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

Abstract

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.

Perineural application of resiniferatoxin on uninjured L3 and L4 nerves completely alleviates thermal and mechanical hypersensitivity following L5 nerve injury in rats

“…In a subsequent study, we used phosphorylated extracellular regulated kinase (pERK) as a pain marker to investigate the development of thermal hyperalgesia after L5 nerve injury. The results showed significantly more pERK immnuoreactive neurons in the ipsilateral side of the L4 spinal segment suggesting that the uninjured L4 nerve is important in the development of thermal hyperalgesia at the spinal cord level after L5 nerve injury (Shehab et al, ). Therefore, in the present study, we aimed to determine whether the unmyelinated primary afferents of the L4 nerve would be responsible for the development and maintenance of neuropathic pain after L5 nerve injury.…”

“…Both mechanical and thermal hyperalgesia require intact nerves to transmit the sensory information from the periphery to the spinal cord. Since the plantar skin of the hind paw of rats and the tested area in the L5 nerve spinal ligation and section model are supplied mainly by the L4 and L5 spinal nerves (Takahashi & Nakajima, ; Takahashi, Nakajima, & Sakamoto, ), the L4 nerve may be involved in the development of hyperalgesia (Fukuoka & Noguchi, ; Shehab et al, ). We demonstrated that the central terminations of the unmyelinated primary afferents of L4 and L5 nerves extend two segments rostrally (L3 and L4) and one segment caudally (L6) intermingling with one another in the dorsal horn at the L3–L5 spinal levels (Shehab, Al‐Marashda, Al‐Zahmi, Abdul‐Kareem, & Al‐Sultan, ).…”

The greater occipital nerve and its spinal and brainstem afferent projections: A stereological and tract‐tracing study in the rat

“…This antiserum showed no binding to Staphylococcus enterotoxins A or B, or Pseudomonas exotoxin, but it bound specifically to cholera toxin, a widely used axonal tracer (manufacturer's datasheet). In several studies, the specificity was demonstrated by the presence of immunolabeling in specific locations on the ipsilateral side of the spinal cord and lack of immunolabeling on the contralateral side (reviewed in Shehab et al, ). This antibody was used with Alexa Fluor 647 donkey anti‐rabbit secondary antibody.…”

A single GABA neuron receives contacts from myelinated primary afferents of two adjacent peripheral nerves. A possible role in neuropathic pain

“…We have previously characterized CTb and IB4 antibodies (Shehab, ; Shehab et al, ). Pretreatment of rabbit anti‐CTb antibody with CTb (10 μg/1 mL) completely abolished the corresponding immunoreactivity.…”

“…In this situation, L4 nerve would be most likely involved in the SNL model of neuropathic pain (Fukuoka & Noguchi, ). Indeed, we have recently shown that the uninjured L4 nerve plays an important role in the development of heat hyperalgesia at the spinal cord level after L5 nerve injury (Shehab et al, ).…”