Rat brain cortex slices and synaptosomes (in a few experiments also hippocampal synaptosomes) preincubated with 3H-noradrenaline, 3H-5-hydroxytryptamine, 3H-choline, 3H-glutamate or 3H-γ-aminobutyric acid were used to investigate the 3H-transmitter release in response to exposure to N-methyl-d-aspartate (NMDA) and other excitatory amino acids. The slices and synaptosomes were superfused with Mg2+-free, otherwise physiologically composed salt solution.
In cortical slices preincubated with 3H-noradrenaline, NMDA concentration-dependently stimulated 3H overflow, whereas no such effect occurred in slices preincubated with 3H-5-hydroxytryptamine, 3H-choline, 3H-λutamate or 3H-γ-aminobutyric acid. In cortical slices preincubated with 3H-noradrenaline, the NMDA-evoked 3H overflow was abolished by tetrodotoxin, presence of Mg2+ 1.2 mmol/l or absence of Ca2+. 2-Amino-5-phosphonovaleric acid produced a parallel shift to the right of the NMDA concentration-response curve, whereas (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohept-5,10-imine hydrogen maleate (MK-801) not only shifted the concentration-response curve to the right but also reduced the maximum effect of NMDA. Other excitatory amino acid receptor agonists also stimulated 3H overflow, yielding the following rank order of potency: NMDA > l-glutamate > l-aspartate. Kainate and, in particular, quisqualate exhibited only low potencies and/or intrinsic activities. Prolonged (25 min) exposure of 3HNA-preincubated cortical slices to a high NMDA concentration produced a short-lasting peak of 3H overflow, followed by a second phase lasting as long as the compound was present; in this phase, 3H overflow was clearly less pronounced and gradually decreased with time. The stimulatory effect of a high NMDA concentration was concentration-dependently reduced by 20 min of pre-superfusion with NMDA or l-glutamate at concentrations which by themselves produced either no or, at the most, moderate increase in 3H efflux in the two 5-min periods before application of the NMDA stimulus; in contrast, the veratridine-evoked 3H overflow was increased by pre-exposure to these NMDA concentrations. Neither in 3H-noradrenaline-preincubated synaptosomes prepared from the cortex or hippocampus nor in cortical synaptosomes preincubated with 3H-5-hy-droxytryptamine did NMDA evoke 3H overflow. The veratridine-evoked 3H overflow from 3H-noradrenaline-preincubated cortical synaptosomes was not affected by simultaneous administration of NMDA.
It is concluded that NMDA selectively stimulates noradrenaline release in the rat brain cortex via NMDA receptors which appear not to be located on the noradrenergic nerve terminals. The NMDA receptor is rapidly desensitized in response to continuous application of NMDA (“tachyphylaxis”) or l-glutamate (“cross-tachyphylaxis”).