Uptake of γ-Aminobutyric Acid and Glycine by Synaptosomes from Postmortem Human Brain

Synaptosomes prepared from frozen postmortem human brain accumulated the neurotransmitter gamma-aminobutyric acid (GABA) and the conformationally restricted GABA analogue cis-3-aminocyclohexanecarboxylic acid (ACHC) by a sodium-dependent, temperature-sensitive, high-affinity transport process into an osmotically sensitive compartment. This transport process could be inhibited by GABA analogues (ACHC, 2,4-diaminobutyric acid, nipecotic acid, arecaidine, guvacine) that have been shown in studies on other species to be relatively selective for neuronal rather than glial uptake systems, whereas the glial uptake inhibitor beta-alanine was ineffective. Synaptosomes prepared from frozen post-mortem human medulla and spinal cord, but not cerebral cortex, took up the neurotransmitter glycine by a sodium-dependent high-affinity transport process. The kinetic parameters for the high-affinity uptake of GABA, ACHC, and glycine were Km = 10 +/- 3, 49 +/- 19, and 35 +/- 19 microM; and Vmax = 98 +/- 15, 84 +/- 25, and 5.5 +/- 2.5 nmol/min/100 mg protein, respectively. These results demonstrate the feasibility of using human CNS preparations for studying GABA and glycine uptake, and suggest that such studies may be useful neurochemical markers for transmitter-specific presynaptic terminals in health and disease.     

A method for studying synaptosomal release of neurotransmitter candidates, as evaluated by studies on cortical cholecystokinin octapeptide release

A method for perfusing rat cortical synaptosomes for studying the regulation of cholecystokinin octapeptide (CCK-8) release has been developed and was found to have advantages over the static incubation system. Synaptosomes isolated from rat cortex were suspended in Biogel P2 columns and perfused with Krebs Ringer Bicarbonate buffer. One hundred mM KCl and 75 microM veratine stimulated CCK-8 release, which was Ca++-dependent. The synaptosomes were functionally viable for at least 135 min of incubation as indicated by multiple 100 mM KCl depolarizations and uptake of (3H)-norepinephrine and (14C)-choline. Dopamine and acetylcholine (10(-6)M) stimulated CCK-8 release while serotonin and norepinephrine were without effect. Approximately 20% of total occluded CCK-8 was released from synaptosomes by 100 mM KCl and degradation of CCK-8 was less than 10%. Perfusion of synaptosomes has several advantages over static incubation systems and allows systematic studies on the role of neurotransmitter in the regulation of neuropeptide secretion.     

Effect of Oleate on Neurotransmitter Transport and Other Plasma Membrane Functions in Rat Brain Synaptosomes

The effects of fatty acids, oleate and palmitate, on gamma-aminobutyric acid (GABA), aspartate, and 3,4- dihydroxyphenylethylamine (dopamine) transport and a variety of other membrane functions were studied in rat brain synaptosomes at a constant lipid-to-protein ratio. Under the conditions utilized oleate, but not palmitate, caused statistically significant changes in synaptosomal functions. Oleic acid inhibited the uptake of the amino acid neurotransmitters and dopamine in a tetrodotoxin-insensitive manner; it also induced the release of neurotransmitters from synaptosomes. The synaptosomal membrane potential decreased and the maximum GABA accumulation ratio [( GABA]i/[GABA]o) declined in parallel. The same depolarizing effect was seen in the presence of 50 microM verapamil or when chloride was replaced by propionate. The rate of respiration was stimulated by the unsaturated fatty acid; neither verapamil (50 microM) nor ouabain (100 microM) was effective in preventing the increase in oxygen consumption. By contrast, ruthenium red substantially decreased the stimulatory effect of oleate. The intrasynaptosomal [Ca2+] was increased by 40%, whereas [Na+]i remained unaltered. It is postulated that under the conditions used the inhibition of neurotransmitter uptake and the decrease in their accumulation caused by oleate result from the depolarization of synaptosomes that arises, at least in part, from increased permeability of the plasma membrane to calcium ions.     

