Caloric restriction increases HDL2 levels in rhesus monkeys (Macaca mulatta)

Caloric restriction (CR) prolongs the life of rodents and other small animals, but the benefits of CR for primates and people are as yet unknown, and mechanisms by which CR may slow aging remain unidentified. A study of rhesus monkeys, Macaca mulatta, is underway to determine if CR might prolong life span in primates and to evaluate potential mechanisms for life prolongation. Thirty rhesus monkeys in three age cohorts, restricted to 70% of ad libitum calorie intake for 6-7 yr, were compared with 30 controls. Plasma lipid, lipoprotein, and high-density lipoprotein (HDL) apolipoproteins and subfractions were measured and compared with weight, percent fat, glucose, and insulin level. CR caused decreased triglyceride levels in adult monkeys and increased levels of HDL2b, the HDL subfraction associated with protection from atherosclerosis. Multivariate statistical analyses showed that differences in lipid and lipoprotein levels occurring with CR could be accounted for, at least in part, by decreased body mass and improved glucose regulation. These studies have used a novel dietary modification paradigm in nonhuman primates focused on calorie reduction. Results suggest that CR, as mediated by its beneficial effect on body composition and glucose metabolism, could prolong human life by decreasing the incidence of atherosclerosis.

Dietary restriction in rhesus monkeys: lymphopenia and reduced mitogen-induced proliferation in peripheral blood mononuclear cells

Dietary restriction (DR) markedly extends mean and maximal life span, and retards the rate of biological aging in rodent models; however, it is unknown if these results occur in primate species. The purpose of the current study was to investigate selected immunologic outcomes in Rhesus monkeys subjected to DR for a period of seven years. Similar to observations in mice on DR, lymphopenia occurred in the restricted monkeys. Compared to normally fed controls, the mitogen-induced proliferative responses of peripheral blood mononuclear cells (PBMC) were reduced in monkeys subjected to DR very early in life (up to 1 year), but not in others restricted in young adulthood (3-5 years). These data indicate that lymphopenia is a shared occurrence in rodents and primates on DR. However, the mitogen-induced proliferative data accumulated in rodents and primates cannot now be compared because PBMC have not been studied long enough or in comparable detail in primates fed restricted diets.

Dehydroepiandrosterone sulfate: a biomarker of primate aging slowed by calorie restriction

The adrenal steroids, dehydroepiandrosterone (DHEA) and its sulfate (DHEAS), have attracted attention for their possible antiaging effects. DHEAS levels in humans decline markedly with age, suggesting the potential importance of this parameter as a biomarker of aging. Here we report that, as seen in humans, male and female rhesus monkeys exhibit a steady, age-related decline in serum DHEAS. This decline meets several criteria for a biomarker of aging, including cross-sectional and longitudinal linear decreases with age and significant stability of individual differences over time. In addition, the proportional age-related loss of DHEAS in rhesus monkeys is over twice the rate of decline observed in humans. Most important is the finding that, in rhesus monkeys, calorie restriction, which extends life span and retards aging in laboratory rodents, slows the postmaturational decline in serum DHEAS levels. This represents the first evidence that this nutritional intervention has the potential to alter aspects of postmaturational aging in a long-lived species.

