DIFFERENTIAL DESENSITIZATION OF IONOTROPIC NON-NMDA RECEPTORS HAVING DISTINCT NEURONAL LOCATION AND FUNCTION

The release of tritium from rat hippocampal synaptosomes prelabeled wi th [H-3]noradrenaline ([H-3]-NA) or [H-3]5-hydroxytryptamine ([H-3]5-H T) and from rat neocortex synaptosomes prelabeled with [H-3]choline an d the release of endogenous GABA and glutamate from rat neocortex syna ptosomes were monitored during superfusion with media containing varyi ng concentrations of lpha-amino-3-hydroxy-5-methyl-4-isoxazolepropioni c acid (AMPA) or kainic acid. Concentration-dependent release potentia tions were elicited by both excitatory amino acids (EAAs) in all the t ransmitter systems investigated. The releases evoked by 100 mu M AMPA were, in all cases, almost totally dependent on external Ca2+ and sens itive to 6,7-dinitroquinoxaline-2,3-dione (DNQX), indicating involveme nt of non-NMDA receptors. When cyclothiazide, a drug able to prevent d esensitization of AMPA-preferring receptors, was added to the superfus ion medium (at 1 or 10 mu M) concomitantly with 100 mu M AMPA or kaina te, the EAA-evoked release of [H-3]NA was significantly enhanced. Conc anavalin A, a lectin thought to prevent desensitization of kainate-pre ferring receptors, had no effect (up to 10 mu M) on the release of [H- 3]NA evoked by AMPA or kainate. The effect of cyclothiazide was lost i f, after an 8-min pretreatment, the drug was removed just before the A MPA stimulus. When added concomitantly with the EAAs, cyclothiazide po tentiated the release of [H-3]5-HT elicited by AMPA and, less so, that evoked by kainate. Concanavalin A was ineffective. Neither cyclothiaz ide (1 or 10 mu M) nor concanavalin A (3 or 10 mu M) could affect the release of [H-3]ACh or endogenous GABA provoked by 100 mu M AMPA or ka inate, suggesting that the receptors involved do not desensitize. Expo sure of neocortex synaptosomes to AMPA or kainate concomitantly with c yclothiazide caused endogenous glutamate release that did not differ f rom that evoked by the EAAs alone. In contrast, the effects of AMPA an d kainate were potentiated by concanavalin A. The activity of the lect in (3 mu M) persisted when it was applied for 8 min and then removed b efore the AMPA or kainate (100 mu M) pulse. When hippocampal synaptoso mes prelabeled with [H-3]NA were subjected to three subsequent AMPA (1 00 mu M) stimuli (S-1, S-2 and S-3), the release of [H-3]NA decreased dramatically from S-1 to S-3 (S-3/S-1 = 0.14 +/- 0.04); a significant 'protection' of the AMPA effect was offered by 1 mu M cyclothiazide (S -3/S-1 = 0.36 +/- 0.06). This value did not differ from the S-3/S-1 ra tio (0.38 +/- 0.04) obtained in parallel experiments with 12 mM K+. Th e release evoked by high-K+ was insensitive to cyclothiazide. Finally, the effect of AMPA on the release of [H-3]ACh did not respond to cycl othiazide also during three subsequent stimuli with 100 mu M AMPA. To conclude: a) ionotropic non-NMDA receptors mediating enhancement of NA , 5-HT, ACh, GABA and glutamate release exist on the corresponding ner ve terminals; b) the receptors present on noradrenergic and serotonerg ic neurons are AMPA-preferring receptors, whereas the glutamate autore ceptors resemble most the kainate-preferring subtype; the receptors me diating ACh and GABA release can not be subclassified at present; c) d esensitization may not be a property of all non-NMDA ionotropic recept ors. The receptors here characterized represent five models of native non-NMDA receptors suitable for pharmacological and molecular studies.