Effect of elevated extracellular potassium on the release of labelled noradrenaline, glutamate, glycine, beta-alanine and other amino acids from rat brain cortex slices

Electrophysiological field potential identification of an intact GABAergic system in mouse cortical slices

In brain slice experiments there's currently no validated electrophysiological method for differentiating viability between GABAergic and glutamatergic cell populations. Here we investigated the neurophysiology of high frequency field potential activity - and its utility for probing the functional state of the GABAergic system in brain slices. Field potentials were recorded from mouse cortical slices exposed to 50 mM potassium ("elevated-K") and the induced high frequency (>20 Hz) response characterized pharmacologically. The elevated-K responses were also related to the high frequency activity imbedded in no-magnesium seizure-like events (SLE) from the same slices. The elevated-K response, comprising a transient burst of high frequency activity, was strongly GABAA-dependent. The size of the high frequency response was reduced by 71% (p = 0.001) by picrotoxin, but not significantly attenuated by either APV or CNQX. High frequency activity embedded in no-magnesium SLEs correlated with the elevated-K response. The success rate for generating an elevated-K response - and high frequency SLE activity - declined rapidly with increasing time since slicing. These findings support the hypothesis that in cortical slices, a functioning synaptic GABAergic system is evidenced by a strong high frequency component to no-magnesium SLE activity - and that the integrity of the GABAergic system degrades quicker than the excitatory glutamatergic system in this preparation.

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

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)

The effects of strychnine on neurons in cat somatosensory cortex and its interaction with the inhibitory amino acids, glycine, taurine and beta-alanine

In area 3b of primary somatosensory cortex, neurons may be classified as either rapidly adapting or slowly adapting to sustained stimuli and may be differentiated further by the presence or absence of a receptive field and by their threshold of activation. It is also possible to use the rate of adaptation of the background activity to a sustained stimulus to divide the cortex into slowly adapting regions or rapidly adapting regions. By blocking GABA-mediated inhibition with iontophoretically administered bicuculline methiodide, others have observed an increase in receptive field size in rapidly adapting regions but not in slowly adapting regions. The present study was designed to look for a different inhibitory transmitter which might control receptive field size in slowly adapting regions. Iontophoretically delivered strychnine was employed as an antagonist because it interferes with glycine-like inhibitory transmitters such as glycine, taurine and beta-alanine. Pharmacological tests were performed on 157 neurons in two series of experiments. In the first series three effects were documented. (i) In rapidly adapting regions, the size of the receptive field increased in 11 out of 25 cases whereas none of the 20 receptive fields tested in slowly adapting regions enlarged. (ii) In 13 of 24 cases a receptive field was revealed for previously unresponsive neurons in rapidly adapting regions whereas only 5 of 22 unresponsive cells tested in slowly adapting regions developed a receptive field. (iii) In 15 of 25 cells with receptive fields tested in rapidly adapting zones, strychnine reduced the threshold for somatic stimuli but only 8 of 20 cells isolated in slowly adapting zones showed this effect. In a second series of experiments, the effect of beta-alanine, glycine and taurine was examined on neurons of the rapidly adapting regions. beta-Alanine and taurine reduced the excitability of all neurons tested. Glycine inhibited most neurons. However, strychnine only antagonized the inhibitory effects of beta-alanine on responses to peripheral stimuli (9 of 11 cases). When neurons could not be driven by peripheral stimuli, the inhibition of spontaneous or glutamate-induced activity could not be blocked by strychnine (0 of 18 cases). We suggest that glycine-like amino acids contribute to the control of receptive field size and the control of neuronal excitability in rapidly adapting regions but not in slowly adapting regions. Our data suggest that strychnine-sensitive synapses are limited only to a subset of cortical neurons driven by somatic inputs.

