Download Ketaset-Rompun Anesthesia Induces a Conditioned Taste Aversion

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associated with administration of Ketaset-Rompun, many researchers
have adopted the use of this anesthetic for surgical and/or analgesic
purposes in rodent preparations. However, despite the recent popularity
of Ketaset-Rompun anesthesia, there have been few investigations
examining the behavioral effects of this drug.
Several investigations have been reported for effects of anesthetics
other than Ketaset-Rompun, but the results have been mixed. Rabin and
Rabin (1984) investigated whether a conditioned taste aversion (eTA)
(Le., a rejection of a novel flavor after that flavor is paired with an illnessinducing substance) would occur under Pentobarbital anesthesia. Of
particular interest in this experiment was a control group that was
presented with a novel sucrose solution, immediately anesthetized with
Pentobarbital, and maintained under a surgical plane of anesthesia for 47 hr. When tested for an aversion 1 day later, rats in this group did not
demonstrate a eTA for the sucrose solution, whereas rats that were
anesthetized and further administered radiation or lithium-chloride did
show the eTA effect. This result suggests that Pentobarbital itself did not
produce a eTA, and also indicates that anesthetization with
Pentobarbital did not attenuate the induction of eTA. Additionally,
Aguado, San Antonio, Perez, Del Valle, & Gomez (1994) examined
whether the administration of Ketaset prior to conditioning would retard
the acquisition of gustatory learning. One result from this study was that
administration of Ketaset prior to conditioning impaired one-trial learning
of an aversion to sucrose with a eTA paradigm (for similar results, see
Welzl, Alessandri, & Battig, 1990). These results suggest that
administration of certain anesthetic agents do not produce eTA, and
they also indicate that certain anesthetic agents may in fact interfere with
learning of taste aversions.
In contrast to studies that report a failure to obtain taste aversions
with anesthetics, other investigations have reported that administration
of anesthetics can induce weak aversions to novel flavors. BureSova and
BureS (1977) investigated the effect of Allobarbital anesthesia on eTA in
rats and reported that injections of this anesthetic following consumption
of a novel saccharin solution produced a weak eTA effect. Similarly,
Bermudez-Rattoni, Forthman, Sanchez, Perez, and Garcia (1988)
reported that administration of Rompun produced a mild aversion to
tomato juice when injected after consumption of the novel flavor. These
results, in contrast to those reported by Rabin and Rabin (1984) and
Aguado et al. (1994), suggest that certain types of anesthetics may
induce mild taste aversions if paired with a novel flavor.
The aim of the present study was to examine further the effects of
an anesthetic (Ketaset-Rompun) as the conditioning agent (US) in the
eTA paradigm. As Ketaset-Rompun has recently become popular for use
in rodent surgical preparations, the methodological implications for
investigators using this drug within the eTA paradigm would be of
potential importance, particularly if the anesthetic itself produces
aversion to novel flavors. Experiment 1 examined whether Ketaset-
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Figure 1. Median difference scores (test ml - train ml) for animals in the SAL and ANES
groups in Experiment 1. (Error bars =semi-interquartile range)
minus the amount consumed at training) and were used as the
dependent measure.
Results and Discussion
Because of skewed distributions, nonparametric Kruskal-Wallis
ANOVAs and Mann-Whitney U tests were computed to detect any
differences between the groups in these experiments. (Figure 1 presents
the median difference scores of the SAL and ANES group on the 24-hr
retention test.) The amount of sucrose consumed by the CTRL and
ANES groups on the conditioning day did not reliably differ, as confirmed
by statistical analysis (U = 44.5, P > .10). However, there were clear
differences in the amount of sucrose intake between the groups on the
24-hr retention test. As seen in Figure 1, following testing the ANES
group demonstrated reliably lower difference scores compared to the
SAL group, suggesting that administration of Ketaset-Rompun
anesthesia resulted in an aversion to the sucrose solution at testing.
These impressions were confirmed by statistical analysis, as a MannWhitney U test computed on the difference scores yielded a significant
treatment effect (U = 16, P < .02).
