REPORTS
Regarding the Inadvisability of Administering
Postoperative Analgesics in the Drinking
Water of Rats (Rattus norvegicus)
ROBERT C. SPETH, PHD,1†* M. SUSAN SMITH, PHD,2 AND REBECCA S. BROGAN, PHD2‡
The feasibility of administering the pain reliever acetaminophen to rats via their water bottles was examined in this study. Two
different preparations of acetaminophen were used, a cherry-flavored suspension and an alcohol-containing solution. Both preparations of acetaminophen were diluted to 6 mg/ml by using normal drinking water. When healthy unmanipulated rats were exposed
to either of the acetaminophen preparations for the first time, the animals showed a dramatic reduction in fluid intake. A marked
reduction in food intake also was associated with the cherry-flavored preparation. These reductions appear to be an expression of
the well-characterized neophobic response that can be demonstrated by rodents when they encounter a novel taste. This neophobic
behavior suggests that administering pain relievers to rats via their drinking water is counterproductive as a means of providing
pain relief.
Neophobia is a phenomenon in which animals, especially rodents, initially consume small amounts of a novel substance (1).
The adaptive value of this behavior is that if the substance is
toxic to the animal, the animal is less likely to consume a lethal
amount of the substance. If it becomes sick within a short time
after the consumption of the substance, the animal develops a
conditioned taste aversion to that substance and will no longer
consume substances that have this taste. This phenomenon was
demonstrated first in wild rats (2) and later in domestic rats (3);
this behavior is also known as bait-shyness (1, 4). In addition,
consumption of novel substances at a time when animals are
already sick, or become sick soon after ingesting the novel-tasting substance, cause them to develop a conditioned taste aversion
to the novel substance (3).
It has been suggested that analgesic agents should be administered postoperatively to rodents recovering from surgery (for
a review of this subject, see [5]). Various strategies have been
proposed, but most involve either injecting the analgesics or
providing them in the animals’ water bottles. Although the injection of analgesics provides the most reliable means of
administering the drugs, it is accompanied by a number of drawbacks, including: 1) the additional stress of handling of the
animals to administer the drug, 2) the possibility of disrupting
the diurnal rhythms of the treated animals, and 3) disrupting
other animals in the same room (5). Administration of analgesics in animals’ water bottles has been shown to produce effective
analgesia (6), and dosing regimens for oral administration have
been tabulated (5). However, when analgesics are given in water bottles, the animals must consume a sufficient volume of fluid
to provide an efficacious dose of the analgesic. Because the animals’ consumption of fluids may be diminished during recovery
from anesthesia and further reduced by postoperative malaise,
it is important to determine whether animals drink enough to
provide adequate postoperative analgesia.
Visiting Scientist, Division of Neuroscience, Oregon Regional Primate Research Center, Oregon Health Sciences University, Beaverton, Oregon, 970061; Division of Neuroscience, Oregon
Regional Primate Research Center, Oregon Health Sciences University, Beaverton, Oregon,
970062
†Present address: Department of Veterinary and Comparative Anatomy, Pharmacology
& Physiology, Washington State University, Pullman, Washington, 99164-6520
‡Present address: Department of Physiology, Medical College of Georgia, Augusta, Georgia, 30912-2300
*Corresponding author
Volume 40, No. 6 / November 2001
In our studies of lactating rats, opioid analgesics are contraindicated because of their ability to disrupt maternal behavior (7,
8). Therefore, we explored the use of a liquid nonsteroidal antiinflammatory drug (NSAID) as a perioperative analgesic in
untreated random-cycling rats.
Materials and Methods
All animal procedures were approved by the Oregon Regional
Primate Research Center Institutional Animal Care and Use
Committee.
