Effects of dopaminergic agents on carrageenan hyperalgesia after intrathecal administration to rats

The present study explored the role of dopaminergic transmission in spinal cord in a model of carrageenan-induced inflammatory pain by examining the effects of selective agonists and antagonists of dopamine receptors. The results were as follows: (1) trans-(-)-4aR-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-1H-pyrazolo[3,4-g] quinoline hydrochloride (LY171555), a dopamine D(2) receptor agonist, produced anti-hyperalgesia (150 and 300 nmol) or hypoalgesia (300 nmol) in the inflamed hindpaws and non-inflamed hindpaws, respectively; spiperone hydrochloride (8-[4-(4-fluorophenyl)-4-oxobutyl]-1-phenyl-1,3,8-triazaspiro[4,5]decan-4-one hydrochloride), a dopamine D(2) receptor antagonist, decreased the pain threshold of non-inflamed hindpaws (300 nmol). (2) (+/-)-SKF38393 hydrochloride ((+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride), a dopamine D(1) receptor agonist, had no effect on either hindpaw, even at a higher dose (300 nmol); R(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (R(+)-SCH23390 hydrochloride), a dopamine D(1) receptor antagonist, induced anti-hyperalgesia in the inflamed hindpaws (300 nmol). The present results suggest that the dopaminergic system in the spinal cord is involved in the central modulation of inflammatory hyperalgesia, and that the different effects are probably induced by different receptors.