Experiment 2
Whereas Experiment 1 demonstrated that immediate injections of
the Ketaset-Rompun anesthetic produced a reliable CTA, Experiment 2
sought to further examine this effect. One characteristic of CTA learning
is that the strength of the aversion decreases as the time interval
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consumed was recorded to the nearest ml. Following sucrose
consumption, animals in the SAL (n = 9) and AN-O (n = 6) groups
received immediate injections of bacteriostatic water or anesthesia,
respectively. The remaining groups were injected with the anesthetic
after delays of 1 hr (AN-1; n = 9), 2 hr (AN-2; n = 9), 4 hr (AN-4; n = 9),
or 6 hr (AN-6; n = 7). The volume, ratio of stock/vehicle, and route of
administration for the anesthesia groups was identical to that described
in Experiment 1. All subjects were tested for an aversion to the sucrose
solution 24 hr after the beginning of conditioning, and the amount
consumed was recorded to the nearest ml. As in Experiment 1,
difference scores were computed to account for variability between the
subjects on fluid intake.
Results and Discussion
A Kruskal-Wallis one-way ANOVA indicated that there were no
differences among the groups on the amount of sucrose consumed on
the conditioning day (H = 7.52, P > .05). However, subsequent planned
comparisons indicated that there were reliable differences in the
difference scores between the groups after testing. Planned
comparisons using Mann-Whitney U tests confirmed that the difference
scores of the AN-O (U = 9; P < .05), AN-1 (U = 18.5, P < .05), and AN-2
(U = 8, P < .05) groups were reliably lower than the scores of the group
injected with saline (SAL) on the conditioning day, suggesting that rats
injected with the anesthetic up to 2 hr after presentation of the CS
demonstrated CTA. However, the groups injected with the anesthetic
either 4 (AN-4; U = 20.5, p> .05) or 6 hr (AN-6; U = 18.5, p> .05) after
sucrose consumption exhibited difference scores similar to those of the
saline-injected group. Additionally, subsequent analysis indicated that
the 2-hr delay group demonstrated the largest CTA, as the AN-2 group
reliably differed from both the AN-O (U = 10.5, P < .05) and the AN-1 (U
= 7.5, P < .05) groups. Figure 2 presents the median difference scores
for the immediate and delay groups in Experiment 2.
General Discussion
This study consisted of two experiments that used Ketaset-Rompun
anesthesia as the conditioning agent in the CTA paradigm. Experiment 1
demonstrated that rats injected with the anesthetic immediately after the
presentation of a novel sucrose solution consumed reliably less sucrose
than control rats on a 24-hr retention test. Additionally, Experiment 2
showed that, when compared to the control group, anesthetic-injected
animals demonstrated CTA if the CS-US interval was either immediate, 1
hr, or 2 hr, whereas no CTA was evident when the CS-US interval was
lengthened to 4 or 6 hr. Taken together, the results of these experiments
suggest that injections of Ketaset-Rompun anesthesia can produce
aversions to novel flavors and that these aversions can occur with CSUS intervals of at least 2 hr.
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The results of this study are similar to earlier reports of the induction
of a CTA when different anesthetics were used as the conditioning
agents, and the present study replicates and extends these findings to a
different class of anesthetic (Ketaset-Rompun). BureSova and BureS
(1977) have reported that Allobarbitol anesthesia produces a CTA, and
Bermudez-Rattoni et al. (1988) have reported similar effects with
Rompun alone. Although this study utilized different anesthetic agents
than these earlier reports, our findings demonstrate the generalized
effect of the induction of CTA with different anesthetic agents.
In contrast to earlier reports of an induction of CTA with different
anesthetics, the findings from our experiments differ from previous
reports of a failure to obtain a CTA with Pentobarbitol anesthesia (Rabin
& Rabin, 1984) and also differ from reports which have demonstrated an
attenuation of CTA with Ketaset (Aguado et aL, 1994). Aguado et aL
(1994) and Welzl et aL (1990) reported an attenuation of CTA when
Ketaset was injected 30 minutes prior to flavor presentation. When
Ketaset is injected prior to the conditioning trial and CTA learning is not
manifested, it is plausible that Ketaset blocks processing of the
conditioned (flavor) stimulus, thus resulting in an attenuated or retarded
CTA effect. We did not administer Ketaset prior to the conditioning trial,
but instead, incorporated Ketaset as part of the conditioning agent. In
any event, whether administration of Ketaset prior to conditioning would
block the induction of Ketaset-Rompun-induced CTA in our paradigm
remains to be determined.
One unexpected result from Experiment 2 was the finding that the 2hr delay between the CS and the US produced the strongest CTA effect.
