Matthew E. Nelson

Effects of a Saturated Fat and High Cholesterol Diet on Memory and Hippocampal Morphology in the Middle-Aged Rat

Diets rich in cholesterol and/or saturated fats have been shown to be detrimental to cognitive performance. Therefore, we fed a cholesterol (2%) and saturated fat (hydrogenated coconut oil, Sat Fat 10%) diet to 16-month old rats for 8 weeks to explore the effects on the working memory performance of middle-aged rats. Lipid profiles revealed elevated plasma triglycerides, total cholesterol, HDL, and LDL for the Sat-Fat group as compared to an iso-caloric control diet (12% soybean oil). Weight gain and food consumption were similar in both groups. Sat-Fat treated rats committed more working memory errors in the water radial arm maze, especially at higher memory loads. Cholesterol, amyloid-beta peptide of 40 (Abeta40) or 42 (Abeta42) residues, and nerve growth factor in cortical regions was unaffected, but hippocampal Map-2 staining was reduced in rats fed a Sat-Fat diet, indicating a loss of dendritic integrity. Map-2 reduction correlated with memory errors. Microglial activation, indicating inflammation and/or gliosis, was also observed in the hippocampus of Sat-Fat fed rats. These data suggest that saturated fat, hydrogenated fat and cholesterol can profoundly impair memory and hippocampal morphology.

Frontal cortex BDNF levels correlate with working memory in an animal model of Down syndrome

Individuals with Down syndrome (DS) develop most neuropathological hallmarks of Alzheimers disease early in life, including loss of cholinergic markers in the basal forebrain. Ts65Dn mice, an animal model of DS, perform poorly on tasks requiring spatial memory and also exhibit basal forebrain pathology beginning around 6 months of age. We evaluated memory as well as brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) protein levels in basal forebrain, frontal cortex, hippocampus, and striatum in Ts65Dn mice at the age when cholinergic degeneration is first observed, and compared values to normosomic controls. Six-month-old Ts65Dn mice exhibited impairments in working and reference memory as assessed on a water radial-arm maze. The working memory deficit was related to the inability of Ts65Dn mice to successfully sustain performance as the working memory load increased. Coupled with cognitive performance deficiencies, Ts65Dn mice also exhibited lower frontal cortex BDNF protein levels than controls. Further, BDNF levels were negatively correlated with working memory errors during the latter portion of testing in Ts65Dn mice, thereby suggesting that lower BDNF protein levels in the frontal cortex may be associated with the observed working memory impairment.

Age-related deficits as working memory load increases: relationships with growth factors

Young and aged female rats were tested on a water radial-arm maze designed to measure performance as working memory load increased, followed by brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin 3 (NT3) protein assessments in hippocampus and frontal cortex. Aged rats showed deficiencies in both working and reference memory. There were also profound age-related working memory load effects. Aged rats made more errors as working memory load increased and showed learning only during early trials when memory load was low, while young rats exhibited learning over all trials. Neurotrophin assessment showed that frontal cortex NGF and BDNF levels were positively, and hippocampal NT3 negatively, correlated with number of errors made during specific trials in aged animals. Comparison to untested rats showed that testing increased NT3, but not BDNF or NGF, protein levels in both age groups. Findings suggest that young rats learn to handle a higher working memory load as testing progresses, while aged rats do not, and that frontal cortex and hippocampal neurotrophin levels may relate to working memory proficiency in aged female rats.

Testosterone, but not nonaromatizable dihydrotestosterone, improves working memory and alters nerve growth factor levels in aged male rats.

Recent studies have suggested that testosterone levels are lower in men with Alzheimers disease and that testosterone treatment improves cognition in older men. Since testosterone can be aromatized to estrogen, testosterones effects could be due to conversion into estrogen. We treated aged male rats with either testosterone or dihydrotestosterone (DHT), the latter of which is not aromatized to estrogen, in order to determine whether these treatments improve spatial working and reference memory as assessed in the water radial arm maze. We also tested whether such effects are related to beta-amyloid levels in the hippocampus or neurotrophin levels in the hippocampus, entorhinal cortex, frontal cortex, or striatum. Aged rats made more errors than young rats on all memory measures. Testosterone, but not DHT, improved working memory and decreased hippocampal NGF protein in aged rats, while having no effect on beta-amyloid. However, higher beta-amyloid levels were correlated with poorer working memory performance in young rats. Neurotrophin levels in entorhinal cortex were positively correlated with errors for all memory measures in androgen-treated rats. Similar to findings in human studies, in our study androgen treatment lowered circulating estradiol levels in aged rats, suggesting that androgen treatment exerts feedback to the hypothalamic pituitary axis and that conversion to estrogen may not be the underlying biological mechanism of testosterones effects on memory and growth factor levels. The ratio of estradiol to testosterone, or the actions of the aromatase enzyme itself, may be responsible for the observed effects. These data support the hypothesis that testosterone therapy in aging men may provide positive effects on cognition and that neural regions that are linked to cognition, such as the hippocampus and/or entorhinal cortex, may be involved in such effects.

