Release of endogenous amino acid neurotransmitter candidates from rat olfactory cortex slices: possible regulatory mechanisms and the effects of pentobarbitone

Measured increase in intracellular Ca(2+) during stimulated release of endogenous glutamate from human cerebrocortical synaptosomes

Presynaptic terminals (synaptosomes) prepared from guinea pig and rat cerebral cortex release endogenous glutamate in a Ca(2+)-dependent manner in response to membrane depolarisation. In the present study, synaptosomes were prepared from human cerebral cortex removed in association with temporal lobe resections in epileptic patients. The cytosolic free Ca(2+) concentration increased from 474+/-66 before to 649+/-89 nM after 2 min depolarisation. The basal level of free cytosolic Ca(2+) is higher and the increase in response to depolarisation is more pronounced in human synaptosomes than observed in animal experiments. The Ca(2+)-dependent glutamate release, estimated as the difference between total - and the Ca(2+)-independent glutamate release, increased from 0 to 5.4+/-1.9 nmol/mg protein. The released amount of glutamate is larger than reported in animal models. These results demonstrate that membrane depolarisation of synaptosomes from human brain evokes a rapid rise in cytosolic free Ca(2+) and a more prolonged rise in synaptic, Ca(2+)-dependent glutamate release.

What the actions of anaesthetics on fast synaptic transmission reveal about the molecular mechanism of anaesthesia

1. Synapses with the brain are important components of the networks responsible for higher nervous function and current evidence suggests that general anaesthetics modulate synaptic transmission in the brain. 2. Analysis of anaesthetic action on these synapses not only defines the cellular mechanisms involved in anaesthesia but also reveals much about the molecular targets of anaesthetic action. 3. It appears that while anaesthetics affect a wide variety of processes, the most sensitive are those which are directly linked to the activity of ligand-gated ion channels. Moreover, both single channel patch clamp studies and the molecular biological investigations of the sub-unit specificity of the sensitivity to anaesthetics indicate that anaesthetics interact directly with these functional proteins.

Comparison of anaesthetic and non-anaesthetic effects on depolarization-evoked glutamate and GABA release from mouse cerebrocortical slices

1. Investigation with substances that are similar in structure, but different in anaesthetic properties, may lead to further understanding of the mechanisms of general anaesthesia. 2. We have studied the effects of two cyclobutane derivatives, the anaesthetic, 1-chloro-1,2,2-trifluorocyclobutane (F3), and the non-anaesthetic, 1,2-dichlorohexafluorocyclobutane (F6), on K+-evoked glutamate and gamma-aminobutyric acid (GABA) release from isolated, superfused, cerebrocortical slices from mice, by use of h.p.l.c. with fluorescence detection for quantitative analysis. 3. At clinically relevant concentrations, the anaesthetic, F3, inhibited 40 mM K+-evoked glutamate and GABA release by 72% and 47%, respectively, whereas the structurally similar non-anaesthetic, F6, suppressed evoked glutamate release by 70% but had no significant effects on evoked GABA release. A second exposure to 40 mM KCl after a approximately 30 min washout of F3 or F6 showed recovery of K+-evoked release, suggesting that F3 and F6 did not cause any non-specific or irreversible changes in the brain slices. 4. Our findings suggest that suppression of excitatory neurotransmitter release may not be directly relevant to the primary action of general anaesthetics. A mechanism involving inhibitory postsynaptic action is implicated, in which a moderate suppression of depolarization-evoked GABA release by the anaesthetic may be consistent with the enhancement of postsynaptic GABAergic activities.

CI-966, a GABA uptake inhibitor, antagonizes ischemia-induced neuronal degeneration in the gerbil

1. Cerebral ischemia of 5 min duration was induced in unanesthetized gerbils by bilateral occlusion of the carotid arteries. 2. The extent of cerebral damage was assessed by the elevation of motor activity in comparison with pre-ischemic levels and by a histological assessment of the extent of neuronal degeneration of the CA1 area of the hippocampus. 3. The GABA transport inhibitor CI-966 (10 mg/kg i.p.) was tested for cerebroprotective activity in a gerbil stroke model. CI-966 reduced the extent of stroke injury as assessed by locomotor activity and measurement of hippocampal CA1 pyramidal cell injury. 4. It is proposed that enhancement of extracellular GABA levels during ischemia accounts for the cerebroprotective actions of CI-966.

Etomidate reduces ischemia-induced glutamate release in the hippocampus in rats subjected to incomplete forebrain ischemia

