Background & Aims
Neuropathic pain is an intractable disease which in case opioids provide inadequate analgesia. So far, the mechanism of neuropathic pain has been sought in the spinal cord and the anterior cingulate cortex (ACC). Our previous studies have shown that muscarinic M1, GABA and NMDA receptors are associated with the mechanism of antinociception in the ACC of sciatic nerve ligation (PSNL) mice (1,2). In addition, our recent studies have also shown that the mechanical hypersensitivity using von Frey filament in the PSNL mice persisted more than 4 weeks, then the hypersensitivity gradually?disappeared, and after 3 months the hypersensitivity disappeared. However, the mechanism by which hypersensitivity disappears is unknown. Therefore, we searched for factors associated with reduced hypersensitivity in ACC of PSNL mice. In this study, we show the results of RNA sequencing of ACC in PSNL mice and age-matched mice.
Methods
A partial sciatic nerve ligation mouse was produced by tying a tight ligature with a No. 8 nylon thread around approximately 1/3–1/2 the diameter of the sciatic nerve located on the left side. On the right side, the nerve was exposed without ligation. Von Frey filament with bending forces of 0.4 g was pressed perpendicularly against the plantar skin of a hind-paw. The paw withdrawal responses to the mechanical stimulus were evaluated by scoring as follows: 0, no response; 1, lifting the hind-paw over 3 s; and 2, lifting the hind-paw within 3 s. The stimulation of the same intensity was applied five times to each hind-paw at several seconds intervals and the total served as the pain-related score. RNA sequencing was performed using ACC tissue isolated from PSNL mice 2 weeks and 3 months after surgery.
Results
Of the 488 differentially expressed genes in the ACC at 2 weeks after PSNL, 228 genes were upregulated. Among these genes, 25 genes were downregulated in the PSNL at 3 months. These genes were applied for Gene Ontology analysis using DAVID to determine enriched functional annotation in terms of biological processes. Among 25 genes, 4 genes (BTG anti-proliferation factor 2, CCAAT/enhancer binding protein beta, nuclear receptor subfamily 4, group A, member 2, immediate early response 2) were involved in neuron differentiation.
Conclusions
These results suggest that the factor related to neuron differentiation in the ACC might be critical for attenuation of neuropathic pain.
References
1. Migita, K. et al. Involvement of GABAB receptor in the antihypersensitive effect in anterior cingulate cortex of partial sciatic nerve ligation model. J Pharmacol Sci 137, 233-236 (2018).
2. Koga, K. et al. Activations of muscarinic M1 receptors in the anterior cingulate cortex contribute to the antinociceptive effect via GABAergic transmission. Mol Pain 13, 1744806917692330 (2017).