The effect of hexobarbital on the duration of the recurrent IPSP in cat motoneurons

Pain assessment by pupil dilation reflex in response to noxious stimulation in anaesthetized adults

BACKGROUND

In response to noxious stimulation, pupillary dilation reflex (PDR) occurs even in anaesthetized patients. The aim of the study was to evaluate the ability of pupillometry with an automated increasing stimulus intensity to monitor intraoperative opioid administration.

METHODS

Thirty-four patients undergoing elective surgery were enrolled. Induction by propofol anaesthesia was increased progressively until the sedation depth criteria (SeD) were attained. Subsequently, a first dynamic pupil measurement was performed by applying standardized nociceptive stimulation (SNS). A second PDR evaluation was performed when remifentanil reached a target effect-site concentration. Automated infrared pupillometry was used to determine PDR during nociceptive stimulations generating a unique pupillary pain index (PPI). Vital signs were measured.

RESULTS

After opioid administration, anaesthetized patients required a higher stimulation intensity (57.43 mA vs 32.29 mA, P < .0005). Pupil variation in response to the nociceptive stimulations was significantly reduced after opioid administration (8 mm vs 28 mm, P < .0005). The PPI score decreased after analgesic treatment (8 vs 2, P < .0005), corresponding to a 30% decrease. The elicitation of PDR by nociceptive stimulation was performed without changes in vital signs before (HR 76 vs 74/min, P = .09; SBP 123 vs 113 mm Hg, P = .001) and after opioid administration (HR 63 vs 62/min, P = .4; SBP 98.66 vs 93.77 mm Hg, P = .032).

CONCLUSIONS

During propofol anaesthesia, pupillometry with the possibility of low-intensity standardized noxious stimulation via PPI protocol can be used for PDR assessment in response to remifentanil administration.

Calcium dependent release of gamma-aminobutyric acid (GABA) from human cerebral cortex

The release of the amino acids GABA, taurine, glycine, glutamine and leucine from human neocortex was investigated in vitro by utilizing brain tissue removed during 8 standard temporal lobectomies for epilepsy or tumor. Slices (0.5 mm thick) were cut from each biopsy and randomly placed in three different chambers. After 90 min preincubation, the three sets of slices were incubated for 60 s in wells containing, respectively, (A) regular ACSF (control), (B) ACSF with 50 mM K+ (to depolarize the cell membrane) and (C) ACSF with 50 mM K+, 0 mM Ca2+ and 4 mM Mg2+ (depolarization during blocked synaptic transmission). The content of amino acids in the wells was determined by high-performance liquid chromatography after pre-column derivatization of the amino acids with o-phthalaldehyde. Membrane depolarization (well B) increased the GABA release to 650% (620 pmol/mg) of control (well A, 95 pmol/mg). Blocking synaptic transmission (well C) reduced the evoked release by 50% (360 pmol/mg). The release of glycine, taurine, glutamine and leucine during membrane depolarization was not significantly different from the control values. The data provide evidence for a Ca(2+)-dependent release of GABA, supporting a possible role of this amino acid as a neurotransmitter in human neocortex.

An experimental study of the effect of isoflurane on epileptiform bursts

The effect of isoflurane on penicillin- and picrotoxin-induced epileptiform activity was tested using hippocampal slice preparations. Isoflurane reduced both the frequency of spontaneous epileptiform bursts and the number of population spikes within each burst in a dose-dependent manner. The last population spikes in the burst were most sensitive to the anesthetic, whereas the first 4-6 spikes were quite resistant and persisted until spontaneous activity was abolished at 3% isoflurane. Isoflurane increased the stimulus current required to evoke epileptiform bursts and shifted the relationship between stimulus current and population spike amplitude to the right. At 3% isoflurane, a dose that usually causes iso-electric EEG and abolishes all spontaneous epileptiform activity, responses could still be evoked, and then invariably had an epileptiform pattern. The maximum response was reduced compared to control and 1.5% isoflurane. With isoflurane there was a reduced tendency for activity to be transmitted from one region within the hippocampus to the other. This effect was also dose-dependent. However, transmitted activity always retained a typical epileptiform character, although the number of population spikes within a train to some extent decreased with increasing concentrations of isoflurane.

