Amyloidosis increase is not attenuated by long-term calorie restriction or related to neuron density in the prefrontal cortex of extremely aged rhesus macaques.

As human lifespan increases and the population ages, diseases of aging such as Alzheimer's disease (AD) are a major cause for concern. Although calorie restriction (CR) as an intervention has been shown to increase healthspan in many species, few studies have examined the effects of CR on brain aging in primates. Using postmortem tissue from a cohort of extremely aged rhesus monkeys (22-44 years old, average age 31.8 years) from a longitudinal CR study, we measured immunohistochemically labeled amyloid beta plaques in Brodmann areas 32 and 46 of the prefrontal cortex, areas that play key roles in cognitive processing, are sensitive to aging and, in humans, are also susceptible to AD pathogenesis. We also evaluated these areas for cortical neuron loss, which has not been observed in younger cohorts of aged monkeys. We found a significant increase in plaque density with age, but this was unaffected by diet. Moreover, there was no change in neuron density with age or treatment. These data suggest that even in the oldest-old rhesus macaques, amyloid beta plaques do not lead to overt neuron loss. Hence, the rhesus macaque serves as a pragmatic animal model for normative human aging but is not a complete model of the neurodegeneration of AD. This model of aging may instead prove most useful for determining how even the oldest monkeys are protected from AD, and this information may therefore yield valuable information for clinical AD treatments.

Dietary restriction retards the age-associated loss of rat striatal dopaminergic receptors.

In male Wistar rats subjected to dietary restriction by alternate days of feeding and fasting the normal age-associated loss of striatal dopamine receptors in the brain was substantially retarded. The mean survival time of the rats on the restricted diet was increased by approximately 40 percent compared to control rats given free access to food. Dopamine receptor concentrations in striata of 24-month-old rats that had been on a restricted diet since weaning were 50 percent higher than those of control animals of the same age, and essentially comparable to 3- and 6-month-old control rats.

Acquisition of conditioned taste aversion is impaired in the amyloid precursor protein/presenilin 1 mouse model of Alzheimer's disease.

Research into the underlying mechanisms of cognitive dysfunction in Alzheimer's disease (AD) has relied traditionally on tasks such as the water maze which evaluate spatial learning and memory. Since non-spatial forms of memory are also disrupted by AD, it is critical to establish other paradigms capable of investigating these deficits. Utilizing a non-spatial learning task, acquisition of conditioned taste aversion (CTA) was evaluated in a mouse model of AD. This line of transgenic mice encode a mutated allele of the human amyloid precursor protein (APP) and presenilin 1 (PS1) genes and exhibit extensive amyloid plaque deposition in the brain by 6-7 mo of age. Compared with wild-type mice, 10-17 month old APP/PS1 mice failed to acquire CTA to saccharin. Mice that only possessed one of the two mutations were able to acquire CTA to the saccharin. In 2-5 month old APP/PS1 mice acquisition of CTA was disrupted despite the lack of extensive plaque deposition. However, further analysis indicated a potential gender difference in both the CTA deficit and onset of plaque deposition with females showing greater conditioned aversion.

Differential gender effects of a reduced-calorie diet on systemic inflammatory and immune parameters in nonhuman primates.

BACKGROUND AND OBJECTIVE: Dietary manipulation, including caloric restriction, has been shown to impact host response capabilities significantly, particularly in association with aging. This investigation compared systemic inflammatory and immune-response molecules in rhesus monkeys (Macaca mulatta). MATERIAL AND METHODS: Monkeys on continuous long-term calorie-restricted diets and a matched group of animals on a control ad libitum diet, were examined for systemic response profiles including the effects of both gender and aging. RESULTS: The results demonstrated that haptoglobin and alpha1-antiglycoprotein levels were elevated in the serum of male monkeys. Serum IgG responses to Campylobacter rectus, Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis were significantly elevated in female monkeys. While only the antibody to Fusobacterium nucleatum was significantly affected by the calorie-restricted diet in female monkeys, antibody levels to Prevotella intermedia, C. rectus and Treponema denticola demonstrated a similar trend. CONCLUSION: In this investigation, only certain serum antibody levels were influenced by the age of male animals, which was seemingly related to increasing clinical disease in this gender. More generally, analytes were modulated by gender and/or diet in this oral model system of mucosal microbial challenge.