As the strength of taste aversion learning has been reported to decrease
as the CS-US interval is lengthened (Garcia et aL, 1966; Kalat & Rozin,
1971; Logue, 1979), it is not clear why the 2-hr delay produced a
stronger CTA than the immediate or 1-hr delayed injection in Experiment
2; however, it may reflect an important characteristic of Ketaset-Rompuninduced CTA. Typically, studies that have reported anesthesia-induced
CTA have not examined the time-dependent characteristics of this type
of learning. Although Bermudez-Rattoni et al. (1988) and Buresova and
Bures (1977) reported that the administration of anesthetic agents
produced CTA in their preparations, neither investigation reported the
time-dependent effects of anesthesia-induced aversions. Therefore, with
the lack of evidence reporting the time-dependent functions of
anesthesia-induced CTA, we cannot conclude whether the increased
CTA at the 2-hr delay in Experiment 2 is specific to Ketaset-Rompuninduced CTA, or if this relationship occurs for other types of anesthetic
agents as well.
One interpretation of the data from these experiments is that
because the retention test occurred after a relatively short time interval
(24 hr after conditioning), it is possible that there was a residual effect on
fluid consumption that had nothing to do with CTA learning. That is, if
subjects injected with Ketaset-Rompun were still under the influence of
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the drug at the time of testing it may have created a nonspecific effect on
consumption that did not reflect taste aversion learning. Although
measuring the time required to metabolize the anesthetic was beyond
the scope of this study, the subjects did appear to be behaviorally "back
to normal" by the time of testing. With the volume of anesthetic injected
in the present experiments, the rats typically regained the righting reflex
within 1.5 to 2 hr of administration of the drug and displayed normal
behavior soon after righting. Additionally, if there was a nonspecific effect
of the anesthetic influencing the results in the present study, subjects
with a shorter injection-to-testing interval (Le., subjects in the AN-4 and
AN-6 groups) should have demonstrated a greater aversion than
subjects in the other conditions, as the injection-test interval for the AN-4
and AN-6 groups was shorter than for rats in the other delay conditions.
However, statistical analysis confirmed that eTA occurred only with
subjects in the AN-O, AN-1, and AN-2 groups, but subjects in the AN-4
and AN-6 groups did not differ from subjects injected with saline on the
conditioning day, an effect opposite than would be expected from a
nonspecific drug effect interpretation. Therefore, although we cannot
entirely rule out a nonspecific drug effect interpretation of the data, such
a conclusion does seem unlikely.
In this study we did not administer the conditioning agent in a mg/kg
injection, but rather we injected all subjects with a constant volume of the
drug. This procedural manipulation was utilized in our experiments
because of a common method used to administer the anesthetic during
surgical preparations. Often, a diluted stock mixture of Ketaset-Rompun
anesthesia is formulated, and subjects that are similar in body weight
are administered the same volume of the drug. One concern with this
approach may be that the amount of eTA produced by the anesthetic in
this study may have been dependent on the body weight of the subjects;
that is, the drug may have differentially affected subjects of different
sizes and weights. However, a correlation computed on the body weights
of the subjects and their difference scores for rats immediately injected
with the anesthetic revealed that there was absolutely no relationship
between body weight and the amount of aversion produced by the drug
(r = .00015, p> .05). Had the aversions been dependent on the size of
the animals, a significant correlation between body weight and amount of
aversion should have been revealed.
The results of the present study are of potential importance to
researchers using Ketaset-Rompun anesthesia for surgical and/or
analgesic purposes in rodent research, especially if flavor presentations
or the eTA paradigm are to be incorporated in the experimental design.
These data indicate that injections of Ketaset-Rompun anesthesia can
induce taste aversions to novel flavors that are presented in close
temporal proximity to administration of the anesthetic, a finding that may
offer alternative explanations to the results of studies employing this
pharmacological agent. Although other studies have investigated the
specific effects (onset, duration, recovery time, etc.) of Ketaset-Rompun
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anesthesia (Wixson et aI., 1987), this is the first study examining the
behavioral effects of administration of this drug. Because this study
demonstrated that Ketaset-Rompun-induced eTA did not follow the
characteristic time-dependent function of taste aversion learning, other
anomalies of this drug may become evident as more research is
conducted with Ketaset-Rompun. Until more is known about the
consequences of utilizing this anesthetic in learning experiments,
researchers incorporating this drug should consider the possible
behavioral effects of Ketaset-Rompun administration in their research.
References
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