High pressure effects on reflexes in isolated spinal cords of newborn rats.

High pressure induces hyperexcitability and convulsions in both intact and decerebrated animals. However, pressure suppresses synaptic transmission in isolated invertebrate preparations. We examined the effect of high pressure on monosynaptic (MSR) and polysynaptic (PSR) reflexes in isolated spinal cords of newborn rats. Reflex activity was recorded extracellularly from the cut ends of the lumbar ventral roots L3-L5 following stimulation of the corresponding dorsal roots. Increasing the stimulus frequency from 0.1 to 2.0 Hz reduced the amplitude of both reflexes by 75%. High pressure (10.1 MPa helium) did not affect this phenomenon. Pressure had no effect on MSR amplitude, but increased PSR amplitude by 30%. For MSR, pressure increased the latency by 25%, duration by 20%, and rise time by 25%. Pressure abolished the moderate (119 +/- 4.5%, mean +/- SEM) posttetanic potentiation observed at 0.1 MPa. For the curve relating MSR amplitude to [Ca2+]o, high pressure produced a slight rightward shift of 0.25 mM without affecting its saturation level. These data suggest that the response of vertebrate central synapses to pressure may be different from previously described invertebrate synapses. Alternatively, if synaptic potentials are reduced at high pressure, other processes that determine excitability must be invoked to account for the relatively stabile reflex response.