In vivo evaluation of (+)-MR200 as a new selective sigma ligand modulating MOP, DOP and KOP supraspinal analgesia

The compound (+)-MR200 [(+)-methyl (1R,2S)-2-{[4-(4-chlorophenyl)-4-hydroxypiperidin-1-yl]methyl}-1-phenylcyclopropanecarboxylate] is a sigma ligand with increased affinity and selectivity compared to the structurally related ligand haloperidol. From the results of a previous study on the modulation of a systemically injected KOP opioid agonist analgesia by (+)-MR200, we analysed the influence of this sigma ligand on the antinociceptive effect of centrally injected MOP, DOP, and KOP selective agonists using the tail-flick test in rats. The results obtained confirmed that systemic administration of (+)-MR200 (1 mg/Kg s.c.) did not modify basal tail-flick latency. Pre-treatment with 1 mg/Kg s.c. of (+)-MR200 provided a significant increase in the antinociceptive effect of DAMGO (100ng/rat i.c.v.) and DPDPE (20 mu g/rat i.c.v.). Conversely to previous reports, pre-treatment with (+)-MR200 reversed, in these experimental conditions, U-50488H (100 mu g/rat i.c.v.) analgesia. The mechanism involved in these effects was not clear, but provided additional data on a diverging modulator role of selective sigma-1 antagonists on KOP analgesia.