SITE OF ACTION OF THIAMYLAL SODIUM ON THE MONOSYNAPTIC SPINAL REFLEX PATHWAY IN CATS

Interactions between hormones and epilepsy

There is a complex, bidirectional interdependence between sex steroid hormones and epilepsy; hormones affect seizures, while seizures affect hormones thereby disturbing reproductive endocrine function. Both female and male sex steroid hormones influence brain excitability. For the female sex steroid hormones, progesterone and its metabolites are anticonvulsant, while estrogens are mainly proconvulsant. The monthly fluctuations in hormone levels of estrogen and progesterone are the basis for catamenial epilepsy described elsewhere in this issue. Androgens are mainly anticonvulsant, but the effects are more varied, probably because of its metabolism to, among others, estradiol. The mechanisms for the effects of sex steroid hormones on brain excitability are related to both classical, intracellularly mediated effects, and non-classical membrane effects due to binding to membrane receptors. The latter are considered the most important in relation to epilepsy. The different sex steroids can also be further metabolized within the brain to different neurosteroids, which are even more potent with regard to their effect on excitability. Estrogens potentiate glutamate responses, primarily by potentiating NMDA receptor activity, but also by affecting GABA-ergic mechanisms and altering brain morphology by increasing dendritic spine density. Progesterone and its main metabolite 5α-pregnan-3α-ol-20-one (3α-5α-THP) act mainly to enhance postsynaptic GABA-ergic activity, while androgens enhance GABA-activated currents. Seizures and epileptic discharges also affect sex steroid hormones. There are close anatomical connections between the temporolimbic system and the hypothalamus controlling the endocrine system. Several studies have shown that epileptic activity, especially mediated through the amygdala, alters reproductive function, including reduced ovarian cyclicity in females and altered sex steroid hormone levels in both genders. Furthermore, there is an asymmetric activation of the hypothalamus with unilateral amygdala seizures. This may, again, be the basis for the occurrence of different reproductive endocrine disorders described for patients with left-sided or right-sided temporal lobe epilepsy.

The inhibitory action of noradrenaline and other monoamines on spinal neurones

1. L-Noradrenaline (NA), 5-hydroxytryptamine (5-HT) and acetylcholine (ACh) were administered micro-electrophoretically to feline lumbar neurones while recording their spike potentials extracellularly.2. There was no evidence to suggest that NA acts as an excitatory transmitter in the spinal cord.3. NA had potent inhibitory effects on some interneurones as revealed by a depression of spontaneous and synaptic firing and on the firing to a local application of an excitant amino acid. The effects on Renshaw cells and motoneurones were less marked.4. The depressant actions of 5-HT were less marked than those of NA. ACh and carbamylcholine had depressant effects on some NA-sensitive interneurones but were invariably far less potent and on other NA-sensitive cells were completely inactive.5. NA had no detectable effect on the normal spike amplitude but when the action potentials were reduced by excessive depolarization then both NA and synaptic inhibition increased the spike amplitude; this effect could be due to a hyperpolarization of the cell membrane.6. There was a correlation between the distribution of NA-sensitive cells and the relative densities of NA-containing terminals in various layers of the grey matter.7. It was postulated that NA acts as an inhibitory transmitter released from the terminals of descending pathways in the spinal cord. Other possible mechanisms were discussed but lacked experimental support.

The progesterone metabolite 5 alpha-pregnan-3 alpha-ol-20-one reduces K(+)-induced GABA and glutamate release from identified nerve terminals in rat hippocampus: a semiquantitative immunocytochemical study

5 alpha-Pregnan-3 alpha ol-20-one (3 alpha-OH-DHP) reduced the depolarization-induced loss of GABA and, to a lesser extent, the glutamate-like immunoreactivities from nerve terminals in the in vitro hippocampal slice. Phenobarbital (PB) had similar effects. These results suggest that 3 alpha-OH-DHP affects presynaptic transmitter release, possibly in a barbiturate-like manner.

