Elsevier

Neuroscience

Volume 162, Issue 4, 15 September 2009, Pages 933-945
Neuroscience

Behavioural Neuroscience
Research Paper
The effects of aging on N-methyl-d-aspartate receptor subunits in the synaptic membrane and relationships to long-term spatial memory

https://doi.org/10.1016/j.neuroscience.2009.05.018Get rights and content

Abstract

There are declines in the protein expression of the NR2B (mouse ε2) and NR1 (mouse ζ1) subunits of the N-methyl-d-aspartate (NMDA) receptor in the cerebral cortex and hippocampus during aging in C57BL/6 mice. This study was designed to determine if there is a greater effect of aging on subunit expression and a stronger relationship between long-term spatial memory and subunit expression within the synaptic membrane than in the cell as a whole. Male, C57BL/6JNIA mice (4, 11 and 26 months old) were tested for long-term spatial memory in the Morris water maze. Frontal cortex, including prefrontal regions, and hippocampus were homogenized and fractionated into light and synaptosomal membrane fractions. Western blots were used to analyze protein expression of NR2B and NR1 subunits of the NMDA receptor. Old mice performed significantly worse than other ages in the spatial task. In the frontal cortex, the protein levels of the NR2B subunit showed a greater decline with aging in the synaptic membrane fraction than in the whole homogenate, while in the hippocampus a similar age-related decline was observed in both fractions. There were no significant effects of aging on the expression of the NR1 subunit. Within the middle-aged mouse group, higher expression of both NR2B and NR1 subunits in the synaptic membrane of the hippocampus was associated with better memory. In the aged mice, however, higher expression of both subunits was associated with poorer memory. These results indicate that aging could be altering the localization of the NR2B subunit to the synaptic membrane within the frontal cortex. The correlational results suggest that NMDA receptor functions, receptor subunit composition, and/or the environment in which the receptor interacted in the hippocampus were not the same in the old animals as in younger mice and this may have contributed to memory declines during aging.

Section snippets

Animals

Thirty-four, male C57BL/6JNIAHSD mice (National Institute on Aging, Bethesda, MD, USA) from three different age groups (4, 11, and 26 months of age at time of behavioral testing and euthanasia) were used. Mice were ad libitum–fed and housed under 12-h light/dark conditions for 1.5 months prior to and during behavioral testing at University of Idaho. All animal procedures were approved by the Institutional Animal Care and Use Committee at University of Idaho and conformed to NIH guidelines.

Behavioral testing results

There was a significant main effect of age (F(2,31)=7.5, P=0.002) and trial block (F(7,217)=11.6; P<0.0001) on corrected cumulative proximity in the place training trials (Fig. 2A, B). The 26-month-old mice had significantly higher corrected cumulative proximity than both the 4- (P=0.002) and 11- (P=0.017) month-old mice when data were averaged across all blocks of place trials (Fig. 2B). The 4- (P=0.0003) and 11- (P=0.0002) month-old mice had significantly lower corrected cumulative

Discussion

There were four major findings in this study. (1) The results of this study support the hypothesis that there was a greater decline during aging in the protein expression of the NR2B subunit within the synaptic membrane of the frontal cortex than in the tissue as a whole. (2) In the hippocampus, the expression of the NR2B subunit showed similar declines between whole homogenate and synaptic membranes. (3) Both the NR2B and NR1 subunits of the NMDA receptor within the synaptic membranes of the

Conclusion

In conclusion, there were differences in the effects of aging on expression of the NR2B subunit within frontal cortex versus the hippocampus. In the frontal cortex there appeared to be an effect of aging on the localization of the NR2B subunit of the NMDA receptor to the synaptic membrane that was greater than changes in whole cell expression. The changes in synaptic expression of the NR2B subunit in the hippocampus appeared to be more related to declines throughout the cell, suggesting a

Acknowledgments

This work was supported by NIH grant AG16322 to K.R.M. We would also like to thank David G. Standaert for his advice on the subfractionation protocol.

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