A PROPOSED COMMON MECHANISM OF ACTION FOR GENERAL AND LOCAL ANAESTHETICS IN THE CENTRAL NERVOUS SYSTEM

Role of Voltage-Gated Sodium Channels in the Mechanism of Ether-Induced Unconsciousness

Despite continuous clinical use for more than 170 years, the mechanism of general anesthetics has not been completely characterized. In this review, we focus on the role of voltage-gated sodium channels in the sedative-hypnotic actions of halogenated ethers, describing the history of anesthetic mechanisms research, the basic neurobiology and pharmacology of voltage-gated sodium channels, and the evidence for a mechanistic interaction between halogenated ethers and sodium channels in the induction of unconsciousness. We conclude with a more integrative perspective of how voltage-gated sodium channels might provide a critical link between molecular actions of the halogenated ethers and the more distributed network-level effects associated with the anesthetized state across species.

Procaine penicillin alters swimming behaviour and physiological parameters of Daphnia magna

Procaine penicillin (PP) is a β-lactam antibiotic widely used in human and veterinary medicine. Although PP is detected in surface water, little is known on its effects on aquatic invertebrates. Our aim was to determine the influence of PP on swimming behaviour (track density, swimming speed, turning angle, hopping frequency) and physiological activity (oxygen consumption, heart rate, thoracic limb movement) of a freshwater invertebrate Daphnia magna exposed to PP at concentrations of 11.79 mg/L, 117.9 mg/L and 1179 mg/L for 2 h and 24 h. The results showed no mortality; however, reduction of swimming activity manifested by the decreased track density, swimming speed and turning angle noted in Daphnia exposed to all the concentrations of PP. Increase of oxygen consumption was observed after 2-h exposure; however, decrease of this parameter was found after 24 h. PP also reduced heart rate and thoracic limb movement in a concentration-dependent manner. The results suggest that the antibiotic should not induce mortality; however, it may affect swimming behaviour and physiological parameters of Daphnia magna particularly inhabiting aquaculture facilities with intensive antibiotic treatment. On the basis of the present results, we also suggest higher sensitivity of behavioural and physiological parameters of cladocerans than the commonly used endpoints: mortality or immobilisation and their possible application as a part of early warning systems in monitoring of surface water toxicity.

Regional neurodegeneration and gliosis are amplified by mild traumatic brain injury repeated at 24-hour intervals

Most traumatic brain injuries (TBIs) that occur every year are classified as "mild." Individuals involved in high-risk activities may sustain multiple mild TBIs. We evaluated the acute physiologic and histopathologic consequences of mild TBI in a mouse model, comparing sham injury, single impact, or 5 impacts at a 24- or 48-hour interinjury interval. A single closed skull impact resulted in bilateral gliosis in the hippocampus and entorhinal cortex that was proportional to impact depth. Midline impact, at a depth just above the threshold to induce transient unconsciousness, produced occasional axonal injury and degenerating neurons accompanied by astrogliosis in the entorhinal cortex and cerebellum. Mild TBI repeated every 24 hours resulted in bilateral hemorrhagic lesions in the entorhinal cortex along with significantly increased neurodegeneration and microglial activation despite diminished durations of apnea and unconsciousness with subsequent impacts. Astrogliosis and diffusely distributed axonal injury were also observed bilaterally in the cerebellum and the brainstem. When the interval between mild TBIs was increased to 48 hours, the pathologic consequences were comparable to those of a single TBI. Together, these data suggest that, in mice, the brain remains at an increased risk for damage for 24 hours after mild TBI despite reduced acute physiologic responses to subsequent mild impacts.

Isoflurane depresses hippocampal CA1 glutamate nerve terminals without inhibiting fiber volleys

BACKGROUND

Anesthetic-induced CNS depression is thought to involve reduction of glutamate release from nerve terminals. Recent studies suggest that isoflurane reduces glutamate release by block of Na channels. To further investigate this question we examined the actions of isoflurane, TTX, extracellular Ca2+, CNQX and stimulus voltage (stim) on glutamate-mediated transmission at hippocampal excitatory synapses. EPSPs were recorded from CA1 neurons in rat hippocampal brain slices in response to Schaffer-collateral fiber stimulation.

