Splice variants of glutamate receptor subunits 2 and 3 in striatal projection neurons

The distribution of glutamate receptor subunits (GluR) 2 and 3 in relation to striatal efferent neurons was examined. Our results showed that striatonigral and striatopallidal neurons differed in the relative proportion of splice variant messenger RNAs expressed. The rank order of the percentage of striatonigral neurons expressing a particular splice variant was GluR2-flop (approximately 40%) > GluR3-flop (approximately 20%) > GluR2-flip (approximately 10-15%) > GluR3-flip (approximately 10%). For striatopallidal neurons, the pattern of expression was GluR3-flop (approximately 35%) > GluR2-flop (approximately 20%) > GluR3-flip (approximately 15-20%) > GluR2-flip (< 10%). By immunohistochemical methods, we observed that approximately half of striatal projection neurons stained for GluR2/4, with no discernible difference in the distribution or proportion of striatonigral neurons labeled as compared to striatopallidal neurons. In the case of striatonigral neurons, the immunohistochemical data support the results of in situ hybridization studies. However, with regard to striatopallidal neurons, the percentage of neurons expressing GluR2 messenger RNA (< 35%) is lower than the percentage of neurons expressing the receptor protein (approximately 50%). This disparity underscores the importance of looking at both messenger RNA expression and protein abundance rather than relying on a single measure of receptor expression. The results of in situ hybridization and immunohistochemical studies demonstrate inhomogeneity within both striatonigral and striatopallidal neuron populations since some, but not all, of these neurons express GluR2 and/or GluR3. These data suggest that specific populations of striatal projection neurons express unique complements of excitatory amino acid receptors, which may be important in the understanding of both normal striatal function and basal ganglia disease.