Despite their acknowledged efficacy, currently available antidepressants still demonstrate undesirable side effects, shortfalls in effectiveness and a delayed onset of action. All these agents act via monoaminergic mechanisms, although recent studies have begun to note the potential role of the cholinergic system as well as the amino acid pathways in affective isorders. It has been suggested that glutamate NMDA receptor activation may be involved in hippocampal degeneration seen in patients with depression, as well as contributing as a molecular target for the antidepressant action of known antidepressant drugs. Glutamate either separately or via the release of nitric oxide, regulates the release of various transmitters in the brain critical for affective state, e.g. monoamines (noradrenaline, dopamine), indoleamines (5HT), y-aminobutyric acid (GABA) and acetylcholine. The aim of this study was to investigate N-methyl-D-aspartate (I\IMDA) and muscarinic M1 receptor characteristics and also GABA and acetylcholine levels in a genetic animal model of depression, the Flinders Sensitive Line (FSL) rat, with respect to its control, viz. Flinders Resistant Line (FRL) rat, thereby establishing a possible role for the amino acid and cholinergic pathways in the hippocampus and frontal cortex, two brain areas implicated in depression. In addition, anxietylike behaviours were assessed using the open field and social interaction tests. A sensitive liquid chromatography tandem mass spectrometer (LC/MS/MS) method was used in the quantification of acetylcholine as well as high performance liquid chromatography with electrochemical detection (HPLG-EGD) for the quantification of GABA in the above-mentioned brain areas of FSL and FRL rats. NMDA and muscarinic M1 receptor characteristics were expressed in terms of receptor denSity (Bmax) and affinity (Kd) values and were performed using [3H]-MK801 (27.5 Gi/mmol) and quinuclidinyl benzilate (52.0 Gilmmol) for NMDA and M1 receptors, respectively. In addition, to provide evidence for face validity, behavioural assessments were routinely performed using the open field test and social interaction test. Significantly elevated levels of acetylcholine were found in the frontal cortex but with significantly reduced levels in the hippocampus of FSL rats. Cortical and hippocampal muscarinic receptor binding characteristics remained unchanged, while no differences with regard to GABA levels and NMDA receptor binding characteristics were noted in these brain areas. In concordance with studies from the literature, aversive and locomotor behaviour as measured in the open field test, provided evidence of anxiogenic behaviour in the FSL rat, evinced by significantly less social interaction than their FRL counterparts. In addition, evidence for a lack in general activity of the FSL rat in the open field was also noted. Our data therefore suggest the presence of a cholinergic dysfunction in both the frontal cortex and hippocampus of the FSL rat, although this is not accompanied by simultaneous changes in muscarinic M1 receptor binding in key limbic brain regions. Although increased cholinergic drive is a recognised characteristic of FSL rats and is representative of the model's' construct validity, we suggest that the depressive phenotype of these animals is not related to altered cholinergic activity in a single brain region, but instead involves various limbic brain regions, possibly being more dependent on opposing cholinergic deficits in the cortex and hippocampus. / Thesis (M.Sc. (Pharmacology)--North-West University, Potchefstroom Campus, 2009.