Etomidate and thiopental reduce ischemic neuronal injury but the mechanism by which they do so is not clear. Ischemia-induced release of the excitatory neurotransmitters glutamate and glycine is thought to play a major role in the pathophysiology of ischemic injury. To determine how etomidate and thiopental modulate excitatory transmitter release, their effect on the release of glycine and glutamate during ischemia was evaluated by microdialysis in the hippocampus and cortex of rats. Three groups of Wistar-Kyoto rats (n = 5/group) were studied. In the etomidate and thiopental groups, electroencephalogram (EEG) burst-suppression was achieved and maintained by a continuous infusion of either etomidate (0.6 mg.kg-1.min-1) or thiopental (3 mg.kg-1.min-1) 40 min prior to ischemia. Halothane anesthetized (1 minimum alveolar anesthetic concentration [MAC]) rats served as controls. Ischemia was induced in all three groups by bilateral carotid artery occlusion with simultaneous hypotension to 35 mm Hg for 10 min. Pericranial temperature was controlled at 38 degrees C. Dialysate was collected before, during, and after ischemia. The levels of glutamate and glycine in the dialysate were measured by high-performance liquid chromatography. Within the hippocampus, both glutamate and glycine levels increased significantly in the thiopental and control groups. By contrast, in the etomidate group, glutamate and glycine levels did not increase during ischemia, and peak levels were significantly less than those in the thiopental group. Peak glutamate levels in the thiopental group were significantly larger than in the control group, whereas the peak glycine levels were not different among the groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of thiopental, halothane and isoflurane on the calcium-dependent and -independent release of GABA from striatal synaptosomes in the rat

The effects of the anesthetic agents thiopental, halothane and isoflurane on the release of GABA induced by depolarization and/or reversal of the GABA carrier were investigated in a synaptosomal preparation obtained from the rat striatum. Veratridine (1 microM) and KCl (9 mM) elicited a significant, Ca(2+)-dependent release of [3H]GABA. The KCl-evoked release was not significantly modified in the presence of nipecotic acid (10(-5) M), a selective blocker of the neuronal GABA carrier. The [3H]GABA release was significantly decreased by omega-conotoxin (10(-7) M, a blocker of the N voltage-dependent Ca2+ channels, but was affected by neither nifedipine (10(-4) M) nor omega-Aga-IVA (10(-7) M which block the L and P Ca2+ channels, respectively. Thiopental application (10(-5) to 10(-3) M) was followed by a dose-related, significant, decrease in both the veratridine and KCl-induced releases, whether nipecotic acid was present or not. In contrast, halothane and isoflurane (1-3%) failed to alter [3H]GABA release. Altogether, these results suggest that reduction of the depolarization-evoked GABA release might contribute to thiopental anesthesia, but this seems unlikely for volatile anesthetics.

Molecular and cellular mechanisms of general anaesthesia

General anaesthetics are much more selective than is usually appreciated and may act by binding to only a small number of targets in the central nervous system. At surgical concentrations their principal effects are on ligand-gated (rather than voltage-gated) ion channels, with potentiation of postsynaptic inhibitory channel activity best fitting the pharmacological profile observed in general anaesthesia. Although the role of second messengers remains uncertain, it is now clear that anaesthetics act directly on proteins rather than on lipids.

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Effets des anesthésiques intraveineux sur les neurones du système nerveux central : mécanismes d'action cellulaires et moléculaires

The mechanisms of action of intravenous anaesthetics are not yet completely elucidated. Until recently, most of the studies had focused on the interactions between anaesthetics and lipid bilayers. It has been proposed that loss of consciousness is produced by disorganization of the lipid phase of nerve membranes, which impairs the action potential propagation. However, new data obtained with sophisticated neuropharmacological tools such as the patch clamp technique have recently contributed to challenge this hypothesis. Indeed, several lines of evidence suggest that intravenous anaesthetics are thought to induce loss of consciousness by blocking the excitatory synaptic transmission. This can be achieved presynaptically, by inhibiting glutamate release from nerve endings via alterations in the gating properties of voltage-dependent calcium channels. Blockade of excitatory synaptic transmission can also occur at the postsynaptic level by antagonizing the glutamate receptors of the N-methyl D-aspartate subtype. Some anaesthetic agents including ketamine also block the nicotinic receptors, however the relevance of this finding with respect to clinical anaesthesia requires further investigation. Preliminary data also suggest that propofol and etomidate elicit uncoupling of gap junctions between astrocytes, which represent a major nonneuronal cell population in the central nervous system. This phenomenon might indirectly contribute to the hypnotic action of these compounds. Whether loss of consciousness involves preferential target structures within the brain remains to be delineated. A better understanding of the mechanisms of action of general anaesthetics might contribute to generate new agents with more pharmacological selectivity and less undesirable side-effects.

Spontaneous single cell discharge in rat somatosensory cortical slices and its relationship to discharge in the urethane-anaesthetized rat

A slice preparation of rat somatosensory cortex has been used to observe the nature of spontaneous single cell discharge in layer V. On the basis of their discharge pattern, cells recorded using extracellular microelectrodes fell in to one of two categories: 70% of cells discharged single action potentials separated by relatively constant time intervals, while the remainder discharged in bursts of 2-7 action potentials. The firing patterns and the range of discharge frequencies observed were similar to those recorded in the same region and layer of the cortex in urethane-anaesthetized rats. However, unlike cells recorded in vivo, the majority of cells in slices never changed their discharge pattern. This suggests that while these layer V cells in cortical slices have the basic ability to discharge spontaneously, the potential for switching between discharge patterns requires circuitry absent from this isolated preparation. Urethane, over the dose range 6-24 mM, caused a dose-dependent decrease in the discharge frequency of cells with a regular firing pattern but did not affect their basic pattern of discharge.

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.

