INTRATHECAL COADMINISTRATION OF MORPHINE AND EXCITATORY AMINO-ACID AGONISTS PRODUCE DIFFERENTIAL-EFFECTS ON THE TAIL-FLICK OF INTACT AND SPINAL RATS

Previous reports, that intrathecal morphine is more potent on the tail-flick test of acute spinal rats than intact rats, suggested that spinal opiate analgesia was attenuated by neurotransmitter release from descending pathways. To determine if this phenomenon involved excitatory amino acids (EAAs), 0.25 nm of N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) were i.t. co-administered with morphine to Intact and Spinal rats. NMDA potentiated morphine antinociception in Intact but not Spinal rats; AMPA had no effect in Intact rats, but significantly reduced morphine antinociception in Spinal rats. The data suggest a reciprocal descending, modulatory influence on the spinal interaction between EAAs and morphine.