Fragile-X syndrome (FXS) is the most common form of inherited intellectual impairment. Caused by the transcriptional repression of the Fmr1 gene on the X chromosome, FXS results in the loss of the Fragile-X Mental Retardation Protein (FMRP). Human female patients with FXS are heterozygous for the Fmr1 mutation whereas males are hemizygous. FXS has been studied far less in females than in males due to a generally less severe clinical phenotype. Previous research has implicated the metabotropic glutamate receptor (mGluR) in synaptic plasticity alterations in the cornu ammonis area 1 (CA1) region of the juvenile male Fmr1 knock-out (KO) hippocampus. In contrast, our investigations into the young adult dentate gyrus (DG) subfield of the hippocampus have revealed N-methyl-D-aspartate receptor (NMDAR)-associated impairments in synaptic plasticity. The current study sought to extend these investigations to the young adult female Fmr1 heterozygous (Het) and Fmr1 KO mouse as well as investigate NMDAR- and mGluR-mediated long-term depression (LTD) in the DG and CA1 of the young adult male Fmr1 KO mouse. Input-output curves and paired pulse measures of short-term plasticity were also evaluated in all genotypes. Field electrophysiology revealed a significant impairment in long-term potentiation (LTP) and LTD in male Fmr1 KO and female Fmr1 Het mice that was associated with NMDAR alteration. A more robust synaptic protocol was not able to rescue LTP in the male Fmr1 KO DG. Paired-pulse low-frequency stimulation and (RS)-3,5-dihydroxyphenylglycine (DHPG)-induced mGluR-LTD was intact in all genotypes and brain regions examined. Although further investigation will be required to expand our understanding of FXS and to fully elucidate the mechanisms behind intact synaptic plasticity in the female Fmr1 KO mouse, our results suggest that NMDARs may be poised as important contributors to hippocampal pathophysiology in FXS. / Graduate

The dentate gyrus is a region that hosts most of the hippocampal cells in mammals. Nevertheless, its role in spatial memory remains poorly understood, especially in light of the recently-studied phenomenon of adult hippocampal neurogenesis and its possible role in aging and chronic brain disease. We found that chronic over-expression of transforming growth factor beta1 (TGFbeta1), a cytokine involved in neurodegenerative disease, results in several modifications of brain structure, including volumetric changes and persistent astrogliosis. Furthermore, TGFbeta1 over-expression affects the generation of new neurons, leading to an increased number of neurons in the dentate gyrus and deficits in spatial memory acquisition and storage in aged mice. Nonetheless, reducing neurogenesis via pharmacological treatment impairs spatial memory in juvenile mice but not in adult or aged mice. This suggests that the addition of new cells to hippocampal circuitry, and not the increased plasticity of these cells, is the most relevant role of neurogenesis in spatial memory. We tested this idea by modifying proliferation in the dentate gyrus at several ages using multiple techniques and evaluating the incorporation of newborn neurons into hippocampal circuitry. We found that all granule neurons, recently generated or not, have the same probability of being incorporated. Therefore, the number of new neurons participating in memory circuits is proportional to their availability. Our conclusion is that adult-generated cells have the same functional relevance as those generated during development. Together, our data show that the dentate gyrus is important for memory processing and that adult neurogenesis may be relevant to its functionality by optimizing the number of neurons for memory processing. The equilibrium between neurogenesis and optimal dentate gyrus size is disrupted when TGFbeta1 is chronically increased, which occurs in neurodegenerative pathologies, leading to cognitive impairment in aged animals.

Neonatal ventral hippocampus (nVH) lesioned animals show molecular and behavioral abnormalities analogous to those described in schizophrenia. As an extension to previous studies that showed an increase in ligand binding of cortical alpha-1 adrenergic receptors (AR) and a dysfunction in alpha-1 AR regulation of mesolimbic dopamine functions in post-pubertal nVH lesioned rats, we investigated the subcellular expression and activity of protein kinase C (PKC)---a second messenger in alpha-1 AR signaling---in the prefrontal cortex (PFC) and nucleus accumbens (NAcc) of post-pubertal nVH lesioned rats. Western blot analysis of membrane and cytosolic fractions showed complex changes in lesioned animals in the expression of different PKC subtypes following saline or alpha-1 AR agonist (cirazoline i.p.) injection. Among these changes, nVH lesioned animals showed a significant increase in membrane bound PKC alpha and phospho-PKC, and a decrease in cytosolic PKC gamma and PKC betaII in the PFC in comparison to sham-lesioned controls following saline. Cirazoline increased membrane bound PKC alpha in controls but decreased it in lesioned animals. In the NAcc, lesioned animals showed an increase in membrane bound and cytosolic PKC epsilon and PKC lambda levels following saline. Following cirazoline, lesioned animals showed a decrease in membrane bound PKC epsilon and PKC lambda, while controls showed an increase in cytosolic and membrane fractions of PKC epsilon with no change in PKC lambda. In vitro PKC activity assays showed increased basal activity in PFC slices of lesioned animals compared to controls, with no difference in NAcc slices. alpha-1 AR stimulation by the agonist phenylephrine (PE) increased PKC activity in PFC of controls while decreasing activity substantially in lesioned animals. In the NAcc, high concentrations of PE increased activity in controls, but decreased activity in lesioned animals. This abnormal expression and activity of PKC in the PFC and NAcc of nVH lesioned animals may be related to abnormal alpha-1 AR functions and may modulate some of the abnormal neuronal functions in these animals, such as working memory deficits and hyper neuronal excitability of the PFC and the NAcc.