Activity measures in rhesus monkeys on long-term calorie restriction

Calorie restriction (CR), undernutrition without malnutrition, extends the mean and maximal lifespan of several ecologically diverse species. Rodents on CR demonstrate increased activity measured as spontaneous locomotion, wheel running, open field behavior or movement. Activity measures were recorded from 19 male rhesus monkeys (Macaca mulatta) as either controls (C) which were fed a nutritious diet to approximate ad libitum levels, or as experimentals (E) which were fed 30% less than age- and weight-matched controls. Within each diet group, some monkeys (n = 10) began CR at 2.3 years of age (range 2.2-2.4 yrs, J Group) while another group (n = 9) began CR at approximately 4.6 years of age (range 4-5.25, A group). Beginning about 6 years after initiation of the study, behavioral activity was measured via ultrasonic motion detectors and recorded on videotape. Diurnal and circadian activity was clearly discernible. Peaks in activity were associated with mealtime and colony husbandry. Compared to Group A, Group J monkeys exhibited higher overall activity as measured by sensors, and also significantly more circling. Compared to AC monkeys, group AE monkeys demonstrated higher rates of gross motor behavior, pacing, stereotypies and grooming. The increases in motor activity observed in one group of monkeys were consistent with results obtained from rodent studies of CR and aging. CR did not significantly inhibit or negatively influence the display of behavior of rhesus monkeys in the laboratory environment. We report here, for the first time, increases in activity due to CR in a model other than the rodent.

Nitric oxide synthase in rat brain: age comparisons quantitated with NADPH-diaphorase histochemistry

We examined age-related differences in nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) containing neurons and neuropil in the striatum and hippocampus of male Fischer 344 rats at 6, 12, and 26 mo of age. NADPH-d staining is considered to be a marker for neurons and neuronal processes containing nitric oxide synthase. Rat brains were processed for NADPH-d histochemistry and analyzed morphometrically using computerized image analysis. The following NADPH-d histochemical parameters were examined: neuronal density, neuronal size, and neuropil staining optical density of selected regions. In the striatum, significant age-related declines were observed in NADPH-d-positive neuronal density and in neuropil staining, while neuronal size increased between 6 and 12 mo and then declined between 12 and 26 mo. In the hippocampus no significant age-related changes were noted in NADPH-d-positive neuronal density or size, or in the optical density of the molecular layer of the hippocampal dentate gyrus. Thus, age differences in NADPH-d histochemistry appear to be regionally specific in the Fischer 344 rat.

N-methyl-D-aspartate receptor binding in brains of rats at different ages

Potential age differences in N-methyl-D-aspartate (NMDA) receptor binding were evaluated in rat brain using [3H]dizocilpine and [3H]D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentanoic acid (CGP 39653) as the radioligands. Neither [3H]dizocilpine nor [3H]CGP 39653 binding in hippocampus revealed any age difference (7 vs. 23-25 months) in Bmax or Kd. High-affinity [3H]dizocilpine binding showed a developmental increase (8 vs. 2 months) in hippocampus but not in frontal cortex. No senescence-related change (25 vs. 8 months) was observed in either region. The lack of senescence-related differences in these biochemical markers does not preclude the possibility of functional changes in the NMDA receptor with aging.

Murine models of brain aging and age-related neurodegenerative diseases

In the past, structural changes in the brain with aging have been studied using a variety of animal models, with rats and nonhuman primates being the most popular. With the rapid evolution of mouse genetics, murine models have gained increased attention in the neurobiology of aging. The genetic contribution of age-related traits as well as specific mechanistic hypotheses underlying brain aging and age-related neurodegenerative diseases can now be assessed by using genetically-selected and genetically-manipulated mice. Against this background of increased demand for aging research in mouse models, relatively few studies have examined structural alterations with aging in the normal mouse brain, and the data available are almost exclusively restricted to the C57BL/6 strain. Moreover, many older studies have used quantitative techniques which today can be questioned regarding their accuracy. Here we review the state of knowledge about structural changes with aging in outbred, inbred, genetically-selected, and genetically-engineered murine models. Moreover, we suggest several new opportunities that are emerging to study brain aging and age-related neurodegenerative diseases using genetically-defined mouse models. By reviewing the literature, it has become clear to us that in light of the rapid progress in genetically-engineered and selected mouse models for brain aging and age-related neurodegenerative diseases, there is a great and urgent need to study and define morphological changes in the aging brain of normal inbred mice and to analyze the structural changes in genetically-engineered mice more carefully and completely than accomplished to date. Such investigations will broaden knowledge in the neurobiology of aging, particularly regarding the genetics of aging, and possibly identify the most useful murine models.