NET UPTAKE OF L-GLUTAMATE AND GABA BY HIGH AFFINITY SYNAPTOSOMAL TRANSPORT SYSTEMS

Abstract— Reuptake of neuroactive amino acids by high affinity transport systems in the CNS is thought to terminate the neurotransmitter activity of these substances. This notion has been challenged since the homoexchange of synaptosomal and exogenous L-glutamate and the corresponding homoexchange of synaptosomal and exogenous GABA has been demonstrated. We reported that depolarizing media (56 mM-KCl, 1 mM-CaCl₂) lowers the GABA content of synaptosomes. In such synaptosomes, net and apparent (radioactive) GABA uptake are similar. When rat cortical synaptosomes (1 mg protein/ml) are incubated with 10μM-[¹⁴C] L-glutamate, net and apparent (radioactive) uptake are similar. When the synaptosome levels are decreased to 0.5 mg protein/ml or less, then net uptake becomes a fraction of radioactive uptake (exchange ensues). Net L-glutamate uptake is Na ⁺-dependent and temperature-dependent. Furthermore, a 1 mM concentration of KCl or RbCl supports net L-glutamate and GABA uptake. LiCl, NH₄Cl, CsCl and choline chloride are ineffective. In addition, diaminobutyric acid (but not β -alanine) inhibits net and apparent GABA uptake. The demonstration of net uptake of L-glutamate and GABA by their respective high affinity systems is consonant with the idea that these systems may play a role in neurotransmitter inactivation in the synaptic region.     

A fraction enriched in dendrodendritic synaptosomes isolated from rat olfactory bulb: Morphology and transmitter release

A fraction enriched in dendro-dendritic synaptosomes was isolated from rat olfactory bulb by a rapid method. Synaptosomes preserved their ultrastructure and showed configurational changes in relation to incubation in physiological ion medium as described earlier in the case of cortical synaptosomes. Dendro-dendritic synaptosomes were larger and contained more mitochondria than cortical synaptosomes. Doublets of terminals synapsing with each other were frequently seen and each terminal contained synaptic vesicles. Oxygen consumption of dendro-dendritic synaptosomes was decreased by ouabain and increased by 2,4-dinitrophenol. High-potassium medium evoked a considerable release of GABA and dopamine but not of noradrenaline or serotonin in accordance with histochemical published data.     

Stimulation of Synaptosomal γ-Aminobutyric Acid Synthesis by Glutamate and Glutamine

gamma-Aminobutyric acid (GABA) synthesis was studied in rat brain synaptosomes by measuring the increase of GABA level in the presence of the GABA-transaminase inhibitor gabaculine. The basal rate of synaptosomal GABA synthesis in glucose-containing medium (25.9 nmol/h/mg of protein) was only 3% of the maximal activity of glutamate decarboxylase (GAD; 804 +/- 83 nmol/h/mg of protein), a result indicating that synaptosomal GAD operates at only a small fraction of its catalytic capacity. Synaptosomal GABA synthesis was stimulated more than threefold by adding 500 microM glutamine. Glutamate also stimulated GABA synthesis, but the effect was smaller (1.5-fold). These results indicate that synaptosomal GAD is not saturated by endogenous levels of its substrate, glutamate, and account for part of the unused catalytic capacity. The greater stimulation of GABA synthesis by glutamine indicates that the GAD-containing compartment is more accessible to extrasynaptosomal glutamine than glutamate. The strong stimulation by glutamine also shows that the rates of uptake of glutamine and its conversion to glutamate can be sufficiently rapid to support GABA synthesis in nerve terminals. Synaptosomes carried out a slow net synthesis of aspartate in glucose-containing medium (7.7 nmol/h/mg of protein). Aspartate synthesis was strongly stimulated by glutamate and glutamine, but in this case the stimulation by glutamate was greater. Thus, the larger part of synaptosomal aspartate synthesis occurs in a different compartment than does GABA synthesis.     

Inhibition of serotonin uptake in synaptosomes and glia cells by some pharmacological substances

The uptake of serotonin -14 C by glial cells and synaptosomes of the rabbit brain cortex was studied. The Km value of the uptake of serotonin -14 C proved to be equal (0.83 + 0.02 microM) both for synaptosomes and glial cells. Synaptosomes of the rabbit brain cortex take up serotonin -14 C twice as fast as glial cells (uptake rates were compared from protein). Among psychotropic drugs studied the tricyclic antidepressant imipramine and psychostimulant cocaine turned out the most active inhibitors of both synaptosomal and glial uptake of serotonin -14 C. The drugs in 50 microM concentration inhibit the uptake of serotonin -14 C in synaptosomes and glial cells by 90 and 75-80%, respectively.     