In Marcel Proust's most famous novel, In Search of Lost Time, a Madeleine cake elicited in him a nostalgic memory of Combray. Here we present a computational hypothesis of how such an episodic memory is represented in a brain area called the hippocampus, and how the dynamics of the hippocampus allow the storage and recall of such past events. Using the Neural Engineering Framework (NEF), we show how different aspects of an event, after compression, are represented together by hippocampal neurons as a vector in a high dimensional memory space. Single neuron simulation results using this representation scheme match well with the observation that hippocampal neurons are tuned to both spatial and non-spatial inputs. We then show that sequences of events represented by high dimensional vectors can be stored as episodic memories in a recurrent neural network (RNN) which is structurally similar to the hippocampus. We use a state-of-the-art Hessian-Free optimization algorithm to efficiently train this RNN. At the behavioural level we also show that, consistent with T-maze experiments on rodents, the storage and retrieval of past experiences facilitate subsequent decision-making tasks.

The present thesis investigated the effects of a western diet (WD) on cell death and functional outcome following global ischemia in rats. Experiment 1 assessed the effects of a 60-day WD regimen on temperature, activity and glucose levels in normal rats. Experiment 2 evaluated the influence of a 60-day WD regimen on hippocampal CA1 injury and cognition following global ischemia. Results from experiment 1 revealed significant differences in activity levels only; animals fed the WD were less active than control diet animals. Results from experiment 2 suggested that a WD did not aggravate CA1 injury or behavioral deficits. The second portion of my thesis examined the effects of a 120-day WD regimen on stroke severity and cognition following global ischemia. Briefly, the surgical protocol used to induce a global ischemic insult did not produce consistent damage across all animals. Plausible reasons for this surgical variability and future directions are discussed.

