Effects of hindlimb unloading on neurogenesis in the hippocampus of newly weaned rats

In search of stress: analysis of stress-related markers in mice after hindlimb unloading and social isolation

OBJECTIVES

Hindlimb unloading (HU), widely used to simulate microgravity effects, is known to induce a stress response. However, as single-housed animals are usually used in such experiments, social isolation (SI) stress can affect experimental results. In the present study, we aimed to delineate stressful effects of 3-day HU and SI in mice.

METHODS

Three animal groups, HU, SI, and group-housed (GH) control mice, were recruited. A comprehensive analysis of stress-related markers was performed using ELISA, western blotting, and immunohistochemistry.

RESULTS

Our results showed that blood corticosterone and activity of glucocorticoid receptors and cAMP response element-binding protein (CREB) in the hippocampus of SI and HU animals did not differ from GH control. However, SI mice demonstrated upregulation of the hippocampal corticotropin-releasing hormone (CRH), inducible NO synthase (iNOS), vesicular glutamate transporter 1 (VGLUT1), and glutamate decarboxylases 65/67 (GAD65/67) along with activation of Fos-related antigen 1 (Fra-1) in the amygdala confirming the expression of stress. In HU mice, the same increase in GAD65/67 and Fra-1 indicated the contribution of SI. The special HU effect was expressed only in neurogenesis attenuation.

DISCUSSION

Thus, our data indicated that 3-day HU could not be characterized as physiological stress, but SI stress contributed to the negative effects of HU.

Selective effects of perinatal estrogen on proliferation and new neurons in hippocampus and piriform cortex of rats at weaning

BACKGROUND

A recent report links heightened prenatal amniotic estrogen levels to an increased risk of autism spectrum disorder (ASD). In this study, we examined the developmental effects of perinatal estrogen treatment on stem cell activity in weaned rats.

METHODS

Sprague-Dawley rats received ethinyl estradiol (EE2, 10µg/kg/day) or vehicle orally from gestational day 6 until parturition. Offspring were then treated with the same daily dose from postnatal days (PNDs) 1-21. The effects of perinatal estrogen treatment on stem cell activities in the subgranular zone (SGZ) of the hippocampus and the piriform cortex were evaluated in male and female rat pups.

RESULTS

EE₂ treatment increased the total Ki67-immunoreactive (Ki67-ir) cell counts in the SGZ of males and females (p<0.05). However, no treatment or sex differences were detectable in the density of the doublecortin (DCX)-immunoreactive (DCX-ir) deposits in the hippocampus. In the piriform cortex, no treatment or sex differences were detected in Ki67-ir cell counts. However, the EE₂ treatment significantly reduced the DCX-ir cell count in male, but not female rats (male EE₂ group=29₂±22/mm², male vehicle group=402±19/mm², female EE² group=34₂±15/mm², female vehicle group=331±9/mm²).

CONCLUSIONS

Perinatal estrogen treatment increased hippocampal Ki67-ir cell counts in both sexes and selectively reduced DCX-ir cell counts in the piriform cortex of males. These data suggest that exposure to abnormally high levels of estrogens early in life may have an impact on neural cell development. Alterations in development so early in life may have long-term cognitive impact.

Plantar Stimulations during 3-Day Hindlimb Unloading Prevent Loss of Neural Progenitors and Maintain ERK1/2 Activity in the Rat Hippocampus

Adult neurogenesis is a flexible process that depends on the environment and correlates with cognitive functions. Cognitive functions are impaired by various factors including space flight conditions and reduced physical activity. Physically active life significantly improves both cognition and the hippocampal neurogenesis. Here, we analyzed how 3-day simulated microgravity caused by hindlimb unloading (HU) or dynamic foot stimulation (DFS) during HU can affect the hippocampal neurogenesis. Adult Wistar rats were recruited in the experiments. The results demonstrated a decrease in the number of doublecortine (DCX) positive neural progenitors, but proliferation in the subgranular zone of the dentate gyrus was not changed after 3-day HU. Analysis of the effects of DFS showed restoration of neural progenitor population in the subgranular zone of the dentate gyrus. Additionally, we analyzed activity of the cRaf/ERK1/2 pathway, which is one of the major players in the regulation of neuronal differentiation. The results demonstrated inhibition of cRaf/ERK1/2 signaling in the hippocampus of HU rats. In DFS rats, no changes in the activity of cRaf/ERK1/2 were observed. Thus, we demonstrated that the process of neurogenesis fading during HU begins with inhibition of the formation of immature neurons and associated ERK1/2 signaling activity, while DFS prevents the development of mentioned alterations.

