Long-term effects of decreased noradrenergic central nervous system innervation on pain behavior and opioid antinociception

Here we examine whether a permanent reduction in the noradrenergic (NA) innervation of the spinal cord leads to a chronic decreased nociceptive threshold. NA denervation of rats was achieved by intrathecal injection of dopamine beta-hydroxylase antibodies conjugated to the toxin saporin. A subset of animals also underwent unilateral L5 spinal nerve ligature to induce sustained neuropathic pain behavior. NA fibers and terminals were lost throughout the spinal cord 2 weeks after toxin application and were still absent 12 months later, indicating that regeneration did not occur. There was also a widespread loss of NA terminals in the cerebral cortex, whereas innervation of the hypothalamus and amygdala were close to normal and NA innervation of the brainstem was moderately reduced. There was extensive loss of NA cells in the locus coeruleus and A5 and A7 cell groups. Dopaminergic and serotoninergic innervation was normal. Intracerebroventricular injection of the toxin resulted in additional NA reduction in the hypothalamus, amygdala, and A1 and A2 cell groups. Long-term removal of NA afferents did not affect nociceptive thresholds. Neuropathic animals showed greater mechanical hyperalgesia in the affected hindpaw only during the first 60 days after toxin. Rats lacking NA spinal afferents were less responsive to the antinociceptive effects of morphine, especially in the neuropathic hindpaw, and did not display opioid-dependent stress analgesia. Finally, in the spinal cord of toxin-treated rats, immunoreactivity for substance P was decreased, whereas that of its receptor (NK1) was increased. These animals exhibited antinociception to a low dose of an NK1 receptor antagonist. Our results suggest that NA contributes only modestly to determining the nociceptive threshold and that its antinociceptive effects are closely linked to opioidergic and tachykinergic neurotransmission.