We have previously reported that ischemic
spinal cord injury in rats leads to
chronic pain-related behaviors. Thus, rats exhibited aversive reactions to innocuous mechanical stimuli (
mechanical allodynia) applied to a body area at or rostral to the dermatomes innervated by the injured spinal segments. The responses of the rats to cold are also markedly enhanced (cold
allodynia). Interestingly, more than 50% of spinally injured rats did not develop these abnormal
pain-related behaviors after
spinal cord injury. In the present study, we showed that the extent of injury is similar between allodynic and non-allodynic rats. Furthermore, intrathecal (i.t.)
naloxone, a broad-spectrum
opioid receptor antagonist, reversibly provoked mechanical and cold
allodynia-like responses in spinally injured rats that did not develop such behaviors spontaneously. However,
naloxone did not elicit such reactions in normal rats and did not alter the tail-flick latency in normal or spinally injured rats. Furthermore, i.t. D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (
CTOP) or naltridole, selective antagonists of mu and
delta opioid receptors, respectively, also triggered
pain-related behaviors similarly to
naloxone. Although
norbinaltorphimine (nor-BIN), a selective
kappa-receptor antagonist, also elicited such responses, the time course of the effect makes it unlikely that spinal
kappa-receptors were involved. These results suggested that the expression of abnormal
pain-related behaviors in some spinally injured rats is tonically suppressed by the spinal opioidergic system. Interindividual differences that lead to lack or dysfunction of such inhibition may underly the appearence of
pain-related behavior in some, but not all, spinally injured rats. It is suggested that such inhibition is exerted through spinal mu and delta, but not
kappa, opioid receptors. The endogenous opioidergic control appears to be only active against abnormal painrelated behaviors in spinally injured rats. Our results are relevant for the clinical observation that only a subgroup of patients with nerve injury suffers from
neuropathic pain.