RESULTS

Isoflurane (350 microM; 1 MAC) reversibly depressed EPSP amplitudes by ~60% while facilitation increased approximately 20%. Consistent with previous studies, these results indicate a presynaptic site of action that involves reduced excitation-release coupling. EPSPs were depressed to comparable levels by TTX (60 nM) or lowered stim, but facilitation was not changed, indicating a simple failure of axonal conduction. Similarly, partial antagonism of postsynaptic glutamate receptors with CNQX (10 microM) depressed EPSP amplitudes with no change in facilitation. However, EPSP depression by low external Ca2+ (0.8 mM) was accompanied by an increase in facilitation comparable to isoflurane. Isoflurane depression of EPSP amplitudes could also be partly reversed by high external Ca2+ (4 mM) that also decreased facilitation. Isoflurane or low Ca2+ markedly reduced the slopes of fiber volley (FV)-EPSP input-output curves, consistent with little or no effect on FVs. By contrast, TTX didn't alter the FV-EPSP curve slope, indicating that EPSP depression resulted from FV depression. FVs were remarkably resistant to isoflurane. Somatic spike currents were unaffected by 350 microM (1 MAC) isoflurane as well. The EC50 for isoflurane depression of FVs was approximately 2.8 mM (12 vol. %; 8 MAC).

CONCLUSION

Isoflurane appears to depress CA1 synapses at presynaptic sites downstream from Na channels, as evident by the increased facilitation that accompanies EPSP depression. Fiber volleys did not exhibit depression by isoflurane, as has been reported for other brain regions.

Pharmacological analysis of local anaesthetic tolycaine-induced convulsions by modification of monoamines in rat brain

The effects of a local anaesthetic, tolycaine, on brain monoamine levels were investigated during the convulsive process in rats. The influence of central monoamine modifications on tolycaine-induced convulsions was also examined. Tolycaine (140 mg/kg, intraperitoneally) produced a significant elevation of noradrenaline and 5-hydroxytryptamine levels in all brain regions in the convulsive state from the levels in the non-convulsive state. Their levels returned to normal during the postconvulsive state. Dopamine levels were depleted in the cerebral cortex, the striatum, and the ponsmedulla oblongata during the convulsive process and increased in the cerebellum. Pretreatment with alpha-methyl-p-tyrosine, which depletes brain catecholamine, suppresses the tolycaine-induced convulsions, as shown by a decrease in the incidence; L-3,4-dihydroxyphenylalanine and bis-(1-methyl-4-homopiperazinyl-thiocarbonyl)-disulfide, which increase brain catecholamine, intensified the convulsions, as shown by shortening of the latency and increase in the mortality. Antagonists of beta-adrenergic and dopamine receptors, such as propranolol, chlorpromazine and pimozide, markedly suppressed the convulsions, but an antagonist of alpha-adrenergic receptor, phenoxybenzamine, had no effect. Furthermore, 5-hydroxytryptophan, which increases brain 5-hydroxytryptamine, suppressed the convulsions, and DL-p-chlorophenylalanine, which depletes brain 5-hydroxytryptamine, intensified them. Antagonists of 5-hydroxytryptamine receptor, methysergide and methiothepin, suppressed the convulsions. These results suggest that brain noradrenaline and 5-hydroxytryptamine are major regulators in the tolycaine-induced convulsive process and that central catecholaminergic neurones act in a stimulatory way on the tolycaine-induced convulsions, while serotonergic neurones act suppressively.

