Acute and Chronic Tactile Sensory Testing after Spinal Cord Injury in Rats

Summary

We describe two tactile sensory testing methods for acute or chronic periods of spinal cord injury in rats. These validated procedures can detect the development and maintenance of allodynia-like sensations.

Abstract

Spinal cord injury (SCI) impairs sensory systems causing allodynia^1-8. To identify cellular and molecular causes of allodynia, sensitive and valid sensory testing in rat SCI models is needed. However, until recently, no single testing approach had been validated for SCI so that standardized methods have not been implemented across labs. Additionally, available testing methods could not be implemented acutely or when severe motor impairments existed, preventing studies of the development of SCI-induced allodynia³. Here we present two validated sensory testing methods using von Frey Hair (VFH) monofilaments which quantify changes in tactile sensory thresholds after SCI^4-5. One test is the well-established Up-Down test which demonstrates high sensitivity and specificity across different SCI severities when tested chronically⁵. The other test is a newly-developed dorsal VFH test that can be applied acutely after SCI when allodynia develops, prior to motor recovery^4-5. Each VFH monofilament applies a calibrated force when touched to the skin of the hind paw until it bends. In the up-down method, alternating VFHs of higher or lower forces are used on the plantar L5 dermatome to delineate flexor withdrawal thresholds. Successively higher forces are applied until withdrawal occurs then lower force VFHs are used until withdrawal ceases. The tactile threshold reflects the force required to elicit withdrawal in 50% of the stimuli. For the new test, each VFH is applied to the dorsal L5 dermatome of the paw while the rat is supported by the examiner. The VFH stimulation occurs in ascending order of force until at least 2 of 3 applications at a given force produces paw withdrawal. Tactile sensory threshold is the lowest force to elicit withdrawal 66% of the time. Acclimation, testing and scoring procedures are described. Aberrant trials that require a retest and typical trials are defined. Animal use was approved by Ohio State University Animal Care and Use Committee.

Protocol

1. Up/Down Plantar Von Frey Testing

The purpose of the test is to determine tactile sensitivity by measuring how much force is required to elicit paw withdrawal from the stimulus. Commercially available calibrated Von Frey Hair monofilaments between the ranges of 4.56 and 6.10 are used to stimulate the paw. Other sensory input like visualization of the stimulus or proprioception can confound tactile threshold measures so responses based on visual or proprioceptive input are invalid. To prevent visualization of the monofilament touching the skin, we test only while the rat is eating cereal. This cognitive distracter, also ensures that all rats....

Discussion

The VFH testing methods presented here represent two testing paradigms that are sensitive and valid for use in rat SCI models. Importantly, the dorsal VFH test was created for assessment of the recovery of tactile sensation of the hind paws acutely following mid-thoracic SCI when rats are unable to stand on their hindquarters. This testing paradigm may also be used when rats regain the ability to support their weight and for the hind limbs after cervical SCI.

The two paradigms demonstrated he.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Semmes-Weinstein Monofilaments	Stoelting Co.	58011
Fish Breeder Box	Any Supplier		Hole drilled in side for cereal delivery
Elevated wire mesh (1/4" X 1/4" grid)	Hardware Store
Elastic to hold fish breeder box in place	Any Supplier
Sugared Cereal	Grocery Store
Surgical Huck Towel 16"x 23"	Medical Supply Store