## 2 Results
### Changes in NMDA Receptors Subunits and NOS Activities Related to Chronic SD (Experiment III)  
Next, we investigated the impact of long-term SD on NR1, NR2A, and NR2B expression in the frontal cortex, and NOS isoforms activity in the R and L parietal cortices in two age groups. The results of these experiments are summarized in Figure 6A-C, Figure 7A-C, and in Table 1. With regard to NMDA receptor subunits, significant decline in NR1 and NR2A expressions (to 95.8% and 96.7%, respectively) were found in old compared to young adult controls. Mild decrease to 98.7% in NR2B reached only to borderline significance. Chronic SD-related changes in NMDA receptor subunits were not observed in young or old rats (Figure 6A-C). Figure 5: Activities of NO synthases in the right and left parietal cortex of young adult and old rats exposed to acute sleep deprivation. Activities of various NOS isoforms (**A–C**) expressed as nmoles/30 min/mg of proteins. Results are presented as means \(\pm\) SEM. Statistical significance (Student’s \(t\)-test) was calculated with respect to young adult or old controls (\({}^{*}p<0.05\), \({}^{***}p<0.001\)). RH—right hemisphere, LH—left hemisphere, nNOS—neuronal NOS, eNOS—endothelial NOS, iNOS—inducible NOS, SD—sleep deprivation, YCTRL—young adult controls (\(n=12\)), YSD—young adult rats exposed to acute SD (\(n=12\)), OCTRL—old controls (\(n=12\)), OSD—old rats exposed to acute SD (\(n=12\)). Figure 6: Expressions of NMDA receptor subunits in the frontal cortex from both hemispheres in young adult and old rats exposed to chronic sleep deprivation. Summary histograms illustrating the expressional changes in NR1 (**A**), NR2A (**B**) and NR2B (**C**) subunits. The optical density of samples of respective NMDA subunit bands was related to that of \(\alpha\)-tubulin, results are presented as means \(\pm\) SEM. Statistical significance (Student’s \(t\)-test) was calculated with respect to young adult controls (\({}^{***}p<0.001\)). SD—sleep deprivation, YCTRL—young adult controls (\(n=12\)), YSD—young adult rats exposed to chronic SD (\(n=12\)), OCTRL—old controls (\(n=12\)), OSD—old rats exposed to chronic SD (\(n=12\)). Results of ANOVA with repeated measures, one-way ANOVA, and t-test supported age-dependent changes only in eNOS activity (bilateral drops to 55.7% and to 60.6%, respectively) in experiment III (Figure 6A-C). In young rats exposed to chronic SD, changes in eNOS activity (reduction to 66.7% in the L side and 72.8% in the R side) were found when compared to age-related controls. The lowest levels of eNOS activity were observed in old rats exposed to chronic SD. Nevertheless, the values of t-test did not reach statistical significance as compared to old controls. Moreover, a notable R/L laterality was observed in nNOS and iNOS activity but not in iNOS activity in young adult controls (Table 2). Correlation analysis performed on these experimental data did not reveal changes in young adult rats exposed to chronic SD as compared to age-related controls (In old rats exposed to chronic SD, however, two significant changes involving NR2A were observed: (i) the association of NR2A and NR2B subunits was reduced (old controls: CC = +0.978, \(p<0.001\), old exposed rats: CC = +0.674, \(p=0.016\), two-tailed test \(p=0.002\)) and (ii) the association between NR2A and eNOS was enhanced (the R side--old controls: CC = \(-0.257\), \(p=0.419\), old exposed rats: CC = +0.798, \(p=0.002\), two-tailed test \(p=0.004\); the very similar results with borderline significance were found also in the L side).