Des-acyl ghrelin attenuates pilocarpine-induced limbic seizures via the ghrelin receptor and not the orexin pathway

Differential impact of a ghrelin receptor antagonist or inverse agonist in the electrical kindling model of epilepsy

Ghrelin is a peptide, which has been shown to affect seizures. However, there is not a consensus about its real impact on the control of seizure severity. We assessed the influence of intra-amygdala injections of a ghrelin receptor (GHSR) antagonist, as well as a GHSR inverse agonist on the electrical kindling-induced seizures. Two unipolar electrodes and a tripolar electrode twisted with a guide cannula were implanted in the skull surface or the basolateral amygdala of adult male rats, respectively. A rapid electrical kindling protocol was applied for kindling epileptogenesis. The stimulations were applied until rats showed three consecutive stage five seizures. Each rat was considered as its control. D-Lys-3-GHRP-6 (1, 12.5, and 25 μg/rat) or [D-Arg, D-phe, D-Trp, heu] substance P (D-SP) (50, 500 and 5000 ng/rat) as the GHSR antagonist or inverse agonist were injected into the basolateral amygdala. Seizure parameters including after-discharge duration (ADD), stage five duration (S5D), and seizure stage (SS) were documented thirty minutes following administration of the drugs or saline. Antagonism of the GHSR in the amygdala, significantly increased seizure induction in the kindled rats, in a dose-dependent manner, and induced spontaneous seizures leading to status epilepticus. Conversely, D-SP had a dose-dependent anticonvulsant activity, indicated by decreased ADD and S5D. The results show that GHSR inverse agonism suppressed seizure severity in the rat amygdala kindling model, whereas GHSR antagonism made seizures more severe. Therefore, when considering the ghrelin system to modulate seizures, it is crucial to note the differential impact of various GHSR ligands.

Down regulation of the hippocampal ghrelin receptor type-1a during electrical kindling-induced epileptogenesis

Numerous studies have shown that the ghrelin hormone is involved in epileptic conditions. However, the profile of ghrelin or its functional receptor mRNAs in seizure-susceptible brain areas was not assessed during epileptogenesis. Here, we measured the expression levels of the hippocampal ghrelin or its receptor mRNAs during electrical kindling-induced epileptogenesis. The study was conducted on twenty adult male Wistar rats. One tri-polar and two uni-polar electrodes were stereotaxically implanted in the baso-lateral amygdala or skull surface, respectively. Animals were divided into four groups, consisting of two sham groups (sham1 and sham2), and two other groups, which were partially or fully kindled. After the establishment of partial or full kindling, the hippocampi of the animals and that of the corresponding sham groups were removed. A quantitative real-time PCR technique was used to measure the expression levels of ghrelin or its functional receptor mRNAs. The results indicated that the expression levels of ghrelin did not alter in either of the partially or fully kindled rats compared to the corresponding sham groups. Ghrelin receptor (ghrelinR) down regulated, significantly in the fully-kindled rats, compared to sham2 group. Meanwhile, the mRNA expression levels of ghrelinR did not change in partially-kindled rats compared to sham1 group. The outcomes of the current study highlight the crucial, unknown impact of the hippocampal ghrelinR through the development of electrical kindling epileptogenesis, and points out the importance of ghrelinR as a goal to adjust epileptogenic progression.

Hippocampal orexin-1 and endocannabinoid-1 receptors underlie the kainate-induced occlusion in theta-burst long- term potentiation

INTRODUCTION

Seizures may result from the hyperexcitable neuronal activity of the brain. Multiple neurotransmitter receptors, including orexin (OX) and endocannabinoids interfere with forming the synaptic responses linked to the seizures. Therefore, this study investigates the involvement of OX-1 (OX1R) and endocannbinoid-1 (CB1R) receptors in the kainate- induced excitability in the synaptic field responses.

