Noradrenaline increases pain facilitation from the brain during inflammatory pain

Antidepressants that inhibit the recapture of noradrenaline have variable effects in chronic pain which may be related to the complex role of noradrenaline in pain modulation. Whereas at the spinal cord noradrenaline blocks nociceptive transmission, both antinociception and pronociception were reported after noradrenaline release in the brain. To study the role of noradrenaline in pain modulatory areas of the brain, we elected the dorsal reticular nucleus (DRt), a key pain facilitatory area located at the medulla oblongata. Three studies were performed. First, we show that the infusion in the DRt of nomifensine, which increases local extracellular levels of noradrenaline as shown by in vivo microdialysis, also enhances pain behavioral responses during both phases of the formalin test, a classic inflammatory pain model. Then, we demonstrate that the formalin test triggers the release of noradrenaline in the DRt in a biphasic pattern that matches the two phases of the test. Finally, we show that reducing noradrenaline release into the DRt, using an HSV-1 vector which decreases the expression of tyrosine hydroxylase in noradrenergic DRt-projecting neurons, attenuates pain behavioral responses in both phases of the formalin test. The increased noradrenaline levels induced by the infusion of nomifensine at the DRt, along with the hyperalgesic effects of noradrenaline released at the DRt upon noxious stimulation, indicates that noradrenaline may enhance pain facilitation from the brain. It is important to evaluate if antidepressants that inhibit noradrenaline recapture enhance pain facilitation from the brain herein attenuating their analgesic effects.