The dolognawmeter: A novel instrument and assay to quantify nociception in rodent models of orofacial pain

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Abstract

Rodent pain models play an important role in understanding the mechanisms of nociception and have accelerated the search for new treatment approaches for pain. Creating an objective metric for orofacial nociception in these models presents significant technical obstacles. No animal assay accurately measures pain-induced orofacial dysfunction that is directly comparable to human orofacial dysfunction. We developed and validated a high throughput, objective, operant, nociceptive animal assay, and an instrument to perform the assay termed the dolognawmeter, for evaluation of conditions known to elicit orofacial pain in humans. Using the device our assay quantifies gnawing function in the mouse. We quantified a behavioral index of nociception and demonstrated blockade of nociception in three models of orofacial pain: (1) TMJ inflammation, (2) masticatory myositis, and (3) head and neck cancer. This assay will be useful in the study of nociceptive mediators involved in the development and progression of orofacial pain conditions and it will also provide a unique tool for development and assessment of new therapeutic approaches.

Introduction

More than 20% of adults are afflicted by orofacial pain (Lipton et al., 1993). Some of the most severe and difficult to treat forms of orofacial pain result from temporomandibular joint (TMJ) disorders, masticatory muscle inflammation and head and neck cancer. Patients with TMJ disorders or masticatory muscle inflammation report that chewing induces the highest levels of pain (Bush et al., 1989, Dworkin et al., 1990, Fricton et al., 1985, Gavish et al., 2000, Winocur et al., 2001, Zarb and Thompson, 1970). Patients with TMJ disorders experience significant and prolonged pain compared to normal subjects after chewing for an extended period of time (Karibe et al., 2003). In patients with masticatory myositis, jaw opening and closing while chewing significantly exacerbates muscle pain (Bowley and Gale, 1987, Christensen, 1976, Christensen and Radue, 1985, Dao et al., 1994, Molin, 1972, Plesh et al., 1998, Scott and Lundeen, 1980). Duration of chewing is associated with development of pain. Patients with TMJ and masticatory muscle pain experience a significant increase in pain after chewing for 9 min (Gavish et al., 2002). The American Academy of Orofacial Pain recommends resting the jaw and limiting jaw movement in patients with TMJ or masticatory muscle pain (Pain).

Like joint and muscle disorders, head and neck cancer causes functional allodynia (Connelly and Schmidt, 2004, Kolokythas et al., 2007). Patients with head and neck cancer report pain as the worst symptom (Shedd et al., 1980), and the primary determinant of a poor quality of life (Chaplin and Morton, 1999, Hodder et al., 1997). In their final months of life 85% of these patients report pain as the most common problem (Shedd et al., 1980). Half of all head and neck cancers are incurable and many patients suffer from intractable cancer pain for extended periods. Oral cancer patients experience significant debilitation of chewing and oral function secondary to pain (Connelly and Schmidt, 2004, Rogers et al., 2006). To investigate orofacial pain ultimately develop treatments for patients we have created an assay that elicits and measures pain-induced oral dysfunction in an animal model.

Masticatory dysfunction is one of the hallmarks of orofacial pain. An objective animal assay that quantifies nociception-induced dysfunction, as seen clinically, is not available. We hypothesize that nociception-induced gnawing dysfunction can be used as an index of orofacial nociception in an animal model. Gnawing involves incisor occlusion and molar disclusion but shares nearly all of the anatomical and physiological components of chewing in humans. To test our hypothesis we designed and validated a novel assay and device (a dolognawmeter-dolor for pain, gnawmeter for measurement of gnawing) that together objectively quantify gnawing function in mice. We quantified a behavioral index of nociception and demonstrated blockade of nociception in three models of orofacial pain: (1) TMJ inflammation, (2) masticatory myositis, and (3) head and neck cancer.

Section snippets

Design of the dolognawmeter

The dolognawmeter is designed for high throughput. Experimental productivity is limited only by the number of devices employed. A dolognawmeter is fabricated as described below (Fig. 1, Fig. 2).

  • The confinement tube is manufactured from a 180 mm section of commercially available schedule-80 polyvinyl chloride (PVC) tubing with an internal diameter of 24 mm and an external diameter of 33 mm. A set of radially oriented holes is drilled entirely through the tube at 30 mm and 50 mm from the end of the

TMJ inflammatory pain model

CFA injection into the left TMJ produced a mean 567.0 ± 273.8 (SEM) % increase in gnaw time compared to the TMJ saline-injected model at 10 and 58 h following injection (Fig. 3). Pretreatment with indomethacin prevented the increase in gnaw time seen in animals receiving only CFA injection into the left TMJ (Fig. 3). Histologic evaluation of the TMJ demonstrated that CFA injection produced an inflammatory infiltrate (Fig. 4a) while saline injection into the TMJ did not (Fig. 4b).

Masticatory muscle inflammatory pain model

The masseter

Discussion

We developed a high throughput, objective, operant assay for orofacial function and validated its ability to detect acute and chronic functional allodynia. The dolognawmeter quantified a behavioral index of nociception in three separate mouse models of orofacial pain (TMJ inflammation, masticatory myositis and head and neck cancer). To confirm that the behavioral dependent measure (gnaw time) reflects nociception, we restored gnawing function by blocking nociception with the same analgesics

Acknowledgment

Supported by NIH/NIDCR R21 DE018561.

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