MATERIAL AND METHODS

Theta pattern used to stimulate Schaffer collaterals and then metal microelectrodes to record the CA1 field excitatory postsynaptic potentials (fEPSPs). Input/ output stimulation and responses and paired- pulse (PP) stimuli employed to measure the state of synaptic activity in normal and kainate- induced seizure-like hyperexcitable activities and the slope of fEPSPs used as a measure of the change in the synaptic activity. Furthermore, agonists and antagonists of OX and endocannabinoids infused to investigate the involvement of their receptors.

RESULT

The results showed that kainate application increased the fEPSP slope either in input stimuli with different intensities/output synaptic responses (I/O), or test pulse stimulated baseline synaptic responses (BSR) and, hence, increased the excitability of field responses in the CA1 region. However, neither kainate nor theta burst stimulation (TBS) could alter the PP stimuli -induced synaptic responses. TBS increased the fEPSP slope of the kainate-applied I/O and BSR, however, the increase was not high enough in BSR to be classified as long-term potentiation (LTP). The single-antagonist OX1R and CB1R administration prevented TBS- induced potentiation and partially recovered the effect by adding eCB or OX agonists in kainate-injected animals. In contrast, OX or combined eCB-OX antagonist application group demonstrated nearly full recovery of LTP induction.

CONCLUSION

Our study concludes that blockade of OX1 or CB1 prevents the induction of LTP, and OX infusion or both receptor blockade recovers the LTP.

Hippocampal orexin receptors: Localization and function

Orexin (hypocretin) is secreted from the perifornical/lateral hypothalamus and is well known for sleep regulation. Orexin has two, orexin A and B, transcripts and two receptors, type 1 and 2 (OX1R and OX2R), located in the plasma membrane of neurons in different brain areas, including the hippocampus involved in learning, memory, seizures, and epilepsy, as physiologic and pathologic phenomena. OX1R is expressed in the dentate gyrus and CA1 and the OX2R in the CA3 areas. Orexin enhances learning and memory as well as reward, stress, seizures, and epilepsy, partly through OX1Rs, while either aggravating or alleviating those phenomena via OX2Rs. OX1Rs activation induces long-term changes of synaptic responses in the hippocampus, an age and concentration-dependent manner. Briefly, we will review the localization and functions of hippocampal orexin receptors, their role in learning, memory, stress, reward, seizures, epilepsy, and hippocampal synaptic plasticity.

Antagonism of the ghrelin receptor type 1a in the rat brain induces status epilepticus in an electrical kindling model of epilepsy

Studies have shown the anti-seizure properties of the ghrelin hormone in different models of epilepsy. Nevertheless, the role of the endogenous ghrelin is unknown in the electrical kindling model of epilepsy. In this study, we evaluated the effect of the antagonism of the ghrelin receptors in the brain of fully kindled rats. Adult male Wistar rats weighing 300 g were used. Animals were stereotaxically implanted with two uni-polar electrodes in the skull surface and a tri-polar electrode in the basolateral amygdala, and a guide cannula in the left lateral ventricle. Animals underwent a rapid kindling protocol. After showing three consecutive stages of five seizures, the animals were considered fully kindled. D-Lys-3-GHRP-6 (1, 50, and 100 μg/rat) was injected intracerebroventricularly (i.c.v.) in the kindled animals. Each rat was considered as its control and received a single dose of D-Lys-3-GHRP-6. Seizure parameters including after discharge duration (ADD), seizure stage (SS), stage four latency (S4L), and stage five duration (S5D) were recorded. The paired t test indicated a significant increase in seizure induction. D-Lys-3-GHRP-6 (1 μg/rat; i.c.v.) prolonged ADD in the kindled rats, significantly. D-Lys-3-GHRP-6 (50 and 100 μg/rat; i.c.v.) induced spontaneous seizures, which led to status epilepticus in the kindled rats. The results indicate that the antagonism of the ghrelin functional receptors prolongs seizures and induces status epilepticus in the kindling model of epilepsy, and propose that the endogenous ghrelin signaling has crucial antiepileptic properties.

