Avoidance learning, behavior genetics, and aging: a critical review and comment on methodology

Does hypertension alone lead to cognitive decline in spontaneously hypertensive rats?

Spontaneously hypertensive rats (SHR) represent an animal model of cognitive decline associated with hypertension. Few studies have systematically investigated this decline in aging. We assessed spatial learning performances of SHR, the genetically similar Wistar-Kyoto rats (WKY), and Sprague-Dawley rats (SD), in a water maze (WM) task. The following age ranges were studied: 6, 12, and 20 months. The results demonstrated that all three strains show the same level of impairment at the age of 20 months. The rate of cognitive decline, however, is different: both SHR and WKY show moderate degrees of impairment at all age ranges, while SD display good cognitive abilities at 6 months, declining at 12, and reaching the performance level of the other two groups at 20 months. Besides, the nine groups showed substantial differences in swim velocity and WKY exhibited a peculiar motor behavior. These results suggest the following: (a) the decline in cognitive level exhibits different trends in the three strains; (b) learning impairment of aged SHR might not be entirely explained by hypertension; (c) WKY should be used cautiously as normotensive control for SHR, due to their unusual behavior and low learning abilities; (d) analysis of escape distances is mandatory for the comparison of different strains in the WM test.

Dissociation of learning and performance deficits in aged mice

The present study was undertaken to determine whether an age-related learning deficit would occur in a complex visual discrimination task and whether the learning impairment could be separated from performance deficits. The study also sought to determine whether treatment with an inhibitor of protein synthesis, anisomycin, would impair learning in this task. Two age groups (7-10 mo; 27-30 mo) of C56BL/6j mice were given training in a five-choice, simultaneous, visual discrimination task. Errors, freezing, avoidances, and response latencies were recorded. Results revealed that the difference in errors between the two groups disappeared during the middle part of training whereas the difference in the performance measures persisted until the end of training. Anisomycin caused increased errors in the adult but not the old mice. These results indicate that old mice can learn a discrimination task as well as adults but the rate of learning is slower, whereas their physical performance on the task is persistently inferior to adult mice.

Behavioral assessment of forgetting in aged rodents and its relationship to peripheral sympathetic function

Observation of age-related memory deficits in rodents is often dependent on the behavioral task used to assess these changes, rather than being universal to all memories. A review of studies using aversively-motivated multiple trial training paradigms suggests that the apparent acquisition deficits common to older animals may instead be due to a confounding tendency of these animals to behavioral rigidity or perseveration. Data obtained using single trial training paradigms, such as the one-trial passive avoidance task, indicate that young and old rodents can learn tasks with equal facility, that retention in young and old animals is similar at short training-testing intervals, but that retention is impaired in aged animals at longer training-testing intervals. We suggest that the adrenal medulla, a peripheral source of catecholamines, secretes catecholamines that act outside the blood-brain barrier to modulate memory processes. Further, we review evidence suggesting that the ability of the adrenal medulla to respond to the stresses of footshock during aversively-motivated training are impaired in aged rodents, and that this impairment may contribute to the rapid forgetting observed in senescent animals.

Hippocampal neurobiological mechanisms of age-related memory dysfunction

Studies are reviewed which indicate that hippocampal frequency potentiation (the growth of neural responses during repetitive synaptic stimulation) is impaired in aged rats, and that this impairment may be important in learning and memory deficits found in these aged animals. Intracellular recording and ultrastructural studies suggest that both hippocampal frequency potentiation and the age deficit in such potentiation are synaptic processes (probably presynaptic), and that the deficit may be due to an age-related increase in calcium influx during depolarization. The latter could in some way result from alterations in the function of a Ca-mediated inactivation of Ca current mechanism recently found in hippocampal neurons. Since major hippocampal changes occur with aging in both rodents and humans, it seems possible that these data are also relevant to human brain aging. Consequently, it is suggested that Alzheimer's disease results from an acceleration of normal age-related neuronal calcium conductance changes by some unknown process (e.g., viruses, aluminum, genetic factors, etc.), leading to a rapid deterioration of brain structure.

Short-term memory, exploration and locomotor activity in aged rats

Behavioral profiles of young (3-6 months) and aged (24-27 months) rats were compared in three respects: (1) short-term memory, (2) exploration and (3) locomotor activity. Compared to young rats, aged rats were impaired in the 8-arm radial maze acquisition, but not in the delayed reinforced alternation acquisition. They had lower scores of spontaneous alternation, of exploration of a novel object and of a novel environment. Their exploratory activity was lower in a simple environment but similar in a complex environment. Their spontaneous locomotor activity was lower during the dark part of the cycle (8 p.m.-8 a.m.) but not different during the light part of the cycle (8 a.m.-8 p.m.). These results suggest that with respect to short-term memory and exploration, differences between aged and young rats may be influenced by a "complexity" factor and may be secondary to differences in motivation and reactions to novelty.

Measurement of cognitive abilities in senescent animals

The behavioral paradigms used in investigating for differences in the cognitive abilities of young and aged animals are critically reviewed with regard to their power to discriminate between young and aged animals' mental capacity. Irrespective of the kind of task, geronto-behavioral research especially is afflicted with difficulties in controlling motivational and emotional influences on cognitive processes. It is hypothesized-somewhat provocatively-that most of the findings indicating an age-related decline are better attributed to the altered motivational status and/or emotional reactiveness than to impaired cognitive processes of senescent animals. Of the common tasks used in this field, it is concluded that complex mazes and different delayed response tasks seem to represent appropriate paradigms in order to study changed capacities in short- and long-term memory (working- and reference-memory, respectively).