Effect of age and caloric restriction on cutaneous wound closure in rats and monkeys

Cutaneous wounds close more slowly in rats and monkeys as age increases. Caloric restriction of 40% in rats and 30% in monkeys did not significantly affect healing rates, although it did exert a trend toward faster closure. Similarly, voluntary exercise did not significantly alter healing rates in rats. Thus, impaired wound healing appears to be a generalized physiological manifestation of aging, but its possible amelioration by "anti-aging" interventions remains to be established.

Similarities in the age-related hippocampal deposition of periodic acid-schiff-positive granules in the senescence-accelerated mouse P8 and C57BL/6 mouse strains

With advancing age clusters of abnormal granules positive for periodic acid-Schiff appear in the hippocampus of C57BL/6 (B6) mice and the senescence-accelerated mouse (SAM) P8. The granules can also be visualized with a polyclonal antibody to a 110,000 mol. wt laminin-binding protein and stain specifically with a monoclonal antibody to heparan sulfate proteoglycan. The present study used light and electron-microscopic analysis to compare the staining and morphological properties of these granules in SAM P8 hippocampus with those in B6 hippocampus at different ages. The results of the light-microscopic analysis revealed that granules in SAM P8 and B6 had similar morphology, staining characteristics and distribution patterns, and appeared to have a close association with astrocytic process. The onset of granules in SAM P8 mice (at two to three months of age) was earlier than that observed in B6 mice (at four to six months of age), but the maximum incidence was similar in both strains. Electron-microscopic analysis revealed that the granules in SAM P8 and B6 mice also had a very similar ultrastructure. Granules in both strains were surrounded by a discontinuous membrane and contained mostly crystalline-like, degenerated material. The successive ultrastructural changes from the exterior to interior of the granules suggest that the degenerative process was initiated outside the granules and that degenerative structures migrate inward. Astrocytes and heparan sulfate proteoglycan are closely associated with beta-amyloid deposits in Alzheimer's disease. The presence of astrocyte-associated heparan sulfate proteoglycan-positive material in aged SAM P8 and B6 mice might model age-related alterations in glia function possibly involved in human cerebral amyloidogenesis.

Cognitive enhancement. New strategies for stimulating cholinergic, glutamatergic, and nitric oxide systems

The development of treatments for AD is being pursued along many diverse lines. While the ACh hypothesis has generated abundant development efforts, little clinical progress has been achieved to date. Recent efforts aimed at developing more potent, more specific, and safer ChE inhibitors appear to offer greater potential for therapeutic success than achieved to date. Treatments aimed at the NMDA Glu system lag much further behind in their development. Progress in this area must be tempered by the potential for glutamate excitotoxicity mediated through this neurotransmitter system. Development of indirect agonists operating at the glycine and polyamine modulatory sites on the NMDA receptor might offer the safest alternative to applying more direct agonists. While a great degree of interest had been generated by the reports of NO involvement in signal transduction through the NMDA system, this area of research has been complicated by conflicting reports regarding NO involvement in learning and LTP. Moreover, the interaction of drugs acting on NOS with the vascular effects mediated by eNOS has also complicated development of drugs that act specifically on the neural actions of NO. This area will continue to receive extensive research attention; but similar to the development of Glu agonists, attention must be given to the potential neurotoxic effects of overstimulating this system. Perhaps targeting other presynaptic mechanisms that effect glutamate release might be a safer strategy to pursue. Considerable progress has been made over the last two decades in identifying the genetic and neural mechanisms involved in AD. Progress in developing treatments will remain highly correlated with this effort, and with basic research geared to comprehending how memories are formed and why neurons degenerate and regenerate.

