James R. Misanin

Age dependency in neophobia: Its influence on taste-aversion learning and the flavor-preexposure effect in rats

Five experiments were conducted to evaluate various aspects of stimulus preexposure effects on conditioned saccharin aversion in rats of three age groups: weanling (19–25 days), young-adult (92–170 days), and old-age (680–850 days). In Experiment 1, flavor neophobia was examined. Only the young-adult and old-age animals showed evidence of neophobia. Furthermore, habituation of the neophobic reaction differed for these two age groups. Using a brief to moderately long flavor-preexposure period and an intense US (Experiment 2), we demonstrated that the youngest age group was most likely to exhibit retarded conditioning as a result of preexposure to the flavor CS. Using a weaker US and a moderately long saccharin-preexposure period (Experiment 3), age differences in conditioning resulting from preexposure to the flavor CS were reduced. When animals were preexposed to saccharin continuously for 48 h (Experiment 4), age differences in the preexposure effect were not evident. In Experiment 5, the intensity of the US was reduced to determine whether floor effects in the previous experiments had masked age differences in the ability of nonpreexposed rats to acquire an aversion to saccharin. Results indicated that taste-aversion learning was directly related to age. Although open to other interpretations, the results support the notion that the flavor-preexposure effect is influenced by the initial level of flavor neophobia. They also suggest that systematic parametric variation is sometimes necessary to obtain an accurate description of age differences in learning.

Escape behavior in neonatal rats

Rats, 5–11 days old, showed improved, performance over trials on a shock-escape task, as indicated by a decrease in competing responses. While rate of improvement did not differ at these ages, the number of competing responses did, being indirectly related to age. This finding plus the observation that Ss exhibited a definite direction preference suggest that the recorded response may be an inborn reaction to aversive stimulation. In contrast to competing responses, speed on trials without competing responses remained constant during training, suggesting that Ss move at an asymptotic speed for their level of maturation and a fixed level of motivation.

Retention of conditioned taste aversion in weanling, adult, and old-age rats

Rats ranging in age from weanling to old age were given a single saccharin/lithium chloride pairing and were tested for saccharin aversion 1 or 28 days later. Weanling, adult, and old-age rats did not differ in their retention of the conditioned aversion at the 1-day retention interval, but weanling rats showed substantially less aversion than adult and old-age rats at the 28-day interval. The old-age rats did not differ from the adults at either retention interval. As old-age rats generally forget a passive avoidance task sooner than adults do, these results lend limited support to the hypothesis that retention of conditioned taste aversion may be mediated by a different memory system than the system that mediates retention of more conventional types of learning.

Aging facilitates long-trace taste-aversion conditioning in rats

In order to examine age-related changes in long-trace conditioning, five age groups (0.25, 1, 1.5, 2, and 2.5 years) of Wistar-derived female albino rats were subjected to taste-aversion conditioning at one of five conditioned stimulus-conditioned stimulus (CS-US) intervals (0, 45, 90, 180, and 360 min). Age differences in the strength of the aversion were evident at CS-US intervals greater than 0 min and the strength of the aversion was directly related to age. An aversion was conditioned in only the two oldest age groups when the CS-US interval was 360 min. The age differences in taste-aversion and the superior long-trace conditioning in old-age rats were attributed to factors that accompany aging, for example, the gradual slowing down of a metabolic pacemaker.

The effect of CS-preexposure on conditioned taste aversion in young and adult rats

Young and adult rats were given nonreinforced exposures to the flavor-CS prior to a taste aversion conditioning session. CS-preexposure prevented the conditioning of a taste aversion in young rats but only attenuated conditioning in adults. These results suggest that an enhanced CS-preexposure effect may account for the previously reported weaker conditioned taste aversion observed in young rats, as compared to adults, when there is a protracted interstimulus interval.

Age differences in the outcome of long-delay taste-aversion conditioning in rats

Weanling, young-adult, and aged rats were subjected to taste-aversion conditioning with a relatively weak US (1% body weight i.p. injection of.15 M LiCl) at one of four CS-US intervals (0, 0.75, 1.5, and 3.0 h). Age differences occurred at all intervals, with aged rats showing superior conditioning. Increasing the CS-US interval was more disruptive the younger the rat. Although not unequivocal, the results suggest that the processes involved in associative memory during taste-aversion conditioning may involve different processes from those that mediate long-delay learning in more conventional learning tasks.

EFFICACY OF LITHIUM CHLORIDE IN THE TASTE-AVERSION CONDITIONING OF YOUNG-ADULT AND OLD-AGE RATS

An absolute amount of a LiCl US was administered to 24 young-adult and 24 old-age rats during taste-aversion conditioning to determine whether the superior performance of old-age rats, when a 1% body-weight injection of a LiCl US is administered 3 hr. after a saccharin CS, is due to age-related differences in US intensity or the efficacy of LiCl. The aversion conditioned in old-age rats with a 3-hr. CS-US interval and an absolute amount of LiCl was still significantly greater than that in young adults suggesting that age-related differences in US intensity or the efficacy of LiCl cannot explain the superior performance of old-age rats. Possibly, old-age rats forget the specifics of the CS more than young adults and, thus, older animals show greater stimulus generalization to the substantially different test stimuli.

Effects of pretraining experience on retention of a passive avoidance task following ECS

Abstract Two experiments were performed to determine what effect pretraining experience has on retention of a passive avoidance task following posttraining electroconvulsive shock (ECS). The first experiment examined the effects of a pretraining ECS on retention in rats, and the second, the effect of a pretraining noncontingent footshock. Both pretraining experiences improved the rats retention of the passive avoidance task, reducing or eliminating the amnesia typically produced by posttraining ECS. It was suggested that such pretraining experiences facilitate memory retrieval by preestablishing memory systems into which memories for subsequent related events are readily incorporated, or by strengthening the neuronal representation of the learning experience.

Recovery rate as a determinant of the amnesic-like effect of hypothermia.

Abstract Two experiments were performed to determine the role of depth and duration of hypothermia in hypothermia-produced impairment of the rats retention of a one-trial passive avoidance task. The first experiment varied depth of hypothermia and held duration constant by varying recovery rate. The results indicated a graded impairment of memory for animals cooled in 5–15°C water. No difference in memory impairment obtained in animals cooled in 0°C and 5°C water. These results suggest that depth of and/or rate of recovery from hypothermia play a significant role in hypothermia-produced amnesia. The second experiment varied duration of hypothermia, holding depth constant. The results indicated that rate of recovery was a prime determinant of hypothermia produced forgetting. Animals whose body temperatures were kept below normal for an extended period of time displayed little or no amnesic-like behavior, whereas animals whose body temperatures were returned to normal at a relatively rapid rate showed extensive forgetting. These results were attributed to retrieval failure rather than to a disruption of memory consolidation.

Context familiarity and delayed conditioned taste aversion in young-adult and old-age rats.

Familiarity with a conditioning context different from the home-cage environment was examined in immediate and delayed (3-hr.) conditioned taste aversion (CTA) learning for young-adult (90—120 days) and old-age (680—750 days) female Wistar albino rats. Context familiarity increased CTA for young adults at the 3-hr. delay. Old-age rats showed no aversion at 3-hr. delays. Results suggest that home-cage cues may be used in mediating long-delay CTA and that the role of these cues may differ with age.