The effect of progesterone and its metabolite 5 alpha-pregnan-3 alpha-ol-20-one on focal epileptic seizures in the cat's visual cortex in vivo

The acute effects of progesterone and its brain metabolite 5 alpha-pregnan-3 alpha-ol-20-one (3 alpha-OH-DHP) on focal epileptic seizures in the cat's visual cortex was studied in vivo using an unanesthetized cervaux-isolé preparation. This model made it possible to study in parallel the effect of the drugs on ictal activity and synaptic transmission. A dose-dependent increase in seizure threshold was observed after i.v. injections of both 3 alpha-OH-DHP and progesterone, 3 alpha-OH-DHP being about 20 times as potent as the latter. I.v. injections of 3 alpha-OH-DHP 1.0 mg/kg increased the median seizure threshold to 265% of baseline. While 3 alpha-OH-DHP exerted an immediate effect on seizure thresholds, the maximal effect of progesterone was delayed about 20 min. Concerning the mechanisms underlying the antiepileptic effect, three changes occurred within the effective dose range: (1) a small, but significant reduction in the presynaptic nerve volleys, (2) a reduction in the postsynaptic excitatory field potentials in the dorsal lateral geniculate nucleus and cortex, and (3) an enhanced postsynaptic inhibition. Taken together, these observations point to both pre- and postsynaptic effects, supporting the hypothesis of a barbiturate-like mechanism of action of progesterone and its brain metabolites.

Mechanisms of spinal reflex depressant effects of CS-722, a newly synthesized centrally acting muscle relaxant, in spinal rats

The mechanisms of the depressant action of (R)-4-chloro-2-(2-hydroxy-3-morpholinopropyl)-5-phenyl-4-isoxaz olin-3-one hydrochloride (CS-722), a newly synthesized centrally acting muscle relaxant, on spinal reflexes were investigated in spinal rats. The drug CS-722 (50 mg/kg, i.v.) depressed the polysynaptic reflex but was less effective on the monosynaptic reflex. Eperisone-HCl (10 mg/kg, i.v.) and baclofen (2 mg/kg, i.v.) markedly decreased the monosynaptic and polysynaptic reflexes, with longer durations than CS-722; CS-722, eperisone and baclofen depressed the dorsal root reflex. The excitability of the motoneurone was reduced by CS-722 and eperisone. Excitability of the primary afferent fibres was reduced by CS-722, while eperisone and baclofen had no effect. Both CS-722 and eperisone did not have a depressant influence on the focal synaptic potential. These results suggest that CS-722 and eperisone but not baclofen, have a common motoneurone-membrane-stabilizing action and that this action may contribute, in part, to the spinal reflex depressant effects of CS-722 and eperisone.

Selective actions of anesthetic agents on membrane potential trajectory in bulbar respiratory neurons of cats

The effects of two anesthetic agents, halothane and thiopental, on the membrane potential trajectory of respiratory-related neurons in the ventral respiratory group were investigated in decerebrate cats, of which the carotid sinus and vagal afferents were denervated. Infusion of halothane (2% for 90 s) depolarized the membrane in nearly half of the inspiratory (12/21), post-inspiratory (10/26) and expiratory (4/6) neurons and caused hyperpolarization in the rest of the population. Thiopental (2.5 mg/kg i.v.) produced depolarization in 11 inspiratory and 10 post-inspiratory neurons and hyperpolarization in 1 expiratory, 4 inspiratory and 7 post-inspiratory neurons. In both hyperpolarized and depolarized neurons, reduction of the respiratory membrane potential fluctuations and an increase of input resistance were commonly observed. Both drugs depressed spontaneous firing in most of the neurons studied. An increase of firing was observed in 9 out of 47 depolarized cells. These two contrasting effects on the membrane potential trajectory occurred similarly in the known groups of respiratory neurons, but the response of a given cell was consistent for the two anesthetic agents. The present results demonstrate that the anesthetic drugs exert various influences on the ventral respiratory group neuron population in maintaining the membrane potential trajectory and discharge activity. This may reflect a functional heterogeneity in the bulbar respiratory network of neurons.