Calorie restriction in nonhuman primates: assessing effects on brain and behavioral aging.

Dietary caloric restriction (CR) is the only intervention repeatedly demonstrated to retard the onset and incidence of age-related diseases, maintain function, and extend both lifespan and health span in mammals, including brain and behavioral function. In 70 years of study, such beneficial effects have been demonstrated in rodents and lower animals. Recent results emerging from ongoing studies of CR in humans and nonhuman primates suggest that many of the same anti-disease and anti-aging benefits observed in rodent studies may be applicable to long-lived species. Results of studies in rhesus monkeys indicate that CR animals (30% less than controls) are healthier than fully-fed counterparts based on reduced incidence of various diseases, exhibit significantly better indices of predisposition to disease and may be aging at a slower rate based on analysis of selected indices of aging. The current review discusses approaches taken in studies of rhesus monkeys to analyze age-related changes in brain and behavioral function and the impact of CR on these changes. Approaches include analyses of gross and fine locomotor performance as well as brain imaging. In a related study it was observed that short-term CR (6 months) in adult rhesus monkeys can provide protection against a neurotoxic insult. Increasing interest in the CR paradigm will expand its role in demonstrating how nutrition can modulate the rate of aging and the mechanisms responsible for this modulation.

Vaccine development for Alzheimer's disease: a shot of good news.

Studies in transgenic mouse models of Alzheimer's disease suggest the potential for vaccine development for this disease. Specifically, inoculation with Abeta peptide reduces Abeta plaque formation. However, this vaccination strategy has raised safety concerns. Recent studies have reduced these concerns by demonstrating that long-term Abeta vaccination in transgenic mice does not produce detrimental behavioral effects and in fact appears to protect against age-related functional decline in spatial memory tasks.

Enhancement of amphetamine-induced locomotor response in rats on different regimens of diet restriction and 2-deoxy-D-glucose treatment.

Diet restriction (DR) in rodents increases lifespan, reduces age-related disease and pathology, increases stress responses, and maintains better function later into life compared with conventional ad libitum (AL) feeding. We have been investigating different DR regimens and also DR mimetics that stimulate stress response pathways that are activated by DR. By inhibiting glycolysis, feeding or injection of 2-deoxy-D-glucose (2DG) has been proposed as a DR mimetic and has been shown to provide neuroprotection. In the current study, we examined whether 2DG treatment produces behavioral changes similar to those observed in DR rats following stimulation of the dopaminergic (DA) system by D-amphetamine (AMPH). Male Fischer 344 rats were maintained on different dietary regimens: 40% daily DR (40% DR); every-other-day feeding (EOD); or AL with some groups provided food containing 0.4% 2DG or injected i.p. with 2DG. In addition, we examined the persistence of effects of DR or 2DG feeding after switching rats to AL. When locomotor activity was assessed at different time points following initiation of dietary treatments, we noted that the enhancement of AMPH-induced locomotor responses emerged earlier in DR rats than observed in 2DG fed rats, but 40% DR and EOD rats responded in a similar manner. Enhanced locomotor responses persisted in 2DG fed rats even when returned to normal diet for 1 month and in the case of DR rats even after 2 months of AL feeding. Three weeks of 2DG injections also enhanced AMPH response, but this effect was transient. The most important finding was that 2DG did not affect body weight or diet intake yet had effects similar to DR. Thus, 2DG appears to activate DA pathways in the same direction as DR does but without the necessity of reducing caloric intake.

Accommodative function in rhesus monkeys: effects of aging and calorie restriction.

