Intraperitoneal (IP) injection in mice is a widely used method in scientific research for administering various substances. This technique involves injecting a substance directly into the peritoneal cavity, which is the space within the abdomen that contains the internal organs. Researchers frequently employ this route for a range of experimental purposes, allowing for the systemic distribution of compounds throughout the mouse’s body.
What is Intraperitoneal Injection?
Intraperitoneal injection refers to the administration of a substance into the peritoneal cavity, a large, membrane-lined space within the abdominal region that surrounds organs like the liver, spleen, and most of the gastrointestinal tract. This cavity is a potential space filled with a small amount of fluid. Its lining, a serous membrane, has a large surface area with a rich network of blood and lymphatic vessels.
This extensive vascularization facilitates the absorption of injected substances into the systemic circulation. It is used in laboratory settings for various research purposes, including drug delivery, cell transplantation, and imaging agent administration. This route allows substances to enter the bloodstream without direct intravenous injection.
Why Intraperitoneal Injections Are Used
Researchers choose intraperitoneal injections for several reasons, including rapid systemic absorption. The peritoneal cavity’s large surface area and extensive vascular supply allow for quick uptake into the bloodstream, which is beneficial for studies requiring a prompt onset of drug effects.
Another advantage is the ability to administer larger fluid volumes compared to other routes like intravenous (IV) or subcutaneous injections. For instance, a 25-gram mouse can receive up to 0.25 ml via IP injection, equating to 10 ml/kg. IP administration can also bypass first-pass metabolism in the liver, which often reduces the bioavailability of orally administered drugs. This ensures more of the substance reaches systemic circulation in its active form.
Considerations for Intraperitoneal Injections
Performing intraperitoneal injections requires careful technique to avoid complications. Improper needle placement can injure internal organs like the bladder, intestines, or major blood vessels. If the bladder is punctured, yellowish fluid may appear in the syringe; greenish-brown fluid suggests intestinal penetration. Such events require discarding the syringe and starting over with a new, sterile needle and substance.
Complications also include peritonitis, an inflammation or infection of the peritoneal cavity, which can occur if the injection is not sterile or if irritating substances are administered. Using a new, sterile needle for each animal and ensuring the substance is sterile and warmed to body temperature minimizes discomfort and infection risk. The substance’s pH or irritating properties can cause pain during and after injection; for instance, a pH not close to neutral can irritate the peritoneal lining.
Ethical Oversight and Animal Welfare
The use of animals in research, including intraperitoneal injections, is subject to strict ethical oversight. Institutional Animal Care and Use Committees (IACUCs) ensure humane treatment and adherence to regulations like the Animal Welfare Act and Public Health Service Policy. These committees review and approve all animal research protocols, balancing potential research benefits against potential harm to animals.
Ethical animal research follows the “3Rs” principle: Replacement, Reduction, and Refinement. Replacement encourages using non-animal methods or less sentient organisms when possible. Reduction minimizes the number of animals used without compromising statistical validity. Refinement improves techniques and husbandry practices to minimize animal pain, distress, and suffering. These principles guide researchers in designing scientifically sound and ethically responsible studies.