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

Age, sex and pre-exposure effects on acquisition and generalization of conditioned taste aversion in rats

The main aim of the present study was to assess the effect of sex and aging in two pre-exposure learning effects, latent inhibition (LI) and perceptual learning (PL), with a conditioned taste aversion paradigm. Young adult (90 days) and aged (more than 18 months) males and females received 8 pre-exposure trials either with stimulus AX (LI conditions) or BX (PL conditions). Then, all animals received a conditioning trial with AX and two test trials, one with AX and other with BX. The level of generalization between AX and BX was assessed by means of the absolute level of consumption of BX and by the difference in consumption between both stimuli. The results showed an attenuation of latent inhibition as well a stronger generalization of conditioned taste aversion in females when generalization is inferred from the BX consumption. A facilitation of conditioning for the aged animals was also found regardless of the pre-exposed stimulus. Pre-exposures to BX resulted in little generalization, but pre-exposures to AX resulted in a very similar consumption of both compounds, indicating a strong generalization between them. Overall, the study provided novel evidence about the effect of sex and aging on taste aversion, raising at the same time some relevant questions about perceptual learning and how such pre-exposure effect has been typically assessed.

Conditioned taste aversions

When one becomes ill after consuming a meal, there is a propensity to target a particular taste as the cause of the illness. The qualities of the taste most likely targeted include more novel, less preferred, and higher protein content. This association between a particular taste and illness is a form of learning that is termed conditioned taste aversion (CTA). A consequence of the learned association is that the taste will become aversive. When experiencing the taste again, individuals will show aversive reactions such as expressions of loathing, will experience mimicked illness sensations such as nausea, and subsequently, will avoid further exposure to the taste. The ability to acquire CTA occurs across species and across ages within a species. In the rat animal model, however, age differences exist in the capability of acquiring CTAs when increasingly longer intervals are imposed between consumption of a novel sweet solution and onset of illness. Pups have a decreased ability compared to young adults while aged rats have an increased ability. Evidence suggests that the failure of pups to acquire CTA at longer intervals is due to an immature retrieval mechanism and the facilitated ability of aged rats is due to a compromised clock mechanism that tracks the passage of time. Learned taste-illness association serves the critical function of informing individuals of the toxic nature of certain foods, thus preventing further illness and potentially death. Additionally, it contributes to the hypophagia observed during cancer chemotherapy and may contribute to the hypophagia found while suffering from bacterial infection, chronic medical conditions such as cancer, and restrictive food intake disorders such as anorexia nervosa.

Age and interstimulus interval in forward and backward long-trace taste-aversion conditioning

Rats, 70-79 days old and 477-557 days old, experienced either a forward or backward taste-aversion conditioning trial with a 15-min. or 45-min. interstimulus interval. Forward conditioning was evident in both age groups at both interstimulus intervals. Backward conditioning was evident in both age groups only at the 15-min. interstimulus interval. The failure to obtain backward conditioning at the 45-min. interval was attributed to the effect of stress, produced by injection of the US, on long-trace conditioning. As reported with forward conditioning, stress may alter metabolic rate such that the intervals at which associations can be formed are shortened.

Amphetamine-induced taste aversion learning in young and old F-344 rats following exposure to 56Fe particles

Exposure to (56)Fe particles produces changes in dopaminergic function and in dopamine-dependent behaviors, including amphetamine-induced conditioned taste aversion (CTA) learning. Because many of these changes are characteristic of the changes that accompany the aging process, the present study was designed to determine whether or not there would be an interaction between age and exposure to (56)Fe particles in the disruption of an amphetamine-induced CTA. One hundred and forty F-344 male rats 2-, 7-, 12-, and 16-months old, were radiated with (56)Fe particles (0.25-2.00 Gy, 1 GeV/n) at Brookhaven National Laboratory. Three days following irradiation, the rats were tested for the effects of radiation on the acquisition of a CTA produced by injection of amphetamine (3 mg/kg, i.p.). The main effect of age was to produce a significant decrease in conditioning day sucrose intake; there was no affect of age on the acquisition of the amphetamine-induced CTA. Exposing rats to (56)Fe particles disrupted the acquisition of the CTA produced by injection of amphetamine only in the 2-month-old rats. These results do not support the hypothesis of an interaction between age and exposure to (56)Fe particles in producing a disruption of amphetamine-induced CTA learning. As such, these results suggest that the aging produced by exposure to (56)Fe particles may be endpoint specific.