海馬迴不僅在學習與記憶中扮演重要的角色，在許多神經退化性疾病中亦佔有重要的地位。海馬迴的齒迴內側區是哺乳動物大腦中成體幹細胞主要來源區域之一，其所新生的海馬迴粒細胞會往上遷移至海馬迴粒細胞層並與固有神經細胞形成功能性連結。 過去的研究發現太少或過量的壓力荷爾蒙均會造成海馬迴粒細胞的死亡，而一定量濃度的皮質固醇對於維持海馬迴粒細胞的生存亦扮演非常重要的角色。在摘除兩側的腎上腺後，海馬迴粒細胞在幾週後會逐漸死亡且造成認知功能的缺損。本實驗即利用雙側腎上腺摘除術建立動物模式，企圖了解海馬迴粒細胞在凋亡的過程中所產生的生理層面的改變。 壓力荷爾蒙（包含皮質固醇，在老鼠稱為corticosterone，在人類稱為cortisol）為腎上腺皮質分泌激素，已知會參與並調控BK 離子通道的選擇性剪接。BK離子通道的孔道形成α次單元由單一基因 (Slo) 負責轉錄，含有STREX外顯子的剪接變異體之α次單元藉由加速神經細胞的再極化，增強過極化電位以及促進鈉離子通道自去活化狀態中回復可造成神經細胞重複激發，而先前的研究已發現過度的激發會對神經細胞產生興奮性毒殺作用。本實驗即探討BK 鉀離子通道選擇性剪接在海馬迴粒細胞凋亡的過程中所扮演的角色。 實驗結果發現，與對照組相比，雙側腎上腺摘除的老鼠海馬迴細胞中含有STREX外顯子的剪接變異體在mRNA含量上確實有改變，而BK 鉀離子通道蛋白質含量亦有所變化。由上述結果推測，含有STREX外顯子的剪接變異體含量可能與海馬迴粒細胞的凋亡機制有關。 / The hippocampus is a brain region central to learning and memory and is a key target of many neurological diseases that have dramatic cognitive consequences, including Alzheimer’s and other forms of dementia, stroke, epilepsy, and chronic stress. Hippocampal granule cells are one of the two cell pools that contain newborn neurons continuously generated from the subgranular zone in adult mammalian brains. The newborn neurons will migrate to the granule cell layer and integrate into preexisting neuron network. Previous studies have indicated that both an excessive and insufficient levels of stress hormones can lead to neuron death. Corticosterone, an adrenal stress hormone, is essential for the survival of granule cells. Bilateral removal of adrenal glands leads to extensive granule cell death over a period of several weeks and gradually causes cognitive deficits. To understand the mechanisms underlying the granule cell death in the hippocampal formation, adrenalectomy (ADX, removal of adrenal glands) was used to specifically eliminate granule cells in the hippocampus, and the subsequent physiological changes in the hippocampal neurons including dentate granule cells are investigated. Stress hormones (corticosterone in rats and cortisol in human) , secreted from the adrenal cortex regulate the alternative splicing of BK channels (big potassium, calcium-voltage activated potassium channels) in adrenal medulla. An inclusion of STREX (stress axis-regulated exon) exon in pore-forming α subunit encoded by Slo gene promotes repetitive firing by speeding action potential repolarization and augmenting the afterhyperpolarization, as well as facilitating sodium channels de-inactivation. In the present study, the role of BK channel alternative splicing in the ADX-induced granule cell death in the hippocampus was explored. The results indicate that BK channel alternative splicing was regulated by stress hormones in the hippocampus including dentate gyrus. The expression patterns of STREX variant in hippocampus were altered after granule cells death induced by ADX, whilst the expression of total slo gene was changes only in translational level. These observations suggest that the alternation in STREX abundance might be involved in the induction of dentate granule cell death.

Alzheimer�s disease (AD) is a neurodegenerative disorder, charaeterised by progressive loss of memory. It is important to understand what factors initiate the onset of AD so that effective therapeutic treatments can be developed to target the precise mechanisms that initiate this disease. Currently, synaptic dysfunction is widely believed to be the first significant alteration preceding the onset of AD, and is thought to be initiated by an intracellular accumulation of amyloid-β (Aβ), or a free radical-induced increase of oxidative stress. As Aβ levels rise during the onset of AD, a concomitant reduction of secreted amyloid precursor protein-α (sAPPα) is observed, as the two proteins exist in equilibrium. Intriguingly, the neuroprotective and neurotrophic properties of sAPPα indicate that it is intimately involved in the physiological pathways of the major hypotheses for the cause of AD, and may also be involved in the mechanisms that underlie learning and memory. Therefore, it is possible that during the onset of AD, the decrease of sAPPα may contribute to synaptic dysfunction by disrupting the mechanisms of synaptic plasticity. Long-term potentiation (LTP) is the leading experimental model for investigating the neural substrate of memory formation, and describes the molecular mechanisms that underlie an increase in the strength of synaptic transmission. The role sAPPα may play in the induction and maintenance of LTP has not previously been addressed in vivo. Therefore, the aim of this thesis was to investigate whether sAPPα affects the induction of LTP in the hippocampus of the anaesthetised rat. The present findings are the first to suggest that sAPPα may modulate the induction of LTP in vivo. Decreasing the function of endogenous sAPPα (with sAPPα-binding antibodies and a pharmacological inhibition of α-secretase) significantly reduced the magnitude of LTP induced in the dentate gyrus. Therefore, the reduction of sAPPα during AD is likely to have a detrimental impact on the mechanisms of synaptic plasticity, and by extension, learning and memory. The present investigation has also found that the application of recombinant, purified sAPPα to the rat hippocampus has an �inverted U-shaped� dose-response effect on the magnitude of LTP. Low concentrations of sAPPα significantly enhanced LTP, supporting previous findings that exogenous sAPPα can facilitate in vitro LTP and enhance memory performance in animals. On the other hand, comparatively high concentrations of sAPPα significantly decreased the magnitude of LTP. This observation is also consistent with previous findings, in which high concentrations of sAPPα have been shown to be less synaptogenic and memory enhancing than lower doses. These results are the first to suggest that sAPPα modulates in vivo synaptic plasticity, and have important implications for the development of strategies to treat AD.

