N-methyl-D-aspartic acid (NMDA) and non-NMDA receptors regulating hippocampal norepinephrine release. I. Location on axon terminals and pharmacological characterization.

UNLABELLED

The effects of endogenous and exogenous agonists at excitatory amino acid receptors mediating enhancement of [3H]norepinephrine [( 3H]NE) release have been investigated using superfused rat hippocampal synaptosomes. In Mg(++)-free medium L-glutamic acid (L-Glu), L-aspartic acid (L-Asp), N-methyl-D-aspartic acid (NMDA), kainic acid, (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and quisqualic acid (QA) all increased the release of [3H]NE. L-Glu produced the largest effect. In the presence of Mg++ (1.2 mM), the effect of L-Glu decreased by about 40%; L-Asp and NMDA lost completely their activity while the effects of kainic acid, QA and AMPA did not change significantly. Similarly to NMDA, the effect of L-Asp was augmented by glycine and blocked by NMDA receptor antagonists, while it was insensitive to the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). The effect of L-Glu on [3H] NE release was partly decreased by the NMDA receptor channel blocker (+)-5-methyl-10,11-dihydro-5-H-dibenzo[a,d]cycloepten-5,10-imine (MK-801) and partly by CNQX; when present together, the two antagonists completely abolished the L-Glu effect. The QA enhancement of [3H]NE release was antagonized by CNQX but it was insensitive to other classical non-NMDA receptor antagonists.

IN CONCLUSION

1. release-enhancing NMDA and non-NMDA receptors exist on noradrenergic axon terminals of rat hippocampus; 2) L-Asp appears to be a potent selective NMDA receptor agonist while L-Glu can activate also non-NMDA receptors; 3) the NE-releasing receptor activated by QA may represent a QA/AMPA receptor subtype.