The action of anaesthetics on synaptic transmission

Anesthetic agent-specific effects on synaptic inhibition

BACKGROUND

Anesthetics enhance γ-aminobutyric acid (GABA)-mediated inhibition in the central nervous system. Different agents have been shown to act on tonic versus synaptic GABA receptors to different degrees, but it remains unknown whether different forms of synaptic inhibition are also differentially engaged. With this in mind, we tested the hypothesis that different types of GABA-mediated synapses exhibit different anesthetic sensitivities. The present study compared effects produced by isoflurane, halothane, pentobarbital, thiopental, and propofol on paired-pulse GABAA receptor-mediated synaptic inhibition. Effects on glutamate-mediated facilitation were also studied.

METHODS

Synaptic responses were measured in rat hippocampal brain slices. Orthodromic paired-pulse stimulation was used to assess anesthetic effects on either glutamate-mediated excitatory inputs or GABA-mediated inhibitory inputs to CA1 neurons. Antidromic stimulation was used to assess anesthetic effects on CA1 background excitability. Agents were studied at equieffective concentrations for population spike depression to compare their relative degree of effect on synaptic inhibition.

RESULTS

Differing degrees of anesthetic effect on paired-pulse facilitation at excitatory glutamate synapses were evident, and blocking GABA inhibition revealed a previously unseen presynaptic action for pentobarbital. Although all 5 anesthetics depressed synaptically evoked excitation of CA1 neurons, the involvement of enhanced GABA-mediated inhibition differed considerably among agents. Single-pulse inhibition was enhanced by propofol, thiopental, and pentobarbital, but only marginally by halothane and isoflurane. In contrast, isoflurane enhanced paired-pulse inhibition strongly, as did thiopental, but propofol, pentobarbital, and halothane were less effective.

CONCLUSIONS

These observations support the idea that different GABA synapses use receptors with differing subunit compositions and that anesthetics exhibit differing degrees of selectivity for these receptors. The differing anesthetic sensitivities seen in the present study, at glutamate and GABA synapses, help explain the unique behavioral/clinical profiles produced by different classes of anesthetics and indicate that there are selective targets for new agent development.

Methylphenidate modulates the locus ceruleus neuronal activity in freely behaving rat

The electrophysiological properties of the locus coeruleus (LC) neurons in response to acute and chronic administration of methylphenidate (MPD) were investigated. The extracellular LC neuronal activities were recorded from non-anesthetized, freely behaving rats previously implanted bilaterally with permanent semi microelectrodes. The main findings were: (1) On experimental day 1 (ED1), 87% (94/108) of LC units significantly changed their firing rate after initial (acute) MPD (2.5mg/kg, i.p.) administration. The majority of the responsive units (80%, 75/94) increased their firing rate; (2) Daily MPD (2.5mg/kg) injection was given on ED2 through ED6 followed by 3 washout days (ED7 to 9). On ED10, all LC units exhibited a significant change of their baseline activity compared to their baseline activity on ED1; (3) MPD rechallenge on ED10 elicits 94% (101/108) of LC units significantly changed their firing rate; the majority of them (78%, 79/101) increased their firing rate; (4) The effect of rechallenge MPD administration on ED10 were compared to the effect of initial MPD on ED1, 98% of the LC units exhibited a significant change in their firing rate. 41% (43/106) of them exhibited a significant increase in their firing rate while 59% (63/106) units significantly decreased their firing rate which can be interpreted as electrophysiological sensitization or tolerance respectively. In conclusion, the majority of LC neurons significantly increased their firing rate after acute and chronic MPD administration. This data demonstrated that enhanced LC neuronal activities play important role in the effect of MPD.

Local anesthetics inhibit kinesin motility and microtentacle protrusions in human epithelial and breast tumor cells

Detached breast tumor cells produce dynamic microtubule protrusions that promote reattachment of cells and are termed tubulin microtentacles (McTNs) due to their mechanistic distinctions from actin-based filopodia/invadopodia and tubulin-based cilia. McTNs are enriched with vimentin and detyrosinated α-tubulin, (Glu-tubulin). Evidence suggests that vimentin and Glu-tubulin are cross-linked by kinesin motor proteins. Using known kinesin inhibitors, Lidocaine and Tetracaine, the roles of kinesins in McTN formation and function were tested. Live-cell McTN counts, adhesion assays, immunofluorescence, and video microscopy were performed to visualize inhibitor effects on McTNs. Viability and apoptosis assays were used to confirm the non-toxicity of the inhibitors. Treatments of human non-tumorigenic mammary epithelial and breast tumor cells with Lidocaine or Tetracaine caused rapid collapse of vimentin filaments. Live-cell video microscopy demonstrated that Tetracaine reduces motility of intracellular GFP-kinesin and causes centripetal collapse of McTNs. Treatment with Tetracaine inhibited the extension of McTNs and their ability to promote tumor cell aggregation and reattachment. Lidocaine showed similar effects but to a lesser degree. Our current data support a model in which the inhibition of kinesin motor proteins by Tetracaine leads to the reductions in McTNs, and provides a novel mechanism for the ability of this anesthetic to decrease metastatic progression.

