Thr136Ile polymorphism of human vesicular monoamine transporter-1 (SLC18A1 gene) influences its transport activity in vitro. Neural Plast. 2012;33:546-551. [PMID: 23213575 PMCID: PMC3504448 DOI: 10.1155/2012/945373]

Comprehensive analysis of genetic risk loci uncovers novel candidate genes and pathways in the comorbidity between depression and Alzheimer's disease

There is growing evidence of a shared pathogenesis between Alzheimer's disease and depression. Therefore, we aimed to further investigate their shared disease mechanisms. We made use of publicly available brain-specific eQTL data and gene co-expression networks of previously reported genetic loci associated with these highly comorbid disorders. No direct genetic overlap was observed between Alzheimer's disease and depression in our dataset, but we did detect six shared brain-specific eQTL genes: SRA1, MICA, PCDHA7, PCDHA8, PCDHA10 and PCDHA13. Several pathways were identified as shared between Alzheimer's disease and depression by conducting clustering pathway analysis on hippocampal co-expressed genes; synaptic signaling and organization, myelination, development, and the immune system. This study highlights trans-synaptic signaling and synaptoimmunology in the hippocampus as main shared pathomechanisms of Alzheimer's disease and depression.

Individuals with cannabis use are associated with widespread morphological alterations in the subregions of the amygdala, hippocampus, and pallidum

BACKGROUND

Cannabis is the most frequently used illicit drug worldwide. Although multiple structural MRI studies of individuals with cannabis use (CB) have been undertaken, the reports of the volume alterations in the amygdala, hippocampus, and pallidum are not consistent. This study aims to detect subregion-level morphological alterations, analyze the correlation areas with cannabis usage characteristics, and gain new insights into the neuro mechanisms of CB.

METHODS

By leveraging the novel surface-based subcortical morphometry method, 20 CB and 22 age- and sex-matched healthy controls (HC) were included to explore their volumetric and morphological differences in the three subcortical structures. Afterward, the correlation analysis between surface morphological eigenvalues and cannabis usage characteristics was performed.

RESULTS

Compared with volumetric measures, the surface-based subcortical morphometry method detected more significant global morphological deformations in the left amygdala, right hippocampus, and right pallidum (overall-p < 0.05, corrected). More obvious morphological alterations (atrophy or expansion) were observed in specific subregions (vertex-based p-value<0.05, uncorrected) of the three subcortical structures. Both positive and negative subregional correlation areas were reported by the correlation analysis.

CONCLUSIONS

The current study illuminated new pathophysiologic mechanisms in the amygdala, hippocampus, and pallidum at the subregion level, which may inform the subsequent smaller-scale CB research.

TET1-induced DNA demethylation in dentate gyrus is important for reward conditioning and reinforcement

Neuroadaptations in neurocircuitry of reward memories govern the persistent and compulsive behaviors. The study of the role of hippocampus in processing of reward memory and its retrieval is critical to our understanding of addiction and relapse. The aim of this study is to probe the epigenetic mechanisms underlying reward memory in the frame of dentate gyrus (DG). To that end, the rats conditioned to the food baited arm of a Y-maze and subjected to memory probe trial. The hippocampus of conditioned rats displayed higher mRNA levels of Ten-eleven translocase 1 (Tet1) and brain-derived neurotrophic factor (Bdnf) after memory probe trial. The DNA hydroxymethylation and TET1 occupancy at the Bdnf promoters showed concomitant increase. Stereotactic administration of Tet1 siRNA in the DG before and after conditioning inhibited reward memory formation and recall, respectively. Administration of Tet1 siRNA impaired the reward memory recall that was reinstated following administration of exogenous BDNF peptide or after wash-off period of 8 days. Infusion of a MEK/ERK inhibitor, U0126 in the DG inhibited reward memory retrieval. The TET1-induced DNA demethylation at the Bdnf promoters raised BDNF levels in the hippocampus, thereby setting the stage for reward memory retrieval. The study underscores the causative role of TET1 in the DG for reward memory formation and recall.

