Biomethodology of the Rat
The Norway rat or laboratory rat rattus norvegicus, is a mammal
of the order Rodentia. The laboratory rat was the first animal where the
primary reason for domestication was for use in scientific endeavors.
Rats have several unique biological characteristics. The acute hearing
of rats makes them sensitive to ultrasounds and high pitched sounds. The
vision of rats is very poor and they are unable to detect color and are
blind to long-wave (red) light. The tail of the rat is the principal organ
for heat exchange.
There are two common methods by which to characterize
laboratory rodents; genetics and microbial flora. Common genetic
categories are "random-bred" which are managed to maintain genetic
diversity by mating unrelated animals; "Inbred" rodents which
are managed to maintain genetic homozygosity by breeding siblings;
"F1 hybrid rodent" in which two inbred strains are crossbred
for one generation; "transgenic" in which specific genetic material has
been introduced into the genome of another inbred rodent strain; "mutant"
rodents which are inbred with developed genetic mutations. The microbial
flora of rodents are used for grouping: Specific Pathogen Free rodents (SPF) are free from
known bacterial, viral, and parasitic mouse pathogens, as opposed
to "conventional" mice, which are not known to be free of
pathogens. Other less common microbial groups are axenic, which
are free from all microbial organisms, and gnotobiotic, which have a known microbial flora.
Inbred, outbred, hybrid, transgenic or mutant rodents are
available from many commercial sources. Vivarium
personnel are familiar with many of the commonly available strains
and will assist in locating animals available for purchase.
To order
animals the investigator must provide the UC Riverside Animal Use
Protocol number.
The rat can become accustomed to handling providing they are not upset
by the experience. Rats will bite without warning, but not repeatedly.
Unlike mice, groups of the same sex can be housed together without
fighting. Rats are active primarily during the night at which time they
feed; the light hours are used primarily for rest, sleep and digestion.
Handling animals during the night phase can be more difficult due to this
increase in activity. The diurnal rhythm can be changed by a 12 hour shift
in the light cycle. It takes approximately two weeks for rats to adjust to
this shift.
| Adult body weight: male |
450-520 gm5 |
| |
300-800 gm21 |
| Adult body weight: female |
250-320 gm5 |
| |
250-400 gm21
|
| Body surface area |
10.5 (wt. in grams)2/35
|
| |
9.1 kg0.66 B.W.21
|
| Life span |
2-3.5 years5 |
| Food consumption |
10 g/100 g/ day5 |
| Water consumption |
10-12 ml/100 g/day5 |
| Breeding onset: male |
65-110 days5 |
| Breeding onset: female |
65-110 days5 |
| Gestation Period |
21-23 days5 |
| Body Temperature |
38-39 C21 |
| |
35.9-37.5 C5 |
| Heart rate |
320-480 beats per minute21 |
| |
250-450 beats per minute5 |
| Respiratory rate |
85-110 per minute21 |
| |
70-115 per minute5 |
| Tidal volume |
0.6-2.0 ml5 |
| |
1.6 (1.5-1.8 ml)21 |
Most rats are housed in shoebox cages composed of polypropylene (opaque) or polycarbonate material (clear) with a wire bar lid used to hold the water bottle and feed
(figure 1). Bedding is
placed directly into the shoe box cage allowing the absorption of urine
and the animal to burrow. This type of cage will hold 1-3 adult rats
depending on the size of the cage. When removing the lid from this type of
cage, it is important to remove the water bottle to prevent spillage. If
the cage is to be transported the bottle should be turned sipper tube up
to prevent spillage during transport. However, you should remember to turn
the bottle back over to allow access to water after transport (figure
1). The
caretaking staff change the cages twice per week, thereby providing the
animal a clean cage with new bedding, food and water. Water bottles and
feed hoppers are checked daily by caretakers to insure the provision of
food and water. Some rats are housed on wire mesh bottom cages to allow
collection of feces and urine or to prevent contact with bedding. This
type of housing is not preferred and is used only when dictated by
experimental design. Suspended cages are occasionally provided with
automatic watering. Typically a nipple valve is located in the back of the
cage which can be operated by contact. When replacing a suspended cage
that is provided with automatic watering it is important to push the cage
fully into the rack care to insure that the lixit fully extends into the
cage, allowing the animal access to water. Occasionally a rat will jam the
nipple open resulting in a constant dripping of water from the nipple,
saturating the bedding material. Upon observation the bedding will appear
much darker. This problem should be reported to husbandry personnel
immediately to allow correction.
