ChiHye Chung

Synaptic potentiation onto habenula neurons in the learned helplessness model of depression

The cellular basis of depressive disorders is poorly understood. Recent studies in monkeys indicate that neurons in the lateral habenula (LHb), a nucleus that mediates communication between forebrain and midbrain structures, can increase their activity when an animal fails to receive an expected positive reward or receives a stimulus that predicts aversive conditions (that is, disappointment or anticipation of a negative outcome). LHb neurons project to, and modulate, dopamine-rich regions, such as the ventral tegmental area (VTA), that control reward-seeking behaviour and participate in depressive disorders. Here we show that in two learned helplessness models of depression, excitatory synapses onto LHb neurons projecting to the VTA are potentiated. Synaptic potentiation correlates with an animal’s helplessness behaviour and is due to an enhanced presynaptic release probability. Depleting transmitter release by repeated electrical stimulation of LHb afferents, using a protocol that can be effective for patients who are depressed, markedly suppresses synaptic drive onto VTA-projecting LHb neurons in brain slices and can significantly reduce learned helplessness behaviour in rats. Our results indicate that increased presynaptic action onto LHb neurons contributes to the rodent learned helplessness model of depression.

Increased expression of the receptor for advanced glycation end products in neurons and astrocytes in a triple transgenic mouse model of Alzheimer's disease

The receptor for advanced glycation end products (RAGE) has been reported to have a pivotal role in the pathogenesis of Alzheimer’s disease (AD). This study investigated RAGE levels in the hippocampus and cortex of a triple transgenic mouse model of AD (3xTg-AD) using western blotting and immunohistochemical double-labeling to assess cellular localization. Analysis of western blots showed that there were no differences in the hippocampal and cortical RAGE levels in 10-month-old adult 3xTg-AD mice, but significant increases in RAGE expression were found in the 22- to 24-month-old aged 3xTg-AD mice compared with those of age-matched controls. RAGE-positive immunoreactivity was observed primarily in neurons of aged 3xTg-AD mice with very little labeling in non-neuronal cells, with the notable exception of RAGE presence in astrocytes in the hippocampal area CA1. In addition, RAGE signals were co-localized with the intracellular amyloid precursor protein (APP)/amyloid beta (Aβ) but not with the extracellular APP/Aβ. In aged 3xTg-AD mice, expression of human tau was observed in the hippocampal area CA1 and co-localized with RAGE signals. The increased presence of RAGE in the 3xTg-AD animal model showing critical aspects of AD neuropathology indicates that RAGE may contribute to cellular dysfunction in the AD brain.

Synaptic enhancement induced by gintonin via lysophosphatidic acid receptor activation in central synapses

Lysophosphatidic acid (LPA) is one of the well-characterized, ubiquitous phospholipid molecules. LPA exerts its effect by activating G protein-coupled receptors known as LPA receptors (LPARs). So far, LPAR signaling has been critically implicated during early development stages, including the regulation of synapse formation and the morphology of cortical and hippocampal neurons. In adult brains, LPARs seem to participate in cognitive as well as emotional learning and memory. Recent studies using LPAR1-deficient mice reported impaired performances in a number of behavioral tasks, including the hippocampus-dependent spatial memory and fear conditioning tests. Nevertheless, the effect of LPAR activation in the synaptic transmission of central synapses after the completion of embryonic development has not been investigated. In this study, we took advantage of a novel extracellular agonist for LPARs called gintonin to activate LPARs in adult brain systems. Gintonin, a recently identified active ingredient in ginseng, has been shown to activate LPARs and mobilize Ca(2+) in an artificial cell system. We found that the activation of LPARs by application of gintonin acutely enhanced both excitatory and inhibitory transmission in central synapses, albeit through tentatively distinct mechanisms. Gintonin-mediated LPAR activation primarily resulted in synaptic enhancement and an increase in neuronal excitability in a phospholipase C-dependent manner. Our findings suggest that LPARs are able to directly potentiate synaptic transmission in central synapses when stimulated exogenously. Therefore, LPARs could serve as a useful target to modulate synaptic activity under pathological conditions, including neurodegenerative diseases.

Hippocampus-dependent cognitive enhancement induced by systemic gintonin administration.

Background A number of neurological and neurodegenerative diseases share impaired cognition as a common symptom. Therefore, the development of clinically applicable therapies to enhance cognition has yielded significant interest. Previously, we have shown that activation of lysophosphatidic acid receptors (LPARs) via gintonin application potentiates synaptic transmission by the blockade of K+ channels in the mature hippocampus. However, whether gintonin may exert any beneficial impact directly on cognition at the neural circuitry level and the behavioral level has not been investigated. Methods In the current study, we took advantage of gintonin, a novel LPAR agonist, to investigate the effect of gintonin-mediated LPAR activation on cognitive performances. Hippocampus-dependent fear memory test, synaptic plasticity in the hippocampal brain slices, and quantitative analysis on synaptic plasticity-related proteins were used. Results Daily oral administration of gintonin for 1 wk significantly improved fear memory retention in the contextual fear-conditioning test in mice. We also found that oral administration of gintonin for 1 wk increased the expression of learning and memory-related proteins such as phosphorylated cyclic adenosine monophosphate-response element binding (CREB) protein and brain-derived neurotrophic factor (BDNF). In addition, prolonged gintonin administration enhanced long-term potentiation in the hippocampus. Conclusion Our observations suggest that the systemic gintonin administration could successfully improve contextual memory formation at the molecular and synaptic levels as well as the behavioral level. Therefore, oral administration of gintonin may serve as an effective noninvasive, nonsurgical method of enhancing cognitive functions.

