Ghrelin signaling in dCA1 suppresses neuronal excitability and impairs memory acquisition via PI3K/Akt/GSK-3β cascades

Nanocurcumin prevents memory impairment, hippocampal apoptosis, Akt and CaMKII-α signaling disruption in the central STZ model of Alzheimer's disease in rat

The central route of streptozotocin (STZ) administration has been introduced as a rat model of sporadic Alzheimer's disease (AD). Curcumin was suggested to possess possible neuroprotective effects, which may be profitable in AD. However, the low bioavailability of curcumin hinders its beneficial effects in clinical studies. Earlier studies suggested that a bovine serum albumin-based nanocurcumin, produces superior neuroprotective effects compared to natural curcumin. In the present study, the protective effect of nanocurcumin in rat model of central STZ induced memory impairment was assessed. In addition, due to the importance of the hippocampus in memory, the amounts of hippocampal active caspase-3, Akt, and CaMKII-α were evaluated. Adult male Wistar rats weighing 250-300 g were used. STZ (icv) was injected during days 1 and 3 (3 mg/kg in divided), and nanocurcumin or curcumin 50 mg/kg/oral gavage was administered daily during days 4-14. Morris water maze training was performed on days 15-17, and the retention memory test was achieved on the 18th day. Following memory assessment, the rats were sacrificed and the hippocampi were used to assess caspase-3 cleavage, Akt, and CaMKII-α signaling. The findings revealed that nanocurcumin ingestion (but not natural curcumin) in the dose of 50 mg/kg was capable to prevent the impairment of water maze learning and memory induced by central STZ. Molecular assessments indicated that STZ treatment increased the caspase-3 cleavage in the hippocampus while deactivating Akt and CaMKII-α. Nanocurcumin reduced caspase-3 cleavage to a non-significant level compared to control group and restored Akt and CaMKII-α within the hippocampus while natural curcumin exerted no significant effect. These findings might suggest that nanocurcumin can restore memory deficit, hippocampal apoptosis as well as Akt and CaMKII-α signaling disruption associated with brain insulin resistance.

GHS-R1a deficiency protects against lipopolysaccharide-induced spatial memory impairment in mice

Neuroinflammation has been implicated in cognitive deficits of neurological and neurodegenerative diseases. There is abundant evidence that the application of ghrelin, an orexigenic hormone regulating appetite and energy balance, abrogates neuroinflammation and rescues associated memory impairment. However, the underlying mechanism is uncertain. In this study, we find that both intraperitoneal (i.p.) and intracerebroventricular (i.c.v.) administration of lipopolysaccharide (LPS) impairs spatial memory in mice. LPS treatment causes neuroinflammation and microglial activation in the hippocampus. Ghsr1a deletion suppresses LPS-induced microglial activation and neuroinflammation, and rescued LPS-induced memory impairment. Our findings thus suggest that GHS-R1a signaling may promote microglial immunoactivation and contribute to LPS-induced neuroinflammation. GHS-R1a may be a new therapeutic target for cognitive dysfunction associated with inflammatory conditions.

Transcranial Magneto-Acoustic Stimulation Protects Synaptic Rehabilitation from Amyloid-Beta Plaques via Regulation of Microglial Functions

Transcranial magneto-acoustic stimulation (TMAS), which is characterized by high spatiotemporal resolution and high penetrability, is a non-invasive neuromodulation technology based on the magnetic-acoustic coupling effect. To reveal the effects of TMAS treatment on amyloid-beta (Aβ) plaque and synaptic plasticity in Alzheimer's disease, we conducted a comparative analysis of TMAS and transcranial ultrasound stimulation (TUS) based on acoustic effects in 5xFAD mice and BV2 microglia cells. We found that the TMAS-TUS treatment effectively reduced amyloid plaque loads and plaque-associated neurotoxicity. Additionally, TMAS-TUS treatment ameliorated impairments in long-term memory formation and long-term potentiation. Moreover, TMAS-TUS treatment stimulated microglial proliferation and migration while enhancing the phagocytosis and clearance of Aβ. In 5xFAD mice with induced microglial exhaustion, TMAS-TUS treatment-mediated Aβ plaque reduction, synaptic rehabilitation improvement, and the increase in phospho-AKT levels were diminished. Overall, our study highlights that stimulation of hippocampal microglia by TMAS treatment can induce anti-cognitive impairment effects via PI3K-AKT signaling, providing hope for the development of new strategies for an adjuvant therapy for Alzheimer's disease.

