LABORATORY ANIMAL MEDICINE AND SCIENCE _ SERIES II
RATS AND MICE: Introduction and Use in Research Part II
V-9040
William P. Porter, DVM
Marion Merrell Dow Inc. Cincinnati, Ohio
The Laboratory Animal Medicine and Science - Series II - has
been developed by the Autotutorial Committee of the American
College of Laboratory Animal Medicine (ACLAM): C. W. McPherson,
DVM, Chair; J. E. Harkness; DVM;J. F. Harwell, Jr., DVM; J. M.
Linn, DVM; B. J. McGough, BS Medical Communication; A. F. Moreland,
DVM; G. L. Van Hoosier, Jr., DVM Instructional development and
production assistance provided by Barbara Macfadden. The
development of this program is supported by a grant from The
Burroughs Wellcome Fund.
Laboratory Animal Medicine and Science Series II is produced by
the Health Sciences Center for Educational Resources University of
Washington
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LABORATORY ANIMAL MEDICINE AND SCIENCE -
AUDIENCE
Veterinary and biomedical students, animal care technicians, and
research investigators. RATS AND MICE: Introduction and Use in
ResearchPart II is one of a series of autotutorial programs
designed to assist in teaching veterinary and other biomedical
science students basic information concerning laboratory animal
medicine and science. At the conclusion of Part II of this program
you should be able to: 1. discuss the advantages and disadvantages
of using rodents as research models and give several examples. 2.
list several signs of pain or distress in rats or mice. 3. describe
preferred analgesics, anesthetics, and methods of euthanasia for
rats and mice. Both Part I and Part II of Introduction and Use in
Research of rats and mice are designed to stand alone; however,
together they form a more complete overview and introduction to the
other programs in this series.
GOAL
OBJECTIVES
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1. Series 2. Program title 3. Objectives
Laboratory Animal Medicine and Science Series II Rats and Mice:
Introduction and Use in ResearchPart II At the conclusion of this
program you should be able to: discuss the advantages and
disadvantages of using rats and mice as research models and give
several examples. list several signs of pain or distress in rats or
mice. describe preferred analgesics, anesthetics, and methods of
euthanasia for rats and mice.
4. Section title 5. History of rat use
SUMMARY OF PART I In Part I of this program, the history of the
use of rats and mice in research was discussed. The ancestors of
the common laboratory rat, Rattus norvegicus, probably followed the
lines of human migration throughout the world. Its early
domestication was most likely a byproduct of a 19th century
sporting event known as rat-baiting. The sport provided an
opportunity for inquisitive individuals to tame the more
distinctive ratsthe albino, black, and piebald mutants. By the late
1800's, stocks developed in Europe were being used for research
purposes in the United States. For example, in the 1890's rats were
used in neuroanatomical studies at the University of Chicago. By
1930 there were more than 12 inbred strains of rats established,
and rats were accepted widely as standard research animals.
6. History of mouse useThe laboratory strains of mice used today
(Mus musculus) are descendants of the European house mouse (Mus
domesticus). By the 17th and 18th centuries, mice were being used
in studies of anatomy and respiration. In the 19th century, albino
and color mutants were generated, and investigations of varietal
characteristics and inheritance progressed. By the 1920's, many of
the strains of mice used in research today were already
established. 7. Part I included Part I also included a discussion
of the general characteristics of rats and mice that make them good
research animals. These rodents are available from commercial
producers. There are large data bases available as a result of
years of selective breeding to meet specific research requirements.
There are a number of inbred and outbred varieties, as well as many
mutant stocks, that are models of human diseases.
8. Section title 9. Introduction
RESEARCH USE Part II of this program will describe specific
examples of inbred, outbred, and hybrid rodents used in research.
Concerns, techniques, analgesics, anesthetics, and euthanasia will
also be covered.