Peroxidation induced changes in synaptosomal transport of dopamine and gamma-aminobutyric acid

The effects of iron-dependent peroxidation on respiration and neurotransmitter transport of brain nerve endings has been studied. Rat brain synaptosomes were peroxidized by exposure to an ADP-Fe/ascorbate system and the protective effect of added Se, Cd, or Zn was investigated with regard to dopamine and gamma-aminobutyric acid (GABA) transport. Peroxidation impaired the respiration of synaptosomes by about 20% and caused a marked increase in dopamine uptake; but in contrast, peroxidation induced a large decrease in synaptosomal uptake of GABA. The increased dopamine transport into synaptosomes was partially prevented by the presence of Zn, Se, or Cd. The presence of Zn, Cd, or Se, in order of decreasing effectiveness, also slowed down ADP-Fe/ascorbate mediated peroxidation of synaptosomes. Peroxidation caused a significant inhibition of veratridine-dependent release of both dopamine and GABA from synaptosomes, but the KCl-dependent release of these neurotransmitters was not effected by peroxidation. These results implicate that peroxidation damage of nerve endings may lead to large changes in neurotransmitter transport thus resulting in an alteration in the function of the central nervous system.     

Uptake of Exogenous Gangliosides by Rat Brain Synaptosomes

Synaptosomes incorporated mixed brain gangliosides at a rapid initial rate followed by a slower phase of net movement from the protein-associated fraction into the membrane core. The pattern of incorporated gangliosides reflected the pattern available for incorporation. Intact synaptosomes incorporated ~100 pmol GM1/mg protein. Synaptosomes preincubated with proteolytic enzymes (trypsin, chymotrypsin, and papain) at different pH values (6.2, 7.4, 7.8) incorporated more exogenous gangliosides than synaptosomes preincubated in buffer alone. This effect was maximal at pH 7.8, though analysis of variance revealed that the proteolytic treatment and pH effects were probably independent processes. Overall uptake of exogenous gangliosides correlated significantly with amount of membrane protein loss, indicating that initial access of exogenous gangliosides to synaptosomal membranes is retarded by cell-surface proteins. These results suggest synaptosomes as a useful alternative to cultured cells for investigating the interaction of gangliosides with other cell surface constituents.     

γ-Hydroxybutyric Acid in Subcellular Fractions of Rat Brain

The presence of gamma-hydroxybutyric acid (GHB) in synaptosome-enriched fractions of rat brain was ascertained using a GLC technique. The stability of GHB in synaptosomes was evaluated by addition of various gamma-aminobutyric acid (GABA) transaminase (GABA-T) inhibitors, GHB, or ethosuximide to the homogenizing medium. Furthermore, changes in whole brain GHB levels were compared with those in the synaptosomal fraction in animals treated with GABA-T inhibitors, GABA, or ethosuximide. GHB was present in synaptosome-enriched fractions in concentrations ranging from 40 to 70 pmol/mg of protein. There was no evidence for redistribution, leakage, or metabolism of GHB during the preparation of synaptosomes. The elevations of whole brain GHB level associated with GABA-T or ethosuximide treatment were reflected by a parallel increase in synaptosomal GHB content. These data add to the growing evidence that GHB may have neurotransmitter or neuromodulator function.     

Functional characteristics of nerve endings isolated from brain by the Hajos method

A study was made of the functional potentialities of synaptosomes isolated from the brain cortex and lumbar enlargement of the spinal cord. The yield of synaptosomes from the brain cortex amounted to 10 mg (with reference to protein) from 1 g of wet tissue, and that of synaptosomes from the spinal cord was equal to 1/3 of the yield from the brain, with the preparation being strongly contaminated with myelin scraps. Brain synaptosomes were marked by high level of respiration whose magnitude was affected by the agents (ouabain, high concentrations of K+ and benzylpenicillin) that change ion membrane transport. Synaptosomes maintained higher GABA gradient across the plasmatic membrane. Ouabain and potassium depolarization produced a considerable release of GABA and 3H-GABA into the incubation medium. A conclusion is made that the method of Hajos should be rather used for rapid isolation of the synaptosomal fraction from the rat brain cortex.     