Antagonism of the actions of glutamate by pentobarbitone or midazolam in the frog optic tectum in vitro

Excitatory synaptic transmission, induced by electrical stimulation of optic nerve fibres on relay neurones, was recorded from in vitro preparations of the optic tectum of the frog. Bath-applied glutamate (the putative excitatory transmitter of the optic nerve) produced transient enhancement of tectal field potentials, followed by a depression, presumably caused by sustained neuronal depolarization. Pentobarbitone potently antagonized the depressant effect of glutamate, producing an approximate 50% reduction in the response of the tectum to glutamate at 25 microM. Midazolam also decreased the effect of glutamate with an IC50 value of 5 nM. Since, in the optic tectum of the frog, neither pentobarbitone nor midazolam enhance responses to bath-applied GABA, it is suggested that this area of the brain is a useful preparation in which to investigate the interaction of barbiturates and benzodiazepines with glutamate receptor mechanisms, without concurrent interactions with GABAergic processes.

Effects of NMDA antagonists, MK-801 and CPP, upon local cerebral glucose use

The effects upon cerebral glucose utilisation of (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801, a non-competitive N-methyl-D-aspartate (NMDA, receptor antagonist) and 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, a competitive NMDA receptor antagonist) were examined in conscious, lightly restrained rats. Cerebral glucose utilisation was assessed quantitatively in 74 brain regions with [14C]2-deoxyglucose autoradiography. The intravenous (i.v.) administration of MK-801 (0.05-5 mg/kg) induced heterogeneous patterns of altered cerebral glucose utilisation with statistically significant increases being observed in 21 brain areas and statistically significant decrease in 8 brain regions. Pronounced dose-related increases in glucose use were observed after MK-801 in the subicular complex, hippocampus molecular layer, dentate gyrus, limbic system (posterior cingulate cortex; mamillary body; anteroventral thalamic nucleus), olfactory areas and substantia nigra (pars reticulata). Glucose use in the neocortex and inferior colliculus was particularly sensitive to reduction by MK-801 administration. The pattern of altered glucose use after administration of CPP (3-30 mg/kg, i.v.) differed markedly from that observed after MK-801 treatment. Statistically significant increases in glucose use after CPP were noted in 11 brain areas and statistically significant decreases in 5 of the regions examined. Regions in which increases were noted after CPP included hippocampus molecular layer, olfactory areas, cochlear nucleus, vestibular nucleus, cerebellar nucleus, superior olives and substantia nigra (pars reticulata). These data indicate that widespread, anatomically organised alterations in cerebral function are associated with the administration of NMDA receptor antagonists despite the minor role normally ascribed to these receptors in conventional fast synaptic transmission. The distinct patterns of response to competitive and non-competitive antagonists may be a reflection of the differential responses of the two modes of receptor blockade to increased glutaminergic transmission.

Effects of anticonvulsants on responses to excitatory amino acids applied topically to rat cerebral cortex

1. Five clinically used anticonvulsants were examined to find out if they block the effects of excitatory amino acids in the cerebral cortex of urethane anaesthetised rats. 2. Compounds were tested by topical application to the cortical surface and following their intraperitoneal injection at anticonvulsant doses. 3. Pentobarbital and diphenylhydantoin blocked the effect of quisqualic acid but only at concentrations higher than the therapeutically relevant levels. 4. Pentobarbital and diphenylhydantoin did not alter the effects of N-methyl-D,L-aspartic acid (NMDLA) or kainic acid. 5. Diazepam prevented the kainic acid-induced development of distorted somatosensory evoked potentials (SEPs) at therapeutically relevant levels. 6. Diazepam had no effect on NMDLA or quisqualic acid. 7. Carbamazepine and chlormethiazole had no effect on NMDLA, kainic acid or quisqualic acid. 8. The anticonvulsive effects of these drugs, with the exception of diazepam, probably do not involve antagonism of endogenous EAAs.

Pentobarbital antagonizes the A1 adenosine receptor-mediated inhibition of hippocampal neurotransmitter release

Barbiturates have been shown to be competitive antagonists at A1 adenosine receptors in radioligand binding studies. The present study investigates the effects of pentobarbital on the A1 receptor-mediated inhibition of neurotransmitter release from rabbit hippocampal slices. The inhibition of the electrically evoked release of [3H]noradrenaline by the A1 receptor agonist (R)-N6-phenylisopropyladenosine (R-PIA) was antagonized by pentobarbital with an apparent pA2 value of 3.5. Low concentrations of pentobarbital alone altered neither basal nor evoked release of [3H]noradrenaline, whereas 1,000 microM pentobarbital enhanced the basal and reduced the evoked release. In the presence of 8-phenyltheophylline, pentobarbital (200 microM and 1,000 microM) reduced the evoked noradrenaline release. Pentobarbital also antagonized the inhibition of [3H]acetylcholine release by R-PIA. In contrast to the noradrenaline release model, the evoked release of acetylcholine was enhanced by the presence of pentobarbital (50-500 microM), an effect that was lost in the presence of 8-phenyltheophylline. These results indicate that pentobarbital, in addition to a direct inhibitory action at higher concentrations, has a facilitatory effect on neurotransmitter release by blocking presynaptic A1 adenosine receptors. The possible relevance of these findings for the excitatory effects of barbiturates is discussed.

