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Injection Sites

Intraperitoneal (i.p.) injections should be administered in the lower right quadrant of the abdomen to avoid vital organs. The animal's head should be tipped downward to allow organs to fall out of the way. Only the tip of the needle should be introduced as penetration of the abdomen too deeply may result in puncture of the intestines.

If the intestine is punctured fluid will often be observed leaking from the rectum. The smallest needle possible should be used, and will depend on the viscosity of the agent to be injected. A maximum of 5 - 10 ml may be administered to rats by this route. This is the most common method of administering drugs and anesthetics to rats. (Intraperitoneal injection using the Camilla restraint method ).

Subcutaneous (s.c.) injections are given under the skin and may be administered dorsally or ventrally. The most common location is under the skin of the back or sides. The skin over the back should be tented while the needle is introduced at a shallow angle to the skin.

A successful s.c. injection results in the formation of a small bleb in the skin if the injection volume is 0.5 ml or more. (Subcutaneous injection using the Camilla restraint method ).

Intradermal (i.d.) injections are administered within the skin. The usual sites for i.d. injections are the skin over the thorax or abdomen. The area must be prepared by removing the hair with clippers. The skin should be held taught while a small (23 gauge or less) needle is introduced at an acute angle to the skin. The needle should be pushed into the skin for 2 - 3 mm before injecting, at which time resistance to the passage of the needle will be felt. If no resistance is encountered the needle is probably in a s.c. location instead. Only a small volume can be injected i.d. (50 - 100 Fl) and will result in the formation of a small bleb at the site.

Intramuscular (i.m.) injections are most commonly administered in the muscles of the hind limb (biceps femoris,

semitendinosus, or gluteus maximus

). The injection should not be too deep to avoid hitting the femoral nerve or bone, and no more than 0.3 ml should be injected into a muscle site. Extremely irritating substances should not be injected into the muscle as it can result in self-mutilation of the limb if muscle or sciatic nerve damage occurs. (Intramuscular injection using the Camilla restraint method ).

Intravenous (i.v.) injections are feasible for a number of sites in the rat. Because all of these veins are relatively small, considerable practice is required to gain proficiency in i.v. injection techniques. The tail or caudal vein may be used for i.v. injection in young rats but may be difficult to use in older animals due to the thickness of the tail skin.

A tail vein is present at 90° on either side of the central tail artery. Warming the tail with water or a heat lamp induces vasodilation and helps in visualization of the vessel (use care not to overheat the rat). Use a 23 - 25 gauge needle to inject and start distally on the tail in relation to the heart. A “flash” of blood into the hub of the needle may or may not occur when the needle enters the vein; likewise, aspirating the needle may not reveal the presence of blood even though the needle is in the correct position. Injection of the contents will be easy if the needle is intravascular, and formation of a bleb indicates that the needle is extravascular. After injection, apply pressure with a piece of gauze until bleeding has stopped.

Injection may be made into the two sublingual veins located on the ventrolateral aspects of the tongue. The rat must be anesthetized for this procedure, and is laid on its back with its head toward the worker. The tongue is pulled out with forceps and then held at the tip with the thumb and first finger to produce a slight convex curvature. The veins are superficial and must be entered almost horizontal to the tongue surface. It may be helpful to dry the surface of the tongue with a piece of gauze or the needle may slide when injection is attempted. A 25 gauge or smaller needle should be used. Adequate pressure should be applied to the injection site to prevent the formation of hematomas and to ensure clotting.

The penile vein may be used in male rats and also requires anesthesia. The rat should be placed on its back, and the glans penis extruded by sliding the prepuce downwards and pressing the base of the penis. The penile vein is visualized as a distinct central vein or as a diffuse, blue sinus. After injection pressure should be applied to the site to effect hemostasis.

The femoral vein may be used for injection but it requires a cutdown for access to the vessel, which represents a minor surgical procedure. Other vessels which may be accessed for i.v. injections include the jugular, lateral saphenous, and dorsal metatarsal veins.