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Remote and Proximal US Preexposure and Aging Effects in Taste Aversion Learning in Rats
Physiology & Behavior, Vol. 61, No. 2, pp. 221–224, 1997 Copyright q 1997 Elsevier Science Inc. Printed in the USA. All rights reserved 0031-9384/97 $17.00 / .00
PII S0031-9384(96)00371-X
Remote and Proximal US Preexposure and Aging Effects in Taste Aversion Learning in Rats JAMES R. MISANIN,* 1 THOMAS D. HOEFEL,* CHRISTINE A. RIEDY* AND CHARLES F. HINDERLITER† *Department of Psychology, Susquehanna University, Selinsgrove, PA 17870-1001 USA; and †University of Pittsburgh at Johnstown, Johnstown, PA 15904 USA Received 20 December 1995; Accepted 26 July 1996 MISANIN, J. R., T. D. HOEFEL, C. A. RIEDY AND C. F. HINDERLITER. Remote and proximal US preexposure and aging effects in taste aversion learning in rats. PHYSIOL BEHAV 61(2) 221–224, 1997.—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. Copyright q 1997 Elsevier Science Inc. Conditioned taste aversion
US preexposure effect
Aging
TASTE aversion in rats can be conditioned in a single pairing of a novel taste with an illness inducing unconditioned stimulus (US). Preconditioning experience with the US, however, either attenuates or eliminates conditioned taste aversion (CTA). It has been suggested (2) that this US-preexposure effect actually consists of two distinct effects: A proximal US-preexposure effect that is a transient effect caused by a single preconditioning exposure to the US occurring within 24 h of the conditioning trial, and a remote US-preexposure effect that is a relatively durable effect brought about by multiple US preexposures experienced more than 24 h before the conditioning trial. In a related study (12), weanling, young adult, and old rats were given 5 preconditioning injections of a 0.15 M LiCl US. It was found that the US preexposure effect was a decreasing function of age. The US preexposure began 2 days before and ended 1.5 h before the conditioning trial; thus, rats experienced both remote and proximal US preexposures. These results were interpreted in terms of age differences in familiarity with the conditioning environment, in conjunction with an information-processing mechanism (13,14) for a retrieval-generated depression in performance. Alternatively, it has been suggested that, although the behavioral effects of remote and proximal US preexposure on CTA may be the same, they may depend upon different underlying mechanisms (2). If remote and proximal US preexposure effects do depend upon different mechanisms, it is possible that the reported age differences in the US preexposure effect (12) are due to age differences in remote-US preexposure, proximal-US preexposure, or some interaction of the two. Because our previous research examining age differences in the US
Learning
preexposure effect involved combined effects of remote and proximal US preexposures (12), the purpose of the present study was to determine the independent effects of each US preexposure procedure on CTA in weanling, young adult, and old rats. EXPERIMENT 1 The effect of remote US preexposures was examined in this experiment. METHOD
Subjects Thirty-two weanling (20–25 days), 32 young adult (90–105 days), and 32 aged (635–725 days) female Wistar albino rats were used. The rats were born and reared in the university animal colony. They were individually housed in standard suspended rodent cages and maintained on a 14-h light/10-h dark cycle. Drinking adaptation, conditioning, US preexposure, and testing occurred during the light phase of the cycle. The rats had ad lib access to food at all times, except during drinking sessions that were 1 h or less. Apparatus The drinking adaptation, conditioning, and testing sessions took place in standard suspended rodent cages (similar to home cages) that had three spring-grip clamps attached to the center and 35 mm in from each side of the front of the cage to hold 100 ml graduated (to 1 ml) cylinders. Cylinders were equipped with
1
To whom requests for reprints should be addressed. E-mail: [email protected]
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rubber stoppers and stainless-steel sipper tubes. During all sessions, except the test session when two cylinders were used, the cylinder was attached with the center clamp. Procedure All rats were given 6 daily 1% body weight IP injections of either 0.15 M LiCl (preexposed) or 0.9% NaCl (nonpreexposed). During the first 3 days of the preconditioning injections, they received ad lib food and water. During the last 3 days of the preconditioning injections, the rats were 23-h water-deprived and the injection was administered immediately after a 1-h drinking adaptation session, during which the rats had access to room temperature tap water in the drinking cages. All preconditioning injections were administered at about the same time of day. Prior to any treatment, 8 rats at each of the 3 age levels had been randomly assigned to 4 groups differentiated on the basis of preexposure and conditioning treatments. The design used was a 3 (age) by 2 (preexposure) by 2 (conditioning treatment) factorial. More specifically, 2 of the groups at each age level [Groups L-L and L-N, where the first letter designates the preexposure injection(s), lithium chloride (L)iCl or sodium chloride (N)aCl, and the second letter designates the conditioning (L) or control (N) injection] were given remote US preexposures. The 2 additional groups (Groups N-L and N-N) were not preexposed to the US. One of the preexposed (Group L-L) and one of the nonpreexposed (Group N-L) groups at each age level underwent conditioning. The remaining groups (Groups L-N and N-N) received the control treatment. The conditioning and control treatments took place 23 h after the last drinking adaptation session. Conditioning consisted of 10-min access to a 0.1% (w/v in tap water) saccharin solution followed by 20-min access to room temperature tap water. Immediately after that, a 1% body weight IP injection of 0.15 M LiCl was administered. For the control treatment, a 1% body weight IP injection of 0.9% NaCl was given instead of LiCl immediately after the 20-min access to water that followed the saccharin. The following day, all rats underwent a 6-h 2-cylinder (saccharin v. water) test with the side on which saccharin was presented counterbalanced for each group. The amount of saccharin and water consumed was recorded.
