At times, different experimental approaches necessitate the use of less commonly employed routes of compound administration in rodents. Intradermal, intranasal, and intracranial are three of such alternate routes that the biomedical researchers use in labs today.
As their name suggests, intradermal is delivering compounds into the outer layers of the dermis. Intranasal is placing the solution in the animal's nares. And intracranial involves inserting the needle directly into the rodent's brain.
Specialized training is essential to perform these procedures successfully. Here, we'll first illustrate the considerations for each of these methods and then we will demonstrate the techniques that will help you learn the procedures while ensuring safety of the animal and success of the experiment.
Let's begin with the discussion of when these routes are usually employed and things one should bear in mind before beginning to perform these specialized administration techniques.
The intradermal injections are used to deliver an article into the space between the epidermis and the dermis This route is usually reserved for the assessment of inflammation, cutaneous blood flow diagnostics, or allergenic reactions to an antigen. Similar to other routes, the intradermal solution should also be prepared using the sterile technique. And it must be physiologically buffered to have a neutral pH in order to avoid tissue necrosis at the injection site. A hub-less system with a 25-30 gauge needle is often used for this injection. This system helps in preserving the volume of administration, which is in the range of 50-100 microliters per injection site. Injecting excess can result in necrosis or undesirable compound leakage due to pressure.
Intranasal route is often chosen for local delivery of vaccinations or decongestant spray as well as systemic and CNS delivery. The mucosa that lines the nasal cavity has a rich supply of blood vessels and nerves that allow for rapid systemic absorption and direct targeting to the CNS. This is a non-invasive method that requires minimal training and skill, and simple equipment - a calibrated micropipette and some disposable tips. The administration volumes for rats should not exceed 40-100 microliters given in 6-10 microliter-drops. And for mice, the maximum total volume is 24 microliters given in 3-4 microliter-drops.
Although anesthesia is not required for this procedure, it has some advantages over intranasal administration in conscious animals 1) it facilitates proper placement of the compound at the nares, ensuring accurate dosing 2) eliminates the possibility of the animal biting the dosing equipment 3) ensures that there is no injury to the animal's nasal tissue, eyes, or facial skin due to jerking of the head, and 4) animal is less likely to snort and spray the compound from the nares upon administration.
Intracranial injections in adult mice and rats employ the use of stereotaxic equipment, which is described in a video in the "Essentials of Neuroscience" collection. The equipment assures proper positioning and correct depth of injection. Here, we will focus on intracranial delivery in neonatal mice and rats in whom the skull is thin enough to inject directly through it, and may be too fragile to support the stereotaxic device. The primary purposes of this technique are to deliver CNS pharmacologic agents directly into to the CNS, and to avoid the effects encountered via any systemic route. The needle gauge, length and volume of administration are determined based on the species and the age of the pups. Note that as the animal's age increases, the gauge number decreases, the required needle length increases, and the maximum recommended administration volume also increases.
With this background information in mind, let's delve into the procedures of these injection methods. First up is the intradermal administration technique. This procedure has to be performed in anesthetized animals. Review another video in this collection to understand the procedures for anesthesia induction and maintenance.
Once the animal is anesthetized, shave the injection site using an electric razor or a depilatory cream. With a water-dampened gauze, thoroughly remove the lingering hair from the site. Then, with another gauze pad, apply a topical antiseptic solution to the shaved area. For administration, first stabilize the skin at the injection site by stretching it between your thumb and index finger.
Now place the needle bevel up on the skin and gently insert it just beyond the bevel so that the opening is between the epidermis and the dermis layers. Then inject slowly and note that it creates a bleb in the skin. If the needle is inserted too deeply then no bleb will be formed. After injecting, pause to allow the skin to stretch and adjust, and then withdraw the needle slowly. Do not pull back on the plunger at anytime, as you would draw up the tissue and cause trauma at the injection site. Also, do not wipe or blot the injection site, as this may cause the injected substance to leak. When performing multiple injections, be sure to space them apart wide enough so that the blebs do not overlap with one another.