A STUDY OF THE METABOLISM AND RELEASE OF DOPAMINE AND AMINO ACIDS FROM NERVE ENDINGS ISOLATED FROM SHEEP CORPUS STRIATUM

Abstract— Synaptosomes prepared from sheep corpus striatum showed a linear rate of respiration over a 90 min period of incubation in Krebs-bicarbonate medium containing glucose (10 mm ) and the rate of respiration was stimulated by electrical pulses. Dopamine was released from synaptosome beds to the medium by either electrical pulses or 56mm -K⁺ (10min), increasing 108% and 76% respectively above control levels of release. The presence of d- or 1-amphetamine (0.12mm ) in the incubation medium (40 min) increased the accumulation of dopamine in the medium by 310 and 275% respectively and 56mm -K⁺ also caused a significant increase in the release of glutamate, GABA and aspartate. Radioactively labelled dopamine was synthesized by the synaptosomes from l -[¹⁴C]tyrosine, l -DOPA or dl -DOPA, and electrical pulses caused a 35% increase in the rate of dopamine production from [U-¹⁴C] tyrosine. No increased release of [¹⁴C]dopamine in response to depolarizing stimuli was found to occur when synaptosome beds were transferred from medium containing radioactive precursors to fresh medium for further incubation (20 min). In the presence of 1- and d-amphetamine, accumulation of ¹⁴C-labelled doparnine in the incubation media was increased 129% and 380% respectively, the latter was partially depressed by absence of calcium from the medium. Three radioactively labelled metabolites formed by synaptosomes during incubation in dl -[2-¹⁴C]DOPA were detected; the major ones were dihydroxyphenylacetic acid and homovanillic acid and the third was unidentified. When the synaptosome beds were transferred to medium containing no radioactive precursors, it was found that labelled dihydroxyphenylacetic acid was 7 times more abundant than labelled dopamine in the incubation medium (20 min) and one-third as abundant in the synaptosomes. The dihydroxyphenylacetic acid n Ci/dopamine n Ci ratio was greatly affected by K⁺ stimulation, decreasing 52% and 34% in the incubation medium and synaptosomes respectively. A pathway of dihydroxyphenylacetic acid degradation was shown to occur through decarboxylation. These results are discussed in terms of the compartmentation of dopamine and its metabolism. It is proposed that one pool of dopamine is released by depolarizing agents and during the period of incubation it is replaced by synthesis from the endogenous tyrosine (19.5 nmol/100 mg protein) and not by the labelled dopamine in the synaptosome. The synaptosomal pool of dopamine which is radioactively labelled after pulse labelling with dl -[2-¹⁴C]DOPA appears to be prone to oxidation to DOPAC and homovanillic acid which are preferentially released from the synaptosomes.     

Neuronal Glutamine Utilization: Pathways of Nitrogen Transfer tudied with [15N]Glutamine

Gas chromatography-mass spectrometry was used to evaluate the metabolism of [15N]glutamine in isolated rat brain synaptosomes. In the presence of 0.5 mM glutamine, synaptosomes accumulated this amino acid to a level of 25-35 nmol/mg protein at an initial rate greater than 9 nmol/min/mg of protein. The metabolism of [15N]glutamine generated 15N-labelled glutamate, aspartate, and gamma-aminobutyric acid (GABA). An efflux of both [15N]glutamate and [15N]aspartate from synaptosomes to the medium was observed. Enrichment of 15N in alanine could not be detected because of a limited pool size. Elimination of glucose from the incubation medium substantially increased the rate and amount of [15N]aspartate formed. It is concluded that: (1) With 0.5 mM external glutamine, the glutaminase reaction, and not glutamine transport, determines the rate of metabolism of this amino acid. (2) The primary route of glutamine catabolism involves aspartate aminotransferase which generates 2-oxoglutarate, a substrate for the tricarboxylic acid cycle. This reaction is greatly accelerated by the omission of glucose. (3) Glutamine has preferred access to a population of synaptosomes or to a synaptosomal compartment that generates GABA. (4) Synaptosomes maintain a constant internal level of glutamate plus aspartate of about 70-80 nmol/mg protein. As these amino acids are produced from glutamine in excess of this value, they are released into the medium. Hence synaptosomal glutamine and glutamate metabolism are tightly regulated in an interrelated manner.     

High Specific Binding of [3H]GABA and [3H]Muscimol to Membranes from Dendrodendritic Synaptosomes of the Rat Olfactory Bulb