FIG. 1. Percent preference for saccharin as a function of age, and preexposure and conditioning treatments. The first letter in the Fig. legends (e.g., L-L) designates the remote preexposure treatment, LiCl (L) or NaCl (N); and the second letter in the legends designates the conditioning (L) or control (N) treatment.
preexposed (Group L-N) and nonpreexposed (Group N-N) controls, Fs(1,60) õ 0.75. The N-L groups, however, differed significantly from their respective N-N controls, Fs(1,60) ú 5.87. Thus, in contrast to previously reported results (12), in which a combination of remote and proximal US preexposure attenuated conditioning at all age levels, remote preexposure alone had no significant effect on taste aversion conditioning in old rats and eliminated any evidence of conditioning in the weanling and young adult rats. EXPERIMENT 2 Because the results of Experiment 1 suggested that age interacts with remote preexposure, the purpose of Experiment 2 was to examine whether or not proximal preexposure also interacts with age.
RESULTS
The amount drunk was converted to a percent-preference score by dividing the amount of saccharin consumed by the total fluid (saccharin plus water) consumption and multiplying by 100. A 3 (age) 1 2 (preexposure) 1 2 (conditioning treatment) ANOVA was performed on the test data. Significance levels were set at p õ 0.05 for all comparisons. The analysis of the data reported in Fig. 1 yielded significant main effects of age, F(2,60) Å 8.40, preexposure, F (1,60) Å 10.45, and treatment, F (1,60) Å 26.46, and a significant Preexposure 1 Treatment interaction, F (1,60) Å 6.23. Although the overall analysis did not reveal any age interactions, group comparisons involving age were completed as suggested by Winer (15) to evaluate the major theoretical issue examined in this experiment (pages 340–342; 384). Group comparisons indicated that, at the old age level there was no significant difference in the preexposed and nonpreexposed groups that underwent conditioning (Groups L-L and N-L, respectively), F (1,60) Å 1.87, and that both groups differed significantly from their respective controls (Groups L-N and N-N), Fs(1,60) ú 4.35. In contrast, at both young adult and weanling levels, the preexposed group that underwent conditioning (Group L-L) differed significantly from the nonpreexposed group that underwent conditioning (Group N-L), Fs(1,60) ú 6.78, and failed to differ from the
METHOD
Subjects and Apparatus Subjects and apparatus for this experiment were identical to that described in Experiment 1. Procedure All rats were given 6 daily 1% body weight IP injections of 0.9% NaCl comparable to the controls’ treatment in Experiment 1. As in Experiment 1, during the first 3 days of preconditioning injections, subjects received ad lib food and water. During the last 3 days of the preconditioning injections, the rats were 23-hr water-deprived and the injection was administered immediately after a 1-h drinking adaptation session, during which the rats had access to room temperature tap water in the drinking cages. The proximal US preexposed groups received their preconditioning US 1 h before the conditioning or control treatments. Nonpreexposed rats received a NaCl injection at this time. As in Experiment 1, four groups (L-L, L-N, N-L, and N-N, where the first letter indicated preconditioning injection and the second letter indicated conditioning injection) were used at each of the 3 age levels. Thus, a 3 (age) 1 2 (preexposure) 1 2 (conditioning treatment) factorial was the design of the experiment.
