ACTIVITY-DEPENDENT REGULATION OF N-METHYL-D-ASPARTATE RECEPTOR SUBUNIT EXPRESSION IN RAT CEREBELLAR GRANULE CELLS

The glutamate receptor channels of the N-methyl-D-aspartate (NMDA) sub type are composed of different subunits named NR1 and NR2A-D, These su bunits can combine in different oligomers with diverging properties an d their expression is developmentally regulated. We have used rat cere bellar slice cultures to test the involvement of bioelectrical activit y and synaptic transmission in the changes in NR2A-C expression observ ed in developing granule coils. A correlation between the functional p roperties of the NMDA receptors and expression of the NR2A-C mRNAs was obtained in single granule cells by coupling patch-clamp recording an d reverse transcription followed by polymerase chain reaction. Granule cells grown under standard culture conditions expressed mainly NR2A m RNA when examined after 15-40 days in vitro. Consistent with this obse rvation, their responses to NMDA were only weakly reduced by 3 mu M if enprodil, a non-competitive antagonist which discriminates between NR2 A and NR2B subunits in expression systems. In cerebellar cultures chro nically exposed to tetrodotoxin to eliminate spontaneous electrical ac tivity, granule cells maintained a predominant expression of NR2B subu nits and their responses to NMDA were largely inhibited by 3 mu M ifen prodil. These results provide evidence that the expression of the NR2A and B subunits is regulated through an activity-dependent mechanism l eading to the formation of NMDA receptors with different pharmacologic al properties. Finally the NR2C subunit, abundantly expressed in vivo by adult granule cells, was only rarely detected in slice cultures, ev en when excitatory synapses were formed between granule cells and fibr es originating from cocultured brainstem explants. These data suggest that the induction of NR2C expression observed in vivo requires an add itional factor(s) that remains to be identified.