Reorganization in the auditory cortex of the rat induced by intracortical microstimulation: a multiple single-unit study

Many manipulations are able to change or perturb various aspects of single neuron properties and interneuronal relationships. Changes of cerebral cortex organization have been observed in different cortical areas and at different time scales in relation to peripheral stimulation, peripheral damage, associative learning, and electrical stimulation. Here we describe studies on separable multineuron recordings in the rat's auditory cortex under two different anesthetics. Acoustic stimuli were used as a normal, physiological input, and weak electrical intracortical microstimulation (ICMS) as a perturbation that forces a rapid cortical reorganization. ICMS induced fast changes in the cortical map and in the receptive field properties of cells at the electrically stimulated and adjacent electrodes. In effect there was an enlargement of the cortical domain tuned to the acoustic frequency that had been represented at the stimulating electrode. ICMS also incremented afterdischarge responses; these consisted of an initial response to the auditory stimulus followed by less intense repetitive activity that was stimulus-time locked and had a period of 8-12 Hz, similar to that of the spontaneous synchronous activity. Cortical activity under ketamine differed from that under pentobarbital sodium, although in both situations we observed that cortical neurons were highly synchronous.

Effects of halothane on the efflux of [3H]D-aspartate from rat brain slices

The in vitro effects of halothane on the potassium-stimulation-induced efflux of [3H]D-aspartate in rat cerebral cortex slices were studied. The slices were initially incubated with Krebs-Ringer's solution containing [3H]D-aspartate, a putative excitatory transmitter. The slices were then stimulated with high concentrations of K+ in the presence and absence of halothane, and the efflux was measured using a scintillation counter. Halothane, 1% and 2%, had little effect on the potassium-stimulation-induced efflux, but that of 4 and 8% increased the efflux significantly. The spontaneous efflux was unaffected by all concentrations of halothane studied. The control study of pentobarbital, in the concentration of 0.05 to 1.00 mmol/l, reduced the efflux in a dose-related manner. These findings indicate that the release of an excitatory transmitter, aspartate, may not be involved in the mechanism of halothane anaesthesia.

Effects of acetylcholine on single cortical somatosensory neurons in the unanesthetized rat

Experiments have been performed on unanesthetized rats using a chronic restraint system. The animal's head was held in a stereotaxic apparatus by means of two metallic tubes fixed on the skull with dental cement. Electrodes consisted of a recording micropipette (filled with 1 M NaCl and 2% Pontamine Blue) attached to a multibarreled micropipette for iontophoresis. Electrode penetrations were reconstructed on camera lucida drawings of frontal brain sections. The overall percentage of spontaneously active somatosensory neurons was 77% with a mean spontaneous activity of 5.9 impulse/s (n = 405). Yet differences were observed in the proportions of active cells as well as in the mean spontaneous activity between cortical layers (both parameters being significantly higher in layers V and VI). Comparison with results obtained under urethane anesthesia [Dykes R. W. and Lamour Y. (1988) J. Neurophysiol. 60, 703-724] shows that the percentage of the spontaneously active neurons and the mean spontaneous activity were both significantly higher in unanesthetized rats (77 vs 36%; 5.9 vs 2.6 impulse/s). Nevertheless, the laminar distribution of the most active cells was similar under both conditions. In the present study, 52.3% of the neurons (n = 380) were excited by acetylcholine and 46% (n = 198) by carbachol. Significantly larger percentages of neurons excited by acetylcholine were found in layers Vb and VIb. These effects of cholinergic agonists--observed for the first time in unanesthetized rats--differed significantly from those previously obtained under anesthesia (33 and 34% of neurons excited by acetylcholine and carbachol, respectively) [Lamour Y. et al. (1982) Neuroscience 7, 1483-1494].(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in spontaneous firing patterns of rat hippocampal neurones induced by volatile anaesthetics