Laminins in the adult and aged brain

Only recently have we become aware of the diversity of laminins in adult brain. In vascular basement membranes, the expression of at least five laminin chains has been demonstrated, suggesting the presence of several laminin variants. Recent ultrastructural evidence for heterogeneity of laminin expression in vascular basement membranes is an exciting finding, and points to structural and functional diversity of the basement membranes around cerebral blood vessels. Neuronal laminin-like immunoreactivity in the adult brain is a consistent observation, but does not fit well in the current understanding of the physiology and biochemistry of the heterotrimeric laminins. Nevertheless, the unique localization of putative neuronal laminins warrants their further characterization. The structure and function of laminins produced by reactive astrocytes in the lesioned adult brain and that seen in the brains of Alzheimer disease (AD) patients are not yet resolved. The possibility that these laminins play an important role in the CNS response to injury and pathophysiology of AD is expected to be a fruitful investigation. The next decade should see very significant advances in the characterization of brain laminins and, hopefully, in the elucidation of functional correlates to the structural diversity of laminins in brain.

Effects of aging and chronic nimodipine on hippocampal binding of [3H]CGS 19755

Previous studies have suggested that aging is associated with impaired behavioral performance and with decrements of N-methyl-D-aspartate (NMDA) receptors in the rat hippo-campus. Other studies have indicated that chronic treatment with nimodipine, a Ca2+ channel antagonist, prevents the age-related decline in performance by rats in behavioral tasks. Therefore, we tested whether nimodipine altered binding of [3H]CGS 19755 to hippocampal NMDA receptors in rats whose performance on a 14-unit T maze had been tested previously (14). No significant age difference was observed in [3H]CGS 19755 binding in hippocampi from old Fischer-344 rats (27 months) as compared with mature but not senescent rats (9 months); however, old rats that received chronic treatment with a low dose of nimodipine (20 mg pellets implanted subcutaneously twice during 70 days of treatment) showed higher levels of binding. A high dose of nimodipine (40 mg pellets implanted by the same route and at the same times as the low dose) was without effect on [3H]CGS 19755 binding, although aged rats given this treatment performed better in the maze than rats that received no nimodipine or the low dose. In a second experiment comparing hippocampi of young (4 months) and old (24 months) rats, saturation studies confirmed the lack of an age difference in [3H]CGS 19755 binding. The findings suggest that neither the age-related decline in maze performance nor the enhancement of behavior by nimodipine depend upon changes in hippocampal NMDA receptors.

Calorie restriction lowers body temperature in rhesus monkeys, consistent with a postulated anti-aging mechanism in rodents

Many studies of caloric restriction (CR) in rodents and lower animals indicate that this nutritional manipulation retards aging processes, as evidenced by increased longevity, reduced pathology, and maintenance of physiological function in a more youthful state. The anti-aging effects of CR are believed to relate, at least in part, to changes in energy metabolism. We are attempting to determine whether similar effects occur in response to CR in nonhuman primates. Core (rectal) body temperature decreased progressively with age from 2 to 30 years in rhesus monkeys fed ad lib (controls) and is reduced by approximately 0.5 degrees C in age-matched monkeys subjected to 6 years of a 30% reduction in caloric intake. A short-term (1 month) 30% restriction of 2.5-year-old monkeys lowered subcutaneous body temperature by 1.0 degrees C. Indirect calorimetry showed that 24-hr energy expenditure was reduced by approximately 24% during short-term CR. The temporal association between reduced body temperature and energy expenditure suggests that reductions in body temperature relate to the induction of an energy conservation mechanism during CR. These reductions in body temperature and energy expenditure are consistent with findings in rodent studies in which aging rate was retarded by CR, now strengthening the possibility that CR may exert beneficial effects in primates analogous to those observed in rodents.

