Increased Excitability and Heightened Magnitude of Long-Term Potentiation at Hippocampal CA3-CA1 Synapses in a Mouse Model of Neonatal Hyperoxia Exposure

Abstract

Preterm infants exposed to supraphysiological oxygen (hyperoxia) during the neonatal period have hippocampal atrophy and cognitive dysfunction later in childhood and as adolescents. Previously, we reported that 14-week-old adult mice exposed to hyperoxia as newborns had spatial memory deficits and hippocampal shrinkage, findings that mirror those of human adolescents who were born preterm. The area CA1 region of the hippocampus that is crucial for spatial learning and memory is highly vulnerable to oxidative stress. In this study, we investigated the long-term impact of neonatal hyperoxia exposure on hippocampal CA3-CA1 synaptic function. Male and female C57BL/6J mouse pups were continuously exposed to either 85% normobaric oxygen or air between postnatal days 2-14. Hippocampal slice electrophysiology at CA3-CA1 synapses was then performed at 14 weeks of age. We observed that hyperoxia exposed mice have heightened strength of basal synaptic transmission measured in input-output curves, increased fiber volley amplitude indicating increased axonal excitability, and heightened LTP magnitude at CA3-CA1 synapses, likely a consequence of increased postsynaptic depolarization during tetanus. These data demonstrate that supraphysiological oxygen exposure during the critical neonatal developmental period leads to pathologically heightened CA3-CA1 synaptic function during early adulthood which may contribute to hippocampal shrinkage and learning and memory deficits we previously reported. Furthermore, these results will help shed light on the consequences of hyperoxia exposure on the development of hippocampal synaptic circuit abnormalities that could be contributing to cognitive deficits in children born preterm.

Keywords: early-life insult; hyperoxia; long-term hippocampal dysfunction; neuronal excitability; preterm; synaptic transmission.

Figure 1

Long-term effect of neonatal hyperoxia exposure on CA3–CA1 synapses basal synaptic transmission. (A) Input-Output curves. Blue circles represent the air-exposed group, and red triangles represent the hyperoxia-exposed group. The maximum field excitatory postsynaptic potentials (fEPSP) slope is significantly larger at CA3–CA1 synapses from the young adult mice exposed to neonatal hyperoxia compared to air-exposed mice. Unpaired Student’s t-test at 320 μA means ± SEM; Air = 0.31 ± 0.06, Hyperoxia = 0.67 ± 0.15, p = 0.04, n = 6 slices/6 animals in Air, and 6 slices/6 animals in 85% O₂. *Represents p < 0.05; Air vs. Hyperoxia. (B) Fiber volley amplitude. Blue circles represent the air-exposed group, and red triangles represent the hyperoxia-exposed group. Compared to air-exposed mice, fiber volley amplitude at the CA3–CA1 synapse was significantly larger in the young adult mice exposed to neonatal hyperoxia. Unpaired Student’s t-test at 320 μA; mean ± SE; Air = 0.75 ± 0.19, Hyperoxia = 1.44 ± 0.23, p = 0.04, n = 6 slices/6 animals in Air, and 6 slices/6 animals in 85% O₂. *Represents p < 0.05; Air vs. Hyperoxia.

Figure 2

Long-term effect of neonatal hyperoxia on presynaptic probability and the magnitude of long-term potentiation in young adult mice. (A) Paired Pulse Ratio (PPR). A blue bar with horizontal lines represents the air-exposed group. The solid red bar represents the hyperoxia-exposed group. No differences in the PPR were detectable between the adult mice that had either neonatal hyperoxia or air exposure at the CA3–CA1 synapse. Unpaired Student’s t-test mean ± SE; Air = 1.48 ± 0.08, Hyperoxia = 1.54 ± 0.15, p = 0.75, n = 5 slices/5 animals in Air, and 6 slices/6 animals in 85% O₂. (B) Long-Term Potentiation (LTP). Blue circles represent the air-exposed group, and red triangles represent the hyperoxia-exposed group. The magnitude of theta-burst stimulation (TBS) induced LTP was significantly greater at the CA3–CA1 synapses in the young adult mice exposed to neonatal hyperoxia compared to air-exposed mice. Unpaired Student’s t-test: means ± SEM%; Air-exposed: 134 ± 12% of baseline fEPSP slope vs. Hyperoxia-exposed 217 ± 23% of baseline fEPSP slope, n = 5 slices/5 animals in Air, and 6 slices/6 animals in 85% O₂. *Represents p < 0.01; Air vs. Hyperoxia. (C) Maximum depolarization at tetanus. A blue bar with horizontal lines represents the air-exposed group. The solid red bar represents the hyperoxia-exposed group. Young adult mice that had hyperoxia exposure as neonates had a greater magnitude of the postsynaptic depolarization during the TBS. Unpaired Student’s t-test: mean ± SE; Air = 8.81 ± 0.73, Hyperoxia = 12.66 ± 1.34, p = 0.03, n = 5 slices/5 animals in Air, and 6 slices/6 animals in 85% O₂. ^*Represents p < 0.05; Air vs. Hyperoxia.

Figure 3

Long-term effect of neonatal hyperoxia on CA3–CA1 synapse excitability in young adult mice. Blue circles represent the air-exposed group, and red triangles represent the hyperoxia-exposed group. Young adult mice exposed to neonatal hyperoxia had a higher mean coastal burst index compared to air-exposed mice. Unpaired Student’s t-test: mean ± SE; Air = 1.06 ± 0.03, Hyperoxia = 1.27 ± 0.05, p = 0.02. n = 5 slices/5 animals in Air, and 6 slices/6 animals in 85% O₂. *Represents p < 0.05; Air vs. Hyperoxia.