NMDA and kainate receptor expression, long-term potentiation, and neurogenesis in the hippocampus of long-lived Ames dwarf mice

Abstract

In the current study, we investigated changes in N-methyl D-aspartate (NMDA) and kainate receptor expression, long-term potentiation (LTP), and neurogenesis in response to neurotoxic stress in long-living Ames dwarf mice. We hypothesized that Ames dwarf mice have enhanced neurogenesis that enables retention of spatial learning and memory with age and promotes neurogenesis in response to injury. Levels of the NMDA receptors (NR)1, NR2A, NR2B, and the kainate receptor (KAR)2 were increased in Ames dwarf mice, relative to wild-type littermates. Quantitative assessment of the excitatory postsynaptic potential in Schaffer collaterals in hippocampal slices from Ames dwarf mice showed an increased response in high-frequency induced LTP over time compared with wild type. Kainic acid (KA) injection was used to promote neurotoxic stress-induced neurogenesis. KA mildly increased the number of doublecortin-positive neurons in wild-type mice, but the response was significantly enhanced in the Ames dwarf mice. Collectively, these data support our hypothesis that the enhanced learning and memory associated with the Ames dwarf mouse may be due to elevated levels of NMDA and KA receptors in hippocampus and their ability to continue producing new neurons in response to neuronal damage.

Fig. 1

NMDA receptor mRNA expression in the hippocampus of wild-type and Ames dwarf mice. The mRNA levels of NR1, NR2A, and NR2B were assessed in the hippocampi of both genotypes. The levels of NR1 and NR2A were higher in the Ames dwarf as compared with wild-type mice. Data are presented as mean ± SEM. *Indicates significance (p < 0.05) between the wild-type and Ames dwarf mice. n = 5–6/genotype

Fig. 2

NMDA receptor protein levels in hippocampal tissues of untreated wild-type and Ames dwarf mice. The protein levels of NR1, NR2A, and NR2B were assessed in the hippocampi of both genotypes. The levels of NR2A and NR2B were higher in the Ames dwarf as compared with wild-type mice. Data are presented as mean ± SEM. *Indicates significance (p < 0.05) between the wild-type and Ames dwarf mice. n = 5–6/genotype

Fig. 3

Kainate receptor mRNA expression levels in hippocampus of wild-type and Ames dwarf mice. The mRNA levels of KAR1, KAR2, GluR5, GluR6, and GluR7 were assessed in the hippocampi of both genotypes. The expression levels of KAR2 were higher in the Ames dwarf as compared with wild-type mice. Following KA injection, KAR2, GluR5, GluR6, and GluR7 mRNA decreased. Data are presented as mean ± SEM. In both genotypes, n = 5–6 for each treatment group

Fig. 4

Kainate receptor protein levels in hippocampus of untreated wild-type and Ames dwarf mice. The protein levels of KAR2, GluR5, and GluR6/7 were assessed in hippocampi of both genotypes. The levels of KAR2 were higher in Ames dwarf as compared with wild-type mice. Data are presented as mean ± SEM. *Indicates significance (p < 0.05) between the wild-type and Ames dwarf mice. n = 5–6/genotype

Fig. 5

Long-term potentiation: The excitatory post synaptic potential (EPSP) slope, measured as a percentage of baseline, is shown for the last 10 min before HFS and 20 min after HFS. The Ames dwarf (closed circles; n = 4 mice, slices—14) tended to show higher LTP as compared with wild-type mice (open squares; n = 4 mice, slices—8). Data are presented as mean ± SEM

Fig. 6

Quantification of neurogenesis using doublecortin (DCX) as a marker of committed, proliferating cells at the end of neurogenesis. The top panel (4A-J) shows the dentate gyrus immunolabeling with DCX. Mice treated with KA showed increased neurogenesis in both genotypes. Stereological quantification of DCX-positive neurons is graphed for wild-type and Ames dwarf mice. The number of DCX-positive neurons was significantly higher in Ames dwarf mice with KA 15 mg/kg as compared with age-matched saline injected animals (**p < 0.01) and when compared with wild-type mice with KA doses of 15 mg/kg (#p < 0.05) and 30 mg/kg (#p < 0.05). Data are presented as mean ± SEM (n = 4 for wild-type mice in each treatment group, n = 3 for Ames dwarf mice in each treatment group). Scale bars represent 200 μm from a–e and 50 μm from f–j. The experimenter was masked to treatment groups while conducting cell counts