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Test Data Experiment 2 The analysis of the test data reported in Fig. 2 yielded significant main effects of preexposure and treatment, Fs(1,60) ú 41.23, and a significant interaction of these variables, F (1,60) Å 30.94. Individual group comparisons made at the 0.05 level of significance indicated that, at all age levels, Group L-L differed significantly from Group N-L, Fs(1,60) ú 16.49 and failed to differ from its respective control (Group L-N), Fs(1,60) õ 1.37. At all age levels, the conditioning treatment without LiCl preexposure (N-L) significantly reduced the preference for saccharin compared to the control treatment (N-N), Fs(1,60) ú 11.79. It should also be noted that there were no significant differences among age groups that were preexposed to LiCl prior to conditioning, Fs(1,60) õ 0.03. Thus, unlike the combined effects of proximal and remote US preexposures, where their attenuating effects on CTA decreased with age (12), proximal preexposure alone eliminated any evidence of conditioning at all age levels. Conditioning Data Experiments 1 and 2 Because strength of conditioning is influenced by the magnitude of the CS, ml of solution drunk on conditioning day was analyzed using a 2 (saccharin, water) 1 3 (weanling, young adult, old) 1 2 (remote preexposure - Experiment 2, proximal preexposure -Experiment 1) ANOVA. The only significant effect obtained was that rats at all ages consumed more saccharin (mean Å 3.7 ml) than water (mean Å 1.1 ml), F (1,30) Å 40.47, p õ 0.01. Age, preexposure treatment, and their interaction did not appear to influence the amount drunk on training day, 0.73 ° Fs ° 2.5. Results indicate that subjects of all ages and treatments appeared to have equal exposure to the CS during conditioning as assessed by amount drunk. DISCUSSION The present results add to a growing body of research that indicates that procedures that weaken conditioning bring out age differences in the outcome of taste-aversion conditioning in rats. For example, manipulating the intensity of the US (6) or the CS (5) can bring out age difference that might otherwise be obscured. Lengthening the CS-US also weakens conditioning (4) and leads to age differences in CTA in rats (5,7). Also, like combined proximal and remote US preexposure (12), and like remote-US preexposure in the present experiment, preexposing rats to the CS prior to the conditioning session reduces the strength of CTA and results in age differences in the effectiveness of the CS-US pairing (6,8). A significant factor that may help explain many of the reported age differences in CTA and preexposure effects is familiarity with the conditioning context (e.g., 3,6). In many CTA experiments, as in the present experiments, the conditioning context is very similar to, or is, the rat’s home cage (see, e.g., 1). The older the animals are, the more experience they have had with this type of environment. Although these age differences in context familiarity can be used to explain the effects of remote-US preexposure observed in the present research in a number of ways, for example, in terms of relative CS novelty (3,9,11) or affective value of the conditioning context (10), we prefer an explanation in terms of Wagner’s information-processing model (13,14) for a number of reasons. First, this model explains the currently reported age differences in the effect of remote-US preexposure on CTA in rats as a result of age-related difference in familiarity with the conditioning context (e.g., 14). Wagner proposed that the formation
FIG. 2. Percent preference for saccharin as a function of age, and preexposure and conditioning treatments. The first letter in the figure legends (e.g., L-L) designates the proximal preexposure treatment, LiCl (L) or NaCl (N); and the second letter in the legends designates the conditioning (L) or control (N) treatment.