The effects of thiamylal sodium on the tonic vibration reflex in man

The effects of barbiturate on the tonic vibration reflex (TVR) in man were studied. Motor unit spikes elicited during the TVR were recorded before and after an intravenous injection of thiamylal sodium. Thiamylal sodium at a dose between 1.5 and 5.8 mg/kg depressed the motor unit response to muscle vibration. Analysis using a cross-correlation between vibratory stimuli and motor unit spikes confirmed the previously reported existence of two types of motor unit response, i.e. spikes that are locked and those that are not locked to the vibratory stimuli. In addition, the latter type was susceptible to the depressant effects of thiamylal than the former type. Some possible explanations for the difference in susceptibility of the two types of motor unit to barbiturate are discussed, placing special emphasis on the probable participation of polysynaptic connections from group Ia afferents to motoneurons in the generation of the human TVR.

Quantitative comparison of cell loss and thiopental-induced EEG changes in human epileptic hippocampus

Thiopental-induced EEG beta activity recorded from mesial and lateral temporal lobe sites was analyzed both visually and by computer in 30 patients with complex partial seizures. All patients later received anterior temporal lobectomy. Volumetric cell densities and percentage of principal cell loss relative to normal controls were determined in several different areas of each resected temporal lobe. The percentage of drug-induced beta activity was then compared with the percentage of cell loss by (a) making precise topographic correlations between induced beta and cell loss, (b) relating the overall degree of left/right beta asymmetry to cell loss averaged over wide regions, and (c) comparing the likelihood of left/right beta asymmetries in patients with and without extreme cell loss. The results obtained were consistently negative and were unaffected by whether EEG data were analyzed by computer or visual inspection. These results indicate that the well-known loss of drug-induced beta activity found in epileptogenic regions is not an indicator of the degree of cell loss or sclerosis. The underlying focal dysfunction measured by EEG beta activity remains unknown, and factors such as focal hypoperfusion, hypometabolism, and the microanatomic features of the cells remaining in the focus should be investigated.

Blocking action of pentobarbital on receptors for excitatory amino acids in the guinea pig hippocampus

The actions of pentobarbital sodium (Pent) on receptors for glutamate (Glu) and related compounds were studied in thin sections of the guinea pig hippocampus. Depolarizations induced by Glu and quisqualate (Quis) in CA3 neurons were reduced in amplitude during iontophoretic administration of Pent. This action of Pent was not accompanied by any noticeable changes in membrane potential or neuron input resistance. Depolarizations induced by N-methyl-D-aspartate were less sensitive to Pent. The fast kainate (KA) response was as susceptible as the Glu response, whereas the slow KA response was unaffected by Pent in three quarters of the neurons examined. Pent suppressed the Glu response at lower concentrations than required to potentiate responses to gamma-amino butyric acid. Excitatory postsynaptic potentials (EPSPs) elicited by stimulation of mossy fibers were suppressed by Pent. The EPSPs were a little more resistant to Pent than were the Glu responses. These results indicate that Pent blocks receptors for excitatory amino acids in the hippocampus. Of the three different populations of the receptors, Quis receptors are the most sensitive to Pent and KA receptors are the least sensitive. The suppression of the EPSPs is in accordance with the notion that Glu is the transmitter released from mossy fibers.