Oxytocin Protects Hippocampal Memory and Plasticity from Uncontrollable Stress

The hippocampus is vulnerable to uncontrollable stress and is enriched with oxytocin receptors, but their interactive influences on hippocampal functioning are unknown. This study aimed to determine the effects of intranasal oxytocin administration on stress-induced alterations in synaptic plasticity and spatial memory in male rats. While vehicle-administered stressed rats showed impairment in long-term potentiation, enhancement in long-term depression, and weakened spatial memory, these changes were not observed in oxytocin-administered stressed rats. To reveal the potential signaling mechanism mediating these effects, levels of phosphorylated extracellular signal-regulated kinases (pERK) in the hippocampus was examined. Western blotting showed that oxytocin treatment blocked stress-induced alterations of pERK. Additionally, the oxytocin receptor antagonist L-368,899 inhibited the oxytocin’s protective effects on hippocampal memory to stress. Thus, intranasal administration of oxytocin reduced stress effects on hippocampal synaptic plasticity and memory in rats via acting on oxytocin receptors and regulating ERK activity. This study suggests that exogenous oxytocin may be a therapeutically effective means to counter the detrimental neurocognitive effects of stress.

Chronic brain inflammation causes a reduction in GluN2A and GluN2B subunits of NMDA receptors and an increase in the phosphorylation of mitogen-activated protein kinases in the hippocampus

Neuroinflammation plays a key role in the initiation and progression of neurodegeneration in Alzheimer’s disease (AD). Chronic neuroinflammation results in diminished synaptic plasticity and loss of GluN1 N-methyl-D-aspartate (NMDA) receptors in the hippocampus, leading to the cognitive deficits that are the most common symptoms of AD. Therefore, it is suggested that chronic inflammation may alter expression levels of GluN2A and GluN2B subunits of NMDA receptors and associated intracellular signalling. Chronic neuroinflammation was induced by chronic infusion of lipopolysaccharide (LPS) into the fourth ventricle in Fischer-344 rats. The status of hippocampus-dependent memory was evaluated in control rats and rats chronically infused with LPS. Microglial activation in the hippocampus was examined using immunohistochemical staining. Western blot analysis was used to measure membrane levels of GluN2A and GluN2B subunits of NMDA receptors and mitogen-activated protein kinase (MAPK) in the hippocampi of these rats, and immunofluorescent double labeling was used to assess the cellular location of MAPK. Microglial activation was observed in the hippocampi of rats that showed memory impairments with chronic LPS infusion. Chronic LPS infusion reduced the levels of GluN2A and GluN2B and increased the levels of phosphorylated MAPKs in the hippocampus. MAPK-positive immunoreactivity was observed mostly in the neurons and also in non-neuronal cells. Reductions in GluN2A and GluN2B subunits of NMDA receptors coupled with altered MAPK signaling, in response to inflammatory stimuli may be related to the cognitive deficits observed in AD.

Oral Administration of Gintonin Attenuates Cholinergic Impairments by Scopolamine, Amyloid-β Protein, and Mouse Model of Alzheimer’s Disease

Gintonin is a novel ginseng-derived lysophosphatidic acid (LPA) receptor ligand. Oral administration of gintonin ameliorates learning and memory dysfunctions in Alzheimer’s disease (AD) animal models. The brain cholinergic system plays a key role in cognitive functions. The brains of AD patients show a reduction in acetylcholine concentration caused by cholinergic system impairments. However, little is known about the role of LPA in the cholinergic system. In this study, we used gintonin to investigate the effect of LPA receptor activation on the cholinergic system in vitro and in vivo using wild-type and AD animal models. Gintonin induced [Ca2+]i transient in cultured mouse hippocampal neural progenitor cells (NPCs). Gintonin-mediated [Ca2+]i transients were linked to stimulation of acetylcholine release through LPA receptor activation. Oral administration of gintonin-enriched fraction (25, 50, or 100 mg/kg, 3 weeks) significantly attenuated scopolamine-induced memory impairment. Oral administration of gintonin (25 or 50 mg/kg, 2 weeks) also significantly attenuated amyloid-β protein (Aβ)-induced cholinergic dysfunctions, such as decreased acetylcholine concentration, decreased choline acetyltransferase (ChAT) activity and immunoreactivity, and increased acetylcholine esterase (AChE) activity. In a transgenic AD mouse model, long-term oral administration of gintonin (25 or 50 mg/kg, 3 months) also attenuated AD-related cholinergic impairments. In this study, we showed that activation of G protein-coupled LPA receptors by gintonin is coupled to the regulation of cholinergic functions. Furthermore, this study showed that gintonin could be a novel agent for the restoration of cholinergic system damages due to Aβ and could be utilized for AD prevention or therapy.

