Enhanced NR2A subunit expression and decreased NMDA receptor decay time atthe onset of ocular dominance plasticity in the ferret

Enhanced NR2A subunit expression and decreased NMDA receptor decay time at the onset of ocular dominance plasticity in the ferret. J. Neurophysiol. 81 : 2587-2591, 1999. The NMDA subtype of glutamate receptor is known to exhib it marked changes in subunit composition and functional properties during n eural development. The prevailing idea is that NMDA receptor-mediated synap tic responses decrease in duration after the peak of cortical plasticity in rodents. Accordingly, it is believed that shortening of the NMDA receptor- mediated current underlies the developmental reduction of ocular dominance plasticity. However, some previous evidence actually suggests that the dura tion of NMDA receptor currents decreases before the peak of plasticity. In the present study, we have examined the time course of NMDA receptor change s and how they correlate with the critical period of ocular dominance plast icity in the visual cortex of a highly binocular animal, the ferret. The ex pression of NMDA receptor subunits NR1, NR2A, and NR2B was examined in anim als ranging in age from postnatal day 16 to adult using Western blotting. F unctional properties of NMDA receptors in layer IV cortical neurons were st udied using whole cell patch-clamp techniques in an in vitro slice preparat ion of ferret primary visual cortex. We observed a remarkable increase in N R1 and NR2A, but not NR2B, expression after eye opening. The NMDA receptor- mediated synaptic currents showed an abrupt decrease in decay time concurre nt with the increase in NR2A subunit expression. Importantly, these changes occurred in parallel with increased ocular dominance plasticity reported i n the ferret. In conclusion, molecular changes leading to decreased duratio n of the NMDA receptor excitatory postsynaptic current may be a requirement for the onset, rather than the end, of the critical period of ocular domin ance plasticity.