Hyperoxic Stimulation of Synchronous Orthodromic Activity and Induction of Neural Plasticity Does Not Require Changes in Excitatory Synaptic Transmission

The first study, described in the companion article, reports that acute exposure of rat hippocampal slices to either hyperbaric oxygen (HBO: 2.84 and 4.54 atmospheres absolute, ATA) or normobaric reoxygenation (NBO _reox ; i.e., normobaric hyperoxia: 0.6 or 0.0 → 0.95 ATA) stimulates synchronous orthodromic activity in CA1 neurons, which includes activation of O ₂ -induced potentiation (OxIP) and, in some cases, hyperexcitability (secondary population spikes, sPS). In this second study we tested the hypothesis that HBO and NBO _reox increase orthodromic activity of CA1 neurons (oPS, orthodromic population spike) and OxIP via a combination of both increased excitatory synaptic transmission (field excitatory postsynaptic potential, fEPSP) and intrinsic excitability (antidromic population spike, aPS). HBO and NBO _reox increased the oPS but rarely increased or potentiated the fEPSP. HBO exposure produced epileptiform antidromic activity, which was abolished during inhibition of fast GABAergic and glutamatergic synaptic transmission. Decreasing O ₂ from 0.95 ATA (control) to 0.6 ATA (intermediate O ₂ ) or 0.0 ATA (hypoxia) reversibly abolished the fEPSP, and reoxygenation rarely induced potentiation of the fEPSP or aPS. Intracellular recordings and antidromic field potential recordings, however, revealed that synaptic transmission and neuronal excitability were preserved, albeit at lower levels, in 0.60 ATA O ₂ . Together, these data indicate that 1 ) the changes in excitatory postsynaptic activity are not required for stimulation of the oPS during and HBO/NBO _reox or for activation of OxIP, suggesting the latter is a form of intrinsic plasticity; 2 ) HBO disinhibits spontaneous synaptic transmission to induce epileptiform activity; and 3 ) although synchronous synaptic activation of the CA1 neuronal population requires hyperoxia (i.e., 0.95 ATA O ₂ ), synaptic activation of individual CA1 neurons does not.