of an association between CS and US depends upon active representation of the CS and US in short-term memory. If the presentation of the CS and US at the time of conditioning is responsible for this representation, an association between the CS and US will be made. If, however, either stimulus has been prerepresented in memory because of the retrieval action of cues with which one or the other of the stimuli have been associated, a CSUS association is less likely to be formed. When the US is preexposed in the conditioning context, it is assumed that an association between the US and context is formed. The contextual cues, because of their retrieval action, prerepresent the US in shortterm memory prior to the conditioning trial and thereby prevent or weaken the CS-US association. Thus, the greater familiarity with the conditioning context could weaken the US-preexposure effect for our old subjects. It could, for example, result in the contextual cues becoming latent inhibitors that would interfere with the context-US association. A weak or nonexistent contextUS association would result in little retrieval action on the part of the contextual cues during the conditioning trial and, thereby, would lead to stronger conditioning, as with our old animals in Experiment 1. A second reason we prefer Wagner’s model over other explanations is that it can also explain the proximal-US preexposure results obtained in the present study. Unlike the effect of remoteUS preexposure, the model does not consider the effect of proximal-US preexposure to depend upon an associative mechanism. The model assumes that, not only can the CS or US be prerepresented in short-term memory by the retrieval action of cues with which they have been previously associated, but also by the stimulus (CS or US), itself. Thus, when, as in Experiment 2, a US is given shortly before the conditioning trial, it is already represented in short-term memory at the time of conditioning, thereby reducing the effectiveness of the CS-US pairing. Because the effect of proximal-US preexposure is nonassociative in nature, differential familiarity with the conditioning context should exert little influence. In the present study, there were no differential age-related effects of proximal-US preexposure as there were with remote-US preexposure. A single proximal-US preexposure eliminated any evidence of conditioning at all age levels. The third reason we prefer Wagner’s model to others is that it can also explain the apparent interaction between proximal and remote preexposure. The present results, together with previously
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reported results (12), suggest that remote-US preexposure interacts with proximal-US preexposure in a way that reduces the overall effectiveness of the proximally preexposed US. In the present study, there was no evidence of CTA in weanling and young adult rats that were either remotely or proximally exposed to the US. In contrast, combined effects of remote and proximal US preexposure have been shown only to attenuate CTA at these age levels (12). Thus, it would appear that remote-US preexposure curtails the effect of proximal-US preexposure or vice versa. In the present study, at the old age level, a single proximally preexposed US eliminated all evidence of CTA, whereas 6 daily remote US preexposures had no significant effect on conditioning. The combined effect of remote and proximal US preexposure at the old age level (12) was an attenuation of CTA, that is, some intermediate effect between the effect of proximal-US preexposure and the effect of remote-US preexposure in the present study. Apparently, then, remote-US preexposure diminishes the effect of proximal-US preexposure, not vice versa. Wagner’s information-processing model suggests that remote US-preexposure would diminish the proximal-US preexposure effect in the following way. As the number of remote-US preexposures increases, contextual stimuli should limit rehearsal of themselves in association with the preexposed US. Thus, after repeated remote-US preexposures, if the US is proximally preex-
posed in the same context, its rehearsal should also be curtailed. Curtailing rehearsal of the proximally preexposed US should, in turn, diminish its effect on conditioning. Although interpretation of the present results has relied upon age differences in familiarity with the conditioning context, they can also be interpreted in terms of age similarities and differences in retention processes. Accordingly, the results of Experiment 2 can be explained on the basis of similar rehearsal capability of weanling, young adult, and old rats. In contrast, the retrieval capability of old rats, as indicated in the results of Experiment 1, would be seen as deficient in comparison with that of weanlings and young adults. Although tenable, such an explanation of agerelated preexposure effects relies more on hypothesizing differences in internal processes rather than on differences in past experiences, such as exposure to the conditioning context. ACKNOWLEDGEMENTS
This research was supported by Grant HD 21161 from the National Institute of Child Health and Human Development. This research was approved by an institutional animal care and use committee and, in all phases and aspects of the research, the treatment of the participants was in accordance with the ethical standards of the American Psychological Association and the Public Health Service Guide for the Care and Use of Laboratory Animals.
REFERENCES 1. Batsell, W. R.; Best; M. R. The role of US novelty in retention interval effects in single-element taste-aversion learning. Anim. Learn. Behav. 22:332–340; 1994. 2. Domjan, M.; Best, M. R. Paradoxical effects of proximal unconditioned stimulus preexposure: Interference with and conditioning of a taste aversion. J. Exp. Psychol. Anim. Behav. 3:310–321; 1977. 3. Hinderliter, C. F.; Misanin, J. R. Context familiarity and delayed conditioned taste aversion in young-adult and old-age rats. Percept. Mot. Skills 77:1403–1406; 1993. 4. Kalat, J. W.; Rozin, P. ‘‘Learned Safety’’ as a mechanism in longdelay taste-aversion learning in rats. J. Comp. Physiol. Psychol. 83:149–155; 1973. 5. Martin, G. N.; Timmins, W. K. Taste-sickness associations in young rats over varying delays, stimulus, and test conditions. Anim. Learn. Behav. 8:529–533; 1980. 6. Misanin, J. R.; Blatt, L. A.; Hinderliter, C. F. Age dependency in neophobia: Its influence on taste-aversion learning and the flavorpreexposure effect in rats. Anim. Learn. Behav. 13:69–76; 1985. 7. Misanin, J. R.; Grieder, D. L.; Hinderliter, C. F. Age differences in the outcome of long-delay taste-aversion conditioning in rats. Bull. Psychon. Soc. 26:258–260; 1988. 8. Misanin, J. R.; Guanowsky, V.; Riccio, D. C. The effect of CSpreexposure on conditioned taste aversion in young and adult rats. Physiol. Behav. 30:859–862; 1983.
