Age-related decrease of nerve growth factor-like immunoreactivity in the basal forebrain of senescence-accelerated mice

SAMP8 mice as a neuropathological model of accelerated brain aging and dementia: Toshio Takeda's legacy and future directions

Senescence accelerated mice P8 (SAMP8) show significant age-related deteriorations in memory and learning ability in accordance with early onset and rapid advancement of senescence. Brains of SAMP8 mice reveal an age-associated increase of PAS-positive granular structures in the hippocampal formation and astrogliosis in the brain stem and hippocampus. A spongy degeneration in the brain stem appears at 1 month of age and reaches a maximum at 4-8 months. In addition, clusters of activated microglia also appear around the vacuoles in the brain stem. β/A4(Aβ) protein-like immunoreactive granular structures are observed in various regions and increase in number markedly with age. Other age-associated histological changes include cortical atrophy, neuronal cell loss in locus coeruleus and lateral tegmental nuclei, intraneuronal accumulation of lipopigments in Purkinje cells and eosinophilic inclusion bodies in thalamic neurons. A blood-brain barrier dysfunction and astrogliosis are also prominent with advancing age in the hippocampus. These changes are generally similar to the pathomorphology of aging human brains and characterized by their association with some specific glioneuronal reactions. As for the hallmarks of Alzheimer brains, tau morphology has not yet been confirmed regardless of the age-related increase in phosphorylated tau in SAMP8 mice brains, but early age-related Aβ deposition in the hippocampus has recently been published. SAMP8 mice are, therefore, not only a senescence-accelerated model but also a promising model for Alzheimer's disease and other cognitive disorders.

Effect of vitamin K2 on the development of stress-induced osteopenia in a growing senescence-accelerated mouse prone 6 strain

Vitamin K2 (VK2) has been used as a therapeutic agent for osteoporosis, since it has been suggested to be able to reduce the frequency of fractures by improving bone quality; however, bone turnover is strictly regulated by various cytokines and hormones. In the present study, the effect of menaquinone-4 (MK-4) on bone turnover was investigated using the senescence-accelerated mouse prone 6 (SAMP6) strain. Since water-immersion restraint stress (WRS) causes a significant decrease in bone mineral density (BMD), WRS was used as the bone resorption model in the SAMP6 strain. Six-week-old SAMP6 male mice were divided into the following three groups: Control, WRS and WRS + MK-4. WRS was performed for 6 h per day, 5 times a week, for 4 weeks. Following WRS, MK-4 (30 mg/kg) was injected subcutaneously 3 times a week for 4 weeks. No growth retardation was observed in the WRS groups as compared with the control group. In the WRS groups, the BMD was significantly lower than that in the control group. The levels of bone formation and resorption markers were increased in the WRS groups, indicating that WRS reduced the BMD by promoting high bone turnover. A bone histomorphometrical examination showed that the trabecular (Tb) bone mass in the secondary spongiosa at the distal femur was significantly reduced in the WRS mice, and this reduction was abrogated by MK-4 treatment. Specifically, the Tb bone reduction was caused by the activation of osteoclasts (Ocs), and Oc activity was suppressed by MK-4. The number of osteoblasts and the mineral apposition rate were significantly increased in the WRS and WRS + MK-4 mice, suggesting that WRS triggered a significantly higher mineral apposition rate. These results indicate that MK-4 can induce recovery from the bone mineral loss caused by WRS treatment. Further studies are required to clarify the association between bone quality and MK-4.

Effect of intermittent treatment with human Parathyroid Hormone 1-34 in SAMP6 senescence-accelerated mice

We examined trabecular and cortical bone in the senescence-accelerated mouse prone 6 (SAMP6) murine model of senile osteoporosis after treatment with human PTH 1-34. Sixteen-week-old female SAMP6 mice were assigned to control and PTH groups. PTH (20 microg/kg) was administered sc 3 times a week for 12 weeks. The control mouse strain, senescence-accelerated mouse resistant 1 (SAMR1), was used for comparison. The femoral metaphysis and diaphysis were used to measure bone mineral density (BMD), analyze the trabecular and the cortical structure by micro-computed tomography, and for conducting the bone strength test. PTH significantly attenuated the loss of BMD, improved the trabecular bone microstructure, and increased the bone strength in the femoral metaphysis. We did not find any differences in the bone strength of the femoral diaphysis after PTH treatment, although the cortical bone volume and cortical thickness were improved. Although the cortical thickness increased, the cortical bone density decreased, likely because of the increase of cortical porosity in the distal metaphysis after administration of PTH.

