Development of NR1, NR2A and NR2B mRNA in NR1 immunoreactive cells of rat visual cortex.

In cells marked for N-methyl-D-aspartate receptors (NMDARs), we studied the relationship between the sensitive period for monocular deprivation and the expression of rat NMDAR subunits, NR2A and NR2B. In the rat the sensitive period ends sometime after postnatal day 50 (P50). Previous studies of the development of these subunit mRNAs focused on animals prior to the end of the sensitive period and did not examine the visual cortex specifically. We used a monoclonal antibody to the NR1 subunit of the receptor to identify cells containing NMDARs. We then used in situ hybridization to label the same sections for NR2A or NR2B mRNA. In an additional experiment we labeled sections for NR1 mRNA to see if the developmental profile was similar at both the mRNA and protein level. We used five animals at each of four ages: P22, P30, P45 and P90. Staining for NR2B mRNA, but not for NR2A mRNA, decreased dramatically from P22 to P45. Staining for NR1 mRNA declined dramatically between P22 and P45 even though most cells remained strongly immunopositive for the NR1 protein during this time. This discrepancy suggests that significant NR1 regulation occurs after gene transcription. Because most of the decrease in NR1 mRNA and NR2B mRNA occurs by P30, transcriptional regulation of these subunits does not easily explain the loss of sensitivity to monocular deprivation, which occurs around P50. The changes are, in fact, more closely synchronized with the beginning of experience-dependent plasticity than with its end.