Olfactory bulbs contain dendrodendritic synapses, which occur between granule cells and mitral cells, and gamma-aminobutyric acid (GABA) is thought to act as an inhibitory neurotransmitter at these synapses. Synaptosomes derived from the dendrodendritic synapses of the olfactory bulb were shown previously to contain considerable L-glutamate decarboxylase activity. The subcellular distribution and binding parameters of [3H]GABA and [3H]muscimol binding sites have now been determined in the rat olfactory bulb. Of all fractions examined, crude synaptic membranes (CSM) prepared from the dendrodendritic synaptosomes were shown to have the highest specific binding activity and accounted for nearly all of the total binding activity for both ligands. The specific binding activities for [3H]GABA and for [3H]muscimol were greatly increased after treating the CSM with 0.05% Triton X-100. Binding was shown to be Na+-independent, reversible, pharmacologically specific, and saturable. High- and low-affinity sites were detected for both ligands, and both classes of sites had appreciably lower KD values for muscimol (KD1 = 3.1 nM, KD2 = 25.1 nM) than for GABA (KD1 = 8.6 nM; KD2 = 63.7 nM). The amounts of the high-affinity binding sites for muscimol and GABA were similar (Bmax = 1.7 and 1.5 pmol/mg protein, respectively). The results of the present experiments indicate that the GABA and muscimol binding sites represent the GABA postsynaptic receptor, presumably on mitral cell dendrites, and provide further support for the hypothesis that GABA functions as a neurotransmitter at the dendrodendritic synapses in the olfactory bulb.     

Effect of parathyroid hormone on the transport of 45Ca2+ and 3H-GABA in nerve endings isolated from the rat cerebral cortex

Parathyroid hormone (PTH) (0.1-10 ng/ml) evokes a dose-dependent increase in 45Ca2+ accumulation in synaptosomes isolated from the rat brain cortex. In the presence of PTH the fast (I sec) potential-dependent 45Ca2+ uptake was less than in the control. PTH had no effect on 3H-GABA uptake by synaptosomes (P2 fraction). Synaptosomes preincubated in the presence of PTH in Ca2+-free medium and transferred into Ca2+-containing normal medium released more 3H-GABA than control synaptosomes. In this case depolarization-evoked 3H-GABA release was diminished.     

Actions of Tremorgenic Fungal Toxins on Neurotransmitter Release

The neurochemical effects of the tremorgenic mycotoxins Verruculogen and Penitrem A, which produce a neurotoxic syndrome characterised by sustained tremors, were studied using sheep and rat synaptosomes. The toxins were administered in vivo, either by chronic feeding (sheep) or intraperitoneal injection 45 min prior to killing (rat), and synaptosomes were subsequently prepared from cerebrocortical and spinal cord/medullary regions of rat, and corpus striatum of sheep. Penitrem A (400 mg mycelium/kg) increased the spontaneous release of endogenous glutamate, GABA (gamma-aminobutyric acid), and aspartate by 213%, 455%, and 277%, respectively, from cerebrocortical synaptosomes. Verruculogen (400 mg mycelium/kg) increased the spontaneous release of glutamate and aspartate by 1300% and 1200%, respectively, but not that of GABA from cerebrocortical synaptosomes. The spontaneous release of the transmitter amino acids or other amino acids was not increased by the tremorgens in spinal cord/medullary synaptosomes. Penitrem A pretreatment reduced the veratrine (75 microM) stimulated release of glutamate, aspartate, and GABA from cerebrocortical synaptosomes by 33%, 46%, and 11%, respectively, and the stimulated release of glycine and GABA from spinal cord/medulla synaptosomes by 67% and 32% respectively. Verruculogen pretreatment did not alter the veratrine-induced release of transmitter amino acids from cerebrocortex and spinal cord/medulla synaptosomes. Penitrem A pretreatment increased the spontaneous release of aspartate, glutamate, and GABA by 68%, 62%, and 100%, respectively, from sheep corpus striatum synaptosomes but did not alter the synthesis and release of dopamine in this tissue. Verruculogen was shown to cause a substantial increase (300-400%) in the miniature-end-plate potential (m.e.p.p.) frequency at the locust neuromuscular junction. The response was detectable within 1 min, rose to a maximum within 5-7 min, and declined to the control rate over a similar period. No change in the amplitude of the m.e.p.p.'s was observed. These effects of the tremorgens on transmitter release are interpreted in terms of their mode of action.     

The Differential Effect of Ischemia on the Active Uptake of Dopamine, γ-Aminobutyric Acid, and Glutamate by Brain Synap to somes