Targeting the Ghrelin Receptor as a Novel Therapeutic Option for Epilepsy

Epilepsy is a neurological disease affecting more than 50 million individuals worldwide. Notwithstanding the availability of a broad array of antiseizure drugs (ASDs), 30% of patients suffer from pharmacoresistant epilepsy. This highlights the urgent need for novel therapeutic options, preferably with an emphasis on new targets, since “me too” drugs have been shown to be of no avail. One of the appealing novel targets for ASDs is the ghrelin receptor (ghrelin-R). In epilepsy patients, alterations in the plasma levels of its endogenous ligand, ghrelin, have been described, and various ghrelin-R ligands are anticonvulsant in preclinical seizure and epilepsy models. Up until now, the exact mechanism-of-action of ghrelin-R-mediated anticonvulsant effects has remained poorly understood and is further complicated by multiple downstream signaling pathways and the heteromerization properties of the receptor. This review compiles current knowledge, and discusses the potential mechanisms-of-action of the anticonvulsant effects mediated by the ghrelin-R.

Translational potential of the ghrelin receptor agonist macimorelin for seizure suppression in pharmacoresistant epilepsy

BACKGROUND

Current drugs for epilepsy affect seizures, but no antiepileptogenic or disease-modifying drugs are available that prevent or slow down epileptogenesis, which is characterized by neuronal cell loss, inflammation and aberrant network formation. Ghrelin and ghrelin receptor (ghrelin-R) agonists were previously found to exert anticonvulsant, neuroprotective and anti-inflammatory effects in seizure models and immediately after status epilepticus (SE). Therefore, the aim of this study was to assess whether the ghrelin-R agonist macimorelin is antiepileptogenic in the pharmacoresistant intrahippocampal kainic acid (IHKA) mouse model.

METHODS

SE was induced in C57BL/6 mice by unilateral IHKA injection. Starting 24 h after SE, mice were treated intraperitoneally with macimorelin (5 mg/kg) or saline twice daily for 2 weeks, followed by a 2-week wash-out. Mice were continuously electroencephalogram-monitored, and at the end of the experiment neuroprotection and gliosis were assessed.

RESULTS

Macimorelin significantly decreased the number and duration of seizures during the treatment period, but had no antiepileptogenic or disease-modifying effect in this dose regimen. While macimorelin did not significantly affect food intake or body weight over a 2-week treatment period, its acute orexigenic effect was preserved in epileptic mice but not in sham mice.

CONCLUSIONS

While the full ghrelin-R agonist macimorelin was not significantly antiepileptogenic nor disease-modifying, this is the first study to demonstrate its anticonvulsant effects in the IHKA model of drug-refractory temporal lobe epilepsy. These findings highlight the potential use of macimorelin as a novel treatment option for seizure suppression in pharmacoresistant epilepsy.

Potentiality of ghrelin as antioxidant and protective agent

Background

Oxidative stress is the result of cellular troubles related to aerobic metabolism. Furthermore, this stress is always associated with biological responses evoked by physical, chemical, environmental, and psychological factors. Several studies have developed many approaches of antioxidant defense to diminish the severity of many diseases. Ghrelin was originally identified from the rat stomach, and it is a potent growth hormone-releasing peptide that has pleiotropic functions.

Methods

A systematic review was conducted within PubMed, ScienceDirect, MEDLINE, and Scopus databases using keywords such as ghrelin, antioxidant, oxidative stress, and systemic oxidative stress sensor.

