The effect of spaceflight on mouse olfactory bulb volume, neurogenesis, and cell death indicates the protective effect of novel environment

Abstract

Space missions necessitate physiological and psychological adaptations to environmental factors not present on Earth, some of which present significant risks for the central nervous system (CNS) of crewmembers. One CNS region of interest is the adult olfactory bulb (OB), as OB structure and function are sensitive to environmental- and experience-induced regulation. It is currently unknown how the OB is altered by spaceflight. In this study, we evaluated OB volume and neurogenesis in mice shortly after a 13-day flight on Space Shuttle Atlantis [Space Transport System (STS)-135] relative to two groups of control mice maintained on Earth. Mice housed on Earth in animal enclosure modules that mimicked the conditions onboard STS-135 (AEM-Ground mice) had greater OB volume relative to mice maintained in standard housing on Earth (Vivarium mice), particularly in the granule (GCL) and glomerular (GL) cell layers. AEM-Ground mice also had more OB neuroblasts and fewer apoptotic cells relative to Vivarium mice. However, the AEM-induced increase in OB volume and neurogenesis was not seen in STS-135 mice (AEM-Flight mice), suggesting that spaceflight may have negated the positive effects of the AEM. In fact, when OB volume of AEM-Flight mice was considered, there was a greater density of apoptotic cells relative to AEM-Ground mice. Our findings suggest that factors present during spaceflight have opposing effects on OB size and neurogenesis, and provide insight into potential strategies to preserve OB structure and function during future space missions.

Keywords: activated caspase-3; doublecortin; environmental enrichment; low Earth orbit; neuroblasts.

Fig. 1.

AEM-Ground mice have increased olfactory bulb (OB) volume relative to AEM-Flight mice and Vivarium mice. A: representative image of the main olfactory bulb (+4.3 mm from bregma) showing the main cell layers: granule cell layer (GCL), internal plexiform layer (IPL), mitral cell layer (MCL), external plexiform layer (EPL), glomerular layer (GL), and olfactory nerve layer (ONL). All volume measurements were quantified from sections stained for doublecortin (DCX) and counterstained with Fast Red. Scale bar = 100 μm. B, D, F: total OB (B), GCL (D), and GL (F) volumes as estimated by the Cavalieri method. Data analyzed by one-way ANOVA with Tukey's post hoc test. All data represent means ± SE. N = 4–6/group. *P < 0.05, **P < 0.01. C, E, G: total OB (C), GCL (E), and GL (G) volumes plotted at different distances from bregma (+5.96 to 3.56 mm from bregma). Data analyzed by two-way ANOVA with Sidak's post hoc test. All data represent means ± SE. N = 4–6/group. *P < 0.05, **P < 0.01, AEM-Flight vs. AEM-Ground. #P < 0.05, ##P < 0.01, AEM-Ground vs. Vivarium. AEM, animal enclosure modules.

Fig. 2.

AEM-Ground mice have more DCX+ immature neurons and reduced cell death relative to AEM-Flight mice and Vivarium mice. A: representative photomicrograph of the GCL in the OB stained for DCX and counterstained with Fast Red. Scale bar = 50 μm. B: no. of immature neurons, assessed by DCX immunoreactivity, in the GCL is significantly increased in AEM-Ground vs. Vivarium mice. This AEM-induced increase in DCX+ cell number is significantly decreased in AEM-Flight mice. C: DCX+ cells in the GCL plotted at different distances from bregma (+5.96 to 3.56 mm from bregma). D: no. of DCX+ cells/mm³ in the GCL was not significantly different among the 3 groups. E: representative photomicrograph of the GL stained for DCX and counterstained with Fast Red. Scale bar = 50 μm. F: DCX+ cells in the GL are significantly increased in AEM-Ground vs. Vivarium mice. This AEM-induced increase in DCX+ cell number is significantly decreased in AEM-Flight mice. G: DCX+ cell numbers in the GL plotted at different distances from bregma (+5.96 to 3.56 mm from bregma). H: no. of DCX+ cells/mm³ in the GL was not significantly different among the 3 groups. I: representative low-magnification photomicrograph (100× magnification) of the GCL in the OB stained for activated caspase 3 (AC3) and counterstained with Fast Red. Scale bar = 100 μm. I, inset: a higher magnification (400× magnification) image of the boxed area. Scale bar = 20 μm. J: no. of apoptotic cells, as assessed by AC3+ immunoreactivity, in the GCL is significantly decreased in AEM-Ground vs. Vivarium mice. AEM-Flight mice also exhibited a strong trend for increased AC3+ cell number vs. AEM-Ground mice. K: AC3+ cell numbers in the GCL plotted at different distances from bregma (+5.96 to 3.56 mm from bregma). L: AEM-Ground mice had significantly fewer AC3+ cells/mm³ in the GCL vs. Vivarium and AEM-Flight. M: representative low-magnification photomicrograph (100× magnification) of the GL stained for AC3 and counterstained with Fast Red. Scale bar = 100 μm. M, inset: a higher magnification (400× magnification) image of the boxed area. Scale bar = 20 μm. N: there is a trend for fewer AC3+ cells in the GL of AEM-Ground mice vs. Vivarium mice, which is not seen in AEM-Flight mice. O: AC3+ cells in the GL plotted at different distances from bregma (+5.96 to 3.56 mm from bregma). P: AEM-Flight mice had significantly more AC3+ cells vs. AEM-Ground but not Vivarium mice. All data represent means ± SE. All total and cell density DCX+ and AC3+ data (B, D, F, H, J, L, N, P) analyzed by one-way ANOVA with Tukey's post hoc test. N = 4–6/group. *P < 0.05, **P < 0.01. All bregma data (C, G, K, O) analyzed by two-way ANOVA with Sidak's post hoc test. N = 4–6/group. *P < 0.05, **P < 0.01, AEM-Flight vs. AEM-Ground. #P < 0.05, ##P < 0.01, AEM-Ground vs. Vivarium.