Random-cycling female Sprague-Dawley rats (Rattus
norvegicus) weighing 290 to 370 g were obtained from B & K
Universal (Kent, Wash.). The rats were certified murine pathogen-free and housed in a room separate from other rats in the
colony. Rats were housed singly in AAALAC-certified polycarbonate shoe-box cages with recycled cardboard bedding and
maintained on a 12:12-h light:dark cycle. The temperature in
the room in which the rats were housed was maintained at 2022°C and the humidity was maintained between 40 and 60%.
Food (Purina 5001 Rodent Chow, Purina Mills, St. Louis, Mo.)
and water were available ad libitum. Food and water consumption, monitored for 4 days prior to exposure of the animals to
acetaminophen, was consistent from day to day. Random-cycling rats were used for these studies to avoid the additional
stressor of lactation and the confounding factor of increasing
food and water intake during lactation.
Two groups of 5 rats each that had no prior manipulations
were given two different commercial liquid acetaminophen
preparations: Children’s Acetaminophen (Cherry flavor, 32 mg/
ml; Rite-Aid, Camp Hill, Pa.; additional ingredients listed on the
container included benzoic acid, citric acid, flavors, glycerin, polyethylene glycol, propylene glycol, sodium benzoate, sorbitol,
sucrose, D&C red #33, and FD&C red #40); and Tylenol Adult
Liquid (33 mg/ml; Johnson and Johnson, New Brunswick, N.J.;
additional ingredients listed on the container included alcohol
[7%], citric acid, D&C yellow #10, FD&C Blue #1, FD&C Yellow
#6, flavor, glycerin, polyethylene glycol, water, sodium benzoate,
sorbitol, and sucrose). These two preparations were diluted to 6
mg/ml by using tap water, thereby yielding a dosage of approximately 700 mg/kg if the daily water consumption were 40 ml.
This dosage of acetaminophen was estimated in light of allometric conversion for a 350-g rat from the recommended maximum
CONTEMPORARY TOPICS © 2001 by the American Association for Laboratory Animal Science
15
groups was similar to pre-treatment levels (data not shown).
In addition, exposure to the cherry-flavored acetaminophen
solution significantly reduced food intake (F2,14 = 74.7, P <
0.0001). Compared to those of the preceding and succeeding
day, food consumption was significantly decreased during exposure to the cherry-flavored acetaminophen preparation (P < 0.001
for both comparisons). Food intake tended to be reduced during exposure to the acetaminophen solution containing alcohol,
but the effect was not statistically significant.
Discussion
Figure 1. Fluid (left panels) and food (right panels) consumption of
rats on the day prior to exposure to acetaminophen (Pre), during exposure to acetaminophen, and after exposure to acetaminophen (Post).
(A) cherry-flavored elixir of acetaminophen. (B) alcohol-containing acetaminophen solution. *, P < 0.01 versus Post; **, P < 0.01 versus both
Pre and Post.
human analgesic dose (4 g daily, [9]) for a 70-kg person. The
power factor used for the allometric equation was the ratio of
body weight to surface area (0.667) as described by SchmidtNielsen (10). Using this equation, we obtained a daily dosage of
334 mg/kg. However, because fluid intake during the light period is considerably less than that during the dark period, we
targeted a dosage twice that derived from the recommended
human dosage.
Consumption of the acetaminophen solutions and food intake was measured during a 24-h period. Afterward, the rats
were again given normal tap water to drink, and their daily food
and fluid consumption was monitored for five days. A one-way,
repeated measures analysis of variance was used to analyze the
effects of adding the flavored acetaminophen to the water being consumed by the rats. Post hoc comparisons were made
with the Bonferroni test (Prism, GraphPad Software Inc., San
Diego, Calif.).
Results
Fluid consumption of the rats was significantly decreased after their exposure to cherry-flavored (F2,14 = 46.4, P < 0.001) or
alcohol-containing (F2,14 = 39.1, P < 0.001) preparations of acetaminophen (Fig. 1). Compared to the previous day’s
consumption, the consumption of the water containing the
cherry-flavored suspension fell by 51% (P < 0.01), and that of
the alcohol-containing preparation dropped by 41% (P < 0.01).
