Release of previously incorporated gamma-[3H]aminobutyric acid in rabbit caudate nucleus slices

Uptake and release of [(3)H]GABA in human dental pulp

The purpose of this study was to determine whether (a) an uptake system for gamma-aminobutyric acid (GABA) exists in human dental pulp, (b) GABA can be released from nerves in this tissue, and (c) GABA(B) autoreceptors modulate release of this transmitter. Segments of vital pulp were incubated in [(3)H]GABA (0.1-10 microM) for up to 120 min, washed, and the retained [(3)H] extracted and assayed. Some tissues were treated with GABA uptake inhibitors (nipecotic acid or NO-711) prior to incubation. At concentrations of 0.1 and 1.0 microM the uptake of [(3)H]GABA was saturated after 90 min of incubation. At 10 microM, at least two uptake compartments were apparent, and the amount of [(3)H]GABA retained was five-fold greater than 0.1 microM. The uptake inhibitors reduced [(3)H]GABA accumulation by more than 80%. In the release study, pulp was incubated in [(3)H]GABA (0.5 microM) for 90 min, and superfused with Krebs solution containing NO-711 (5 microM). Electrical stimulation increased the overflow of [(3)H]; a GABA(B) autoreceptor agonist (baclofen) inhibited, whilst an antagonist, Sch 50911, enhanced this release. The effects of baclofen were reversed by Sch 50911. These results imply that GABA can be taken up and bound firmly in compartments within human dental pulp, GABA can be released from isolated pulp segments by electrical stimulation, and this release is modulated by GABA(B) autoreceptors.

Cerebral creatine deficiency syndromes: clinical aspects, treatment and pathophysiology

Cerebral creatine deficiency syndromes (CCDSs) are a group of inborn errors of creatine metabolism comprising two autosomal recessive disorders that affect the biosynthesis of creatine--i.e. arginine:glycine amidinotransferase deficiency (AGAT; MIM 602360) and guanidinoacetate methyltransferase deficiency (GAMT; MIM 601240)--and an X-linked defect that affects the creatine transporter, SLC6A8 deficiency (SLC6A8; MIM 300036). The biochemical hallmarks of these disorders include cerebral creatine deficiency as detected in vivo by 1H magnetic resonance spectroscopy (MRS) of the brain, and specific disturbances in metabolites of creatine metabolism in body fluids. In urine and plasma, abnormal guanidinoacetic acid (GAA) levels are found in AGAT deficiency (reduced GAA) and in GAMT deficiency (increased GAA). In urine of males with SLC6A8 deficiency, an increased creatine/creatinine ratio is detected. The common clinical presentation in CCDS includes mental retardation, expressive speech and language delay, autistic like behaviour and epilepsy. Treatment of the creatine biosynthesis defects has yielded clinical improvement, while for creatine transporter deficiency, successful treatment strategies still need to be discovered. CCDSs may be responsible for a considerable fraction of children and adults affected with mental retardation of unknown etiology. Thus, screening for this group of disorders should be included in the differential diagnosis of this population. In this review, also the importance of CCDSs for the unravelling of the (patho)physiology of cerebral creatine metabolism is discussed.

Exocytotic release of creatine in rat brain

The guanidino compound creatine has been shown to occur throughout the brain affecting energy metabolism and mental performance and to act at central GABAA receptors as a partial agonist. Therefore, we examined the possibility that creatine may in fact represent a neuromodulator that is released in the brain in an action-potential dependent manner. To that end, we studied the uptake of [3H]creatine and its electrically evoked release from superfused rat brain slices as well as the evoked release of endogenously synthesized creatine. [3H]creatine was accumulated in neocortex slices in a Na+-dependent manner, consistent with the involvement of the Na+-dependent SLC6A8 creatine transporter. Most importantly, the electrically evoked release of [3H]creatine from neocortex slices (like that from caudate putamen and hippocampus slices) as well as the evoked release of endogenous (unlabeled) creatine was abolished when Ca2+ was omitted from the superfusion medium or in the presence of the Na+-channel blocker tetrodotoxin (TTX). Moreover, blockade of K+-channels by 4-aminopyridine (4-AP) strongly enhanced the electrically evoked release of [3H]creatine as well as that of endogenous creatine. These in vitro data indicate that creatine is not only synthesized and taken up by central neurons, but also released in an action-potential dependent (exocytotic) manner, providing strong evidence for its role as a neuromodulator in the brain.