A time contraction effect of acute tail-pinch stress on the associative learning of rats

The effect of tail-pinch stress interpolated between the saccharin conditioned stimulus (CS) and the illness-inducing unconditioned stimulus (US) during long-trace taste-aversion conditioning was examined in young- and old adult rats with a two-cylinder (saccharin versus water) test. A 2 x 2 x 4 factorial ANOVA was performed on percent-preference-for-saccharin data, with age (young, old), stress condition (stressed, non-stressed), and CS-US interval (22.5-, 45-, 90-, and 180-min) being the factors under consideration. The ANOVA yielded only significant main effects of stress condition and CS-US interval. These findings indicate that stress weakens the CS-US association as evidenced by a higher percent preference for saccharin in the stressed rats than in non-stressed rats at all CS-US intervals. A comparison of the stressed and non-stressed conditioned rats with pseudo-conditioned controls showed that the non-stressed rats formed strong aversions up to the 45-min CS-US interval whereas the stressed rats showed no conditioning beyond the 22.5 min CS-US interval, indicating that stress decreases the effective CS-US interval. Results were interpreted in terms of time-contraction and an internal biological countdown timer hypothesized to govern processes involved in associative learning over long delays.

Low body temperature, time dilation, and long-trace conditioned flavor aversion in rats

Conditioned flavor aversion was examined in Wistar-derived albino rats that were immersed in cold water for 0, 2.5, 5, or 10 min immediately following 10-min exposure to a.1% saccharin solution and given an intraperitoneal (i.p.) injection of 0.15 M lithium chloride (LiCl) either 90, 135, 180, or 225 min later. Cold water immersion for 2.5, 5, and 10 min led to body temperature decreases of approximately 4.5, 7, and 10 degrees C, respectively. Rats whose body temperatures were not reduced (0 min immersion) showed no saccharin aversion when the LiCl was delayed 90 min. Rats whose body temperatures were reduced 4.5, 7, and 10 degrees C displayed conditioned aversions at LiCl delays up to 135, 180, and 225 min, respectively. These results were interpreted in terms of a cold-induced slowing of a biochemical clock that may uniquely govern specific timing processes involved in associative learning over long delays, such as long-trace conditioned flavor aversion, learned safety, and certain types of learning that involve an extensive time lapse (e.g., extinction of fear).

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 interaction of age and unconditioned stimulus intensity on long-trace conditioned flavor aversion in rats

To see if the neural representation of the conditioned stimulus (CS) is available to old-age rats beyond the time it is available to young adults, the intensity of the unconditioned stimulus (US) and the length of the CS-US interval were systematically varied in a trace conditioning experiment. Results indicated that increasing US intensity extends the interval over which trace conditioning is evident in old-age rats but not in young adults, suggesting that trace decay occurs more rapidly in young rats. Results were interpreted in terms of age differences in the workings of hypothesized biochemical timing mechanisms that may directly influence the ability to associate stimuli over trace intervals in conditioned taste-aversion procedures.

Conditioned blocking is re-established by neurotransplantation in mature rats

Aging has been associated with a decay of hippocampal function that may begin well before senescence. Conditioned blocking is a complex learning phenomenon that requires an intact hippocampus in young-adult rats and is absent in middle-aged rats. The aim of the present study was to test the possibility of re-establishing conditioned blocking in 17-month-old Wistar rats by neurotransplantation. Solid embryonic hippocampal or nigral tissue was bilaterally transplanted in the proximity of the dorsal hippocampus (lateral ventricle and alveus). Conditioned blocking of an aversion to a cider vinegar (3%) solution presented in compound with a previously conditioned saccharin solution (0.1%) appeared 14 days after transplantation and persisted 3 months later only in the hippocampal grafted group, showing the possibility of restoring age-related cognitive deficits.