The effect of isoflurane on excitatory synaptic transmission in the rat hippocampus

The purpose of this investigation was to study the effect of isoflurane on excitatory synaptic transmission. Rat hippocampal slices maintained in vitro were used as a model. Isoflurane caused a dose-dependent reduction of the excitatory postsynaptic potential (EPSP); 1.5% isoflurane reduced the EPSP by 35 +/- 9% (mean +/- s.d.) and 3% by 57 +/- 11%. Neither spontaneous nor potassium-stimulated efflux of the glutamate analogue D-(3H)aspartate was changed, but the content of D-(3H)aspartate in slices loaded during isoflurane was reduced to 83 +/- 12% of control (P less than 0.05). The intracellularly recorded response to direct application of glutamate increased by 37 +/- 20% during isoflurane (3%) and 50 +/- 5% during halothane (2%). Isoflurane (3%) enhanced the response to the glutamate receptor agonist quisqualate by 44 +/- 19%, whereas the N-methyl-D-aspartate response was unchanged. Isoflurane enhanced the tetanic depression of the population spike. The present results suggest that isoflurane reduces excitatory synaptic transmission by a presynaptic mechanism.

Differing actions of convulsant and nonconvulsant barbiturates: an electrophysiological study in the isolated spinal cord of the rat

The effects of various pairs of convulsant and nonconvulsant barbiturates on mono- and polysynaptic activity were studied in the isolated spinal cord of the immature rat, using extracellular recording. The convulsant barbiturates, 5-ethyl-5-(3-methylbut-2'-enyl) barbituric acid (3M2B), 5-ethyl-5-(1,3-dimethylbut-1'-enyl) barbituric acid (1,3M1B) and (+)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(+) DMBB] all increased the monosynaptic reflex at concentrations between 5 and 50 microM with no change in polysynaptic activity. When the concentration was raised to between 100 and 300 microM, however, the convulsants all reduced the monosynaptic reflex, thus producing a biphasic dose-response relationship. The nonconvulsant barbiturates phenobarbital, 5-ethyl-5-(3-methylbut-1'-enyl) barbituric acid (3M1B), amylobarbital (3MB) and (-)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(-)DMBB] produced only a decrease in mono- and polysynaptic reflexes. At concentrations which enhanced the monosynaptic reflex, the responses of motoneurones to glycine and eledoisin-related peptide (an analogue of substance P) were reduced by (+)DMBB, while 1,3M1B and 3M2B had no significant effects upon any of the neurotransmitters tested. At concentrations which depressed the monosynaptic reflex, the convulsants all reduced the response to glycine whereas the nonconvulsant barbiturates all increased the response to GABA. With the exception of phenobarbital, both convulsant and nonconvulsant barbiturates produced a direct depolarisation of the presynaptic terminal membrane, with only the convulsants producing a depolarisation of the membrane of the motoneurone. Using another convulsant barbiturate, 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB), this direct depolarising action was found to be calcium-dependent.

The effect of thiopentone on somatosensory evoked responses and EEGs in comatose patients

EEGs and somatosensory evoked responses from the brachial plexus, neck and scalp were recorded in seven comatose patients on continuous thiopentone infusion. Although pathological in five of the patients, the evoked responses were present in all. Additional amounts of thiopentone producing a full suppression of all spontaneous EEG activity had no effects either on the configuration of the evoked responses or on the central conduction times. This resistance of the somatosensory evoked responses to a deep and sustained thiopentone narcosis makes it a useful test in comatose patients receiving this treatment.

