EFFECTS OF DECAHYDROISOQUINOLINE-3-CARBOXYLIC ACID MONOHYDRATE, A NOVEL AMPA RECEPTOR ANTAGONIST, ON GLUTAMATE-INDUCED CA2+ RESPONSES AND NEUROTOXICITY IN RAT CORTICAL AND CEREBELLAR GRANULE NEURONS

In this study, we examined the effects of a novel water-soluble, putat ive AMPA receptor antagonist, 4,4a,5,6,7,8,8a-decahydroisoquinoline-3- carboxylic acid monohydrate (LY326325), on glutamate-, N-methyl-D-aspa rtic acid (NMDA), lpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic a cid (AMPA)-, and kainic acid (KA)-induced elevations of intracellular Ca2+ concentrations ([Ca2+](i)) and Ca-45(2+) uptake, as well as gluta mate agonist-induced neurotoxicity in primary cultures of intact rat c ortical and cerebellar granule neurons. In some experiments, the actio ns of LY326325 were tested in the presence of cyclothiazide, a compoun d that is known to block glutamate-induced desensitization of AMPA-pre ferring subtypes of glutamate receptors, thereby largely potentiating the functional effects of AMPA. LY326325 fully blocked the elevations of [Ca2+](i) induced by NMDA and non-NMDA glutamate receptor agonists in both cortical and cerebellar granule neurons. The application of in creasing concentrations of cyclothiazide was not able to reverse the L Y326325-induced blockade of glutamate receptors in cortical neurons. I n contrast, the same cyclothiazide treatment fully reversed the blocka de produced by the noncompetitive AMPA/KA receptor antagonist -4-methy l-7,8-methylenedioxy-5H-2,3-benzodiazepine HCl (GYKI 52466). In Ca-45( 2+) uptake studies. LY325325 inhibited the NMDA-, AMPA-, and KA-induce d enhancement of Ca-45(2+) uptake in a concentration-dependent fashion in both cortical and cerebellar granule cells. In analogy to the resu lts obtained with [Ca2+](i) recordings, cyclothiazide failed to counte ract the LY326325-induced blockade of KA-stimulated Ca-45(2+) uptake i n cerebellar granule neurons, whereas the blockade induced by the nonc ompetitive AMPA/KA receptor blocking agent GYKI 52466 was fully revers ed by cyclothiazide. Because a similar, although not identical pattern of actions was seen following the application of the competitive AMPA /KA receptor antagonist 6-nitro-7-sulphamoyl-benzo(f)quinoxaline-2-3-d ione (NBQX), it is suggested that the inhibitory actions of LY326325 a re similar to those produced by NBQX but clearly differ from those cau sed by the noncompetitive AMPA/KA receptor antagonist GYKI 52466. Fina lly, when the neuroprotective actions of LY326325 on glutamate agonist -induced neurotoxicity were examined in cerebellar granule neurons, we found that LY326325 almost completely blocked the neurotoxic actions of NMDA, AMPA, and KA, respectively, whereas it produced only a partia l blockade of glutamate-induced neurotoxicity. Taken together, our cur rent results suggest that although LY326325 blocked both nonNMDA and N MDA-induced Ca2+ responses, it still displayed a preferential affinity for nonNMDA receptors as compared to NMDA receptors. However, LY32632 5 appears to be a less selective AMPA/KA receptor antagonist than NBQX and GYKI 52466, respectively.