Potassium-evoked release of endogenous primary amine-containing compounds from the trout saccular macula and saccular nerve in vitro

An in vitro preparation of the trout saccular macula, containing a large number of hair cells, served as a potential source of neurotransmitter(s) released at the acousticolateralis hair cell-afferent nerve synapse. An in vitro preparation of the saccular nerve, maintained in parallel, served to indicate the potential neural contribution to overall release from the macula. Efflux of 27 primary amine-containing compounds from the macula and nerve fractions was monitored by cation-exchange HPLC with fluorescence detection, and release by 53.5 mM potassium was determined at 1.45 mM calcium, 0.35 mM magnesium or 0 mM calcium, 10.1 mM magnesium. Taurine was released from the saccular macula in the greatest amount, accounting for 72% of the total evoked release of primary amine-containing compounds. Its release was calcium dependent and its time course prolonged. The contribution by myelinated nerve and associated Schwann cells within the macula to overall release of taurine from the macula in the presence of calcium, as determined from the saccular nerve preparation, was only 2%. Other components specifically released from the macula included ethanolamine, phosphoserine, beta-alanine, and glycine. Glutamate and aspartate were released from both the macula and saccular nerve fractions by potassium in the presence of calcium and in a ratio of 6:1 (glutamate:aspartate) for the macula and 7.5:1 for the nerve. The release of aspartate, but not that of glutamate, was lowered in saline containing 0 mM calcium, 10.1 mM magnesium. The calculated contribution from neural elements to overall release from the macula was 10% for aspartate and 18% for glutamate. These studies demonstrate that both the macula and saccular nerve fractions release the 'excitatory neurotransmitter' candidates aspartate and glutamate. Calcium-dependent, potassium-evoked release of taurine appears to be specific to the hair cell-supporting cell population of the saccular macula, and taurine may, therefore, be involved directly or indirectly in hair cell neurotransmission in labyrinthine organs. This study represents the first detailed biochemical characterization of efflux and release for an in vitro hair cell system of relatively high purity with respect to hair cells.

Endogenous serotonin release from rat cerebellar slices

Endogenous serotonin (5-HT) content in cerebellar slices and in the incubation medium was measured before and after depolarizing stimulus with high-K+ concentration in the medium. Small amount of 5-HT was spontaneously released from tissue slices incubated in low-K+ and Ca2+-free medium. A significant increase of endogenous 5-HT release was obtained after depolarizing stimulus with high-K+ plus Ca2+ in the medium. It is suggested that the endogenous 5-HT release in cerebellum induced by depolarizing stimuli is a Ca2+-dependent phenomenon.

Potassium-stimulated efflux of radiolabeled products formed from L-[14C(U)]-glutamine in vitro by the saccule of the rainbow trout (Salmo gairdnerii R.)

Samples of saccular macula from the rainbow trout were incubated in vitro with uniformly-labeled L-[14C]-glutamine, and radiolabeled products, released by potassium-induced depolarization in the presence of calcium, were examined. Most of the effluxed radioactivity was distributed in six (of 17) thin-layer chromatographic fractions. Fractions corresponding to aspartate and glutamate showed highly significant increases in radioactivity (as percent of total recovered radioactivity) during high-potassium treatment. Radioactivity in a fraction with an RF close to that of ornithine also significantly increased during potassium, and dropped sharply after potassium. The origins of the thin-layer fractions, with respect to sensory and neural elements in the saccular macula samples, are discussed.

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.

Isoguvacine binding, uptake, and release: relation to the GABA system

Isoguvacine (1,2,3,6-tetrahydropyridine-4-carboxylic acid) is a GABA (gamma-aminobutyric acid) agonist with limited conformational flexibility. In these studies we investigated the binding, uptake, and release of [3H] isoguvacine by use of tissue preparations of rat CNS, comparing the results with similar studies of [3H]GABA. The results from these investigations indicate that isoguvacine binds to membrane preparations of rat forebrain with pharmacological characteristics similar to the post-synaptic GABA recognition site; that it is transported into synaptosomal preparations by an uptake system similar to the high-affinity GABA uptake system; and that recently accumulated isoguvacine is released in a Ca2+-dependent manner and by heteroexchange with external GABA. The ability of isoguvacine and gamma-hydroxybutyric acid to decrease the K+-stimulated Ca2+-dependent release process was also investigated. The results indicate that isoguvacine interactions have many of the biochemical features of GABA synaptic function, isoguvacine being, however, less potent than GABA.

