Decreased sympathetic innervation of spleen in aged Fischer 344 rats

Splenic noradrenergic innervation in young adult and aged Fischer 344 rats was examined using fluorescence histochemistry for catecholamines and high performance liquid chromatography with electrochemical detection (LCEC) for the quantitation of norepinephrine (NE). In young adult rats, abundant noradrenergic plexuses followed the vasculature and trabeculae into splenic white pulp. In aged rats, noradrenergic innervation was reduced in density and in overall intensity of fluorescence, and splenic NE levels were significantly lower. The relationship between diminished noradrenergic innervation and diminished immune responsiveness in aging mammals, while not clear on a causal level, is presented as a hypothesis for further testing.     

Tolerance of aged Fischer 344 rats against chlordecone-amplified carbon tetrachloride toxicity

We have investigated the effects of chlordecone 1(CD)+CCl4 combination in adult (3 months), middle aged (14 months), and old aged (24 months) male Fischer 344 (F344) rats. After a non-toxic dietary regimen of CD (10 ppm) or normal powdered diet for 15 days, rats received a single non-toxic dose of CCl4 (100 microl/kg, i.p., 1:4 in corn oil) or corn oil (500 microl/kg, i.p.) alone on day 16. Liver injury was assessed by plasma ALT, AST, and histopathology during a time course of 0-96 h. Liver tissue repair was measured by [3H-CH3]-thymidine (3H-T) incorporation into hepatic nuclear DNA and proliferating cell nuclear antigen (PCNA) immunohistochemistry. Hepatomicrosomal CYP2E1 protein, enzyme activity, and covalent binding of 14CCl4-derived radiolabel were measured in normal and CD fed rats. Exposure to CCl4 alone caused modest liver injury only in 14- and 24-month-old rats but neither progression of injury nor mortality. The CD+CCl4 combination led to 100% mortality in 3-month-old rats by 72 h, whereas none of the 14- and 24-month-old rats died. Both 3- and 14-month-old rats exposed to CD+Cl4 had identical liver injury up to 36 h indicating that bioactivation-mediated CCl4 injury was the same in the two age groups. Thereafter, liver injury escalated only in 3-month-old while it declined in 14-month-old rats. In 24-month-old rats initial liver injury at 6 h was similar to the 3- and 14-month-old rats and thereafter did not develop to the level of the other two age groups, recovering from injury by 96 h as in the 14-month-old rats. Neither hepatomicrosomal CYP2E1 protein nor the associated p-nitrophenol hydroxylase activity or covalent binding of 14CCl4-derived radiolabel to liver tissue differed between the age groups or diet regimens 2 h after the administration of 14CCl4. Compensatory liver tissue repair (3H-T, PCNA) was prompt and robust soon after CCl4 liver injury in the 14- and 24-month-old rats. In stark contrast, in the 3-month-old rats it failed allowing unabated progression of liver injury. These findings suggest that stimulation of early onset and robust liver tissue repair rescue the 14- and 24-month-old F344 rats from the lethal effect of the CD+CCl4 combination.     

Effects of dietary taurine supplementation or deprivation in aged male Fischer 344 rats

Taurine is a sulfur amino acid that is present in high concentration in mammalian tissues and previously has been reported to decline in a number of tissues with advancing age. The aims of the present study were to examine: (1) the effects of dietary taurine supplementation; (2) the effects of taurine-free diets; (3) the ability of aged rats to conserve urinary taurine; and (4) the consequences of these dietary manipulations on some biochemical parameters. Male F344 rats (n = 30/group) 18 months of age were placed on control diets, diets supplemented with 1.5% taurine in the drinking water, or a taurine-free diet for 10 months. An adult control group (12 months old at the end of the study) on normal diets was included for comparison purposes. Significant (P < 0.05) age-related declines in taurine content were observed in the spleen, kidney, eye, cerebellum and serum. Taurine supplementation corrected these deficits in tissue content in aged rats and in many cases increased taurine content above that of adult controls. Urinary excretion of taurine was significantly (P < 0.05) reduced in aged rats indicating an increased need to conserve taurine. Taurine-deficient diets did not further exacerbate the age-related decline in tissue taurine content, suggesting biosynthetic adaptations to the lack of dietary taurine. Dietary taurine supplementation blunted age-related declines in serum IGF-1 and increases in serum creatinine and blood urinary nitrogen (BUN). These studies suggest that advanced aging results in a taurine-deficient state that can be corrected by dietary supplementation.     

