IN VIVO RELEASE OF ENDOGENOUS GABA FROM RAT SUBSTANTIA NIGRA MEASURED BY A NOVEL METHOD

Ethanolamine: A Potential Promoiety with Additional Effects in the Brain

Ethanolamine is a bioactive molecule found in several cells, including those in the central nervous system (CNS). In the brain, ethanolamine and ethanolamine-related molecules have emerged as prodrug moieties that can promote drug movement across the blood-brain barrier. This improvement in the ability to target drugs to the brain may also mean that in the process ethanolamine concentrations in the brain are increased enough for ethanolamine to exert its own neurological ac-tions. Ethanolamine and its associated products have various positive functions ranging from cell signaling to molecular storage, and alterations in their levels have been linked to neurodegenerative conditions such as Alzheimer's disease. This mini-review focuses on the effects of ethanolamine in the CNS and highlights the possible implications of these effects for drug design.

GABAergic transmission in rat pontine reticular formation regulates the induction phase of anesthesia and modulates hyperalgesia caused by sleep deprivation

The oral part of the pontine reticular formation (PnO) contributes to the regulation of sleep, anesthesia and pain. The role of PnO γ-aminobutyric acid (GABA) in modulating these states remains incompletely understood. The present study used time to loss and time to resumption of righting response (LoRR and RoRR) as surrogate measures of loss and resumption of consciousness. This study tested three hypotheses: (i) pharmacologically manipulating GABA levels in rat PnO alters LoRR, RoRR and nociception; (ii) propofol decreases GABA levels in the PnO; and (iii) inhibiting GABA synthesis in the PnO blocks hyperalgesia caused by sleep deprivation. Administering a GABA synthesis inhibitor [3-mercaptopropionic acid (3-MPA)] or a GABA uptake inhibitor [nipecotic acid (NPA)] into rat PnO significantly altered LoRR caused by propofol. 3-MPA significantly decreased LoRR for propofol (-18%). NPA significantly increased LoRR during administration of propofol (36%). Neither 3-MPA nor NPA altered RoRR following cessation of propofol or isoflurane delivery. The finding that LoRR was decreased by 3-MPA and increased by NPA is consistent with measures showing that extracellular GABA levels in the PnO were decreased (41%) by propofol. Thermal nociception was significantly decreased by 3-MPA and increased by NPA, and 3-MPA blocked the hyperalgesia caused by sleep deprivation. The results demonstrate that GABA levels in the PnO regulate the time for loss of consciousness caused by propofol, extend the concept that anesthetic induction and emergence are not inverse processes, and suggest that GABAergic transmission in the PnO mediates hyperalgesia caused by sleep loss.

Ethanolamine and related amino alcohols increase basal and evoked release of [3H]-D-aspartic acid from synaptosomes by enhancing the filling of synaptic vesicles

This research examines the effects of ethanolamine and other amino alcohols on the dynamics of acridine orange (AO), oxonol V, and [3H]-D-aspartic acid in synaptic preparations isolated from mammalian brain. Ethanolamine concentration-dependently enhanced AO release from synaptosomes. Similar effects were observed with methylethanolamine and dimethylethanolamine, but not choline. The enhancement of AO efflux by ethanolamine was independent of extrasynaptosomal calcium (in contrast to KCl-induced AO efflux), was unaffected by tetrodotoxin and did not involve depolarization of the synaptosomal plasma membrane. KCl was unable to release AO from synaptosomes following exposure to ethanolamine, however ethanolamine and other amino alcohols were found to enhance both basal and KCl-evoked release of [3H]-D-aspartic acid from synaptosomes. Using isolated synaptic vesicles we demonstrate that amino alcohols are able to 1) abolish the ATP-dependent intravesicular proton concentration (i.e. stimulate efflux of AO) in a similar way to carbonyl cyanide m-chlorophenylhydrazone (CCCP), 2) increase the ATP-supported transvesicular membrane potential (i.e. quench oxonol V fluorescence) in contrast to CCCP and 3) enhance intravesicular uptake of [3H]-D-aspartic acid. These results suggest that positively charged, membrane impermeant amino alcohol species are generated within synaptic vesicles as they sequester protons. Cationic forms of these amino alcohols boost the transvesicular electrical potential which increases transmitter uptake into synaptic vesicles and facilitates enhancement of basal and evoked release of transmitter. Our data suggest a potential role for ethanolamine and related amino alcohols in the regulation of synaptic vesicle filling. These findings may also have relevance to neuropathophysiological states involving altered production of ethanolamine.