1. The effects of the volatile anaesthetics, halothane, isoflurane and enflurane, on rat hippocampal CA1 and CA3 neurones in in vitro preparations were studied by intracellular recording methods. 2. The three anaesthetics, at concentrations similar to those used clinically (0.2-1.2 mM), initially increased and then subsequently decreased the spontaneous firing of CA1 neurones without affecting the resting membrane properties or the EPSPs evoked by focal stimuli. 3. The anaesthetics at these concentrations depressed both the fast after-hyperpolarization of the soma spike and the post-tetanic hyperpolarization induced by repetitive stimulation. They also decreased the IPSPs evoked by focal stimuli. 4. The threshold for spike generation was gradually elevated by as much as 4-6 mV during application of the anaesthetics at these concentrations. The subthreshold potential oscillations (which are likely to be associated with periodic alterations in non-inactivating Ca2+ and Na+ currents) were enhanced in the low concentrations (0.2-0.5 mM), but were depressed in the high concentrations (0.8-1.2 mM). 5. The results suggest that the transient increase in the firing frequency was caused by a depression of both the spike after-hyperpolarization and the post-tetanic hyperpolarization, and that the reduction of spontaneous firing was mainly due to an elevated threshold for spike generation. 6. The three anaesthetics altered the pattern of spontaneous spike-firing in CA3 neurones from solitary spiking to burst firing without affecting the resting membrane properties. 7. The effects of the anaesthetics on the active membrane properties and the postsynaptic potentials in CA3 neurones were similar to the effects in CA1 neurones. 8. In the majority of CA3 neurones, soma spikes elicited by depolarizing current pulses were followed by a Ca2+-dependent after-depolarization, which was in turn followed by a prolonged after-hyperpolarization (post-burst hyperpolarization). The anaesthetics facilitated the after-depolarizing potential, while they depressed the post-burst hyperpolarization. Combination of the two effects would give rise to the highly stereotyped burst (about 1 Hz in frequency) in the presence of the volatile anaesthetics.

Cardiovascular responses elicited by stimulation of neurons in the central amygdaloid nucleus in awake but not anesthetized rats resemble conditioned emotional responses

Cardiovascular responses elicited by electrical stimulation of the central amygdaloid nucleus were examined in awake and anesthetized rats. Stimulation through chronically implanted electrodes evoked increases in arterial pressure and heart rate in awake, freely behaving rats. The responses, which were dependent upon the frequency and the intensity of the stimulus, were not consistently related to the presence of evoked amygdaloid afterdischarges or to evoked behavioral reactions. Following induction of anesthesia, stimuli delivered to the same rats through the same fixed electrodes produced decreases in blood pressure and heart rate. Microinjection of L-glutamate into the amygdala of freely behaving rats also elicited increases in arterial pressure and heart rate, indicating that the cardiovascular changes evoked by electrical stimuli are due to excitation of local neurons rather than fibers of passage. The timing and pattern of the response elicited by electrical stimulation of the amygdala in the awake but not the anesthetized rat closely corresponds with that evoked by an acoustic conditioned emotional stimulus.

Isoflurane effects in rat hippocampal cortex: a quantitative evaluation of different cellular sites of action

In order to evaluate quantitatively the effects of an inhalation anaesthetic on neuronal excitability and on synaptic transmission in the central nervous system, we have examined the action of isoflurane on slices from rat hippocampal cortex. Isoflurane 1.5% (1.38% anaesthetize 50% of tested rats (MAC)) reduced orthodromically evoked activity in pyramidal cells by 62%. This was due to the combined effects on afferent fibres, excitatory synapses and pyramidal cells. The effect on the postsynaptic neurones was almost as strong as the effect on the excitatory synapses: the population spike evoked by a given synaptic current was reduced by 24%, and the field-EPSP in response to a given afferent fibre volley by 27%. The presynaptic fibre volley was reduced by 17%.

Pentobarbital and halothane hyperpolarize cat alpha-motoneurons

The effect of anesthetics on the membrane potential of spinal alpha-motoneurons was studied in awake-intact cats that were sequentially anesthetized with either halothane or pentobarbital. Pentobarbital (35 mg/kg) and halothane (5%) both induced a 5 mV hyperpolarization and a reduction in spontaneous synaptic activity of motoneurons. We suggest that a reduction in the urinary EPSP frequencies contributes to the hyperpolarization and the membrane potential criterion of acceptable impalements of motoneurons in awake-intact preparations should be 5 mV more depolarized than that in anesthetized preparations.