Swelling and ion uptake in cat cerebrocortical slices: control by neurotransmitters and ion transport mechanisms

Cat cerebrocortical slices incubating in medium containing normal K+ concentrations were exposed to a number of different transmitters. Norepinephrine, histamine and adenosine or 2-chloroadenosine caused increased swelling of the slices associated with an increased Na+ and Cl- content. These effects were seen only when both Cl- and HCO3- were present in the medium, and were inhibited by a number of anion transport inhibitors. These characteristics were identical to those of the HCO3(-)-dependent component of the swelling induced by high K+ levels in the medium. Other transmitters, namely 5-hydroxytryptamine, dopamine, and gamma-amino butyric acid, were ineffective. The effects of norepinephrine, histamine and 2-chloroadenosine were antagonised by propranolol and phentolamine, chlorpheniramine and diphenhydramine, and theophylline respectively. These antagonists also inhibited HCO3(-)-dependent, K+-stimulated swelling. The transmitters which induced swelling also stimulated the carbonic anhydrase activity of cerebrocortical slices. We conclude from these data that the HCO3(-)-dependent component of K+-stimulated swelling may be due to K+-stimulated release of transmitters. Furthermore, the fact that the transmitters which induce swelling have also been reported to be most effective in increasing cAMP content in both brain slices or cultured astrocytes is consistent with the swelling response being mediated via cAMP-induced changes and being predominantly localized to astrocytes.

A study of calcium compartments in rat brain cortex thin slices: effects of veratridine, lithium and of a mitochondrial uncoupler

The efflux kinetics of 45Ca from rat brain cortex thin slices previously equilibrated with it, was studied in a superfusion system. Two first order kinetic components of efflux from the tissue were found: k2 = 0.0667 min-1, that was unchanged by lowering the temperature from 37 degrees C to 15 degrees, and k3 = 0.0167 min-1 at 37 degrees C, that was reduced to 0.0897 min-1 at 15 degrees C. This suggests that k2 represents efflux from the extracellular space, and k3 that from the cellular compartment. Addition of the mitochondrial uncoupler carbonyl cyanide, m-chlorophenylhydrazone (CCCP) (10(-5)M) increased the efflux fractional rate constant of 45Ca by 35%, while no change in efflux was induced by 10 mM caffeine. Veratridine (10(-5)M) drastically reduced 45Ca efflux if superfusion was with physiological salt solution (150 mM sodium present), but not if 50 mM lithium replaced an equivalent amount of sodium in the superfusion fluid. This lithium-containing solution did not affect 45Ca efflux in the absence of veratridine. These results indicate that mitochondria accumulate only a minor fraction of intracellular 45Ca; that 45Ca possibly turns over very rapidly in the endoplasmic reticulum, and that most of 45Ca is present in a different, non-mitochondrial, non endoplasmic reticular compartment, the nature of which can be only conjectured.

Electrically induced release of radiolabeled histamine from rat hippocampal slices: opposing roles for H1- and H2-receptors

Rat hippocampal slices were preloaded with 3H-histamine and superfused with physiological medium and electrically stimulated in the absence (S1) and in the presence (S2) of drugs. The electrically evoked 3H-overflow consisted mainly of histamine, was Ca++ dependent and completely blocked by tetrodotoxin, all pointing towards an impulse triggered neuronal release. Mepyramine, promethazine and diphenhydramine the H1-antagonists, inhibited the stimulation evoked histamine release in a dose dependent manner. Burimamide and cimetidine, the H2-antagonists, enhanced the stimulation induced release of histamine whereas dimaprit, the H2-antagonist, had the opposite effect. Histamine by itself did not influence its own release. The observations indicate an opposing role for H1- and H2-receptors in modulating spike induced histamine release and represents a functional consequence of the stimulation of the receptors.

