Ghrelin triggers the synaptic incorporation of AMPA receptors in the hippocampus

Abstract

Ghrelin is a peptide mainly produced by the stomach and released into circulation, affecting energy balance and growth hormone release. These effects are guided largely by the expression of the ghrelin receptor growth hormone secretagogue type 1a (GHS-R1a) in the hypothalamus and pituitary. However, GHS-R1a is expressed in other brain regions, including the hippocampus, where its activation enhances memory retention. Herein we explore the molecular mechanism underlying the action of ghrelin on hippocampal-dependent memory. Our data show that GHS-R1a is localized in the vicinity of hippocampal excitatory synapses, and that its activation increases delivery of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic-type receptors (AMPARs) to synapses, producing functional modifications at excitatory synapses. Moreover, GHS-R1a activation enhances two different paradigms of long-term potentiation in the hippocampus, activates the phosphatidylinositol 3-kinase, and increases GluA1 AMPAR subunit and stargazin phosphorylation. We propose that GHS-R1a activation in the hippocampus enhances excitatory synaptic transmission and synaptic plasticity by regulating AMPAR trafficking. Our study provides insights into mechanisms that may mediate the cognitionenhancing effect of ghrelin, and suggests a possible link between the regulation of energy metabolism and learning.T he appetite-stimulating peptide ghrelin is a 28-aa peptide predominantly produced by X/A-like cells in the oxyntic glands of the stomach as well as in the intestine (1), and secreted into the blood stream. This peptide promotes pituitary growth hormone secretion, through activation of the growth hormone secretagogue type 1a receptor (GHS-R1a) or ghrelin receptor (2). Additionally, ghrelin is involved in the regulation of energy balance by increasing food intake and reducing fat utilization (3). Plasma ghrelin levels rise before meals and decrease thereafter (4), a pattern which is consistent with the implication of ghrelin in preprandial hunger and meal initiation. Ghrelin is secreted into the circulation and crosses the blood-brain barrier (5, 6), but there is also evidence for ghrelin synthesis locally in the brain (2,7,8). The GHS-R1a receptor mRNA was initially found in the hypothalamus and in the pituitary gland (9), and later detected in the hippocampus (10). GHS-R1a is a G protein-coupled seventransmembrane domain receptor (3), which can signal through guanine nucleotide-binding protein (G protein) subunit alpha 11 (Gq class) to activate phosphatidylinositol-specific phospholipase C, generating 1,4,5-triphosphate (IP 3 ) responsible for Ca 2+ intracellular release from endoplasmic reticulum, and diacylglicerol, which in turn activates protein kinase C (PKC) (11). Ghrelin receptor activation is also coupled to the phosphatidylinositol 3 (PI3)-kinase signaling cascade in different cellular systems through a pertussis toxin-sensitive G protein (G i/o α) (11), and to protein kinase A (PKA) in isolated hypothalamic neurons, modulating N-type Ca 2+ channels (12).T...

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“…This is consistent with a study in healthy older subjects in which serum ghrelin was linked to poorer WM function (85). Ghrelin has also been associated with the synaptic accumulation of the AMPA glutamate receptors and increases in LTP, a form of synaptic plasticity that is thought to underlie learning and memory (86).…”

Novel Regulation of the Synthesis of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptor Subunit GluA1 by Carnitine Palmitoyltransferase 1C (CPT1C) in the Hippocampus

“…We speculate that CPT1C is a sensor of the energetic status of the neuron through its ability to bind malonyl-CoA (2), an intermediate in the fatty acid synthesis, whose cellular levels in hippocampus fluctuate during fasting and feeding (40). In fact, it has been described that the regulation of energy metabolism contributes to AMPAR synaptic incorporation (41), synaptic plasticity (42,43), and memory processes (44).…”

Carnitine palmitoyltransferase 1C: From cognition to cancer

“…Furthermore, metabolic hormones such as ghrelin and leptin also contribute to the synaptic incorporation of AMPARs and learning processes. It has been demonstrated that ghrelin increases memory retention in rodents [81], enhances longterm potentiation in the hippocampus [82], and increases the delivery of AMPARs to synapses [83]. In addition, leptin regulates AMPAR trafficking [84] and synaptic plasticity [85].…”