Ghrelin receptor activity amplifies hippocampal N‐methyl‐d‐aspartate receptor‐mediated postsynaptic currents and increases phosphorylation of the GluN1 subunit at Ser896 and Ser897

Abstract

Although ghrelin and its cognate receptor GHSR1a are highly localized in the hypothalamic nuclei for the regulation of metabolic states and feeding, GHSR1a is also highly localized in the hippocampus suggesting its involvement in extra-hypothalamic functions. Indeed, exogenous application of ghrelin is reported to improve hippocampal learning and memory. However, the underlying mechanism of ghrelin regulation of hippocampal functions is poorly understood. Here we report ghrelin promoted phosphorylation of GluN1 and amplified NMDA receptor (NMDAR)-mediated EPSCs in the CA1 pyramidal cell of hippocampus in slice preparations. The ghrelin-induced responses were sensitive to a GHSR1a antagonist and inverse agonist, and were absent in GHSR1a homozygous (-/-) knock-out mice. These results indicated that activation of GHSR1a was critical in the ghrelin-induced enhancement of the NMDAR function. Interestingly, heterozygous (+/-) mouse hippocampi were also insensitive to ghrelin treatment, suggesting a slight reduction in the availability of GHSR1a may be sufficient to negate the effect of ghrelin on GluN1 phosphorylation and NMDAR channel activities. In addition, NMDAR-mediated spike current, which is of dendritic origin, was blocked by the GHSR1a antagonist, suggesting the presence of GHSR1a on the pyramidal cell dendrites with physical proximity to NMDAR. Together with our findings on the localization of GHSR1a in the CA1 region of the hippocampus, which was evidenced by fluorescent ghrelin binding, immunoreactivity, and eGFP reporter gene expression, we conclude that the activation of GHSR1a favors a rapid modulation of the NMDA receptor-mediated glutamatergic synaptic transmission by phosphorylating GluN1 subunit in the hippocampus.

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

“…The present observation of the localization of GHSR1a in CA1 hippocampal neurons is in agreement with reports by Cabral et al (2013), Mani et al (2014), and Muniz and Isokawa (2015); all identified GHSR1a in the hippocampus using either a fluorescent ghrelin or a transgenic mouse model carrying eGFP-tagged GHSR1a. The present study also revealed that GHSR1a was expressed and localized in small punctate structures on the somatic and dendritic membranes of pyramidal neurons, or possibly on axon terminals that synapsed on the pyramidal cell soma and dendrites.…”

“…It has been demonstrated that GluN2B-containing NMDA receptors undergo more robust endocytosis (Roche et al, 2001) and have higher surface mobility (Groc et al, 2006) than GluN2A-containing receptors. Phosphorylation of GluN1 at Ser 896/897 was also reported to facilitate NMDAR surface expression (Scott et al, 2001), and ghrelin was reported to facilitate GluN1 phosphorylation at Ser896/897 (Muniz and Isokawa, 2015). Together this evidence suggests that ghrelin and GHSR1a receptor-mediated signaling may preferentially target phosphorylation sites that regulate receptor trafficking and surface expression on both subunits (GluN1 and GluN2B).…”

“…Phosphorylation of the GluN1 subunit at Ser896 by PKC and Ser897 by PKA was shown to facilitate receptor trafficking and surface expression by disrupting GluN1 clustering in the endoplasmic reticulum and Golgi apparatus (Scott et al, 2001). We previously reported that the exogenous application of ghrelin facilitated the phosphorylation of GluN1 at Ser896 and Ser897 and increased NMDAR-mediated synaptic currents in hippocampal slices (Muniz and Isokawa, 2015). This finding led us to investigate whether ghrelin is also involved in the phosphorylation of GluN2B, which could lead to the synergistic or cooperative upregulation of NMDAR function and receptor-mediated synaptic activity.…”

Preclinical evidence of ghrelin as a therapeutic target in epilepsy

“…Ghrelin can increase delivery of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic-type receptors (AMPARs) to synapses and produce functional modifications at excitatory synapses via activating GHS-R1a [ 71 ]. Ghrelin can promote the phosphorylation of GluN1 and amplified N-methyl-D-aspartate receptor (NMDAR)-mediated excitatory postsynaptic currents in the CA1 pyramidal cells of the hippocampus [ 72 ]. However, activation of AMPARs play a critical role in the generation of acute seizures and the antagonist of AMPARs, talampanel, is supposed to be an AED [ 73 ].…”

Estudio de la inhibición de los canales de calcio presinápticos tipo CaV2 por la actividad constitutiva del receptor de ghrelina (GHSR) y su impacto sobre la actividad sináptica en neuronas hipocampales

“…En primer lugar, la administración sistémica de ghrelina promueve la reorganización (Berrout y Isokawa, 2012) e incrementa la densidad de espinas sinápticas a la vez que ratones nulos para ghrelina presentan una menor densidad de espinas sinápticas con respecto a animales salvajes en esta área y presentan problemas para desempeñarse en pruebas de memoria (Diano et al, 2006). Por otro lado, ghrelina desencadena el reclutamiento de receptores AMPA a las sinapsis (Ribeiro et al, 2014), y aumenta la expresión de las subunidades NR2B (Ghersi et al, 2015), y la fosforilación de las subunidades NR1 (Muniz y Isokawa, 2015) las cuales juegan un rol importante en la plasticidad a largo plazo y el aprendizaje regulando la tasa de apertura y la expresión del receptor NMDA (Chen y Roche, 2007;Valenzuela Harrington et al, 2007). La aplicación de ghrelina exógena también induce la potenciación a largo plazo en esta área así como en el GD (Diano et al, 2006).…”