Amino acid neurotransmitters in the CNS: effect of thiopental

Thiopental, a thiobarbiturate which partitions prefentially into the hydrophobic environment, inhibited transport of amino acid neurotransmitters, GABA, aspartate and glutamate, and of biogenic amine, dopamine, across the synaptosomal membrane. At a given protein and thiopental concentration GABA transport was more sensitive to the barbiturate than were the movements of aspartate and glutamate although the uptake of each amino acid was inhibited essentially to the same extent as was its K+-stimulated release. By contrast, inhibition of dopamine uptake was larger than that of its release. Thiopental also inhibited the release of amino acid neurotransmitters caused by anaerobiosis. It is suggested that the barbiturate modifies the properties of the synaptosomal lipids and/or hydrophobic segments of proteins and thereby, simultaneously and independently, affects various membrane functions. The equal inhibition of uptake and release of amino acid neurotransmitters is consistent with the postulate that their transport occurs through the reversible membrane carriers which function efficiently in both the inward and outward directions.

Brain-stem auditory evoked potentials in rats with high-dose pentobarbital

Brain-stem auditory evoked potentials (BAEPs) are relatively resistant to alteration by barbiturate drugs, but the effects of the high doses that are used clinically to produce deep barbiturate coma for the treatment of intracranial hypertension and postischemic anoxic encephalopathy on BAEPs are unknown. We gave high-dose pentobarbital infusions to mechanically ventilated rats while recording serial BAEPs from the scalp. Pentobarbital progressively suppressed and then abolished all peaks. First the later waves, then all but the first wave, and finally all waves were abolished at intravenous doses of 120, 220, and 260 mg/kg, respectively, in addition to the initial anesthetic dose of 60 mg/kg i.p. The changes were at least partially reversible; peaks returned in reverse order of their disappearance. Peak latencies increased with dose. The results show a significant effect of pentobarbital on BAEPs in the rat, suggesting that BAEPs may be useful in assessing the depth of and recovery from barbiturate coma.

Effect of methamphetamine on rat spinal cord. Dopamine receptor-mediated depression of monosynaptic reflex

The experiments were performed on spinal rats transected at Cl. Intravenous administration of methamphetamine-HCl (MA-HCl, 2 mg/kg) and apomorphine-HCl (5 mg/kg) reduced the amplitude of the monosynaptic reflex (MSR), while the polysynaptic reflex was increased by methamphetamine. Depression of the monosynaptic reflex by both drugs was antagonized by haloperidol, but not by phentolamine. Depression of the monosynaptic reflex by methamphetamine was not antagonized by pretreatment with reserpine; however, the result was explained by the assumption that methamphetamine releases newly-synthesized dopamine or that methamphetamine may act directly on dopamine receptors. Depression of the monosynaptic reflex induced by methamphetamine was independent of peripheral changes in blood pressure. Oxygen tension in the spinal cord was slightly reduced by methamphetamine in rats treated with phentolamine and a change of pO2 in the spinal cord was ruled out as a possible mechanism of action. These results suggest that dopaminergic neurons in the spinal cord of the rat depress the transmission of monosynaptic spinal reflexes.

Effects of intrathecally administered pentobarbital and naloxone on the activity evoked in ascending axons of the rat spinal cord by stimulation of afferent A and C fibres. Further evidence for a tonic endorphinergic inhibition in nociception

The effects of intrathecally administered pentobarbital and naloxone on activity in ascending axons were determined in decerebrate rats with the spinal cord transected at the lower thoracic level. Activity in ascending axons of the spinal cord was recorded below the site of transection and evoked by electrical stimulation of afferent A beta, A delta or C fibres in the sural nerve. Pentobarbital 250 micrograms depressed activity evoked by stimulation of non-nociceptive A beta and nociceptive C fibres; it did not change activity in response to stimulation of A delta fibres. A low dose (100 micrograms) had no effect of A beta and C fibre-evoked activity but depressed spontaneous activity in the ascending axons. Naloxone 5 micrograms enhanced the spontaneous and evoked activities only in those ascending axons which responded to C fibre stimulation. Pretreatment with pentobarbital 250 micrograms prevented the facilitation by naloxone of C fibre-evoked activity. Naloxone was ineffective even when it was administered in a dose of 100 micrograms simultaneously with pentobarbital. Intrathecal injections of magnesium chloride depressed spontaneous and C fibre-evoked activities and markedly reduced the facilitatory effect of naloxone. It is concluded that nociceptive C fibre-evoked activity is subject to the inhibitory control of endorphinergic neurones and that naloxone facilitates this activity by producing release from inhibition.

