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

The Effect of Ghrelin on the Maturation of Sheep Oocytes and Early Embryonic Development In Vitro

Simple Summary

Different gradients of ghrelin (0, 100, 200, and 300 ng/mL) were added to the IVM system of sheep oocytes to observe their changes, and 200 ng/mL ghrelin was found to be the optimal concentration. The RNA-seq analysis showed that the Cell cycle signaling pathway was enriched. The results suggest that adding ghrelin shortens the duration of IVM of sheep oocytes and hinders early embryonic development.

Abstract

In vitro maturation (IVM) of sheep oocytes and early embryonic development are of great scientific importance for the study of reproductive development in sheep. Ghrelin is an important hormone that regulates the secretion of the growth hormone (GH). In this study, different gradients of ghrelin (0, 100, 200, and 300 ng/mL) were added to the IVM system of sheep oocytes to observe their cell morphology, and Hosesth 33342 staining was used to determine the time taken for oocytes to reach different developmental stages. We found 200 ng/mL ghrelin to be the optimal concentration. The RNA-seq analysis showed that many signaling pathways were significantly altered by ghrelin. Cell cycle, Wnt, and oxidative phosphorylation were activated; the P53 was inhibited. These pathways together regulate the maturation of oocytes and early embryonic development in vitro. The effects of the addition of ghrelin were verified by the expression of GLUT1 in early embryonic development. The results suggest that adding ghrelin shortens the duration of the IVM of sheep oocytes and hinders early embryonic development. This study provides new insights into the effects of exogenous ghrelin on sheep oocyte maturation and early embryonic development in vitro.

Ghrelin-O-acyltransferase (GOAT) acylates ghrelin in the hippocampus

Ghrelin is an appetite-stimulating peptide hormone and produced in the stomach. Serine 3 on ghrelin must be acylated by the lipid transferase known as Ghrelin-O-acyltransferase (GOAT) in order for the peptide to become physiologically-active and bind to the cognate receptor, growth hormone secretagogue receptor type 1a (GHSR1a). GHSR1a has been known to be expressed in the feeding center of the hypothalamus. However, the interest in GHSR1a increased dramatically among researchers in various biomedical fields when GHSR1a mRNA was found wide-spread in the brain including the hippocampus. Current understanding is that GHSR1a has multifaceted functions beyond the regulation of metabolism. In the blood, a nonacylated form of ghrelin (des-acyl ghrelin) exists in far greater amounts. Des-acyl ghrelin can cross the blood-brain barrier (BBB), but it cannot bind to GHSR1a in the brain. Thus, the identification of the source for acyl ghrelin in the brain became the critical and urgent quest. Here, we discuss the presence of GOAT in the hippocampus and its ability to acylate ghrelin locally within the hippocampus. We will show that GOAT is localized specifically at the base of the dentate granule cell layer in the rat and wild-type mouse, but not in the GHSR1a knockout mouse. This evidence points the possibility that the expression of GHSR1a may be a prerequisite for the synthesis of GOAT in the hippocampus. We will also show that: (1) the activation of GHSR1a by acyl ghrelin upregulates the cAMP and CREB phosphorylation, (2) amplifies the NMDA receptor-mediated synaptic transmission by phosphorylating GluN1 subunit at Ser896/897, and (3) activates Fyn kinase and induces GluN2B phosphorylation at Tyr1336. In summary, GOAT is a critical molecule that acts as the master switch in the initiation of ghrelin-induced hippocampal synapse and neuron plasticity.

Therapeutic potential of GHSR-1A antagonism in alcohol dependence, a review

Growth hormone secretagogue receptor type 1A (GHSR-1A) is a functional receptor of orexigenic peptide ghrelin and is highly expressed in mesolimbic dopaminergic systems that regulate incentive value of artificial reward in substance abuse. Interestingly, GHSR-1A has also shown ligand-independent constitutive activity. Alcohol use disorder (AUD) is one of the growing concerns worldwide as it involves complex neuro-psycho-endocrinological interactions. Positive correlation of acylated ghrelin and alcohol-induced human brain response in the right and left ventral striatum are evident. In the last decade, the beneficial effects of ghrelin receptor (GHSR-1A) antagonism to suppress artificial reward circuitries and induce self-control for alcohol consumption have drawn significant attention from researchers. In this updated review, we summarize the available recent preclinical, clinical, and experimental data to discuss functional, molecular actions of central ghrelin-GHSR-1A signaling in different craving levels for alcohol as well as to promote "GHSR-1A antagonism" as one of the potential therapies in early abstinence.

