We report behaviours suggesting the presence of
allodynia elicited by non-noxious brushing and mechanical pressure following photochemically induced ischaemic
spinal cord injury in the rat. Female rats were intravenously injected with
Erythrosin B and the T10 vertebra was irradiated with a
laser beam for 1, 5 or 10 min. These procedures initiated an intravascular photochemical reaction, resulting in ischaemic
spinal cord injury. After irradiation a clear
allodynia was observed in most rats. The animals vocalized intensely to light touch during gentle handling and were clearly agitated to light brushing of the flanks. The vocalization threshold in response to the mechanical pressure measured with von Frey hairs was markedly decreased during this period. In some animals the existence of spontaneous
pain was suggested by spontaneous vocalization. The duration of the
allodynia varied among animals from several hours to several days. The severity and duration of
allodynia seemed not to be related to the duration of irradiation. In
sham-operated rats a slight, transient
allodynia was also noted around the
wound within a few hours after surgery, which was effectively relieved by systemic
morphine (2 mg/kg, i.p.).
Morphine (2 mg/kg, i.p.) also partially relieved the
allodynia in spinally injured rats 4 h after irradiation. However,
morphine, even at a higher dose (5 mg/kg, i.p.), failed to alleviate the
allodynia in spinally injured rats 24-48 h after the injury. Systemic injection of the GABAB agonist
baclofen (0.01-0.1 mg/kg, i.p.), but not the GABAA agonist
muscimol (1 mg/kg, i.p.), effectively relieved
allodynia during this period. Pretreatment with
guanethidine 24 h and just prior to the irradiation (20 mg/kg, s.c.) did not prevent the occurrence of
allodynia in spinal cord injured rats. The present observation is the first to show that ischaemic
spinal cord injury could result in cutaneous
mechanical allodynia. This phenomenon is resistant to
morphine and may not involve the sympathetic system. Histological examination of allodynic animals 3 days after
spinal cord injury revealed considerable morphological damage in the dorsal spinal cord of a rat irradiated for 5 min. The related dorsal roots were also slightly affected in this animal, while the dorsal root ganglia were normal. However, in rats irradiated for 1 min, despite the existence of strong
allodynia, no damage could be found at this time in the spinal cord, dorsal roots or dorsal root ganglia. It is suggested that functional deficits in the GABAB system in the spinal cord may be related to this
allodynia-like phenomenon.(ABSTRACT TRUNCATED AT 400 WORDS)