Sulindac improves memory and increases NMDA receptor subunits in aged Fischer 344 rats

Inflammatory processes in the central nervous system are thought to contribute to Alzheimer's disease (AD). Chronic administration of nonsteroidal anti-inflammatory drugs (NSAIDs) decreases the incidence of Alzheimer's disease. There are very few studies, however, on the cognitive impact of chronic NSAID administration. The N-methyl-d-aspartate (NMDA) receptor is implicated in learning and memory, and age-related decreases in the NMDA NR2B subunit correlate with memory deficits. Sulindac, an NSAID that is a nonselective cyclooxygenase (COX) inhibitor was chronically administered to aged Fischer 344 rats for 2 months. Sulindac, but not its non-COX active metabolite, attenuated age-related deficits in learning and memory as assessed in the radial arm water maze and contextual fear conditioning tasks. Sulindac treatment also attenuated an age-related decrease in the NR1 and NR2B NMDA receptor subunits and prevented an age-related increase in the pro-inflammatory cytokine, interleukin 1beta (IL-1beta), in the hippocampus. These findings support the inflammation hypothesis of aging and have important implications for potential cognitive enhancing effects of NSAIDs in the elderly.     

Acetylcholine release from striatal slices of young adult and aged Fischer 344 rats

The release of endogenous and newly synthesized acetylcholine (ACh) was examined in neostriatal slices prepared from young adult (10-month) and aged (28-month) Fischer 344 rats. Both spontaneous and potassium-stimulated release were tested after various in vitro incubation times (1, 3 or 5 hr). The potassium-stimulated release of ACh from slices of 28-month rats was decreased by 53% when tested after incubating the slices 1 hr. The age-related differences in ACh release lessened if the slices were incubated for longer times (3 or 5 hr) before monitoring release. The spontaneous release of ACh was similar among the slices from both age-groups and at all times points monitored. When the neostriatal slices were incubated in medium supplemented with deuterated choline, the release of both the endogenous and newly synthesized ACh from slices of 28-month rats was decreased by 33% when tested after a 1-hr incubation, but was again similar to that released from slices of 10-month rats when tested after a 3-hr incubation. Choline release from slices of the 28-month rats was similar to that released from the slices of the 10-month rats when acetylcholinesterase (AChE) was inhibited during the release incubation. In slices with intact AChE activity, however, the age-related difference in choline release was similar to that observed for ACh release when AChE was inhibited. That is, when AChE activity was intact, the potassium-stimulated choline release from slices of 28-month rats was less than that released from slices of 10-month rats when release was tested after a 1-hr incubation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Spatial reference and working memory across the lifespan of male Fischer 344 rats

Loss of mnemonic function is among the earliest and most disconcerting consequences of the aging process. This study was designed to provide a comprehensive profile of spatial mnemonic abilities in male Fischer 344 (F344) rats across the lifespan. Young, middle-aged, and aged F344 rats were trained in spatial reference and working memory versions of the water maze task. There was a progressive age-related decline in spatial reference memory across the lifespan. Reliable individual differences were observed among aged rats, with some aged rats performing as well as young cohorts and others performing outside this range. An age-related delay-dependent decline was observed on a working memory version of the water maze task although no relationship between performance on reference and working memory tasks was present. Notably, middle-aged rats were impaired relative to young on both tasks. Together these data demonstrate that individual differences in spatial reference memory exist among aged F344 rats and provide novel data demonstrating an unrelated decline in working memory across the lifespan, suggesting that age-related mnemonic dysfunction may occur across multiple brain systems.     

Deficits in nitric oxide production after tetanic stimulation are related to the reduction of long-term potentiation in Schaffer-CA1 synapses in aged Fischer 344 rats

In the present study, we investigated whether nitric oxide (NO) production after tetanic stimulation is involved in long-term potentiation (LTP) in Schaffer-CA1 synapses in both young adult and aged rats. The changes in both the population spike amplitude and NO metabolites, nitrite (NO2-) and nitrate (NO3-), in the CA1 region were simultaneously determined before and after tetanic stimulation. Increases in NOx (NO2- plus NO3-) levels in the CA1 region were observed after tetanic stimulation in young adult rats as well as increase in the population spike amplitude. In aged rats, LTP was significantly inhibited compared with that in young adult rats. No increase in NOx level after tetanic stimulation was observed in aged rats. These findings directly demonstrated that NO production might be involved in the process of LTP formation in Schaffer-CA1 synapses of the rat hippocampus, and that the deficiency of hippocampal NO production might be responsible for reduction of LTP formation in aged rats.     