Longevity and the genetic determination of collagen glycoxidation kinetics in mammalian senescence

A fundamental question in the basic biology of aging is whether there is a universal aging process. If indeed such a process exists, one would expect that it develops at a higher rate in short- versus long-lived species. We have quantitated pentosidine, a marker of glycoxidative stress in skin collagen from eight mammalian species as a function of age. A curvilinear increase was modeled for all species, and the rate of increase correlated inversely with maximum life-span. Dietary restriction, a potent intervention associated with increased life-span, markedly inhibited glycoxidation rate in the rodent. On the assumption that collagen turnover rate is primarily influenced by the crosslinking due to glycoxidation, these results suggest that there is a progressive age-related deterioration of the process that controls the collagen glycoxidation rate. Thus, the ability to withstand damage due to glycoxidation and the Maillard reaction may be under genetic control.

Utilization of the rat as a model of mammalian aging: impact of pathology on behavior

Because pathology is concomitant to aging in rat strains, extraneous variance can be added to studies of aging at all levels of analysis. Thus, several gerontologists have made recent requests for characterization of pathology in aging studies including not only investigators' reports of diseases commonly observed (e.g., Sendai virus) and the occurrence of prevalent age-related lesions (e.g., nephropathy, leukemia, radiculoneuropathy) in rodent colonies, but also how specific disease processes might impact on the variable of interest in their investigation. Reported here are simple techniques (e.g., physical examination, necropsy to identify lesions, hematocrit, Wright stain) used routinely by our laboratory to screen for the presence of age-related disease in studies using Fischer 344 and Wistar rats. Routine health screening by physical examination and blood testing in our studies has allowed us either to eliminate moribund rats or to assess whether deficient performance was related to health status when these animals had been included in behavioral investigations. Additional health screens (e.g., antibodies for specific tumors) need to be developed. Investigators should be encouraged to utilize existing techniques, such as those reported here, and new technologies either to screen moribund animals from studies or to demonstrate that the pathology observed does or does not impact on the variable under investigation.

Chronic treatment with human recombinant erythropoietin increases hematocrit and improves water maze performance in mice

Erythropoietin is a glycoprotein produced endogenously in the kidney, which stimulates red blood cell production. We evaluated the effects of chronic treatment with recombinant human erythropoietin (epoetin alfa: EPO) on the performance of 6-month-old male C57BL/6J mice in a spatial learning task, the Morris water maze. Mice were treated with either EPO (1.5 U injected SC every other day) or vehicle (PBS also injected SC every other day). Results indicated that the treatment had no effect on maze performance after 8 weeks, but after 19 weeks the EPO-treated mice showed better performance compared to controls as measured by mean distance (centimeters) to reach the goal platform. The improved performance in EPO-treated mice at 19 weeks was accompanied by an increased hematocrit. After 32 wk of EPO-treatment, the hematocrit returned to baseline levels even though the size and density of the red blood cells were increased.

Adenovirus-mediated gene transfer of dopamine D2 receptor cDNA into rat striatum

A robust feature of mammalian aging associated with diminished motor control is the loss of dopamine D2 receptors from the neostriatum. Decline in this neurotransmitter receptor is also observed in neurodegenerative disorders, such as Huntington's disease and late-stage Parkinson's disease. We have constructed a replication-deficient adenoviral vector to transfer rat dopamine D2 receptor cDNA to brain as a possible therapeutic strategy. Using tissue culture cells infected with this vector, we detected dopamine D2 receptor mRNA by Northern analysis and functional receptor protein in membrane preparations as specific binding of the dopamine D2 receptor ligand, [3H]spiperone. In vivo demonstration involved autoradiographic analysis of [3H]spiperone binding in rat striatum following injection of the adenoviral vector. Dopamine D2 receptor expression was amplified markedly above normal concentrations in the injection site, whereas no increased expression was observed in sites receiving control treatments. These results demonstrate the potential of gene therapy using adenoviral vectors to transfer neurotransmitter receptor proteins to the brain to reverse deficiencies in specific neurodegenerative disorders.

