Jung-Soo Han

Hypothalamic-pituitary-adrenal axis function and corticosterone receptor expression in behaviourally characterized young and aged Long-Evans rats: Glucocorticoids, memory dysfunction and ageing

In the current investigation, hypothalamic–pituitary–adrenal (HPA) axis function was examined in young and aged male Long‐Evans rats that were initially assessed on a version of the Morris water maze sensitive to cognitive impairment during ageing. In behaviourally characterized rats, a 1‐h restraint stress paradigm revealed that plasma corticosterone concentrations in aged cognitively impaired rats took significantly longer to return to baseline following the stressor than did those in young or aged cognitively unimpaired rats. No differences in basal or peak plasma corticosterone concentrations, however, were observed between young or aged rats, irrespective of cognitive status. Using ribonuclease protection assays and in situ hybridization, we evaluated mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) mRNA abundance in young and aged rats characterized on the spatial task. Abundance of MR mRNA was decreased as a function of age in stratum granulosum but not hippocampus proper, and the decrease in MR mRNA was largely unrelated to cognitive status. However, GR mRNA was significantly reduced in several hippocampal subfields (i.e. stratum granulosum and temporal hippocampus proper) and other related cortical structures (medial prefrontal and olfactory regions) of aged cognitively impaired rats compared to either young or aged cognitively unimpaired cohorts, and was significantly correlated with spatial learning ability among the aged rats in each of these brain regions. In agreement with previous stereological data from this ageing model, no changes were detected in neuron density in the hippocampus of the rats used in the in situ hybridization analysis. These data are the first to describe a coordinated decrease in GR mRNA in a functional brain system including hippocampus and related cortical areas that occurs in tandem with impairments of the HPA response to stress and cognitive decline in ageing.

ROS inhibit the expression of testicular steroidogenic enzyme genes via the suppression of Nur77 transactivation.

Steroidogenesis decreases with aging in the testis, whereas the levels of reactive oxygen species (ROS) increase. In addition, ROS have been reported to inhibit testicular steroidogenesis. Here, we investigated the effects of ROS on the transcriptional activity of Nur77, one of the major transcription factors that regulate the expression of steroidogenic enzyme genes. ROS signaling inhibited Nur77 transactivation, which was diminished by either treatment with c-Jun N-terminal kinase (JNK) inhibitor or the expression of a dominant negative form of JNK. This suggests the involvement of JNK signaling, which elevates the expression of c-Jun as well as its phosphorylation in Leydig cells. In transient transfection assays, c-Jun suppressed Nur77 transactivation in a dose-dependent manner. Further studies using c-Jun mutants revealed that the protein level of c-Jun, but not phosphorylation itself, was important for the suppression of Nur77 transactivation. Nur77 directly interacted with c-Jun in vivo, which blocked the DNA binding activity of Nur77. Together, these results suggest that ROS signaling-mediated c-Jun upregulation suppresses the expression of steroidogenic enzyme genes by inhibiting Nur77 transactivation, resulting in the reduction of testicular steroidogenesis. These findings may provide a mechanistic explanation for the age-related decline in testicular steroid hormone production.

Sulforaphane alleviates scopolamine-induced memory impairment in mice

Sulforaphane, an organosulfur compound present in cruciferous vegetables, has been shown to exert neuroprotective effects in experimental in vitro and in vivo models of neurodegeneration. To determine whether sulforaphane can preserve cognitive function, we examined its effects on scopolamine-induced memory impairment in mice using the Morris water maze test. Sulforaphane (10 or 50mg/kg) was administered to C57BL/6 mice by oral gavage for 14 days (days 1-14), and memory impairment was induced by intraperitoneal injection of scopolamine (1mg/kg) for 7 days (days 8-14). Mice that received scopolamine alone showed impaired learning and memory retention and considerably decreased cholinergic system reactivity in the hippocampus and frontal cortex, as indicated by a decreased acetylcholine (ACh) level and an increased acetylcholinesterase (AChE) activity. Sulforaphane significantly attenuated the scopolamine-induced memory impairment and improved cholinergic system reactivity, as indicated by an increased ACh level, decreased AChE activity, and increased choline acetyltransferase (ChAT) expression in the hippocampus and frontal cortex. These effects of sulforaphane on cholinergic system reactivity were confirmed in vitro. Sulforaphane (10 or 20μM) increased the ACh level, decreased the AChE activity, and increased ChAT expression in scopolamine-treated primary cortical neurons. These observations suggest that sulforaphane might exert a significant neuroprotective effect on cholinergic deficit and cognitive impairment.