The effects of anaesthetics and high pressure on the responses of the rat superior cervical ganglion in vitro

The effects of helium pressure and of general anaesthetics were studied on the responses of the isolated superior cervical ganglion of the rat, to determine how far these reflected the pressure reversal of anaesthesia seen in vivo. The method of Brown & Marsh (1974) for extracellular recording of surface potentials was adapted for use in a high-pressure chamber. Helium alone, at 130 atm, did not alter the responses of the ganglion to gamma-aminobutyric acid (GABA) but significantly depressed the depolarizing and hyperpolarizing components of the nicotinic responses, and the muscarinic responses. The potentiation of the responses to GABA caused by pentobarbitone was not altered by the application of helium, at 130 atm. This pressure also decreased further the nicotinic responses which were depressed by pentobarbitone. Nitrogen, at 34 atm (the anaesthetic ED50 in vivo) and at 68 atm, significantly decreased the nicotinic responses of the ganglia, and the addition of helium to a total of 130 atm further increased this depression. At pressures of 3.3-68 atm, nitrogen caused small decreases in the responses to GABA. Nitrous oxide at 1.5 atm (the ED50 for loss of righting reflex in mice) and at 3 atm, significantly depressed the responses to GABA and to the nicotinic agonist, but did not alter the responses to methylfurmethide. The effects of nitrous oxide were unaltered when helium was added to a total of 130 atm, although this pressure of helium added alone significantly depressed the cholinergic responses. A mixture of 50% nitrous oxide and 50% oxygen, when added to the pressure chamber, at normal atmospheric pressure, caused transient increases in the responses to GABA. The effects of temperature on GABA responses and on nicotinic responses were very different from those of pressure. Preliminary evidence suggested that raising the temperature may decrease the extent of potentiation of GABA responses by pentobarbitone. The results are discussed in relation to the pressure reversal of anaesthesia in vivo. It was concluded that there was no evidence that the basis of this interaction lay in the potentiation of GABA responses by general anaesthetics, or the depression of cholinergic responses, although the changes seen were not in all cases simply additive. It was considered that effects of general anaesthetics such as the potentiation of GABA may contribute to the effects used to measure general anaesthesia in vivo, such as loss of righting reflex, but may not be related to the non-specific actions which cause anaesthesia.

The effect of isoflurane on unmyelinated and myelinated fibres in the rat brain

The effect of isoflurane on unmyelinated and myelinated fibres of the rat brain was investigated in vitro. The amplitude of the unmyelinated fibre potential (the prevolley of the hippocampal CA1 region) was reduced in a dose-dependent manner with increasing isoflurane concentrations (up to 5%). The conduction velocity was slightly decreased. The minimal alveolar concentration (MAC) anesthetizing 50% of the animals is 1.38%. At this concentration the presynaptic volley was reduced by 18% and the conduction velocity was decreased by about 1%. The effect on myelinated fibres (fimbria) was small and significantly different from the effect on unmyelinated fibres (P = 0.03 and 0.005 at 1 and 2% isoflurane, respectively.

Selective depression of hippocampal inhibitory postsynaptic potentials and spontaneous firing by volatile anesthetics

The effects of halothane, isoflurane and enflurane on the rat hippocampal CA1 neurons in in vitro preparations were studied by intracellular recordings. All volatile anesthetics, at the concentrations which are likely to be in the range of clinical doses, depressed the IPSPs and the spontaneous firing without affecting the resting membrane properties and the EPSPs. The results suggest that the simultaneous blocking effects of the anesthetics on both the spontaneous firing and IPSPs are responsible for production of general anesthesia rather than their blocking actions on the EPSPs in terms of behavior of hippocampal neurons.

Intrinsic mechanisms of pain inhibition: activation by stress

Portions of the brain stem seem normally to inhibit pain. In man and laboratory animals these brain areas and pathways from them to spinal sensory circuits can be activated by focal stimulation. Endogenous opioids appear to be implicated although separate nonopioid mechanisms are also evident. Stress seems to be a natural stimulus triggering pain suppression. Properties of electric footshock have been shown to determine the opioid or nonopioid basis of stress-induced analgesia. Two different opioid systems can be activated by different footshock paradigms. This dissection of stress analgesia has begun to integrate divergent findings concerning pain inhibition and also to account for some of the variance that has obscured the reliable measurement of the effects of stress on tumor growth and immune function.