Convergence of muscle spindle afferents on single neurons of the cat dorsal spino-cerebellar tract and their synaptic efficacy

By means of tungsten microelectrodes, action potentials from axons within the dorsal spino-cerebellar tract (DSCT) and from muscle spindle afferents were recorded. A quantitative study was performed in monomuscular DSCT neurons which were excited predominantly by Ia fibers originating in the gastrocnemius muscles. In some experiments single Ia fibers were stimulated electrically while the impulse sequence of a DSCT neuron postsynaptic to the respective afferent fiber was recorded. The gastrocnemius DSCT neurons receive excitatory inputs from 10-18 Ia muscle spindle afferents. The efficacy of each of these inputs is very similar. Thus the neuronal activation decreased approximately linearly with the number of the excitatory afferents cut. Cross-correlograms between the impulse sequence of a Ia gastrocnemius muscle spindle afferent and a DSCT neuron postsynaptic to it exhibited an increased discharge probability of the DSCT neuron from 3-4 ms to 10 ms after the Ia action potential. With increasing impulse rates of the Ia afferent fibers, the excitatory efficacy of the single action potential decreased, but the overall excitation increased with the presynaptic discharge frequency, according to a hyperbolic function. This effect was tested by electrical stimulation of a single Ia axon exciting the DSCT neuron recorded. Interval histograms computed from DSCT neuron impulse trains at steady stretch conditions were predominantly monomodal. They can be well approximated by a Gaussian distribution. The coefficient of variation was independent of the mean activity. At impulse rates above 25 imp X s-1 a negative correlation between successive intervals was observed in first order joint interval diagrams. With an increasing mean discharge rate this correlation (expressed as the serial linear correlation coefficient of the first order r1,2) became stronger up to--0.62 at 90 imp X s-1. Only in a few neurons did the higher order linear correlation coefficients deviate significantly from zero. In 15% of the observed histograms double discharging (mean interval 3-5 ms) produced bimodal distributions. Under steady-state conditions the response of Ia-activated DSCT cells are linearly related to muscle stretch within a middle range of extensions. The differences between Ia impulse pattern and DSCT neuron impulse pattern at steady stretch are discussed. The number of large dendrites of the principal cells in the nucleus dorsalis (Clarke's column) corresponds to the number of excitatory afferent muscle fibers. It is assumed that each excitatory Ia axon sends one axon collateral to the DSCT neuron, forming a climbing type terminal mainly on one of the large dendrites of a DSCT cell.

Effect of barbiturates on the GABA receptor of cat primary afferent neurones

1. The effects of the barbiturate anaesthetics, pentobarbitone and thiopentone, on the membrane properties and the gamma-aminobutyric acid (GABA)-induced responses of cat primary afferent neurones were studied with intracellular recording and voltageclamp techniques.2. At low concentrations (10(-7)-10(-5) M) both barbiturates slightly enhanced and prolonged GABA-induced depolarizations or currents without affecting the membrane properties. At these concentrations, barbiturates have no effect on the apparent dissociation constant of the GABA-GABA receptor interaction or the reversal potential for GABA-induced depolarizations or currents.3. At high concentrations (10(-4)-10(-3) M) barbiturates produced a few millivolts reduction in the resting membrane potential. Voltage-clamp analysis revealed that the depolarization was associated with one of the three types of conductance change, i.e., an initial increase followed by a decrease (40% of neurones examined), only an increase (40%) and only a decrease (20%).4. Analysis in different ionic media indicated that the depolarization with a reduced membrane resistance is associated with an increased chloride conductance and that the one with an increased membrane resistance is accompanied by a reduction in potassium conductance. Bath-application of GABA (10(-3) M) or picrotoxin (10(-5) M) inhibited the increase in chloride conductance but not the reduction in potassium conductance.5. Barbiturates at these high concentrations initially caused a marked augmentation and prolongation of GABA responses; this was followed by a depression. The depressant action did not appear to be voltage-dependent. These actions of barbiturates were not accompanied by changes in the apparent dissociation constant of the GABA-current dose-response curve or the reversal potential for GABA currents. In addition, the single exponential decay of GABA current was not changed despite a marked prolongation of its decay time.6. Picrotoxin (10(-5) M) antagonized the depressant effect of barbiturates at high concentrations on GABA currents, and barbiturates (5 x 10(-6) M) reduced the inhibitory action of picrotoxin (5 x 10(-6) M) on the GABA-currents.7. From all these results, it is suggested that the site of barbiturate actions on GABA-responses is mainly the allosteric site (the ionic conductance regulatory subunit) but not the agonist recognition site or the chloride channels linked with GABA receptors.

