Irradiation of Rodents

Introduction

Boston University is committed to observing federal guidelines and AAALAC international guidelines for the humane care and use of animals.

This policy deals with ionizing irradiation of mice and rats exposed to gamma irradiation emanating from a 137cesium source. Experimental reasons for exposing rodents to ionizing radiation are used in the fields of immunology and cancer biology. Irradiation to produce myeloablation preparatory to bone marrow transplantation (BMT) is frequently used. BMT procedures are addressed elsewhere in a separate IACUC policy.

Definitions

A. Gamma irradiation is one type of ionizing irradiation. The source may be 137cesium, 60cobalt, or high-energy X-rays. This policy refers to the 137cesium irradiator at BUMC.

B. Gray (Gy) is the SI unit of absorbed radiation dose due to ionizing radiation.

C. Rad is a largely obsolete unit of absorbed radiation dose. It is now superseded in the SI by the “gray”. 100 rads = 1 gray (Gy).

D. Fractionation of dose: The total irradiation dose is split into two or more equal parts separated by a time interval in order to minimize morbidity and mortality.

Post-Procedure Monitoring Form for Rodents

Policies for Animals on Studies Exposed to Irradiation

A. Animals exposed to irradiation must be monitored and findings documented daily for the first 14 days. However, after protocols have been established and the principal investigator (PI) has experience with the particular strain of mice and dose, three/week monitoring and documentation is acceptable. If animals experience morbidity or mortality, daily checks are required. Procedures for care of irradiated rodents must be observed.

B. Irradiation must be scientifically justified in the IACUC protocol.

C. The planned dose of irradiation, or dose range, must be specified.

D. Fractionated doses may be useful in certain studies if practical, since animal morbidity and mortality will be lower.

E. Unless literature references are available, a pilot study to determine the best dose is recommended if the PI is starting a new study or using a new strain of mice.

F. If control animals (animals that are lethally irradiated but not reconstituted with bone marrow) will be used, the PI must justify the use of these animals, use as few animals as possible, use historical controls as much as possible, and comply with the Humane Endpoints Policy.

G. The IACUC will place non-reconstituted, lethally irradiated control rodents with death as an endpoint in USDA Category E. Scientific justification of unrelieved pain and distress or potential pain or distress of lethal radiation doses is required.

H. Prior to starting an irradiation experiment, the PI is required to submit a form to Boston University Animal Science Center (BU ASC): Intent to Irradiate Rodents (request for supplies needed for post-irradiation animal care).

Effects of Whole-Body Irradiation

Ionizing irradiation causes breaks in the DNA double-strand, thus it mostly affects mitotically active cells. This damage leads to cell death through either necrosis or apoptosis, primarily affecting the cells of the hematopoietic and gastrointestinal tract (Duran-Struuck, R. and Dysko, R, 2009).

A. Weight loss: Lethargy, inappetence, diarrhea resulting in anorexia, and body weight loss, which peaks at about seven days post-irradiation. Depending on the dose and whether immune reconstitution had been provided, recovery will usually occur in 2–3 weeks. Mice may never regain the body weight they had before irradiation.

B. Anemia

C. Infection: Severe bacteremia may occur as a result of translocation of bacteria from the GI tract (Duran-Struuck, R. et.al., 2008).

D. Intestinal bleeding

E. Transplant failure: Successful survival of a bone marrow graft requires suppression of the host’s immune system. If the irradiation dose has been too low, Graft-versus-host Disease (GVHD) will ensue. As in humans, older mice are more prone to develop GVHD.

F. Graying of hair coat: Black mice, such as C57BL, will frequently turn gray after irradiation.

G. Development of secondary neoplasias: The development of neoplasia after irradiation has been reported in humans and many large animal species. This may occur in mice on long-term studies as well.

H. Incisor damage: One non-neoplastic illness reported in mice is incisor damage and subsequent difficulty in eating. Giving softened food during the recovery phase is required.