Alternations of Septal-hippocampal System in the Adult Wistar Rat with Spatial Memory Impairments Induced by Chronic Cerebral Hypoperfusion

In the current investigation, the status of the septo-hippocampal cholinergic pathway and hippocampal mitogen-activated protein kinase (MAPK) signaling was examined in male Wistar rats with chronic cerebral hypoperfusion, which showed cognitive deficits based on assessment on a version of the Morris water maze. Chronic cerebral hypoperfusion was induced by bilateral common artery occlusion and maintained for 12 weeks until behavioral testing. Chronic cerebral hypoperfusion was shown to induce memory impairments and microglial activation in regions of white matter, including the fimbria of hippocampus. Choline acetyltransferase expression of the basal forebrain and expression of hippocampal MAPKs was decreased in rats with BCCAo compared to control rats. The results of this study suggest that cognitive decline induced by chronic cerebral hypoperfusion could be related to dysfunction of the basal forebrain cholinergic system and reduction of hippocampal MAPK activities.

Diabetes augments cognitive dysfunction in chronic cerebral hypoperfusion by increasing neuronal cell death: implication of cilostazol for diabetes mellitus-induced dementia.

Many patients with diabetes are at increased risk of cognitive dysfunction and dementia. Diabetes mellitus is a vascular risk factor that may increase the risk of dementia through its associations with vascular dementia. We tested whether cognitive impairment could be exacerbated in combined injury using a rat model of chronic cerebral hypoperfusion with diabetes. We also determined whether a potent inhibitor of type III phosphodiesterase could prevent the cognitive decline caused by this combined injury. We used Otsuka Long-Evans Tokushima Fatty (OLETF) rats as a model of type II diabetes (T2DM) and Long-Evans Tokushima Otsuka (LETO) rats as a control. Chronic cerebral hypoperfusion was modeled by permanent bilateral common carotid artery occlusion (BCCAO). At 24weeks, the non-diabetic and T2DM rats were randomly assigned into groups for the following experiments: analysis I (1) sham non-diabetic rats (n=8); (2) hypoperfused non-diabetic rats (n=9); (3) sham T2DM rats (n=8); (4) hypoperfused T2DM rats (n=9); analysis II- (1) sham T2DM rats without treatment (n=8); (2) cilostazol-treated T2DM rats (n=8); (3) hypoperfused T2DM rats (n=9); and (4) hypoperfused T2DM rats and cilostazol treatment (n=9). The rats were orally administered cilostazol (50mg/kg) or vehicle once a day for 2weeks after 24weeks. Rats performed Morris water maze tasks, and neuronal cell death and neuroinflammation were investigated via Western blots and histological investigation. Spatial memory impairment was exacerbated synergistically in the hypoperfused T2DM group compared with the hypoperfused non-diabetic group and sham T2DBM group (P<0.05). Compared with the control group, neuronal cell death was increased in the hippocampus of the hypoperfused T2DM group. Cilostazol, a PDE-3 inhibitor, improved the memory impairments through inhibition of neuronal cell death, activation of CREB phosphorylation and BDNF expression in the hypoperfused T2DM group. Our experimental results support the hypothesis that there are deleterious interactions between chronic cerebral hypoperfusion and T2DM. That is, metabolic diseases such as diabetes may exacerbate cognitive impairment in a rat model of vascular dementia. We also suggest that surprisingly, the phosphodiesterase III inhibitor, cilostazol may be useful for the treatment of cognitive impairment in diabetes mellitus-induced dementia. In conclusion, diabetes can aggravate cognitive dysfunction in vascular dementia, and PDE-3 inhibitors, such as cilostazol, may form the basis of a novel therapeutic strategy for diabetes-associated cognitive impairment or vascular dementia.

