Inactivation of the Constitutively Active Ghrelin Receptor Attenuates Limbic Seizure Activity in Rodents

Abstract

Ghrelin is a pleiotropic neuropeptide that has been recently implicated in epilepsy. Animal studies performed to date indicate that ghrelin has anticonvulsant properties; however, its mechanism of anticonvulsant action is unknown. Here we show that the anticonvulsant effects of ghrelin are mediated via the growth hormone secretagogue receptor (GHSR). To our surprise, however, we found that the GHSR knockout mice had a higher seizure threshold than their wild-type littermates when treated with pilocarpine. Using both in vivo and in vitro models, we further discovered that inverse agonism and desensitization/internalization of the GHSR attenuate limbic seizures in rats and epileptiform activity in hippocampal slices. This constitutes a novel mechanism of anticonvulsant action, whereby an endogenous agonist reduces the activity of a constitutively active receptor.

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

“…The knock-out mice and their littermates were generated as explained previously (Portelli et al, 2012b). All experiments were carried out in accordance with the national rules on animal experiments and were approved by the Ethics Committee on Animal Experiments of the Vrije Universiteit Brussel, Belgium.…”

“…Each collection period was of 20 min duration. DAG concentrations were chosen based on ghrelin experiments performed in the same model (Portelli et al, 2012b). The concentration of almorexant to fully block the hippocampal orexin receptors was determined by taking into account the pKi values (orexin 1 receptor (OX1R) = 7.8 nM; orexin 2 receptor (OX2R) = 8.0 nM) towards the orexin receptors as well as microdialysis probe recovery.…”

“…Intravenous pilocarpine infusion induces limbic seizures with secondary generalization in mice, and this model is normally used as a screening model to identify potential compounds for further screening in epilepsy-related models (De Bundel et al, 2011;Portelli et al, 2012b). Having already established that ghrelin significantly increased pilocarpine thresholds at the concentration of 1.8 μg/g (Portelli et al, 2012b), we first chose this concentration to test DAG in this mouse model.…”

“…Intravenous pilocarpine infusion induces limbic seizures with secondary generalization in mice, and this model is normally used as a screening model to identify potential compounds for further screening in epilepsy-related models (De Bundel et al, 2011;Portelli et al, 2012b). Having already established that ghrelin significantly increased pilocarpine thresholds at the concentration of 1.8 μg/g (Portelli et al, 2012b), we first chose this concentration to test DAG in this mouse model. Administration of 1.8 μg/g DAG failed to result in seizure threshold alterations when compared to the saline group, whereas the higher concentration of 3 μg/g DAG significantly increased all behavioural thresholds (Shivering: one-way ANOVA F = 4.077, df = 16, P = 0.0402; Rearing: one-way ANOVA F = 8.263, df = 16, P = 0.0043; Falling: one-way ANOVA F = 11.59, df = 16, P = 0.0011; Tonus: one-way ANOVA F = 10.5, df = 16, P = 0.0016; Death: one-way ANOVA F = 10.91, df = 16, P = 0.0014) (Fig.…”

Receptors of Peptides as Therapeutic Targets in Epilepsy Research

“…Alternatively, it acts as an anti-inflammatory agent, as demonstrated in the periphery [223]. Recently, another interesting hypothesis was suggested based on the finding that the GHSR1a receptor has significant basal activity [224] and that the GHSR knockout mice had a higher seizure threshold than their wild-type littermates [225]: an endogenous agonist can have anticonvulsant action by reducing the activity of the constitutively active receptor with a combination of inverse agonism and desensitization/internalization of the receptor [225]. This hypothesis should be tested by newly developed inverse agonists of the GHSR1a receptor [226,227].…”