Influence of normobaric hypoxia on learning capacity of different aged rats

Astrocyte HIF-2α supports learning in a passive avoidance paradigm under hypoxic stress

BACKGROUND

The brain is extensively vascularized, useŝ20% of the body's oxygen, and is highly sensitive to changes in oxygen. While synaptic plasticity and memory are impaired in healthy individuals by exposure to mild hypoxia, aged individuals appear to be even more sensitive. Aging is associated with progressive failure in pulmonary and cardiovascular systems, exposing the aged to both chronic and superimposed acute hypoxia. The HIF proteins, the "master regulators" of the cellular response to hypoxia, are robustly expressed in neurons and astrocytes. Astrocytes support neurons and synaptic plasticity via complex metabolic and trophic mechanisms. The activity of HIF proteins in the brain is diminished with aging, and the increased exposure to chronic and acute hypoxia with aging combined with diminished HIF activity may impair synaptic plasticity.

PURPOSE

Herein, we test the hypothesis that astrocyte HIF supports synaptic plasticity and learning upon hypoxia.

MATERIALS AND METHODS

An Astrocyte-specific HIF loss-of-function model was employed, where knock-out of HIF-1α or HIF-2α in GFAP expressing cells was accomplished by cre-mediated recombination. Animals were tested for behavioral (open field and rotarod), learning (passive avoidance paradigm), and electrophysiological (long term potentiation) responses to mild hypoxic challenge.

RESULTS

In an astrocyte-specific HIF loss-of-function model followed by mild hypoxia, we identified that the depletion of HIF-2α resulted in an impaired passive avoidance learning performance. This was accompanied by an attenuated response to induction in long-term potentiation (LTP), suggesting that the hippocampal circuitry was perturbed upon hypoxic exposure following HIF-2α loss in astrocytes, and not due to hippocampal cell death. We investigated HIF-regulated trophic and metabolic target genes and found that they were not regulated by HIF-2α, suggesting that these specific targets may not be involved in mediating the phenotypes observed.

CONCLUSION

Together, these results point to a role for HIF-2α in the astrocyte's regulatory role in synaptic plasticity and learning under hypoxia and suggest that even mild, acute hypoxic challenges can impair cognitive performance in the aged population who harbor impaired HIF function.

The effects of prenatal exposure to hypoxia on the behavior of rats during their life span

The aim of this study was to investigate the influence of moderate prenatal damage on adaptability during the juvenile, adult, and senile phases. Pregnant rats were exposed to a 12% normobaric hypoxia from day 1 to 17 postconception. Pregnancy was normal in both the treated animals and the controls. Erythrocytes, hemoglobin, and hematocrit did not increase in the treated pregnant animals. During the first 3 weeks, the F1 generation showed developmental deviations in physiological characteristics. Throughout subsequent ontogeny, motor performance, cognitive ability, and adaptability to physical stress were determined with a test battery of varying demands. Some of the differences (e.g., locomotor activity, learning ability) between juvenile untreated and treated rats disappeared during the adult phase. Motor and coordinative abilities, however, remained partially impaired in the old rats, especially under high demands. This study, and previous findings with alcohol (37), indicate that prenatal exposure to a noxa may result in a highly differentiated brain injury pattern. Depending on the different functions, damage may intensify age-dependent adaptive disorders or provoke impairment without influencing the course of development.