Acylated Ghrelin Activates PI3K/mTOR Signaling Pathway by Promoting ThPOK Acetylation to Promote Milk Fat Synthesis in Bovine Mammary Epithelial Cells

Ghrelin regulates diverse physiological activities. However, the effects of this hormone on the milk fat synthesis remain unknown. This study aimed to investigate the effect of acylated ghrelin (AG) on milk fat synthesis by modifying the expression (knockdown or overexpression) of growth hormone secretagogue receptor 1a (GHSR1a) and Th-inducing POK (ThPOK) in primary bovine mammary epithelial cells (BMECs). The results showed that AG significantly increased the triglyceride relative content from 260.83 ± 9.87 to 541.67 ± 8.38 in BMECs via GHSR1a. ThPOK functions as a key regulatory target downstream of AG, activating the PI3K and mTOR signaling pathways to promote milk fat synthesis in BMECs. Moreover, AG-regulated ThPOK by increasing the EP300 activity, which promoted ThPOK acetylation to protect it from proteasomal degradation. In conclusion, AG increases ThPOK acetylation and stabilizes ThPOK through GHSR1a, thereby activating the PI3K/mTOR signaling pathway and ultimately promoting the milk fat synthesis in BMECs.

GHSR1a deficiency suppresses inhibitory drive on dCA1 pyramidal neurons and contributes to memory reinforcement

Growth hormone secretagogue receptor 1a (GHSR1a)-the receptor for orexigenic hormone ghrelin-is a G protein-coupled receptor that is widely distributed in the brain, including the hippocampus. Studies have demonstrated that genetic deletion of GHSR1a affects memory, suggesting the importance of ghrelin/GHSR1a signaling in cognitive control. However, current reports are controversial, and the mechanism underlying GHSR1a modulation of memory is uncertain. Here, we first report that global GHSR1a knockout enhances hippocampus-dependent memory, facilitates initial LTP in dorsal hippocampal Schaffer Collateral-CA1 synapses, and downregulates Akt activity in the hippocampus. Moreover, we show that the intrinsic excitability of GAD67+ interneurons-rather than neighboring pyramidal neurons in the dCA1-is suppressed by GHSR1a deletion, an effect that is antagonized by acute application of the Akt activator SC79. In addition, the inhibitory postsynaptic currents (IPSCs) on dCA1 pyramidal neurons are selectively reduced in mice with a GHSR1a deficiency. Finally, we demonstrate that selectively increasing the excitability of parvalbumin-expressing interneurons by hM3Dq-DREADDs increases IPSCs on dCA1 pyramidal neurons and normalizes memory in Ghsr1a KO mice. Our findings thus reveal a novel mechanism underlying memory enhancement of GHSR1a deficiency and herein support an adverse effect of GHSR1a signaling in hippocampus-dependent memory processes.

Ghrelin signaling in the cerebellar cortex enhances GABAergic transmission onto Purkinje cells

Ghrelin, an orexigenic peptide ligand for growth hormone secretagogue receptor 1a (GHS-R1a), occurs not only in the stomach but also in the brain, and modulates neuronal activity and synaptic efficacy. Previous studies showed that GHS-R1a exists in the cerebellum, and ghrelin facilitates spontaneous firing of Purkinje cells (PCs). However, the effects of ghrelin on cerebellar GABAergic transmission have yet to be elucidated. We found that ghrelin enhanced GABAergic transmission between molecular layer interneurons (MLIs) and PCs using electrophysiological recordings in mouse cerebellar slices. This finding was consistent with the possibility that blocking synaptic transmission enhanced the ghrelin-induced facilitation of PC firing. Ghrelin profoundly increased the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) in PCs without affecting miniature or stimulation-evoked IPSCs, whereas it significantly facilitated spontaneous firing of MLIs. This facilitation of MLI spiking disappeared during treatments with blockers of GHS-R1a, type 1 transient receptor potential canonical (TRPC1) channels and KCNQ channels. These results suggest that both activating TRPC1 channels and inhibiting KCNQ channels occur downstream the ghrelin-GHS-R1a signaling pathway probably in somatodendritic sites of MLIs. Thus, ghrelin can control PC firing directly and indirectly via its modulation of GABAergic transmission, thereby impacting activity in cerebellar circuitry.