Contribution of hypothermia to effects of chloral hydrate on flash evoked potentials of hooded rats

This study examined the contribution of hypothermia to the effects of chloral hydrate on the flash evoked potential (FEP) of hooded rats. Three experiments were performed, all employing intraperitoneal injections of saline, and of 75, 150 and 300 mg chloral hydrate/kg body weight. In the first experiment, body temperature was measured in a standard (23 degrees C) environment for 6 hr following injection. Rats were hypothermic following administration of the 150 and 300 mg/kg dosages for up to 1 and 2 hr, respectively. In the second experiment, FEPs were recorded from the visual cortex of chronically implanted rats 30 min after injection (22 degrees C environment). P1N1, N1P2 and P2N2 amplitudes and P1, N1, P2, N2 and P3 peak latencies were significantly increased by the 300 mg/kg dosage. Increased latencies were also noted for the primary components with the 150 mg/kg dosage. The final experiment replicated the second experiment, but at an ambient temperature of 30 degrees C, which prevented hypothermia. Amplitudes were unaffected by chloral hydrate. Significantly increased peak latencies were observed, even with the 75 mg/kg dose for some components. However, the magnitude of the latency increases of the primary components was less than half of that found with a standard environment. These results indicate that depending upon ambient temperature, hypothermia may contribute to chloral hydrate-induced alterations in FEPs.

Effects of some depressant drugs on synaptic responses to glutamate at the crayfish neuromuscular junction

Excitatory junction currents produced by glutamate were recorded with an extracellular electrode at the neuromuscular junction of the crayfish. Pentobarbitone, phenobarbitone, diazepam, chlordiazepoxide and procaine had only minimal effects on current decay at concentrations which are highly effective in other preparations. The glutamate synapse in the crayfish appears relatively resistant to these drugs. In contrast, ether and halothane increased the rate of decay of the currents at concentrations which are comparable to those occurring during anaesthesia.

Pressure reversal of the action of octanol on postsynaptic membranes from Torpedo

Octanol increases the binding of [3H]-acetylcholine to the desensitized state of the nicotinic receptor in postsynaptic membranes prepared from Torpedo californica. This increase in binding results from an increase in the affinity of [3H]-acetylcholine for its receptor without any change in the number of sites or the shape of the acetylcholine binding curve. High pressures of helium (300 atm) decrease [3H]-acetylcholine binding by a mechanism that changes only the affinity of acetylcholine binding. Helium pressure reverses the effect of octanol on the affinity of [3H]-acetylcholine for its receptor. This pressure reversal of the action of octanol at a postsynaptic membrane is consistent either with pressure counteracting an octanol-induced membrane expansion or with independent mechanisms for the actions of octanol and pressure. The data do not conform with a mechanism in which pressure displaces octanol from a binding site on the receptor protein.

Chloral hydrate anesthesia alters the responsiveness of central serotonergic neurons in the cat

The influence of chloral hydrate anesthesia on the spontaneous activity and responsiveness of serotonergic neurons was examined by administering chloral hydrate (300 mg/kg, i.p.) to freely moving cats from which serotonergic unit activity in the dorsal raphe nucleus (DRN) was being recorded. Although chloral hydrate administration produced a surgical level of anesthesia within 15 min following injection, it produced only a small decrease (approximately 20%) in the spontaneous activity of DRN serotonergic neurons. In contrast, the responsiveness of these same neurons was greatly altered by chloral hydrate administration. By examining the same neuron before and after chloral hydrate injection, it was found that chloral hydrate anesthesia completely abolished the excitatory responses of DRN serotonergic neurons to auditory and visual stimuli, as well as their excitatory response to electrical stimulation of the gigantocellular tegmental field (FTG) in the pontine reticular formation. On the other hand, the inhibition of serotonergic neuron firing resulting from systemic administration of WB 4101 (1.0 mg/kg, i.p.), a selective alpha 1 adrenergic receptor antagonist, was greatly potentiated by chloral hydrate anesthesia. Therefore, these data indicate that chloral hydrate anesthesia produces profound changes in the physiological and pharmacological responses of central serotonergic neurons which are not predictable by examination of spontaneous activity alone. Furthermore, as discussed, it it not clear to what extent these confounding influences might generalize to other anesthetized or immobilized preparations. Thus, beyond the obvious advantage which allows for the study of relationships between neuronal activity and behavior, single unit studies conducted in awake, freely moving animals also may be of greater value for basic physiological and pharmacological studies.

