Effects of intrathecal morphine, baclofen, clonidine and R-PIA on the acute allodynia-like behaviours after spinal cord ischaemia in rats

The present study assessed the efficacy and potency of intrathecal (i.t.) a dministration of the opiate morphine, the gamma -aminobutyric acid-B (GABA( B)) receptor agonist baclofen, the alpha (2)-adrenoceptor agonist clonidine and the adenosine A(1)-receptor agonist R-phenylisopropyl-adenosine (R-PIA ) on the acute allodynia-like behaviour after photochemically induced spina l cord injury (SCI) in rats. Rats displaying allodynia-like behaviours to b rushing, von Frey hairs and cold stimulation 1-2 days after photochemically induced SCI were Studied. In a Cumulative dose regime, morphine (0.1-10 mu g), baclofen (0.1-1 mug), clonidine (0.1-10 mug) and R-PIA (0.01-10 nmol) w ere administered i.t. through an implanted catheter at the lumbar spinal co rd. All tested drugs dose-dependently reduced the brushing, von Frey hairs and cold stimulation-induced allodynia-like behaviour. No increase in adver se effects such as motor deficits was found for morphine, clonidine and R-P IA. There was a slight increase in motor impairments at the highest dose of baclofen. For the mechanical allodynia, morphine appeared to be most effec tive, whereas baclofen, clonidine and R-PIA only provided a partial allevia tion. For the cold allodynia, morphine and baclofen were more effective tha n clonidine and R-PIA. In relieving acute mechanical and cold allodynia-lik e behaviours in rats 1-2 days after SCI, i.t. morphine and baclofen were su perior to clonidine and R-PIA. (C) 2001 European Federation of Chapters of the International Association for the Study of Pain.