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.

Blockade of orexin receptors in the hippocampal dentate gyrus reduced the extinction latency of morphine-induced place preference in male rats

Relapse to drugs such as opioids is a major challenge in addiction therapy. It has been known that the orexinergic system has a significant role in mediating reward processing and addiction, as shown by the conditioned place preference (CPP). The dentate gyrus (DG) of the hippocampus receives orexinergic projections from the lateral hypothalamus that has been approved as a critical area arbitrating the maintenance of drug-seeking behavior following the extinction. The present study aimed to investigate the effects of intra-DG administration of the orexin-1 receptor (OX1R) and orexin-2 receptor (OX2R) antagonists on the extinction of morphine-induced CPP in male rats. Animals received different doses of SB334867 (as OX1R antagonist) or TCS OX2 29 (as OX2R antagonist) (0.5, 2.5, and 12.5 nM/0.5 μl DMSO 12 %) bilaterally into the DG during the extinction phase, after CPP had been induced by subcutaneous injection of morphine (5 mg/kg) during a 3-day conditioning phase. The conditioning scores were recorded during the test. The results demonstrated that intra-DG administration of the highest dose of OX1R antagonist (12.5 nM/0.5 μl DMSO 12 %) shortened the extinction latency of morphine-CPP compared to the DMSO group, while the OX2R antagonist did not significantly alter the latency. Findings imply that the blockade of OX1R, but not OX2R, within the DG facilitates the extinction of morphine-induced reward. In conclusion, the OX1R antagonist might be kept in mind as a convenient therapeutic factor in repressing drug-seeking behaviors in an optimum amount of treatment considering the low dose-treatments applied.

In vivo reduction of hippocampal Caveolin-1 by RNA interference alters morphine addiction and neuroplasticity changes in male mice

Prescription opioids are powerful pain-controlling medications that have both benefits and potentially serious risks. Morphine is one of the preferred analgesics that are widely used to treat chronic pain. However, chronic morphine exposure has been found to cause both functional and structural changes in several brain regions, including the medial prefrontal cortex (mPFC), ventral tegmental area (VTA), and hippocampus (HPC), which lead to addictive behavior. Caveolin-1 (Cav-1), a scaffolding protein of membrane lipid rafts (MLRs), has been shown to organize GPCRs and multiple synaptic signaling proteins within the MLRs to regulate synaptic signaling and neuroplasticity. Previously, we showed that in vitro morphine treatment significantly elevates Cav-1 expression and causes neuroplasticity changes. In this study, we confirmed that chronic morphine exposure can significantly increase Cav-1 expression (P < 0.05) and microtubule-associated protein (MAP-2)-positive neuronal dendritic growth in the hippocampus. Moreover, the rewarding effect and dendritic growth in the HPC induced by chronic morphine exposure were significantly inhibited by hippocampal Cav-1 knockdown. Together, these data suggest that Cav-1 in the hippocampus plays an essential role in the neuroplasticity changes that underlie morphine addiction behaviors.

Hippocampal neural cell loss in high-fat diet-induced obese rats-exploring the protein networks, ultrastructure, biochemical and bioinformatical markers

OBJECTIVE

Obesity, which has become one of the main health problems, results from irregular and unhealthy nutrition. In particular, an increase in the intake of high-fat foods leads to obesity and associated disorders. It is noteworthy to specify that obese individuals have memory problems. This study aims to examine the effects of high-fat diet on hippocampus, with stereological, histopathological methods and STRING bioinformatic tool.

METHODS

Female Adult Sprague Dawley rats (n = 20) were equally divided into control (CONT) and high-fat diet (HFD) groups. The control group was given standard rat pellet feed, while the high-fat diet group was fed with a 40 % fat content for 2 months. Following the feeding program, rats were sacrificed. The collected blood samples were analyzed biochemically to determine the level of oxidative stress while performing a stereological and histopathological examination of the brain tissues. Functional protein-protein networks for BDNF, C-Fos, CAT, LPO, SOD and MPO by gene ontology (GO) enrichment analysis were evaluated.