Effects of cholecystokinin tetrapeptide (CCK(4)) and of anxiolytic drugs on GABA outflow from the cerebral cortex of freely moving rats

The effect of cholecystokinin tetrapeptide (CCK(4)) and of different anxiolytic drugs on GABA outflow from the cerebral cortex was investigated in freely moving rats, by using the epidural cup technique. CCK(4) (3-30 microg/kg, i.p.) increased GABA outflow and induced objective signs of anxiety. These neurochemical and behavioral responses were prevented by the CCK(B) antagonist GV150013 at 0.1 microg/kg (i.p.). At higher doses (up to 30 microg/kg) this compound per se reduced GABA release and caused sedation, suggesting the presence of a CCKergic positive tonic modulation on GABA interneurons. Similarly the GABA(A) receptors modulator, diazepam (2mg/kg, i.p.) and the 5-HT(1A) agonist buspirone (3mg/kg, i.p.) reduced GABA outflow and caused the expected behavioral effects (reduced muscle tone, mild 5-HT syndrome) which were prevented by the respective, selective antagonists, flumazenil (1mg/kg, i.p.) and NAN-190 (3mg/kg, i.p.). These findings support the idea that GV150013, diazepam and buspirone inhibit GABAergic cortical activity, through the respective receptors. This neurochemical effect may represent the end-effect of various anxiolytic compounds affecting the cortical circuitry.

Release of gamma-[(3)H]aminobutyric acid in rat brain cortical slices by alpha-scorpion toxin

In this paper, the effect of the alpha-scorpion toxin tityustoxin (TsTX) in the release of gamma-[(3)H]aminobutyric acid ([(3)H]GABA) from rat brain cortical slices is described. The TsTX-stimulatory effect on the release of [(3)H]GABA was dependent on incubation time and TsTX concentration, having an EC(50) of 0.33 microM. Tetrodotoxin (TTX) completely inhibited the TsTX action on [(3)H]GABA release. The scorpion toxin effect was calcium-dependent and involves P/Q calcium channels. beta-Alanine also induces the release of [(3)H]GABA that was not inhibited by TTX but was additive in the presence of TsTX. The data suggest a neuronal origin for the release of [(3)H]GABA by TsTX.

Serotonin-1B receptor-mediated inhibition of [(3)H]GABA release from rat ventral tegmental area slices

In order to assess a role of 5-HT(1B) receptors for regulation of GABA transmission in the ventral tegmental area (VTA), VTA slices from the rat were incubated with [(3)H]GABA and beta-alanine, and superfused in the presence of nipecotic acid and aminooxyacetic acid. [(3)H]GABA release was induced by exposures to the medium containing 30 mM potassium for 2 min. The results showed that high potassium-evoked [(3)H]GABA release was sensitive to calcium withdrawal or blockade of sodium channels by tetrodotoxin, suggesting that tritium overflow induced by high potassium derived largely from neuronal stores. Administration of CP 93129 (0.15 and 0.45 microM), a 5-HT(1B) receptor agonist, or RU 24969 (0.15 and 0.45 microM), a 5-HT(1B/1A) receptor agonist, but not 8-OH-DPAT (0.45 microM), a 5-HT(1A) receptor agonist, inhibited high potassium-evoked [(3)H]GABA release in a concentration-related manner. The RU 24969-induced inhibition of [(3)H]GABA release was antagonized by either SB 216641, a 5-H(1B) receptor antagonist, or cyanopindolol, a 5-HT(1B/1A) receptor antagonist, but not by WAY 100635, a 5-HT(1A) receptor antagonist. Pre-treatment with SB 216641 also antagonized CP 93129-induced inhibition of [(3)H]GABA release. The results support the hypothesis that 5-HT(1B) receptors within the VTA can function as heteroreceptors to inhibit GABA release.

Beta-scorpion toxin induces the release of gamma-[3 H]aminobutyric acid in rat brain slices

The effect of the beta-scorpion toxin, TiTX gamma on the release of [3H]GABA from rat brain cortical slices is described. The stimulatory effect of TiTX gamma on the release of [3H]GABA was dependent on incubation time and TiTX gamma concentration with an EC50 of 0.19 microM. The scorpion toxin effect was calcium dependent and was completely inhibited by tetrodotoxin. beta-Alanine also induced the release of [3H]GABA and this effect was not inhibited by tetrodotoxin but was additive in the presence of TiTX gamma. The data suggest a neuronal origin for the release of [3H]GABA by TiTX gamma.