Taurine and GABA release from mouse cerebral cortex slices: effects of structural analogues and drugs

The effects of structural analogues, excitatory amino acids and certain drugs on spontaneous and potassium-stimulated exogenous taurine and GABA release were investigated in mouse cerebral cortex slices using a superfusion system. Spontaneous efflux of both amino acids was rather slow but could be enhanced by their uptake inhibitors. Taurine efflux was facilitated by exogenous taurine, hypotaurine, beta-alanine and GABA, whereas GABA, nipecotic acid and homotaurine effectively enhanced GABA release. The stimulatory potency of the analogues closely corresponded to their ability to inhibit taurine and GABA uptake, respectively, indicating that these efflux processes could be mediated by the carriers operating outwards. Glutamate induced GABA release, whereas taurine efflux was potentiated by aspartate, glutamate, cysteate, homocysteate and kainate. The centrally acting drugs, including GABA agonists and antagonists, as well as the proposed taurine antagonist TAG (6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide), had no marked effects on spontaneous taurine and GABA release. Potassium ions stimulated dose-dependently both taurine and GABA release from the slices, the responses of taurine being strikingly slow but sustained. Exogenous GABA and nipecotic acid accelerated the potassium-stimulated GABA release, whereas picrotoxin and bicuculline were ineffective. The potassium-stimulated taurine release was unaltered or suppressed by exogenous taurine and analogues, differing in this respect from GABA release. The apparent magnitude of the depolarization-induced GABA release is thus influenced by the function of membrane transport sites, but the same conclusion cannot be drawn with regard to taurine. Haloperidol and imipramine were able to affect the evoked release of both taurine and GABA.

Effects of barbiturates on responses evoked by excitatory amino acids in slices of rat olfactory cortex

A study has been made of the effects of ranges of concentrations of phenobarbitone, pentobarbitone and thiopentone on responses evoked by gamma-aminobutyric acid (GABA), L-glutamate, L-aspartate, N-methyl-D-aspartate, kainate and quisqualate in slices of olfactory cortex of the rat. All three barbiturates affected GABA-evoked depolarizations similarly in that responses were potentiated by small doses but markedly inhibited at greater concentrations. Responses to L-aspartate and L-glutamate were little affected except at the largest dose of phenobarbitone tested (10 mM). The responses evoked by the selective agonists of excitatory amino acid receptors were inhibited by the barbiturates, the relative sensitivities being quisqualate greater than or equal to N-methyl-D-aspartate greater than or equal to kainate with phenobarbitone, quisqualate greater than or equal to kainate greater than N-methyl-D-aspartate with pentobarbitone and quisqualate greater than kainate = N-methyl-D-aspartate with thiopentone. The possible significance of these findings is discussed.

The action of pentobarbitone on stimulus-secretion coupling in adrenal chromaffin cells

The action of pentobarbitone on stimulus-secretion coupling was studied in bovine isolated adrenal medullary cells. Pentobarbitone inhibited catecholamine release evoked by 500 microM carbachol with half maximal inhibition (IC50) around 50 microM. It also inhibited catecholamine release induced by depolarization with 77 mM potassium (IC50 100 microM). These effects of pentobarbitone were observed with concentrations that lie within the range encountered during general anaesthesia. Evoked secretion required the presence of calcium in the extracellular medium and was associated with an influx of Ca2+ through voltage-sensitive channels. Pentobarbitone inhibited 45Ca influx in response to both carbachol (IC50 50 microM) and K+-depolarization (IC50 150 microM). The action of pentobarbitone on the relationship between intracellular free Ca and exocytosis was examined using electropermeabilised cells which were suspended in solutions containing a range of concentrations of ionised calcium between 10(-8) and 10(-4)M. Catecholamine secretion was measured in the presence of 0, 50, 200 or 500 microM pentobarbitone. The anaesthetic had no effect on the activation of exocytosis by intracellular free calcium. When catecholamine secretion in response to potassium or carbachol was modulated by varying extracellular calcium or by adding pentobarbitone to the incubation medium, the amount of catecholamine secretion for a given Ca2+ entry was the same. Pentobarbitone inhibited the secretion and 45Ca uptake induced by carbachol in a non-competitive manner. The secretion evoked by nicotinic agonists was associated with an increase in 22Na influx. Pentobarbitone inhibited this influx with an IC50 of 100 microM. We concluded that: (a) Pentobarbitone inhibits the catecholamine secretion from bovine adrenal chromaffin cells induced by nicotinic agonists by non-competitive inhibition of the nicotinic receptor. (b) The decrease in Ca influx caused by pentobarbitone accounts fully for the decrease in secretion in response to depolarization with potassium.

An analysis of the action of pentobarbitone on the excitatory postsynaptic potentials and membrane properties of neurones in the guinea-pig olfactory cortex

Intracellular recordings were made from neurones in slices of guinea-pig olfactory cortex maintained in vitro at 37 degrees C. The average membrane potential was 63 +/- 12 mV and the input resistance of these cells was 42 +/- 20 M omega (mean +/- s.d.). Stimulation of the lateral olfactory tract (l.o.t.) generated a transient depolarization in these cells which had the characteristics of an excitatory postsynaptic potential (e.p.s.p.). If the e.p.s.p. was of sufficient amplitude it culminated in an action potential. The e.p.s.p. was potentiated by repetitive stimulation at 10-50 Hz and showed post-tetanic potentiation after a prolonged period of high frequency stimulation (50-100 Hz for 30-60 s). Pentobarbitone (0.1-0.5 mM) depressed the e.p.s.p. reversibly but was without effect on the resting membrane potential, input resistance or time constant of the neurones. Pentobarbitone did not inhibit potentiation of the e.p.s.p. by a preceding conditioning shock. It is concluded that pentobarbitone does not affect the passive membrane properties of neurones in the olfactory cortex. The depressant action of pentobarbitone on synaptic transmission results from a decrease in the amount of transmitter released in response to a nerve impulse, or a decrease in the sensitivity of the postsynaptic membrane to the transmitter or a combination of both effects.