FINDINGS

The number of neurons decreased in the HFD group compared to the CONT group. Damage to the histological structure of the hippocampus region; such as degenerate neurons, damaged mitochondria and extended cisterns of the endoplasmic reticulum was observed. Although C-Fos level and oxidative stress parameters increased in HFD group, BDNF level decreased. While BDNF and C-Fos were observed in pathways related to neuron death, oxidative stress and memory, BDNF was pronounced in the mitochondria, and C-Fos in the endoplasmic reticulum.

DISCUSSION

This study shows that changes in both BDNF and C-Fos levels in obesity due to high-fat diet increase oxidative stress and cause neuron damage in the hippocampus.

Overexpression of AMPD2 indicates poor prognosis in colorectal cancer patients via the Notch3 signaling pathway

BACKGROUND

AMPD2 is a critical enzyme catalyzing smooth muscle energy supply and metabolism; however, its cellular biological function and clinical implication in colorectal cancer (CRC) are largely unknown.

AIM

To clarify the role of AMPD2 in CRC and study the pathway and prognostic value of its role.

METHODS

AMPD2 expression was analyzed by integrated bioinformatics analysis based on TCGA data sets and immunohistochemistry in tissue microarrays, and the correlation between AMPD2 expression and clinicopathological parameters, Notch3 expression, and prognostic features was assessed. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analysis were then performed to investigate the regulatory pathway involved. The effects of AMPD2 expression on CRC cells and Notch3 protein expression were investigated by downregulation and overexpression of AMPD2.

RESULTS

AMPD2 mRNA was significantly overexpressed in tumor tissue when compared with normal tissue in a cohort of the TCGA-COAD data set. Biological function enrichment analysis indicated that the Notch pathway strongly correlated with AMPD2 expression, and that the expression of Notch3 and JAG2 mRNA was positively associated with AMPD2 in CRC tissues. In vitro, AMPD2 overexpression markedly reduced Notch3 protein expression in CRC cells, while knockdown of AMPD2 showed the opposite findings. In addition, protein expression was significantly up-regulated in our CRC cohort as indicated by tissue microarray analysis. High expression of AMPD2 protein correlated with advanced depth of tumor and poor differentiation. Furthermore, high AMPD2 expression in CRC tissues was an indicator of poor outcome for CRC patients.

CONCLUSION

AMPD2 is commonly overexpressed in CRC, and acts as a metabolism oncogene to induce CRC progression through the Notch signaling pathway. Thus, AMPD2 may be a novel prognostic biomarker for CRC.

Training on an Appetitive Trace-Conditioning Task Increases Adult Hippocampal Neurogenesis and the Expression of Arc, Erk and CREB Proteins in the Dorsal Hippocampus

Adult hippocampal neurogenesis (AHN) plays an essential role in hippocampal-dependent memory consolidation. Increased neurogenesis enhances learning, whereas its ablation causes memory impairment. In contrast, few reports suggest that neurogenesis reduces after learning. Although the interest in exploring the role of adult neurogenesis in learning has been growing, the evidence is still limited. The role of the trace- and delay-appetitive-conditioning on AHN and its underlying mechanism are not known. The consolidation of trace-conditioned memory requires the hippocampus, but delay-conditioning does not. Moreover, the dorsal hippocampus (DH) and ventral hippocampus (VH) may have a differential role in these two conditioning paradigms. Here, we have investigated the changes in: (A) hippocampal cell proliferation and their progression towards neuronal lineage; and (B) expression of Arc, Erk1, Erk2, and CREB proteins in the DH and VH after trace- and delay-conditioning in the rat. The number of newly generated cells significantly increased in the trace-conditioned but did not change in the delay-conditioned animals compared to the control group. Similarly, the expression of Arc protein significantly increased in the DH but not in the VH after trace-conditioning. Nonetheless, it remains unaltered in the delay-conditioned group. The expression of pErk1, pErk2, and pCREB also increased in the DH after trace-conditioning. Whereas, the expression of only pErk1 pErk2 and pCREB proteins increased in the VH after delay-conditioning. Our results suggest that appetitive trace-conditioning enhances AHN. The increased DH neuronal activation and pErk1, pErk2, and pCREB in the DH may be playing an essential role in learning-induced cell-proliferation after appetitive trace-conditioning.

