Chronic Exposure to High Altitude: Synaptic, Astroglial and Memory Changes

Abstract

Long-term operations carried out at high altitude (HA) by military personnel, pilots, and astronauts may trigger health complications. In particular, chronic exposure to high altitude (CEHA) has been associated with deficits in cognitive function. In this study, we found that mice exposed to chronic HA (5000 m for 12 weeks) exhibited deficits in learning and memory associated with hippocampal function and were linked with changes in the expression of synaptic proteins across various regions of the brain. Specifically, we found decreased levels of synaptophysin (SYP) (p < 0.05) and spinophilin (SPH) (p < 0.05) in the olfactory cortex, post synaptic density-95 (PSD-95) (p < 0.05), growth associated protein 43 (GAP43) (p < 0.05), glial fibrillary acidic protein (GFAP) (p < 0.05) in the cerebellum, and SYP (p < 0.05) and PSD-95 (p < 0.05) in the brainstem. Ultrastructural analyses of synaptic density and morphology in the hippocampus did not reveal any differences in CEHA mice compared to SL mice. Our data are novel and suggest that CEHA exposure leads to cognitive impairment in conjunction with neuroanatomically-based molecular changes in synaptic protein levels and astroglial cell marker in a region specific manner. We hypothesize that these new findings are part of highly complex molecular and neuroplasticity mechanisms underlying neuroadaptation response that occurs in brains when chronically exposed to HA.

Figure 1

CEHA (12 weeks) results in hippocampal memory impairment. (A) No significant differences were observed during training. (B) HA mice spent significantly less time freezing when returned to the context test after 24hrs. (C) No significant differences were observed in the cued test. Data are expressed as mean ± SEM, n = 18 animals per group, *p < 0.05.

Figure 2

CEHA has no significant effect on synaptic density, PSD length or spine head diameter in the CA1 striatum radiatum of the hippocampus. (A) Total synapse density, (B) non-perforated synapse density (inset: arrows indicate single synapses), (C) perforated synapse density (inset: arrows indicate perforated-synapses), (D) PSD length (inset: black line indicate PSD), and (E) spine head diameter (inset: arrows indicate measured HD). Data represent group means ± SEM, n = 6 animals per group.

Figure 3

CEHA results in a significant decrease in the expression levels of SYP and SPH in the olfactory cortex. WB analysis of (A) SYP, (B) SPH, (C) PSD-95, (D) GAP43, (E) GFAP, (F) MBP (G) GLUR2, and (H) NMDAR1 protein levels. No significant changes were observed in PSD-95, GAP43, GFAP, MBP, GLUR2 and NMDAR1 protein levels. Data are expressed as mean ± SEM, n = 6 animals per group. *p < 0.05, **p < 0.01. Full length blots are presented in Supplementary Fig. 1 (GAPDH was done on each blot after stripping the protein of interest on each corresponding blot).

Figure 4

CEHA does not cause significant changes in synaptic protein expression levels in the hippocampus. WB analysis of (A) SYP, (B) SPH, (C) PSD-95, (D) GAP43, (E) GFAP, (F) MBP, (G) GLUR2, and (H) NMDAR1 protein levels. Data are expressed as mean ± SEM, n = 6. Full length blots are presented in Supplementary Fig. 2 (GAPDH was done on each blot after stripping the protein of interest on each corresponding blot).

Figure 5

CEHA results in a significant decrease in the expression levels of PSD-95, GAP43, and GFAP in the cerebellum. WB analysis of (A) SYP, (B) SPH, (C) PSD-95, (D) GAP43, (E) GFAP, (F) MBP, (G) GLUR2, and (H) NMDAR1. No significant changes were observed in SYP, SPH, MBP, GLUR2 and NMDAR1 expression levels. Data are expressed as mean ± SEM, n = 6 animals per group, *p < 0.05, **p < 0.01. Full length blots are presented in Supplementary Fig. 3 (GAPDH was done on each blot after stripping the protein of interest on each corresponding blot).

Figure 6

CEHA results in a significant decrease in the expression levels of SYP and PSD-95 in the brainstem. WB analysis of (A) SYP, (B) SPH, (C) PSD-95, (D) GAP43, (E) GFAP, (F) MBP, (G) GLUR2, and (H) NMDAR1. No significant changes were observed in SPH, GAP43, GFAP, MBP, GLUR2 and NMDAR1. Data are expressed as mean ± SEM, n = 6 animals per group, *p < 0.05. Full length blots are presented in Supplementary Fig. 4 (GAPDH was done on each blot after stripping the protein of interest on each corresponding blot).