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 spin al opiate analgesia was attenuated by neurotransmitter release from de scending pathways. To determine if this phenomenon involved excitatory amino acids (EAAs), 0.25 nm of N-methyl-D-aspartate (NMDA) or pha-ami no-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) were i.t. co-a dministered with morphine to Intact and Spinal rats. NMDA potentiated morphine antinociception in Intact but not Spinal rats; AMPA had no ef fect in Intact rats, but significantly reduced morphine antinociceptio n in Spinal rats. The data suggest a reciprocal descending, modulatory influence on the spinal interaction between EAAs and morphine.