Developmentally regulated expression of CaMKII and iGluRs in the rat retina

Calcium/calmodulin-dependent protein kinase II (CaMKII) and the ionotropic glutamate receptors (iGluRs) have been shown to be pivotal in the maturation of synapses during development of the central nervous system. The purpose of the current study was to assay the expression profiles of these molecules during the development of the rat retina. The mRNA levels of CaMKII were determined by the semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) method. The protein levels of CaMKII were assayed in slot blots. The CaMKII enzyme activity was also measured. In addition, the protein levels of iGluRs in a retinal membrane-enriched fraction were evaluated in Western blots. The results show that the levels of CaMKII (mRNA, protein, and activity) and distinct subunits of iGluR proteins increased during the first 2 weeks after birth. The highest level of CaMKII was reached during the second postnatal week, coincident with the peak of synaptogenesis in the inner plexiform layer of the rat retina. The expressions of NMDAR-NR1 and -NR2A were relatively low in the first postnatal week but rose quickly thereafter. However, NMDAR-NR2B was relatively high at postnatal day 5 (P5) and increased steadily during the postnatal period. Thus, the subunit compositional profile of the retinal NMDARs was altered during retinal maturation. The developmental pattern of AMPAR-GluR1 was similar to that of NMDAR-NR2B, with high expression at P5, and modest increases thereafter. The patterns of CaMKII and NR1/NR2A were better correlated than were CaMKII and NR2B, or CaMKII and GluR1. The temporal differences in subunit expression of these synaptically relevant molecules suggest that they play distinct roles during the development of the retina.