Caerulein and morphine in a model of visceral pain. Effects on the hypotensive response to renal pelvis distension in the rat

In pentobarbital-anaesthetized rats (60 mg/kg, i.p.) renal pelvis distension with a pressure of 80 cm H2O caused a decline in mean arterial blood pressure. This pressure response, which disappeared rapidly after cessation of the distension, was used to study the effects of analgesic drugs known to be effective in renal colic pain in man. Morphine (0.75 and 1 mg/kg, s.c.) and the decapeptide caerulein (1.6, 4 and 8 microgram/kg, s.c.) abolished the pressure response. The effects of the largest doses lasted for at least 30 min. Ineffective in this respect were (a) desulphated caerulein (40 microgram/kg, s.c.) and (b) additional doses of pentobarbital (20 and 40 mg/kg, s.c.). This shows (a) the importance of the sulphated tyrosine (known from previous studies on central effects) and (b) the missing influence of the depth of anaesthesia. Naloxone (0.5 mg/kg, s.c.) abolished the effect of morphine (1 mg/kg, s.c.) but failed to influence that of caerulein (8 microgram/kg, s.c.). Even a fourfold dose of naloxone (2 mg/kg, s.c.) did not weaken the effect of caerulein. Naloxone, per se, was ineffective. These results suggest different mechanisms of the present effects of morphine and caerulein. It appears that renal pelvis distension in the anaesthetized rat can serve as a model of renal colic.

Barbiturate reduction of calcium-dependent action potentials: correlation with anesthetic action

Calcium-dependent action potentials were recorded from mouse spinal cord neurons in primary dissociated cell culture following addition of the potassium channel blockers tetraethylammonium ion and 3-aminopyridine. The pharmacologically active barbiturates, pentobarbital and phenobarbital, but not the pharmacologically inactive barbiturate, barbituric acid, produced reversible, dose-dependent reduction of action potential duration at sedative-hypnotic and anesthetic concentrations. Pentobarbital reduced action potential duration at concentrations from 25 to 600 microM (50% reduction at 170 microM) while phenobarbital reduced action potential duration at concentrations from 100 to 5000 microM (50% reduction at 900 microM). The barbiturate concentrations which reduced calcium-dependent action potential duration in this study correlate with reduction of neurotransmitter release from other neuronal preparations and with reduction of calcium uptake by synaptosomes. The results suggest that barbiturates may produce anesthesia in part by reduction of presynaptic calcium entry and consequent reduction of neurotransmitter release in addition to postsynaptic increase of membrane chloride ion conductance. Barbiturate anticonvulsant actions are probably due to postsynaptic augmentation of GABA-mediated inhibition and depression of excitatory synaptic transmission. The major difference between anticonvulsant (phenobarbital) and anesthetic (pentobarbital) barbiturates was the dose-dependency of these actions. Phenobarbital produced postsynaptic modulation of neurotransmitter responses at low concentrations and decreased calcium-dependent action potential duration and increased chloride ion conductance at high concentrations. In contrast, pentobarbital produced all actions at low concentrations. Thus for phenobarbital there would be a large therapeutic index for anticonvulsant activity compared to anesthetic activity but for pentobarbital there would be a small therapeutic index.