Presynaptically located CB1 cannabinoid receptors regulate GABA release from axon terminals of specific hippocampal interneurons

To understand the functional significance and mechanisms of action in the CNS of endogenous and exogenous cannabinoids, it is crucial to identify the neural elements that serve as the structural substrate of these actions. We used a recently developed antibody against the CB1 cannabinoid receptor to study this question in hippocampal networks. Interneurons with features typical of basket cells showed a selective, intense staining for CB1 in all hippocampal subfields and layers. Most of them (85.6%) contained cholecystokinin (CCK), which corresponded to 96.9% of all CCK-positive interneurons, whereas only 4.6% of the parvalbumin (PV)-containing basket cells expressed CB1. Accordingly, electron microscopy revealed that CB1-immunoreactive axon terminals of CCK-containing basket cells surrounded the somata and proximal dendrites of pyramidal neurons, whereas PV-positive basket cell terminals in similar locations were negative for CB1. The synthetic cannabinoid agonist WIN 55,212-2 (0.01-3 microM) reduced dose-dependently the electrical field stimulation-induced [3H]GABA release from superfused hippocampal slices, with an EC50 value of 0. 041 microM. Inhibition of GABA release by WIN 55,212-2 was not mediated by inhibition of glutamatergic transmission because the WIN 55,212-2 effect was not reduced by the glutamate blockers AP5 and CNQX. In contrast, the CB1 cannabinoid receptor antagonist SR 141716A (1 microM) prevented this effect, whereas by itself it did not change the outflow of [3H]GABA. These results suggest that cannabinoid-mediated modulation of hippocampal interneuron networks operate largely via presynaptic receptors on CCK-immunoreactive basket cell terminals. Reduction of GABA release from these terminals is the likely mechanism by which both endogenous and exogenous CB1 ligands interfere with hippocampal network oscillations and associated cognitive functions.

Separation of carrier mediated and vesicular release of GABA from rat brain slices

In this study the temperature dependence of [3H]GABA release from brain slices evoked by electrical field stimulation and the Na+/K+ ATPase inhibitor ouabain was investigated. [3H]GABA has been taken up and released from hippocampal slices at rest and in response to electrical field stimulation (20 V, 10 Hz, 3 msec, 180 pulses) at 37 degrees C. When the bath temperature was cooled to 7 degrees C, during the sample collection period, the tissue uptake and the resting outflow of [3H]GABA were not significantly changed. In contrast, the stimulation-induced tritium outflow increased both in absolute amount (Bq/g) and in fractional release and the S2/S1 ratio was also higher at 7 degrees C. Perfusion of the slices with tetrodotoxin (TTX, 1 microM) inhibited stimulation-induced [3H]GABA efflux indicating that exocytotic release of vesicular origin is maintained under these conditions. 15 min perfusion with ouabain (10-20 microM) induced massive tritium release both in hippocampal and in striatal slices. However, the fraction of [3H]GABA outflow evoked by ouabain was much higher in the hippocampus than in the striatum. Sequential lowering the bath temperature from 37 degrees C to 17 degrees C completely abolished ouabain-induced [3H]GABA release in both brain regions, indicating that it is a temperature-dependent, carrier-mediated process. When the same experiments were repeated under Ca2+ free conditions, cooling the bath temperature to 17 degrees C, although substantially decreased the release but failed to completely abolish the tritium outflow evoked by ouabain, a significant part was maintained. Our results show that vesicular (field stimulation-evoked) and carrier-mediated (ouabain-induced) release of GABA is differentially affected by low temperature: while vesicular release is unaffected, carrier-mediated release is abolished at low bath temperature. Therefore, lowering the temperature offers a reliable tool to separate these two kinds of release and makes possible to study exclusively the pure neuronal release of GABA of vesicular origin.

Postsynaptic integration of cholinergic and dopaminergic signals on medium-sized GABAergic projection neurons in the neostriatum

The effects of cholinergic drugs and the interaction between cholinergic and dopaminergic compounds were studied on electrically evoked [3H]gamma-aminobutyric acid (GABA) overflow in slices of the rat neostriatum. Slices were prepared and loaded with [3H]GABA in the presence of beta-alanine and then superfused with Krebs-bicarbonate buffer containing aminooxyacetic acid and nipecotic acid to inhibit GABA uptake and metabolism, respectively. The nonselective muscarinic agonist oxotremorine (0.1-10 microM) increased the release of [3H]GABA and the selective M1 receptor agonist McN-A-343 (0.1-10 microM) exerted similar effect. The stimulatory effect of oxotremorine (10 microM) on [3H][GABA overflow was antagonized by the nonselective muscarinic antagonist atropine (1 microM) and the selective M1 receptor antagonist pirenzepine (0.1-1.0 microM). The M2 receptor antagonist methoctramine (1.0 microM) did not alter the stimulatory effect of oxotremorine. Of the muscarinic receptor antagonists atropine, pirenzepine, and methoctramine (1.0 microM) failed to affect [3H]GABA overflow. The M3 receptor antagonist p-F-HHSiD (1 microM) increased [3H]GABA overflow and p-F-HHSiD and oxotremorine were found to be additive in increasing this effect. The D2 dopamine receptor antagonist sulpiride (10 microM) increased the electrical stimulation-induced [3H]GABA overflow, and this stimulation was counteracted by concomitant administration of atropine (1 microM). McN-A-343 and sulpiride also increased the KCl-induced [3H]GABA overflow from superfused neostriatal slices and tetrodotoxin (1 microM) did not affect these stimulations. These data indicate that the release of GABA in the neostriatum is under the control of M1 stimulatory and M3 inhibitory muscarinic receptors. Dopamine, which exerts inhibition on GABA release via D2 receptors, may counteract the M1 facilitation, and M1 and D2 receptors involved in the cholinergic-dopaminergic interaction may be located postsynaptically on medium-sized spiny GABAergic projection neurons.