Hypothermia and chloropent anesthesia differentially affect the flash evoked potentials of hooded rats

Anesthetics and body temperature alterations are both known to alter parameters of sensory-evoked responses. However few studies have quantitatively assessed the contributions of hypothermia to anesthetic-induced changes. Two experiments were performed. In the first, chronically implanted rats were injected with either 0, 0.05, 0.10 or 0.20 ml Chloropent/100 g b.w., while body temperature was maintained. Flash evoked potentials recorded 30 min later showed increased latencies but only minor (not statistically significant) changes in amplitude. In the second experiment the same rats were anesthetized with 0.35 ml Chloropent/100 g b.w. and rectal temperature was systematically varied between 31 degrees C and 37 degrees C. Over the ranges of temperature and anesthetic employed, latencies increased more extensively with hypothermia than with anesthesia. P1N1 amplitude doubled when temperature was lowered to 31C, but P2N2 and N2P3 amplitudes declined over the same temperature range. Anesthetic-induced changes in peak-to-peak amplitude did not reach statistical significance when body temperature was constant. The findings suggest that previously reported alterations in evoked potentials following anesthesia may have been confounded with hypothermia.

The effects of apomorphine upon local cerebral glucose utilization in conscious rats and in rats anesthetized with chloral hydrate

The effects of the dopaminergic agonist apomorphine (1 mg . kg-1 i.v.) upon local cerebral glucose utilization in 43 anatomically discrete regions of the CNS were examined in conscious, lightly restrained rats and in rats anesthetized with chloral hydrate by means of the quantitative autoradiographic [14C]2-deoxyglucose technique. In animals anesthetized with chloral hydrate, glucose utilization was reduced throughout all regions of the CNS from the levels observed in conscious animals, although the magnitude of the reductions in glucose use displayed considerable regional heterogeneity. With chloral hydrate anesthesia, the proportionately most marked reductions in glucose use (by 40-60% from conscious levels) were noted in primary auditory nuclei, thalmaic relay nuclei, and neocortex, and the least pronounced reductions in glucose use (by 15-25% from conscious levels) were observed in limbic areas, some motor relay nuclei, and white matter. In conscious, lightly restrained rats, the administration of apomorphine (1 mg . kg-1) effected significant increased in glucose utilization in 15 regions of the CNS (e.g., subthalamic nucleus, ventral thalamic nucleus, rostral neocortex, substantia nigra, pars reticulata), and significant reductions in glucose utilization in two regions of the CNS (lateral habenular nucleus and anterior cingulate cortex). In rats anesthetized with chloral hydrate, the effects of apomorphine upon local glucose utilization were less widespread and less marked than in conscious animals. In only two of the regions (the globus pallidus and septal nucleus), which displayed increased glucose use following apomorphine in conscious rats, were significant increases in local glucose utilization observed with this agent in chloral hydrate-anesthetized rats. In the pars compacta of the substantia nigra, in which apomorphine increased glucose utilization in conscious animals, significant reductions in glucose utilization were observed following apomorphine in rats anesthetized with chloral hydrate. The profound effects of chloral hydrate anesthesia upon local cerebral glucose use, and the modification by this anesthetic regime of the local metabolic responses to apomorphine, emphasize the difficulties which exists in the extrapolation of data from anesthetized animals to the conditions which prevail in the conscious animal.

General anesthetics hyperpolarize neurons in the vertebrate central nervous system

The effect of general anesthetics on frog motoneurons and rat hippocampus pyramidal cells was examined with sucrose gap and intracellular recording, respectively. A number of volatile and intravenous anesthetics directly hyperpolarized the motoneurons. The potency of these agents in hyperpolarizing motoneurons was strongly correlated with their anesthetic potency. While the responses to barbiturates and alpha-chloralose were blocked by gamma-aminobutyric acid antagonists and were dependent on the chloride gradient, the responses to all the other anesthetics tested were generated by a separate mechanism. Intracellular recording from hippocampal pyramidal cells suggested that an increase in potassium conductance accounts for these responses. Such a nonsynaptic action would contribute to the decreased neuronal responsiveness observed for these compounds and thus to their anesthetic action.