Retrograde transfer of replication deficient recombinant adenovirus vector in the central nervous system for tracing studies

We assessed the application of a replication deficient recombinant adenovirus vector as a retrograde tracer in neural pathway studies. The adenovirus vector, Ad. RSV betagal, containing the intracellular marker gene, beta-galactosidase, was injected directly into the laterodorsal striatum of rats. The retrograde transport of the vector from the injection site was clearly visible in the cerebral cortex, thalamic nucleus, and substantia nigra. No evidence for anterograde transport of the vector was found. When the vector was injected into the genu of the corpus callosum, little uptake of the vector by fibers was noted which suggested that uptake by fibers-of-passage should not be a problem in tracing studies. The present study demonstrates that adenoviral vectors can be useful retrograde tracers in the study of afferent connections within the central nervous system.

Energy balance in rhesus monkeys (Macaca mulatta) subjected to long-term dietary restriction

Male rhesus monkeys of various age groups representative of the species life span were fed ad libitum amounts (controls) or 30% less food than control monkeys of comparable age and body weight. Despite significantly lowered energy intake and body weight, the amount of energy lost in the feces, and fecal energy density (concentration) were not altered in diet-restricted (DR) monkeys, compared to age- and weight-matched controls. Absolute energy expenditure (EE; 24-hr) was consistently lower in DR monkeys, but this trend was not statistically significant. Expressed as a function of metabolic mass (body weight, metabolic body size, lean mass), 24-hr EE was not different in monkeys subjected to long-term DR, compared to controls. Calculations of net energy (intake-loss), as an index of energy balance, revealed that energy expenditure generally exceeded energy intake in all juvenile and adult group monkeys. However, this discrepancy was not statistically different from zero, suggesting that most animals were in energy balance. Also, there was no difference between control and DR animals with respect to energy balance. Diet restriction induced significant reductions in the absolute amount of lean body mass; however, percent (of total weight) lean and fat mass did not differ from controls.

In vivo assessment of striatal dopamine release in the aged male Fischer 344 rat

A microdialysis probe was implanted into the striatum of young (4- to 5-month-old) and aged (26- to 27-month-old) Fischer 344 male rats to assess age-related alterations in striatal dopamine (DA) release. Basal levels of DA and the magnitude of DA response evoked by 50 mM and 100 mM high potassium (K+) in aged rats were similar to those in young rats. Furthermore, K(+)-evoked DA release did not correlate with motor performance within either age group. In contrast, amphetamine (250 microM) evoked-DA release of aged rats was significantly lower than that of young rats. Moreover, the enhancement of K(+)-evoked DA release by oxotremorine (500 microM) was significantly attenuated in aged rats. These results indicate that a putative DA release mechanism and its cholinergic modulation of the aged striatum are impaired.

Aging and food restriction alter some indices of bone metabolism in male rhesus monkeys (Macaca mulatta)

Food restriction increases life span, reduces aging rate and affects a wide variety of biological functions. In rats, food restriction delays bone growth and reduces bone density and mineral content. We report the effects of aging and long-term (> 6.0 y) food restriction on several indices of bone growth and metabolism in rhesus monkeys (Macaca mulatta). Food allotments for controls approximated free access consumption, whereas food-restricted monkeys received 30% less food on a body weight basis. Cross-sectional and longitudinal age effects on serum alkaline phosphatase paralleled those reported for humans. Food restriction induced a significant delay in the developmental decline (to adult levels) in total alkaline phosphatase and significantly suppressed serum interleukin 6 concentrations, particularly in younger monkeys. Also, food restriction slowed skeletal growth, as reflected by shorter crown-rump length, and significantly reduced total body bone mineral content, but not bone mineral density, measured by dual energy X-ray absorptiometry. Analyses of serum parathyroid hormone, calcium, phosphate and osteocalcin concentrations suggested that the effects on skeletal growth were not related to alterations in calcium and phosphate homeostasis or a primary defect in bone formation. These findings suggest that long-term food restriction delays skeletal development in male rhesus monkeys while allowing the development of a reduced but otherwise normal skeleton.