Cardiovascular responses to acute footshock stress in adult and aged Fischer 344 male rats

Tail artery catheters were surgically implanted in Fischer 344 male rats to allow for measurement of mean arterial pressure (MAP, mm Hg) and heart rate (HR, beats/min) in conscious, unrestrained rats. Basal values of MAP and HR were similar for groups of 4, 12 and 24 month old rats. Increments in Map did not differ among rats of the 3 ages following handling and transfer to a shock chamber or immediately or 5 minutes after exposure to inescapable footshock (2.0 mA, 0.6 sec duration, every 6 sec for 1 min). In contrast, there was a significant age-related attenuation of the tachycardia following handling and transfer of rats to the shock chamber and at the end of footshock. These data are consistent with previous findings of a reduced sensitivity of the aged myocardium to stress-induced sympathetic stimulation.     

Quantitation of synaptic density in the septal nuclei of young and aged Fischer 344 rats

Synaptic density in the medial and lateral septal nuclei was examined in 3 and 24-28 months of age Fischer 344 rats. The lateral nucleus had a higher synaptic density than the medial region in both age groups. There were no statistically significant differences in synapse density in either region as a function of age, but the data suggested a subpopulation of aged animals which did show an age-related decline in synaptic density in the lateral, but not medial area of the septum. These data indicate that sample size may be an important variable in assessing possible age-related differences in synaptic density, since a broad range of values, some significantly below the range of young animals, exists in the aged brain.     

Vasopressin in aged rats: longitudinal studies of vasopressin excretion in Sprague-Dawley and Fischer 344 strains

In order to provide physiological baseline values for future experimental procedures, indices of vasopressin secretion were assessed in male Sprague-Dawley (SD) and Fischer 344 (F344) rats at 3 and 20 months of age. Daily water intake, urine volume, urine osmolality, and urinary vasopressin excretion were monitored in SD rats for 30 days, and in F344 rats for 60 days. In the SD strain, daily water and urine volumes, expressed as ml/24 hr/100 g b.wt., were consistently lower in aged animals, as was a calculation of water balance (water intake-urine output volumes/24 hr). Although mean VP concentration in urine appeared higher in aged rats (33.9 +/- 20.4 pg/ml) than in young (16.3 +/- 7.7 pg/ml), total daily VP excretion was comparable for both ages when expressed as a function of body weight [80.6 +/- 37.3 pg for 3 months old (m.o.) and 81.9 +/- 47.2 pg/24 hr/100 g b.wt. for 3 and 20 m.o. respectively]. Young and old F344 males showed comparable daily drinking and urine volumes, and water balance, during two months of monitoring, but VP excretion was lower (p less than 0.025) in aged rats (83.8 +/- 19.0 pg/24 hr/100 g b.wt.) than in 3 m.o. rats (213.0 +/- 48.1 pg/24 hr/100 g b.wt.). Urine VP concentration was comparable (69.6 +/- 20.6 for 3 m.o.; 59.8 +/- 25.6 pg/ml for 20 m.o.). Mean urine osmolality was not significantly different among groups. Urine osmolality and daily urine volumes showed a significant correlation with daily VP excretion among young, but not aged, rats of both strains.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hippocampal neuronal necrosis in control Fischer 344 rats

Brain tissue sections from control male and female Fischer 344 (F344) rats from ten National Toxicology Program (NTP) Bioassays were histomorphologically reviewed for non-neoplastic lesions of the hippocampus. Unilateral segmental hippocampal neuronal necrosis, which has not been reported in normal rats, was observed in 9 of 433 (2.1 per cent) males and 1 of 454 (0.2 per cent) females. Significant coexisting lesions were left-sided atrial and/or valvular thrombosis, metastatic mesothelioma, and large lymphocyte leukaemia. These data suggest that this naturally occurring lesion of predominantly aged male rats may result from an impairment of cerebral perfusion secondary to vascular obstruction by thrombotic emboli or leukaemic cells and haemolytic anaemia concomitant with large lymphocyte leukaemia, which commonly occurs in F344 rats.     

Stress-induced changes in brain-derived neurotrophic factor expression are attenuated in aged Fischer 344/N rats

Aging and stress can sometimes result in a decline in brain function. We addressed the question whether changes in the expression of neurotrophic factors, which are necessary for the survival and maintenance of neurons, might occur during aging and stress. Therefore, we used in situ hybridization to investigate the effects of aging and stress on neurotrophic factor expression in young (3–4 month) and old (24 month) male Fischer 344/N rats. The ability of acute immobilization stress (2 h) to modulate BDNF mRNA levels in old rats was significantly reduced both in the hippocampus (a smaller decrease in BDNF) and the PVN (a smaller increase in BDNF) compared to young rats. In contrast, the induction of nerve growth factor and neurotrophin 3 (NT-3) by stress was not influenced by age. The diminished BDNF responses to stress in aged rats may be relevant to difficulties in adaptation to stress encountered during old age.     

