behavioral testing and mechanical stimulation assessment in distal infraorbital nerve chronic constriction injured mouse

Establishing the Mouse Model for Trigeminal Neuropathic Pain

Neuropathic pain arises from damage to the somatosensory nervous system, leading to abnormal transmission of sensory signals to the brain. Somatosensory nerve damage does not always lead to neuropathic pain, but the prevalence increases with the severity of clinical neuropathy1,2. Neuropathic pain patients experience specific symptoms such as spontaneous sensations (burning, pins and needles, electric sensations) and abnormally intense or prolonged pain to innocuous or noxious stimulation that tend to become chronic and resistant to treatment with conventional pain medication3. Significant progress in the field of neuropathic pain research stems from the discovery that loosely constricting ligatures around the sciatic nerve in rats leads to behaviors resembling human neuropathic pain conditions4. The animals display reduced thresholds to heat, cold, and mechanical stimulation, and exhibit nocifensive behaviors. Despite the inherent biological differences in pain processing between humans and rodents, animal models are a valuable tool for studying the underlying mechanisms in the development of neuropathic pain and testing newly proposed treatment strategies.
Sensory reflex-based pain testing paradigms have been extensively used in neuropathic pain models, but measuring ongoing pain or other frequently accompanied disturbances (sleeping disorder, depression, anxiety) has not received sufficient attention considering that these are common clinical symptoms affecting quality of life5,6,7,8. Face grooming behavior in rats has been documented as a measure of spontaneous neuropathic pain following chronic constriction injury (CCI) of the infraorbital nerve (IoN)9,10. In addition, rats also develop hyperresponsiveness to mild tactile stimulation of the IoN territory, which is indicative of mechanical allodynia.
Compared to mice, because of their larger size, rats are better suited for surgical injuries. However, mice offer cost and space efficiency and require smaller drug quantities. Also, the advent of transgenic technology has further boosted the use of mice11,12. Therefore, the overall goal of this procedure is to perform a surgical infraorbital nerve injury in mice, similar to that in rats, that induces changes in spontaneous and evoked behavior for the study of trigeminal neuropathic pain.

Author Spotlight: Utilizing Infraorbital Nerve Ligation in Mice for Investigating Trigeminal Neuropathic Pain and Treatment Strategies

Chronic Constriction Injury of the Distal Infraorbital Nerve (DIoN-CCI) in Mice to Study Trigeminal Neuropathic Pain

The overall goal of this procedure is to perform an infraorbital nerve injury in mice to study the development of trigeminal neuropathic pain and to test new treatment strategies. Isolated phase grooming behavior after infraorbital nerve ligation is one of the few animal models to study spontaneous neuropathic pain. But its use has long been hampered by the complexity of performing an infraorbital nerve ligation.Compared to sensory reflex based pain testing, such as from fray or withdrawal responses from heat or cold stimulation, measuring spontaneous or ongoing pain has not received sufficient attention, considering that this is an important clinical symptom affecting quality of life. The distal approach to the infraorbital nerve has the advantage of being a surgically less demanding procedure than the intraorbital approach. It is minimally invasive, and can be performed without using a stereotaxic frame.

Behavioral Testing and Mechanical Stimulation Assessment in Distal Infraorbital Nerve Chronic Constriction Injured Mouse