Doses of Irradiation

There are numerous literature references reporting differences in sensitivity in radioresistance or radio susceptibility to radiation damage in different strains of mice. If the PI is unfamiliar with the strain to be used or the radiation source, it is advisable to irradiate a small number of animals in a pilot study to determine to optimal dose for the project and strain under study. The age of the mouse is important as well, older mice being more resistant. For myeloablation in preparation for BMT, total whole-body irradiation (TBI) is used. Doses of 700 to 1300 cGy are myeloablative (Duran-Struuck, R. and Dysko, R., 2009). Mice of the C57BL strain are more resistant to irradiation damage, whereas BALB/c mice are more sensitive.

Table 1
Published doses of irradiation for various rat and mouse stocks and strains
(2X indicates fractionated doses are given three hours apart)

Mouse Strain

Protocol

Dose (Gy)

Response to Dose

Reference

C57BL TBI and
BMT
2X 5.5 Survival The Mouse in Biomedical Research. Vol 3. Fox J.G et. Al. Eds. 2nd Ed. 2007. Academic Press. ACLAM Series. P. 453.
C57BL Hematopoietic ablation 3.5–6.0
2X 5.5 G
UCSF IACUC Policy
Source: 60Co irradiator
C57BL/6BALB/c

B6CF1

LD50/6
LD50/30
Source
60Co
0.8 Gy
1.4 Gy
The F1 hybrid is similar in radiosensitivity to the C57BL/6 and different from the BALB/c Hanson et. al. 1987. Comparison of intestine and bone marrow radio sensitivity of the BALB/c and the C57BL/6 mouse strains and their B6CF1 offspring. Radiat Res 110:340–352.
FVB/NC57BL/6J

Rag1-/-iNOS

FVB/N Tie2-GFP

TBI and BMT Source 137Cs
10–22 Gy
Germ-free (GF) mice were much less sensitive to radiation enteritis than conventional mice Crawford, PA and Gordon, JI Microbial regulation of intestinal radiosensitivity. PNAS 102(37); Sept. 13, 2005.
A/JC3H/HeMs

C57BL/6J

C.B.17/lcr-scid

C3H-scid

Source 137Cs10 – 60 Gy10 – 50 Gy10 – 50 Gy Skin sensitivity Long-term damage
Mild damage Less sensitive than A/J
Severe skin damage
Severe skin damage
Iwakawa, M. et. al. 2003. Different radiation susceptibility among five strains of mice detected by skin reaction. J Radiat Res (Tokyo) 44: 7–13.
C57BL/6NCrB6.Cg-

Ptpcrca/NCr(Ly5.2)

TBI and BMT More sensitive and more prone to bacteremia Less sensitive Duran-Struuck, R. et.al. 2008. Differential susceptibility of C57BL/6NCr and B6.Cg-Ptprca mice to commensal bacteria after whole-body irradiation in translational bone marrow transplantation studies. J Transl Med: 6:10

 

Rat Stock

Protocol

Dose (Gy)

Response to Dose

Reference

Wistar rats M 100g TBI 14.4 Gy Increased serum Corticosterone 60 minutes and 3 days after exposure. Rats began to die on day 5 post-exposure. Kassayova, E. et.al. 1999. Two-Phase Response of Rat Pineal melatonin to Lethal Whole-Body Irradiation with Gamma Rays. Physiol. Res. 48:227–230.
Wistar rats M 100g TBI 9.6 Gy Rats began to die on day 10 post-exposure. Same
Wistar rats M 100g TBI 4.8 Gy Same

Procedures

Selection and preparation of mice
Animals to be lethally irradiated should be healthy and free from infectious agents, including Pseudomonas aeruginosa (Fox et al., 2007). Mice are tested for P. aeruginosa at the vendor and at the annual sentinel surveillance at BU ASC. The drinking water is tested for P. aeruginosa annually as well. Immunocompetent mice are not made sick even though they may carry commensal microbial agents. After exposure to TBI, mice are immunocompromised and subject to infections not previously clinically important. If post-irradiation infections occur and interfere with recovery of the animals, this possibility should be taken into consideration.