Role of orexin-1 receptors in the dorsal hippocampus (CA1 region) in expression and extinction of the morphine-induced conditioned place preference in the rats

Orexinergic system is involved in reward processing and drug addiction. Objectives here, we investigated the effect of intra-hippocampal CA1 administration of orexin-1 receptor (OX1r) antagonist on the expression, and extinction of morphine-induced place preference in rats. Conditioned place preference (CPP) was induced by subcutaneous injection of morphine (5 mg/kg) during a 3-day conditioning phase. Two experimental plots were designed; SB334867 as a selective OX1r antagonist was dissolved in 12% DMSO, prepared in solutions with different concentrations (3, 30, and 300 nM), and microinjected into the CA1 and some neighboring regions (0.5 μl/side), bilaterally. CPP score and locomotor activity were recorded during the CPP test. Results demonstrated that intra-CA1 administration of the OX1r antagonist attenuates the expression of morphine-induced CPP. Furthermore, higher concentrations of SB334867 facilitated the extinction period of morphine-induced CPP and reduced its latency. Nevertheless, solely administration of DMSO did not have any influence on the CPP scores and locomotion in both phases. Our findings suggest that OX1rs in the CA1 region of the hippocampus are involved in the expression of morphine CPP. Moreover, blockade of OX1rs could facilitate extinction and may extinguish the ability of drug-related cues. It seems that the antagonist might be considered as a propitious therapeutic agent in suppressing drug-seeking behaviors.

Effects of dorsal hippocampal orexin-2 receptor antagonism on the acquisition, expression, and extinction of morphine-induced place preference in rats

RATIONALE

Orexinergic system is involved in reward processing and drug addiction.

OBJECTIVES

Here, we investigated the effect of intrahippocampal CA1 administration of orexin-2 receptor (OX2r) antagonist on the acquisition, expression, and extinction of morphine-induced place preference in rats.

METHODS

Conditioned place preference (CPP) was induced by subcutaneous injection of morphine (5 mg/kg) during a 3-day conditioning phase. Three experimental plots were designed; TCS OX2 29 as a selective antagonist of orexin-2 receptors (OX2rs) was dissolved in DMSO, prepared in solutions with different concentrations (1, 3, 10, and 30 nM), and was bilaterally microinjected into the CA1 and some neighboring regions (0.5 μl/side). Conditioning scores and locomotor activities were recorded during the test.

RESULTS

Results demonstrate that intra-CA1 administration of the OX2r antagonist attenuates the induction of morphine CPP during the acquisition and expression phases. Effect of TCS OX2 29 on reduction of morphine CPP was dose-dependent and was more pronounced during the acquisition than the expression. Furthermore, higher concentrations of TCS OX2 29 facilitated the extinction of morphine-induced CPP and reduced extinction latency period. Nevertheless, administration of TCS OX2 29 solutions did not have any influence on locomotor activity of all phases.

CONCLUSIONS

Our findings suggest that OX2rs in the CA1 region of hippocampus are involved in the development of the acquisition and expression of morphine CPP. Moreover, blockade of OX2rs could facilitate extinction and may abrogate or extinguish the ability of drug-related cues, implying that the antagonist might be considered as a propitious therapeutic agent in suppressing drug-seeking behavior.

