A, different N-terminal (a and b) and C-terminal (1 to 4) variants of the NR1 subunit. B, primers used to identify the different NR1 variants, and ethidium bromide stained agarose gel showing RT-PCR products for the mRNA for the different variants from adult rat optic nerve. C, agarose gel showing RT-PCR products for the different NR2 subunits. D, agarose gel showing RT-PCR products for the different NR3 subunits.