9. Mitchell, D.; Winter, W.; Moffit, T. Cross-modality contrast: exteroceptive context habituation enhances taste neophobia and conditioned taste aversions. Anim. Learn. Behav. 8:524–528; 1980. 10. Pumo, J.; Spear, N. E. The affective value of the context determines retention of Pavlovian conditioning in the 7-day old rat. Paper presented at the meeting of the International Society for Developmental Psychobiology, Annapolis, MD, November 1986. 11. Richardson, R.; Siegel, M. A.; Campbell, B. A. Unfamiliar environments impair information processing as measured by behavioral and cardiac orienting responses to auditory stimuli in preweanling and adult rats. Dev. Psychobiol. 21:491–503; 1988. 12. Valliere, W. A.; Misanin, J. R.; Hinderliter, C. F. Age-related differences in the US preexposure effect on conditioned taste aversion in rats. Bull. Psychonom. Soc. 26:64–66; 1988. 13. Wagner, A. R. Priming in STM: An information processing mechanism for self-generated or retrieval-generated depression in performance. In: Tighe, T. J.; Leaton, R. N., eds. Habituation: Perspectives from child development, animal behavior, and neurophysiology. Hillsdale, NJ: Erlbaum; 1976. 14. Wagner, A. R. Expectancies and the priming of STM. In: Hulse, S. H.; Fowler, H.; Honig, W. K., eds. Cognitive processes in animal behavior. New York: Wiley; 1978. 15. Winer, B. J. Statistical principles in experimental design. 2 ed. New York: McGraw-Hill; 1971.
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Remote and Proximal US Preexposure and Aging Effects in Taste Aversion Learning in Rats JAMES R. MISANIN,* 1 THOMAS D. HOEFEL,* CHRISTINE A. RIEDY* AND CHARLES F. HINDERLITER† *Department of Psychology, Susquehanna University, Selinsgrove, PA 17870-1001 USA; and †University of Pittsburgh at Johnstown, Johnstown, PA 15904 USA Received 20 December 1995; Accepted 26 July 1996 MISANIN, J. R., T. D. HOEFEL, C. A. RIEDY AND C. F. HINDERLITER. Remote and proximal US preexposure and aging effects in taste aversion learning in rats. PHYSIOL BEHAV 61(2) 221–224, 1997.—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. Copyright q 1997 Elsevier Science Inc. Conditioned taste aversion
US preexposure effect
Aging
TASTE aversion in rats can be conditioned in a single pairing of a novel taste with an illness inducing unconditioned stimulus (US). Preconditioning experience with the US, however, either attenuates or eliminates conditioned taste aversion (CTA). It has been suggested (2) that this US-preexposure effect actually consists of two distinct effects: A proximal US-preexposure effect that is a transient effect caused by a single preconditioning exposure to the US occurring within 24 h of the conditioning trial, and a remote US-preexposure effect that is a relatively durable effect brought about by multiple US preexposures experienced more than 24 h before the conditioning trial. In a related study (12), weanling, young adult, and old rats were given 5 preconditioning injections of a 0.15 M LiCl US. It was found that the US preexposure effect was a decreasing function of age. The US preexposure began 2 days before and ended 1.5 h before the conditioning trial; thus, rats experienced both remote and proximal US preexposures. These results were interpreted in terms of age differences in familiarity with the conditioning environment, in conjunction with an information-processing mechanism (13,14) for a retrieval-generated depression in performance. Alternatively, it has been suggested that, although the behavioral effects of remote and proximal US preexposure on CTA may be the same, they may depend upon different underlying mechanisms (2). If remote and proximal US preexposure effects do depend upon different mechanisms, it is possible that the reported age differences in the US preexposure effect (12) are due to age differences in remote-US preexposure, proximal-US preexposure, or some interaction of the two. Because our previous research examining age differences in the US
Learning
preexposure effect involved combined effects of remote and proximal US preexposures (12), the purpose of the present study was to determine the independent effects of each US preexposure procedure on CTA in weanling, young adult, and old rats. EXPERIMENT 1 The effect of remote US preexposures was examined in this experiment. METHOD
