Development of 24 hour retention of escape learning in neonatal C3H mice

The development of 24-hr memory of an escape response in neonatal C3H mice was examined in 2 experiments. In experiment 1, separate groups of mice received 25 trials of escape training for 3 consecutive days (24 hour Inter-session-interval) beginning at 5, 7, and 9 days of age. Several different measures demonstrated witin-session improvement for all ages on all test days. However, between-session improvement which was not due to maturation did not occur until after 9 days of age. In experiment 2, separate groups of mice received 25 escape training trials at either 8 or 10 days of age, and were retested 24 hrs later. Comparisons of the retest scores with original training scores as well as with littermate controls without prior training experience, showed that 8-day old mice failed to show retention effects of prior training, whereas 10-day old mice performed better on the retention test than they had on original training, as well as better than controls without previous training. The results suggest that 24-hr memory of the escape task develops at 9–10 days of age in the C3H mouse.     

Sex differences in the behavioral consequences of inescapable footshocks depend on time since shock

In two experiments, the effects of inescapable shock on subsequent shuttle-box escape performance were studied in male and female rats. Effects of treatment with short-duration shocks (2 s) were studied after 1- and 24-hour intervals (Experiment 1), and effects of long-duration shocks (6 s) were studied after 24- and 72-hour intervals (Experiment 2). Experience with inescapable shock resulted in a serious disruption of escape performance in both males and females. A large increment in escape latencies was found both during fixed ratio 1 and fixed ratio 2 escape training; however, effects of inescapable shock were more pronounced in males than in females. In Experiment 1, sex differences were most obvious after the short 1-hour interval whereas, in Experiment 2, sex differences were only present after 24 hours and not after 72 hours. Shuttle activity during 2-min adaptation prior to shock-escape training was reduced in both males and females treated with IS, and this effect was somewhat stronger in males than in females. The data of these experiments show that male rats are more sensitive to the consequences of exposure to inescapable aversive stimulation than female rats. It is proposed that the time-dependency of the sex differences in behavioral consequences of treatment with inescapable shock may be related to sex differences in transient neurochemical or hormonal changes induced by inescapable shock.     

The role of homecage environmental stimuli in the facilitation of shock-motivated spatial discrimination learning in rat pups

In three experiments we examined the role of homecage environmental stimuli on learning an aversively motivated spatial discrimination task in 11-day-old rats. Varying the presence and absence of nest shavings in the correct and incorrect arms of a T-maze in Experiment 1 revealed that nest shavings had both nondirective facilitation and approach-eliciting properties. Training the 11-day-old rats in the presence or absence of shavings over three daily sessions and comparing their performance on the third day of training with that of a maturation control in Experiment 2 indicated that there is a residual training effect of the approach-eliciting property of nest shavings on shock-escape behavior. In Experiment 3, using nest shavings either as an irrelevant stimulus or as a redundant relevant cue in an aversively motivated T-maze reversal task suggested that 11-day-old rats acquire the discrimination task using non-nest spatial or directional cues as well as those provided by nest shavings. Overall, these results suggest that nest shavings introduced into aversively motivated tasks create a learning environment comparable to that of appetitively motivated instrumental tasks utilizing suckling from an anesthetized dam as a reinforcer.     

Cross-stressor immunization against the behavioral deficits introduced by uncontrollable shock

Exposure to inescapable shock disrupted performance in both shock- and water-escape tasks. These deficits were prevented in mice that were previously trained in the same task. However, an asymmetrical immunization effect was seen in a cross-stressor paradigm. Whereas deficits of water-escape performance engendered by inescapable shock were prevented by prior shock-escape training, the deficits of shock-escape performance were not eliminated by prior water-escape training. Evidently, the immunization effect occurs when initial training and subsequent testing are conducted in the same task, or when the initial training and uncontrollable stress session involve the same aversive stimulus. Norepinephrine determinations revealed that reductions of the amine introduced by inescapable shock were unaffected by prior shock-escape training and were enhanced by prior exposure to the stress of water immersion. Thus, although the performance deficit introduced by inescapable shock may be related to variations of norepinephrine, the immunization effect probably was unrelated to alterations of this transmitter. Rather, the data provisionally suggested that the immunization stems from two independent factors: Namely, initially training animals in an active escape task may (a) disrupt subsequent learning that the inescapable stress actually is uncontrollable and (b) limit the influence of the motor deficits introduced by uncontrollable shock on subsequent escape performance.     

