Elsevier

Neuroscience

Volume 158, Issue 2, 23 January 2009, Pages 904-914
Neuroscience

Pain Mechanism
Effects of distal nerve injuries on dorsal-horn neurons and glia: Relationships between lesion size and mechanical hyperalgesia

https://doi.org/10.1016/j.neuroscience.2008.10.010Get rights and content

Abstract

Penetrating limb injuries are common and usually heal without long-lasting effects, even when nerves are cut. However, rare nerve-injury patients develop prolonged and disabling chronic pain (neuralgia). When pain severity is disproportionate to severity of the inciting injury, physicians and insurers may suspect exaggeration and limit care or benefits, although the nature of the relationship between lesion-size and the development and persistence of neuralgia remains largely unknown. We compared cellular changes in the spinal dorsal-horn (the initial CNS pain-processing area) after partial or total tibial-nerve axotomies in male Sprague–Dawley rats to determine if these changes are proportional to the numbers of peripheral axons cut. Unoperated rats provided controls. Plantar hind-paw responses to touch, pin, and cold were quantitated bilaterally to identify hyperalgesic rats. We also compared data from nerve-injured rats with or without hyperalgesic responses to mechanical hind-paw stimulation to evaluate concordance between pain behaviors and dorsal-horn cellular changes. Hyperalgesia was no less prevalent or severe after partial than after total axotomy. L5 spinal-cord sections from rats killed 7 days postoperatively were labeled for markers of primary afferents (substance P calcitonin gene-related peptide isolectin B4, gamma aminobutyric acid, and glial fibrillary acidic protein), then labeled cells were stereologically quantitated in somatotopically defined dorsal-horn regions. Total axotomy reduced markers of primary afferents more than partial axotomy. In contrast, GABA-immunoreactive profiles were similarly reduced after both lesions, and in rats with sensory loss versus hyperalgesia. Numbers of GFAP-immunoreactive astrocytes increased independently of lesion size and pain status. Small nerve injuries can thus have magnified and disproportionate effects on dorsal-horn neurons and glia, perhaps providing a biological correlate for the disproportionate pain of post-traumatic neuralgias (including complex regional pain syndrome-I) that follow seemingly minor nerve injuries. However, the presence of similar dorsal-horn changes in rats without pain behaviors suggests that not all transcellular responses to axotomy are pain-specific.

Section snippets

Animal care and distal nerve injury (DNI) surgery

Male Sprague–Dawley rats (200–230 g, Charles River Laboratories, Wilmington, MA, USA) were housed in smooth-bottomed cages with free access to food and water. Rats were randomly assigned to undergo partial-DNI (n=32), total-DNI (n=28), or to serve as unoperated controls (n=8). Rats were rested for at least 48 h after arrival and then underwent 3 days of behavioral testing, the median of which defined the baseline value. All procedures were approved by the Institutional Animal Care and Use

Results

DNI rats appeared to ambulate and explore as much control rats, and their feeding was judged unimpaired because weight gain in both groups of DNI rats was similar to that of unoperated rats (P≥0.49). Neither autotomy nor tremors were detected in any rats. Some DNI rats developed sustained (tonic) abnormal postures of their ipsilesional hind paws, specifically lateral hind-paw-margin elevation and paw eversion, or plantar-flexion of all digits with ambulation on the volar surface of the digit.

Discussion

We generated a nerve injury that models the epidemiological characteristic of neuralgia, namely, that it does not always follow nerve injury. Most extant rodent models of neuralgia produce pain-behaviors in all or most lesioned animals. While efficient for preclinical testing, this precludes exploration of which post-lesioning changes are specifically associated with pain development. Seven days after our tibial-nerve lesions, 63% of rats had mechanical allodynia; 29% had prolonged withdrawal

Conclusion

In summary, this study demonstrates that partial axotomies of one distal nerve can be as effective as or more effective than total axotomies at producing long-lasting intraterritorial and extraterritorial hind-paw mechanical and thermal pain behaviors. A smaller, partial lesion was as effective as total axotomy at causing trans-synaptic and widespread effects on GABAergic interneurons and astrocytes in the dorsal-horn. This cellular “magnification” appears to develop within the dorsal horn

Acknowledgments

The technical assistance of Li Zheng, Ralph Gott, YangFeng Li, and statistical support of Dr. Yuchiao Chang are gratefully acknowledged. Supported in part by the Public Health Service (R01NS42866, K24NS059892, P30 EY 12196), the Beatrice and Roy Backus Foundation, and the RSDSA Rachel Tobias Young Investigator Award. Presented to the Society for Neuroscience in abstract form.

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