Pentobarbitone interference with inhibitory synaptic transmission in crayfish stretch receptor neurones

1. The effect of pentobarbitone (PB) on GABA-ergic inhibition was investigated in the isolated crayfish stretch receptor. The soma of the slowly adapting neurone was impaled with two micro-electrodes to give an accurate determination of membrane conductances. 2. Application of PB in concentrations from 10(-6) to 10(-3) M increased the rise time constant of the inhibitory post-synaptic potential (i.p.s.p.). The i.p.s.p. percentage amplitude and decay time constant were also increased in eight out of twelve neurones. On prolonged exposure, the percentage amplitude declined at a rate dependent upon the dose and the frequency of stimulation until the i.p.s.p. became undetectable. 3. The response to ionophoretically applied GABA remained essentially unaltered in the presence of PB, but the falling phase was prolonged by up to 8% in four of the ten neurones tested. Resting membrane conductance, i.p.s.p. driving force (i.p.s.p. reversal potential minus resting membrane potential), and parameters of the anti- and orthodromic action potential were not significantly affected. 4. Removal of PB after prolonged exposure usually caused an immediate increase in i.p.s.p. percentage amplitude but the i.p.s.p. rising phase remained slowed. 5. Application of excess extracellular GABA only affected the i.p.s.p. percentage amplitude after it had been reduced by PB. It transiently increased the attenuated i.p.s.p. percentage amplitude in the presence of PB, and after the removal of PB permanently increased the amplitude to its original value. 6. Nipecotic acid and cis-1,3-aminocyclohexane carboxylic acid, inhibitors of GABA re-uptake, slightly increased the i.p.s.p. percentage amplitude, and prolonged the falling phase but did not affect the rising phase. The percentage amplitude declined on prolonged exposure. 7. We conclude that PB has no electrophysiologically demonstrable post-synaptic action in the crayfish stretch receptor neurone, but it inhibits the presynaptic release and re-uptake of GABA.

(-)Pentobarbital opens ion channels of long duration in cultured mouse spinal neurons

Intracellular recordings from voltage-clamped mouse spinal neurons in tissue culture were used to study the membrane mechanisms underlying inhibitory responses to gamma-aminobutyric acid and the (-) isomer of pentobarbital. Fluctuation analysis suggested that both substances activated ion channels in the membranes. However, the channels activated by pentobarbital remained open five times longer than those activated by gamma-aminobutyric acid.

Pentobarbital: stereospecific actions of (+) and (-) isomers revealed on cultured mammalian neurons

Stereoisomers of the barbiturate anesthetic pentobarbital were applied to mouse spinal neurons growing in tissue culture. Intracellular recordings of neuronal membrane properties revealed that the (+) and (-) isomers caused direct changes in membrane potential and conductance on some but not all of the cells tested. The action of the (+) isomer was predominantly excitatory, whereas the (-) isomer produced predominantly inhibitory responses. The (-) isomer was considerably more effective in potentiating inhibitory responses to the transmitter gamma-aminobutyric acid. The results show that pentobarbital has multiple effects on neuronal excitability and demonstrate the presence of stereospecific sites of barbiturate action on central neurons.

Pentobarbitone pharmacology of mammalian central neurones grown in tissue culture

1. The effects of the barbiturate anaesthetic pentobarbitone on the membrane properties and amino acid pharmacology of mammalian C.N.S. neurones grown in tissue culture were studied using intracellular recording coupled with bath application, extracellular ionophoresis, or focal diffusion. 2. The addition of an anaesthetic concentration of pentobarbitone to the bathing medium abolished all spontaneous synaptic activity, but did not render individual cells electrically inexcitable nor prevent evoked synaptic acitivity. 3. Focal ionophoresis of pentobarbitone or diffusion from blunt micropipettes reversibly increased membrane conductance, effectively dampening excitability without directly affecting individual action potential characteristics. 4. Pentobarbitone-induced membrane conductance was reversibly blocked by picrotoxin. The inversion potential of the pentobarbitone voltage response depended on Cl- ion gradients and was similar to that of GABA. 5. Pentobarbitone reversibly enhanced the conductance increase produced by GABA with a variable slowing of response kinetics, shifting GABA dose-response curves to the left. Responses to glycine and beta-alanine were not affected. 6. Higher ionophoretic currents of pentobarbitone, which measurably increased membrane conductance, attenuated and markedly slowed GABA responses. Similar effects on GABA responses were observed by superimposing GABA pulses on low level GABA currents. 7. Pentobarbitone, in the absence of an increase in membrane conductance, reversibly depressed depolarizing responses to glutamate without changing response kinetics. Slower responses to acetylcholine which were associated with an apparent decrease in membrane conductance were not affected by the drug. 8. Analysis of double-reciprocal plot data suggested a non-competitive type of antagonism between pentobarbitone and glutamate. Pentobarbitone depression of glutamate was not affected by picrotoxin. 9. Both GABA and glutamate responses appeared to be equally sensitive to pentobarbitone. Specific interaction of the drug with amino acid receptor-coupled events is indicated by the requirement for pentobarbitone pipette placement close to the amino acid response site. 10. The results suggest that pentobarbitone depresses neuronal excitability by (1) directly activating post-synaptic GABA-receptor coupled Cl- conductance, (2) potentiating post-synaptic GABA-induced conductance events, probably at the level of the GABA receptor, and (3) depressing post-synaptic glutamate-induced excitation, probably at the level of the conductance mechanism.