Influence of dopamine on GABA release in striatum: evidence for D1-D2 interactions and non-synaptic influences

Striatal slices from the rat were preincubated with [3H]GABA and superfused in the presence of nipecotic acid and aminooxyacetic acid, inhibitors of high-affinity GABA transport and GABA aminotransferase, respectively. GABA efflux was estimated by monitoring tritium efflux, 98% of which was in the form of [3H]GABA. The following three major observations were made: (1) The overflow of GABA evoked by electrical field stimulation (8 Hz) was increased two-fold by SKF-38393 (10 microM), an agonist at the D1 family of dopamine receptors. This increase was completely blocked by the D1 receptor antagonist SCH-23390 (10 microM). However, SCH-23390 had no effect on GABA overflow when given alone. Thus, dopamine agonists appear to exert an excitatory influence on GABA release; however, this effect was not elicited by endogenous dopamine under the conditions of this experiment. (2) Electrically evoked GABA overflow was reduced 50% by quinpirole (10 microM), an agonist at the D2 family of dopamine receptors, and this effect was blocked by the D2 antagonist sulpiride (10 microM). Moreover, exposure to sulpiride alone caused a 60% increase in GABA overflow, and this effect was abolished by 3-iodotyrosine (2 mM), a dopamine synthesis inhibitor. Thus, D2 agonists appear to exert an inhibitory influence on dopamine release, an effect that can be exerted by endogenous stores of dopamine. (3) The stimulatory effect of SKF-38393 was attenuated by quinpirole, whereas the sulpiride-induced increase in GABA efflux was attenuated by SCH-23390. Sulpiride also increased [3H]GABA efflux during KCl-induced depolarization, an effect that was antagonized by SCH-23390 as in the case of electrical stimulation. However, although tetrodotoxin did not alter the stimulatory effect of sulpiride, it did block the ability of SCH-23390 to antagonize the sulpiride-induced increase in GABA overflow. These latter results suggest that there is an interaction between D1 and D2 receptors whereby the effects of dopamine mediated via D1 sites are inhibited by an action on D2 sites. In conclusion, our results suggest that (i) dopamine agonists can exert an excitatory influence on depolarization-induced GABA release within neostriatum via D1 receptors and an inhibitory influence via D2 receptors; (ii) under the conditions of these experiments, endogenous dopamine fails to act on D1 sites but does exert an inhibitory influence via D2 sites; and (iii) there is an interaction between D1 and D2 receptors such that the actions of dopamine mediated via D1 sites are inhibited as a result of the concomitant actions exerted via D2 sites.

Cholecystokinin-GABA interactions in rat striatum

The release of [3H]gamma-aminobutyric acid ([3H]GABA) from rat striatal slices before and during electrical field stimulation (EFS) was measured. Electrical stimulation (10 Hz) induced an increase of Ca(++)- and tetrodotoxin-sensitive [3H]GABA release from the striatal slices. In the presence of sulphated octapeptide of cholecystokinin, CCK-8S (10(-9) M, 10(-8) M and 10(-7) M) both the basal and the electrically (10 Hz)-evoked release of [3H]GABA were dose-dependently increased. These effects of CCK-8S were abolished by tetrodotoxin (10(-6) M) and were not influenced by the CCK-A receptor antagonist loxiglumide (CR1505) (10(-7) M and 10(-6) M). The stimulant effect of CCK-8S was antagonized by the newly synthesized CCK-B selective receptor antagonist PD134308 (10(-7) M and 10(-6) M). These findings suggest that CCK-8 plays a neuromodulatory role in the regulation of GABAergic neuronal activity in the striatum. The activation of CCK-B receptors located on GABAergic neurons is involved in the GABA release-potentiating effect of CCK-8S in rat striatum.