Numerous degenerative changes in the visual system occur with age, including a loss of accommodative function possibly related to hardening of the lens or loss of ciliary muscle mobility. The rhesus monkey is a reliable animal model for studying age-related changes in ocular function, including loss of accommodation. Calorie restriction (CR) is the only consistent intervention to slow aging and extend lifespan in rodents, and more recently the beneficial effects of CR have been reported in nonhuman primates. The goal of the present study was to evaluate age-related changes in ocular accommodation and the potential effect of long-term (>8 years) CR on accommodation in male and female rhesus monkeys. Refraction, accommodation (Hartinger coincidence refractometer), and lens thickness (A-scan ultrasound) were measured in 97 male and female rhesus monkeys age 8-36 years under Telazol/acepromazine anesthesia. Refraction and accommodation measurements were taken before and after 40% carbachol corneal iontophoresis to induce maximum accommodation. Half the animals were in the control (CON) group and were fed ad libitum. The CR group received 30% fewer calories than age- and weight-matched controls. Males were on CR for 12 years and females for eight years. With increasing age, accommodative ability declined in both CON and CR monkeys by 1.03 ± 0.12 (P = 0.001) and 1.18 ± 0.12 (P = 0.001) diopters/year, respectively. The age-related decline did not differ significantly between the groups (P = 0.374). Baseline lens thickness increased with age in both groups by 0.03 ± 0.005 mm/year (P = 0.001) and 0.02 ± 0.005 mm/year (P = 0.001) for the CON and CR groups, respectively. The tendency for the for the lens to thicken with age occurred at a slower rate in the CR group vs. the CON group but the difference was not statistically significant (P = 0.086). Baseline refraction was -2.8 ± 0.55 and -3.0 ± 0.62 diopters for CON and CR, respectively. Baseline refraction tended to become slightly more negative with age (P = 0.070), but this trend did not differ significantly between the groups (P = 0.587). In summary, there was no difference in the slope of the age-related changes in accommodation, lens thickness, or refraction in the carbachol-treated eyes due to diet. These data are consistent with previous findings of decreased accommodative ability in aging rhesus monkeys, comparable to the age-dependent decrease in accommodative ability in humans. This study is the first to indicate that the accommodative system may not benefit from calorie restriction.

An overview of neurobiological comparisons in mouse strains.

A literature review of research dealing with mouse strain comparisons along neurobiological parameters is presented in tabular form which provides a rank-ordering of strains and estimates of percentage differences in individual parameters. Drawing from 86 references, three tables deal respectively, with comparisons of neurochemical, neuroanatomical, and neuropsychophysiological parameters.

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.

Dietary caloric restriction prevents the age-related decline in plasma melatonin levels of rhesus monkeys.

Rhesus monkeys exhibit an age-associated decrease in peak plasma melatonin levels analogous to that reported for humans. This decrease is essentially abolished in monkeys subjected to a 30% reduction in caloric intake over a 12-yr period. The caloric restriction (CR) effect does not seem to be a reversal, but rather a long-term prevention, of the age-related decline in hormone concentrations. The age effect does not seem to be due to a phase shift in the peak of melatonin secretions, as has been observed in some populations of aged humans. It is also extremely unlikely that the CR effect simply reflects a phase shift, since old monkeys on the diet have nocturnal melatonin levels equal to or greater than adult fully fed controls. Thus, if peak times (approximately 0200 h) were actually shifted, maximal levels in old CR monkeys would be even higher. These findings, coupled with previous observations in humans, suggest that peak plasma melatonin levels may represent a possible candidate "biomarker of aging" in primates. Moreover, this index of age-associated physiological decrement seems to be inhibited by dietary CR.

The experimental Alzheimer drug phenserine: preclinical pharmacokinetics and pharmacodynamics.

