Antinociceptive synergism between supraspinal and spinal sites after subcutaneous morphine evidenced by CNS morphine content.

Morphine antinociception after various administration routes was estimated by the tail-flick method in rats. The antinociceptive ED50 (AD50) values for i.c.v., i.t., i.c.v. + i.t. (4:1 dose ratio) and s.c. were 6.9 micrograms, 0.49 + 0.12 micrograms and 2.7 mg/kg, respectively. Isobolographic analysis of AD50 (except s.c.) suggested that concurrent administration of i.c.v. and i.t. morphine interacted multiplicatively to produce antinociception. Morphine content in the CNS after administration of AD50 morphine for each route was estimated. Isobolographic analysis of morphine content revealed that supraspinal and spinal morphine interacted multiplicatively to produce antinociception after i.c.v. + i.t. and s.c. administration. Comparison of the dose-response curves (i.c.v. alone, i.t. alone, various i.c.v. + fixed i.t., fixed i.c.v. + various i.t.) suggested that supraspinal and spinal morphine can potentiate the antinociception induced by the other site, and that they have almost equal importance in the antinociceptive synergism. These results provide direct evidence for the synergism between supraspinal and spinal morphine to play an important role in the antinociception of systemically administered morphine.