Decreases in endomorphin-2-like immunoreactivity concomitant with chronic pain after nerve injury

Nerve injury often leads to chronic, sometimes excruciating, pain. The mech anisms contributing to this syndrome include neurochemical plasticity in ne urons involved in the earliest stages of pain transmission. Endomorphin-2 ( Tyr-Pro-Phe-Phe-NH2) is an endogenous morphine-like substance that binds to the mu -opioid receptor with high affinity and selectivity. Endomorphin-2- like immunoreactivity (LI) is present in the superficial layers of the dors al horn in the spinal cord and in primary afferents, suggesting a role for this peptide in pain transmission. To determine whether spinal endomorphin- 2-LI is altered in an animal model of chronic pain, the left sciatic nerve of Swiss Webster and ICR mice was ligated in a modified Seltzer model of ne rve injury. Changes in endomorphin-2-LI were assessed by immunocytochemistr y at 2, 4 and 14 days after nerve injury. The side of the spinal cord ipsil ateral to the nerve injury exhibited a dramatic decrease in endomorphin-2-L I relative to the contralateral side and to control animals. The change was restricted to the medial dorsal horn in the lumbar segments innervated by the sciatic nerve. Substance P-LI showed a small decrease, while calcitonin gene-related peptide-LI was unchanged. Both thermal hyperalgesia, as evide nced by significantly decreased paw withdrawal latencies, and decreased end omorphin-2-LI were observed within 2 days of injury and were most pronounce d at 2 weeks after injury. The decrease in endomorphin-2-LI during the deve lopment of chronic pain is consistent with the loss of an inhibitory influe nce on pain transmission. These results provide the first evidence that reduction of an endogenous op ioid in primary afferents is associated with injury-induced chronic pain. P ublished by Elsevier Science Ltd on behalf of IBRO.