Phenserine, a phenylcarbamate of physostigmine, is a new potent and highly selective acetylcholinesterase (AChE) inhibitor, with a > 50-fold activity versus butyrylcholinesterase (BChE), in clinical trials for the treatment of Alzheimer's disease (AD). Compared to physostigmine and tacrine, it is less toxic and robustly enhances cognition in animal models. To determine the time-dependent effects of phenserine on cholinergic function, AChE activity, brain and plasma drug levels and brain extracellular acetylcholine (ACh) concentrations were measured in rats before and after phenserine administration. Additionally, its maximum tolerated dose, compared to physostigmine and tacrine, was determined. Following i.v. dosing, brain drug levels were 10-fold higher than those achieved in plasma, peaked within 5 min and rapidly declined with half-lives of 8.5 and 12.6 min, respectively. In contrast, a high (> 70%) and long-lasting inhibition of AChE was achieved (half-life > 8.25 h). A comparison between the time-dependent plasma AChE inhibition achieved after similar oral and i.v. doses provided an estimate of oral bioavailability of 100%. Striatal, in vivo microdialysis in conscious, freely-moving phenserine-treated rats demonstrated > 3-fold rise in brain ACh levels. Phenserine thus is rapidly absorbed and cleared from the body, but produces a long-lasting stimulation of brain cholinergic function at well tolerated doses and hence has superior properties as a drug candidate for AD. It selectively inhibits AChE, minimizing potential BChE side effects. Its long duration of action, coupled with its short pharmacokinetic half-life, reduces dosing frequency, decreases body drug exposure and minimizes the dependence of drug action on the individual variations of drug metabolism commonly found in the elderly.

Loss of D2 receptor binding with age in rhesus monkeys: importance of correction for differences in striatal size.

The relation between striatal dopamine D2 receptor binding and aging was investigated in rhesus monkeys with PET. Monkeys (n = 18, 39 to 360 months of age) were scanned with 11C-raclopride; binding potential in the striatum was estimated graphically. Because our magnetic resonance imaging analysis revealed a concomitant relation between size of striatum and age, the dynamic positron emission tomography (PET) data were corrected for possible partial volume (PV) artifacts before parameter estimation. The age-related decline in binding potential was 1% per year and was smaller than the apparent effect if the age-related change in size was ignored. This is the first in vivo demonstration of a decline in dopamine receptor binding in nonhuman primates. The rate of decline in binding potential is consistent with in vitro findings in monkeys but smaller than what has been measured previously in humans using PET. Previous PET studies in humans, however, have not corrected for PV error, although a decline in striatal size with age has been demonstrated. The results of this study suggest that PV correction must be applied to PET data to accurately detect small changes in receptor binding that may occur in parallel with structural changes in the brain.

Complex maze learning in rodents as a model of age-related memory impairment.

Research is reviewed concerning the age-related learning deficit observed in a 14-unit T-maze (Stone maze). Rats and mice of several strains representing different adult age groups are first trained to criterion in one-way active avoidance in a straight runway. Then training in the Stone maze is conducted which involves negotiation of five maze segments to avoid footshock. Results indicate a robust age-related impairment in acquisition observed in males and females, and in outbred, inbred, and hybrid strains. Pharmacological studies using the muscarinic antagonist, scopolamine, in young and aged rats indicate cholinergic involvement for accurate encoding during acquisition of this task. Retention aspects of storage and retrieval do not appear to be affected by scopolamine treatment. Bilateral electrolytic lesions to the fimbria-fornix of young rats also produce an acquisition deficit to implicate involvement of the septo-hippocampal cholinergic system in Stone maze learning. A salient feature of Stone maze performance is the tendency to demonstrate an alternation strategy in solving the maze. This strategy is exacerbated by impairment of cholinergic neurotransmission with either scopolamine treatment or fimbria-fornix lesions. Various models of hippocampal function are applied toward the psychological characterization of the Stone maze task without complete success. Future research is outlined to provide more thorough psychological characterization of maze performance, to analyze the specificity of cholinergic involvement in the task, and to test possible therapeutic interventions for alleviating the age-related impairments observed.

Developing mouse models of aging: a consideration of strain differences in age-related behavioral and neural parameters.

Increased interest is emerging for using mouse models to assess the genetics of brain aging and age-related neurodegenerative diseases. Despite this demand, relatively little information is available on aging in behavioral or neuromorphological parameters in various mouse strains that are being used to create transgenic and null mutant mice. We review several issues regarding selection of appropriate strains as follows: (1) Does the behavioral parameter exhibit a significant age by strain interaction? (2) Do the strains differ in lifespan? (3) Are there potential intervening variables, such as strain-specific performance strategies or disease, in the behavioral task being investigated that would confound the desired conclusions? (4) Does the behavioral difference have an underlying neural correlate? In this context we present a conceptual model pertaining to the selection of mouse strains and behavioral parameters for genetic analyses. We also review the importance of applying stereological techniques for determining age-related structural changes in the mouse brain as well as the potential value of a database that would catalog this information. Thus, our intention is to underscore the growing importance of mouse models of brain aging and the concomitant need for additional information about mouse aging in general.

