M. Di Benedetto, D. Mercatelli, C. D’Addario, M. Canossa, P. Romualdi and S. Candeletti

Dept. of Pharmacology, University of Bologna, Bologna

It is now accepted that neuropathic pain, caused by different triggering events or diseases, underlies a series of maladaptative plasticity changes within the nociceptive transmission system, that result in a state of neural disease [1]. Hence, the possibility to develop better therapeutic tools and/or strategies for its control requires a better comprehension of these underlying neuroplasticity phenomena.

The aim of this study was to investigate the alterations of a series of genes involved in pain transmission and neuroplasticity, in a murine experimental model of neuropathic pain, during the development of the pain condition.

To this end, ICR CD-1 or BDNF (+/+) and BDNF (+/-) mice were subjected to a right sciatic nerve lesion, according to the chronic constriction injury (CCI) model [2], adapted to the mouse.

The presence and development of behavioural signs of neuropathic pain were assessed the Hargreaves’ plantar test (hyperalgesia) and by the acetone and cold plate tests (allodynia), performed at different intervals after the sciatic nerve lesion.

The measurement of mRNA levels for proenkephalin (pENK), prodynorphin (pDYN), pronociceptin (pN/OFQ), pronociceptin opioid receptor (pNOP) and pro brain derived neurotrophic factor (pBDNF) in the L4-L6 portion of the spinal cord, was performed by Real Time RT-PCR, at different intervals after nerve injury.

In ICR CD-1 mice, a significant increase of pDYN and pBDNF gene expression was detected at seven days after CCI whereas no changes were observed for the other investigated genes. In addition, no significant alterations of all investigated mRNA levels were observed at 14 days.

A later onset of hyperalgesia was observed in BDNF (+/-) mice (seven days after CCI) compared with BDNF (+/+) and with ICR CD-1 mice (three days).

Both BDNF (+/+) and BDNF (+/-) animals exhibited allodynia at seven days after CCI with BDNF (+/-) mice showing significantly higher signs, compared to BDNF(+/+), in both behavioural tests.

Furthermore, in the L4-L6 spinal cord segment of BDNF (+/-) mice subjected to CCI, the pDYN gene expression increase observed in ICR CD-1 mice at seven days after CCI was not detected.

These results indicate a role for BDNF in the mechanisms underlying the development of allodynia and hyperalgesia, thus confirming its involvement in neuroplasticity leading to neuropathic pain conditions. At the same time, pDYN gene expression data suggest a role for this opioid peptide in this neural disease and strengthens the hypothesis that dynorphin could act as a downstream effector of BDNF actions [3].

Furthermore, the elucidation of the epigenetic phenomena leading to the observed gene expression alterations could help to identify possible novel targets for the control of neuropathic pain.

[1] Costigan et al. (2009) Neuropathic pain: a maladaptive response of the nervous system to damage. Annual Review of Neuroscience 32, 1-32.

[2] Bennett and Xie (1988) A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man. Pain 33(1), 87-107.

[3] Logrip et al. (2008) Dynorphin is a downstream effector of striatal BDNF regulation of ethanol intake. FASEB Journal 22, 2393-2404.