Methamphetamine-induced enhancement of hippocampal long-term potentiation is modulated by NMDA and GABA receptors in the shell-accumbens

Addictive drugs modulate synaptic transmission in the meso-corticolimbic system by hijacking normal adaptive forms of experience-dependent synaptic plasticity. Psychostimulants such as METH have been shown to affect hippocampal synaptic plasticity, albeit with a less understood synaptic mechanism. METH is one of the most addictive drugs that elicit long-term alterations in the synaptic plasticity in brain areas involved in reinforcement learning and reward processing. Dopamine transporter (DAT) is one of the main targets of METH. As a substrate for DAT, METH decreases dopamine uptake and increases dopamine efflux via the transporter in the target brain regions such as nucleus accumbens (NAc) and hippocampus. Due to cross talk between NAc and hippocampus, stimulation of NAc has been shown to alter hippocampal plasticity. In this study, we tested the hypothesis that manipulation of glutamatergic and GABA-ergic systems in the shell-NAc modulates METH-induced enhancement of long term potentiation (LTP) in the hippocampus. Rats treated with METH (four injections of 5 mg/kg) exhibited enhanced LTP as compared to saline-treated animals. Intra-NAc infusion of muscimol (GABA receptor agonist) decreased METH-induced enhancement of dentate gyrus (DG)-LTP, while infusion of AP5 (NMDA receptor antagonist) prevented METH-induced enhancement of LTP. These data support the interpretation that reducing NAc activity can ameliorate METH-induced hippocampal LTP through a hippocampus-NAc-VTA circuit loop. Synapse 70:325-335, 2016. © 2016 Wiley Periodicals, Inc.

RACK1 promotes maintenance of morphine-associated memory via activation of an ERK-CREB dependent pathway in hippocampus

Existence of long-term drug-associated memories may be a crucial factor in drug cravings and relapse. RACK1 plays a critical role in morphine-induced reward. In the present study, we used conditioned place preference (CPP) to assess the acquisition and maintenance of morphine conditioned place preference memory. The hippocampal protein level of RACK1 and synaptic quantitation were evaluated by Western blotting, immunohistochemistry and electron microscopy, respectively. Additionally, shRACK1 (shGnb2l1) was used to silence RACK1 in vivo to evaluate the role and the underlying mechanism of RACK1 in maintenance of morphine CPP memory. We found that morphine induced CPP was maintained for at least 7 days after the last morphine treatment, which indicated a positive correlation with hippocampal RACK1 level, and was accompanied simultaneously by increases in the synapse density and hippocampal expression of synaptophysin (SYP), phosphorylation of extracellular signal-regulated kinase1/2 (pERK1/2) and the phosphorylation of cyclic adenosine monophosphate response element-binding (pCREB). ShGnb2l1 icv injection significantly suppressed the expression of all above proteins, decreased the synapse density in the hippocampus and attenuated the acquisition and maintenance of morphine CPP. Our present study highlights that RACK1 plays an important role in the maintenance of morphine CPP, likely via activation of ERK-CREB pathway in hippocampus.

Cocaine-induced locomotor sensitization in rats correlates with nucleus accumbens activity on manganese-enhanced MRI

A long-standing goal of substance abuse research has been to link drug-induced behavioral outcomes with the activity of specific brain regions to understand the neurobiology of addiction behaviors and to search for drug-able targets. Here, we tested the hypothesis that cocaine produces locomotor (behavioral) sensitization that correlates with increased calcium channel-mediated neuroactivity in brain regions linked with drug addiction, such as the nucleus accumbens (NAC), anterior striatum (AST) and hippocampus, as measured using manganese-enhanced MRI (MEMRI). Rats were treated with cocaine for 5 days, followed by a 2-day drug-free period. The following day, locomotor sensitization was quantified as a metric of cocaine-induced neuroplasticity in the presence of manganese. Immediately following behavioral testing, rats were examined for changes in calcium channel-mediated neuronal activity in the NAC, AST, hippocampus and temporalis muscle, which was associated with behavioral sensitization using MEMRI. Cocaine significantly increased locomotor activity and produced behavioral sensitization compared with saline treatment of control rats. A significant increase in MEMRI signal intensity was determined in the NAC, but not AST or hippocampus, of cocaine-treated rats compared with saline-treated control rats. Cocaine did not increase signal intensity in the temporalis muscle. Notably, in support of our hypothesis, behavior was significantly and positively correlated with MEMRI signal intensity in the NAC. As neuronal uptake of manganese is regulated by calcium channels, these results indicate that MEMRI is a powerful research tool to study neuronal activity in freely behaving animals and to guide new calcium channel-based therapies for the treatment of cocaine abuse and dependence.