Irradiating the mice—preparation
A. Purchasing the pie cage: Each investigator is required to purchase their own pie cage. The type recommended is a pie cage with filters which holds up to 20 mice and which can be autoclaved between uses. Pie cages must be sanitized between uses.

  • These may be obtained from Braintree Scientific. The dimensions of the irradiator slot is circular with a diameter of 12 inches and a depth of 4 inches. There is plenty of space for either type of pie cage used for mice or rats.

B. Scheduling: The PI must call the Radiation Protection office (RPO) at 617-638-7052 to schedule a time. If the dose is to be fractionated, this must be specified and time must be scheduled for three hours later as well.

C. The approved IACUC protocol number is given to the RPO staff.

D. The RPO staff sets and runs the irradiator.

E. Dose: The PI must provide the RPO staff with a suitable dose required for their study.

F. Time of exposure: Animals are generally irradiated for a short period of time (<15 minutes). The amount of time spent inside the irradiator varies depending upon the dose and the radioisotope decay charts.

G. Rodents are restrained in a rotating pie-shaped holder (to limit mobility and ensure equal doses of irradiation). This may be done in the PI’s laboratory in a biosafety cabinet (BSC), or in the rodent housing room in a BSC, and the rodents then transported to the irradiator inside the pie cage. Alternatively, rodents are transported to the irradiator in their home cage and transferred to the pie cage once inside the irradiator room. This may be necessary if more rodents are irradiated than can be contained inside one pie cage.

H. Rodents may or may not be anesthetized for irradiation.

I. Transporting the mice to the RPO: The IACUC guidelines for Transportation of Rodents must be followed with the proviso that mice may be transported inside the pie cage.

J. Rodents can only stay in the pie cage for a maximum of one hour.

Use of the RPO irradiation room
A. The PI is responsible for bringing the following items:

  1. Pie cage and rodents

B. The following items are available in the irradiator room:

  1. Chucks (diapers) to cover the work counter
  2. Paper towels
  3. MB-10
  4. Trash bags
  5. PPE as needed. If mice are handled, the minimum required PPE include:
    • Disposable gown
    • Surgical mask
    • Gloves
    • Bonnet

Procedures for handling rodents in the irradiator room
A. If animals are transferred from home cage to pie cage in the irradiator room, the PI or research staff are required to first don PPE.

B. After the irradiation is completed, the PI or research staff are required to sanitize the counter and place all trash in a trash bag and remove the trash bag from the irradiator room, take it back to the BU ASC, and dispose of it according to BU ASC policy.

C. If animals are transported in the pie cage, it is not necessary to don PPE or sanitize the room.

Irradiating the mice
A. After the pie cage is loaded, it is placed in the irradiator.

B. Mice must be returned to the animal housing room or laboratory as soon as the irradiation is completed.

Care of Irradiated Mice

A. After irradiator exposure, mice must be placed in a cage and kept clean and quiet. Sterilized caging, food, and water is recommended. The PI must order these from BU ASC prior to the start of irradiation.

B. The PI or designated research staff are responsible for caring for the irradiated animals. BU ASC staff will provide special care in an emergency and in such a case the PI will be charged accordingly. For the first 14 days, animals must be checked daily, or three times a week, their condition and care documented on the rodent post-procedure monitoring sheet. Care, especially during the first week, must be dedicated to making animals as comfortable as possible, keeping them clean and quiet, and making sure that they stay hydrated and have ready access to moistened food and Napa Nectar if indicated. Please refer to the section Policies for Animals on Studies Exposed to Irradiation above.

C. Use of antibiotics in the drinking water
The PI is responsible for placing rodents on SulfaTrim antibiotic water a few days before the planned irradiation in order for the animals to get used to the taste. Rodents are kept on the SulfaTrim water for at least 14 days and up to 28 days post-irradiation.