Advance in Tissue Differentiation and its Regulatory Mechanisms by Master Proteins of Nervous System during Weaning

Weaning is a critical period for the growth and development of mammals, in which various physiological and biochemical indicators of the body have undergone great changes. The development, differentiation, and maturation of the nervous system are regulated by many proteins. Changes in related proteins affect the physiological functions of the nervous system. However, the regulation of selfrenewal and differentiation of the nervous system at this stage is still poorly understood. The mechanism of differentiation and regulation of the major proteins in the nervous system during this special period of weaning remains to be investigated. Therefore, this paper aims to summarize the alteration of the nervous system during weaning and provide the basis for subsequent research.

Involvement of Cholinergic Dysfunction and Oxidative Damage in the Effects of Simulated Weightlessness on Learning and Memory in Rats

The present study aimed to determine how the learning and memory gradually change with the prolonged hindlimb unloading (HU) treatment in rats. Different HU durations (7 d, 14 d, 21 d, and 28 d) in Sprague-Dawley (SD) rats were implemented. Cognitive function was assessed using the Morris water maze (MWM) and the shuttle box test. Additionally, parameters about cholinergic activity and oxidative stress were tested. Results showed that longer-than-14 d HU led to the inferior performances in the behavioral tasks. Besides, acetylcholine esterase (AChE) activity, malondialdehyde (MDA) level in brain, reactive oxygen species (ROS), and 8-hydroxy-2-deoxyguanosine (8-OHdG) concentrations of HU rats were significantly increased. Furthermore, choline acetyltransferase (ChAT), superoxide dismutase (SOD), and catalase (CAT) activity in brain were notably attenuated. Most of these effects were more pronounced after longer exposure (21 d and 28 d) to HU, although some indicators had their own characteristics of change. These results indicate that cholinergic dysfunction and oxidative damage were involved in the learning and memory impairments induced by longer-than-14 d HU. Moreover, the negative effects of HU tend to be augmented as the HU duration becomes longer. The results may be helpful to present possible biochemical targets for countermeasures development regarding the memory deficits under extreme environmental conditions.

Negative rebound in hippocampal neurogenesis following exercise cessation

Physical exercise can improve brain function, but the effects of exercise cessation are largely unknown. This study examined the time-course profile of hippocampal neurogenesis following exercise cessation. Male C57BL/6 mice were randomly assigned to either a control (Con) or an exercise cessation (ExC) group. Mice in the ExC group were reared in a cage with a running wheel for 8 wk and subsequently placed in a standard cage to cease the exercise. Exercise resulted in a significant increase in the density of doublecortin (DCX)-positive immature neurons in the dentate gyrus (at week 0). Following exercise cessation, the density of DCX-positive neurons gradually decreased and was significantly lower than that in the Con group at 5 and 8 wk after cessation, indicating that exercise cessation leads to a negative rebound in hippocampal neurogenesis. Immunohistochemistry analysis suggests that the negative rebound in neurogenesis is caused by diminished cell survival, not by suppression of cell proliferation and neural maturation. Neither elevated expression of ΔFosB, a transcription factor involved in neurogenesis regulation, nor increased plasma corticosterone, were involved in the negative neurogenesis rebound. Importantly, exercise cessation suppressed ambulatory activity, and a significant correlation between change in activity and DCX-positive neuron density suggested that the decrease in activity is involved in neurogenesis impairment. Forced treadmill running following exercise cessation failed to prevent the negative neurogenesis rebound. This study indicates that cessation of exercise or a decrease in physical activity is associated with an increased risk for impaired hippocampal function, which might increase vulnerability to stress-induced mood disorders.

Evaluation of gene, protein and neurotrophin expression in the brain of mice exposed to space environment for 91 days

Effects of 3-month exposure to microgravity environment on the expression of genes and proteins in mouse brain were studied. Moreover, responses of neurobiological parameters, nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF), were also evaluated in the cerebellum, hippocampus, cortex, and adrenal glands. Spaceflight-related changes in gene and protein expression were observed. Biological processes of the up-regulated genes were related to the immune response, metabolic process, and/or inflammatory response. Changes of cellular components involving in microsome and vesicular fraction were also noted. Molecular function categories were related to various enzyme activities. The biological processes in the down-regulated genes were related to various metabolic and catabolic processes. Cellular components were related to cytoplasm and mitochondrion. The down-regulated molecular functions were related to catalytic and oxidoreductase activities. Up-regulation of 28 proteins was seen following spaceflight vs. those in ground control. These proteins were related to mitochondrial metabolism, synthesis and hydrolysis of ATP, calcium/calmodulin metabolism, nervous system, and transport of proteins and/or amino acids. Down-regulated proteins were related to mitochondrial metabolism. Expression of NGF in hippocampus, cortex, and adrenal gland of wild type animal tended to decrease following spaceflight. As for pleiotrophin transgenic mice, spaceflight-related reduction of NGF occured only in adrenal gland. Consistent trends between various portions of brain and adrenal gland were not observed in the responses of BDNF to spaceflight. Although exposure to real microgravity influenced the expression of a number of genes and proteins in the brain that have been shown to be involved in a wide spectrum of biological function, it is still unclear how the functional properties of brain were influenced by 3-month exposure to microgravity.