Transmitter release by non-receptor activation of the alpha-subunit of guanine nucleotide regulatory protein in rat striatal slices

The effects of 5 mM NaF + 10 microM AlCl3, a direct activator of guanine nucleotide-binding proteins (G proteins), on the release of [3H]dopamine ([3H]DA), [3H]gamma-aminobutyric acid ([3H]GABA), and [3H]acethylcholine ([3H]ACh) were investigated in slices of rat striatum. When the tissue was exposed to NaF + AlCl3 the release of [3H]DA, [3H]GABA, and [3H]ACh was enhanced significantly. In a calcium-free solution the release of [3H]GABA and [3H]DA was increased by NaF+AlCl3 much more than in the presence of [Ca2+]o. In slice preparations taken from reserpinized animals, in which the vesicular storage of [3H]DA was therefore prevented, NaF + AlCl3 had no effect on [3H]DA release. HPLC analysis of the radioactivity of the perfusate showed that, in the presence of NaF + AlCl3, the content of dihydroxyphenylacetic acid (DOPAC) in perfusate samples increased significantly, while in pargyline-treated animals only the DA content was increased. Inhibition of DA carriers by nomifensine or low temperature prevented the effect of NaF + AlCl3. N-ethylmaleimide (NEM) preincubation did not modify the effect of NaF + AlCl3 on [3H]DA release Neomycin (0.1 mM), a phospholipase C (PLC) inhibitor, significantly decreased the effect of NaF + AlCl3 on [3H]DA and [3H]GABA release. The internal concentration of Ca2+ in synaptosomes was enhanced by NaF + AlCl3 in normal solution. However, [Ca2+]i was not influenced by NaF + AlCl3 in Ca(2+)-free medium. It is concluded that a non-receptor-mediated activation, by NaF + AlCl3, of the alpha-subunit of a G protein, results in a [Ca2+]o-independent release of DA and GABA, but not that of ACh.

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)

Opposing effects of dopamine D2 receptor stimulation on the spontaneous and the electrically evoked release of [3H]GABA on rat prefrontal cortex slices

The spontaneous and the electrically evoked release of [3H]GABA were studied in vitro on slices of rat medial prefrontal cortex. The slices were preincubated with [3H]GABA and then superfused with a Krebs' solution. The superfusion with a Ca(2+)-free medium progressively increased the spontaneous [3H]GABA release and strongly decreased the electrically evoked release of [3H]GABA (-65%). The effects of three dopaminergic D2 receptor agonists (RU24926, lisuride and LY171555) were studied on both the spontaneous and the electrically evoked [3H]GABA release. The spontaneous release of [3H]GABA was increased by exposure to each of these three D2 agonists. RU24926 produced a dose-dependent increase from 10(-9) to 3 x 10(-8) M and the maximal effect was totally abolished by the dopaminergic D2 receptor antagonist sulpiride (10(-5) M). With lisuride a progressive increase of [3H]GABA release was observed and a plateau value was reached with concentrations between 10(-7) and 10(-6) M. These effects were totally reversed by 10(-5) M sulpiride. The dose-response relation for LY171555 was bell-shaped, with a maximal effect being obtained with 10(-9) M) LY171555. This effect decreased with a higher concentration (10(-8) M) and finally was no longer observed for 10(-7) M LY171555. The maximal increase induced by LY171555 was totally abolished by 10(-5) M sulpiride. In contrast, the electrically evoked release of [3H]GABA was inhibited by these three D2 agonists. The IC50 value of the inhibition was 4.1 x 10(-8) M for RU24926 and 2 x 10(-7) M for lisuride. Sulpiride (10(-5) M) totally abolished the effect of 10(-7) M RU24926. In the concentration range of lisuride examined, a 50% reduction of the lisuride inhibition was obtained in the presence of sulpiride (10(-5) M). The dose-response curve obtained with LY171555 had a U-shape, with a maximal inhibition reached with 10(-8) M, whereas no effect was observed with 10(-6) M. The inhibition induced by 10(-8) M LY171555 was completely antagonized by 10(-5) M sulpiride. The D2 agonist-induced inhibition of the electrically evoked release of [3H]GABA was mimicked by dopamine endogenously released by 10(-5) M amphetamine. This effect was reversed by 10(-5) M sulpiride. Our data provide further evidence for a dopaminergic control of GABA interneurons in the prefrontal cortex. This regulation implies the activation of D2 dopaminergic receptors. The possible mechanisms underlying the opposite effects of D2 agonists on the spontaneous and the electrically evoked release of [3H]GABA are discussed.

