In conclusion this is the
In conclusion, this is the first study performed in acute limbic seizure models that shows the ability of DAG to dose-dependently attenuate pilocarpine-induced seizures, albeit at a higher concentration as ghrelin (Portelli et al., 2012b). We also establish that DAG's anticonvulsant mechanism of action seems to involve the ghrelin receptor and not the orexin pathway. Moreover we show that simultaneous blockade of both orexin receptors in the hippocampus does not modulate limbic seizures.
Acknowledgements We thank G. De Smet and R. Berckmans for their technical assistance. This study was supported by the Research Council (OZR 2102) of the Vrije Universiteit Brussel and the FWO (G.0163.10N).
Introduction The growth hormone secretagogue receptor (GHS-R), which is a G-protein-coupled receptor (GPC-R), is an orphan receptor cloned from pig pituitaries in 1996 (Howard et al., 1996). GHS-R1α is pharmacologically active form of GHS-R (Kojima and Kangawa, 2005). The growth hormone secretagogues (GHSs) include a large number of synthetic peptidyl and non-peptidyl molecules. These GHSs have the ability to promote the release of growth Cy7.5 NHS ester (non-sulfonated) synthesis (GH) through GHS-R1α in a wide spectrum of species (Howard et al., 1996). In the last three decades, many synthetic GHSs have been extensively investigated, for example, GHRP-2, Hexarelin, MK-0677(Bednarek et al., 2000), FHI-2571 (Howick et al., 2017) etc. [D-Lys3]-GHRP-6 is the selective antagonist of GHS-R1α (Muccioli et al., 2007). But these GHSs are ectogenous ligands of GHS-R1α. Many efforts have been made to explore the endogenous ligand of GHS-R1α. In 1999, the endogenous ligand for GHS-R1α was isolated from a rat stomach and named ghrelin (Kojima et al., 1999). Ghrelin is an unique hormone with 28 amino acids, in which the serine 3 residue is n-octanoylated (Kojima et al., 1999; Sato et al., 2012). Ghrelin and GHS-R1α transcripts have been found in many organs, including the heart, liver, lung, thyroid gland, pancreas, testes and ovaries, T-cells, (Chanclón et al., 2012; Okuhara et al., 2018; Ueberberg et al., 2009) etc. The activation of GHS-R1a by ghrelin activates phospholipase C to produce diacylglycerol (DAG) and inositol triphosphate (IP3), which results in a transient increase in the concentrations of intracellular calcium (Yamazaki et al., 2012). Ghrelin displays several physiological and pathological functions by binding to GHS-R1α. Indeed, apart from stimulating GH secretion and food intake (Wren et al., 2000), ghrelin plays many other roles, such as promoting prolactin (PRL), adrenocorticotropic hormone (ACTH) secretion (Sato et al., 2012), regulating of cardiovascular roles (Eid et al., 2018), modulating of immune system (Hattori, 2009), treating of cancer, (Lin and Hsiao, 2017) etc. Furthermore, the mRNAs or proteins of ghrelin and GHS-R1α have been detected in the central nervous system in areas such as the medulla oblongata, the midbrain, the hypothalamus, the spinal cord and the sensorimotor area of the cortex, regions for controlling pain transmission (Ferrini et al., 2009). In recent years, a number of studies have focused on the roles and mechanisms of ghrelin in the regulation of pain perception. It has been shown that ghrelin inhibits inflammatory pain in rats through the central opioid receptors (Sibilia et al., 2006). Other researches have been revealed that ghrelin prevents hyperalgesia (Farajdokht et al., 2016), reduces chronic neuropathic pain (Zhou et al., 2014), induces visceral antinociception, (Okumura et al., 2018), etc. In our previous researches, intracerebroventricular (i.c.v.) administration of ghrelin could produce antinociceptive effects in the acute pain in mice (Wei et al., 2013). Our further study has revealed that i.c.v. injection of ghrelin initially activated the GHS-R1α, then increased the release of endogenous δ-opioid peptide PENK to activate the of δ-opioid receptor OPRD to produce antinociception (Liu et al., 2016). It can be seen that there is a close relationship between the ghrelin system and opioid system in pain regulation.