Effects of small nonpolar molecules on membrane compressibility and permeability. A theoretical study of the effects of anesthetic gases

We explore from a theoretical perspective the effects of small nonpolar molecules, such as anesthetic gases, on membrane compressibility and permeability. As a model system we expand a previously proposed generalization of Nagle's model for biomembrane phase transitions. In this model anesthetic gases alter membrane compressibility, causing profound changes in membrane permeability. Anesthetics either increase or decrease membrane permeability, depending on whether the membrane lipid is originally in the solid or melted state, or in a two-phase region. These changes are reversed by high pressure, in agreement with experimental results. Anesthetic-induced changes in compressibility are predicted to inhibit fusion of phospholipid vesicles to each other and to planar bilayers, and thus might be expected to inhibit the fusion of presynaptic vesicles with the presynaptic nerve membrane. This work provides a detailed molecular theory for many of the effects of anesthetic gases on both synapse and axon, and provides a coherent framework for understanding diverse experimental results.

The actions of pentobarbitone, procaine and tetrodotoxin on synaptic transmission in the olfactory cortex of the guinea-pig

1 It has been suggested that the depression of excitatory synaptic potentials produced by general anaesthetics can be attributed to a partial blockade of impulse conduction in the terminal branches of axons. This hypothesis has been tested by comparing the actions of pentobarbitone, procaine and tetrodotoxin (TTX) on synaptic transmission in the guinea-pig olfactory cortex. 2 Pentobarbitone (0.1-0.3mM) depressed the evoked synaptic potentials without any significant depression of impulse conduction in the afferent fibres of the lateral olfactory tract (1.o.t). It had no effect on the electrical excitability of either the l.o.t axons or the postsynaptic neurones. 3 Tetrodotoxin (TTX; 1-5x10(-8 M) slowed conduction of impulses in the l.o.t. and decreased the amplitude of the l.o.t compound action potential in proportion to the concentration applied. All concentrations of TTX elevated the electrical threshold of the l.o.t. axons and there was evidence to suggest that the threshold of the postsynaptic neurones was also elevated. The synaptic potentials were depressed in direct proportion to the depression of the l.o.t. compound action potential. 4 Procaine (0.1-0.5 mM) exhibited a pattern of activity intermediate between pentobarbitone and TTX. The most marked effect, seen at all concentrations tested, was a slowing of impulse conduction and a decrease in the electrical excitability of the l.o.t. axons. 5 It is concluded that general anaesthetics (exemplified by pentobarbitone) depress synaptic transmission by interfering with the processes involved in chemical transmission and not by blocking impulse conduction in the terminal branches of afferent nerves.

Inhibitory postsynaptic currents at Aplysia cholinergic synapses: effects of permeant anions and depressant drugs

Inhibitory postsynaptic potentials (i.p.s.ps) and, under voltage-clamp conditions, inhibitory postsynaptic currents (i.p.s.cs) were recorded in neurons in buccal ganglia of Aplysia juliana. The decay of i.p.s.cs was exponential with a single time constant, tau, which decreased with membrane depolarization. In external solutions containing iodide or bromide ions instead of chloride ions, tau varied according to the sequence tau (I) greater than tau (Br) greater than tau (Cl), and the voltage sensitivity of tau was altered. In iodide solution, the voltage sensitivity of tau was reversed. Furthermore, the foreign halides depressed the peak current amplitude and shifted the reversal (zero-current) potential to more positive membrane potentials. In low concentrations of sodium pentobarbitone (100-200 microns), the decay of i.p.s.cs became biphasic. Increasing drug concentration and membrane hyperpolarization had differential effects on the rates and relative amplitudes of the two components of i.p.s.c. decay. Octanol (0.5-1 mM) reduced the amplitude of i.p.s.ps and increased the rate of decay of i.p.s.cs without changing the voltage sensitivity of tau. The effect of foreign halides and barbiturates on i.p.s.c. decay were interpreted in terms of a reaction between the anion and an ion-binding site(s) associated with the anion-selective channel, which affects the probability of anions entering the channel and normal channel closure.

[14C]Deoxyglucose mapping of first-order projections activated by stimulation of lateral hypothalamic sites eliciting gnawing, eating, and drinking in rats

