Glutamate binding in the rat cerebral cortex during ontogeny.

Two binding sites for L-glutamate have been identified on adult rat brain cortical membranes. One of these sites is Na+-dependent with Kd of 1.3 micro M and a Bmax of 210 pmol/mg protein. The other is Na+-independent with a Kd of 0.37 micro M and Bmax of 6.2 pmol/mg protein. There is a sharp rise in total number of Na+-independent sites per cortex up to 20 days postnatally followed by a more gradual rise to adult levels at 50 days. Na+-dependent binding is also low at birth rising to a peak at 20 days followed by a drop in total levels of binding to 30 days and then a very sharp rise up to 50 days. The kinetics of binding at 20 days gives a Kd for the Na+-dependent site of 1.77 micro M and a Bmax of 82 pmol/mg protein. The Na+-independent site at 20 days has a Kd = 1.3 micro M and Bmax of 8.47 pmol/mg protein. The ability of several acidic amino acid analogues to displace specifically bound L-glutamate was investigated by estimating IC50 values at 20 and 50 days of age. The Na+-independent site is stereospecific for L-glutamate at both ages, but will also interact with L-aspartate at 20 days. The Na+-dependent site has a similar affinity for L- and D-glutamate and L-aspartate at 50 days. The L-glutamate analogue kainate will not displace any bound L-glutamate.