Learning and memory in young and aged Fischer 344 rats

Changes in learning and memory processes that occur with senescence were investigated in male and female Fischer 344 rats, 3-26 mth of age. Age-related impairments were seen in retention of inhibitory avoidance learning, acquisition of a Y-maze discrimination task, and in a swim escape task with short intertrial training intervals. In contrast, old animals performed better than the young rats in an active avoidance task. No age differences were observed in either open field activity or in flinch or jump thresholds to footshock. These results indicate that impairments in learning and memory processes of aged rats are task-specific, and that memory deficits in old rats are best seen following one-time-only events or with weak training. The behavioral baselines described will help in the design of further research to correlate memory and neurobiological changes observed during the aging process in the rat.

Effects of age on some behavioral characteristics of novel auditory stimuli in the rat

Low intensity novel auditory stimuli have two behavioral characteristics. First, such stimuli elicit exploratory behavior directed at the sound source. Second, novel stimuli gain control of responding in a discriminative task more rapidly than familiar stimuli. Experiments were carried out to investigate these two characteristics of novel stimuli. In the first experiment 3, 12 and 30 month old rats were given three sessions, 24 hours apart, of exposure to an initially novel low intensity series of noise pulses; the number of exploratory responses of the sounding speaker per session was counted. In the second experiment, 12 and 30 month old rats were trained in a sound location discrimination in which the stimuli were made familiar prior to discrimination training. Age had no effect on the amount, habituation and retention of habituation of exploration of the sounding speaker. In the second experiment, the 12 month and 30 month old animals acquired the discrimination at the same rate; again no age effect was found.

Effects of age on acquisition and maintenance of a location discrimination in rats

Twelve-and thirty-month old Sprague Dawley rats were trained in an auditory discrimination using a discrete trial procedure with food as the reinforcer. Two thirty-month an two twelve-month animals acquired the discrimination in one trail. The asymptotic level of correct responding was lower for the old animals and this was attributed to changes in auditory threshold with age. The thirty-month animals gave more intertrial interval responses than the twelve-month animals. There was no difference in the average response latency for the two age groups.

Brain aging correlates: retardation by hormonal-pharmacological treatments

Mid-aged rats were either adrenalectomized and chronically maintained, or left intact and treated daily for a 9- to 10-month period with a potent analog of the peptide adrenocorticotropin (residues 4 to 9), which has some stimulant properties, or with the neural stimulant pentylenetetrazole. All three treatments reduced hippocampal morphologic correlates of brain aging (neuronal loss, glial reactivity). The pentylenetetrazole and peptide treatments also improved reversal learning. These results suggest that certain endogenous peptides, with stimulant properties, may also exert long-term, trophic effects on brain structure and function.

Failure of piracetam to facilitate acquisition or retention in younger or older rats

The performance of Long-Evans hooded rats of two age groups (450 and 75 days) was observed in open field activity, one-way active avoidance, and passive avoidance conditioning. One-half of the subjects in each group were administered 100 mg/kg of piracetam two hours prior to all testing trials. Younger rats required fewer acquisition trials to reach active avoidance criterion and made more active avoidances on 7-day retention tests than did older rats. Thus, younger rats performed better on both acquisition and retention of active and passive avoidance conditioning. Piracetam did not produce significant behavioral changes in any of the groups on any of the tasks. Neither age nor piracetam was shown to affect activity.

Conditioned taste aversion as a function of age in mature male rats

Experimental groups of young mature (90-120 days), mature (365-395 days), and aged (730-760 days) Fischer-344 rats were allowed to drink a saccharin solution followed by lithium chloride toxicosis initiated at one of three intervals, either 15, 60, or 240 minutes. Control groups were given saline placebos according to the same schedule. In a preference test conducted 48 hours after conditioning, there was little evidence of age differences in the acquisition of a saccharin aversion. Age differences were noted in the extinction of the aversion which was tested by monitoring preference over a period of 32 days of continuous access to saccharin and water. Older animals tended to show greater resistance to extinction.

Failure of B6 deficiency to affect performance of aging rats in a passive avoidance task

Male Fischer-344 rats from three age groups (6-7, 15-16, and 27-29 months) were given either a pyridoxine deficient diet or control diet for five weeks. Differences in body weight produced by the diets were evident in the youngest age groups but not in the oldest group in which all animals tended to lose weight. There was no evidence that diet or age affected performance during passive avoidance learning.

Age pigments, cell loss and hippocampal function

The specific aims of this study were to perform direct correlational analyses of age differences in learning, short-term memory and arousal in relation to cell loss and lipofuscin increase in the hippocampus CA1 zone and in visual area 17 of the Fisher 344 rat. The following tentative conclusions can be made from the results presented in this study: (1) significant age differences in 2 and 6 hour passive-avoidance retention or memory between mature and old rats were related to non-significant age differences in days to criterion learning, starting latencies, running distance and time in original approach learning, and (2) significant age differences in 2 and 6 hours retention of old, compared to mature rats were correlated significantly with loss of neurons, and very significantly with increases in intraneuronal lipofuscin in the hippocampus CA1 zone and in visual area 17.