Learning and retention of a discriminated escape response in infant mice

Separate groups of mice received 25 training trials in a shock-escape T-maze at 7,9,11, or 13 days of age, followed by retention trials 24 hr later. During original training, all mice were trained to the goal opposite their 1st-trial choice-point turn. During the retraining sessions, half of the mice in each age group received an additional 25 trials to the same goal as original training, while the other half was trained to the goal opposite that of original training. All age groups demonstrated improved performance during training in reaching the choice point, although only mice 9 days of age and older indicated an increase in correct choice-point turns. During retraining, only the groups trained at 11 and 13 days of age showed significant effects of prior training.     

Eyeblink conditioning in the developing rat.

Long-Evans rat pups, 17-18 or 24 days of age, were trained with an eyeblink conditioning (EBC) procedure that was used previously with adult rats (Skelton, 1988). Pups received 3 sessions of delay conditioning in a single day at about 4-hr intervals (100 trials/session). Trials involved pairings of an auditory conditioned stimulus (2.8-kHz, 82-dB tone) and a periocular-shock unconditioned stimulus (US; 100 ms, 2 mA), which were presented 280 ms apart. EBC was observed at both ages, but older pups learned much more rapidly. Subsequent experiments established that this effect is associative (Experiment 2), that age differences in EBC cannot be attributed to differences in ability to respond or in sensitivity to the US (Experiment 3), and that EBC rate can be modulated by motivational state (Experiment 4). This preparation may help elucidate the relation between neural development and the ontogeny of learning.     

An evaluation of spatial information processing in aged rats

The spatial learning abilities of young, middle-age, and senescent rats were investigated in two experiments using several versions of the Morris water maze task. In Experiment 1, Long-Evans hooded rats were trained to find a submerged escape platform hidden within the water maze. During this phase of testing, aged rats exhibited acquisition deficits compared with either young or middle-age subjects. With continued training, however, all age groups eventually achieved comparable asymptotic levels of performance. Subsequent testing in Experiment 1 revealed that following original training, aged rats were not impaired in learning a novel escape location or in their ability to locate a visible, cued escape platform. In an attempt to identify the basis of the age-related impairments observed in Experiment 1, naive young and aged rats in Experiment 2 were initially tested for their ability to locate a cued escape platform in the water maze. During this phase of testing, the escape latencies of both young and aged rats rapidly decreased to equivalent asymptotic levels. Subsequent analyses revealed that following cue training, young subjects exhibit a significant spatial bias for the region of the testing apparatus where the platform was positioned during training. In contrast, aged rats showed no spatial bias. Training was continued in Experiment 2 using a novel submerged platform location for each subject. During these place training trials, the escape latencies of senescent rats were longer than those of young subjects. These impairments were also accompanied by a lack of spatial bias among aged rats relative to young control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inception of a 24-hr capacity memory in two mouse strains

Separate groups of C3H and Swiss-Webster (S-W) mice received 25 escape training trials in a straight alley at 8 or 10 days of age. Each of these groups was then divided into 3 retention groups, receiving an additional 25 training trials, 1, 6, or 24 hr following original training. Comparisons of the retest scores with original training scores, as well as with litter-mate controls without prior training, indicated that both strains trained at 8 days of age failed to demonstrate retention of learning when retested 24 hr after training, although they showed reliable 1-hr retention and some evidence of 6-hr retention. However, mice trained at 10 days of age performed better on the retention tests at all retest intervals than they had on original training, as well as better than maturation controls without prior training. These results indicate that a 24-hr retention capacity for escape training develops between 8-10 days of age in both mouse strains.     

Straight-alley escape behavior in infant mice: effect of shock intensity

Separate groups of Swiss-Webster mice between 5-15 days of age received 25 training trials in a straight-alley escape task at 1 of 4 shock intensities, ranging from .1-.8 mA. An increase in shock level led to better escape performance at all ages, although the amount of improvement varied directly as a function of age. The results supported earlier work suggesting that the competing response measure was the most useful index of improved escape behavior at the early ages. Running speeds did not indicate improved within-session escape performance at any age or shock level, although speeds clearly reflected maturational and motivational differences between groups.     

Ontogeny of two-way avoidance in male and female rats

In Experiment I, 17-, 21-, 36-, 51-, 90-, and 200-day-old male and female rats were given a single session of 100 two-way avoidance (TWA) trials. In the 2nd experiment, males and females of these ages and 15 and 28 days of age that were obtained from a different source, weaned at a later age, and housed differently received TWA training. Results of both studies showed that avoidance of 15-, 17-, and 21-day-old rats is low, but avoidance increases from 21 to 51 days of age. Avoidance of 95- and 200-day-old animals was generally lower than 51-day-old rats. No significant gender differences appeared until 90 days of age; at this age avoidance of males was lower than females. In Experiment III, CS intensity was varied and the US intensity was lower than that used in Experiment I and II. Avoidance of 21- and 90-day-old rats was higher with a more intense CS, but 21-day-olds were still lower than adults. Avoidance of 17-day-old rats was not affected by CS intensity.     

