Reversal of neuropathic pain by HSV-1-mediated decrease of noradrenaline in a pain facilitatory area of the brain.

Descending modulation of nociceptive transmission depends on the release of noradrenaline at the spinal cord. The role of noradrenaline in the control of nociceptive transmission at the supraspinal pain control system remains understudied. As chronic pain is associated with enhanced descending facilitation of nociceptive transmission, we sought to determine the role of noradrenaline in pain facilitation from the brain during neuropathic pain. We determined the action of the noradrenergic input to the dorsal reticular nucleus (DRt), a unique pain facilitatory area, using the spared nerve injury model. Injections of the α1-adrenoreceptor agonist phenylephrine into the DRt induced hyperalgesia and allodynia, indicating that α1-adrenoreceptors enhance the facilitatory action of the nucleus. This led us to reduce noradrenaline release at the DRt using a viral vector derived from the Herpes Simplex virus type 1 (HSV-1) which carried the tyrosine hydroxylase (TH) transgene in antisense orientation. The reduction of noradrenaline release, confirmed by microdialysis experiments, induced a long-lasting attenuation of pain responses, which was reverted by the local administration of phenylephrine. The present study indicates that the noradrenergic modulation of a pronociceptive area at the supraspinal pain control system accounts for pain facilitation, through the activation of α1-adrenoreceptors. The study also shows that sustained effects on chronic pain can be achieved by decreasing the release of noradrenaline in a pain facilitatory centre of the brain using gene transfer.