D. Making drinking water readily available
Irradiated mice will suffer from irradiation sickness and will not feel well for the first 7–14 days. They may lose up to 25% of their body weight, which in the case of a successful bone marrow transplant will be mostly regained by day 14–21 post-irradiation. It is important to provide easy access to water.

E. Napa Nectar must be provided on the bottom of the cage during the first 14 days if morbidity or mortality is seen. Napa Nectar is available in the animal room free of charge. Placing a new Napa Nectar in the cage daily can be done by the PI or research staff or can be done by BU ASC for fee for service. The Napa Nectar becomes contaminated with fecal material and must be replaced daily.

F. Provision of softened food
Giving softened food during the recovery phase is required. Powdered chow is available in the rodent housing rooms and should be mixed with water and served in a small petri dish on the cage floor. Pellets moistened with water dry up easily and are not recommended. Placing a new moistened food dish in the cage daily must be done by the PI or research staff or can be done by BU ASC for fee for service.

G. Housing
It is important to realize that even after bone marrow transplantation, lethally irradiated mice are severely immunosuppressed for the first two weeks and providing a totally sterile housing environment is recommended (Fox et al. 2007, & Duran-Struuck, R and Dysko, R., 2009). Sterile caging, food, and water is strongly recommended and is required if post-irradiation complications occur.

Humane Endpoints for Irradiated Rodents

In the case of BMR, transplanted mice undergo a 5–10 day irradiation sickness period from which they generally recover within 14 days (Duran-Struuck, R and Dysko, R., 2009). Depending upon radiation dose, they may lose as much as 25% of their body weight by day seven, but if successfully reconstituted will regain most of this weight during the following week. If mice have not received a bone marrow transplant they will remain sick and/or die, the time of death depending upon the dose.

A. If animals are not well on their way to recovery by day 14, considerations should be given to euthanasia. Consultation with veterinary staff is highly recommended. Moribund animals must be euthanized.

B. If not recovered by day 21 post-irradiation, animals must be euthanized.

References

1. The Mouse in Biomedical Research. Vol 3. Fox J.G et. Al. Eds. 2nd Ed. 2007. Academic Press. ACLAM Series. Pp. 453–454.
2. Duran-Struuck, R. and Dysko, R. Principles of Bone Marrow Transplantation (BMT): Providing Optimal Veterinary and Husbandry Care to Irradiated Mice in BMT Studies. JAALAS 48(1): 2009. 11–22. Highly recommended.
3. APHIS Policy 11. Painful procedures.
4. The Guide for the Care and Use of Laboratory Animals. NRC. ILAR. 2011. P.12.
5. Boston University IACUC Policy for Humane Endpoints.
6. Duran-Struuck, R. et.al. 2008. Differential susceptibility of C57BL/6NCr and B6.Cg-Ptprca mice to commensal bacteria after whole-body irradiation in translational bone marrow transplantation studies. J Transl Med: 6:10.

BU ASC SulfaTrim (Septra) Doses and Dilutions
Revised April 2008

SulfaTrim stock solution (oral suspension):
200mg Sulfamethoxazole and 40mg Trimethoprim/5ml = 240mg SulfaTrim/5ml

Mouse drinks 15ml/100g/24 hours = 150ml/kg/24 hours

Rat drinks 10ml/kg/24 hours = 100ml/kg/24 hours

For Mice:
3.5ml stock → 250ml water = _3.5 x 240mg __ = 0.6720mg/ml drinking soln.
5ml x 250ml

Mouse dose = 0.6720mg x 150mg = 100.8mg/kg ~ 100mg/kg/24 hours POml x kg x 24 hours

For Rats:
2.78ml stock → 250ml water = _2.78 x 240mg __ = 0.5358mg/ml drinking soln.
5ml x 250ml

Rat dose = 0.5358mg x 100mg = 53.58mg/kg ~ 54mg/kg/24 hours POml x kg x 24 hours

BU IACUC approved May 2009, Revised January, 2014