Degenerate perturbations of protein structure as the mechanism of anaesthetic action

The interaction of the n-alkanols with lipid bilayers and excitable membranes shows that there is no simple correlation between conduction block and any of the perturbations of bilayer structure currently proposed as unitary mechanisms of local anaesthetic action. We propose instead that the n-alkanols act by direct interaction with target proteins to cause perturbations which depend directly on the precise structure of the alcohol.

Anaesthetics depress the sensitivity of cortical neurones to L-glutamate

1 The effects of general anaesthetics on the responses of neurones to iontophoretically applied L-glutamate have been examined in slices of the guinea-pig olfactory cortex in vitro. 2 Concentrations of pentobarbitone, ether, methoxyflurance, trichloroethylene and alphaxalone that are known to depress synaptic transmission in the prepiriform cortex also depressed the sensitivity of prepiriform neurones to L-glutamate. 3 Halothane, in concentrations that depress synaptic transmission (less than 1%) did not alter sensitivity of neurones to glutamate. Higher concentrations (greater than 1% produced a dose-related depression of the glutamate sensitivity of neurones. 4 All four volatile anaesthetics tested caused some cells to alter their glutamate-evoked firing pattern to one in which the spike discharges were more closely grouped. Pentobarbitone and alphaxalone had no such effect. 5 If the sensitivity of the neurones to the endogenous excitatory transmitter is affected by anaesthetics in the same way as the glutamate-sensitivity, these results suggest that halothane depresses synaptic transmission by decreasing the amount of transmitter released from the nerve terminals, whereas the other anaesthetics depress the sensitivity of the post-synaptic membrane to the released transmitter.

The actions of volatile anaesthetics on synaptic transmission in the dentate gyrus

1. The action of four volatile anaesthetics on the evoked synaptic potentials of in vitro preparations of the hippocampus were examined. 2. All four anaesthetics (ether, halothane, methoxyflurane and trichloroethylene) depressed the synaptic transmission between the perforant path and the granule cells at concentrations lower than those required to maintain anaesthesia in intact animals. 3. The population excitatory post-synaptic potential (e.p.s.p.) and massed discharge of the cortical cells (population spike) were depressed at concentrations of the anaesthetics lower than those required to depress the compound action potential of the perforant path nerve fibres. None of the anaesthetics studied increased the threshold depolarization required for granule cell discharge. Furthermore, frequency potentiation of the evoked cortical e.p.s.p.s was not impaired by any of the anaesthetics studied. 4. It is concluded that all four anaesthetics depress synaptic transmission in the dentate gyrus either by reducing the amount of transmitter released from each nerve terminal in response to an afferent volley, or by decreasing the sensitivity of the post-synaptic membrane to released transmitted or by both effects together.

The action of ether and methoxyflurane on synaptic transmission in isolated preparations of the mammalian cortex

1. The actions of ether and methoxyflurane on the evoked potentials of in vitro preparations of the guinea-pig olfactory cortex were studied. Following stimulation of the lateral olfactory tract (l.o.t.) evoked potentials could be recorded from the cortical surface; these potentials consisted of an initial wave (the compound action potential of the l.o.t.) followed by a negative field potential which was associated with the synchronous excitation of many superficial excitatory synapses (population e.p.s.p.). Superimposed on the population e.p.s.p. was a number of positive peaks. These positive peaks reflect the synchronous discharge of many neurones and so have been called population spikes. 2. When ether or methoxyflurane was added to the gas stream that superfused the surface of the preparations, the population e.p.s.p.s. and population spikes were depressed at lower concentrations than those required to depress the compound action potential of the afferent fibres. 3. The evoked activity of individual cells in the cortex was depressed by ether and methoxyflurane. However, five of the twelve cells tested in ether showed an increase in their evoked activity at concentrations below 4-5%, but at higher concentrations these cells also became depressed. 4. Both ether and methoxyflurane depressed the sensitivity of cortical neurones to iontophoretically applied L-glutamate and may similarly depress the sensitivity of the post-synaptic membrane to the released transmitter substance. 5. Neither anaesthetic appeared to increase the threshold depolarization required for nerve impulse generation. Thus, the decrease of the discharge of the post-synaptic cells was primarily caused by a depression of chemical transmission. 6. Ether caused some cells in the cortex to alter their normal pattern of synaptically evoked discharge and both anaesthetics induced similar changes during excitation by glutamate.

On the mechanism of halothane anaesthesia

1. The effects of halothane on the evoked potentials of in vitro preparations of guinea-pig olfactory cortex were studied.2. The evoked potentials recorded from the cortical surface comprised an initial diphasic wave - the lateral olfactory tract (l.o.t.) compound action potential - followed by a negative wave of 1-3 mV amplitude and about 10 msec duration. Superimposed on the negative wave was a number of positive peaks. The negative wave has been identified as an extracellularly recorded, monosynaptic, excitatory post-synaptic potential (e.p.s.p.) and the positive peaks have been shown to reflect the discharge of the cortical cell population in response to the evoked e.p.s.p. and are therefore termed ;population spikes'.3. When halothane (0.4-1.5%) was added to the gas stream that superfused the surface of the preparation the evoked e.p.s.p.s became smaller in amplitude and the size of the population spikes diminished. The l.o.t. compound action potential was unaffected by these levels of halothane. Higher levels of halothane (above 2%) further reduced the amplitude of the evoked e.p.s.p.s, abolished the population spikes, decreased the amplitude of the l.o.t. compound action potential and slowed its time course. The effects of halothane on the evoked potentials were dose-related and were independent (after the first 10 min of treatment) of the duration of the exposure to halothane.4. The decrease in the size of the population spike caused by the exposure to halothane implied that transmission through the cortical relay had been impaired. This was also shown by the decrease in the evoked activity of units in the prepiriform cortex. Of eleven units, eight were depressed by halothane (0.5-1.5%) two were unaffected and one showed a transient increase in the number of spikes generated in response to a l.o.t. volley.5. Halothane (up to 1.5%) had no effect on the threshold of the l.o.t. fibres to electrical stimulation or on that of the post-synaptic cells to synaptic excitation.6. Post-tetanic potentiation and frequency potentiation of the evoked e.p.s.p.s were enhanced in the presence of 1% halothane.7. It is concluded that halothane reduces excitatory synaptic transmission not by an increase in the electrical threshold of the post-synaptic cells to synaptic excitation but by interference with the process of chemical transmission either by reducing the output of transmitter from the pre-synaptic nerve terminal or by reducing the sensitivity of the post-synaptic membrane to the released transmitter substance.