Calcium-independent GABA release from striatal slices: the role of calcium channels

We have investigated the role of Ca2+ and Ca2+ channels in the modulation of GABA release. Brain slices prepared from rat striatum were preincubated with [3H]GABA, superfused with Krebs bicarbonate buffer, and exposed to electrical field stimulation (2 Hz for 3 min). Tritium efflux was measured as an index of GABA release. Both resting and evoked efflux were greatly accelerated by deleting Ca2+ from the medium and adding EGTA (1 mM). However, when the concentration of Mg2+ in the buffer was elevated to 10 mM, no effect of the Ca2(+)-deficiency was observed on resting release and its impact on evoked overflow was diminished. Moreover, addition of verapamil (10 microM), a Ca2+ channel blocking agent, reduced evoked overflow even in the absence of external Ca2+, while 4-aminopyridine (10 microM), a K+ channel inhibitor, enhanced GABA efflux in normal buffer but had no effect in the absence of Ca2+. Finally, we have shown previously that nipecotic acid, an inhibitor of high affinity GABA transport, increases GABA overflow in normal buffer, but blocks it in Ca2(+)-free buffer. Collectively, these results suggest that Ca2+ channels may play two roles in the regulation of depolarization-induced GABA release. Firstly, these channels permit a depolarization-induced influx of Ca2+ which then promotes GABA release. In addition, these channels influence GABA release through a mechanism that does not involve external Ca2+. Although the precise nature of this latter involvement is unclear, we propose that the Ca2+ channels serve to permit an influx of Na+, which in turn promotes Ca2(+)-independent release through an influence on the high affinity GABA transport system.

Activation of arginine metabolism to glutamate in rat brain synaptosomes in thioacetamide-induced hepatic encephalopathy: an adaptative response?

Crude (P2) synaptosomes derived from rats with acute hepatic encephalopathy (HE) induced with thioacetamide showed a slightly increased uptake of radiolabeled arginine (ARG) and a 2.5-fold enhanced conversion of newly taken-up ARG to both glutamate (GLU) and gamma-aminobutyric acid (GABA) as compared with control synaptosomes. Pulse treatment of the preloaded synaptosomes with a high potassium medium decreased their radioactive GLU and GABA content without affecting the content of the precursor ARG. This result, which was identical with control or HE preparations, appears to indicate that ARG contributes at least, in part, to the synthesis of neurotransmitter GLU or GABA. As measured in purified synaptosomal preparations, HE increased by about 50% the activities of arginase and ornithine-delta-aminotransferase--the two enzymes of the ARG to GLU shunt. It is postulated that increased conversion of ARG to GLU may compensate for excessive utilization of the latter amino acid as an ammonia trap during HE and, as such, may be considered as an adaptative response of the synaptic compartment to this pathological condition.

L-glutamate stimulates the efflux of newly taken up glutamine from astroglia but not from synaptosomes of the rat

Bulk isolated rat astrocytes loaded with [14C] glutamine (Gln) were superfused on glass fiber filters with a standard Krebs-Henseleit medium until the basal efflux of the radioactivity became constant. The samples were then pulse-treated with: L-glutamate (Glu), gamma-aminobutyric acid (GABA), L-aspartate (Asp), cold Gln, or catecholamines, each at 0.5 mM concentration. Of the neurotransmitters tested, only Glu stimulated the efflux of radio-labelled Gln. The effect of Glu was sodium-dependent and was not mimicked by the Glu-receptor agonists: N-methyl-D-aspartate (NMDA), quisqualate (Quis) or kainate (KA), indicating a transport-mediated mechanism. Glu did not stimulate the efflux of newly taken up glutamine from either crude or purified brain synaptosomes, which is in keeping with the existing evidence that astrocytes function as a glutamine donor to other compartments of the central nervous system.

A role for protein kinase C in the electrically evoked release of [3H] gamma-aminobutyric acid in rabbit caudate nucleus

A possible participation of protein kinase C (PKC) in depolarization-induced release of gamma-aminobutyric acid (GABA) in rabbit caudate nucleus was examined by means of phorbol esters and staurosporine. Slices of caudate nucleus were loaded with [3H]GABA, then superfused and stimulated electrically (3 ms, 5 Hz, 24 mA, 5 V/cm) for 2 min. Aminooxyacetic acid and the uptake inhibitor nipecotic acid were present throughout. The PKC activator 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB) markedly enhanced the evoked [3H]GABA release. In contrast, its biologically inactive isomer, 4 alpha-PDB, did not facilitate transmitter release. Staurosporine, an inhibitor of PKC, diminished [3H]GABA release and counteracted the effects caused by 4 beta-PDB. The above results suggest a participation of PKC in depolarization-induced GABA release in rabbit caudate nucleus. The mechanism underlying the modulation of GABA release by PKC seems to be independent of presynaptic GABA, dopamine and 5-hydroxytryptamine receptors.

Dopamine may influence striatal GABA release via three separate mechanisms

We have examined the influence of dopamine (DA) on the spontaneous and electrically evoked release of gamma-aminobutyric acid (GABA) from striatal slices prepared from rat brain. We observe that DA has at least 3 separate effects on GABA release. These effects include both excitatory and inhibitory influences, and involve DA receptors of the D1- and D2-subtypes as well as a mechanism that is not mediated by either of these receptors.

