PLANTAR INCISIONAL PAIN MODEL IN RAT (BRENNAN MODEL)

Animals

Studies are conducted in adult male and female Sprague Dawley rats (Envigo) in an AAALAC-accredited facility with approval from an Institutional Animal Care and Use Committee, and implemented in accordance with the Guide for the Care and Use of Laboratory Animals 8th Edition (National Research Council, 2011) with standards set by the National Institutes of Health.

Plantar incision surgery

The plantar incision model in rats is an established model of acute post-operative pain induced by incision of the skin and of the plantaris muscle (Figure 1) (Brennan et al., 1996). The model is characterized by transient hind paw mechanical allodynia and spontaneous guarding behaviors which are believed to be indicative of post-operative pain. Animals are carefully monitored throughout the experiment and rats showing any indication of infection or autophagia are removed from the study.

Schematic representation of the plantar incision surgery of the right hind paw based on Brennan et al., 1996
Figure 1: Schematic representation of the plantar incision surgery of the right hind paw based on Brennan et al., 1996. A- Longitudinal skin incision, B- Isolation, elevation, and incision of the plantaris muscle, C- Two horizontal mattress sutures close the skin.

Study design

In the plantar incision model, mechanical allodynia using von Frey filaments and guarding behavior are assessed using appropriate group sizes determined by power analysis. Male and female rats (n=8-10) are evaluated in separate cohorts. Dose-responses are investigated, and vehicle and positive control groups are included to establish the validity of the experiment. The experimenter is blinded to the treatments, and dosing is performed by an independent experimenter.

Because incisional pain resulting from the plantar incision surgery only induces transient pain, testing occurs 24 hours after surgery, and mechanical allodynia and guarding behavior are assessed in independent cohorts of animals.

Experimental outcomes

Hind paw mechanical allodynia testing using von Frey filaments

In the plantar incision model, von Frey filaments are applied medial to the incision between sutures.

Paw withdrawal threshold (PWT) is assessed using the up-down method (Chaplan et al., 1994) for both the ipsilateral and contralateral hind paws 3 times prior to surgery. The peak tactile mechanical allodynia is observed 24 hours after surgery. At that time, PWT is assessed again and animals with ipsilateral PWT ≤3.0g are included in the subsequent testing of the asset’s efficacy to treat mechanical allodynia in rats with plantar incision. In the plantar incision model, von Frey filaments are applied medial to the incision between sutures (X on Figure 2).

Figure 2: Filaments are applied medial to the incision between the sutures, marked with an X on the diagram.

Guarding behavior assessment

Guarding scores are assessed prior to surgery and 24 hours post-surgery (Brennan et al., 1996). Animals with a guarding score ≥ 10 at 24 hours post-surgery are included in the subsequent testing of the asset’s efficacy to treat guarding behavior in rats with plantar incision.

References

Brennan TJ, Vandermeulen EP, Gebhart GF (1996). Characterization of a rat model of incisional pain. Pain 64: 493-501. PMID: 8783314 DOI: 10.1016/0304-3959(95)01441-1

Chaplan SR, Bach FW, Pogrel JW, Chung JM, and Yaksh TL (1994). Quantitative assessment of tactile allodynia in the rat paw. J Neurosci Methods, 53 (1): 55-63. PMID: 7990513 DOI: 10.1016/0165-0270(94)90144-9

National Research Council (2011). Guide for the Care and Use of Laboratory Animals: Eighth Edition. Washington, DC: The National Academies Press

This work was conducted by PsychoGenics Inc. (Paramus, NJ) in collaboration with PSPP, NINDS, NIH under contract # 75N95019D00026