REACTION OF LOCAL ANESTHETICS WITH PHOSPHOLIPIDS. A POSSIBLE CHEMICAL BASIS FOR ANESTHESIA

Local anesthetics (LA) have been found to interact with phospholipids and lipids extracted from nerve and muscle. This reaction is demonstrated by: (a) Inhibition by LA of phospholipid (and tissue lipid) facilitated transport of calcium from a methanol: water phase into chloroform. This action is dependent upon the cationic form of the LA. (b) LA increase the electrical resistance of "membranes" prepared by impregnating Millipore filters with cephalin:cholesterol or tissue lipid extracts and bathed with NaCl or KCl solutions. (c) LA coagulate aqueous dispersions of cephalin, phosphatidyl serine, phosphatidyl ethanolamine, and inositide, an action shared by calcium. The order of potency in coagulating cephalin sols is tetracaine > calcium > butacaine > procaine. Na⁺ and K⁺ do not coagulate phospholipid dispersions at 0.1 M concentration and antagonize the effect of Ca²⁺. (d) LA produce a marked fall in the pH of cephalin sols equivalent to that produced by calcium, (e) Ca²⁺ and LA form 1:2 molar complexes with phospholipids probably by ion-ion and ion-induced polar type of binding at the phosphate groups of the lipid. It is suggested that such reactions with cell membrane phospholipids may underlie inhibitory effects of LA on cellular ion fluxes and provide a chemical basis for anesthetic action.

Oral flunitrazepam in the prevention of local anaesthetic-induced convulsions in mice

The present study determined whether oral flunitrazepam was effective reducing CNS toxicity of lidocaine and bupivacaine. Pretreatment of mice with flunitrazepam, 0.065-0.25 mg X kg-1, significantly reduced or prevented convulsions and mortality induced by lidocaine 106 and 209 mg X kg-1 or bupivacaine 58 and 90 mg X kg-1 injected intraperitoneally. The doses of flunitrazepam used did not cause measurable sedation in mice. The efficacy of oral flunitrazepam in preventing local anaesthetic-induced convulsions is similar to that previously reported by intraperitoneal or intramuscular injections in mice. Flunitrazepam could be useful for oral premedication of patients before regional anaesthesia.

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.

Role of central beta-adrenoceptors in the control of pentylenetetrazol-induced convulsions in rats

1 The role of central beta-adrenoceptors in the anticonvulsant effect of beta-adrenoceptor antagonists has been examined. 2 Oral administration of (-)- and (+)-propranolol (0.05-1 mg/kg) and (+/-)-pindolol (0.025-0.5 mg/kg) produced a dose-dependent decrease in duration of convulsions produced by pentylenetetrazol (PTZ 50 mg/kg, i.p.) in rats. 3 At the EC50 level, (-)-propranolol is seven times more effective than the (+)-isomer. 4 Oral administration of (-)-, (+)- or (+/-)-practolol (1-10 mg/kg) or (-)- or (+)-timolol (1-10 mg/kg), two beta-adrenoceptor antagonists that do not penetrate the blood brain barrier, had no significant effect on the duration of PTZ-induced convulsions. 5 Intracerebroventricular administration of (-)-propranolol (0.5 microgram/kg) or (-)-timolol (0.25 microgram/kg) produced highly significant anticonvulsant effects whereas the (+)-isomers at the same dose level were ineffective. (+/-)-Pindolol (0.25 microgram/kg) was also much more effective given by this route than when given orally. The (+)- and (-)-isomers of the beta 1-adrenoceptor selective antagonist practolol (10 microgram/kg) exerted only weak anticonvulsant effects. 6 This study provides evidence that beta-adrenoceptor antagonists exert an anticonvulsant effect through central beta 2-adrenoceptors. At high dose levels, additional anticonvulsant activity is associated with membrane stabilization in those antagonists which possess this property.

Lidocaine and procaine alter rat brain amines

Lidocaine HCl (100 mg/kg i.p.) or procaine HCl (240 mg/kg i.p.) was injected into rats, which were killed after zero to 12 min. Six brain parts were analyzed for serotonin, norepinephrine, and dopamine. Lidocaine induced depletion of serotonin and of dopamine in some regions and elevation in others; however, it produced elevated levels of norepinephrine. Not all brain regions showed a significant change of amines. Procaine induced elevations of serotonin, norepinephrine, and dopamine; norepinephrine elevation occurred in a few brain parts, whereas serotonin and dopamine levels were elevated in all brain regions.

