Background

Damaged peripheral nerves cause persisting overactivation referred to as “neuropathic pain”. A serious consequence of persisting nerve injury pain is the transition to chronic pain. Neuropathic pain is the result of changes in the signaling molecules in the peripheral nerve cells that eventually cause central sensitization and changes in the brain’s pain and affective circuitry. A major obstacle to better understanding of pathophysiological mechanisms of chronic neuropathic pain and development of effective therapeutics is lack of available experimental animal models that mimic established chronic pain. To emulate chronic orofacial neuropathic pain, the FRICT-ION (Foramen Rotundum Inflammatory Constriction of Trigeminal InfraOrbital Nerve) nerve injury model was developed in mice to study behavioral, pharmacological, cellular, and molecular mechanisms persisting during chronic neuropathic pain.

While many nerve injury models have been developed that allow study of nociceptive mechanisms and pain-related behaviors in the short term, the resilience of natural healing processes typically reverses nerve injury models within 3-4 weeks in neuropathic pain models. The method described below is one of the few models that allow long-term studies suitable for better understanding of clinical pain states and for testing experimental therapies. Several innate differences in the trigeminal nerve are likely responsible for the chronic persistence of the neuropathic model. The great importance of the trigeminal nerve to survival is postulated to be one characteristic responsible for the ease in producing persistence of trigeminal nerve injury pain and its effects on psychological well-being (Carlson, 2007). Anatomical proximity to the brain and the unique circuitry of the trigeminal system with one less synapse forming more direct connectivity with the limbic circuitry are also likely responsible for the intensity and persistence of neuropathic pain associated with the trigeminal nerve (Rodriguez et al., 2017).

Previous successful chronic trigeminal pain models described have been induced by loose chromic gut suture tie of the infraorbital nerve (ION) in rats (CCI-ION) behind the eye (Vos et al., 1994; Kniffin et al., 2015) or on the snout in rats (dIoN-CCI) and mice (DIONI) (Ding et al., 2017; Hardt et al., 2019) (Figure 1). Alternatively, the intraoral approach has also been used to induce the CCI-ION model in rats (Immamura et al., 1997). We previously introduced a trigeminal nerve hypersensitivity model induced by sliding the irritative chromic gut suture into the ION fissure behind the eye in mice creating a trigeminal inflammatory compression (TIC) (Figure 1) (Ma et al., 2012 and 2015; Lyons et al., 2015 and 2018). The surgical approach to the ION behind the eye is technically challenging and the vasculature prone to bleeding, often severely. After induction of trigeminal nerve injury models, mechanical hypersensitivity develops within a week and persists indefinitely in the ION’s whisker pad receptive field. After trigeminal nerve injury, cognitive deficits are reported within 3 weeks in rats and anxiety- and depression-related symptoms emerge in subsequent weeks after injury (>6 weeks) in rodents indicating brain origin and potential circuitry neuroplasticity within brain regions are responsible for these behaviors accompanying chronic pain (Yalcin et al., 2011 and 2014; Kniffin et al., 2015; Lyons et al., 2015 and 2018). Estimates are that in weeks 6-8 post-surgery, mice have experienced pain equivalent to 6-8 human years and persistence of models in this time frame can easily be considered chronic (Dutta and Sengupta, 2016; Hannaman et al., 2016). The interplay between psychologic and physical functioning is a particular consideration in patients with orofacial pain (Carlson, 2007).


Figure 1. Surgical sites utilized in trigeminal neuropathic pain models. 1. TIC, 2. CCI-ION, 3. FRICT-ION, 4. dIoN-CCI and DIONI.

Better models for the study of chronic neuropathic pain are needed to discover more effective treatments since the current treatment of choice, microvascular decompression surgery, in patients with chronic trigeminal nerve pain or multiple sclerosis pain wanes over time with reoccurrence typically at 3 years (Xia et al., 2014). The pain-free rate at 5 and 10 years decreases to 61% and 44% after decompression surgery to place a Teflon spacer and after stereotaxic radioablation is only 47%, and 27%, respectively (Wang et al., 2018). Teflon granulomas are reported in 5.6% of patients after microvascular decompression (Chen et al., 2000).

The FRICT-ION method provides a simpler intraoral approach (Figure 1). The benefits of using the mouse FRICT-ION model of trigeminal nerve injury for study of chronic neuropathic pain in general are many. The model induces long-term chronic neuropathic pain with no other apparent side effects or outward physical indication. The mouse experiences no health or weight gain issues. The method exposes the rodent to only minutes of isoflurane anesthesia but sensitivity in the receptive field on the whiskerpad persists indefinitely. If done correctly, the minimally invasive surgery has little or no bleeding. The model is quickly and easily mastered following the instructions provided below.

Surgical Induction of the FRICT-ION Model
We induce the model in either BALBc or C57Bl/6 mice (20 to 25 g; 8-10 weeks; Harlan Laboratories, Indianapolis, IN). We have performed the procedure in older mice with no complications. The surgery is performed open mouth through a tiny slit to align chromic gut suture along the trigeminal nerve. The constant but minimal nerve irritation likely produced with chewing is similar to one known cause of trigeminal neuropathic pain, irritation by proximity of the nerve to pulsating brainstem vasculature which is relieved by insertion of a Teflon sheet (Dandy, 1934; Xia et al., 2014). The extended duration of orofacial neuropathic pain models provides an optimal platform for preclinical testing of potential therapeutics since mechanical and cold sensitivities, anxiety, and depression-like behaviors are measurable.

The model illustrated here is induced by inserting 3 mm of chromic gut suture intraorally into the tight space where the infraorbital nerve (ION) passes through the bony infraorbital foramen. Similar to nerve biopsies from patients (Weis et al., 2012), the TIC model we reported previously using the intraorbital surgical approach did not cause the axonal degeneration seen in tied or cut nerve models (Ma et al., 2012). As a result, the TIC model induced hypersensitivities similar to those in the human condition, i.e., mechanical and cold allodynia, but not heat hypersensitivity, along with anxiety and depression. Though not tested as yet, we are hopeful that these results will be duplicated using the intraoral approach to induce the FRICT-ION model. The model is illustrated here induced on the right side of the mice in all figures. The model has been induced in both males and females, BALBc and C57Bl/6, and while not yet tested by our lab, likely could also be done in rats. The intraoral approach has been used previously in rats to induce the nerve tied CCI-ION model (Immamura et al., 1997).