We examined a
pain-related syndrome, which includes
mechanical allodynia and autotomy, in rats after ischemic
spinal cord injury photochemically induced by
laser irradiation for 5-20 min. This procedure results in an acute
allodynia-like phenomenon which lasts for several days and is possibly related to dysfunction of the GABAB system in the spinal cord. In some animals this is followed by a chronic
allodynia-like symptom with an onset varying between 1 week and 1.5 months after injury, expressed as a clearly painful reaction to light pressure applied to a skin area at or near the dermatome of the injured spinal segments. In the majority of rats the
allodynia persists over several months, in some cases accompanied by autotomy of the hind paws. Pharmacological studies indicated that the
allodynia in the majority of rats could be relieved by systemic
tocainide (75 mg/kg).
Morphine was only effective at a
sedative dose (5 mg/kg). The
allodynia was not relieved by
baclofen,
muscimol,
clonidine or
carbamazepine. Low-dose systemic
pentobarbital (5 mg/kg) had a slight beneficial effect.
Guanethidine (20 mg/kg, s.c.) did not abolish the
allodynia in most of the rats. Histological examination revealed massive damage in the spinal cord. The dorsal roots of the irradiated segments were also injured. No morphological abnormalities were seen in the dorsal root ganglia. The mechanism that may account for this
chronic pain-related syndrome in spinally injured rats probably involves abnormalities in the central nervous system. The
allodynia seen in chronic spinally injured rats was similar to some painful symptoms in patients after
spinal cord injury or
stroke. It is suggested that the chronic
allodynia-like phenomenon may represent an animal model for studying the mechanisms of chronic central
pain.