Developmental aspects of passive and active avoidance learning in rats

In Experiment 1, groups of rats 16, 19, 25, 32, and 90 to 120 days of age, were tested for retention of a passive avoidance response 2 min or 24 hr following a single training trial. Passive avoidance learning improved markedly with age, and retention over a 24-hr interval was complete for all age groups. In Experiment 2, rats 19, 25, 32, and 90 to 120 days of age were trained in a simple, active avoidance task. A trials-to-criterion measure indicated that learning was relatively independent of age, although 19-day rats were somewhat inferior to older rats. The occurrence of differences in passive avoidance learning through developmental ranges in age where simple active avoidance is little affected suggests that inhibition of responding may be selectively influenced by maturational variables.     

Ontogenesis of learning: V. Variation in associative and nonassociative control of an operant forelimb response in infant rats

To study ontogenetic variation in the mechanisms that control operant responding, infant rats 4-16 days old were trained to lift a forelimb in order to receive an infusion of a sucrose-milk solution (reinforcer). Although the learning processes that contribute to this behavior were to some extent functional in pups 5-6 days old (Experiment V), there were major age-related constraints on the performance of this response. We were unable to observe selective responding with the reinforced forelimb until pups were 9 days old (Experiment I). This was in part because pups tested when 7, but not 9 days old, were highly activated (as measured by responding with both forelimbs) by both the reinforcer (Experiment II) and cues associated with the reinforcer during training (Experiment IV). Implications of these findings are discussed.     

Study of suckling behavior in the rat: the effects of food deprivation of pups and lactational age of mother

The effects of food deprivation (0-1, 2-3, and 22-23 h) of pups and of lactational age of mother (1-7 days, 16-22 days and own mother) upon suckling behavior of pups were investigated with the mother anesthetized. The effect of food deprivation was not evident prior to 11 days of age but deprived pups showed higher level of suckling than non-deprived pups at 16 and 21 days of age (Experiment 1). The effect of lactational age of mother was not found at 6-17 days of age but pups at 1-2 and 21-22 days of age showed lower level of suckling to the anesthetized mother which differed in lactational age from their own mother (Experiment 2).     

Pavlovian excitation,internal inhibition,and their interaction with free operant avoidance as a function of age in rats

The central hypothesis was that, in young as compared with adult rats, Pavlovian excitation is weaker and extinguishes more slowly, whereas internal inhibition is weaker but extinguishes more rapidly. In Experiment I, the suitability of free operant (Sidman) avoidance as a baseline for measuring Pavlovian effects was assessed in 5 age groups between 19 and 98 days of age trained in a shuttle box. Although clear age-related increases were evident in avoidance capacity, efficient “high criterion” avoiders were found in all age groups, indicating that the response was suitable. In Experiment II, groups of 26-, 38-, and 95-day-old rats first received Pavlovian aversive excitatory and differential inhibitory conditioning, or the conditional stimuli and shock randomly, or no treatment. Three days later they received free avoidance training. Prior Pavlovian conditioning facilitated subsequent avoidance learning in young subjects, but depressed it in adults. Among “high criterion” avoiders, such effects were absent. Following the avoidance training phase, 3 age groups of matched “high criterion” avoiders received conditioned stimuli superimposed on the baseline (a conditioning test). These results confirmed the central hypothesis.     

Development of learning and memory in mice genetically selected for differences in brain weight

Mice selected for high (H), medium (M), and low (L) brain weight were trained to the goal arm opposite their preference in a shock-escape T-maze at 7, 9, 11, or 13 days of age. Twenty-four hours later, half of the trained groups at each age received additional training to the original goal, whereas the other half was trained to the opposite goal. Maturational control groups without prior training were trained on the selected retest days for the previously trained groups. Improvement in correct choice-point turns during training was suggested for the H-line by 9 days of age and by 11 days of age for the M- and L-lines. During retraining, H-line mice demonstrated 24-hr retention effects by 10 days of age, whereas 24-hr memory was not indicated for M- and L-lines at any of the ages investigated. These results indicate that the onsets of learning and memory were influenced by genes affecting brain weight, in that H-line mice demonstrated more rapid brain growth as well as heavier brain weights at every age studied as compared with M- and L-line mice which showed similar brain growth functions.     