Evidence in favor of a neurotransmitter role of glycine in the rat cerebral cortex

In the present study we analyze whether glycine satisfies some electrophysiological and biochemical criteria to consider it as a putative transmitter in the rat cerebral cortex. Intracellular recordings from rat sensory-motor cortex showed that in 15-20% of the tested neurons glycine hyperpolarized the cell membrane, decreased the firing rate and flattened the evoked EPSP-IPSP sequence by increasing the membrane conductance. The iontophoretic application of strychnine antagonized the block of 'spontaneous' firing and the membrane hyperpolarization induced by glycine. Moreover, in a group of neurons, strychnine decreased the amplitude and duration of the IPSP and brought back the membrane potential to resting values. Previously accumulated [3H]glycine and endogenous glycine were released from cortical synaptosomal preparations by depolarizing stimuli in a Ca2+-dependent way. The release pattern of glycine was qualitatively similar in cortical and in spinal synaptosomes. [14C]Glycine was rapidly synthetized from [14C]serine in cortical synaptosomal preparations, and the newly formed [14C]glycine was released by depolarizing stimuli in a Ca2+-dependent way. It is concluded that glycine, which is generally considered as an inhibitory neurotransmitter in the spinal cord, medulla and pons, may also have a transmitter role in a discrete number of cortical neurons of some mammalian species.U

Differential effects of ouabain and 2,4-dinitrophenol on contractile tension of and on sodium and calcium efflux from frog heart ventricular strips

1 The efflux, from heart ventricular strips of Rana pipiens, of sodium ((22)Na) and calcium ((45)Ca) was measured simultaneously.2(22)Na efflux could be resolved into two first order kinetic components: k(I) = 0.105 min(-1), thought to represent efflux from the extracellular space, and k(II) = 0.0182 min(-1) representing efflux from the cells.3(45)Ca efflux was also resolved into an extracellular component, k(I) = 0.1216 min(-1); and an intracellular one, k(II) = 0.0102 min(-1). (45)Ca k(II) was greatly increased by 2,4-dinitrophenol (DNP), but unchanged by caffeine. This suggests that it represents a mitochondrial calcium compartment.4(22)Na k(II) was not changed by DNP. This indicates that, at the time of DNP addition, (22)Na was passively bound to undefined intracellular components.5 Ouabain (10(-6) M) decreased (45)Ca efflux (k(II)) initially but at later periods slightly increased it. The former effect is thought to be due to an action at the plasma membrane level, while the latter probably represents an increased exchangeability of mitochondrial calcium. The same effects were always found when ouabain was applied at different times of strip superfusion.6 Ouabain (0.25 to 4 muM) did not decrease the k(II) of (22)Na efflux. Kinetic reasons are presented which indicate that, in this preparation, the activity of the sodium pump may be too fast to be measured by means of (22)Na efflux, therefore these findings do not necessarily mean that ouabain does not inhibit active sodium transport.7 The time course of the inotropic effect of ouabain was also studied in ventricular strips of Rana pipiens heart that were stimulated at 0.2 Hz with biphasic, 2 ms pulses of supramaximal intensity, and incubated in Ringer solution containing 1.1 mM calcium, or in ;calcium-free' Ringer (residual calcium: 5.2 muM), or in ;calcium-free' Ringer with 0.1 mM of the calcium chelator ethyleneglycol bis (beta-aminoethylether) N,N'-tetraacetic acid (EGTA).8 In Ringer, the inotropic effect of ouabain was already observed at 5-10 s after steroid addition, even with the lowest concentration tested (0.25 muM), while signs of toxicity appeared only after 15 min in 4 muM ouabain, the highest concentration used.9 When the strips were incubated in ;calcium-free' Ringer solution, force of contraction decayed to 1-2% of that in 1.1 mM calcium. Addition of 4 muM ouabain to these hypodynamic strips led to a progressive increase in contractile force of up to 300%, that started after a 50 s latency period. No signs of toxicity were observed.10 Incubation of the strips in EGTA-Ringer also reduced contractile force to about 2% of that in Ringer, and 4 muM ouabain also increased force of contraction by approximately the same amount as seen in ;calcium-free' Ringer, but the effect began after a 10 min latency period. The concentration of calcium ion (Ca(2+)) in the extracellular space of strips incubated in EGTA-Ringer, was approximately 800 fold lower than in Ringer, and 60 fold lower than in ;calcium-free' Ringer solution.11 Caffeine (20 mM) induced, in strips previously incubated for 1 h in 4.4 mM calcium Ringer solution plus 10(-6) M ouabain, a marked initial contracture, that relaxed spontaneously, and was followed by slow waves of contracture. This was not observed if the strips were incubated, prior to caffeine, in 4.4 mM calcium Ringer without ouabain, or in 1.1 mM calcium Ringer solution that contained 10(-6) M ouabain.12 Based on these findings, a hypothesis that can explain the inotropic effect of cardioactive steroids is presented.