Cephalic phase, reflex insulin secretion neuroanatomical and physiological characterization

Using chronically catheterized, freely moving male Wistar rats, we have shown that the sweet taste of a saccharin solution reliably triggers a rapid cephalic phase insulin response (CPIR), in the absence of any significant change of glycemia. To establish the neural mediation of this reflex response we used rats that were cured from streptozotocin diabetes by intrahepatic islet-transplantation as a denervated B-cell preparation. The complete lack of any saccharin-induced CPIR in these rats suggests that it is indeed mediated by the peripheral autonomic nervous system, and that the insulin-stimulating gastrointestinal hormones are not involved in this response. It was further found that this reflex insulin secretion is not easily extinguishable and thus might have an unconditioned component. To investigate the central neural pathways involved in this reflex response we used both electrophysiological methods in anesthetized and semi-micro CNS manipulations in freely moving rats. On the basis of our preliminary results, and several reports, using the decerebrate rat preparation for measuring behavioral or saliva secretory oral taste reactivity, it appears that CPIR might be organized at the brain stem/midbrain level, receiving strong modulatory influences from the diencephalon. But much further work has to be done to establish the central nervous circuitry. Finally, in two experiments, aiming at the question of how important and physiologically relevant the CPIR might be, we found that, on one hand, its lack can result in pathological oral glucose tolerance and on the other hand its exaggeration might contribute to the behavioral reaction to highly palatable sweet food and the resulting development of dietary obesity.

Alteration of lidocaine- or procaine-induced convulsions by manipulation of brain amines

Lidocaine and procaine seizure thresholds were studied. The i.p. median convulsant dose (CD50) of lidocaine and procaine with saline pre-treatment was 95 and 240 mg/kg, respectively. Dopamine depletion by pre-treatment with d1-alpha-methyl-p-tyrosine plus dihydroxyphenylserine resulted in a significant drop of CD50 to 69 and 175 mg/kg for lidocaine and procaine, respectively. Serotonin depletion by pre-treatment with p-chlorophenylalanine resulted in a significant drop of CD50 to 68 mg/kg for lidocaine, but no significant change for procaine.

Sleep during neuromuscular blockade in cats

The purpose of the experiment was to determine whether normal sleep patterns can occur during neuromuscular blockade. Electrographic variables for determining the states of sleep and wakefulness, the electrocorticogram, lateral geniculate nucleus potentials, and dorsal hippocampal potentials, were recorded before, during and after the administration of gallamine triethiodide to cats with chronically implanted electrodes. When respiratory muscles became paralyzed, artificial ventilation commenced through a chronic tracheal fistula. The electrographic wave forms of the states (wakefulness, NREM sleep and REM sleep) in paralyzed cats were indistinquishable by visual observation from those of freely moving animals. As compared to freely moving cats, paralyzed cats had more wakefulness at the expense of both states of sleep (about 33% NREM and 3% REM compared to 45% NREM and 15% REM respectively). REM sleep wasdemonstrated to occur, albeit increase across repeated session in the same cats nor was the distribution uneven within the average session. Large percentages of REM sleep with respect to total recording time were associated with large percentages of NREM sleep (correlation coefficient = 0.58). The sequence of sleep states was like that of freely behaving animals. The main conclusion is that this preparation, depsite low amounts of REM sleep, is useful in neural studies of sleep and wakefulness.

Postoperative convulsions due to iophendylate (Myodil). Report of a case and review of the causes of postoperative convulsions

A previously fit 58-year-old man underwent lumbar laminectomy following a myelogram. Postoperatively convulsions occurred. He was successfully treated and made a complete recovery. Large quantities of intracranial iophendylate (Myodil) were seen on postoperative skull X-rays. In the absence of other likely causes, a diagnosis of iophendylate-induced encephalopathy was made. A review of the aetiology of convulsions in the intra- and postoperative period is presented.

Lidocaine's influence on brain amines in mice

This series of experiments suggests that a relationship exists between convulsive doses of lidocaine and central depletion of 5-HT and DA. Since DA and usually 5-HT are regarded as central inhibitory transmitters, this study further supports the postulate that lidocaine's convulsive activity is by inhibition of central inhibitory pathways-possibly a dopaminergic or tryptaminergic pathway or both.

