NMDA AND AMPA RECEPTORS EVOKE TRANSMITTER RELEASE FROM NORADRENERGIC AXON TERMINALS IN THE RAT SPINAL-CORD

N-methyl-D-aspartate (NMDA) stimulated release of [H-3]noradrenaline ( NA) from prelabelled rat spinal cord slices. The release Mras partiall y insensitive to tetrodotoxin (TTX) and was inhibited by the NMDA anta gonist MK-801. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate ( AMPA) also evoked release of [H-3]NA, which was enhanced by blocking A MPA receptor desensitization with cyclothiazide. AMPA-evoked release w as inhibited by the non-NMDA antagonist 2,3-dihydroxy-6-nitro-7-sulfam oyl-benzo(NBQX) but was not affected by TTX. NMDA and AMPA showed syne rgistic effects, indicating co-existence of NMDA and AMPA receptors on noradrenergic terminals. Kainate evoked [H-3]NA release only at high concentrations and the release was not potentiated by blocking kainate receptor desensitization with concanavalin A. Thus, the results indic ate that there are stimulatory presynaptic NMDA and AMPA receptors on noradrenergic axon terminals in the spinal cord and that they interact synergistically to evoke release of [H-3]NA.