Subjects Thirty-two weanling (20–25 days), 32 young adult (90–105 days), and 32 aged (635–725 days) female Wistar albino rats were used. The rats were born and reared in the university animal colony. They were individually housed in standard suspended rodent cages and maintained on a 14-h light/10-h dark cycle. Drinking adaptation, conditioning, US preexposure, and testing occurred during the light phase of the cycle. The rats had ad lib access to food at all times, except during drinking sessions that were 1 h or less. Apparatus The drinking adaptation, conditioning, and testing sessions took place in standard suspended rodent cages (similar to home cages) that had three spring-grip clamps attached to the center and 35 mm in from each side of the front of the cage to hold 100 ml graduated (to 1 ml) cylinders. Cylinders were equipped with
1
To whom requests for reprints should be addressed. E-mail: [email protected]
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rubber stoppers and stainless-steel sipper tubes. During all sessions, except the test session when two cylinders were used, the cylinder was attached with the center clamp. Procedure All rats were given 6 daily 1% body weight IP injections of either 0.15 M LiCl (preexposed) or 0.9% NaCl (nonpreexposed). During the first 3 days of the preconditioning injections, they received ad lib food and water. During the last 3 days of the preconditioning injections, the rats were 23-h water-deprived and the injection was administered immediately after a 1-h drinking adaptation session, during which the rats had access to room temperature tap water in the drinking cages. All preconditioning injections were administered at about the same time of day. Prior to any treatment, 8 rats at each of the 3 age levels had been randomly assigned to 4 groups differentiated on the basis of preexposure and conditioning treatments. The design used was a 3 (age) by 2 (preexposure) by 2 (conditioning treatment) factorial. More specifically, 2 of the groups at each age level [Groups L-L and L-N, where the first letter designates the preexposure injection(s), lithium chloride (L)iCl or sodium chloride (N)aCl, and the second letter designates the conditioning (L) or control (N) injection] were given remote US preexposures. The 2 additional groups (Groups N-L and N-N) were not preexposed to the US. One of the preexposed (Group L-L) and one of the nonpreexposed (Group N-L) groups at each age level underwent conditioning. The remaining groups (Groups L-N and N-N) received the control treatment. The conditioning and control treatments took place 23 h after the last drinking adaptation session. Conditioning consisted of 10-min access to a 0.1% (w/v in tap water) saccharin solution followed by 20-min access to room temperature tap water. Immediately after that, a 1% body weight IP injection of 0.15 M LiCl was administered. For the control treatment, a 1% body weight IP injection of 0.9% NaCl was given instead of LiCl immediately after the 20-min access to water that followed the saccharin. The following day, all rats underwent a 6-h 2-cylinder (saccharin v. water) test with the side on which saccharin was presented counterbalanced for each group. The amount of saccharin and water consumed was recorded.
FIG. 1. Percent preference for saccharin as a function of age, and preexposure and conditioning treatments. The first letter in the Fig. legends (e.g., L-L) designates the remote preexposure treatment, LiCl (L) or NaCl (N); and the second letter in the legends designates the conditioning (L) or control (N) treatment.
preexposed (Group L-N) and nonpreexposed (Group N-N) controls, Fs(1,60) õ 0.75. The N-L groups, however, differed significantly from their respective N-N controls, Fs(1,60) ú 5.87. Thus, in contrast to previously reported results (12), in which a combination of remote and proximal US preexposure attenuated conditioning at all age levels, remote preexposure alone had no significant effect on taste aversion conditioning in old rats and eliminated any evidence of conditioning in the weanling and young adult rats. EXPERIMENT 2 Because the results of Experiment 1 suggested that age interacts with remote preexposure, the purpose of Experiment 2 was to examine whether or not proximal preexposure also interacts with age.