The Na+,K+-ATPase: a plausible trigger for voltage-independent release of cytoplasmic neurotransmitters

A comparison was made between the releasability of eight neurotransmitters from eight regions of mouse brain in response to either 60 mM-K+ or 20 microM-ouabain, a specific inhibitor of the Na+,K+-ATPase. With few exceptions, all transmitters were released by either or both agents from each brain region examined. Potassium was superior in releasing the biogenic amines and acetylcholine, while the putative amino acid transmitters were generally releasable by both agents. Measurements of tissue depolarization using [3H]-tetraphenylphosphonium uptake indicated that 60 mM-K+ is capable of depolarizing brain tissue above the threshold necessary for initiating an action potential, but 20 microM-ouabain is not. The pattern of release by ouabain coupled with its failure to depolarize brain tissue at 20 microM suggests that inhibition of the Na+,K+-ATPase is capable of releasing cytoplasmic neurotransmitters in a voltage-independent manner.

Electrically induced release of amino acids formed from (U14C) glucose in rat brain cortex slices, studied by a simplified dansylation procedure

The nonessential amino acids glutamate, aspartate, glutamine, gamma-aminobutyrate (GABA), alanine, glycine, and proline present in rat thin brain cortex slices were labeled by in vitro incubation of these with [U-14C]glucose, and the efflux of such endogenous radioactive amino acids and of lactate was studied in a superfused system, under control conditions or when the slices were depolarized by varous procedures. When electrical stimuli known to induce selective neurotransmitter release (1 or 1.5 volt, sine wave 60 Hz) were applied for 10 sec to the slices, no significant increase in amino acid efflux was found. When more intense stimuli (4 volt, 60 Hz) were applied for 60 sec, or extracellular potassium was raised to 56 mM, both conditions being known to induce nonselective substance release, the efflux of essentially all amino acids and of lactate was markedly increased. Increases in efflux were proportionately larger for glutamate, aspartate, and gamma-aminobutyrate, and this could be accounted for by their greater intracellular chemical (or electrochemical) potentials, but not because of a selective release mechanism for them. Amino acids were analyzed as their 1-dimethylaminoaphthalene-5-sulfonyl (dansyl) derivatives, by a modification of existing procedures in which the dansyl (DNS) derivatives were efficiently extracted from acidified incubation fluid into an organic phase. This rapidly desalted the derivatives and allowed their concentration and chromatographic separation on thin-layer silica gel sheets with little loss.