Estrogen restores cognition and cholinergic phenotype in an animal model of Down syndrome

Estrogen maintains normal function of basal forebrain (BF) cholinergic neurons and estrogen replacement therapy (ERT) has therefore been proposed as a therapy for Alzheimers disease (AD). We provide evidence to support this hypothesis in an animal model of Down syndrome (DS), a chromosome 16 segmental trisomy (Ts65Dn) mouse. These mice develop cholinergic degeneration similar to young adults with DS and AD patients. ERT has not been tested in women with DS, even though they are more likely than normosomic women to develop early menopause and AD. Female Ts65Dn and normosomic mice (11-15 months) received a subcutaneous estrogen pellet or a sham operation. After 60 days, estrogen treatment improved learning of a T-maze task and normalized behavior in the Ts65Dn mice in reversal learning of the task, a measure of cognitive flexibility. Stereological evaluation of choline acetyltransferase (ChAT) immunopositive BF neurons showed that estrogen increased cell size and total number of cholinergic neurons in the medial septum of Ts65Dn mice. In addition, estrogen increased NGF protein levels in the BF of trisomic mice. These findings support the emerging hypothesis that estrogen may play a protective role during neurodegeneration and cognitive decline, particularly in cholinergic BF neuronal systems underlying cognition. The findings also indicate that estrogen may act, at least partially, via endogenous growth factors. Collectively, the data suggest that ERT may be a viable therapeutic approach for women with DS coupled with dementia.

Progesterone counteracts estrogen-induced increases in neurotrophins in the aged female rat brain

Neurotrophin alterations have been associated with normal aging and age-related neurodegenerative disease, as well as cognitive status. Estrogen influences expression of mRNA and protein of neurotrophins and their receptors, and affects cognitive performance in young ovariectomized (Ovx) rats. The current investigation evaluated whether estrogen or estrogen plus progesterone affects neurotrophin protein levels in cognitive brain regions in the aged Ovx rat. While estrogen treatment increased BDNF, NGF, and NT3 levels in entorhinal cortex, progesterone abated the effects of estrogen resulting in neurotrophin levels comparable to aged Ovx rats not given hormone. Our findings suggest that the aged female brain is responsive to estrogen in cognitive brain regions, and that progesterone can reverse these estrogen effects.

Regional alterations in amyloid precursor protein and nerve growth factor across age in a mouse model of Down's syndrome

Individuals with Downs syndrome (DS) develop the pathological hallmarks of Alzheimers (AD) disease at an early age, subsequently followed by memory decline and dementia. We have utilized an animal model for DS, mice with segmental trisomy of chromosome 16 (Ts65Dn), to study biological events linked to memory loss. Previous studies demonstrated a cognitive decline and loss of cholinergic markers after 6-8 months of age. In the current study, we found increased levels of amyloid precursor protein (APP) in the striatum by 6-8 months of age, and in the hippocampus and parietal cortex by 13-16 months of age in Ts65Dn but not in normosomic mice. Additionally, Ts65Dn mice exhibited alterations in nerve growth factor (NGF) levels in the basal forebrain and hippocampus. Ts65Dn mice demonstrated a significant decline in NGF levels in the basal forebrain with age, as well as a reduction in hippocampal NGF by 13-16 months of age. These findings demonstrate that elevated APP and decreased NGF levels in limbic areas correlate with the progressive memory decline and cholinergic degeneration seen in middle-aged trisomic mice.

Chronic nicotine improves working and reference memory performance and reduces hippocampal NGF in aged female rats.

The cholinergic system is involved in cognition and several forms of dementia, including Alzheimers disease, and nicotine administration has been shown to improve cognitive performance in both humans and rodents. While experiments with humans have shown that nicotine improves the ability to handle an increasing working memory load, little work has been done in animal models evaluating nicotine effects on performance as working memory load increases. In this report, we demonstrate that in aged rats nicotine improved the ability to handle an increasing working memory load as well as enhanced performance on the reference memory component of the water radial arm maze task. The dose required to exert these effects (0.3mg/kg/day) was much lower than doses shown to be effective in young rats and appears to be a lower maintenance dose than is seen in light to moderate smokers. In addition, our study reports a nicotine-induced reduction in nerve growth factor (NGF) protein levels in the hippocampus of the aged rat. The effects of nicotine on hippocampal NGF levels are discussed as a potential mechanism of nicotine-induced improvements in working and reference memory.

PATTERNS OF NEUROTROPHIN PROTEIN LEVELS IN MALE AND FEMALE FISCHER 344 RATS FROM ADULTHOOD TO SENESCENCE: HOW YOUNG IS “YOUNG” AND HOW OLD IS “OLD”?

The current study assessed neurotrophin protein levels in male and female rat brain tissues at four different ages ranging from postpuberty to senescence. In both sexes nerve growth factor (NGF) increased, and brain-derived neurotrophic factor (BDNF) decreased, from 4 to 24 months of age. Using a slightly older age for the young group, or a slightly younger age for the aged group, had profound effects on whether age effects were realized. There were no sex differences in the pattern of change in neurotrophin levels across age, and neurotrophin levels did not correlate with estrogen levels in females or estrogen or testosterone levels in males. The current findings suggest that profound changes in neurotrophin protein levels can occur within only a few months time, and that these changes influence whether age-related neurotrophin alterations are realized.

Stimulated neuronal expression of brain-derived neurotrophic factor by Neurotropin

Expression of brain-derived neurotrophic factor (BDNF) was stimulated in human neuroblastoma SH-SY5Y cells by a nonprotein extract of inflamed rabbit skin inoculated with vaccinia virus (Neurotropin), an analgesic widely used in Japan for treatment of disorders associated with chronic pain, with the optimal dosage at 10mNU/mL. This stimulation was accompanied by activations of p42/44 MAP kinase, CREB and c-Fos expression. Inhibitors of MAP kinases or PI 3-kinase prevented the stimulatory action of Neurotropin, indicating that neuronal TrkB/CREB pathway mediates the action. Repetitive oral administration of Neurotropin (200NU/kg/day, 3months) prevented the age-dependent decline in hippocampal BDNF expression in Ts65Dn mice, a model of Downs syndrome. This effect was associated with the improvement of spatial cognition of the mice. These results open an intriguing new strategy in which Neurotropin may prove beneficial treatment for neurodegenerative disorders.