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.

The actions of barbiturates on release of noradrenaline from rat hippocampal synaptosomes

Barbiturates are believed to work both by augmenting GABA action and independently perhaps by decreasing neurotransmitter release. By studying the effect of pentobarbitates on the release of 3H noradrenaline from rat hippocampal synaptosomes it was found that pentobarbitone (10(-6)M and 10(-5)M) augmented the GABA evoked release of 3H noradrenaline but 10(-4)M depressed this GABA effect. This latter concentration of pentobarbitone also depressed the K+-evoked release of 3H noradrenaline by a Ca++ dependent but picrotoxin insensitive mechanism.

Veratridine-induced release in vivo and in vitro of amino acids in the rabbit olfactory bulb

Free amino acids were studied in the olfactory bulb of the rabbit during basal conditions and veratridine-induced depolarization, in vitro with a tissue slice preparation and in vivo with a perfusion-dialysis technique. In vivo, basal extracellular concentrations of GABA, beta-alanine and aspartate were low, while glutamine showed the highest level. The basal steady-state concentration ratio between the total tissue pool of free amino acids and amino acids in the extracellular fluid was high for GABA, aspartate and glutamate, while low for glutamine and other 'non-transmitter' amino acids. Veratridine induced a marked TTX-sensitive release of GABA (40-50 times the control) both in vivo and in vitro. In vivo, the GABA release showed a peak during the first minutes of veratridine perfusion. The TTX-sensitive release of aspartate and glutamate, on the other hand, was approximately 5 times higher in vitro than in vivo. Furthermore, a prolonged response to veratridine was seen for glutamate and aspartate in vivo consisting of an early peak, followed by a sustained release. Taurine showed a time-delayed veratridine response, both in vivo and in vitro, whereas glutamine displayed a slow, TTX-sensitive decrease. No effect of veratridine was seen on beta-alanine or carnosine-threonine levels.

Kainate-enhanced release of D-[3H]aspartate from cerebral cortex and striatum: reversal by baclofen and pentobarbital

A study was made of the actions of the excitant neurotoxin, kainic acid, on the uptake and the release of D-[2,3-3H]aspartate (D-ASP) in slices of guinea pig cerebral neocortex and striatum. The slices took up D-ASP, reaching concentrations of the amino acid in the tissue which were 14-23 times that in the medium. Subsequently, electrical stimulation of the slices evoked a Ca2+-dependent release of a portion of the D-ASP. Kainic acid (10(-5)-10(-3) M) produced a dose-dependent inhibition of D-ASP uptake. The electrically evoked release of D-ASP was increased 1.6-2.0 fold by 10(-5) and 10(-4)M kainic acid. The kainate-enlarged release was Ca2+-dependent. Dihydrokainic acid, an analogue of kainic acid with little excitatory or toxic action, did not increase D-ASP release but depressed D-ASP uptake. Attempts were made to block the action of kainic acid with baclofen and pentobarbital, compounds which depress the electrically evoked release of L-glutamate (L-GLU) and L-aspartate (L-ASP). Baclofen (4 X 10(-6)M), an antispastic drug, and pentobarbital (10(-4)M), an anesthetic agent, each inhibited the electrically evoked release of D-ASP and prevented the enhancement of the release above control levels usually produced by 10(-4)M kainic acid. It is proposed that 10(-5) and 10(-4)M kainic acid may enhance the synaptic release of L-GLU and L-ASP from neurons which use these amino acids as transmitters. This action is prevented by baclofen and pentobarbital. In view of the possibility that cell death in Huntington's disease could involve excessive depolarization of striatal and other cells by glutamate, baclofen might be effective in delaying the loss of neurons associated with this condition.

Desmethylimipramine enhances the release of endogenous GABA and other neurotransmitter amino acids from the rat thalamus

The influence of desmethylimipramine (DMI) on the release of endogenous gamma-aminobutyric acid (GABA) and some other amino acids from the rat thalamus was studied with a push-pull perfusion technique. Following HPLC the amino acids were fluorimetrically estimated. Added to the perfusion medium at a concentration of 10 mumol L-1, DMI caused a 5- to 10-fold increase in the release of GABA. Similar effects were found with imipramine, trimeprimine, haloperidol, and propranolol. The elevation of GABA release induced by DMI was Ca dependent. The release of aspartate and glutamate was also increased by DMI, but in contrast to K ions, DMI did not reduce the thalamic output of glutamine.