RESULTS
The amount drunk was converted to a percent-preference score by dividing the amount of saccharin consumed by the total fluid (saccharin plus water) consumption and multiplying by 100. A 3 (age) 1 2 (preexposure) 1 2 (conditioning treatment) ANOVA was performed on the test data. Significance levels were set at p õ 0.05 for all comparisons. The analysis of the data reported in Fig. 1 yielded significant main effects of age, F(2,60) Å 8.40, preexposure, F (1,60) Å 10.45, and treatment, F (1,60) Å 26.46, and a significant Preexposure 1 Treatment interaction, F (1,60) Å 6.23. Although the overall analysis did not reveal any age interactions, group comparisons involving age were completed as suggested by Winer (15) to evaluate the major theoretical issue examined in this experiment (pages 340–342; 384). Group comparisons indicated that, at the old age level there was no significant difference in the preexposed and nonpreexposed groups that underwent conditioning (Groups L-L and N-L, respectively), F (1,60) Å 1.87, and that both groups differed significantly from their respective controls (Groups L-N and N-N), Fs(1,60) ú 4.35. In contrast, at both young adult and weanling levels, the preexposed group that underwent conditioning (Group L-L) differed significantly from the nonpreexposed group that underwent conditioning (Group N-L), Fs(1,60) ú 6.78, and failed to differ from the
METHOD
Subjects and Apparatus Subjects and apparatus for this experiment were identical to that described in Experiment 1. Procedure All rats were given 6 daily 1% body weight IP injections of 0.9% NaCl comparable to the controls’ treatment in Experiment 1. As in Experiment 1, during the first 3 days of preconditioning injections, subjects received ad lib food and water. During the last 3 days of the preconditioning injections, the rats were 23-hr water-deprived and the injection was administered immediately after a 1-h drinking adaptation session, during which the rats had access to room temperature tap water in the drinking cages. The proximal US preexposed groups received their preconditioning US 1 h before the conditioning or control treatments. Nonpreexposed rats received a NaCl injection at this time. As in Experiment 1, four groups (L-L, L-N, N-L, and N-N, where the first letter indicated preconditioning injection and the second letter indicated conditioning injection) were used at each of the 3 age levels. Thus, a 3 (age) 1 2 (preexposure) 1 2 (conditioning treatment) factorial was the design of the experiment.
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Test Data Experiment 2 The analysis of the test data reported in Fig. 2 yielded significant main effects of preexposure and treatment, Fs(1,60) ú 41.23, and a significant interaction of these variables, F (1,60) Å 30.94. Individual group comparisons made at the 0.05 level of significance indicated that, at all age levels, Group L-L differed significantly from Group N-L, Fs(1,60) ú 16.49 and failed to differ from its respective control (Group L-N), Fs(1,60) õ 1.37. At all age levels, the conditioning treatment without LiCl preexposure (N-L) significantly reduced the preference for saccharin compared to the control treatment (N-N), Fs(1,60) ú 11.79. It should also be noted that there were no significant differences among age groups that were preexposed to LiCl prior to conditioning, Fs(1,60) õ 0.03. Thus, unlike the combined effects of proximal and remote US preexposures, where their attenuating effects on CTA decreased with age (12), proximal preexposure alone eliminated any evidence of conditioning at all age levels. Conditioning Data Experiments 1 and 2 Because strength of conditioning is influenced by the magnitude of the CS, ml of solution drunk on conditioning day was analyzed using a 2 (saccharin, water) 1 3 (weanling, young adult, old) 1 2 (remote preexposure - Experiment 2, proximal preexposure -Experiment 1) ANOVA. The only significant effect obtained was that rats at all ages consumed more saccharin (mean Å 3.7 ml) than water (mean Å 1.1 ml), F (1,30) Å 40.47, p õ 0.01. Age, preexposure treatment, and their interaction did not appear to influence the amount drunk on training day, 0.73 ° Fs ° 2.5. Results indicate that subjects of all ages and treatments appeared to have equal exposure to the CS during conditioning as assessed by amount drunk. DISCUSSION The present results add to a growing body of research that indicates that procedures that weaken conditioning bring out age differences in the outcome of taste-aversion conditioning in rats. For example, manipulating the intensity of the US (6) or the CS (5) can bring out age difference that might otherwise be obscured. Lengthening the CS-US also weakens conditioning (4) and leads to age differences in CTA in rats (5,7). Also, like combined proximal and remote US preexposure (12), and like remote-US preexposure in the present experiment, preexposing rats to the CS prior to the conditioning session reduces the strength of CTA and results in age differences in the effectiveness of the CS-US pairing (6,8). A significant factor that may help explain many of the reported age differences in CTA and preexposure effects is familiarity with the conditioning context (e.g., 3,6). In many CTA experiments, as in the present experiments, the conditioning context is very similar to, or is, the rat’s home cage (see, e.g., 1). The older the animals are, the more experience they have had with this type of environment. Although these age differences in context familiarity can be used to explain the effects of remote-US preexposure observed in the present research in a number of ways, for example, in terms of relative CS novelty (3,9,11) or affective value of the conditioning context (10), we prefer an explanation in terms of Wagner’s information-processing model (13,14) for a number of reasons. First, this model explains the currently reported age differences in the effect of remote-US preexposure on CTA in rats as a result of age-related difference in familiarity with the conditioning context (e.g., 14). Wagner proposed that the formation
FIG. 2. Percent preference for saccharin as a function of age, and preexposure and conditioning treatments. The first letter in the figure legends (e.g., L-L) designates the proximal preexposure treatment, LiCl (L) or NaCl (N); and the second letter in the legends designates the conditioning (L) or control (N) treatment.
