Background & Aims

The neural mechanisms which underpin the transition from acute pain to chronic pain remain poorly understood. Previous human brain imaging studies have reported that chronic neuropathic pain is associated with volumetric differences in numerous pain-related brain regions (Wilcox et al., 2015). It remains unknown if these anatomical differences exist prior to the development of chronic pain, develop during the initial stages following injury or once chronic pain is well established. This study aimed to determine the timing of neuroanatomical changes during the development of neuropathic pain in a preclinical rodent model. We hypothesised that animals which transition to chronic pain will display changes in grey matter density in pain-related regions, which develop gradually during the transition from acute to chronic pain.

Methods

Seventy male Sprague Dawley rats were allocated to either a naïve (n=22), sham nerve injury (n=19), or nerve injury (n=29) group and anaesthetised with 1.5% isoflurane. After 20 minutes, rats in the naïve group were allowed to recover, whilst the sham group had their right infra-orbital nerve (ION) isolated but left undamaged and the injury group had loose ligatures applied to their ION (chronic constriction injury), to induce neuropathic pain. Rats were then randomly allocated to a timepoint; day 2, 7, 14 or 28, the post-injury timepoint at which they were scanned, using in-vivo 9.4T magnetic resonance imaging. T2-weighted anatomical images (voxel size=0.2mm3) were processed and analysed using a voxel-based morphometry pipeline in SPM12. Significant effects of injury were determined using two sample t tests (p<0.001). Brain regions with significant grey matter density changes were identified using a rat brain atlas. The same protocol was applied to a separate longitudinal cohort of rats (n=20), however, they were scanned repeatedly, 7 days pre-“injury”, 2, 7, 14 and 28 days post-“injury”.

Results

Our voxel-by-voxel analysis revealed that chronic constriction of the ION was associated with significant changes in regional grey matter density in the thalamus, substantia nigra and entorhinal cortex at day 2, the hypothalamus and amygdala at day 7 and the cingulate cortex, primary and secondary motor cortices, upper lip region of the primary somatosensory cortex and the frontal/orbital cortex at day 14 (p<0.001). Grey matter density changes in the cingulate cortex and upper lip region of the primary somatosensory cortex persisted at day 28, with additional changes in the caudate/putamen and hippocampus (p<0.001). Our preliminary longitudinal results displayed a significant difference in grey matter density of injured rats in the hypothalamus, between days 7 and 14 (p<0.001, one-way ANOVA) and a decreasing trend in the upper lip region of the primary somatosensory cortex, across time.

Conclusions

Our results demonstrate the neuroanatomical changes associated with the development of neuropathic pain, following damage to the infra-orbital nerve. These grey matter density changes were observed in numerous pain-related brain regions and occurred gradually across time. As grey matter density changes are anatomical indices of astrogliosis, these results contribute to furthering our understanding of the neural-glial basis of the transition from acute to chronic pain and may ultimately allow us to develop more effective treatments aimed at altering this interaction.

References

Wilcox, S. L., Gustin, S. M., Macey, P. M., Peck, C. C., Murray, G. M. & Henderson, L. A. (2015) Anatomical changes at the level of the primary synapse in neuropathic pain: Evidence from the spinal trigeminal nucleus. The Journal of neuroscience, 35(6), 2508-2515.

Presenting Author

Angela Doshen

Poster Authors

Angela Doshen

BSc(Hons)

The University of Sydney

Lead Author

E-mail

Gaelle Emvalomenos

BBiomedEng

The University of Sydney

Lead Author

E-mail

Sabrina Salberg

Monash University

Lead Author

E-mail

Crystal Li

Monash University

Lead Author

James Kang

University of Sydney

Lead Author

E-mail

Richelle Mychasiuk

Monash University

Lead Author

E-mail

Kevin Keay

PhD

The University of Sydney

Lead Author

E-mail

Luke Henderson

PhD

The University of Sydney

Lead Author

E-mail

Topics

  • Pain Imaging