Acute effects of sodium valproate and gamma-vinyl GABA on regional amino acid metabolism in the rat brain: incorporation of 2-[14C]glucose into amino acids

Amino acid concentrations have been determined in rat brain regions (cortex, striatum, cerebellum, and hippocampus) by HPLC after administration of acute anticonvulsant doses of sodium valproate (400 mg/kg, i.p.) and gamma-vinyl-GABA (1 g/kg, i.p.). After valproate administration the GABA level increases only in the cortex; aspartic acid concentration decreases in the cortex and hippocampus, and glutamic acid decreases in the hippocampus and striatum and increases in the cortex and cerebellum. There are no changes in the concentrations of glutamine, taurine, glycine, serine, and alanine following valproate administration. Only the GABA level increases in all the regions after gamma-vinyl-GABA administration. Cortical analyses 2, 4 and 10 minutes after pulse labeling with 2-[14C]glucose, i.v., show no change in the rate of cortical glucose utilization in the valproate treated group. The rate of labeling of glutamic acid is also unchanged, but the rate of labeling of GABA is reduced following valproate administration. After gamma-vinyl-GABA administration there is no change in the rate of labeling of GABA. These biochemical findings can be interpreted in terms of a primary anticonvulsant action of valproate on membrane receptors with secondary effects on the metabolism of amino acid neurotransmitters. This contrasts with the primary action of gamma-vinyl-GABA on GABA-transaminase activity.

Effects of pentobarbitone on 45Ca2+ transport by rat brain mitochondria

The effects of pentobarbitone on the transport of 45Ca2+ by rat brain mitochondria were studied, using the Ruthenium Red-EGTA quench technique. In the presence of succinate and inorganic phosphate, mitochondria rapidly accumulate 45Ca2+. Pentobarbitone (0.1-1.0 mM) stimulates the initial rate of Ca2+ transport. In contrast, pentobarbitone (1 mM) did not affect the NaCl (50 mM)-induced efflux of 45Ca2+ from mitochondria. Dibucaine (60 micro M), a clinically used local anaesthetic, inhibits both 45Ca2+ uptake an efflux. The results suggest that barbiturate stimulation of mitochondrial Ca2+ uptake may, in combination with effects on other Ca2+ sequestering processes, contribute to the inhibitor of transmitter release observed at a number of synapses.

Some effects of excitatory amino acid receptor antagonists on synaptic transmission in the rat olfactory cortex slice

A study has been made of the effects of a series of excitatory amino acid receptor antagonists on the field potentials evoked on electrical stimulation of the lateral olfactory tracts of olfactory cortex slices perfused in vitro. The antagonists studied included (+/-)-2-amino-5-phosphonovaleric acid, a potent, specific antagonist of N-methyl-D-aspartate (NMDA) receptors, gamma-D-glutamylglycine, an antagonist of NMDA and kainate receptors and (+/-)-cis-2,3-piperidine dicarboxylic acid and 2-amino-4-phosphonobutyric acid, drugs which in addition to antagonizing NMDA and kainate receptors also block responses to quisqualic acid. From the patterns of effects of the drugs it is proposed that quisqualate and NMDA but not kainate receptors are involved in mediating excitatory transmission in the olfactory cortex; quisqualate receptors are located at the lateral olfactory tract - superficial pyramidal cell synapse whereas NMDA receptors are present at the synapses of the superficial pyramidal cell collaterals with the deep pyramidal cell dendrites and/or at the synapses of the pyramidal cell collaterals and inhibitory interneurones. The results are discussed in terms of possible presynaptic and/or postsynaptic sites of antagonist action.

Baclofen: effects on evoked field potentials and amino acid neurotransmitter release in the rat olfactory cortex slice

A study has been made of the in vitro effects of (+/-)- and (-)-baclofen on the evoked field potentials and release of endogenous amino acid neurotransmitter candidates (aspartate, glutamate, GABA and possibly taurine) which accompany electrical stimulation of the excitatory input to the olfactory cortex slice, the lateral olfactory tract. Baclofen appears to reduce the excitatory input to the GABA-utilizing inhibitory interneurones; this action was manifest as a drug-induced abolition of the field potential known as the P-wave (IC50 for (-)-baclofen, 1.7 +/- 0.4 microM) together with a simultaneous reduction in the synaptically evoked release of aspartase and glutamate from the cut surface of slices. Both these actions of baclofen exhibited concentration dependence and stereospecificity and were not antagonized by picrotoxin (25 microM) thereby suggesting that they are directly related. The consequence of this action of baclofen was the abolition of GABA-mediated presynaptic and postsynaptic inhibition together with their respective field potential correlates, the late N- and I-waves. (+/-)-Baclofen (5 and 25 microM) also inhibited the potassium-evoked release of aspartate and glutamate from small cubes of tissue but, except at a high concentration (1 mM), had no effect on GABA release. Baclofen (up to 1 mM) did not affect transmission either at the lateral olfactory tract-superficial pyramidal cell synapse, a site where aspartate is the likely neurotransmitter, or at the superficial pyramidal cell collateral-deep pyramidal cell excitatory synapse. It is proposed that: (i) the actions of baclofen on the olfactory cortex are the result of inhibition of aspartate and glutamate release, probably from deep pyramidal cell collaterals; and (ii) not all neurones utilizing excitatory amino acids as their neurotransmitters are subject to the inhibitory action of baclofen.

