Chronic pain-related syndrome in rats after ischemic spinal cord lesion: a possible animal model for pain in patients with spinal cord injury

Abstract

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 GABA B 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.