induce excitation of neurons in the central nervous system. Continuous intake of lycopene-rich food was found to induce accumulation of lycopene in the plasma and liver, and stimulated central neurons in the paraventricular nucleus, ventromedial nucleus and supraoptic nucleus of the hypothalamus, which are known to be involved in the functions of athrocytosis and water drinking. These findings suggest that lycopene may have some influence on feeding and water-drinking behaviors.

Stimulus-specific neuroendocrine responses to osmotic challenges in apelin receptor knockout mice

The expression of the novel peptide apelin and its receptor APJ within specific regions of the brain, in particular the magnocellular neurones of the hypothalamus and the circumventricular organs, has implicated the apelinergic system in mechanisms controlling fluid homeostasis. In addition, apelin and APJ are considered to be involved in controlling arginine vasopressin (AVP) secretion into the circulation and release within the hypothalamic-neurohypophysial system. To clarify the role of APJ during regulation of fluid homeostasis, we compared the effects of osmotic stimulation on the urinary concentrating capacities and central nervous system responses of salt-loaded (SL) and water-deprived (WD) female APJ knockout (APJ(-/-)) mice and wild-type controls. SL resulted in a significantly increased urine volume in APJ(-/-) mice compared to wild-type controls, whereas WD in APJ(-/-) mice failed to reduce urine volume as seen in wild-type controls. AVP transcripts in the supraoptic and paraventricular nuclei and plasma AVP concentrations were significantly attenuated in SL APJ(-/-) mice compared to SL wild-type, but increased comparably in wild-type and APJ(-/-) mice after WD. Analysis of c-fos mRNA expression in the median preoptic nucleus and subfornical organ in response to either WD or SL showed attenuated expression in APJ(-/-) compared to wild-type mice. These findings further implicate the apelinergic system in mechanisms controlling fluid homeostasis, particularly at a neuroendocrine level, and suggest stimulus-specific involvement in vasopressinergic activity.

Neuroprotective effect of the endogenous neural peptide apelin in cultured mouse cortical neurons

The adipocytokine apelin and its G protein-coupled APJ receptor were initially isolated from a bovine stomach and have been detected in the brain and cardiovascular system. Recent studies suggest that apelin can protect cardiomyocytes from ischemic injury. Here, we investigated the effect of apelin on apoptosis in mouse primary cultures of cortical neurons. Exposure of the cortical cultures to a serum-free medium for 24 h induced nuclear fragmentation and apoptotic death; apelin-13 (1.0-5.0 nM) markedly prevented the neuronal apoptosis. Apelin neuroprotective effects were mediated by multiple mechanisms. Apelin-13 reduced serum deprivation (SD)-induced ROS generation, mitochondria depolarization, cytochrome c release and activation of caspase-3. Apelin-13 prevented SD-induced changes in phosphorylation status of Akt and ERK1/2. In addition, apelin-13 attenuated NMDA-induced intracellular Ca(2+) accumulation. These results indicate that apelin is an endogenous neuroprotective adipocytokine that may block apoptosis and excitotoxic death via cellular and molecular mechanisms. It is suggested that apelins may be further explored as a potential neuroprotective reagent for ischemia-induced brain damage.

Abnormal fluid homeostasis in apelin receptor knockout mice

The apelinergic system, comprised of apelin and its G protein-coupled receptor (APJ; APLNR as given in MGI Database), is expressed within key regions of the central nervous system associated with arginine vasopressin (AVP) synthesis and release as well as in structures involved in the control of drinking behaviour, including the magnocellular neurones of the hypothalamus, circumventricular organs, and the pituitary gland. This localisation is indicative of a possible functional role in fluid homeostasis. We investigated a role for APJ in the regulation of fluid balance using mice deficient for the receptor. Male APJ wild-type and knockout (APJ(-/-)) mice were housed in metabolic cages to allow determination of water intake and urine volume and osmolality. When provided with free access to water, APJ(-/-) mice drank significantly less than wild-types, while their urine volume and osmolality did not differ. Water deprivation for 24 h significantly reduced urine volume and increased osmolality in wild-type but not in APJ(-/-) mice. Baseline plasma AVP concentration increased comparably in both wild-type and APJ(-/-) mice following dehydration; however, APJ(-/-) mice were unable to concentrate their urine to the same extent as wild-type mice in response to the V2 agonist desmopressin. Analysis of c-fos (Fos as given in MGI Database) mRNA expression in response to dehydration showed attenuation of expression within the subfornical organ, accentuated expression in the paraventricular nucleus, but no differences in expression in the supraoptic nucleus nor median pre-optic nucleus in APJ(-/-) mice compared with wild-type. These findings demonstrate a physiological role for APJ in mechanisms of water intake and fluid retention and suggest an anti-diuretic effect of apelin in vivo.