Pressure and temperature: time-dependent modulation of membrane properties in a bifurcating axon.

A crustacean bifurcating motor neuron that selectively controls output to its daughter branches was exposed to helium pressure of 1-200 atmospheres (atm) and temperatures of 9-22 degrees C. The membrane responses of this integrative axon were monitored by intra- and extracellular recording, macropatch clamp, and Vaseline gap voltage clamp. The response of the neuron to pressure changes was biphasic and time dependent. Initially there was an increase in action potential amplitude and rate of rise, in magnitude of the inward sodium current, in conduction velocity, and in the ability of the branch point to conduct at high frequency. After 10-20 min at a given pressure above 35 atm, these functional measures declined to levels below control. Action potential duration increased throughout. In addition, membrane resting potential was depolarized by 10-15 mV, and input resistance increased. Pressure-related depolarization was not seen in an axon pretreated with ouabain, a result consistent with pressure inhibition of the electrogenic sodium pump in this axon. Cooling induced changes opposite to the initial effects and similar to the delayed effects of pressure in all measures, including action potential amplitude, rise time, duration, membrane potential, membrane resistance, inward current amplitude, conduction velocity, and ability to conduct at high frequency. This axon differs from other axons that have been studied at hyperbaric pressure in the bimodal nature of its response and in the magnitude of pressure-related depression of membrane properties related to excitability.