Formation of 6-nitro-norepinephrine from nitric oxide and norepinephrine in the spinal cord and its role in spinal analgesia

Spinally released norepinephrine is thought to produce analgesia in part by stimulating alpha(2)-adrenergic receptors, which in turn leads to nitric oxide synthesis. Also, nitric oxide is known to react with norepinephrine in vivo in the brain to form 6-nitro-norepinephrine, which inhibits neuronal norepinephrine reuptake. In the present study, we tested the hypothesis that formation of 6-nitro-norepinephrine occurs in the spinal cord and that intrathecal administration of 6-nitro-norepinephrine produces analgesia by stimulating norepinephrine release. 6-Nitro-norepinephrine was present in rat spinal cord tissue and microdialysates of the dorsal horn and intrathecal space. Intrathecal norepinephrine injection increased 6-nitro-norepinephrine. 6-Nitro-norepinephrine also stimulated norepinephrine release in dorsal spinal cord in vitro. Intrathecal injection of 6-nitro-norepinephrine produced antinociception and interacted additively with norepinephrine for antinociception. Spinal noradrenergic nerve destruction increased antinociception from intrathecally injected norepinephrine, but decreased antinociception from 6-nitro-norepinephrine.These results suggest a functional interaction between spinal nitric oxide and norepinephrine in analgesia, mediated in part by formation of 6-nitro-norepinephrine. Stimulation of auto-inhibitory alpha(2)-adrenergic receptors at noradrenergic synapses decreases norepinephrine release. Paradoxically, alpha(2)-adrenergic agonist injection increases and alpha(2)-adrenergic antagonist injection decreases norepinephrine release in the spinal cord. 6-Nitro-norepinephrine may be an important regulator of spinal norepinephrine release and could explain the positive feedback on norepinephrine release after activation of spinal alpha(2)-adrenergic receptors.