Depolarization-induced release of ethanolamine from brain synaptic preparations in vitro

The release of ethanolamine from mouse brain synaptosomes and synaptoneurosomes has been investigated. The depolarizing agents veratridine (50 microM), KCl (35 mM) and 4-aminopyridine (2 mM) enhanced the release of [3H]ethanolamine from preloaded synaptosomes under superfusion conditions. Tetrodotoxin (2 microM) strongly inhibited veratridine- and 4-aminopyridine-stimulated release of [3H]ethanolamine but had no effect on KCl-evoked or resting release. In the absence of calcium, a reduction in the resting release of [3H]ethanolamine occurred and release evoked by veratridine, and KCl was markedly reduced. Exposure of preloaded synaptosomes to 5 mM ethanolamine (but not 5 mM serine or 5 mM choline) calcium-dependently increased the efflux of [3H]ethanolamine, however, this was not accompanied by membrane depolarization. When these experiments were performed using synaptoneurosomes, qualitatively similar results were obtained. The resting and evoked release of [3H]ethanolamine was however approximately 2.5-fold higher compared to synaptosomes on a brain equivalent basis, suggesting that uptake and release occur at sites in addition to the nerve ending. Our data are consistent with the idea that a significant amount of ethanolamine accumulates presynaptically and undergoes calcium-dependent release upon depolarization possibly via classical exocytosis. In contrast, ethanolamine-induced release of [3H]ethanolamine likely involves mostly diffusional exchange across the neuronal membrane rather than base exchange. The present results add support to the concept that ethanolamine may play a role as a synaptic signaling molecule in mammalian brain.

Alcohol stimulates the release of dopamine in the ventral pallidum but not in the globus pallidus: a dual-probe microdialysis study

The mesoaccumbens dopamine system has been hypothesized to be a common neural substrate mediating the actions of various drugs of abuse, including ethanol. However, the involvement of the mesopallidal dopamine system has received very little attention. The present study examined the effects of intraperitoneal (IP) ethanol administration on the extracellular levels of dopamine in the ventral pallidum (VP) and globus pallidus (GP) of Wistar rats. Rats were bilaterally implanted with microdialysis probes aimed at the VP and GP or nucleus accumbens (NAc) and dorsal striatum (dSTR). During microdialysis testing, rats with probes located in the VP and GP were injected IP with sterile saline or 15% (v/v) ethanol in saline at doses of 0.75, 1.5, or 2.25 g/kg. Rats with NAc and dSTR probes were injected with saline or 2.25 g/kg ethanol. The IP administration of 1.5 and 2.25 g/kg ethanol significantly (p <0.05) elevated the extracellular levels of dopamine in the VP (maximal increase: 136 and 182% of baseline, respectively) but not in the GP. No effects on extracellular dopamine levels were observed following the IP injections of 0.75 g/kg ethanol or saline. The IP administration of 2.25 g/kg ethanol significantly (p <0.05) elevated the extracellular levels of dopamine in the NAc (maximal increase: 198% of baseline) and dSTR (maximal increase: 155% of baseline). Analysis of the effects of 2.25 g/kg ethanol on dopamine release revealed greater increases in the VP, NAc, and dSTR compared to the GP. The data suggest that the mesopallidal, mesoaccumbens, and nigrostriatal dopamine systems are more sensitive to the effects of ethanol than the nigropallidal dopamine system.

Functional neuroanatomy of the nigrostriatal and striatonigral pathways as studied with dual probe microdialysis in the awake rat--I. Effects of perfusion with tetrodotoxin and low-calcium medium

In the present study we employed the dual probe approach to investigate functional interactions between the nigrostriatal dopaminergic and striatonigral GABAergic pathways in the awake, freely moving rat and their role in motor function. One microdialysis probe of concentric design was implanted in the substantia nigra pars reticulata and another in the ipsilateral dorsolateral striatum. Perfusion with a low-Ca2+ (0.1 mM) medium and with the voltage-dependent Na(+)-channel blocker tetrodotoxin (10 microM) was alternatively performed in both brain regions and the dialysate dopamine, glutamate and GABA levels were simultaneously measured in the dorsolateral striatum, whereas GABA levels alone were monitored in the substantia nigra. Perfusion with a low-Ca2+ medium in the substantia nigra pars reticulata did not affect local GABA levels, but transiently increased striatal dopamine release (+40%) without modifying striatal glutamate and GABA levels. Conversely, intranigral perfusion with tetrodotoxin transiently increased local GABA levels (+40%), while it decreased striatal dopamine (-60%) and increased glutamate (+70%) and GABA (+50%) levels. Perfusion with a low-Ca2+ medium in the dorsolateral striatum reversibly decreased local dopamine (-70%), glutamate (-20%) and GABA (-20%) levels, while local perfusion with tetrodotoxin decreased dopamine (-70%), increased glutamate (+30%) but did not affect dialysate GABA levels in this brain area. Neither of these intrastriatal treatments significantly affected GABA levels in the substantia nigra. Intranigral but not intrastriatal perfusion with tetrodotoxin was also associated with an increase in spontaneous locomotor activity as expressed by contralateral turning. Intranigral and intrastriatal perfusion with low-Ca2+ medium did not influence locomotor activity. On the basis of these neurochemical and behavioural findings, we propose a new dynamic model for the study of motor behaviour as mediated by basal ganglia circuitry.