Multiple remote-US preexposures and the blocking effect produced by a proximal-US

Weanling, young-adult, and old-age Wistar albino rats were used to determine whether number of unconditioned stimulus (US) presentations, given 24 h or more (remote preexposure) prior to conditioning, alters the blocking effect of a single-US preexposure given 2 h before (proximal) taste aversion conditioning. As the number of remote-US preexposures increased from 0 to 6, the ability of the proximal-US preexposure to block conditioning initially increased then decreased for all age groups. Of the models put forth to explain US preexposure effects on conditioned taste aversion (CTA), only Wagner's information processing model adequately explained the reduction of the blocking effect of the proximal-US preexposure produced as a result of increasing remote-US preexposures.

Food aversion learning: a risk factor for nutritional problems in the elderly?

Food aversions that are acquired as a result of unpleasant experiences with foods represent a potent defense mechanism against poisoning. However, this powerful and durable form of conditioning can also contribute to avoidance of foods that are not poisonous, and are, in fact, quite nutritious. This is because such foods may be coincidentally associated with unpleasant gastrointestinal symptoms, sometimes due to transient, unrelated illness, or unpleasant drug side effects. Most of the studies of naturally occurring learned food aversions in humans have been focused on subjects of college age, so we have limited information about the extent to which such food aversions occur in the elderly. Additionally, most studies have employed questionnaire or interview methods that may have some significant limitations in the accurate assessment of the incidence of food aversions. Thus, although food aversion learning has been thoroughly documented in the animal laboratory, its role in everyday food selection in humans, including the elderly, remains relatively unclear.

Low body temperature affects associative processes in long-trace conditioned flavor aversion

A series of experiments examined the effect of low body temperature on the associative process in long-trace conditioned flavor aversion. Experiment 1 demonstrated that maintaining a low body temperature between conditioned stimulus (CS) and unconditioned stimulus (US) administration facilitates the associative process and allows a flavor aversion to be conditioned in young rats over an interval that would normally not support conditioning. Experiments 2 and 3 demonstrated that this was due neither to lingering systemic saccharin serving as a CS nor to a cold induced enhancement of US intensity. Experiment 4 demonstrated that inducing hypothermia at various times during a 3-h CS-US interval results in an apparent delay of reinforcement gradient. We propose that a cold induced decrease in metabolic rate slows the internal clock that governs the perception of time and that the CS-US association depends upon perceived contiguity rather than upon an external clock-referenced contiguity.

Remote and proximal US preexposure and aging effects in taste aversion learning in rats

Age as a factor in the effect of proximal and remote unconditioned stimulus (US) preexposure on conditioned taste aversion in weanling, young adult, and old rats was studied in 2 experiments. In Experiment 1, 6 daily US preconditioning exposures attenuated conditioning in weanlings and young adults, but not in old rats. In Experiment 2, exposure to a single US 1 h before the conditioning trial curtailed conditioning at all age levels. These results are explained in terms of age differences in familiarity with the conditioning context and Wagner's information-processing model for self- and retrieval-generated disruption of conditioning.

Age differences and the interstimulus interval context in long-delay taste-aversion conditioning of rats

Young-adult and old-adult rats were allowed to remain in the conditioning context or were returned to their home cages during a 3-hr. interval to assess whether previously observed age differences in long-delay taste-aversion conditioning may be due to age differences in the use of home-cage cues to mediate the CS-US association over a long delay. The old adults but not the young adults showed an aversion irrespective of the context in which they were detained during the interstimulus interval. These results suggest that young-adult rats do not use the interstimulus context cues to mediate the association over a delay interval. They suggest, rather, that context cues, which are more contiguous with the US than taste cues in long-delay conditioning, may be more effective in overshadowing taste cues in young adults than in old adults.

Lack of age differences in context-illness associations in the long-delay taste-aversion conditioning of rats

To evaluate whether previously observed age differences in long-delay taste aversion were due to age-related differences in the shared association of contextual cues and CS with the US, weanling, young-adult, and old-adult rats were given a NaCl or LiCl US immediately after or a LiCl US 3 hr. after a saccharin CS presentation in a black or white context. They were then given a context-preference test in a chamber which was half black and half white. Analysis showed rats, irrespective of age or conditioning context, spent a significantly smaller percentage of time on the white side than on the black side of the test chamber. These results suggest that age differences in long-delay taste-aversion conditioning are not due to age-related differences in the shared association of contextual cues and CS with the US.

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.

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.