Anatomical evidence that the uninjured adjacent L4 nerve plays a significant role in the development of peripheral neuropathic pain after L5 spinal nerve ligation in rats

Rats develop hyperalgesia and allodynia in the hind paw after L5 spinal nerve ligation. Phosphorylated extracellular regulated kinase (pERK) was used as a pain marker to investigate the potential role of adjacent uninjured L4 nerve in the development of heat hyperalgesia after L5 nerve injury. Left L5 nerve was ligated and sectioned in rats. Three days later, rats were randomly assigned to five groups; each had both hind paws immersed in water at different temperatures (no heat, 37, 42, 47, and 52°C) under sevoflurane anesthesia for 2 minutes. Five minutes after stimulation the rats were sacrificed and sections of L3-L6 spinal segments were stained immunocytochemically with pERK antibody. pERK immunoreactivity, which is not detectable in the normal spinal cord, was discernible in neurons (not glia) of the superficial dorsal horn after noxious heat stimuli. pERK-positive neurons clearly overlapped in laminae I-II with normal unmyelinated and thin myelinated afferents labeled with calcitonin gene-related peptide and isolectin B4, and injured unmyelinated afferents labeled with vasoactive intestinal polypeptide. There was a linear increase in pERK immunoreactivity on both sides with an increase in temperature. Importantly, the number of positive pERK neurons was significantly higher in the ipsilateral side of L4 spinal segment, which receives innervation from uninjured L4 nerve, compared with the contralateral control side, which receives both uninjured L4 and L5 spinal nerves. The data demonstrate that the uninjured L4 nerve plays an important role in the development of heat hyperalgesia at the spinal cord level after L5 nerve injury.