Results

In the last decade, many studies show that ghrelin exhibits protection effects against oxidative stress derived probably from its antioxidant effects. Pieces of evidence demonstrate that systemic oxidative stress increase ghrelin levels in the plasma. The expression of ghrelin and its receptor in ghrelin peripheral tissues and extensively in the central nervous system suggests that this endogenous peptide plays an important role as a systemic oxidative stress sensor

Conclusion

The current evidence confirms that ghrelin and its derived peptides (Desacyl-ghrelin, obestatin) act as a protective antioxidant agent. Therefore, stressor modality, duration, and intensity are the parameters of oxidative stress that must be taken into consideration to determine the role of ghrelin, Desacyl-ghrelin, and obestatin in the regulation of cell death pathways.

Ghrelin upregulates the phosphorylation of the GluN2B subunit of the NMDA receptor by activating GHSR1a and Fyn in the rat hippocampus

Ghrelin and its receptor GHSR1a have been shown to exert numerous physiological functions in the brain, in addition to the well-established orexigenic role in the hypothalamus. Earlier work indicated that ghrelin stimulated the phosphorylation of the GluN1 subunit of the NMDA receptor (NMDAR) and enhanced synaptic transmission in the hippocampus. In the present study, we report that the exogenous application of ghrelin increased GluN2B phosphorylation. This increase was independent of GluN2B subunit activity or NMDAR channel activity. However, it depended on the activation of GHSR1a and Fyn as it was blocked by D-Lys3-GHRP-6 and PP2, respectively. Inhibitors for G-protein-regulated second messengers, such as Rp-cAMP, H89, TBB, ryanodine, and thapsigargin, unexpectedly enhanced GluN2B phosphorylation, suggesting that cAMP, PKA, casein kinase II, and cytosolic calcium signaling may oppose to the effect of ghrelin on the phosphorylation of GluN2B. Our findings suggest that 1) GluN2B is likely a molecular target of ghrelin and GHSR1a-driven signaling cascades, and 2) the ghrelin-mediated phosphorylation of GluN2B depends on Fyn activation under complex negative regulation by other second messengers.

Preclinical evidence of ghrelin as a therapeutic target in epilepsy

Ghrelin, an orexigenic peptide synthesized by endocrine cells of the gastric mucosa, plays a major role in inhibiting seizures. However, the underlying mechanism of ghrelin's anticonvulsant action is still unclear. Nowadays, there are considerable evidences showing that ghrelin is implicated in various neurophysiological processes, including learning and memory, neuroprotection, neurogenesis, and inflammatory effects. In this review, we will summarize the effects of ghrelin on epilepsy. It may provide a comprehensive picture of the role of ghrelin in epilepsy.

Obestatin improves oxidative brain damage and memory dysfunction in rats induced with an epileptic seizure

Obestatin was shown to alleviate renal, gastrointestinal and haemorrhage-induced brain injury in rats. In order to investigate the neuroprotective effects of obestatin on seizure-induced oxidative brain injury, an epileptic seizure was induced with a single intraperitoneal (i.p.) dose of pentylenetetrazole (PTZ, 45mg/kg) in male Wistar rats. Thirty minutes before the PTZ injection, rats were treated with either saline or obestatin (1μg/kg, i.p.). Seizure was video-taped and then evaluated by using Racine's scoring (0-5). For the assessment of memory function, passive-avoidance test was performed before seizure induction, which was repeated on the 3rd day of seizure. The rats were decapitated at the 24th or 72nd hour of seizures and brain tissues were obtained for histopathological examination and for measuring levels of malondialdehyde (MDA), glutathione (GSH), reactive oxygen radicals and myeloperoxidase (MPO) activity. Obestatin treatment reduced the average seizure score, decreased the occurrence and duration of generalized tonic-clonic seizures, presenting with a shorter latency to their onset. Increased lipid peroxidation and enhanced generation of oxygen-derived radicals detected at the post-seizure 72nd h were suppressed by the consecutive treatments of obestatin, but no changes were observed by the single obestatin treatment in the 24-h seizure group. Neuronal damage and increased GFAP immunoreactivity, observed in the hippocampal areas and cortex of PTZ-induced rats were alleviated in 3-day obestatin-treated PTZ group. PTZ-induced memory dysfunction was significantly improved in obestatin-treated PTZ group as compared to saline-treated rats. The present data indicate that obestatin ameliorated the severity of PTZ-induced seizures, improved memory dysfunction and reduced neuronal damage by limiting oxidative damage.