Deficits in the expression of the NR2B subunit in the hippocampus of aged Fisher 344 rats

The NMDA receptor (NMDAR) has been implicated in the induction of LTP at hippocampal synapses, and has been proposed to play a significant role in the involvement of the hippocampus with learning and memory. Aged rats are known to have deficits in LTP, learning and memory. We tested the hypothesis that aged rats might have deficits in expression of NMDAR subunits. Aged rats have significantly lower levels of NR2B mRNA and protein compared to young animals. This complements a recent report which showed improved learning and memory in mice which overexpress NR2B. No changes were seen in either the mRNA or the protein levels of the NMDAR subunit NR2A, nor in the alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate receptor (AMPAR) subunit GluR2. Our data support the hypothesis that age related alterations in the expression of the NMDAR NR2B subunit might underlie deficits in LTP and learning and memory in aged animals.     

Differential effects of swimming and running on microsomal metabolism in middle-aged and aged Fischer 344 rats.

Cytochrome P-450 (P-450) content as well as p-nitroanisole (pNA) O-demethylase and UDP-glucuronyltransferase (UDPGT) activities were determined in livers of middle-aged (MA; 12 or 18 months) and aged (24-26 months) rats exercised by either treadmill running or swimming. In addition, aniline hydroxylase activity was measured in MA runners and aged swimmers and compared to respective sham and non-handled controls. Treadmill exercise consisted of running aged and MA rats on a motorized treadmill for 16 and 20 m/min respectively, 60 min/day and 4 times per week, for 8 weeks. Sham rats were placed on the treadmill twice per week for 5 min at 8 m/min. No differences were found in any parameter comparing sham rats to non-handled controls. Running did not affect body weight or hepatic microsomal protein during the 8-week study. A 33-35% decline in microsomal P-450 content in treadmill exercised MA and aged rats was found. PNA O-demethylase activity was decreased 30% in MA and 45% in aged runners and aniline metabolism was inhibited 21% in MA rats. UDPGT activity was not affected by running in MA or aged rats. Swimming exercise was accomplished by placing the rats in a tank of water (32-33 degrees C) filled to a depth of 2 ft. Swim time was 60 min twice daily, 5 times per week. The aged and MA rats were trained for 6 months and 1 year, respectively. Two control groups, non-swimming sedentary (dry control) and 1 min swim/day sham (wet control), were utilized. MA and aged wet controls and swimmers weighted 8% and 15% less respectively, than MA and aged sedentary rats. Microsomal protein was significantly increased in MA swimmers compared to sedentary (20%) and wet control (35%) but no change was found with swimming in the aged rats. The results of the enzymatic studies were variable in the MA rats. Increases in P-450 content were found in wet controls (16%) and swimmers (27%) of the MA group, but only the swimming change was significant. No significant change was determined for pNA metabolism between swimmers and wet (22%) or dry (17%) controls. Aged swimmers and wet controls were more consistent, with no change in any of the parameters except aniline metabolism which was significantly increased in wet controls (25%) and swimmers (28%) as compared to dry controls. No significant change in UDPGT activity was measured in either age group of swimmers.(ABSTRACT TRUNCATED AT 400 WORDS)     

Good things come to those who wait: attenuated discounting of delayed rewards in aged Fischer 344 rats

The ability to make advantageous choices among outcomes that differ in magnitude, probability, and delay until their arrival is critical for optimal survival and well-being across the lifespan. Aged individuals are often characterized as less impulsive in their choices than their young adult counterparts, demonstrating an increased ability to forgo immediate in favor of delayed (and often more beneficial) rewards. Such "wisdom" is usually characterized as a consequence of learning and life experience. However, aging is also associated with prefrontal cortical dysfunction and concomitant impairments in advantageous choice behavior. Animal models afford the opportunity to isolate the effects of biological aging on decision-making from experiential factors. To model one critical component of decision-making, young adult and aged Fischer 344 rats were trained on a two-choice delay discounting task in which one choice provided immediate delivery of a small reward and the other provided a large reward delivered after a variable delay period. Whereas young adult rats showed a characteristic pattern of choice behavior (choosing the large reward at short delays and shifting preference to the small reward as delays increased), aged rats maintained a preference for the large reward at all delays (i.e., attenuated "discounting" of delayed rewards). This increased preference for the large reward in aged rats was not due to perceptual, motor, or motivational factors. The data strongly suggest that, independent of life experience, there are underlying neurobiological factors that contribute to age-related changes in decision-making, and particularly the ability to delay gratification.     

