Subcutaneous injection (s.c.)
It is possible to inject under the skin when holding the animal in a neck-skin hold: either from the front at the base of the skin fold between the thumb and index finger (be careful not to stick your fingers), or beneath the lateral skin in front of the hind leg. The needle must be introduced very obliquely beneath the lateral skin, and in order to ensure the correct location of the needle tip you can lift the skin slightly with a sideways movement of the needle before injection.
A mouse placed on a surface may also be injected above the pelvis. The mouse should be pulled slightly backwards by the tail so that it will remain still. The tail should be held with the other fingers, while thumb and index finger are used to lift the skin on the pelvis. With the other hand, the needle will then be introduced under the skin fold. This technique requires some more practice and it may be difficult especially if the mouse puts up a lot of resistance.
When the injected substance enters the correct location, there is no resistance in the plunger, and the skin may exhibit a bump depending on the point and amount of the injection.
Usually a rat is injected beneath the neck or back skin, but it is possible to inject also beneath the lateral or abdominal skin. Injecting a non-sedated rat is easiest if an assistant is holding the rat. The skin must be collected and lifted into a ”tent” and the needle inserted in the “pocket” of the tent at a 20–30° angle. Instead of pushing the needle, you can also pull the skin over the tip of the needle.
Intramuscular injection (i.m.)
It is not recommended to give mice intramuscular injections due to the small size of their muscles, unless it is necessary for research-related reasons.
IM injections are not recommended for rats either, but their muscles are bigger, and if there is a special need for an intramuscular injection, the front thigh muscles can be used (Musculus quadriceps femoris). At the back of the thigh, there is a risk of hitting large vessels or the sciatic nerve. An assistant holds the rat, and the injector holds the thigh muscle with one hand between the thumb and index finger. Before the injection, you should pull slightly on the plunger to make sure that the needle is not inside a vein.
Intraperitoneal injection (i.p.)
An i.p. injection is made into the right lower quarter of the abdominal cavity. It has been recommended in related literature that the animal be kept head downwards, in which case the organs will be pressed more cranially in the abdominal cavity, ”out of the way”. In practice this does not matter much, as long as the procedure is performed correctly. The needle should be injected from the caudal direction, at a slight angle to the abdominal wall, with a quick and firm movement, but only so far that the tip just barely pierces the abdominal wall. Before the injection you should slightly pull on the plunger: if any fluid enters the syringe, you must inject again with a clean needle and syringe.
Regardless of how skilfully the procedure is done, i.p. injections have a certain risk of failure. Therefore other injection routes are recommended, unless there is a specific need to inject into the abdominal cavity.
Intradermal injection (i.d.)
An i.d. injection is not recommended unless it is necessary for research-related reasons. Intradermal injections can be done on the back or neck or beneath the abdomen. The animal must be under anaesthesia for the injection. The injection area should be shaved. The needle must be very thin and the amount of liquid small. Pull the skin tight with your other hand, and introduce the needle at a very slight angle right beneath the surface layer of the skin. You can feel clear resistance both when inserting the needle and injecting, and a hard bump will appear at the point of the injection. The needle should be pushed some way inside the skin before injecting, and the puncture point pressed with a finger when taking out the needle, to prevent leaking out of the fluid.
Intravenous injection (i.v.)
Injection into the tail vein
The most common place for an i.v. injection is the lateral tail vein. The injection is difficult especially in mice due to the small size of the vein, and this technique requires practice. Warming the tail or the whole animal dilates the tail veins and significantly facilitates the procedure. Good lighting is important. Injecting an awake animal requires an immobilisation device.
The tail is pulled straight so that it is crossways on the index finger, and the thumb presses the tip of the tail behind the index finger. The tail may also be on the edge of a table. The needle should be held parallel to the tail and inserted into the vein at the exact point where the tail starts to bend downwards. The needle bevel should be pointed upwards. The needle is inserted 2-4 mm inside the vein. If the needle is correctly placed inside the vain, no resistance can be felt in the plunger when injecting and you can see through the skin how the injected substance advances in the vein. After the injection is finished, the normal colour of the vein returns when the blood circulates again. You must press your finger on the point of injection when pulling out the needle, and for a moment afterwards, to prevent bleeding.
Other possible IV injection points are the dorsal metatarsal vein (on the dorsal side of the tarsus), the jugular, femoral, sublingual and penile vein.
Intragastric administration or gavage
For the gavage procedure, you should grab the mouse into a firm neck-tail-hold so that the head in particular is as immobile as possible. Optimally the neck would be straight so that the mouth, pharynx and oesophagus form a straight line, but gavage can be done also if neck is not quite straight. Reaching the correct position can be facilitated if the neck-hold is made with your thumb and middle finger, and the index finger is used to turn the head up from the crown skin.
The mouse should be held in an upright position. The ballpoint probe is carefully inserted into the mouth, and the tip advanced along the palate to the back of the mouth. At this point the probe should be in a parallel line with the oesophagus. The probe must not be pushed down the pharynx with force! When the probe is correctly placed, it will go down easily when the mouse swallows. If the mouse is upright, gravity is enough to advance the probe into the stomach, without any pushing.
A rat should be held around the thorax so that the thumb or the index finger can be used to press the lower jaw, or alternatively the neck and back skin. The probe is inserted into the mouth between the teeth and across the tongue into the pharynx.
Dipping the probe in sugar water right before the oral gavage will facilitate and accelerate the procedure. The animal will resist less and swallow the probe more easily when it tastes sweet.
Possible complications are gavage into the lungs, tearing the oesophagus, and damage to the pharynx. No unnecessary force must be used in the procedure. The animal should be allowed to swallow the probe without forcing it down. A probe of an appropriate size will easily be swallowed up to the base, the tip all the way down to the stomach; in the trachea the probe will not travel as far. When the probe travels through the trachea, your fingers may feel a “vibration” caused by the tracheal rings. If the probe does not travel easily deep enough or the animal starts to cough, gag or otherwise resist once the tip has passed the pharynx, the probe must be pulled out. After a successful gavage the animal will behave normally.
Last updated: 5.1.2017