Pharmacological heterogeneity of NMDA receptors in cerebellar granule cells in immature rat slices. A microfluorimetric study with the [Ca2+]i sensitive dye Indo-1.

We have developed a technique for monitoring the internal calcium concentration--[Ca2+]i--in a single selected cell in cerebellar slices of 8-day-old rats. In post-migratory granule cells NMDA induced a rapid, reversible and concentration-dependent increase in [Ca2+]i (+22% at 10 microM and +210% at 100 microM). This effect was dependent on the presence of Ca2+ in the extracellular medium. When Mg2+ was absent from the perfusion buffer the sensitivity to NMDA was greatly increased (+108% at 10 microM). The NMDA response was not affected by glycine site agonists or by tetrodotoxin, a sodium channel blocker. In the absence of magnesium, the NMDA (10 microM)-induced increase in [Ca2+]i was prevented in a monophasic manner by the recognition site antagonist 2-amino-phosphonovalerate (2-APV; IC50 = 13 microM), and in a biphasic manner by the glycine site antagonist 7-chlorokynurenate (IC50s = 25 nM and 5.9 microM) and by the channel blocker dizocilpine (IC50s = 5 nM and 3 microM). In contrast, this NMDA response was only partially antagonized by the polyamine site antagonists ifenprodil and eliprodil (maximal inhibition approximately 50% at concentrations > or = 10 microM, IC50 1 and 2 microM, respectively). These results demonstrate the presence in granule cells from immature rat cerebellum of at least two NMDA receptor populations which can be differentiated by their sensitivity to inhibitors acting upon the different sites of the NMDA receptor complex.