Endogenous GABA release from slices of rat cerebral cortex and hippocampus in vitro

Using a rapid, simple and sensitive radioreceptor assay, a Ca2+-dependent K+-evoked release of endogenous GABA was demonstrated from rat cortical and hippocampal slices in vitro. This evoked-release of endogenous GABA was similar to that of [3H]GABA release (in its Ca2+ dependency) but differed from the latter in having a higher signal to noise level. Neither 5-HT nor a stable enkephalin analogue had any effect on endogenous GABA release from hippocampus slices.

The effects of anaesthetics on the uptake and release of amino acid neurotransmitters in thalamic slices

1 The effect of thiopentone, methohexitone, urethane and ketamine on the uptake and release of gamma-aminobutyric acid (GABA) and D-aspartate by rat thalamic slices has been investigated. 2 A high, supra-anaesthetic concentration of methohexitone increased the uptake of both D-aspartate and GABA. 3 None of the anaesthetics used had any detectable effect upon the spontaneous release of either amino acid. 4 Urethane and ketamine had no effect upon the K+-stimulated release of either amino acid. 5 Methohexitone and thiopentone produced a biphasic dose-response on the K+-stimulated release of both amino acids; low concentrations enhanced release, high concentrations depressed release. 6 Bicuculline hydrochloride and picrotoxin both significantly reduced the barbiturate-induced enhancement of K+-stimulated amino acid release, but did not significantly alter the depression of K+-stimulated release at higher barbiturate concentrations. 7 Baclofen, either alone (1 microM to 1 mM), or tested against the barbiturates, had no detectable effect.

Synaptosomes prepared from fresh human cerebral cortex; morphology, respiration and release of transmitter amino acids

Synaptosomes prepared from fresh human cerebral cortex were shown to be morphologically similar to those from other species. On incubation, they took up oxygen at a high and linear rate and accumulated potassium against a concentration gradient. In response to depolarization by raised extracellular K+ or addition of veratrine, they showed increased respiration, lowered tissue potassium, and enhanced release of glutamate, aspartate and GABA. The preparation may be of value for studies of neurological disorders.

The effects of chlordiazepoxide on synaptic transmission and amino acid neurotransmitter release in slices of rat olfactory cortex

The rat olfactory cortex slice has been used to investigate the effects of chlordiazepoxide on evoked field potentials and the release of endogenous amino acid neurotransmitters (aspartate, glutamate, GABA and possibly taurine) which accompany electrical stimulation of the lateral olfactory tract. When single, low frequency stimuli were employed, chlordiazepoxide (2 microM-1 mM) depressed the amplitude of the field potential correlate of the depolarizing actions of the lateral olfactory tract excitatory transmitter (aspartate?) although aspartate release was unaffected. The field potential correlate of GABA-mediated presynaptic inhibition (late N-wave) was also depressed in amplitude but low drug concentrations (between approximately 2 and 50 microM) increased its peak duration . Effects of chlordiazepoxide on evoked inhibition were analyzed by giving paired stimuli such that the second stimulus occurred during the field potentials evoked by the first stimulus. Chlordiazepoxide (1-20 microM) increased the depression in amplitudes of the presynaptic massed action potential and late N-wave evoked by the second of a pair of stimuli compared with those evoked by the first stimulus suggesting that presynaptic inhibition was potentiated. These effects of chlordiazepoxide were accompanied by a significant reduction in aspartate release from the lateral olfactory tract terminals. Moreover, the drug effects on presynaptic inhibition and aspartate release were antagonized by picrotoxin (5 microM). On the other hand, chlordiazepoxide (1-50 microM) had no significant effect on postsynaptic inhibition. The results are discussed in terms of both the sites (presynaptic or postsynaptic) and mechanisms of action of chlordiazepoxide.

The GABA postsynaptic membrane receptor-ionophore complex. Site of action of convulsant and anticonvulsant drugs

The function of the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), has been implicated in the mode of action of many drugs which excite or depress the central nervous system. Many convulsant agents appear to block GABA action whereas anticonvulsants enhance GABA action. Some of these drug effects involve altered GABA-mediated synaptic transmission at the level of GABA biosynthesis, release from nerve endings, uptake into cells, and metabolic degradation. A greater number of agents of diverse classes appear to affect GABA action at the postsynaptic membrane, as determined from both electrophysiological and biochemical studies. The recently developed in vitro radioactive receptor binding assays have led to a wealth of new information about GABA action and its alteration by drugs. GABA inhibitory transmission involves the regulation, by GABA binding to its receptor site, of chloride ion channels. In this GABA receptor-ionophore system, other drug receptor sites, one for benzodiazepines and one for barbiturates/picrotoxinin (and related agents) appear to form a multicomponent complex. In this complex, the drugs binding to any of the three receptor categories are visualized to have an effect on GABA-associated chloride channel regulation. Available evidence suggests that the complex mediates many of the actions of numerous excitatory and depressant drugs showing a variety of pharmacological effects.

Direct action of pentobarbitone in potentiating the responses to GABA of rat dorsal root ganglion neurones in vitro

Pentobarbitone (PB) was tested for effects on responses to GABA recorded intracellularly in rat dorsal root ganglion neurones. Concentrations of over 1 mM PB elicited small depolarizations, whereas at greater than or equal to 10 microM PB readily potentiated depolarizations and increased membrane conductance evoked by GABA. The GABA antagonists bicuculline and picrotoxin reduced PB-potentiated and equiamplitude control responses to the same degree. Since an action of PB on GABA transport is unlikely in this tissue, the PB effects probably occur at the receptor-ionophore complex.