of an association between CS and US depends upon active representation of the CS and US in short-term memory. If the presentation of the CS and US at the time of conditioning is responsible for this representation, an association between the CS and US will be made. If, however, either stimulus has been prerepresented in memory because of the retrieval action of cues with which one or the other of the stimuli have been associated, a CSUS association is less likely to be formed. When the US is preexposed in the conditioning context, it is assumed that an association between the US and context is formed. The contextual cues, because of their retrieval action, prerepresent the US in shortterm memory prior to the conditioning trial and thereby prevent or weaken the CS-US association. Thus, the greater familiarity with the conditioning context could weaken the US-preexposure effect for our old subjects. It could, for example, result in the contextual cues becoming latent inhibitors that would interfere with the context-US association. A weak or nonexistent contextUS association would result in little retrieval action on the part of the contextual cues during the conditioning trial and, thereby, would lead to stronger conditioning, as with our old animals in Experiment 1. A second reason we prefer Wagner’s model over other explanations is that it can also explain the proximal-US preexposure results obtained in the present study. Unlike the effect of remoteUS preexposure, the model does not consider the effect of proximal-US preexposure to depend upon an associative mechanism. The model assumes that, not only can the CS or US be prerepresented in short-term memory by the retrieval action of cues with which they have been previously associated, but also by the stimulus (CS or US), itself. Thus, when, as in Experiment 2, a US is given shortly before the conditioning trial, it is already represented in short-term memory at the time of conditioning, thereby reducing the effectiveness of the CS-US pairing. Because the effect of proximal-US preexposure is nonassociative in nature, differential familiarity with the conditioning context should exert little influence. In the present study, there were no differential age-related effects of proximal-US preexposure as there were with remote-US preexposure. A single proximal-US preexposure eliminated any evidence of conditioning at all age levels. The third reason we prefer Wagner’s model to others is that it can also explain the apparent interaction between proximal and remote preexposure. The present results, together with previously
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reported results (12), suggest that remote-US preexposure interacts with proximal-US preexposure in a way that reduces the overall effectiveness of the proximally preexposed US. In the present study, there was no evidence of CTA in weanling and young adult rats that were either remotely or proximally exposed to the US. In contrast, combined effects of remote and proximal US preexposure have been shown only to attenuate CTA at these age levels (12). Thus, it would appear that remote-US preexposure curtails the effect of proximal-US preexposure or vice versa. In the present study, at the old age level, a single proximally preexposed US eliminated all evidence of CTA, whereas 6 daily remote US preexposures had no significant effect on conditioning. The combined effect of remote and proximal US preexposure at the old age level (12) was an attenuation of CTA, that is, some intermediate effect between the effect of proximal-US preexposure and the effect of remote-US preexposure in the present study. Apparently, then, remote-US preexposure diminishes the effect of proximal-US preexposure, not vice versa. Wagner’s information-processing model suggests that remote US-preexposure would diminish the proximal-US preexposure effect in the following way. As the number of remote-US preexposures increases, contextual stimuli should limit rehearsal of themselves in association with the preexposed US. Thus, after repeated remote-US preexposures, if the US is proximally preex-
posed in the same context, its rehearsal should also be curtailed. Curtailing rehearsal of the proximally preexposed US should, in turn, diminish its effect on conditioning. Although interpretation of the present results has relied upon age differences in familiarity with the conditioning context, they can also be interpreted in terms of age similarities and differences in retention processes. Accordingly, the results of Experiment 2 can be explained on the basis of similar rehearsal capability of weanling, young adult, and old rats. In contrast, the retrieval capability of old rats, as indicated in the results of Experiment 1, would be seen as deficient in comparison with that of weanlings and young adults. Although tenable, such an explanation of agerelated preexposure effects relies more on hypothesizing differences in internal processes rather than on differences in past experiences, such as exposure to the conditioning context. ACKNOWLEDGEMENTS
This research was supported by Grant HD 21161 from the National Institute of Child Health and Human Development. This research was approved by an institutional animal care and use committee and, in all phases and aspects of the research, the treatment of the participants was in accordance with the ethical standards of the American Psychological Association and the Public Health Service Guide for the Care and Use of Laboratory Animals.
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