PHARMACOLOGICAL HETEROGENEITY OF NMDA RECEPTORS IN CEREBELLAR GRANULECELLS IN IMMATURE RAT SLICES - A MICROFLUORIMETRIC STUDY WITH THE [CA2-1(](I) SENSITIVE DYE INDO)

We have developed a technique for monitoring the internal calcium conc entration-[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 1 0 mu M and +210% at 100 mu M). This effect was dependent on the presen ce of Ca2+ in the extracellular medium. When Mg2+ was absent from the perfusion buffer the sensitivity to NMDA was greatly increased (+108% at 10 mu M). The NMDA response was not affected by glycine site agonis ts or by tetrodotoxin, a sodium channel blocker. In the absence of mag nesium, the NMDA (10 mu M)-induced increase in [Ca2+](i) was prevented in a monophasic manner by the recognition site antagonist 2-amino-pho sphonovalerate (2-APV;IC50 = 13 mu M), and in a biphasic manner by the glycine site antagonist 7-chlorokynurenate (IC(50)s = 25 nM and 5.9 m u M) and by the channel blocker dizocilpine (IC(50)s = 5 nM and 3 mu M ). In contrast, this NMDA. response was only partially antagonized by the polyamine site antagonists ifenprodil and eliprodil (maximal inhib ition similar to 50% at concentrations greater than or equal to 10 mu M, IC50 1 and 2 mu M, respectively). These results demonstrate the pre sence in granule cells from immature rat cerebellum of at least two NM DA receptor populations which can be differentiated by their sensitivi ty to inhibitors acting upon the different sites of the NMDA receptor complex.