Both groups manifested a prominent rebound increase in water
consumption during the first day after acetaminophen exposure;
the rebound intake was significantly greater than water consumption during the pre-exposure period as well as on the day of
acetaminophen exposure (P < 0.01 for both periods). During
days 2 through 5 post-treatment, fluid consumption in both
16
CONTEMPORARY TOPICS © 2001 by the American Association for Laboratory Animal Science
Based upon these results no further efforts to administer acetaminophen via the rats’ water bottles were attempted, since
the animals showed a neophobic reaction to the change in taste.
Less of the cherry-flavored acetaminophen solution than of the
alcohol-containing preparation was ingested; this finding suggests that the presence of flavoring increased the novelty of the
taste. However, we cannot rule out the possibility that acetaminophen itself, or some other component of the analgesic mixture,
had an aversive taste. Anesthetization and surgical trauma cause
malaise in rats, and coupling a novel taste with this malaise likely
will produce a conditioned taste aversion to the flavored acetaminophen solution in a manner similar that through which Revusky
and Bedarf (3) produced conditioned taste avoidance. Therefore, this phenomenon compromises not only the rats’
consumption of analgesic but that of water as well.
An additional factor mitigating against the administration of
analgesics in drinking water is the pronounced diurnal rhythm in
fluid consumption. Rats consume more than 90% of their water
during the dark phase of the light-dark cycle (11). Therefore, to
provide effective doses throughout the light period, it will be necessary to use high doses that could cause toxic concentrations of
the drugs to be accumulated during the dark period.
Another strategy that has been used to accomplish oral ingestion of analgesics is to lace gelatin cubes with analgesic (12). To
successfully use this method, it is necessary to pre-administer the
unadulterated gelatin cubes prior to surgery to allow the rodents
to adapt to the novel taste and then to administer the adulterated gelatin cubes immediately before surgery (13). Barnett (1)
notes that this type of adaptation first was used by Chitty and
Kempson in 1949 as “pre-baiting” to poison wild rats. However,
administration of the drug-containing gelatin cubes repeatedly
during the dark phase poses the risk of disturbing the diurnal
rhythms of the rats.
Another possible approach to oral administration of analgesics
to rats would be to start them on one of the above noted acetaminophen-containing solutions several days prior to surgery. Unlike
most NSAIDs, acetaminophen does not impair blood clotting (9).
However, the prior exposure to analgesics when they are not indicated increases the likelihood that the animals will suffer adverse
effects, e.g., nephrotoxicity and hepatotoxicity. It might be possible to give the rats the same solution but devoid of the
acetaminophen until after they have surgery. However, because
the mixture is a proprietary preparation, this analgesic-free solution likely would be difficult to procure or prepare. In addition,
the possibility remains that the rats would be able to distinguish a
change in taste when the analgesic was added, nullifying the effort
to develop a familiarity with the solution.
Perhaps a more effective approach will be to administer analgesics of the NSAID class by injection. Carprofen (Rimadyl, Pfizer
Inc., New York, N.Y.) is available in an injectable form that can
be administered subcutaneously at a dose of 5 mg/kg upon
completion of the surgical procedures and prior to the awakening of the rat.
In summary, the inability to reliably provide an efficacious dose
of NSAID analgesic via water bottles because of the reluctance
Volume 40, No. 6 / November 2001
of rats to consume novel fluids combined with postsurgical malaise suggests that this method of administering analgesics to
rodents should be avoided.
Acknowledgments
This study was supported by grants RR00163 and HD14643 from the
National Institutes of Health. We thank Dr. John Fanton (Senior Veterinarian and Head of the Surgery Program, the Oregon Regional Primate
Research Center) for his helpful assistance in these studies and Dr. Muriel
Slattum (Director, Laboratory Animal Resources, Washington State
University) for reviewing this manuscript.
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