Laminar distribution of NMDA receptor subunit (NR1, NR2A, NR2B) expression during the critical period in cat visual cortex.
Changes in NMDA subunit composition may be part of the molecular basis for critical period plasticity. The present study used immunohistochemistry to determine developmental changes in the laminar distribution of the three major cortical NMDA subunits (NR2A, NR2B, NR1) during the critical period in cat visual cortex. For all three subunits, at 1 week staining was concentrated in two bands: an upper band consisting of layer I, the compact zone and the upper half of the cortical plate; a lower band consisting of layers V and VI. In the lower part of the cortical plate (immature layer IV) staining was very low. For NR2A and NR2B, immunoreactivity in layer IV remained low until 10 weeks of age. At 20 weeks and adult, layer IV filled in and NR2A and NR2B label was rather uniform across all layers. NR1 showed a developmental pattern of expression different from NR2A and NR2B after 1 week. At 5 and 10 weeks, label was prominent in layer IV and superficial layers, but low in layers V and VI. The main change after 10 weeks was a progressive decrease in staining, such that in older animals label was markedly densest in superficial layers. Thus, during the rise of the critical period, NR1 is the dominant subtype in layer IV and could play a role in anatomical ocular dominance column formation and plasticity. At the same time, NR2A and NR2B subunits are concentrated outside layer IV, and could be related to physiological plasticity in extragranular layers, which precedes and outlasts plasticity in layer IV. For all three NMDA receptor subunits, the laminar distribution was similar in normal and dark reared visual cortex at 20 weeks, indicating that the developmental changes in laminar pattern of expression are independent of visual input.