Ghrelin system in Alzheimer's disease

Alzheimer's disease (AD) is the most common type of dementia in seniors. Current efforts to understand the etiopathogenesis of this neurodegenerative disorder have brought forth questions about systemic factors in the development of AD. Ghrelin is a brain-gut peptide that is activated by ghrelin O-acyltransferase (GOAT) and signals via its receptor, growth hormone secretagogue receptor (GHSR). With increasing recognition of the neurotropic effects of ghrelin, the role of ghrelin system deregulation in the development of AD has been accentuated in recent years. In this review, we summarized recent research progress regarding the mechanisms of ghrelin signaling dysregulation and its contribution to AD brain pathology. In addition, we also discussed the therapeutic potential of strategies targeting ghrelin signaling for the treatment of this neurological disease.

Potential of Heterogeneous Compounds as Antidepressants: A Narrative Review

Depression is a globally widespread disorder caused by a complicated interplay of social, psychological, and biological factors. Approximately 280 million people are suffering from depression worldwide. Traditional frontline antidepressants targeting monoamine neurotransmitters show unsatisfactory effects. The development and application of novel antidepressants for dissimilar targets are on the agenda. This review characterizes the antidepressant effects of multiple endogenous compounds and/or their targets to provide new insight into the working mechanism of antidepressants. We also discuss perspectives and challenges for the generation of novel antidepressants.

Ozone causes depressive-like response through PI3K/Akt/GSK3β pathway modulating synaptic plasticity in young rats

Ozone pollution has been associated with several adverse effects, including memory impairment, intellectual retardation, emotional disturbances. However, the potential mechanisms remain uncertain. The present study aimed to investigate whether ozone (O₃) regulates synaptic plasticity through PI3K/Akt/GSK3β signaling pathway and induces neurobehavioral modifications among the young rats. In vivo, the newborn rats were used to construct the animal model of early postnatal O₃ treatment. In vitro, this study measured the effect of different concentrations of serum from O₃ treated rats on the viability of the PC12 cells, and investigated the modifications of synaptic plasticity and PI3K/Akt/GSK3β signaling pathway in the hippocampus and PC12 cells after O₃ treated. The results revealed significant depression-like behavior and increased hippocampal histopathological damage in the young rats after O₃ treated. Compared with the control group, the expression levels of synaptic related proteins including Drebrin, PSD95, Synaptophysin and PIK3R1, p-Akt, and p-GSK3β were decreased in the O₃ treated group. In vitro assays, a significant reduction in Drebrin, PSD95, Synaptophysin, PIK3R1, p-Akt, and p-GSK3β was found in PC12 cells after O₃ serum treated. While 740Y-P (a specific PI3K activator) administered, the expression levels of Drebrin, PSD95, Synaptophysin, PIK3R1, p-Akt, and p-GSK3β in the 740Y-P + O₃ group were significantly elevated in vivo and vitro compared with the O₃-only group. In addition, miRNAs modulating PIK3R1 were screened on bioinformatics website, the study found aberrant expression of miR-221-3p in the hippocampus and serum of O₃ treated group. Inhibition of miR-221-3p expression effectively reversed the reduction of Drebrin, PSD95, Synaptophysin, PIK3R1, p-Akt, and p-GSK3β in PC12 cells induced by O₃ treatment. Altogether, these studies indicate that O₃ restrained the expression of PI3K/Akt/GSK3β signaling pathway and impaired synaptic plasticity that resulted in depressive-like behavior in young rats. Moreover, miR-221-3p plays an important role in this procedure by regulating PIK3R1.

Ghrelin infusion into the basolateral amygdala suppresses CTA memory formation in rats via the PI3K/Akt/mTOR and PLC/PKC signaling pathways