Blockade of the reticulospinal inhibitory pathway by anaesthetic agents

1. Adult cats were decerebrated at the intercollicular level. The effect of the anaesthetic agents, pentobarbitone, paraldehyde, tribromethanol, chloralose and procaine on the reticulospinal inhibitory pathway, which produced inhibition of segmental reflex potentials, was analysed.2. The doses which blocked this inhibitory pathway did not exceed the doses required to produce surgical level anaesthesia with any of the drugs.3. After the reticular inhibition of the reflex potentials was abolished, the reflex potentials were augmented by reticular stimulation with a higher intensity. This was thought to be due to spread of current to the excitatory pathways which were not completely depressed by the anaesthetic agent.4. The resistance of the reticular facilitation of the reflex potentials to inhibition by these drugs after abolition of inhibition corresponded in general to the degree of excitement in intact mice produced by the same drugs.5. These findings seem to indicate that the preferential block of the reticulospinal inhibitory pathway may be an important neural mechanism for the excitement stage of anaesthesia.

Transitory procaine-induced Parkinsonism

A case of prolonged, though reversible procaine-induced Parkinsonism treated with l-dopa and trihexyphenidyl (Artane-Sustets) is reported. A pathophysiological explanation is discussed on the basis of recent theories concerning the transmitter systems of the basal ganglia.

Effects of general stimulant drugs on the electrical responses of isolated slabs of cat's cerebral cortex

1. In the neuronally isolated cortex of the cat, local application of bemegride, picrotoxin, nikethamide, caffeine and strychnine facilitated the surface positive response of the isolated cortex and lowered the stimulus threshold for this response. Excepting nikethamide, they all produced convulsive discharge in the isolated cortex unrelated to the applied stimulus.2. Local application of glutamate to the cortex produced spreading depression, which was sometimes preceded by spontaneous positive bursting.3. In contrast to the "general depressants" which produce a relatively consistent pattern of effects on the electrical responses of isolated cortex, the "general stimulants", although they all have excitatory effects on isolated cortex, each produced a greatly different type of electrical response in the isolated cortex, suggesting that several different mechanisms of action are responsible for their effects.

Effects of general depressant drugs on the electrical responses of isolated slabs of cat's cerebral cortex

1. In the neuronally isolated cortex of the cat, local application of diphenhydramine, promethazine, gammahydroxybutyrate, gammabutyrolactone, gamma aminobutyric acid, hyoscine and pethidine, and the intravenous injection of diazepam and meprobamate depressed or abolished the surface negative and surface positive response to direct stimulation and raised the stimulus threshold of the positive burst response. These effects were the same as previously demonstrated for general and local anaesthetics on the same preparation.2. Chlorpromazine produced a similar depression in small concentrations but caused spontaneous activity in higher concentrations.3. In contrast to local application, pethidine when given by intravenous injection in a high dose produced convulsant activity in the isolated cortical slab. The possibility was suggested that the convulsant activity was produced by a metabolite of pethidine.4. The results of this investigation suggest that the central depression produced by a number of structurally unrelated drugs is indicative of an anaesthetic-like property of these drugs.

Effects of drugs acting alone and in combination on the motor activity of intact mice

1. When administered to intact white mice, the central depressants-diphenhydramine, promethazine, chlorpromazine, gammahydroxybutyrate, gammabutyrolactone, hyoscine, and pethidine-produced sedation in small doses, but excitement and convulsions in higher doses. When given to mice pretreated with subanaesthetic doses of phenobarbitone these drugs abolished the righting reflex both in convulsant doses (hyoscine excepted) and in non-convulsant doses. These effects are similar to the effects previously observed with local anaesthetics.2. Meprobamate, diazepam and chlorpromazine produced a loss of righting reflex both when given alone and following phenobarbitone. When given alone in higher doses, chlorpromazine induced convulsions.3. The central stimulants bemegride and picrotoxin antagonized the loss of righting reflex produced by phenobarbitone, but nikethamide, caffeine and strychnine did not alter the depressant effects of phenobarbitone.4. On the basis of these and previous studies with intact white mice a tentative classification of drugs having generalized depressant and stimulant effects on the central nervous system was proposed and discussed.