Abstract: Ischemic stroke was induced in the Mongolian gerbil by left common carotid ligation. No change in uptake of [³H]dopamine, [³H]γ-aminobutyric acid ([³H]GABA), or [¹⁴C]glutamate in synaptosomes obtained from the ischemic hemisphere was observed for up to 8 h. At 16 h after ligation, marked decrements in uptake were observed in animals showing hemiparesis: Uptake values expressed as a percent of the corresponding control hemisphere were 15.2% for dopamine, 28.0% for GABA, and 47.5% for glutamate. The differential sensitivity of dopamine terminals compared with glutamate terminals was highly significant. Separate experiments performed with synaptosomes isolated from the corpus striatum showed that the greater sensitivity to damage was intrinsic to the dopamine nerve terminal and not the result of regional variations in ischemic damage in brain. No bilateral effect of ischemia on dopamine uptake was evident. In animals exhibiting milder behavioral deficits (circling), a smaller and comparable decrement in uptake of dopamine, GABA, and glutamate was evident at 16 h, whereas animals not affected behaviorally showed no decrement at 16 h. Following uptake, the subsequent fractional release of neurotransmitter stimulated by 60 mM-potassium ions was not affected at any time point studied. Therefore, the loss in uptake at 16 h probably represents overt destruction of nerve terminals. Experiments with urethane used in place of pentobarbital for anesthesia during carotid occlusion showed that "protection" by pentobarbital was not a factor in the delayed response to ischemia. These results show that damage or destruction of nerve terminals is a delayed event following ischemia and that dopamine terminals are intrinsically more sensitive than glutamate terminals.     

<Emphasis Type="Italic">Cynomorium songaricum</Emphasis> Extracts Functionally Modulate Transporters of γ-Aminobutyric Acid and Monoamine

Cynomorium songaricum Rupr. (SY) is a central nervous system-oriented herb material that has actions of anti-dementia, anti-epilepsy, and anti-stress. It is unclear whether SY would be biologically active in functionally regulating neurotransmitter transporters. Here, we assessed these potential actions using Chinese hamster ovary cells expressing γ-aminobutyric acid (GABA) transporter (GAT-1), dopamine transporter (DAT), norepinephrine transporter (NET), or serotonin transporter (SERT) (i.e. G1, D8, N1, or S6 cells, respectively). It was shown that SY extracts, such as SYw, SYa, SYp, SYc, SYe, and SYb (SY water, ethanol, petroleum ether, chloroform, ethyl acetate, and n-butyl alcohol extract, respectively) increased dopamine/norepinephrine (DA/NE) uptake by corresponding D8/N1 cells and decreased γ-aminobutyric acid/serotoin (GABA/5HT) uptake by corresponding G1/S6 cells; wherein, the potency or efficacy of SYc for up-regulating DA/NE uptake and that of SYb for inhibiting GABA/5HT uptake were relatively stronger. Additionally, GABA/5HT-uptake inhibition by SY extracts were also seen in cortical synaptosomes, and DA/NE-uptake enhancement by SYc was dependent on the activity of DAT and NET. Thus, SY extracts especially SYc and SYb are novel neurotransmitter-transporter modulators functioning as DAT/NET activators and/or GAT-1/SERT inhibitors.     

Uptake and release of kyotorphin in rat brain synaptosomes

Uptake and release of kyotorphin (TyrArg) in rat brain synaptosomes were studied. Synthetic kyotorphin was taken up into crude synaptosomes (P₂), in a temperature-dependent manner. The Km and Vmax of the uptake were 1.31 ± 0.12 × 10⁻⁴M and 5.9 ± 0.5 pmol/mg protein/min, respectively. Metabolic inhibitors such as dinitrophenol and iodoacetamide and ouabain which is known as an inhibitor of Na⁺ dependent uptake mechanism significantly inhibited the uptake. When the synaptosomes previously preloaded with synthetic kyotorphin at 10⁻⁴M were exposed to high K⁺ medium, kyotorphin was released in a Ca²⁺-dependent manner. These findings support the view that kyotorphin plays a role as neurotransmitter/neuroregulator.     

The use of 3H-gamma-aminobutyric acid for transport studies with isolated nerve-terminals from rat brain

Isolated synaptosomes were used to study the problem of net accumulation of neurotransmitters. The time-course and the kinetics of exogenous and endogenous GABA transport were studied by liquid-scintillation counting and HPLC-amino acid analysis respectively. Different pools of GABA were suggested by a 6-fold difference in tissue-to-medium-ratio of endogenous vs. exogenous GABA. Net accumulation, exchange and net efflux of GABA was found to be a function of the GABA concentration in the incubation medium. The Kms for net accumulation and for 3H-GABA accumulation were 2.68 +/- 1.16 and 6.19 +/- 1.26 microM respectively, whereas the Vmaxs were 5.9 +/- 4.9 and 134 +/- 13 pmol/mg w.w. min respectively. This means that the transport studies which use exogenous substances (e.g. 3H-GABA) considerably overestimate the transport by overlooking the magnitude of the counter transport.