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10. Diabetic mice These mice are both of the 129/J inbred
strain, but the larger mouse on the left carries the genotype for
diabetes (db/db). Homozygous diabetic mice are recognized by 3 or 4
weeks of age by size difference. They increase in weight rapidly
from 3- to 8-weeks of age, reaching a maximum weight of about 45 g.
From this point on, their weight declines steadily until death.
Obesity, associated with overeating, is accompanied by
hyperglycemia. Blood sugar concentrations of 300 mg/dl serum at 4
weeks of age are not uncommon, and by 12 weeks of age, blood sugar
levels are usually greater than 500 mg/dl. 11. Dwarf mice These
mice are used to study pituitary dwarfism. They are from the inbred
strain C3H/HeJ, but the smaller one has a dwarf (dw/dw) genotype
which causes deficiency or absence of growth hormone or prolactin
or both. The deficiency depresses protein synthesis, a consequence
of depressed RNA synthesis.
12. Obese mice Obese mice (ob/ob) were discovered at the Jackson
Laboratory in 1949 and are used to study obesity. They are
distinguishable from their normal littermates (+/+ and ob/+) at
about 4 weeks of age. They gain weight rapidly and reach a maximum
weight of about 70 g at 8- to 9-months of age. Their normal
littermates reach a maximum weight of about 30 g at 3- to 4-months
of age. From the peak of about 70 g, their weight declines slowly
until shortly before death. The lifespan of obese mice is about 20
to 24 months, a few months shorter than their normal littermates.
Obese mice have a variety of metabolic defects including increased
lipogenesis, decreased lipolysis, and obesity itself. All of these
can be attributed to hyperphagia and hyperinsulinemia.
13. New Zealand mice
Hybrid mice resulting from breeding New Zealand Black mice with
New Zealand White mice develop eye, kidney, and joint disease.
These diseases model systemic lupus erythematosus, a condition that
primarily affects women of child-bearing age.
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14. Radiation
Mice can be useful in studies of radiobiology, because they
tolerate high doses of radiation. Research into the effects of
radiation on mice can provide clues on how to avoid problems for
people undergoing radiation treatment for cancer. Also, because of
their small size, the amount of radiation needed for these studies
can be kept at lower levels than if larger animals, such as
rabbits, were used.
15. Pneumocystis infection As seen in this lung section, mice
and rats can carry the silver-staining micro-organism Pneumocystis
sp. in their lungs, yet remain healthy. When rodents are
immunodeficient or immunosuppressed with drugs or irradiation, the
animals develop pneumocystis pneumonia and usually die. This
disease was once rare in man but is now of great importance in
humans with AIDS, who often develop pneumocystis pneumonia.
16. Dental lesions One use of rats in dental research is to
study carious lesions. When the oral flora of the rat is combined
with a cariogenic strain of Streptococcus sp., and the rat is fed a
high sucrose diet, the lesions in the enamel of the rats teeth are
both macroscopically and microscopically identical to carious
lesions of human teeth.
17. Transplantable tumors The tumor types shown here can be
transplanted into the rat. Notice that the types have been grouped
according to the host stock or strain required for perpetuation of
each tumor. The laboratory mouse is the primary animal used in
experimental oncology, but rats, because of their size, can provide
some significant advantages for this type of study.
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18. Murphy-Sturm tumor The lesion shown here resulted from
subcutaneous inoculation of a Wistar albino rat with a Murphy-Sturm
lymphosarcoma. This rapidly growing, invasive tumor provides a
valuable model for studying the pathogenesis and therapy of
tumors.
19. Inhalation chamber When rats are placed intermittently in an
inhalation chamber, they may be exposed to various gas or
particulate mixtures. The heart and lung lesions that may develop,
mimic the lesions found in humans living in various high-risk
environments. This is a valuable model that can lead to further
understanding of how disease develops, and how it can be prevented
or cured in humans.
20. WAG/Rij rat The WAG/Rij rat shown here can be bred to model
the development of a serious condition in humans called retinitis
pigmentosa. Humans develop the disease early in life, as does this
inbred rat.