Next, let's learn the intranasal administration procedure in conscious and anesthetized animals.
For awake animals, restrain them by scruffing the skin at the nape of the neck and then hold the animal in a vertical position with its head immobilized. Be careful not to constrict the chest as that might impede the animal's ability to take sufficiently deep breaths to draw the liquid into the lungs. Using a micropipette, administer part of the solution by placing a small drop of liquid at the nasal opening. The animal will inhale the droplet. Repeat this process, alternating between the two nasal openings until the entire volume to be administered has been given. Recall - the total volume of administration should not exceed 24 μl and 100 μl in mice and rats, respectively.
For anesthetized mice and rats, place the animal in a dorsal recumbency position. This position is ideal for CNS delivery as it allows for better absorption of the compound. Rotate the animal's head and administer half of the compound directly into one side of the nasal opening, timing it with inhalation. Then, rotate the animal's head to position for next administration. After 2 breaths or so, administer the remaining volume into the second nasal opening. After complete administration, return the animal back to its cage.
Next, let's review the intracranial administration procedure for neonatal mice and rats. Prior to beginning the procedure, place the cage with the pups and the dam on an electric heating pad set to low. Make sure that a part of the cage is off of the heating pad. This is to prevent hypothermia and, at the same time, allow the dam to move away from the heat if she desires. Next, select a needle gauge appropriate for the age of the animal. Recall, the needle gauge; needle length, which is used to control the depth of the needle during the intracranial injection; and the volume of administration...all vary with animal's age and species.
The length is adjusted using a guard. To prepare this guard, measure the correct needle against its cap and make a mark. Next, place a second mark on the cap to indicate where it will be cut. The distance between the two marks is the desired needle length. Then, cut the cap with a razor blade. Do not use scissors as they will crush the cap and will not produce a clean level cut. This is the "needle guard." Dispose the needle used for creating the guard, as it is no longer sterile, and instead insert a new needle into the guard and ensure that the correct length is exposed. Next, using a different needle attached to the suitable syringe, draw the injection substance. A different needle is used to do this, because placement into the stopper will significantly dull these fine gauge needles, which is not ideal for intracranial administration. Then, place the filled syringe on the needle with the guard. Now the system is ready for an injection.
For pups over 10 days of age, administer inhalation anesthesia. Pups younger than 10 days old need not be anesthetized. To perform the injection, first locate the site, which is 5 mm behind the eye and approximately 3 mm off the midline of the skull. Next, insert the needle to the depth allowed for by the needle guard. Then, inject in a slow steady manner to avoid trauma to the brain. Remove the needle immediately and with great care to prevent injury to the brain tissue. Finally, place the animal back in with the dam to allow for proper recovery.
Now let's review some experiments being conducted in labs today that utilize these uncommon routes of administration.
An intradermal injection is often used to study skin inflammatory reaction. In this experiment, the researchers used this method to inject an allergen into one ear and a neutral substance into the opposite ear of a pre-sensitized mouse. Next, they delivered a blue dye into the animal's circulatory system to examine the changes in vascular permeability due to allergen injection.
As mentioned earlier, one of the applications of intranasal administration is to administer vaccines. Here, scientists used this route to deliver a genetically modified, live attenuated influenza vaccine into the wild type and transgenic mice and studied mucosal immunity via production of a specific type of T-cells.
Lastly, these biomedical researches used intracranial administration to implant cancer cells in immunocompromised mice, in order to create a human brain tumor model. The efficacy of the injection was then analyzed using an in vivo imaging system.
You have jus watched JoVE's video on some of the special methods of compound administration in laboratory mice and rats. You should now understand when these procedures are helpful, the considerations you should bear in mind before and while performing these techniques, and the essential procedural steps to ensure that the administration has minimal impact on the animal's health and on the experimental data to be collected. As always, thanks for watching!