Minimum alveolar anesthetic concentration of fluorinated alkanols in rats: relevance to theories of narcosis.

UNLABELLED: The Meyer-Overton hypothesis predicts that the potency of conventional inhaled anesthetics correlates inversely with lipophilicity: minimum alveolar anesthetic concentration (MAC) x the olive oil/gas partition coefficient equals a constant of approximately 1.82 +/- 0.56 atm (mean +/- SD), whereas MAC x the octanol/gas partition coefficient equals a constant of approximately 2.55 +/- 0.65 atm. MAC is the minimum alveolar concentration of anesthetic required to eliminate movement in response to a noxious stimulus in 50% of subjects. Although MAC x the olive oil/gas partition coefficient also equals a constant for normal alkanols from methanol through octanol, the constant (0.156 +/- 0.072 atm) is one-tenth that found for conventional anesthetics, whereas the product for MAC x the octanol/gas partition coefficient (1.72 +/- 1.19) is similar to that for conventional anesthetics. These normal alkanols also have much greater affinities for water (saline/gas partition coefficients equaling 708 [octanol] to 3780 [methanol]) than do conventional anesthetics. In the present study, we examined whether fluorination lowers alkanol saline/gas partition coefficients (i.e., decreases polarity) while sustaining or increasing lipid/gas partition coefficients, and whether alkanols with lower saline/gas partition coefficients had products of MAC x olive oil or octanol/gas partition coefficients that approached or exceeded those of conventional anesthetics. Fluorination decreased saline/gas partition coefficients to as low as 0.60 +/- 0.08 (CF3[CF2]6CH2OH) and, as hypothesized, increased the product of MAC x the olive oil or octanol/gas partition coefficients to values equaling or exceeding those found for conventional anesthetics. We conclude that the greater potency of many alkanols (greater than would be predicted from conventional inhaled anesthetics and the Meyer-Overton hypothesis) is associated with their greater polarity. IMPLICATIONS: Inhaled anesthetic potency correlates with lipophilicity, but potency of common alkanols is greater than their lipophilicity indicates, in part because alkanols have a greater hydrophilicity--i.e., a greater polarity.

Evidence for reduced presynaptic Ca2+ entry in a lobster neuromuscular junction at high pressure.

1. Previous studies have shown that hyperbaric pressure depresses synaptic transmission and have suggested that the effect is primarily on transmitter release. The present study analysed the effects of pressure at a crustacean neuromuscular junction. Changes in pressure were compared to changes in extracellular calcium concentration [Ca2+]o with respect to effects on excitatory junction potential (EJP) amplitude, time course, facilitation and potentiation. 2. The effects of 10.1 MPa pressure on EJP amplitude, facilitation and potentiation, but not time course, were mimicked by reducing [Ca2+]o to approximately one-half the normal level. 3. The effects of pressure and the interaction between compression and calcium concentration were analysed in terms of a model of transmitter release. The model assumes that release is dependent on internal calcium concentration, as modulated by both influx and removal processes; that calcium influx is a saturating function of [Ca2+]o; and that release and removal are saturating functions of [Ca2+]i. 4. The results were consistent with the hypothesis that increased pressure acts primarily to reduce calcium influx into the nerve terminal.

Synaptic integrative properties at hyperbaric pressure.

1. Because hyperbaric pressure profoundly depresses excitatory synaptic transmission, it has proved difficult to account for its excitatory effects in the CNS. We tested the hypothesis that hyperbaric pressure might increase excitation by enhancing facilitation and potentiation during repetitive synaptic activation, and/or by selectively depressing inhibitory synaptic transmission. Intracellular microelectrode recordings were obtained from crustacean muscle fibers innervated by single identifiable excitor and inhibitor motor neurons; the preparations were exposed to pressures of 0.1-10.1 MPa. 2. Hyperbaric pressure reduced the amplitude of the singly evoked excitatory junctional potential (EJP), enhanced paired-pulse facilitation, and increased the potentiation elicited by trains of stimuli. The potentiated EJP at 10.1 MPa approached the comparable response evoked at normobaric pressure. 3. Hyperbaric pressure also depressed inhibitory synaptic transmission, measured as depression of the EJP by the inhibitor motor neuron. However, pressure depressed excitatory and inhibitory synaptic transmission to the same extent. Thus there appears to be no selective effect of pressure on the GABA-activated chloride channel. The amplitude of the inhibited EJP at 10.1 MPa remained below that at normobaric pressure, even during repetitive stimulation. 4. The results do not support the hypothesis that pressure increases central excitation by selectively depressing inhibitory transmission per se; enhancement of potentiation, however, probably plays an important role. In this preparation, in which inhibitory transmission also displays facilitation, pressure did not increase overall excitation or alter the balance between excitation and inhibition. 5. These results predict that a pressure-excitable network should encompass excitatory synaptic connections which exhibit pronounced facilitation and inhibitory synapses with little or no facilitation.

