The mechanism of L-DOPA for antinociception was investigated. Nociceptive behaviors in mice after an intrathecal (i.t.) administration of substance P were evaluated. L-DOPA (i.t.) dose-dependently attenuated the substance P-induced nociceptive behaviors. Co-administration of benserazide (i.t.), a DOPA decarboxylase inhibitor, abolished the antinociceptive effect of L-DOPA. The L-DOPA-induced antinociception was antagonized by sulpiride, a D2 blocker, but not by SCH 23390, a D1 blocker. These results suggest that L-DOPA relieves pain after conversion to dopamine, with the dopamine sedating pain transmission by way of the dopamine D2 receptor.
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Shimizu et al. (2004) studied this question.