Age and neurochemical correlates of radial maze performance in rats.

Young adult (8 months) and aged (26 months) female Wister rats were tested in a 12-arm radial maze in which the optimal strategy was to enter all arms without a repetition. In order to determine if possible age-associated alterations in behavior were correlated with defects in cholinergic. GABAergic and adrenergic neurons in the hippocampus and cerebral cortex, the activities of choline acetyltransferase (CAT), glutamic acid decarboxylase (GAD) and tyrosine hydroxylase (TH) were assayed in these regions of all animals after testing in the radial maze. In the maze, the aged rats continued to perform at the chance level after 15 trials, whereas the young rats had virtually mastered the task. The only significant neurochemical age effect was an increase in hippocampal TH. However, analysis of individual differences among rats revealed positive correlations between maze performance and hippocampal CAT in the aged group and cortical GAD in both the young and aged groups.

Life-long dietary restriction affects striatally-mediated behavioral responses in aged rats.

The effects of life-long dietary restriction on rotational behavior and stereotypy induced by intrastriatally administered dopamine-excitatory (dopamine, DA; amphetamine, AMPH) or cholinergic inhibitory (atropine, AT) agents were examined in rats. Three groups of male Wistar rats were used; mature ad lib (YAL, 6 month), old ad lib (OAL 24 month), and old restricted animals (OR). OR animals were fed and fasted on alternate days from weaning. Each rat was unilaterally lesioned in the left substantia nigra with 6-hydroxydopamine and a cannula was implanted in the right caudate nucleus. Several doses of each drug [AT (0, 1, 7, micrograms) DA (0, 5, 50 micrograms), and AMPH (0, 2, 5, 5, 10 micrograms)] were given randomly (1 dose/day with 1 week between drugs). All doses of a particular drug were given before the next drug regimen was started. A combination of AMPH (25 micrograms) and AT (1 microgram) was also administered in one injection at the completion of all other regimens. Results showed that OR animals responded as well as YAL animals and far better than OAL animals following AMPH, DA, or low doses of AT. However, both the high dose of AT and the combined dose of AMPH-AT seemed to suppress responding in OR animals. Neither AT alone or AMPH-AT combinations had effects in OAL animals. Results are discussed in terms of the possible specificity of effects of dietary restriction on striatal functioning and behavior in senescence.

Circadian rhythmicity and sleep: effects of aging in laboratory animals.

This literature review of research on age-related differences in sleep and rhythmic phenomena in laboratory animals covers three general areas: (1) age-related differences in biorhythms in general; (2) age-related differences in sleep patterns as assessed by psychophysiological measures; and (3) neurobiological correlates of biorhythms and sleep, including consideration of possible morphological, chemical, and endocrine bases of age-related defects in animal models. It is concluded that systematic research bridging these areas is lacking although several promising areas have been explored.

Methadone effects on locomotor activity of young and aged mice.

The effect of age on the behavioral response to methadone hydrochloride was examined in male C57BL/6J mice. After 1 hr habituation to an oval runway, locomotor activity of young (6-8 months) and aged (30-32 months) mice was monitored for 3 hr following injections of saline or methadone hydrochloride (2.5, 7.5, 15.0 or 22.5 mg/kg). The three highest doses initially elevated activity which then declined over the next 1 1/2 to 2 1/2 hr. Compared to young mice, activity of aged mice was elevated less extensively, but remained elevated for a longer period of time. The attenuated elevation could be interpreted in terms of reduced physical capacity of aged mice; however, the more prolonged elevation was clearly an age-related difference in reaction to the drug. Although the mechanism accountable for this effect is unknown, it is not likely related to age differences in drug absorption or distribution to active sites.