Differential effects of pentobarbital and ether on the synaptic transmission of the hippocampal CA1 region in the rat

The effect of ether and sodium pentobarbital on the synaptic transmission of the hippocampal CA1 region was studied in chronically implanted rats. Animal behavior, EEG, and the average evoked potentials (AEPs) following electrical stimulation of the alveus or the stratum radiatum in the CA1 region were recorded. Components of the AEPs, interpreted previously as generated by population excitatory postsynaptic potentials (EPSPs), population inhibitory postsynaptic potentials (IPSPs) (Leung 1979a, b, c) or population postsynaptic spikes (Andersen et al. 1971), were differentially sensitive to ether or pentobarbital. Ether reduced the population EPSPs and population spike evoked at all intensities tested (1-4 X threshold); the population IPSP was slightly enhanced at intermediate stimulus intensities. Pentobarbital suppressed the population EPSP evoked by alvear stimulation but not that by radiatum stimulation, reduced the population spike and greatly enhanced and prolonged the population IPSP evoked at low stimulus intensities. At high stimulus intensities, the IPSP was interpreted to be smaller after pentobarbital but neuronal output from the hippocampal CA1 region, as seen from the evoked population spike, remained attenuated. It is concluded that ether and pentobarbital both suppress hippocampal neuronal excitability but the effect of anesthesia differs for different anesthetics, for different synapses and for different levels of activity in the input fibers.

(-)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.

Action of barbiturates on activity of acetylcholinesterase from synaptosomal membranes

The acetylcholinesterase from synaptosomal membranes is inhibited by anesthetics: Nembutal, brietal, and thiopental. Nembutal and brietal decrease the Km for acetylthiocholine, without changes in Vmas. A noncompetitive type of inhibition is produced by thiopental. This anesthetic decreases Arrhenius plot discontinuity by about 4 degrees C and increases activation energies. Nembutal and brietal do not change Arrhenius plot discontinuities, but they increase activation energies. These results suggest that barbiturates change lipid-protein interactions in synaptosomal membranes.

The effects of anaesthetic and convulsant barbiturates on the efflux of [3H]D-aspartate from brain minislices

The effects have been investigated of anesthetic and convulsant barbiturates on the spontaneous and potassium-stimulated efflux of [3H]D-aspartate from rat cortex minislices. In concentrations (100 microM) which have previously been shown to alter calcium ATPase activity in synaptosomes, anaesthetic barbiturates depressed and convulsant barbiturates stimulated the spontaneous efflux of [3H]D-aspartate in a dose-dependent manner. The effects on the efflux of [3H]D-aspartate evoked by 44.7 mM KCl were similar, although one convulsant barbiturate (1,3M1B) was without effect. The results are discussed in terms of entry and removal of calcium ions from nerve terminals.

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.

Barbiturates and calcium-activated adenosine triphosphatase

Anaesthetic barbiturates potentiate and convulsant barbiturates inhibit the calcium-activated adenosine triphosphatase (Ca-ATPase) activity in rat brain synaptosomes. Such differential effects and consequent modification of transmitter release may be important in the contrasting actions of these classes of barbiturates in vivo.