Characterization of [3H]GABA release from striatal slices: evidence for a calcium-independent process via the GABA uptake system

GABA can be released by depolarization even in the absence of external Ca2+. To investigate the underlying mechanism of this phenomenon, GABA release was studied using slices prepared from rat striatum. Slices were preincubated with [3H]GABA in the presence of beta-alanine and superfused with Krebs buffer. Total tritium efflux was measured as an index of GABA release. Electrical stimulation at 2 Hz for 3 min elevated resting tritium efflux approximately two-fold. Decreasing external Ca2+ to 0.1 mM increased basal tritium efflux and reduced electrically evoked overflow, while omitting Ca2+ entirely (and adding 1 mM EGTA) increased both basal efflux and evoked overflow. Tetrodotoxin (5 microM) abolished the evoked release of tritium but did not affect the resting outflow in either normal or Ca2+-deficient conditions. In the presence of normal Ca2+, nipecotic acid (0.1-1 mM), an inhibitor of GABA transport into neurons as well as glia, enhanced both spontaneous efflux and evoked overflow of tritium. Nipecotic acid also increased spontaneous release when external Ca2+ was reduced or removed; however, under these conditions electrically evoked overflow was reduced. These results suggest that the electrically evoked release of [3H]GABA from striatal slices is of neuronal origin, but can occur in part in the absence of external Ca2+. They further suggest that this Ca2+-independent release, which may co-exist with the Ca2+-dependent release, takes place via the same carrier system utilized for high-affinity GABA uptake.

Release of D-[3H]aspartate and [14C]GABA in rat hippocampus slices: effects of fatty acid-free bovine serum albumin and Ca2+ withdrawal

Extended incubation of hippocampus slices in the presence of fatty acid-free bovine serum albumin (FAF-BSA) strongly enhanced the release of D-[3H]aspartate and [14C]GABA induced by veratridine. Saturation of the FAF-BSA with oleic acid abolished the enhancing effect. Spontaneous release and K+-induced release were not significantly changed by the addition of FAF-BSA. Amino-oxyacetic acid in the medium enhanced the veratridine-induced release of D-[3H] aspartate. The spontaneous release of [14C]GABA was greatly increased by Ca2+ withdrawal. With the further addition of EGTA the spontaneous release in the absence of Ca2+ increased more than 8-fold over the measured in the presence of 1.5 mM Ca2+. The enhanced release caused by Ca2+ withdrawal was totally blocked by tetrodotoxin. The toxin was effective even when added after the spontaneous release in the absence of Ca2+ was already proceeding at a high rate. The veratridine-induced release of [14C]GABA was also considerably augmented by Ca2+ withdrawal. D-[3H]aspartate release, studied simultaneously with [14C]GABA by double labeling, did not show enhanced spontaneous release upon Ca2+ withdrawal. The findings provide evidence that the enhanced [14C]GABA release caused by Ca2+ withdrawal is mediated by voltage-dependent Na+ channels.

Potential involvement of a baclofen-sensitive autoreceptor in the modulation of the release of endogenous GABA from rat brain slices in vitro

The effects of the GABAA agonist, muscimol, and of the enantiomers of the GABAB agonist, baclofen, on the release of endogenous GABA from slices of the rat cerebral cortex, striatum and hippocampus were measured by means of a HPLC method with electrochemical detection. Moreover, the effect of the GABAA antagonist, bicuculline, and of the frequency of stimulation were studied in cortical slices. The amount of endogenous GABA released per impulse from cortical slices decreased by about 50% when the frequency was increased from 0.25 Hz to 1 Hz. This might indicate that GABA inhibited its own release (-)-Baclofen at 1 and 10 microM, but not its (+)-enantiomer, markedly inhibited the release of endogenous GABA, to a similar extent in all 3 areas investigated. The effect of (-)-baclofen was dependent on the frequency of stimulation: at lower frequencies (0.25 and 0.5 Hz) it was more marked than at a higher one (4 Hz). This would be expected from the results showing that the release of endogenous GABA decreases with increasing frequency, which suggests that this amino acid inhibits its own release. Muscimol at 10 microM, on the other hand, was ineffective in all 3 areas at a stimulation frequency of 0.5 Hz. Bicuculline (10 microM) at 4 Hz, at which autosuppression of GABA release is maximal did not enhance the release of endogenous GABA from cortical slices. With cerebellar or nigral slices, no adequate stimulation-induced release of endogenous GABA could be obtained under comparable conditions. These data are compatible with, but do not prove the existence of GABAB-type presynaptic autoreceptors modulating the release of this amino acid. More definite conclusions may possibly be drawn when a GABAB antagonist becomes available, which is expected to enhance GABA release under appropriate conditions.