Ghrelin Is Effective on Passive Avoidance Memory by Altering the Expression of NMDAR and HTR1a Genes in the Hippocampus of Male Wistar Rats

Background

Memory-dependent psychological behaviors have an important role in life. Memory strengthening in adulthood to prevent its defects in aging is a significant issue. The ghrelin endogenous hormone improves memory by targeting glutamatergic and serotonergic circuits. Also, citicoline, a memory strengthening drug in aging, is not recommended to adults due to its side effects. The current study aims to test that ghrelin treatment, like citicoline, would improve passive avoidance memory via expression of the genes encoding the N-methyl-D-aspartate receptor (NMDAR1) and the serotonin receptor 1A (HTR1α) involved in this process.

Methods

Five groups of adult male rats received (1) saline (as control), (2) 0.5 mg/kg citicoline, or (3-5) 0.3, 1.5, and 3 nmol/μl ghrelin). The rats received the drugs via intra-hippocampal injection. Passive avoidance memory was determined using a shuttle box device. The latency to enter the dark chamber before (IL) and after (RL) injection and the total duration of the animal's presence in the light compartment (TLC) were evaluated. Then, the gene expression rates of NMDAR1 and HTR1α were measured by the Real-Time PCR.

Results

Ghrelin and citicoline had some similar and significant effects on passive avoidance memory, and both increased NMDAR1 and decreased HTR1α expression.

Conclusion

Ghrelin, like citicoline, improves passive avoidance learning by altering the NMDAR1 and HTR1α expression in the hippocampus.

Glutamate in Male and Female Sexual Behavior: Receptors, Transporters, and Steroid Independence

The survival of animal species predicates on the success of sexual reproduction. Neurotransmitters play an integral role in the expression of these sexual behaviors in the brain. Here, we review the role of glutamate in sexual behavior in rodents and non-rodent species for both males and females. These encompass the release of glutamate and correlations with glutamate receptor expression during sexual behavior. We then present the effects of glutamate on sexual behavior, as well as the effects of antagonists and agonists on different glutamate transporters and receptors. Following that, we discuss the potential role of glutamate on steroid-independent sexual behavior. Finally, we demonstrate the interaction of glutamate with other neurotransmitters to impact sexual behavior. These sexual behavior studies are crucial in the development of novel treatments of sexual dysfunction and in furthering our understanding of the complexity of sexual diversity. In the past decade, we have witnessed the burgeoning of novel techniques to study and manipulate neuron activity, to decode molecular events at the single-cell level, and to analyze behavioral data. They pose exciting avenues to gain further insight into future sexual behavior research. Taken together, this work conveys the essential role of glutamate in sexual behavior.

THE INTRIGUING LIGAND-DEPENDENT AND LIGAND-INDEPENDENT ACTIONS OF THE GROWTH HORMONE SECRETAGOGUE RECEPTOR ON REWARD-RELATED BEHAVIORS

The growth hormone secretagogue receptor (GHSR) is a G-protein-coupled receptor (GPCR) highly expressed in the brain, and also in some peripheral tissues. GHSR activity is evoked by the stomach-derived peptide hormone ghrelin and abrogated by the intestine-derived liver-expressed antimicrobial peptide 2 (LEAP2). In vitro, GHSR displays ligand-independent actions, including a high constitutive activity and an allosteric modulation of other GPCRs. Beyond its neuroendocrine and metabolic effects, cumulative evidence shows that GHSR regulates the activity of the mesocorticolimbic pathway and modulates complex reward-related behaviors towards different stimuli. Here, we review current evidence indicating that ligand-dependent and ligand-independent actions of GHSR enhance reward-related behaviors towards appetitive stimuli and drugs of abuse. We discuss putative neuronal networks and molecular mechanisms that GHSR would engage to modulate such reward-related behaviors. Finally, we briefly discuss imaging studies showing that ghrelin would also regulate reward processing in humans. Overall, we conclude that GHSR is a key regulator of the mesocorticolimbic pathway that influences its activity and, consequently, modulates reward-related behaviors via ligand-dependent and ligand-independent actions.