Alterations in the hippocampal phosphorylated CREB expression in drug state-dependent learning

The present study investigated the possible alterations of hippocampal CREB phosphorylation in drug state-dependent memory retrieval. One-trial step-down passive avoidance task was used to assess memory retrieval in adult male NMRI mice. Pre-training administration of ethanol (1g/kg, i.p.) induced amnesia. Pre-test administration of ethanol (1g/kg, i.p) or nicotine (0.7 mg/kg, s.c.) reversed ethanol-induced amnesia, indicating ethanol- or ethanol-nicotine induced state-dependent learning (STD). Using Western blot analysis, it was found that the p-CREB/CREB ratio in the hippocampus increased in the mice that showed successful memory retrieval as compared with untrained mice. In contrast, pre-training administration of ethanol (1g/kg, i.p.) decreased the hippocampal p-CREB/CREB ratio in comparison with the control group. The hippocampal p-CREB/CREB ratio enhanced in ethanol- and ethanol-nicotine induced STD. Moreover, memory impairment induced by pre-training administration of WIN (1 mg/kg, i.p.) improved in the animals that received pre-test administration of WIN (1 mg/kg, i.p.), ethanol (0.5 g/kg, i.p.) or nicotine (0.7 mg/kg, s.c.), suggesting a cross STD between the drugs. The p-CREB/CREB ratio in the hippocampus decreased in the of WIN-induced amnesia and STD groups in comparison with the control group. In addition, cross state-dependent learning between WIN and ethanol or nicotine was associated with the increase of the hippocampal p-CREB/CREB ratio. It can be concluded that phosphorylation of CREB in the hippocampus is a critical event underlying the interaction of co-administration of drugs on memory retrieval in passive avoidance learning.

Differential expression of CART in feeding and reward circuits in binge eating rat model

Binge eating (BE) disrupts feeding and subverts reward mechanism. Since cocaine- and amphetamine-regulated transcript peptide (CART) mediates satiety as well as reward, its role in BE justifies investigation. To induce BE, rats were provided restricted access to high fat sweet palatable diet (HFSPD) for a period of 4 weeks. Immunoreactivity profile of the CART elements, and accompanying neuroplastic changes were studied in satiety- and reward-regulating brain nuclei. Further, we investigated the effects of CART, CART-antibody or rimonabant on the intake of normal chow or HFSPD. Rats fed on HFSPD showed development of BE-like phenotype as reflected by significant consumption of HFSPD in short time frame, suggestive of dysregulated satiety mechanisms. At the mid-point during BE, CART-immunoreactivity was significantly increased in hypothalamic arcuate (ARC), lateral (LH), nucleus accumbens shell (AcbSh) and paraventricular nucleus of thalamus (PVT). However, for next 22-h post-binge time-period, the animals showed no interest in food, and low CART expression. Pre-binge treatment with rimonabant, a drug recommended for the treatment of BE, produced anorexia, increased CART expression in ARC and LH, but not in AcbSh and PVT. Higher dose of CART was required to produce anorexia in binged rats. While neuronal tracing studies confirmed CART fiber connectivity from ARC and LH to AcbSh, increase in CART and synaptophysin immunostaining in this pathway in BE rats suggested strengthening of the CART connectivity. We conclude that CART bearing ARC-LH-PVT-AcbSh reward circuit may override the satiety signaling in ARC-PVN pathway in BE rats.

Ghrelin increases memory consolidation through hippocampal mechanisms dependent on glutamate release and NR2B-subunits of the NMDA receptor

RATIONALE

Ghrelin (Ghr) is a peptide that participates in the modulation of several biological processes. Ghr administration into the hippocampus improves learning and memory in different memory tests. However, the possible mechanisms underlying this effect on memory have not yet been clarified.