Caloric Restriction Protects against Lactacystin-Induced Degeneration of Dopamine Neurons Independent of the Ghrelin Receptor

Parkinson’s disease (PD) is a neurodegenerative disorder, characterized by a loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). Caloric restriction (CR) has been shown to exert ghrelin-dependent neuroprotective effects in the 1-methyl-4-phenyl-1,2,3,6-tetrathydropyridine (MPTP)-based animal model for PD. We here investigated whether CR is neuroprotective in the lactacystin (LAC) mouse model for PD, in which proteasome disruption leads to the destruction of the DA neurons of the SNc, and whether this effect is mediated via the ghrelin receptor. Adult male ghrelin receptor wildtype (WT) and knockout (KO) mice were maintained on an ad libitum (AL) diet or on a 30% CR regimen. After 3 weeks, LAC was injected unilaterally into the SNc, and the degree of DA neuron degeneration was evaluated 1 week later. In AL mice, LAC injection significanty reduced the number of DA neurons and striatal DA concentrations. CR protected against DA neuron degeneration following LAC injection. However, no differences were observed between ghrelin receptor WT and KO mice. These results indicate that CR can protect the nigral DA neurons from toxicity related to proteasome disruption; however, the ghrelin receptor is not involved in this effect.

Acylated but not des-acyl ghrelin is neuroprotective in an MPTP mouse model of Parkinson's disease

The gut hormone ghrelin is widely beneficial in many disease states. However, ghrelin exists in two distinctive isoforms, each with its own metabolic profile. In Parkinson's Disease (PD) acylated ghrelin administration is neuroprotective, however, the role of des-acylated ghrelin remains unknown. In this study, we wanted to identify the relative contribution each isoform plays using the MPTP model of PD. Chronic administration of acylated ghrelin in mice lacking both isoforms of ghrelin (Ghrelin KO) attenuated the MPTP-induced loss on tyrosine hydroxylase (TH) neuronal number and volume and TH protein expression in the nigrostriatal pathway. Moreover, acylated ghrelin reduced the increase in glial fibrillary acidic protein and Ionized calcium binding adaptor molecule 1 microglia in the substantia nigra. However, injection of acylated ghrelin also elevated plasma des-acylated ghrelin, indicating in vivo deacetylation. Next, we chronically administered des-acylated ghrelin to Ghrelin KO mice and observed no neuroprotective effects in terms of TH cell number, TH protein expression, glial fibrillary acidic protein and ionized calcium binding adaptor molecule 1 cell number. The lack of a protective effect was mirrored in ghrelin-O-acyltransferase KO mice, which lack the ability to acylate ghrelin and consequently these mice have chronically increased plasma des-acyl ghrelin. Plasma corticosterone was elevated in ghrelin-O-acyltransferase KO mice and with des-acylated ghrelin administration. Overall, our studies suggest that acylated ghrelin is the isoform responsible for in vivo neuroprotection and that pharmacological approaches preventing plasma conversion from acyl ghrelin to des-acyl ghrelin may have clinical efficacy to help slow or prevent the debilitating effects of PD. Ghrelin exists in the plasma as acyl and des-acyl ghrelin. We determined the form responsible for in vivo neuroprotection in a mouse model of Parkinson's disease. Although exogenous acyl ghrelin is deacylated in situ to des-acyl, only acyl ghrelin was neuroprotective by attenuating dopamine cell loss and glial activation. Acyl ghrelin is a therapeutic option to reduce Parkinson's Disease progression. Cover Image for this issue: doi: 10.1111/jnc.13316.