PRESYNAPTIC SITE OF ACTION UNDERLYING THE ETHANOL-INDUCED INHIBITION OF NOREPINEPHRINE RELEASE EVOKED BY STIMULATION OF N-METHYL-D-ASPARTATE (NMDA) RECEPTORS IN RAT CEREBRAL-CORTEX

Rat brain cortex synaptosomes pre-incubated with [H-3]norepinephrine were used (1) to provide evidence that part of the NMDA receptors mediating stimulation of norepinephrine (NE) release are located on the noradrenergic varicosities themselves, (2) to characterize these receptors and (3) to examine whether ethanol specifically inhibits the NMDA-evoked NE release via a presynaptic site of action. In synaptosomes superfused with Mg2+-free Krebs-Henseleit solution, NMDA (2-min exposure) stimulated tritium overflow in a concentration- and glycine-dependent manner. The stimulatory effect of NMDA was not altered by tetrodotoxin but was abolished by omission of Ca2+ from the superfusion fluid and was considerably reduced in the presence of 1.2 mM Mg2+. DL-(E)-2-Amino-4-methyl-5-phosphono-3-pentanoic acid (CGP 37849; a competitive NMDA receptor antagonist) produced a parallel shift of the concentration-response curve for NMDA to the right, whereas dizocilpine (MK-801; an antagonist at the phencyclidine, PCP, recognition site of the NMDA-gated ion channel) reduced the maximum effect of NMDA. Ethanol inhibited the NMDA-evoked tritium overflow in a concentration-dependent manner. In contrast, in synaptosomes superfused with Ca2+-free Krebs-Henseleit solution containing 15 mM K+ throughout, ethanol did not affect the tritium overflow evoked by 2 min introduction of 75-mu-M Ca2+ into the superfusion fluid. This Ca2+-evoked overflow was also not altered by tetrodotoxin and dizocilpine, but was inhibited by the inorganic Ca2+ channel antagonist Cd2+. Three main conclusions can be drawn: (1) NMDA receptors mediating stimulation of NE release are also located on the cortical noradrenergic varicosities (and not only on so far unknown excitatory interneurones within the cortex); (2) these receptors exhibit the characteristic pharmacological features of the NMDA receptor system; (3) ethanol selectively inhibits the NE release evoked by stimulation of the presynaptic NMDA receptors, leaving the Ca2+-evoked release promoted by high K+ unaffected. This finding is compatible with the suggestions that the NMDA receptor system itself is a site of action of ethanol.