An electrophysiological analysis of oxygen and pressure on synaptic transmission

The effect of oxygen at high pressure (OHP), helium at 150 PSIG and 100% oxygen at ambient pressure on excitatory synaptic transmission was studied using the lobster walking leg neuromuscular preparation. Both 100% oxygen at 150 PSIG (7135 mm Hg oxygen) and helium at 150 PSIG (7000 mm Hg helium plus 135 mm Hg oxygen) produced a significant decrease in the amplitude of the junction potential (Vejp). The decrease in Vejp induced by OHP, however, was greater than with pressure alone. OHP also produced a significant decrease in short term facilitation. Exposure to 100% oxygen at ambient pressure produced a transient increase in Vejp and a large increase in frequency of miniature junction potentials. In each case the change in Vejp was due to changes in presynaptic release of transmitter since quantal content per fiber (M') was shown to decrease for OHP and helium at 150 PSIG and to transiently rise with 100% oxygen at ambient pressure. In addition, the response to exogenously applied glutamate (the putative neurotransmitter) was not affected by OHP, 150 PSIG helium or 100% oxygen at ambient pressure. This further indicates a presynaptic site of action.