The role of basolateral amygdala in the regulation of stress-induced phosphorylated extracellular signal-regulated kinase expression in the hippocampus.

Previous studies have shown that the amygdala plays a key role in the modulation of uncontrollable stress effect on hippocampal long-term potentiation and memory in rats. This study aimed to determine the effects of selective neurotoxic lesions of the basolateral amygdala (BLA) on stress-induced glucocorticoid receptor (GR) translocation and alteration of phosphorylated extracellular signal-regulated kinases (pERK) in the hippocampus. Intrinsic neurons of the BLA in rats were destroyed using N-methyl-d-aspartate and the rats were subjected to uncontrollable stress induced by restraint and electrical tail shocks. Western blot analyses showed that stress-induced GR translocation occurred in both rats with sham-operated surgery and rats with BLA lesions. As in the Western blot analyses, immunohistological analyses revealed stress-induced reduction of GR expression in CA1 and dentate gyrus (DG) of the hippocampi in control rats and rats with BLA lesions. In addition, the Western blot analyses showed that, in response to stress, the levels of hippocampal pERK were reduced in the sham-operated controls, but not in the rats with BLA lesions. Interestingly, the immunohistological analyses showed that BLA lesions prevented the stress-induced reductions in hippocampal pERK levels, only in the DG. These results suggested that the amygdala modulates stress-induced cognitive impairments by regulating the ERK signaling pathway in the hippocampus.

Exposure to Stressors Facilitates Long-Term Synaptic Potentiation in the Lateral Habenula

The lateral habenula (LHb) is a small part of the epithalamus that projects to monoamine centers in the brain. Previously, neurotransmission onto the LHb was shown to be abnormally potentiated in animal models of depression. However, synaptic plasticity in this brain area and the effect of stressor exposure on synaptic plasticity of the LHb have not been investigated. Thus, we explored whether the LHb undergoes dynamic changes in synaptic efficacy or not. First, we observed that a moderate LTP occurs in a fraction of LHb neurons obtained from naive Sprague Dawley rats. Interestingly, a single exposure to acute stressors, such as inescapable foot shock or restraint plus tail shock (RTS), significantly enhances the magnitude of LTP in the LHb. We also observed an increased number of LHb neurons expressing phosphorylated cAMP response element-binding protein (pCREB) after exposure to stressors, which may contribute to determine the threshold for LTP induction. LTP induction in the LHb resulted in an additional increase in the number of pCREB-expressing neurons in stress-exposed animals but not in naive control animals. Together, we showed that LHb neurons have heterogeneous propensity for synaptic potentiation at rest; however, a single exposure to stressors greatly facilitates LTP induction in the LHb, suggesting that fundamental alterations in synaptic plasticity in the LHb may occur in animal models of depression or post-traumatic stress disorder. SIGNIFICANCE STATEMENT Stress exposure is known to cause depression in human patients and animal models, although explanations at the cellular level remain to be elaborated. Here, we show that the lateral habenula (LHb) exhibits LTP after a pattern of brief strong stimulation. In addition, we show that stress exposure facilitates LTP in the LHb by lowering the threshold for LTP induction. We observed a selective increase in the number of neurons expressing pCREB in the LHb of animal models of depression. LTP induction results in a further increase in the density of pCREB-expressing neurons only after stress exposure. Our study provides the first evidence that animal models of depression exhibit altered synaptic plasticity of the LHb.

Learning strategy preference of 5XFAD transgenic mice depends on the sequence of place/spatial and cued training in the water maze task.

Learning strategy preference was assessed in 5XFAD mice, which carry 5 familial Alzheimers disease (AD) mutations. Mice were sequentially trained in cued and place/spatial versions of the water maze task. After training, a strategy preference test was conducted in which mice were required to choose between the spatial location where the platform had previously been during the place/spatial training, and a visible platform in a new location. 5XFAD and non-transgenic control mice showed equivalent escape performance in both training tasks. However, in the strategy preference test, 5XFAD mice preferred a cued strategy relative to control mice. When the training sequence was presented in the reverse order (i.e., place/spatial training before cued training), 5XFAD mice showed impairments in place/spatial training, but no differences in cued training or in the strategy preference test comparing to control. Analysis of regional Aβ42 deposition in brains of 5XFAD mice showed that the hippocampus, which is involved in the place/spatial learning strategy, had the highest levels of Aβ42 and the dorsal striatum, which is involved in cued learning strategy, showed a small increase in Aβ42 levels. The effect of training protocol order on performance, and regional differences in Aβ42 deposition observed in 5XFAD mice, suggest differential functional recruitment of brain structures related to learning in healthy and AD individuals.