Developmental Changes of Synaptic and Extrasynaptic NMDA Receptor Expression in Rat Cerebellar Neurons In Vitro

Transient expression of different NMDA receptors (NMDARs) plays a role in development of the cerebellum. Whether similar processes undergo during neuronal differentiation in culture is not clearly understood. We studied NMDARs in cerebellar neurons in cultures of 7 and 21 days in vitro (DIV) using immunocytochemical and electrophysiological approaches. Whereas at 7 DIV, the vast majority of neurons were immunopositive for GluN2 subunits, further synaptoginesis was accompanied by the time-dependent loss of NMDARs. In contrast to GluN2B- and GluN2C-containing NMDARs, which at 7 DIV exhibited homogenous distribution in extrasynaptic regions, GluN2A-containing receptors were aggregated in spots both in cell bodies and dendrites. Double staining for GluN2A subunits and synaptophysin, a widely used marker for presynaptic terminals, revealed their co-localization in about 75% of dendrite GluN2A fluorescent spots, suggesting postsynaptic origin of GluN2A subunits. In agreement, diheteromeric GluN2A-containing NMDARs contributed to postsynaptic currents recorded in neurons throughout the timescale under study. Diheteromeric GluN2B-containing NMDARs escaped postsynaptic regions during differentiation. Finally, the developmental switch favored the expression of triheteromeric NMDARs assembled of 2 GluN1/1 GluN2B/1 GluN2C or GluN2D subunits in extrasynaptic regions. At 21 DIV, these receptors represented over 60% of the NMDAR population. Thus, cerebellar neurons in primary culture undergo transformations with respect to the expression of di- and triheteromeric NMDARs that should be taken into account when studying cellular aspects of their pharmacology and functions.