Biomethodology of the Rat

General Biology

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.

Rats can be grouped by their microbial colonization: Specific Pathogen Free (SPF) rats are free from known bacterial, viral, and parasitic rat pathogens, as opposed to "conventional" rats, which are not known to be free of pathogens. Standard housing (i.e. conventional housing) cannot totally prevent the introduction of rodent pathogens. To insure the maintenance of the SPF microbial status of rats requires that animals be housed in more stringent conditions that prevent the introduction of rodent pathogens (i.e. barrier housing). This type of housing is available at the University of Miami. Other less common categories are axenic rats (i.e free from organisms) and gnotobiotic rats, which have a defined microbial flora. Rats can also be categorized genetically. Common genetic categories are "random-bred" rats which are managed to maintain genetic diversity by mating unrelated rats; "Inbred" rats which are managed to maintain genetic homozygosity by breeding siblings; and "F1 hybrid rats" which are produced by crossing two inbred strains for one generation.

Sources and Ordering

Inbred, outbred, hybrid, or mutant rats are available from many commercial sources. The Division of Veterinary Resources personnel are familiar with many of the commonly available strains and will assist in locating animals available for purchase. Typically, for commercially available rats, animals ordered by Wednesday will arrive the following week. Most animals will recover from the stress of shipping within 48 hours.

Behavior

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.

Biological Data

Adult body weight: male

450-520 gm1

300-800 gm2

Adult body weight: female

250-320 gm1

250-400 gm2

Body surface area

10.5 (wt. in grams)2/31

9.1 kg0.66 B.W.2

Life span

2-3.5 years1

Food consumption

10 g/100 g/ day1

Water consumption

10-12 ml/100 g/day1

Breeding onset: male

65-110 days1

Breeding onset: female

65-110 days1

Gestation Period

21-23 days1

Body Temperature

38-39 C2

35.9-37.5 C1

Heart rate

320-480 beats per minute2

250-450 beats per minute1

Respiratory rate

85-110 per minute2

70-115 per minute1

Tidal volume

0.6-2.0 ml1

1.6 (1.5-1.8 ml)2


Basic Husbandry

Most rats are housed in shoebox cages composed of polypropylene (opaque)(future picture) or polycarbonate material (clear)(picture ) with a wire bar lid used to hold the water bottle and feed. 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 (picture ) to prevent spillage during transport. However, you should remember to turn the bottle back over to allow access to water after transport. 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.

Identification

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.

Handling

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 (future picture). 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 (picture ). 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 (picture ). 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 (picture ).

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 (picture ).

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.

Breeding

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 DVR 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.

Restraint Devices

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 (picture ).

Sexing

Male and female rats can be differentiated by observing the distance from the anus and genital papilla which is greater in males (picture ). This difference is also present in neonatal rats (future picture).

Transport

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 (picture ). 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 (2443-6802).

References

1) Harkness JE, Wagner JE 1989, The Biology and Medicine of Rabbits and Rodents; 3rd Edition , Lea and Febiger, Philadelphia

2) Poole TB 1987, The UFAW Handbook on the Care and Management of Laboratory Animals , 6th edition. Longman Scientific & Technical: England