Glia and their cytokines in progression of neurodegeneration

A glia-mediated, inflammatory immune response is an important component of the neuropathophysiology of Alzheimer's disease, of the midlife neurodegeneration of Down's syndrome, and of other age-related neurodegenerative conditions. All of these conditions are associated with early and often dramatic activation of, and cytokine overexpression in, microglia and astrocytes, sometimes decades before pathological changes consistent with a diagnosis of Alzheimer's disease are apparent, as in patients with Down's syndrome or head injury. Brains of normal elderly individuals also often show Alzheimer-type neuropathological changes, although to a lesser degree than those seen in Alzheimer's disease itself. These normal age-related glial changes, likely a response to the normal wear and tear of the aging process, raise the threshold of glial activation and thus may explain the fact that even genetically determined Alzheimer's disease, resulting from genetic mutations such as those in beta-amyloid precursor protein and presenilins or from genetic duplication such as of chromosome 21, only shows the full manifestation of the disease decades after birth. In the more common sporadic form of Alzheimer's disease, age-related increases in glial activation and expression of cytokines may act in synergy with other genetic and acquired environmental risks to culminate in the development of disease.

Age-related expression of ?1 receptors and antidepressant efficacy of a selective agonist in the senescence-accelerated (SAM) mouse

The sigma1 receptor is a unique intracellular receptor whose activation results in an efficient modulation of several neurotransmitter responses. Its role as a target for the rapid nongenomic effects of neuro(active)steroids and the age-related diminutions in steroid levels suggested that targeting the sigma1 receptor might allow alleviation of age-related neuronal dysfunctions. We examined here the expression and behavioral efficacy of sigma1 receptors in the senescence-accelerated (SAM) mouse model. The sigma1 receptor mRNA expression was measured by using comparative RT-PCR in the olfactory bulb, hippocampus, hypothalamus, cortex, or cerebellum of senescence-prone SAMP/8 and senescence-resistant SAMR/1 control animals. No difference was observed between substrains in 6-, 9-, and 12-month-old (m.o.) mice. The sigma1 protein expression was analyzed by using immunohistochemical techniques. Labeling was intense in the olfactory bulb, hippocampus, hypothalamus, and midbrain of both SAMR/1 and SAMP/8 mice, and the distribution appeared unchanged in 6-, 9-, and 12-m.o. animals. The receptor's in vivo availability was examined by using in vivo [3H](+)-SKF-10,047 binding. No age-related difference was observed in the olfactory bulb, hippocampus, hypothalamus, cortex, cerebellum, and brainstem of 6- or 12-m.o. SAMR/1 or SAMP/8 mice. The antidepressant efficacy of the selective agonist igmesine was examined in the forced-swimming test. The compound decreased significantly the immobility duration at 60 mg/kg in 6- and 12-m.o. SAMR/1 and in 6-m.o. SAMP/8 mice. In 12-m.o. SAMP/8 mice, the drug efficacy was facilitated; a significant effect was measured at 30 mg/kg. Decreased neurosteroid levels, particularly of progesterone, were seen in 12-m.o. SAMP/8 mice that might explain the enhanced efficacy of igmesine. Preserved sigma1 receptor expression and enhanced behavioral efficacy of sigma1 agonists were measured in SAM animals, confirming the therapeutic opportunities for selective ligands against age-related mood disorders.