Interaction of basal forebrain cholinergic neurons with the glucocorticoid system in stress regulation and cognitive impairment.

A substantial number of studies on basal forebrain (BF) cholinergic neurons (BFCN) have provided compelling evidence for their role in the etiology of stress, cognitive aging, Alzheimer’s disease (AD), and other neurodegenerative diseases. BFCN project to a broad range of cortical sites and limbic structures, including the hippocampus, and are involved in stress and cognition. In particular, the hippocampus, the primary target tissue of the glucocorticoid stress hormones, is associated with cognitive function in tandem with hypothalamic-pituitary-adrenal (HPA) axis modulation. The present review summarizes glucocorticoid and HPA axis research to date in an effort to establish the manner in which stress affects the release of acetylcholine (ACh), glucocorticoids, and their receptor in the context of cognitive processes. We attempt to provide the molecular interactive link between the glucocorticoids and cholinergic system that contributes to BFCN degeneration in stress-induced acceleration of cognitive decline in aging and AD. We also discuss the importance of animal models in facilitating such studies for pharmacological use, to which could help decipher disease states and propose leads for pharmacological intervention.

Effects of donepezil, an acetylcholinesterase inhibitor, on neurogenesis in a rat model of vascular dementia.

Vascular dementia (VaD) is the second most common form of dementia caused by cerebrovascular disease. Several recent reports demonstrated that cholinergic deficits are implicated in the pathogenesis of VaD and that cholinergic therapies have shown improvement of cognitive function in patients with VaD. However, the precise mechanisms by which donepezil achieves its effects on VaD are not fully understood. Donepezil hydrochloride is an acetylcholinesterase inhibitor (AChEI) currently used for the symptomatic treatment of Alzheimers disease (AD). Several lines of evidence have demonstrated that AChEIs such as donepezil promote neurogenesis in the central nervous system. We investigated whether donepezil regulated hippocampal neurogenesis after bilateral common carotid artery occlusion (BCCAO) in rats, a commonly used animal model of VaD. To evaluate the effect of donepezil on neurogenesis, we orally treated rats with donepezil (10mg/kg) once a day for 3weeks, and injected BrdU over the same 3-week period to label newborn cells. The doses of donepezil that we used have been reported to activate cholinergic activity in rats. After 3weeks, a water maze task was performed on these rats to test spatial learning, and a subsequent histopathological evaluation was conducted. Donepezil improved memory impairment and increased the number of BrdU-positive cells in the dentate gyrus (DG) of BCCAO animals. These results indicated that donepezil improves cognitive function and enhances the survival of newborn neurons in the DG in our animal model of VaD, possibly by enhancing the expression of choline acetyltransferase and brain-derived neurotropic factor.

Macelignan attenuates LPS-induced inflammation and reduces LPS-induced spatial learning impairments in rats

Previous studies have shown that macelignan has anti-inflammatory and neuroprotective effects. Subsequently, in the current study, we demonstrate that oral administrations of macelignan reduce the hippocampal microglial activation induced by chronic infusions of lipopolysaccharide (LPS) into the fourth ventricle of Fisher-344 rat brains. A Morris water maze was used to evaluate the status of the hippocampal-dependent spatial learning in control rats with an artificial cerebrospinal fluid infusion, rats with chronic LPS infusions, and rats with chronic LPS infusions and oral administrations of macelignan. The rats with chronic LPS infusions showed spatial memory impairments relative to the control rats in the performance of the memory task. Daily administration of macelignan reduced the spatial memory impairments induced by the chronic LPS infusions. The results indicate that macelignan may possess therapeutic potential for the prevention of Alzheimers disease.

