A high GluR1 : GluR2 expression ratio is correlated with expression of Ca2+-binding proteins in rat forebrain neurons

alpha-Amino-3-hydroxy-5-methyl-4-isoxazle propionic acid (AMPA) receptors a re ubiquitously expressed; however, their subtypes and abundance vary from region to region. We classified the neurons in various forebrain regions (h ippocampus, striatum, amygdala, piriform cortex and somatosensory cortex) i nto six types: [R1+/R2+], [R1-/R2+], [R1+/R2-], [R1-/R2-], [R1++/R2+] and [ R1++/R2-], and analysed the expression of Ca2+-binding proteins, such as pa rvalbumin and calbindin-D28k, using a triple-staining method. The neurons s howing a high GluR1 : GluR2 expression ratio, [R1+/R2-], [R1++/R2+] and [R1 ++/R2-] neurons, comprised 13-30% of the total neuronal population. In addi tion, the expression of Ca2+-binding proteins was mainly observed in these three types of neurons. The results suggest that Ca2+-binding protein-posit ive neurons express Ca2+-permeable AMPA receptors, because the Ca2+-permeab ility of AMPA receptors is enhanced by the relative scarcity of the GluR2 s ubunit. To directly test the possibility that Ca2+-binding protein-positive neurons express Ca2+-permeable AMPA receptors, we performed Ca2+-imaging e xperiments in cultured cortical neurons. Ca2+ influx through AMPA receptors was measured selectively by addition of AMPA together with cyclothiazide i n the presence of blockers of other Ca2+ influx routes. More than half of t he calbindin-D28k-positive neurons showed a large increase in the intracell ular Ca2+ concentration ([Ca2+](i)), whilst most of the calbindin-D28k-unde tectable neurons exhibited only a slight rise in [Ca2+](i) after AMPA addit ion. These results suggest that the expression of calbindin-D28k is related to the expression of Ca2+-permeable AMPA receptors.