21. Testicular tumor
Approximately 65% of all male Fischer-344 rats develop a
testicular interstitial cell tumor. This tumor develops by 18-to
24-months of age and is associated with atrophy of the secondary
sex glands, such as the prostate. There is a similar disease found
in man.
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22. Hypertensive rat This image shows one variety of
spontaneously hypertensive rat. They develop high blood pressure,
sometimes with obesity. The lifespan of the animals that become
obese is approximately half that of their non-obese littermates.
Hypertension and obesity are serious conditions in humans and have
drawn considerable research attention.
23. Rat kidney section This image is a microscopic view of a
rat's kidney. The rat kidney has superficial nephrons that can be
punctured easily using microscopic techniques. This characteristic
makes rats attractive for kidney research, because other animals
usually have less accessible nephrons.
24. Section title
BIOMETHODOLOGY
25. Nonspecific illness This image shows the hunched posture and
rough hair coat characteristic of a sick mouse. Generally rats and
mice behave similarly if distressed or in pain. While it is true
that many diseases and conditions produce similar clinical signs in
these animals, there are specific etiologies that cause specific
problems.
26. Intrasex aggression Male mice housed together may be
aggressive and will often fight and mutilate one another. Because
of this aggression, male mice should usually be housed
separately.
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27. Acute pain signs
Anorexia can sometimes be the earliest sign of health problems
or pain in an animal. A decrease in food intake can be transient
and not related to pain or discomfort, but it should be considered
in determining the condition of the animal. Rats or mice that have
had surgery may exhibit signs of pain or discomfort by rubbing or
scratching accessible surgical sites, such as the eye or ear.
General signs of acute pain or discomfort can include biting or
shaking an affected limb or foot, vocalization on movement or
palpation, restlessness, lacrimal porphyrin discharge in rats, and
increased respiratory rate. Chronic pain or discomfort may
generally include the following signs: decreased body weight or
reduced weight gain in growing animals, reluctance to move, change
in behavior when approached or handled, accumulation of body
secretions due to poor grooming, change in bowel or urinary
activity, hunched posture, and a rough hair coat. None of the acute
or chronic clinical signs is an absolute indication of pain or
distress. Experience and professional judgment must be used, along
with specific clinical laboratory tests, to determine the cause and
severity of the problem.
28. Chronic discomfort
29. Identification methods This image shows an ear punch, one of
the methods used for individual identification of mice and rats.
Other methods include fur dyes, ear tag, tattooing, micro-chip
implantation, and cage cards. Toe clipping is usually considered
unacceptable, because suitable alternatives are available.
30. IP injection This image shows the proper restraint of rats
and mice during intraperitoneal (IP) injection and anesthesia
induction. Restraint is an important factor, not only for the
comfort of the animal but for the safety of the person
administering anesthesia. An animal that moves suddenly as it is
injected could cause an accidental injection of the anesthetist or
needless injury to the animal.
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31.
Section title
ANESTHESIA, ANALGESIA, AND EUTHANASIA
32. Drug in syringe Anesthetic drug doses usually differ between
rats and mice. It is important to verify the correct dosage for
each species, because overdosing these small animals is relatively
easy. Injectable anesthetics may be diluted to at least half
strength with saline solution to facilitate accurate dose levels
and reduce irritation.
33. List of anesthetics
Injectable anesthetics in rats and mice include sodium
pentobarbital given intraperitoneally (IP), or ketamine in
combination with xylazine or acetylpromazine, given IP or by the
intramuscular (IM) route. IM administration of ketamine at high
doses can cause muscle necrosis. Hypothermia is used sometimes for
brief periods of anesthesia in fetal or newborn animals.
34. Analgesics
Pain relief should be considered, if consistent with study
requirements. Injectable analgesics in rats and mice include
butorphanol given subcutaneously (SC), meperidine given SC or IM,
or pentazocine given SC.