Age- and training-related changes in the collagen metabolism of rat skeletal muscle

Summary The effects of ageing and life-long endurance training on the collagen metabolism of skeletal muscle were evaluated in a longitudinal study. Wistar rats performed treadmill running 5 days a week for 2 years. The activities of collagen biosynthesis enzymes, prolyl-4-hydroxylase and galactosylhydroxylysyl glucosyltransferase, were highest in the muscles of the youngest animals, decreased up to the age of 2 months and from then on remained virtually unchanged. The enzyme activity in young animals was higher in the slow collagenous soleus muscle than in the rectus femoris muscle. The enzyme activity in the soleus muscle was higher for older trained rats than older untrained rats. The relative proportion of type I collagen increased and that of type III collagen decreased with age, suggesting a more marked contribution by type I collagen to the agerelated accumulation of total muscular collagen. The results show that collagen biosynthesis decreases with maturation and that life-long endurance training maintains a higher level of biosynthesis in slow muscles.     

Instrumental learning within the spinal cord: IV. Induction and retention of the behavioral deficit observed after noncontingent shock

Spinalized rats given shock whenever 1 hind leg is extended learn to maintain that leg in a flexed position, a simple form of instrumental learning. Rats given shock independent of leg position do not exhibit an increase in flexion duration. Experiment 1 showed that 6 min of intermittent legshock can produce this deficit. Intermittent tailshock undermines learning (Experiments 2-3), and this effect lasts at least 2 days (Experiment 4). Exposure to continuous shock did not induce a deficit (Experiment 5) but did induce antinociception (Experiment 6). Intermittent shock did not induce antinociception (Experiment 6). Experiment 7 addressed an alternative interpretation of the results, and Experiment 8 showed that presenting a continuous tailshock while intermittent legshock is applied can prevent the deficit.     

Appetitive instrumental training in preweanling rats: I. Motivational determinants

Two experiments tested learning and retention of preweanling rats (10, 15, and 23 days of age) with an appetitive T-maze spatial discrimination task, using 3 different reinforcers for training (Mother Alone, Mother + Milk, and Milk Alone.) The hypothesis was that immature rats are prepared to select and generate effective hypotheses for learning in an environment bearing some similarity to the home nest prior to their exhibiting similar capacities in other contexts. In Experiment I, we found support for this hypothesis in that 10- and 15-day-old rats were able to acquire the maze task when reinforced with Mother Alone or Mother +Milk, but not when reinforced with Milk Alone. In Experiment II, we report that 23-day-old rats weaned at 19 days postpartum were better at learning the maze task with milk as the reinforcer than littermates raised until training with the dam. We hypothesize that the development of foraging strategies might be necessary for the expression of maze behaviors reinforced with milk alone.     

Latent inhibition in the developing rat: an examination of context-specific effects

Latent inhibition (LI) refers to the reduction in conditioned responding when the conditioned stimulus (CS) is preexposed prior to CS-unconditioned stimulus pairings. Experiment 1a demonstrated that preexposure to an odor CS prior to odor-shock pairings markedly reduced conditioned freezing in 25-day-old rats; however, this LI effect was observed only if odor preexposure and odor-shock pairings occurred in the same context (i.e., LI was context-specific at this age). The results of Experiment 1b showed that 18-day-olds also exhibited LI, but this effect was not context-specific at this age. In Experiment 2, rats were preexposed to the odor at 18 days of age and given odor-shock pairings at 25 days of age. These rats exhibited context-specific latent inhibition, suggesting that 18-day-old rats encoded the preexposure context. In Experiment 3, all parameters were identical to Experiment 2, with the exception that odor-shock pairings were given at approximately PN18 and testing occurred at approximately PN25. These rats exhibited latent inhibition at test, but this effect was not context-specific. The results of this study suggest that (a) PN18 rats can exhibit latent inhibition, and (b) the expression of context-specific latent inhibition depends on the age at which conditioning occurs.     

Effects of interrun interval shift in the reward serial learning of the rat on the straight runway

Using runway training procedure, three experiments examined effects of interrun interval (IRI) shift on serial pattern learning in rats. Series consisted of varying number of 0.045 g food pellets given in a goal box of a runway. Groups of rats were trained with 24-10-1-0 or 24-16-10-6-3-1-0 series (Experiment 1), 14-7-3-1-0 series (Experiment 2), and 18-12-6-3-1-0 or 18-7-7-7-1-0 series (Experiment 3), under either short (15-20 s) or long (5-15 min) IRI. In test phases, IRI was shifted from short to long or vice versa. Anticipation of 0 pellet was not disrupted by shortening of IRI, whereas lengthening of IRI eliminated the 0 pellet anticipation of the five- to seven-item monotonic series but did not eliminate that for the four-item series and the nonmonotonic series. These results suggest that rats learn a series with different strategies depending on IRI, pattern length, or structural complexity of a series.