Long-term potentiation (LTP) is an activity-dependent increase in the efficacy of synaptic transmission. In concert with long-term depression (LTD), this synaptic plasticity likely underlies some types of learning and memory. It has been suggested that for LTP/LTD to act as effective memory storage mechanisms, homeostatic regulation is required. This need for plasticity regulation is incorporated into the Bienenstock, Cooper and Munro (BCM) theory by a threshold determining LTD/LTP induction, which is altered by the previous history of activity (Bienenstock et al., 1982). The present work aimed to test key predictions of the BCM model. This was done using field and intracellular recordings in area CA1 of hippocampal slices from young, adult male Sprague-Dawley rats. The first prediction tested was that following a strong, high-frequency priming stimulation all synapses on primed cells will show inhibition of subsequent LTP and facilitation of LTD induction (heterosynaptic metaplasticity). This was confirmed using two independent Schaffer collateral pathways to the same CA1 pyramidal cells. Following priming stimulation to one pathway, LTP induction was heterosynaptically inhibited and LTD facilitated. To more fully investigate whether all synapses show metaplastic changes, the priming stimulation was given in a different dendritic compartment, in stratum oriens, prior to LTP induction in stratum radiatum. This experiment supported the conclusion that all synapses show inhibited LTP following priming. A second prediction of the BCM model is that metaplasticity induction is determined by the history of cell firing. To investigate this, cells were hyperpolarized during priming to completely prevent somatic action potentials. Under these conditions inhibitory priming of LTP was still observed, and thus somatic action potentials are not critical for the induction of the effect. The next aim was to determine the mechanism underlying heterosynaptic metaplasticity. One way in which plasticity induction can be altered is through changes in gamma-aminobutyric acid (GABA)-mediated inhibition of pyramidal cells. For this reason, it was tested whether blocking all GABAergic inhibition, for the duration of the experiment, would prevent priming of LTP. However, priming inhibited subsequent LTP and it was concluded that GABAergic changes do not underlie either the induction, or expression, of the metaplastic state. Proposed revisions to the BCM model predict that postsynaptic elevations in intracellular Ca�⁺ determine the induction of metaplasticity. There are many potential sources for postsynaptic Ca�⁺ elevations, including entry through N-methyl-D-asparate receptors (NMDARs) or voltage-dependent calcium channels (VDCCs), or release from intracellular stores. Results of the present work demonstrate that the inhibition of LTP is dependent on the release of Ca�⁺ from intracellular stores during priming; however this release is not triggered by Ca�⁺ entry through NMDARs or VDCCs, or via activation of metabotropic glutamate receptors. Overall, the present results show that, in accordance with the BCM model, a high level of prior activity induces a cell-wide metaplastic state, such that LTD is facilitated and LTP is inhibited. In contrast to predictions of the BCM model, this is not mediated by cell-firing during priming. Instead the release of Ca�⁺ from intracellular stores is critical for induction of the metaplastic state.

Bibliography: 197-226 p. / xiv, 226 p. : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Examines the alterations in electrophysiological function during hypoxia in the rat hippocampal slice, in particular those alterations induced by extracellular accumulation of adenosine. / Thesis (Ph.D.)--University of Adelaide, Faculty of Science, 1996