Deficits in radial-arm maze learning in aged rats

The present study was designed to investigate the possible deficits in the place learning on the 8-arm radial maze in aged rats. In this task, reward was given in the 4 predetermined arms. Aged rats (27 months old, N = 7) acquired this task more slowly than young rats (12 months old, N = 11), and didn't reach to the performance level of the young rats within 80 training trials. Analysis of error choices revealed that the aged animals first entered in the unbaited arms more often than the young rats, whereas there was no difference in the number of re-entered choices to the baited and unbaited arms between the aged and young animals. Therefore, it was concluded that learning deficits in aged rats were attributed to deficits in the reference memory but not in the working memory.     

Deficits in nitric oxide production after tetanic stimulation are related to the reduction of long‐term potentiation in Schaffer‐CA1 synapses in aged Fischer 344 rats

In the present study, we investigated whether nitric oxide (NO) production after tetanic stimulation is involved in long‐term potentiation (LTP) in Schaffer‐CA1 synapses in both young adult and aged rats. The changes in both the population spike amplitude and NO metabolites, nitrite (NO₂^–) and nitrate (NO₃^–), in the CA1 region were simultaneously determined before and after tetanic stimulation. Increases in NO_x (NO₂^– plus NO₃^–) levels in the CA1 region were observed after tetanic stimulation in young adult rats as well as increase in the population spike amplitude. In aged rats, LTP was significantly inhibited compared with that in young adult rats. No increase in NO_x level after tetanic stimulation was observed in aged rats. These findings directly demonstrated that NO production might be involved in the process of LTP formation in Schaffer‐CA1 synapses of the rat hippocampus, and that the deficiency of hippocampal NO production might be responsible for reduction of LTP formation in aged rats.     

Hippocampal NGF levels are not reduced in the aged Fischer 344 rat.

The sympathetic sprouting response that occurs in the rat hippocampal formation following septal denervation is reduced in aged rats. Since considerable evidence implicates NGF-like activity in eliciting the sprouting, the simplest explanation for the age-related decline in sympathetic sprouting is a reduction in hippocampal NGF levels. In the present study, hippocampal NGF levels were measured using a 2-site ELISA in four different age groups of Fischer 344 rats. There was no decline in NGF levels with age, nor did we find any differences between male and female rats. This contradicts an earlier report in which a 40% reduction in hippocampal NGF protein levels was found in aged rats. Possible reasons for this discrepancy are discussed. The present results do not support the hypothesis that the age-related decline in sympathetic sprouting is due to a reduction in total hippocampal NGF levels.     

Behavior of transplanted large granular lymphocyte leukemia in Fischer 344 rats

The behavior of transplanted large granular lymphocyte leukemia was studied by isogeneic inoculation of 55 young F344 rats. A high percentage of transplants were successful, showing a dose-dependent latent phase followed by weight loss, exponential increase of tumor cells, and widespread infiltration. Serial passage of tumor cells appeared to increase malignancy by decreasing longevity. Grossly, there was splenomegaly, variable lymphadenopathy, and petechial hemorrhages on the lungs, lymph nodes, and brain. Diffuse infiltration of tumor cells caused morphologic changes identical with spontaneous large granular lymphocyte leukemia including splenic lymphoid depletion, hepatocellular necrosis, tumor cell erythrophagocytosis, and femoral medullary endosteal bone proliferation. Clinicopathologic features were also similar to spontaneous large granular lymphocyte leukemia. There was immune-mediated hemolytic anemia and thrombocytopenia, which coincided with development of leukemia, as well as hyperbilirubinemia and elevated serum enzymes, indicative of liver disease. It was concluded that isogeneic transplantation of large granular lymphocyte leukemia is not only highly successful but reproduces the pattern and clinical syndrome of the spontaneous tumor including the important paraneoplastic syndrome of hemolytic anemia and thrombocytopenia.     

Fischer 344 rats display age-related memory deficits in trace fear conditioning

A functional hippocampus is required for trace fear conditioning, which involves learning the association of a tone and shock that are separated over time. Young and aged rats received 10 trace conditioning trials. Twenty-four hours later, rats were tested for fear to the tone in a novel chamber by measuring freezing. The results showed significantly lower levels of freezing in aged rats as compared with young rats, which provides evidence of age-related memory impairments. Pseudorandom conditioning groups showed low levels of freezing, indicative of no associative memory. Age-related memory deficits were not found with delay conditioning, which suggests no age-related sensory-motor deficits. These data suggest that aging hinders the ability of the hippocampus to process information separated over time.