A pharmacological study of Renshaw cell inhibition

1. In cats anaesthetized with pentobarbitone a pharmacological investigation was made of the inhibition of Renshaw cells by dorsal root afferent volleys and ventral root antidromic volleys, and of the inhibition of motoneurones by Renshaw cells. 2. The effects of strychnine, bicuculline and tetanus toxin indicate that both glycine and GABA operate as inhibitory transmitters released on Renshaw cells by dorsal root volleys. 3. The 'mutual' inhibition of Renshaw cells, and the recurrent inhibition of motoneurones by Renshaw cells, are suppressed by strychnine: Renshaw cells are thus glycinergic inhibitory neurones, a proposal consistant with recent evidence for strychnine-sensitive inhibition of Ia interneurones by Renshaw cells. 4. The 'pause' which follows high frequency synaptic excitation of Renshaw cells is insensitive to strychnine, bicuculline and tetanus toxin, and is considered unlikely to be the consequence of synaptic inhibition.

Inhibitory and excitatory effects of CNS depressants on invertebrate synapses

(1) The effects of pentobarbital were studied on the membrane properties and synaptic activity of crustacean neuromuscular junction preparations and molluscan neurons. (2) Pentobarbital selectivity depressed in a dose-dependent, reversible manner the exciatory postynaptic potentials (EPSPs) recorded at crustacean neuromuscular junctions without altering either inhibitory postsynaptic potentials (IPSPs) or post-synaptic membrane properties. (3) Pentobarbital depressed cholinergic EPSPs recorded in an identified molluscan neuron and depressed the depolarizing phase of biphasic PSP without affecting the hyperpolarizing phase of the BPSP on the same cell. Facilitation of the EPSP was not affected. (4) Pentobarbital did not appreciably alter the reversal potentials of the EPSP and IPSP. (5) Low concentrations of pentobarbital did not alter the appearance of spontaneously occurring IPSPs, while high concentrations changed the pattern of regular IPSP input to an irregular, burst-like pattern. (6) Pentobarbital and 5 other CNS depressants (cholralose, chloroform, ethanol, and urethane) increased the excitability and altered the current--voltage relations of a cell whose membrane properties have been proposed as a model of presynaptic terminal membranes. The effects were dependent on the species of external divalent cation present. (7) The results in these invertebrate systems may provide insight into the cellular basis of the depressant and excitatory effects of these agents.

Presynaptic action of barbiturates in the frog spinal cord

The action of pentobarbital on primary afferents of the isolated frog spinal cord was analyzed with sucrose gap and intracellular recordings techniques. Pentobarbital in concentrations generally considered to be in the anesthetic range greatly prolonged presynaptic inhibition and also depolarized primary afferents. The depolarization was accompanied by an increase in excitability and resulted from activation of gamma-aminobutyric acid receptors, possibly by a direct action on these receptors, since the depolarization was reversibly blocked by gamma-aminobutyric acid, but not by glycine, antagonists, and magnesium ions. Furthermore, dorsal root ganglion cells exhibited a reduced sensitivity to both gamma-aminobutyric acid and pentobarbital after a "desensitizing" dose of gamma aminobutyric acid. The prolongation of presynaptic inhibition and the activation of gamma-aminobutyric acid receptors on primary afferents by pentobarbital should act to reduce the amount of transmitter released from the first synapse in sensory pathways.