Pelleted natural ingredient diets are used to feed all rodents and are
composed primarily of cereal grains which are supplemented with additional
protein, vitamins and minerals. Due to the nature of this type of diet the
exact composition can vary substantially from lot to lot. The water
provided to animals is chlorinated tap water.
Some rats are housed to prevent the acquisition of rodent pathogens.
Rodent pathogens often do not produce clinical signs in affected animals
but often have an immunomodulating effect. Therefore, this type of housing
is important for studies that involve the immune system. Immunocomprimised
animals, which are sensitive to opportunistic agents, are also housed in
this fashion to allow for their long term survival. These animals are
housed in sterilized cages and are provided sterile food and water. Access
to this housing area is limited to prevent inadvertent fomite transmission
of rodent pathogens by personnel.
Cage cards are utilized to identify the strain of rat, sex, number,
principal investigator, and research protocol. Cage cards should not be
removed from the cage to avoid misidentification of the animals. Temporary
identification of individual rats can be accomplished by pen marks on the
tail, hair clipping or dyeing the fur. Pen marks will only last 1-2 days
whereas hair clipping may last up to 14 days. Permanent identification
methods can be achieved by tail tattooing which will be performed by
husbandry personnel upon request (future picture). Tail and toe clipping
are not recommended. Ear punch
identification can be utilized but may be obliterated by fighting
between individuals (figure 2).
When handling rats it is advisable to wear latex gloves to prevent the
development of allergies due to direct contact with animal allergens. Rats
typically become accustomed to repeated handling. In a naive animal the
temperament of the animal can be determined by placing the hand into the
cage to allow exploration by the animal prior to touching. Initial gentle
stroking of the animal followed by gradual grasping the animal will
prevent startling the animal and initiating an aggressive response. Avoid
approaching the animal from the front.
Rats are normally lifted by grasping the whole body with the palm over
the back, with forefinger behind the head and the thumb and second finger
under opposite axilla. This extends the rat's forelimbs so that they may
be controlled. Holding with one hand is usually adequate
for control, but the tail, rear legs or lower part of body may be held by
the other hand for close control, treatment or examination (figure
3).
The use of both hands is often necessary for rats weighing over 350 gms.
Young rats may be
handled like mice when body size does not permit ease of handling
within the hand. Investigators should avoid lifting by the tail as they
may strip the skin from the tail. This is particularly likely for heavy
rats (>450 gms), rats that "spin," and when the tail is grasped more than
a couple of centimeters from its base. However, the
"base" of the tail may
be grasped with the thumb and forefinger (figure 4).
With this simple method of holding, they may be transferred to another
cage or a balance, identified, examined casually or sex may be determined.
For transporting short distances it may be helpful to support the rat with
your arm or hand while holding the tail (figure 5).
Rats will bite and certain strains are more aggressive than others
(e.g., F344 rats tend to be more aggressive than Sprague-Dawley), so care
and experience are essential to rapid handling. Various restraint devices are
available for use with rats (figure 6).
Neonatal rats can be handled from the day of birth but care should be
taken to carefully replace the newborn in the nest with the remaining
pups.
Rat colonies tend to be very labor intensive and are discouraged for
the production of commercially available rat strains. For strains not
available commercially or in instances where neonatal or fetal animals are
required breeding colonies are maintained. Investigators requiring the
establishment of breeding colonies should consult with the Animal Care
Unit to assure proper management of the colonies. Breeding animals have
different nutritional requirements, which if not provided, will have a
detrimental effect on reproductive performance. Light cycles are important
in breeding rats and are provided with 12 hours of light and 12 hours of
dark. Deviations from this cycle will effect reproductive performance.
Numerous types of restraint devices are commercially available to
restrain rats. Quality devices prevent the animal from turning around yet
allow easy access to the tail or legs. Devices should also be easy to
clean and provide adequate ventilation.
For tail vein injections in small rats, a wire box cage top can be
turned over and the tail gently passed through the wire bars
preventing the rat from turning (figure 7).
Male and female rats can be differentiated by observing the distance from the anus and genital
papilla which is greater in males (figure 8). This difference is also present in
neonatal rats.
Rats that are moved indoors can be transported in their cage after
removing the water bottle and placing it upright in the cage lid to
prevent spillage. Transport always results in some stress to the animal,
however, animals should recover from indoor transport within their own
cage within an hour. No recovery time may be needed if the animals are
moved with care and have become accustomed to routine transport. It is
recommended that a permeable drape be placed over the cages to darken the
cage and prevent over arousal of the rats during transport. Rats cannot be
transported out of doors or by vehicles by other than animal care unit
husbandry personnel. Investigators can request transfer to other buildings
by contacting the husbandry staff.
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