Fear conditioning performance and NMDA receptor subtypes: NR2A differential expression in the striatum.

While considerable evidence implicates NMDA receptors in the hippocampus in contextual fear conditioning, the role of other brain regions is less well understood. To further investigate this issue, rats were subjected to a contextual fear conditioning task and then classified as high or low responders according to performance. Density of NMDA receptors was evaluated using [3H]MK-801 autoradiography in 52 brain areas and expression of NR2A and NR2B subunits was studied with in situ hybridization in the same brains. Results revealed no differences between high- and low-performance rats in NMDA receptor binding in any of the brain areas studied. Similarly, NR2B subunit expression was also not different between groups. However, NR2A expression was significantly higher in the caudate-putamen of low-performance rats. These results suggest that NMDA receptors in the caudate-putamen may also be involved in contextual fear conditioning performance.