Pressure reversal of anaesthesia: a synaptic mechanism.

Hyperbaric pressure induces seizures and increases anaesthetic requirements ("pressure reversal of anaesthesia"), but both pressure and anaesthetic agents depress excitatory synaptic transmission. The present study has attempted to resolve this paradox. The interaction between helium pressure to 10.1 MPa and anaesthetic agents (pentobarbitone, halothane, methoxyflurane) was investigated at a crustacean glutaminergic excitatory neuromuscular junction which can be modulated by GABA inhibition. Both pressure and the anaesthetics depressed the singly evoked excitatory junctional potential (EJP). During repetitive stimulation, both pressure and pentobarbitone antagonized their own depressant effects by enhancing tetanic potentiation. The additive enhancement at 10.1 MPa was sufficient to increase the pentobarbitone-depressed response above the corresponding normobaric level. No significant antagonism between pressure and any of the anaesthetics was observed on the properties of EJP amplitude and time course, facilitation, potentiation or inhibition. Additivity rather than antagonism is the basis for pressure reversal of anaesthetic depression at this model synapse. The functional antagonism is therefore indirect, and probably involves multiple sites of action for both pressure and anaesthetics.

Modulation of impulse conduction through axonal branchpoint by physiological, chemical and physical factors.

The impulse conduction capability of a crustacean bifurcating motor axon was studied under various physiological and physical conditions, and effects of several pharmacological agents were tested. The passive and active membrane properties were measured by intra- and extracellular recording, macropatch clamp and vaseline gap voltage clamp. Block of condition after high-frequency stimulation is caused by accumulation of K+ in the extracellular space, while its differential nature is attributed to early activation of the Na+-K+ electrogenic pump and increased intracellular Ca2+ in the thinner branch. Decreased Na+ and K+ conductance, or increased K+ conductance induced by various drugs, largely reduced the maximal following frequency of the branchpoint. High pressure initially increased the neuron excitability, and later decreased, to below control levels, the axon ability to conduct at high frequency. Cooling had a similar effect on the delayed effects of high pressure. The physiological significance of the frequency-dependent block and its possible role in anesthesia, epilepsy, high-pressure nervous syndrome and behavior are discussed.

Pressure and temperature modulation of conduction in a bifurcating axon.

A bifurcating crustacean motor neuron, which serves an integrative function by selectively controlling output to its daughter branches, was examined for its behavioral response to changes in pressure and in temperature when each was varied while the other was held constant, and when both were varied together. The neuron was exposed to helium pressure between 1 and 200 ATA and temperatures between 9 and 22 degrees C. The response of the neuron to pressure changes was biphasic and time dependent. Immediately following a pressure change, action potential amplitude and conduction velocity increased, and the ability of the branch point to pass high frequency trains improved; after 15-20 min at pressures above 35 ATA these measures were depressed below control values. The curve relating functional measures to temperature displayed a time-dependent hysteresis, fast warming leading to values for amplitude, velocity, and branchpoint capacity which corresponded to those made at a point 3-5 degrees C higher during slow cooling. The delayed depressant effects of compression and cooling were synergistic. Low temperature significantly enhanced the effects of pressure on amplitude and conduction velocity; high pressure increased the Q10 of both measures. However, slow cooling antagonized the transient compression-induced excitability increase, and prolonged exposure to hyperbaric pressure diminished the temperature hysteresis. The complex time-dependent changes in this branching axon's ability to conduct are related to previously described changes in membrane potential properties. The responses of this axon to pressure changes are different from responses of other axons studied at hyperbaric pressure. Thus, even within relatively stereotyped axon membrane the effects of pressure are not generalizeable among cells. The possible relevance of these findings to the high pressure nervous syndrome (HPNS) is discussed.

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.

Cyclic AMP reduction of frequency following ability in peripheral axons.

Radioimmunoassays for cAMP demonstrated that a beta-adrenergic agonist, isoproterenol, increased cAMP levels in isolated frog sciatic nerve. Dibutyryl cAMP (db-cAMP) and isoproterenol reduced the amplitude of the compound action potential and decreased the ability of the Xenopus sciatic nerve to follow high frequency stimulation. Similar effects of db-cAMP and a phosphodiesterase inhibitor were seen on intracellularly recorded action potentials of single lobster peripheral axons. These results suggest that cAMP can modulate the electrophysiological response properties of both myelinated and unmyelinated axons.