Differential selectivity of several barbiturates on experimental seizures and neurotoxicity in the mouse

Six barbiturates with diverse time-action characteristics--thiopental, pentobarbital, butabarbital, phenobarbital, diphenylbarbiturate, and barbital--were evaluated for "anticonvulsant" and "neurotoxic" effects. For the former, the MES test, clonic seizures induced by pentylenetetrazol, 90 mg/kg, s.c., and maximal seizures produced by pentylenetetrazol, 200 mg/kg, s.c., were employed. For the latter, we used a rotorod technique. Time to peak activity in the MES test was employed as the time for other tests. Pentobarbital required at least neurotoxic doses to produce substantial "anticonvulsant" activity, its protective index ranging from 0.79 to 0.98 in the three tests. Among the drugs tested, phenobarbital and diphenylbarbiturate exhibited the most favorable protective indices, ranging from 2.71 to 3.41 for phenobarbital and from 3.85 to 5.0 for diphenylbarbiturate. Barbital, another drug with a prolonged duration of action, exhibited a range from 0.84 to 2.81. Although a prolonged duration of action is an important characteristic for antiepileptic activity, this property does not confer per se a favorable protective index.

The effect of pentobarbital, chloralhydrate, ether and protoveratrine on the distribution of synaptic vesicles in rat cortical synaptosomes

Rat cortical synaptosomes were incubated in a saline medium. On the addition of pentobarbital, chloralhydrate, ether and protoveratrine a redistribution of synaptic vesicles was revealed by electron microscopy. The anaesthetics decreased the number of synaptic vesicles attached to the presynaptic membrane while protoveratrine increased them. It is assumed that these agents affect the mechanism of vesicle reformation from the synaptosomal membrane and/or vesicle migration towards the synaptic cleft.

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.

Pentobarbital: differential postsynaptic actions on sympathetic ganglion cells

The frog sympathetic ganglion has been used as a model to elucidate the cellular mechanism of barbiturate anesthesia. Anesthetic concentrations of pentobarbital markedly reduced the fast nicotinic excitatory postsynaptic potential while having no effect on the slow excitatory postsynaptic potential or slow inhibitory postsynaptic potential, even though all three synaptic potentials depend on the presynaptic release of acetylcholine. A similar differential effect was seen for nicotinic and muscarinic responses to exogenously applied agonists, while the depolarizing action of gamma-aminobutyric acid (GABA) was enhanced. These results indicate that pentobarbital has remarkably selective actions on the sympathetic ganglion and further indicate that blockade of ganglionic transmission by anesthetic concentrations of pentobarbital can be entirely explained by a postsynaptic action. The present results strengthen the concept that pentobarbital anesthesia results from a postsynaptic blockade of central excitatory synapses which increase sodium conductance coupled with a postsynaptic enhancement of GABA-mediated synaptic inhibition.

Size and duration of inhibition in the medial geniculate body in unanesthetized cats

A method which allows repeated micro-electrode recordings from subcortical structures without using any drugs is described. This method was adopted in combination with convential implantation techniques to study click-evoked potentials and inhibitory processes in the auditory system of the cat. The click-evoked potentials in MG were hardly affected by moderate doses of barbiturate and only to a minor degree in the auditory cortex. In the unanaesthetized animal the most significant contribution to the click-evoked inhibition in the auditory system was due to mechanisms in the MG. The inhibition was diminished both in size and duration as compared with the situation in anaesthetized cats. The MG cells showed a tendency to cyclic inhibition in the unanaesthetized cat, but not so regularly as following administration of sodium pentobarbital. The action of barbiturates on the auditory system is discussed.

Differential effects of phenobarbital and pentobarbital on isolated nervous tissue

Epileptiform after discharges evoked by repetitive electrical stimulation of chronically isolated cortical slabs (cat) were shortened by low doses of phenobarbital but not affected by hypnotic doses of pentobarbital. Both pentobarbital and phenobarbital raised threshold and lowered spike amplitude in isolated sciatic nerves. The action of both drugs was increased by reducing Na in the medium and by decreasing the Ringer's pH. Similar to the action of other general anesthetics, the axonal effect of pentobarbital was enhanced by D2O replacement for H2O in the Ringer's (suggesting that tissue water is involved in pentobarbital action), whereas D2O replacement did not modify the action of phenobarbital or of local anesthetics. These results suggest that the varying in vivo effects of pentobarbital and phenobarbital may be due to a difference in their action upon excitable membranes (rather than to a different regional distribution in brain).

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.

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.