OBJECTIVE

The purpose of the present work is to add new insights about the mechanisms by which Ghr modulates long-term memory consolidation in the hippocampus. We examined Ghr effects upon processes related to increased synaptic efficacy as presynaptic glutamate release and changes in the expression of the NR2B-subunits containing n-methyl-d-aspartate receptors (NMDAR), which are critical for LTP induction. We also attempted to determine the temporal window in which Ghr administration induces memory facilitation and if the described effects depend on GHS-R1a stimulation.

RESULTS

The present research demonstrated that Ghr increased glutamate release from hippocampal synaptosomes; intra-hippocampal Ghr administration increased NR2B-subunits expression in CA1 and DG subareas and also reversed the deleterious effects of the NR2B-subunit-specific antagonist, Ro 25-6981, upon memory consolidation and LTP generation in the hippocampus. These effects are likely to be the consequence of GHS-R1a activation.

CONCLUSION

According to the results above mentioned and previous findings, we can hypothesize some of the mechanisms by which Ghr modulates memory consolidation. At presynaptic level, Ghr stimulates glutamate release, probably by enhancing [Ca(2+)]i. At postsynaptic level, the glutamate released activates NMDAR while Ghr also mediates effects directly activating its specific receptors and increases NR2B-subunit expression.

GIRK Channels: A Potential Link Between Learning and Addiction

The ability of drug-associated cues to reinitiate drug craving and seeking, even after long periods of abstinence, has led to the hypothesis that addiction represents a form of pathological learning, in which drugs of abuse hijack normal learning and memory processes to support long-term addictive behaviors. In this chapter, we review evidence suggesting that G protein-gated inwardly rectifying potassium (GIRK/Kir3) channels are one mechanism through which numerous drugs of abuse can modulate learning and memory processes. We will examine the role of GIRK channels in two forms of experience-dependent long-term changes in neuronal function: homeostatic plasticity and synaptic plasticity. We will also discuss how drug-induced changes in GIRK-mediated signaling can lead to changes that support the development and maintenance of addiction.

The stimulating effect of ghrelin on gastric motility and firing activity of gastric-distension-sensitive hippocampal neurons and its underlying regulation by the hypothalamus

Ghrelin is an acylated peptide originally identified in the rat stomach as the endogenous ligand for growth hormone secretagogue receptor (GHSR) that promotes gastric motility. Our aims were to explore the effects of ghrelin on gastric-distension-sensitive neurons in the hippocampus and the potential for ghrelin to regulate gastric motility through the arcuate nucleus (Arc). Single-unit discharges in the hippocampus were recorded extracellularly, and gastric motility in conscious rats was monitored. The expression of GHSR-1a in the hippocampus was determined by PCR, Western blot and fluo-immunohistochemistry staining. Retrograde tracing and fluo-immunohistochemistry staining were used to determine ghrelin neuron projection. Ghrelin-Fluoro-Gold double-labelled neurons and GHSR-1a expression were observed in the Arc and hippocampus, respectively. There were gastric-distension-sensitive neurons in the hippocampus that could be excited by ghrelin or by electrical stimulation of the Arc. The excitatory effects could be blocked completely or partly by pretreatment with the ghrelin receptor antagonist [d-Lys-3]-GHRP-6. Gastric motility was significantly promoted by the administration of ghrelin into the hippocampus in a dose-dependent manner that could be completely abolished by [d-Lys-3]-GHRP-6. Electrical stimulation of the Arc could promote gastric motility as well. Nevertheless, these effects could be mitigated by pretreatment with [d-Lys-3]-GHRP-6. Electrical lesioning of the hippocampus diminished the excitatory effects on gastric motility that were induced by electrical stimulation the Arc. Our findings suggest that ghrelin plays an important role in promoting gastric motility via the hippocampus. The Arc may be involved in regulation of the influence of the hippocampus on gastric motility.