Repeated episodic exposure to ethanol affects neurotrophin content in the forebrain of the mature rat

Chronic exposure to ethanol can cause deficits in learning and memory. It has been suggested that withdrawal is potentially more damaging than the ethanol exposure per se. Therefore, we explored the effect of repeated episodic exposure to ethanol on key regulators of cortical activity, the neurotrophins. Rats were exposed to ethanol via a liquid diet for 3 days per week for 6-24 weeks. Control rats were pair-fed an isocaloric liquid diet or ad libitum fed chow and water. The concentrations of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) were determined using enzyme-linked immunosorbant assays (ELISAs). Five telencephalic structures were examined: parietal cortex, entorhinal cortex, hippocampus, the basal nucleus, and the septal nuclei. All five areas expressed each of the three neurotrophins; BDNF was most abundant and NGF the least. The parietal cortex was susceptible to ethanol exposure, NGF and BDNF content increased, and NT-3 content fell, whereas no changes were detectable in the entorhinal cortex. In the hippocampus, the amount all three neurotrophins increased following episodic ethanol exposure. Neurotrophin content in the two segments of the basal forebrain was affected; NGF and NT-3 content in the basal forebrain was reduced and NGF and BDNF content in the septal nuclei was increased by ethanol exposure. In many cases where ethanol had an effect, the change was transient so that by 24 weeks of episodic exposure, no significant changes were evident. Thus, the effects of ethanol are site- and time-dependent. This pattern differs from changes caused by chronic ethanol exposure, hence, neurotrophins must be vulnerable to the effects of withdrawal. Furthermore, the ethanol-induced changes do not appear to fit a model consistent with retrograde regulation, rather they suggest that neurotrophins act through autocrine/paracrine systems.

Chronic exposure to ethanol alters neurotrophin content in the basal forebrain-cortex system in the mature rat: Effects on autocrine-paracrine mechanisms

Neurotrophins are broadly expressed in the mammalian forebrain: notably in cerebral cortex and the basal forebrain (e.g., the septal and basal nuclei). These factors promote neuronal survival and plasticity, and have been implicated as key players in learning and memory. Chronic exposure to ethanol causes learning and memory deficits. We tested the hypothesis that ethanol affects neurotrophin expression and predicted that these changes would be consistent with alterations in retrograde or autocrine/paracrine systems. Mature rats were fed a liquid diet containing ethanol daily for 8 or 24 weeks. Weight-matched controls were pair-fed an isocaloric, isonutritive diet. Proteins from five structures (parietal and entorhinal cortices, hippocampus, and the basal and septal nuclei) were studied. ELISAs were used to determine the concentration of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3). All three neurotrophins were detected in each structure examined. Ethanol treatment significantly (p < 0.05) affected neurotrophin expression in time- and space-dependent manners. NGF content was generally depressed by ethanol exposure, whereas NT-3 content increased. BDNF concentration was differentially affected by ethanol: it increased in the parietal cortex and the basal forebrain and decreased in the hippocampus. With the exception of NGF in the septohippocampal system, the ethanol-induced changes in connected structures were inconsistent with changes that would be predicted from a retrograde model. Thus, the present data (a) support the concept that neurotrophins act through a nonretrograde system (i.e., a local autocrine/paracrine system), and (b) that chronic exposure to ethanol disrupts these regulatory mechanisms.

Exogenous NGF restores endogenous NGF distribution in the brain of the cognitively impaired aged rat

Alzheimer's disease and normal aging may impair retrograde transport of nerve growth factor (NGF) from cortical areas to basal forebrain cholinergic neurons. We demonstrate a relationship between performance in a spatial reference memory task and NGF distribution in the aged rat brain. In addition, exogenous NGF restored endogenous NGF distribution in cognitively impaired aged rats. These data suggest that NGF administration restores utilization of endogenous growth factor in the brain of cognitively impaired aged rats.

Treatment of aged rat sensory neurons in short-term, serum-free culture with nerve growth factor reverses the effect of aging on neurite outgrowth, calcium currents, and neuronal survival

