Hydrostatic pressure reduces synaptic efficiency by inhibiting transmitter release.

The amplitude of post-synaptic potentials at an identified synapse in the Aplysia central nervous system can be modulated by the application of hydrostatic pressure. Postsynaptically controlled functions, including responses to acetylcholine and the time course of decay of the synaptic responses, remain unaffected by compression. In contrast, frequency facilitation and posttetanic potentiation, which result from presynaptic processes, are altered by pressure in a manner similar to that of agents that block transmitter release. It is concluded that pressure reduces synaptic efficiency by interfering with certain presynaptic mechanisms associated with transmitter release.

A versatile high-pressure chamber for electrophysiological measurements.

A modular high-pressure chamber is described. This chamber will allow stable microelectrode recordings to be made for a variety of intracellular preparations at pressure of 200-300 atmospheres absolute. Its features include internal temperature control, easy visibility, continuous perfusion, electrical penetrations, and manipulation of several internal controls while the system is at pressure. The small size and high versatility of the Wilson chamber make it a convenient and inexpensive research unit for experiments in the moderately high-pressure ranges that affect biological preparations.

Effect of interaction of volatile anesthetics and high hydrostatic pressure on central neurons.

Autoactive neurons in the central nervous system of Helix and Aplysia were studied after exposure to several volatile anesthetics and under compression by mineral oil. Voltage clamp studies reveal that halothane will eliminate the slow inward current that underlies oscillatory activity in burster neurons, while high pressure shifts the negative resistance region of the current without causing its elimination. Simultaneous application of the anesthetic and pressure results in the loss of inward current over a time course similar to that of halothane application alone. It is concluded that in this system, pressure and anesthetics are not acting antagonistically at the site in the membrane that controls slow wave conductances.

Mechanism of ketamine block of nerve conduction.

The node of action of ketamine on the nerve membrane has been studied in intact and internally perfused squid giant axons at 10-12 degrees C. Voltage clamp techniques have been employed to measure the maximal values of peak transient and steady-state conductances as an index of activity and to measure the apparent reversal potential for peak transient current. When applied externally to intact axons, kketamine decreased the resting membrane potential, suppressed steady-state conductances and slightly decreased theleakage conductance, although the last effect was not statistically significant. Peak transient conductance was not appreciably affected. However, when the drug was applied internally, both peak transient and steady-state conductances were suppressed. Ketamine applied externally either to intact axons or to internally perfused axons with internal flow temporarily suspended shifted the apparent reversalpotential for peak transient current towards hyperpolarization. The shift was estimated to be 28.5 mV for 200 micronM ketamine. Wahing the intact axons with drug-free sea water shifted the reversal potential further towards membrane hyperploarization. However, internal washing quickly returned the reversal potential to near control value. The change in resting sodium influx caused by external exposure to ketamine was also measured by using radioactive sodium in external sea water at 10 degrees C. Ketamine (200 micronM) changed the resting sodium influx from (28.9 +/- 5.6) x 10(-12 mol/cm2-sec to (41.8 +/- 5.6) x 10(-12 mol/cm2-sec (mean +/- S.E.M.). The data presented in this paper strongly suggest that the shift in the reversal potential for peak current caused by ketamine is due partly to sodium ion accumulation inside the nerve and partly to the increase in the PR/PNa ratio during peak current. These changes would have a profound narcotic effect on the electrical activity of nerve fibers and nerve endings in the brain during ketamine anesthesia.

Active form of ketamine in squid giant axons.

The active form of ketamine has been studied with internally perfused squid qiant axons. The drug was applied internally, and decreases in peak transient and steady-state conductances as measured by voltage clamp technique were taken as an index of activity. When the total internal ketamine concentration was maintained constant, the suppression peak transient and steady-state conductances decreased with an increase in internal pH from 7.0 TO 8.4. When the concentration of the internally present charged form of ketamine was kept constant, the suppression of the peak transient conductance remained almost constant at internal pH values of 7.0, 7.3 and 7.7, but increased at pH 8.4. However, the suppression of the stead-state conductance became more prominent as the pH was raised from 7.0 to 8.4. With a constant internal concentration of the uncharged form, the suppression of both conductances decreased as the pH was raised from 7.0 to 8.4. Computation of dissociation constants to suppretamine is more potent than the charged form at all internal pH values examined. These data also show that the potency to suppress the peak transient conductance by the charged and uncharged forms of ketamine decreased as the intertance of the charged form increased, and that of the uncharged form decreased considerably with increase in the internal pH.

Mode of action of trichloroethylene on squid axon membranes.

The mode of action of trichloroethylene on electrical properties of squid giant axons has been studied by means of voltage clamp techniques. Trichloroethylene decreased the resting membrane potential in a manner dependent upon the concentration, the depolarization by 50% saturated trichloroethylene attaining 28.4 and 32.7% of the initial value at 20 and 10 degrees C, respectively. Leakage conductance was decreased to 34.6% of the control by 30% saturated trichloroethylene at 10-12 degrees C. It appears that the trichloroethylene-induced depolarization is at least in part due to a decrease in resting potassium permeability. Both peak transient and steady-state conductance increases were suppressec by trichloroethylene, and the curve relating the steady-state conductance to the membrane potential was shifted in the depolarizing direction while the peak transient conductance curve was not appreciably shifted. The reversal potential for the peak transient current was greatly shifted by trichloroethylene in the direction of hyperpolarization in a manner dependent on the concentration, the maximum shift amounting to 25 mV at 10 degrees C. This effect was less pronounced at 20 degrees C. The shift in the reversal potential is mostly due to a decrease in selectivity of the peak transient channel and partly due to an accumulation of sodium ions inside. Analyes of dose-response relation in suppressing peak transient and steady-state conductances show that trichloroethylene interacts with receptor on a one-to-one stoichiometric basis. Steady-state sodium inactivation curve was shifted by trichloroethylene in the direction of hyperpolarization. All of these effects were partially reversed after washing the axon with anesthetic-free media. The accumulation of sodium ions inside would be much more pronounced in small nerve fibers in the brain than in giant axon and, together with the observed decrease in the selectivity of peak transient channels, would play a significant role in general anesthesia.