Increased Oxygen Hemoglobin Affinity Mitigates Hypobaric Hypoxia‐dependent Spatial Learning Decrements In Sprague‐Dawley Rats
Authors: Williams Brian T. , Johnson W. Rainey , Ciarlone Geoffrey E. , Willoughby Chelsea N. , Zhou Hanbing , Mahon Richard T. 
Published: April 2020

Introduction

High altitude (HA) exposure negatively impairs cognitive performance in humans. Mechanisms for cognitive impairment include hypobaric hypoxia itself, sleep disturbance, and/or the effects of acclimation such as hyperventilation and increased red cell mass. GBT1118 (GBT) allosterically modifies Hemoglobin (Hb) and affects an increase in oxygen (O2) affinity. Using Sprague‐Dawley (SD) rats, we examined the effects of GBT1118 on spatial memory decrements observed with simulated altitude exposure.

Methods

Male SD rats (N=42) underwent 4 Morris Water Maze (MWM) training sessions twice daily over a two day period, with each session including an initial platform‐acquisition followed by three platform re‐acquisitions. Once animals satisfied the initial training, they underwent MWM testing on the third day where initial and subsequent platform acquisitions were again performed collecting time and distance. On the third experimental day animals were randomized to receive GBT1118 100mg/kg by oral gavage or equal volume of saline (control). Two hours after gavage, animals were placed in a chamber and either remained at sea level (SL) or exposed to a simulated altitude of 24,000 ft. for 4 hr. Chambers were constantly flushed with air to maintain CO2 levels of <0.5%. After the 4hr exposure, the chamber pressure was gradually returned to SL by allowing a pre‐mixed hypoxic gas consisting of 8.5% O2 (balance N2) into the chamber, allowing the altitude equivalent partial pressure of O2 to be maintained during descent. Once SL was reached, the animals were removed and placed into a box flushed with 8.5% O2 and then transported for MWM testing. Animals then immediately underwent MWM as previously described, except that they returned to the hypoxic box between re‐acquisition trials. Animals that were exposed to SL for 4hr were maintained in normal room air throughout testing. After MWM testing completion, the animals were anesthetized and underwent an assessment of O2‐Hb affinity by tail vein blood sample and measuring p50.

Results

O2‐Hb affinity as measured by p50 in animals treated with saline was 42.3 in SL‐exposed animals vs. 48.2 in HA‐exposed animals. GBT significantly decreased p50 to 30.4 in SL‐exposed animals compared to saline controls (p=0.0119), and to 39.1 in HA‐exposed animals compared to saline controls (p=0.0421). Spatial learning was assessed by determining the average distance and latency to the platform during the three reacquisition trials, and comparing between groups the percent change of those values from the pre‐exposure assessment and the post‐exposure assessment. Data show that, compared to SL control animals, exposure to HA significantly increases both distance (p=0.0067) and latency (p=0.0079) to find the platform. While GBT only exhibited a strong trend for mitigation of the effects of HA on distance (p=0.0566), it did significantly reduce latency (p=0.0298) compared to HA‐exposed, saline treated animals. GBT had no effect on distance or latency in SL‐exposed animals.

Conclusion

Increasing O2‐Hb affinity with GBT1118 improves spatial learning in rodents exposed to HA.