Impaired NGF production and release has been documented in aged animals, suggesting that decreased NGF receptor stimulation may be one factor contributing to neuronal dysfunction with aging. Other studies have suggested that aging may be associated with impaired intracellular responses to NGF. Because aging-associated neuronal dysfunction contributes to morbidity and mortality in the geriatric population, it is important to determine whether the effects of aging on sensory neuron function and survival are reversible. In the present study, we observed significantly decreased neurite outgrowth and neuronal survival in short-term cultures (0-96 h) of dorsal root ganglion (DRG) neurons from aged (>22 months) Fisher 344 x Brown Norway F1 hybrid rats, compared to young (4-6 month) and middle-aged (14 month) animals. From 24 to 96 h in culture, diminished survival of aged neurons appeared to be due to an increased rate of apoptotic cell death. DRG neurons from aged animals also exhibited significantly decreased whole cell, high-threshold voltage-dependent calcium currents, with a larger proportion of L-type current, compared to youthful and middle-aged animals. Treatment of aged DRG neurons with NGF restored neurite outgrowth, neuronal survival and calcium current amplitude and subtype distribution to those observed in youthful DRG neurons.

Age-related changes in the brains of senescence-accelerated mice (SAM): association with glial and endothelial reactions

Twelve substrains of inbred senescence-accelerated mice (SAM) have been developed, among which the SAMP8 and SAMP10 strains show a significant age-related deterioration in learning and memory for passive and active avoidance tasks. These strains have, respectively, a low and high incidence of systemic senile amyloidosis. Although we found no amyloid deposits in their brain parenchyma, a variety of age-related alterations were identified, involving neurons, glia, and vessels in the brain tissues. Here we review the degenerative changes in aged SAMP8 and SAMP10 brains. These changes are generally similar to the pathology of aging human brain and may be characterized by their association with some specific glial reactions.

The role of neuronal growth factors in neurodegenerative disorders of the human brain

Recent evidence suggests that neurotrophic factors that promote the survival or differentiation of developing neurons may also protect mature neurons from neuronal atrophy in the degenerating human brain. Furthermore, it has been proposed that the pathogenesis of human neurodegenerative disorders may be due to an alteration in neurotrophic factor and/or trk receptor levels. The use of neurotrophic factors as therapeutic agents is a novel approach aimed at restoring and maintaining neuronal function in the central nervous system (CNS). Research is currently being undertaken to determine potential mechanisms to deliver neurotrophic factors to selectively vulnerable regions of the CNS. However, while there is widespread interest in the use of neurotrophic factors to prevent and/or reduce the neuronal cell loss and atrophy observed in neurodegenerative disorders, little research has been performed examining the expression and functional role of these factors in the normal and diseased human brain. This review will discuss recent studies and examine the role members of the nerve growth factor family (NGF, BDNF and NT-3) and trk receptors as well as additional growth factors (GDNF, TGF-alpha and IGF-I) may play in neurodegenerative disorders of the human brain.

Senescence-accelerated overexpression of S100beta in brain of SAMP6 mice

S100beta is an astrocyte-derived protein with paracrine and autocrine effects on neurons and glia. Brain S100beta expression increases progressively with age, and this increased expression has been implicated as a factor underlying the increasing risk of Alzheimer's disease that accompanies aging. Senescence acceleration-prone (SAMP) mice are a group of inbred strains that provide animal models of aging and of various age-related disease processes in the brain and peripheral tissues. One of these strains, the osteopenic SAMP6, has not been previously associated with central nervous system alterations. We used Northern and Western immunoblot analysis and immunohistochemical labeling to examine S100beta expression in brains of SAMP6 mice. Cerebral tissue levels of S100beta and of S100beta mRNA were 2.2-fold and 1.6-fold those of senescence-resistant (control) mice at 4 months of age (p < 0.05 in each case), and were 3.7-fold and 1.9-fold those of control mice at 6 months of age (p < 0.01 in each case). In contrast, levels of glial fibrillary acidic protein (GFAP) in cerebral hemispheres were not different from those of controls. Image analysis of immunohistochemical preparations showed increased numbers and immunoreactive intensity of S100beta-immunoreactive astrocytes in both the hippocampus and cerebral cortex of SAMP6 mice at 4 months of age (p < 0.05 or better in each case). These increases were greater in the hippocampus than in the cerebral cortex. In contrast, increases in numbers of GFAP immunoreactive astrocytes were noted only in the hippocampus. Our finding of increased S100beta gene expression in brains of SAMP6 mice mirror age-associated increases in S100beta expression in human brain and suggest that SAMP6 may provide insights into age-associated brain alterations and diseases.