35. Inhalation anesthesia A bell jar or face mask connected with
a calibrated vaporizer can be used to induce and maintain adequate
anesthesia. The gas anesthetics that can be used include
isoflurane, methoxyflurane, or halothane. Ether was used commonly
in the past but is now discouraged because of its explosive hazard.
Selection of the anesthetic agent and route of administration
should be based on the type of experimental procedure that will be
performed. Further information on anesthesia is available in
programV-9054, LABORATORY ANIMALS: Surgery and Anesthesia in
Rodents.
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36. Euthanasia methods Rats and mice can be euthanatized by
several methods including injection with barbiturates or other
suitable euthanasia solution; inhalation of carbon dioxide,
methoxyflurane, or halothane; and decapitation or cervical
dislocation with or without sedation, depending on the nature of
the study. Because large numbers of these animals are often on
study at one time, euthanatizing several animals at a time could be
necessary. In such cases, crowding of animals should be avoided.
37. Cervical dislocation Physical methods of euthanasia, such as
cervical dislocation shown in this image, can be performed with or
without sedation or anesthesia depending on study requirements.
This method should be used only on small or young animals, and when
the nature of the study precludes the use of chemicals. It should
also be scientifically justified and approved by the Institutional
Animal Care and Use Committee.
38. Mouse in desiccator jar This image shows a mouse in an
inhalation chamber. At no time should animals have direct contact
with chemical anesthetic or euthanasia substances, such as liquid
methoxyflurane or halothane. To prevent epithelial irritation or
burns, there should be no direct contact with dry ice, which is
sometimes used to generate carbon dioxide.
39. Confirmation of death Death of an animal after euthanasia
administration should be confirmed before carcass disposal. This
can be done by checking for respiration a few minutes after a
procedure is completed, by exsanguination, or by opening the chest
cavity to collapse the lungs. The 1993 report of the American
Veterinary Medical Association Panel on Euthanasia provides more
information on this subject (1).
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40. Conclusion
This image concludes this autotutorial session on the
introduction and use in research of rats and mice. There are seven
programs in this series: V-9039 RATS AND MICE: Introduction and Use
in ResearchPart I V-9040 RATS AND MICE: Introduction and Use in
ResearchPart II V-9041 RATS AND MICE: Biology V-9042 RATS AND MICE:
Care and Management V-9043 RATS AND MICE: Bacterial and Mycotic
Diseases V-9044 RATS AND MICE: Viral Diseases V-9045 RATS AND MICE:
Parasitic Diseases
41. ACLAM credits This program was developed for the American
College of Laboratory Animal Medicine. C. W. McPherson, DVM Chair
J. E. Harkness, DVM J. F. Harwell, Jr., DVM J. M. Linn, DVM B. J.
McGough, BS A. F. Moreland, DVM G. L. Van Hoosier, Jr., DVM
Instructional development, editing, and production management
provided by Barbara Macfadden. Guide production supported by Pamela
Young. The development of this program was supported by a grant
from The Burroughs Wellcome Fund.
42. HSCER credits Produced by the Health Sciences Center for
Educational Resources University of Washington Seattle WA 98195
(206) 685-1156 Fax: (206) 543-8051 2000
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ACKNOWLEDGMENTS Images 10 and 11 are provided courtesy of The
Jackson Laboratory, Bar Harbor, Maine. Image 12 is provided
courtesy of J. Russell Lindsey, DVM, University of
Alabama-Birmingham. Image 15 is provided courtesy of Charles W.
McPherson, DVM, North Carolina State University. Image 20 is
provided courtesy of Gail C. Wolz, University of Washington,
Seattle. Image 22 is provided courtesy of William P. Porter, DVM,
Marion Merrell Dow Inc., Cincinnati, Ohio.
REFERENCES 1. American Veterinary Medical Association. 1993.
Report of the AVMA panel on euthanasia. J. Am. Vet. Med. Assoc.
202(2):229-249.