To begin, carry a single mouse from the housing to the test room in a covered plastic cage without bedding material. Place the cage in front of a video camera. Then place a mirror to view the mouse's face when its back is toward the camera.Record the mouse's behavior for 10 minutes. After recording, clean the observation cage before recording the next animal. Have an observer blind to the experimental conditions to analyze the recorded behavior of the mouse.Note each face grooming episode during the 10-minute recording. Make a distinction between isolated face grooming and face grooming behaviors during body grooming. Determine the number of face grooming episodes with a four-second cutoff criterion.The time between grooming actions of less than four seconds is defined as a pause within a single episode. A time greater than four seconds is defined as a full interruption of grooming actions between two episodes. Place the mouse tail in a soft silicone clamp and attach the clamp magnetically to a metal plate on the table.Place a three-walled plastic holder over the mouse so that only the head protrudes from the container. Place a weight on top of the holder to keep it in place. Use a graded series of four von Frey hairs to apply force to the mouse.Habituate the mouse to the restrainer and reaching movements for 10 minutes. When the mouse is relaxed, slowly apply the lightest von Frey hair within the infraorbital nerve territory near the center of the vibrissae until it bends. Score the mouse response to the stimulation into one of the following categories.Calculate the mean score for ipsilateral responses to the four von Frey hairs, then calculate mean scores for the contralateral sides. Distal infraorbital nerve chronic constriction injury mice exhibited a significant postoperative increase in isolated face grooming time and episodes, peaking in the first week and then gradually decreasing, remaining elevated for six weeks. Initially, distal infraorbital nerve chronic constriction injury mice showed almost complete unresponsiveness to mechanical stimulation, which, during subsequent weeks, transformed into hyperresponsiveness lasting for six weeks.A slight increase in responsiveness to contralateral mechanical stimulation was also observed.

behavioral-testing-mechanical-stimulation-assessment-distal

Research

JoVE Journal

Neuroscience

Animal models remain necessary tools to study neuropathic pain. This manuscript describes the distal infraorbital nerve chronic constriction injury (DIoN-CCI) model to study trigeminal neuropathic pain in mice. This includes the surgical procedures to perform the chronic constriction injury and the postoperative behavioral tests to evaluate the changes in spontaneous and evoked behavior that are signs of ongoing pain and mechanical allodynia. The methods and behavioral readouts are similar to the infraorbital nerve chronic constriction injury (IoN-CCI) model in rats. However, important changes are necessary for the adaptation of the IoN-CCI model to mice. First, the intra-orbital approach is replaced by a more rostral approach with an incision between the eye and the whisker pad. The IoN is thus ligated distally outside the orbital cavity. Secondly, due to the higher locomotor activity in mice, allowing rats to move freely in small cages is replaced by placing mice in custom-designed and constructed restraining devices. After DIoN ligation, mice exhibit changes in spontaneous behavior and in response to von Frey hair stimulation that are similar to those in IoN-CCI rats, i.e., increased directed face grooming and hyperresponsiveness to von Frey hair stimulation of the IoN territory.

Chronic constriction injury of the distal infraorbital nerve in mice induces changes in spontaneous behavior (increased face grooming activity) and nocifensive behavior in response to tactile stimulation (hyperresponsiveness to von Frey hair stimulation) that are signs of ongoing pain and allodynia and serves as a model for trigeminal neuropathic pain.

Animal models remain necessary tools to study neuropathic pain. This manuscript describes the distal infraorbital nerve chronic constriction injury ...

Infraorbital Nerve Surgery in Mouse

To begin, gently shave the buccal hair between the whisker pad and the eye of an anesthetized mouse. Fix the head of the mouse in a stereotaxic frame, or otherwise, fixate the head. Apply ointment on both eyes to avoid drying.Scrub the shaved head area with alcohol, and then with Betadine. Under a dissecting microscope, make a four millimeter skin incision perpendicular to the midline, approximately halfway between the edge of the whisker pad and the eye, just rostral to the infraorbital foramen and centered around the line between the center of the eye and the center of the whisker pad. To expose the infraorbital nerve, bluntly separate the superficial connective tissue, minimizing musculature damage and avoiding the motor nerve fibers.Using a rotation motion, slip the head of a hooked ligation aid under the infraorbital nerve. Place the chromic gut ligature through the hole on the tip of the ligation aid and retract the ligation aid so that the ligature remains under the infraorbital nerve, with both ends of the ligature equidistant from the nerve. Tie a slip knot from both ends of the ligature and slide it against the infraorbital nerve.Slide the knot further to constrict the infraorbital nerve. Then place a normal knot on top of the slip knot. Cut the ends of the ligature, leaving approximately 1.5 millimeters of free ends to prevent the knot from becoming undone.Close the skin incision using 6-0 synthetic absorbable sutures. Allow the mouse to recover on a heated pad or under an infrared heating lamp.