Rats having hypothalamic electrodes that elicited gnawing, eating, and drinking in free-moving tests received intermittent electrical stimulation for 45 min following i.v. injection of [14C] deoxyglucose. Autoradiographs of regional brain glucose utilization were made by the method of Sokoloff et al. ('77). To maximize the detectability of first-order neuronal effects and minimize potentially complex transsynaptic effects, baseline metabolism and synaptic transmission were reduced by light barbiturate anesthesia. Laterally asymmetrical increases in glucose utilization indicative of elicited activity were largely coterminous with the known projections of the lateral hypothalamus and some projections of adjoining areas, indicating that most first-order efferents were above threshold for deoxyglucose visualization, while most transsynaptic effects were subthreshold. Although the majority of hypothalamic projections were similarly affected in control rats that received hypothalamic stimulation that elicited other responses, a number were activated significantly less than in the rats whose electrodes induced gnawing, eating, and drinking. Chief among these areas was a continuous descending pathway from the ventral tegmental area through the lateral tegmentum to the cuneiform and parabrachial nuclei. Smaller and/or less reliable increases above controls were found in the dorsomedial caudate-putamen, the posterolateral zona incerta, the anterior lateral central gray, the caudal linear nucleus, the laterodorsal tegmental nucleus, the pontine tegmental nucleus, and a previously undescribed pathway lying medially between the pontine medial lemniscus and cerebral peduncle. These areas, especially the lateral tegmental and parabrachial zone, are the most likely candidates for the pathways and/or destinations of the directly excited efferents or fibers of passage that constitute the first link in the elicitation of gnawing, eating, and drinking by lateral hypothalamic stimulation. Since self-stimulation and exploratory activity were elicited by control as well as experimental electrodes, they are probably dependent on other projections among those affected similarly in both groups.

Succinyldicholine-induced return of the eyes to the basic deviation: a motion picture study

Succinyldicholine 2 mg/kg body weight returned the eyes of 20 anesthetized subjects to the same basic horizontal position as when conscious. The distance between the two eyes, as measured from motion pictures, agreed when the patient was in the conscious state and succinyldicholine-stimulated unconscious state by 99 +/- 10%. Since succinyldicholine has been shown by others to selectively produce a sustained contraction of en grappe muscle fibers, these muscle fibers are implicated as determinants of the basic ocular position and a cause of strabismus.

Mechanisms of anaesthesia: a review

Anaesthesia is a drug-induced reversible perturbation of neuronal activity. Since a wide variety of structurally unrelated substances are capable of producing this phenomenon, it has been generally accepted that anaesthetics produce their effects through non-specific hydrophobic interactions. Results of recent studies in whole animal and cellular (membrane) preparations demonstrate that a unitary theory of action does not exist. Anaesthetics can produce a spectrum of activity in the central nervous system, and different agents produce different patterns of activity. At the cellular and membrane level, differential effects have been observed, structural dependent differences occur and optical isomers display very different activities. The perturbation (fluidity change) of membrane components does not appear to be uniform for all anaesthetics. It is concluded that anaesthetics are selective agents, and produce their effects at multiple sites and through a variety of mechanisms.

Excitability of bulbar respiratory neurones: a study using microiontophoretic applications of depolarizing agents

The discharge of cat's bulbar respiratory neurones (RN) was shown to be modulated by periodic depressions which are characterized by their ability to reduce the effectiveness of microiontophoretically applied depolarizing agents: L-glutamate, acetylcholine and potassium. From the observation of cycle triggered time histograms (CTH), it appeared that these depressions have a determined and invariable phase relationship within the respiratory cycle. They were demonstrated in RN histologically located between and including the nucleus of the tractus solitarius and the nucleus ambiguus. Reproducibility and dose/response relationship of L-glutamate-induced depolarizations enabled an estimation of the functional effectiveness of these periodic depressions. In spontaneously phasic or 'silent' RN, depressions were demonstrated in the majority of cases (71%). Strongest depressions prevented spontaneous and L-glutamate-induced firing. Slighter depressions did not completely abolish L-glutamate effectiveness but reduced it by 20-90%. Conversely, in the majority of spontaneously tonic units (68%) depressions were not identified since the L-glutamate effect remained unchanged throughout the respiratory cycle. Four types of these respiration-related depressions were differentiated on the basis of their length, their phase relation to the respiratory cycle and their potentiation in barbiturate-anaesthetized preparations. A first type suppressed L-glutamate-evoked firing throughout inspiration; it was found in late-expiratory neurones. Two other types of depressions had a more restricted duration in the cycle: one was restricted to a portion of inspiration and was found in early-expiratory neurones; the other restricted to the beginning of expiration, was found in a special group of inspiratory neurones. A fourth type of inhibition was weaker and actively prolonged throughout expiration: it was found in another group of inspiratory neurones including the respiratory neurones located at the level of the nucleus of the tractus solitarius. These periodic depressions are interpreted in terms of synaptic inhibition; it is proposed that they play a major role in the functional organization of the respiratory centers at the bulbar level.