The distribution of vasopressin and oxytocin in the hypothalamoneurohypophysial system of the guinea-pig

1. The ratio of the content of vasopressin to that of oxytocin (V/O ratio) was estimated in the supraoptic nucleus (SON), paraventricular nucleus (PVN) and posterior pituitary gland (PIT) of guinea-pigs.2. Extracts were assayed for antidiuretic activity to estimate vasopressin and for milk-ejecting activity to estimate oxytocin. In assays for milk-ejecting activity, trypsin was used to inactivate vasopressin in the extracts.3. The mean V/O ratios in the SON, PVN and PIT were 28, 8.5 and 7.0 respectively in male guinea-pigs, 6.8, 7.4 and 6.9 in non-lactating females, and 5.1, 3.3 and 6.6 in lactating females.4. The distribution of the hormones within the hypothalamus is discussed in relation to their independent release in response to electrical stimulation of the SON and PVN.

On the mechanism of barbiturate anaesthesia

1. The effects of pentobarbitone (0.05-0.6 mM in saline solution) on the evoked field potentials of in vitro preparations of guinea-pig olfactory cortex were studied.2. The evoked field potentials comprised an initial diphasic wave - the lateral olfactory tract (l.o.t.) compound action potential - followed by a surface negative wave (e.p.s.p) of 1-3 mV amplitude and about 10 msec duration. Superimposed on the negative wave were a number of positive peaks (population spikes).3. Pentobarbitone depressed the e.p.s.p. but not the l.o.t. compound action potential. The number and size of the population spikes were progressively reduced as the e.p.s.p. became depressed, indicating a failure of transmission through the cortical relay. The e.p.s.p. depression increased with increasing concentrations of pentobarbitone.4. Pentobarbitone had no effect on the threshold to electrical stimulation of the l.o.t. fibres or on that of the post-synaptic cells to synaptic excitation.5. Post-tetanic potentiation and frequency potentiation were either of normal magnitude or were enhanced in the presence of 0.2-0.3 mM pentobarbitone.6. It is concluded that pentobarbitone probably reduces the output of transmitter from the presynaptic nerve terminals of the olfactory cortex and that this mechanism could be the basis of the depressant action of the barbiturates.

Potentiation and depression of synaptic transmission in the olfactory cortex of the guinea-pig

1. The extracellular field potentials of the olfactory cortex evoked by stimulation of the lateral olfactory tract (l.o.t.) were studied in in vitro preparations from the olfactory cortex. The field potentials comprised an initial diphasic wave - the l.o.t. compound action potential - followed by a negative wave of about 10 msec duration which in turn was followed by a low amplitude positive wave of long duration (100 msec or more). In this paper, the size of the negative field potential (extracellularly recorded EPSP) has been studied during and after periods of repetitive stimulation of the l.o.t.2. If two identical volleys were delivered to the l.o.t. the second evoked EPSP was not the same size as the conditioning EPSP. At brief conditioning intervals (up to 10 msec) the second (test) EPSP was smaller than the control. For conditioning intervals between 10 and 200 msec, the test EPSP was potentiated over the control. For long conditioning intervals (300 msec up to 5 sec) the test EPSP was again slightly smaller than the control EPSP. After a brief conditioning train, the depression of a test EPSP (elicited 300 msec or more after the conditioning train) was more pronounced and lasted longer. These changes of test EPSP size were attributed to the presence of two opposing processes: an initial potentiation superimposed on a more prolonged but less pronounced depression.3. During prolonged repetitive stimulation the final steady amplitude of an EPSP varied with the frequency of stimulation. At low frequencies (0.5-2/sec) the steady EPSP amplitude was 90-95% of the initial control amplitude. At moderate frequencies (5-20/sec) the steady EPSP amplitude was greater than the initial control. At high frequencies (above 20/sec) the steady amplitude of the EPSPs declined with increasing frequency of stimulation. Potentiation of EPSPs was observed early in a train of impulses when the stimulation frequency was 5-70/sec.4. After a large number of stimuli at frequencies from 20 to 100/sec the amplitude of individual, infrequently evoked, EPSPs passed through a phase of depression that lasted about 30 sec. This depression was followed by a phase of potentiation (post-tetanic potentiation). The amplitude and duration of post-tetanic potentiation appeared to depend on the characteristics of the conditioning train.5. The discussion compares the results obtained with those obtained for other mammalian synapses. It is suggested that the transmitter in the presynaptic terminals could be in three parts, (a) immediately available transmitter (b) conditionally available transmitter requiring a single nerve impulse for its availability and (c) main depot transmitter which replenishes the other two stores. Potentiation and depression of evoked EPSPs were interpreted in terms of changes in the amount of transmitter released by the test volley. According to this analysis, a fixed proportion (about 10%) of the immediately available transmitter is released by each nerve impulse.