The effect of anaesthetics on the uptake and release of gamma-aminobutyrate and D-aspartate in rat brain slices

1 The effect of various concentrations of thiopentone, pentobarbitone, methohexitone, hydroxydione, alphaxalone/alphadolone, ketamine, alpha-chloralose, and urethane on the transport of radiolabelled gamma-aminobutyric acid (GABA) and D-aspartate was investigated. 2 Uptake of the amino acids was weakly inhibited, if at all, by the anaesthetics and it is unlikely that such effects contribute significantly to their physiological function. 3 The spontaneous efflux of GABA and D-aspartate was not detectably altered by any of the drugs tested. 4 Thiopentone, pentobarbitone, methohexitone and hydroxydione inhibited K+-stimulated GABA and D-aspartate release. The other anaesthetics had no effect on K+-stimulated amino acid release. 5 The rank order of potency of the inhibitors of K+-stimulated amino acid release did not correlate with their anaesthetic potency. Furthermore not all inhibitors appeared to be very effective at anaesthetic concentrations. 6 It is concluded that although it is possible that inhibition of excitatory transmitter release may be involved in the anaesthetic action of some anaesthetics, for many of the substances tested in this study such as mechanism does not appear to be implicated.

L-Aspartate binding sites in rat cerebellum: a comparison of the binding of L-[3H]aspartate and L-[3H]glutamate to synaptic membranes

The binding of L-[3H]aspartate to sonicated, extensively washed and preincubated cerebellar synaptic membranes was investigated. Binding was optimal under physiological conditions of pH and temperature, and attained equilibrium within 10 min. Binding was saturable, and Eadie-Hofstee analysis revealed interaction with a single population of binding sites (Kd = 874 nM and Bmax = 44 pmol/mg protein), which displayed no cooperativity (Hill coefficient approx. = 1). Specific [3H]aspartate was readily and reversibly displayed by unlabelled L-aspartate (the D-isomer being less than half as active) with a half-life of dissociation of 32 sec. Quisqualate, 4-fluoroglutamate and 2-amino-4-phosphonobutyrate, which are good displacers of [3H]glutamate binding, were only weakly active against the aspartate system. The excitatory amino acid antagonists, DL-alpha-aminoadipate, DL-alpha-aminosuberate and HA-966 were effective displacers, but the proposed aspartate receptor-preferring agonist, N-methyl-D-aspartate was inactive. Kainic acid exhibited negligible affinity for the aspartate binding site, in common with that for glutamate. While freezing or cold storage of membranes resulted in diminished [3H]-aspartate binding, lyophilization was not only able to confer substantial stability, but induced a marked increase in affinity of the binding site. Differential effects of various cations on [3H]aspartate binding were observed--monovalent cations reduced, while divalent cations enhanced L-[3H]aspartate binding.

Aspartate and not glutamate is the likely transmitter of the rat lateral olfactory tract fibres

The protoveratrine A-evoked release of endogenous amino acid neurotransmitter candidates from olfactory cortex slices taken from control and bulbectomized rats has been monitored. Bulbectomy is accompanied by a statistically significant and specific attenuation of drug-evoked aspartate release suggesting that the excitatory transmitter of the lateral olfactory tract fibres is aspartate rather than glutamate.

Patterns of endogenous amino acid release from slices of rat and guinea-pig olfactory cortex

A study has been made of the effects of depolarizing stimuli on the release of endogenous amino acid neurotransmitter candidates (aspartate, glutamate, GABA and taurine) from in vitro preparations of rat and guinea pig olfactory cortex. Exposure of small cubes of olfactory cortex tissue from either species to potassium chloride (50 mM) was accompanied by a calcium-dependent release of aspartate, glutamate and GABA. A similar release pattern was evoked by protoveratrine A (100 muM) although the release was largely calcium-independent. Neither agent led to increased release of taurine. Electrical stimulation of the excitatory input (lateral olfactory tract) of freshly prepared, synaptically intact olfactory cortex slices of both species induced significant release of aspartate and GABA from the uncut pial surface and of aspartate, GABA and glutamate from the cut surface. Evoked taurine release occurred from both surfaces of rat olfactory cortex slices but no release was detected from guinea pig olfactory cortex slices. These patterns of release were unaffected by changes in stimulus frequency and were mimicked by protoveratrine A (100 muM) applied to one or other surface. Preincubation of slices from rats for 2 led to loss of tissue amino acids and to changes in their release patterns; the presence of glutamine (5 mM) during preincubation prevented the loss of amino acids but did not alter their pattern of release. Because of the close similarities between both the electrophysiological properties and the patterns of amino acid release it is concluded that there is probably an identity of amino acid neurotransmitters (aspartate, glutamate and GABA) in rat and guinea pig olfactory cortex. The role of taurine in the rat olfactory cortex is unknown but would seem unlikely to be that of a neurotransmitter. The results are discussed: (i) in terms of the cellular origins of the released amino acids; and (ii) wit respect to apparent experimental discrepancies which have appeared in the literature.