Pentobarbital: selective depression of excitatory postsynaptic potentials

The effects of pentobarbital (Nembutal) on synaptic transmission and postsynaptic potentials were studied by the use of several invertebrate preparations. Pentobarbital selectively and reversibly depressed both excitatory postsynaptic potentials and sodium-dependent postsynaptic responses to putative excitatory transmitters without affecting either inhibitory postsynaptic potentials or chloride- and potassium-dependent postsynaptic responses to putative transmitters. A selective depression of postsynaptic excitatory events was also observed with other central nervous system depressants (ethanol, chloroform, chloralose, diphenylhydantoin, and urethane). The results suggest that central and peripheral depression observed during general anesthesia is due to a selective depression of excitatory synaptic events.

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.

The effects of anaesthetics on synaptic excitation and inhibition in the olfactory bulb

1. The effects of anaesthetics (pentobarbitone, hexobarbitone, halothane, urethane, chloralose, chloral hydrate and ethanol) on the extracellular field potentials of the olfactory bulb produced by lateral olfactory tract stimulation were analysed.2. Relatively large doses of all the anaesthetics (e.g. pentobarbitone, 40-70 mg/kg) depressed the synaptic excitation of granule cells.3. The antidromic invasion of mitral cell dendrites was only slightly less sensitive to the anaesthetics than was the synaptic excitation of granule cells.4. A wide dose range of anaesthetics (e.g. pentobarbitone, 3-60 mg/kg) prolonged the granule cell post-synaptic inhibition of mitral cells. All the anaesthetics, except ethanol, prolonged the inhibition.5. The action of anaesthetics on post-synaptic inhibition was due to a specific effect on the inhibitory synapses.6. Amino-oxyacetic acid, an inhibitor of GABA catabolism, had little effect on the synaptic inhibition or on the ability of hexobarbital to prolong the inhibition. This suggests that the prolongation seen with anaesthetics is not a result of interfering with GABA catabolism.7. The present results are compared with results obtained with anaesthetics in other areas of the nervous system and it is proposed that prolongation of ;gaba-ergic' inhibition might contribute to an agent's ability to produce general anaesthesia.

Neuronal pathway of the recurrent facilitation of motoneurones

1. The recurrent facilitation of motoneurones is a disinhibition, i.e. a release of the motoneurones from a sustained hyperpolarization evoked by tonically active inhibitory interneurones. Only two groups of interneurones are known to receive recurrent inhibition from motor axon collaterals via Renshaw cells; the interneurones mediating the reciprocal Ia inhibition and the Renshaw cells themselves. The properties of these two groups of neurones were studied to determine if they could produce the tonic inhibition of motoneurones removed during recurrent facilitation.2. It was found that the tonic firing of Ia inhibitory interneurones is sensitive to anaesthetics to the same degree as is recurrent facilitation. The range of frequencies of tonic discharges of Renshaw cells appeared to be similarly low in unanaesthetized and anaesthetized preparations although in individual cells the discharge rates were decreased by anaesthesia.3. The recurrent inhibition of Ia interneurones inhibiting a given group of motoneurones and the recurrent facilitation of the same group of motoneurones were, as a rule, evoked from the same nerves, although in some cats the origin of the recurrent facilitation was somewhat wider. In contrast no evidence could be found that the Renshaw cells which inhibit a functional group of motoneurones are inhibited by volleys in the nerves from which recurrent facilitation is regularly evoked.4. It was concluded that the recurrent facilitation is caused mainly by inhibition of the tonic activity of Ia inhibitory interneurones and that it is thus a manifestation of the recurrent control of Ia reciprocal inhibition of motoneurones.