High pressure modulation of NMDA receptor dependent excitability

Pressure above 1.1 MPa induces in mammals and humans the high pressure neurological syndrome (HPNS). HPNS is characterized by cognitive and motor decrements associated with sleep disorders, EEG changes, tremor, and convulsions that ultimately may lead to death. Previous theories proposed that augmented response of the glutamatergic N-methyl-d-aspartate receptor (NMDAR) or reduced GABAergic inhibition may be involved. Recently, we have reported that isolated NMDAR response was augmented at high pressure. We now test whether this augmentation induces neuronal hyperexcitability. We studied high pressure effects on pharmacologically isolated NMDAR field excitatory postsynaptic potentials (fEPSPs) and on their efficacy in generating population spikes (PSs). Sprague-Dawley male rats were used. Hippocampal coronal brain slices were prepared, constantly superfused with physiological solutions, gas-saturated at normobaric pressure, and compressed up to 10.1 MPa with helium. fEPSPs and PSs were recorded from the dendritic and the somatic layers of CA1 pyramidal neurons in response to Schaefer collaterals stimulation with trains of five stimuli at 25 Hz. Pressure caused PSs to appear earlier in the train. However, PS delay, rise time and decay time were increased and PS amplitude, frequency, and number were decreased in the last responses in the train. The decrease in late fEPSPs was associated with a reduction of the total number of PSs in the train, apparently without a change in the synaptic efficacy. These results may partially explain the neuronal hyperexcitability observed at pressure. Therefore, it is postulated that significant hyperexcitability is attained at pressure only when the normal fast fEPSP is intact.