Induction of Neuropathic Pain in a Rat Model by Constriction of the Infraorbital Nerve

Secure an anesthetized rat with a shaved head in a stereotaxic frame. Apply ointment to the eyes to prevent drying and cover with a surgical drape

Disinfect the head and incise to expose the skull.

Using a dissection microscope, visualize one of the eye orbits, the bony cavity in the skull that houses the eye.

Deflect the surrounding soft tissues to expose the infraorbital nerve, which transmits sensory signals from the face to the brain.

Tie two ligatures around the infraorbital nerve to constrict it and induce nerve damage.

Suture the scalp incision and allow the rat to recover.

Following the nerve constriction, damaged neurons release signaling molecules that recruit immune cells.

These immune cells release mediators that bind to nearby intact nociceptors, sensory neurons that transmit pain.

This mediator binding sensitizes the nociceptors, increasing neuronal excitability and generating pain signals that travel to the brain, contributing to neuropathic pain. 

In preparation, cut two six-centimeter pieces of 5-0 chromic gut ligature. Store them in a dish of sterile saline so that they do not stiffen up. After confirming that the rat is anesthetized by the absence of a reflex response, begin preparing it for surgery by shaving its head for a mid scalp incision. Shave a 25-millimeter radius from the center point of the eyes.

Next, fix the rat's head in a stereotaxic frame and put a heated pad under the rat's body. Then, apply some ophthalmic ointment. And clean the shaved area with alternating alcohol and betadine scrubs. Begin the surgery with a midline scalp incision to expose the skull and nasal bone.

Proceed with the surgery using a dissection microscope. Next, using forceps, free the edge of the orbit formed from the maxillary, frontal, lacrimal, and zygomatic bones. Also, swab up blood as needed.

Now, to access the IoN, gently deflect the orbital contents with a precision cotton swab. In doing this, do not overstretch the anterior ethmoidal nerve, which crosses superiorly to the IoN. Next, with the help of forceps, use a spreading motion to dissect the IoN free from the surrounding connective tissue. Always swab up bleeding as needed.

Now, place the piece of ligature between the IoN and frontal bone by pushing one end downwards along the frontal bone, medial to the IoN, and then advancing it a few millimeters parallel to the IoN, staying inferior to the IoN. Then, slip the tip of a 45-degree angled forceps under the IoN, close to the ligature, and gently retract the IoN laterally, revealing the ligature and tips of forceps.

Grab hold of the ligature and pass it under the IoN, pulling it out until the exposed lengths are even. Then tie the ends with a slip knot and slide the knot onto the IoN. Visibly constrict the nerve just slightly and complete the knot so it can't slip any further.

Now, cut off the free ends of the ligature and place another ligature using the same technique, two millimeters away from the first ligature. Visually confirm the correct placement of the ligatures.

Complete the surgery by closing the scalp with 4-0 polyester suture. Then, monitor the rat as it recovers on a heated pad. When it has regained sternal recumbency, return it to a proper cage. The rat should be housed alone until it has completely recovered.