Behavioral, Biomarker, and Imaging Characterization of the Mouse Spared Nerve Injury Model

Background & Aims

Animal models are important for increasing our understanding of the mechanism of neuropathic pain and for the discovery and development of novel therapies for pain management. This study was carried out to assess behavioral, biomarker, and imaging readouts in the mouse spared nerve injury (SNI) model of neuropathic pain. The work was done to verify and/or identify readouts that could be used in future drug discovery projects to study the efficacy of putative therapies. As a novel and explorative readout, we applied CNS functional ultrasound imaging (fUS) in the model. Functional US is a non-invasive and sensitive functional neuroimaging tool with excellent spatial and temporal resolution. It is applicable in lightly anesthetized mice and feasible for both acute and chronic functional characterization of disease models, and for efficacy and target engagement studies.

Methods

Spared nerve injury was induced in two cohorts of C57Bl6/J mice according to the method described by Decosterd and Woolf with small modifications. Briefly, a small piece of the tibial and common peroneal nerves distal and close to sciatic nerve furcation was dissected. Control animals were sham-operated by exposing the nerves but leaving them intact. The first cohort of sham and SNI mice was followed for one week post-SNI. Short-term tactile allodynia was assessed with electronic von Frey (evF). Plasma was collected at 24 h, 72 h, and 7 days post-SNI for measurement of neurofilament light chain (NfL) levels with Quanterix Simoa. At endpoint 7 days post-SNI a skin punch biopsy sample was collected from the ipsilateral paw and skin intraepidermal nerve fiber (IENF) endings were evaluated using PGP9.5 immunohistochemistry. The second cohort of SNI mice were followed for two weeks to assess tactile and cool allodynia (aceton cooling test) and subjected to fUS imaging (IconeusOne).

Results

In the first cohort of mice, tactile allodynia was assessed by evF at baseline and 72 h and 7 days post-surgery. In SNI mice, the baseline mean paw withdrawal threshold (PWT) was decreased by almost 60% at 72 h post-SNI, with the mean value remaining at a similar level until day 7. SNI mice showed a highly significant increase in plasma NfL levels at 24 h and 72 h post-SNI as compared to baseline. The NfL levels peaked at 24 h, showing a gradual decrease at the 72-h and 7-day timepoints, but without reaching baseline levels by day 7. Post-surgery plasma NfL levels of sham-operated mice did not differ from the baseline (pre-surgery) level. The SNI mice displayed a decrease in IENF counts as compared to the sham mice. The second cohort of SNI mice showed robust and highly significant mechanical and cool allodynia at 2 weeks post-SNI. The mice were subjected to fUS imaging to study neurovascular coupling in response to mechanical stimulation.

Conclusions

The mouse SNI model displays robust changes in the behavioral, biomarker, and imaging readouts that were applied in the study. These readouts can be used in future drug discovery projects to preclinically investigate the efficacy of novel therapies for the management of pain or modulation of pathological processes related to neuropathic pain.

References

Decosterd and Woolf. Spared nerve injury: an animal model of persistent peripheral neuropathic pain. Pain 87(2):149-158, 2000

Presenting Author