SK-PC-B70M from Pulsatilla koreana improves scopolamine-induced impairments of memory consolidation and spatial working memory.

Previous studies have shown that hederacolchiside-E from Pulsatilla koreana has neuroprotective effects and cognition-enhancing effects. Subsequently, in the current study, we demonstrate that oral administrations of oleanolic-glycoside saponins enriched fraction from P. koreana, designated as SK-PC-B70M, improve impairments in memory consolidation and spatial working memory by systemic injection of scopolamine, a muscarinic cholinergic receptor antagonist. In a step-through avoidance task, when the rats stepped through a dark chamber in a shuttle box, an electric shock was given and then SK-PC-B70M was administered 30 min later. Twenty-four hours later, the rats were placed in an illuminated chamber. The rats with SK-PC-B70M treatments showed longer response latencies than rats with only scopolamine. Spatial working memory was measured with a trial-unique matching-to-place task in a water maze which assessed memory for place information over varying lengths of delays. Three delay lengths were used: 1 min, 5 min, and 3 h. In comparison with the control rats, the rats with scopolamine treatments took significantly longer to find the platform in the second trial with 1- and 5-min delays. The rats with both scopolamine and SK-PC-B70M had significantly less search error compared with the rats with scopolamine only. These findings indicate that SK-PC-B70M has effects on reversing impairments of memory consolidation and working memory impairments induced by scopolamine.

Effects of Scutellaria baicalensis on chronic cerebral hypoperfusion-induced memory impairments and chronic lipopolysaccharide infusion-induced memory impairments.

ETHNOPHARMACOLOGICAL RELEVANCE Extracts of the roots of Scutellaria baicalensis Georgi (Labiatae) have been widely used to relieve fever related to bacterial infection and inflammatory diseases in traditional Korean medicine and have been reported to be effective in brain diseases. These experiments were conducted to examine the effects of oral administration of Scutellaria baicalensis extracts on the rescue of memory impairments induced by chronic cerebral hypoperfusion or chronic lipopolysaccharide (LPS) infusion. In addition, the underlying mechanisms of these effects were investigated. MATERIALS AND METHODS In the first experiment, chronic cerebral hypoperfusion was induced in male Wister rats by bilateral common carotid artery occlusion (BCCAo). Daily administration of Scutellaria baicalensis extracts was started on 20 day after BCCAo and given for 40 days. A Morris water maze was then used to evaluate the status of the hippocampal-dependent spatial learning and hippocampal mitogen-activated protein kinase (MAPK) signaling was examined in control rats, rats with chronic cerebral hypoperfusion, and rats with chronic cerebral hypoperfusion that was administered Scutellaria baicalensis. In the second experiment, hippocampal microglial activation was induced by chronic infusions of LPS into the fourth ventricle of Fisher-344 rat brains. Daily administration of Scutellaria baicalensis extracts was started on 7 day after the surgery of LPS infusion and given for 32 days. Spatial memory and hippocampal microglial activation was then examined in control rats with an artificial cerebrospinal fluid infusion, rats with chronic LPS infusion, and rats with chronic LPS infusion that were administered Scutellaria baicalensis. RESULTS Rats that received chronic cerebral hypoperfusion or chronic LPS infusion showed spatial memory impairments relative to their control rats; however, these symptoms were reduced by daily administration of Scutellaria baicalensis. Administration of Scutellaria baicalensis mitigated alterations of hippocampal MAPK signaling by chronic cerebral infusion and microglial activation by chronic LPS infusion. CONCLUSIONS These results indicate that Scutellaria baicalensis may possess therapeutic potential for the prevention of Alzheimers disease and vascular dementia.