The measurement of the release of endogenous GABA from rat brain slices by liquid chromatography with electrochemical detection

A method for the determination of GABA by derivatization with 2,4,6-trinitrobenzenesulphonic acid and subsequent separation and quantitation by HPLC with electrochemical detection was characterized with respect to specificity, reproducibility and sensitivity. No other amino acid occurring in significant amounts in the brain was found to interfere; however, adequate separation of the derivatives of GABA and tryptophan must be carefully checked in each experiment. The sensitivity of the method is essentially determined by baseline noise, which mainly depends on the quality of the HPLC pump; under our conditions, it was about 2 ng/ml analyte. The coefficients of variation determined at two different concentrations relevant for the subsequent experiments were well below 10%. The method proved useful for the assessment of endogenous release of GABA from superfused rat cortical slices by electrical stimulation, which, in contrast to the basal release, was found to be completely calcium-dependent at stimulation frequencies of 5 and 12 Hz, under our conditions. Both stimulated and basal release of GABA was enhanced 4-5-fold by the inhibitor of GABA uptake, SK&F 89976 (10 microM).

Evidence for a neurotransmitter function of acetylcholine in rabbit superior colliculus

Acetylcholinesterase staining and studies on the uptake of [3H]choline into the subsequent efflux of tritium from collicular slices were carried out in order to provide evidence for a neurotransmitter function of acetylcholine in rabbit superior colliculus. Acetylcholinesterase staining was dense and homogeneous in superficial layers whereas the staining was arranged in patches with slightly higher density caudally than rostrally in the intermediate layers. The accumulation of tritium in slices incubated with [3H]choline depended on time, temperature and concentration, and was inhibited by hemicholinium-3. Accumulation was slightly higher in caudal than in rostral slices. Electrical stimulation enhanced tritium outflow from slices preincubated with [3H]choline. Tetrodotoxin and a low calcium medium inhibited the evoked overflow whereas hemicholinium-3 caused an enhancement. Oxotremorine decreased the evoked overflow; atropine prevented this effect. The opioids [D-Ala2, MePhe4, Glycol5]enkephalin, [D-Ala2, D-Leu5]enkephalin and ethylketocyclazocine caused an inhibition. The effects of the latter two agonists were antagonized by naloxone. The GABAB-receptor-agonist (-)-baclofen decreased the evoked overflow at lower concentrations than GABA, whereas the GABAA-receptor-agonist muscimol was ineffective. Serotonin produced an inhibition which was prevented by metitepin, alpha- and beta-adrenoceptor as well as dopamine-receptor ligands caused no change. It is concluded that in the rabbit superior colliculus the pattern of acetylcholinesterase staining is comparable, but not identical to the distribution in other species. The accumulation of [3H]choline, as well as the tetrodotoxin-sensitive and calcium-dependent overflow of tritium upon electrical stimulation (reflecting presumably release of [3H]acetylcholine) indicate that acetylcholine has a neurotransmitter function in this tissue. The release of [3H]acetylcholine was modulated by various transmitter substances and related compounds. The pattern of modulation of release differed from the pattern in other cholinergically innervated tissues.

A search for receptors modulating the release of gamma-[3H]aminobutyric acid in rabbit caudate nucleus slices

Various putative striatal transmitters and related compounds were studied for their effects on the release of gamma-aminobutyric acid (GABA) from slices of the head of the rabbit caudate nucleus. The slices were preincubated with [3H]GABA and then superfused and stimulated electrically at 5 or 20 Hz. Aminooxyacetic acid was present throughout. The main changes observed were the following. The basal and, less consistently, the electrically evoked overflow of [3H]GABA were enhanced by 3,4-dihydroxyphenylethylamine (dopamine), an effect not blocked by cis-flupentixol or domperidone and not mimicked by apomorphine and D1-selective agonists. The electrically evoked overflow was diminished by 5-hydroxytryptamine (serotonin); the inhibition was prevented by methiothepin. The basal but not the electrically evoked overflow was enhanced by carbachol; acetylcholine and nicotine also accelerated the basal outflow whereas oxotremorine caused no consistent change; the effect of carbachol and acetylcholine were blocked by hexamethonium but not by atropine or by tetrodotoxin. These findings indicate that the GABA neurons in the caudate nucleus may be stimulated by dopamine, although the receptor type involved remains unclear; inhibited by serotonin; and stimulated by acetylcholine acting via a nicotine receptor. However, all drug effects observed were relatively small. No evidence was obtained for autoreceptors, alpha 2-adrenoceptors or receptors for opioids, adenosine or substance P at the GABA neurons.