Ghrelin is a circulating orexigenic hormone that promotes feeding behavior and regulates metabolism in humans and rodents. We previously reported that local infusion of ghrelin into the basolateral amygdala (BLA) blocked memory acquisition for conditioned taste aversion (CTA) by activating growth hormone secretagogue receptor 1a. In this study, we further explored the underlying mechanism and signaling pathways mediating ghrelin modulation of CTA memory in rats. Pharmacological agents targeting distinct signaling pathways were infused into the BLA during conditioning. We showed that preadministration of the PI3K inhibitor LY294002 abolished the repressive effect of ghrelin on CTA memory. Moreover, LY294002 pretreatment prevented ghrelin from inhibiting Arc and zif268 mRNA expression in the BLA triggered by CTA memory retrieval. Preadministration of rapamycin eliminated the repressive effect of ghrelin, while Gsk3 inhibitors failed to mimic ghrelin's effect. In addition, PLC and PKC inhibitors microinfused in the BLA blocked ghrelin's repression of CTA acquisition. These results demonstrate that ghrelin signaling in the BLA shapes CTA memory via the PI3K/Akt/mTOR and PLC/PKC pathways. We conducted in vivo multichannel recordings from mouse BLA neurons and found that microinjection of ghrelin (20 µM) suppressed intrinsic excitability. By means of whole-cell recordings from rat brain slices, we showed that bath application of ghrelin (200 nM) had no effect on basal synaptic transmission or synaptic plasticity of BLA pyramidal neurons. Together, this study reveals the mechanism underlying ghrelin-induced interference with CTA memory acquisition in rats, i.e., suppression of intrinsic excitability of BLA principal neurons via the PI3K/Akt/mTOR and PLC/PKC pathways.

Inhibition of Glycogen Synthase Kinase 3β Activity in the Basolateral Amygdala Disrupts Reconsolidation and Attenuates Heroin Relapse

Exposure to a heroin-associated conditioned stimulus can reactivate drug reward memory, trigger drug cravings, and induce relapse in heroin addicts. The amygdala, a brain region related to emotions and motivation, is involved in processing rewarding stimulus. Recent evidence demonstrated that disrupting the reconsolidation of the heroin drug memories attenuated heroin seeking which was associated with the basolateral amygdala (BLA). Meanwhile, neural functions associated with learning and memory, like synaptic plasticity, are regulated by glycogen synthase kinase 3 beta (GSK-3β). In addition, GSK-3β regulated memory processes, like retrieval and reconsolidation of cocaine-induced memory. Here, we used a heroin intravenous self-administration (SA) paradigm to illustrate the potential role of GSK-3β in the reconsolidation of drug memory. Therefore, we used SB216763 as a selective inhibitor of GSK-3β. We found that injecting the selective inhibitor SB216763 into the BLA, but not the central amygdala (CeA), immediately after heroin-induced memory retrieval disrupted reconsolidation of heroin drug memory and significantly attenuated heroin-seeking behavior in subsequent drug-primed reinstatement, suggesting that GSK-3β is critical for reconsolidation of heroin drug memories and inhibiting the activity of GSK-3β in BLA disrupted heroin drug memory and reduced relapse. However, no retrieval or 6 h after retrieval, administration of SB216763 into the BLA did not alter heroin-seeking behavior in subsequent heroin-primed reinstatement, suggesting that GSK-3β activity is retrieval-dependent and time-specific. More importantly, a long-term effect of SB216763 treatment was observed in a detectable decrease in heroin-seeking behavior, which lasted at least 28 days. All in all, this present study demonstrates that the activity of GSK-3β in BLA is required for reconsolidation of heroin drug memory, and inhibiting GSK-3β activity of BLA disrupts reconsolidation and attenuates heroin relapse.

Virus-mediated GHS-R1a expression in the basolateral amygdala blocks extinction of conditioned taste aversion memory in rats

Ghrelin is an orexigenic gastric hormone that promotes feeding behaviors and regulating energy homeostasis in both humans and rodents. Our previous studies have shown that ghrelin, when locally infused into the basolateral amygdala (BLA), blocks both acquisition and extinction of conditioned taste aversion (CTA) memory in rats. In this study, we further investigated the effect of virus-mediated overexpression of ghrelin receptor growth hormone secretagogue receptor 1a (GHS-R1a) in BLA pyramidal neurons on CTA memory processes. We found that upregulation of GHS-R1a expression in BLA pyramidal neurons repressed CTA extinction while it had no effect on CTA acquisition. In addition, we reported that local infusion of the endogenous GHS-R1a antagonist, liver-expressed antimicrobial peptide 2 (LEAP2), in the BLA abolished the inhibitory effect of increased GHS-R1a on CTA memory extinction. Those findings provide new supportive evidence that ghrelin/GHS-R1a signaling in the BLA circuit shapes emotional memory processes.