Antinociceptive mechanisms of platycodin D administered intracerebroventricularly in the mouse.

Platycodin D administered intracerebroventricularly (i.c.v.) showed an antinociceptive effect in a dose-dependent manner as measured by the tail-flick assay. The antinociception induced by platycodin D was maintained at least 1 h. MK-801 [(+/-)-5-methyl-10,11-dihydro-5 H-dibenzo[ a,d]cyclohepten-5,10-imine maleate], a competitive N-methyl- D-aspartic acid (NMDA) receptor antagonist, or CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), a non-NMDA receptor antagonist, muscimol (a GABA(A) receptor agonist), or baclofen (a GABA(B) receptor antagonist), or sulfated cholecystokinin (CCK-8 s; CCK A receptor agonist), injected i.c.v. significantly reduced the inhibition of the tail-flick response induced by platycodin D administered i.c.v. Additionally, intrathecal (i.t.) pretreatment with yohimbine (an alpha 2 -adrenergic receptor antagonist) or methysergide (a serotonin receptor antagonist) dose-dependently attenuated inhibition of the tail-flick response induced by i.c.v. administered platycodin D. However, naloxone (an opioid receptor antagonist) did not affect the inhibition of the tail-flick response induced by platycodin D. Our results suggest that platycodin D has an antinociceptive effect when it is administered supraspinally, and supraspinal GABA(A), GABA(B), NMDA and non-NMDA receptors are involved in platycodin D-induced antinociception. Furthermore, platycodin D administered supraspinally produces antinociception by stimulating descending noradrenergic and serotonergic, but not opioidergic, pathways.