Endogenous ghrelin-O-acyltransferase (GOAT) acylates local ghrelin in the hippocampus

Ghrelin is an appetite-stimulating peptide. Serine 3 on ghrelin must be acylated by octanoate via the enzyme ghrelin-O-acyltransferase (GOAT) for the peptide to bind and activate the cognate receptor, growth hormone secretagogue receptor type 1a (GHSR1a). Interest in GHSR1a increased dramatically when GHSR1a mRNA was demonstrated to be widespread in the brain, including the cortex and hippocampus, indicating that it has multifaceted functions beyond the regulation of metabolism. However, the source of octanoylated ghrelin for GHSR1a in the brain, outside of the hypothalamus, is not well understood. Here, we report the presence of GOAT and its ability to acylate non-octanoylated ghrelin in the hippocampus. GOAT immunoreactivity is aggregated at the base of the dentate granule cell layer in the rat and wild-type mouse. This immunoreactivity was not affected by the pharmacological inhibition of GHSR1a or the metabolic state-dependent fluctuation of systemic ghrelin levels. However, it was absent in the GHSR1a knockout mouse hippocampus, pointing the possibility that the expression of GHSR1a may be a prerequisite for the production of GOAT. Application of fluorescein isothiocyanate (FITC)-conjugated non-octanoylated ghrelin in live hippocampal slice culture (but not in fixed culture or in the presence of GOAT inhibitors) mimicked the binding profile of FITC-conjugated octanoylated ghrelin, suggesting that extracellularly applied non-octanoylated ghrelin was acylated by endogenous GOAT in the live hippocampus while GOAT being mobilized out of neurons. Our results will advance the understanding for the role of endogenous GOAT in the hippocampus and facilitate the search for the source of ghrelin that is intrinsic to the brain.

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

Ghrelin and its receptor GHSR1a have been shown to exert numerous physiological functions in the brain, in addition to the well-established orexigenic role in the hypothalamus. Earlier work indicated that ghrelin stimulated the phosphorylation of the GluN1 subunit of the NMDA receptor (NMDAR) and enhanced synaptic transmission in the hippocampus. In the present study, we report that the exogenous application of ghrelin increased GluN2B phosphorylation. This increase was independent of GluN2B subunit activity or NMDAR channel activity. However, it depended on the activation of GHSR1a and Fyn as it was blocked by D-Lys3-GHRP-6 and PP2, respectively. Inhibitors for G-protein-regulated second messengers, such as Rp-cAMP, H89, TBB, ryanodine, and thapsigargin, unexpectedly enhanced GluN2B phosphorylation, suggesting that cAMP, PKA, casein kinase II, and cytosolic calcium signaling may oppose to the effect of ghrelin on the phosphorylation of GluN2B. Our findings suggest that 1) GluN2B is likely a molecular target of ghrelin and GHSR1a-driven signaling cascades, and 2) the ghrelin-mediated phosphorylation of GluN2B depends on Fyn activation under complex negative regulation by other second messengers.

Preclinical evidence of ghrelin as a therapeutic target in epilepsy

Ghrelin, an orexigenic peptide synthesized by endocrine cells of the gastric mucosa, plays a major role in inhibiting seizures. However, the underlying mechanism of ghrelin's anticonvulsant action is still unclear. Nowadays, there are considerable evidences showing that ghrelin is implicated in various neurophysiological processes, including learning and memory, neuroprotection, neurogenesis, and inflammatory effects. In this review, we will summarize the effects of ghrelin on epilepsy. It may provide a comprehensive picture of the role of ghrelin in epilepsy.

Estrogen affects neuropathic pain through upregulating N-methyl-D-aspartate acid receptor 1 expression in the dorsal root ganglion of rats

Estrogen affects the generation and transmission of neuropathic pain, but the specific regulatory mechanism is still unclear. Activation of the N-methyl-D-aspartate acid receptor 1 (NMDAR1) plays an important role in the production and maintenance of hyperalgesia and allodynia. The present study was conducted to determine whether a relationship exists between estrogen and NMDAR1 in peripheral nerve pain. A chronic sciatic nerve constriction injury model of chronic neuropathic pain was established in rats. These rats were then subcutaneously injected with 17β-estradiol, the NMDAR1 antagonist D(-)-2-amino-5-phosphonopentanoic acid (AP-5), or both once daily for 15 days. Compared with injured drug naïve rats, rats with chronic sciatic nerve injury that were administered estradiol showed a lower paw withdrawal mechanical threshold and a shorter paw withdrawal thermal latency, indicating increased sensitivity to mechanical and thermal pain. Estrogen administration was also associated with increased expression of NMDAR1 immunoreactivity (as assessed by immunohistochemistry) and protein (as determined by western blot assay) in spinal dorsal root ganglia. This 17β-estradiol-induced increase in NMDAR1 expression was blocked by co-administration with AP-5, whereas AP-5 alone did not affect NMDAR1 expression. These results suggest that 17β-estradiol administration significantly reduced mechanical and thermal pain thresholds in rats with chronic constriction of the sciatic nerve, and that the mechanism for this increased sensitivity may be related to the upregulation of NMDAR1 expression in dorsal root ganglia.