Are Ca²⁺-permeable kainate/AMPA receptors more abundant in immature brain?

The permeability properties of kainate/AMPA receptors are determined by subunit composition. The GluR1 and GluR3 subunits form Ca²⁺-permeable channels and exhibit inward rectification; heteromeric receptors containing the GluR2 subunit are Ca²⁺-impermeable and electrically linear. These observations raise the possibily of a developmental 'switch' in which turning on or off of GluR2 expression regulates the level of Ca²⁺ permeable kainate/AMPA receptors. We examined the ratio of GluR1 and GluR3 to GluR2 gene expression in developing and adult rat brain by in situ hybridization. A larger value of this ratio is likely to be associated with greater Ca²⁺ permeability. Our data suggest that in neocortex, striatum and cerebellum the number of Ca²⁺-permeable kainate/AMPA receptors is high at P4 and declines monotonically with age. In hippocampus, the number increases from P7 to P21, after which it declines. These findings provide evidence for a developmental 'switch' in which Ca²⁺ permeable glutamate receptors are turned off following early developmental events.