Impulse-dependent and tetrodotoxin-sensitive release of GABA in the rat's substantia nigra measured by microdialysis

gamma-Aminobutyric acid (GABA) release in the rat substantia nigra pars reticulata (SNR) was studied by microdialysis coupled with high-performance liquid chromatography and fluorimetric detection. Electrical stimulation of striatonigral axons in the internal capsule (IC) increased nigral GABA release in conscious and halothane-anaesthetized rats. This was prevented by intranigral infusion of tetrodotoxin (TTX) while basal GABA release was unaffected. Calcium-free, cobalt-containing (2 mM CoCl2) artificial cerebrospinal fluid reduced basal GABA overflow but not that evoked with high K+ (100 mM). Extracellular levels of glutamate (GLU) and taurine (TAU) were not modified by IC stimulation, TTX or 0 Ca2+ although high K+ promoted GABA and TAU release but not that of GLU. These data demonstrate an impulse-and sodium-dependent release of GABA from nigral afferent neurones which contribute little to the extracellular concentration of GABA under steady-state conditions.

Characterization of extracellular GABA in the substantia nigra reticulata by means of brain microdialysis

Brain microdialysis was used to characterize extracellular gamma-aminobutyric acid (GABA) in the substantia nigra reticulata (SNR) of freely moving rats. The extracellular GABA in the SNR was characterized using acutely implanted probes (4-8 h after surgery; day 1) and chronically implanted probes (24 h after surgery; day 2). 3-Mercaptopropionic acid, a glutamic acid decarboxylase inhibitor, was used to identify GABA. This drug induced an immediate decrease in the extracellular GABA levels to 40% of basal values, suggesting that the detected GABA is, at least in part, newly synthesized. The basal levels of extracellular GABA measured either on day 1 or day 2 were not affected by infusion of micromolar amounts of tetrodotoxin. Therefore, a direct coupling between GABA dialysate concentrations and nerve-impulse flow does not seem to exist. Infusion of the GABA uptake inhibitor nipecotic acid (0.5 mmol/l) resulted in a 4-fold increase in the dialysate levels of GABA lasting at least for 3 h on both days. K+ stimulation (60 mmol/l) increased extracellular GABA levels in the SNR to 450% of basal values. This effect again did not differ significantly on day 1 and day 2. The origin of the extracellular GABA in the SNR, as recorded by microdialysis under the two experimental conditions, is discussed.

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)

In vivo release of newly synthesized [3H]GABA in the substantia nigra of the rat: relative contribution of GABA striato-pallido-nigral afferents and nigral GABA neurons

The release of [3H]gamma-aminobutyric acid ([3H]GABA) continuously formed from [3H]glutamine has been measured with a push-pull cannula implanted in the substantia nigra of the rat anesthetized with ketamine. Consistent with the high density of GABA terminals coming from both the striato-pallido-nigral afferents, and from GABA nigrofugal neurons, our results showed that a large amount of [3H]GABA was spontaneously released in the reticulata, about 4 times higher than in the compacta. In the absence of calcium the spontaneous [3H]GABA release was reduced (-30%), as well as the K(+)-induced release of [3H]GABA (-66%). Bicuculline (10(-4) M) did not affect the K(+)-evoked release of [3H]GABA, suggesting that autoreceptors on GABA afferent fibers are distinct from the GABAA subtype. Partial lesions of striato- and pallido-nigral GABA neurons with kainic acid (1.2 micrograms) decrease by 40% the glutamic acid decarboxylase (GAD) activity in the ipsilateral SN without decreasing the spontaneous release of [3H]GABA; even following extensive lesions with kainic acid (2.5 micrograms), GAD activity (-72%) and spontaneous [3H]GABA release (-83%) were not completely abolished. These results suggest that a non-negligible contribution of GABA nigral neurons accounts for the spontaneous GABA release measured in the substantia nigra. This is further supported by the decrease (-20%), and the increase (+40%) of [3H]GABA release produced by the local application of glycine (10(-6) M), and bicuculline (10(-4) M), which respectively, inhibits and activates the nigral neuron activity. The contribution of nigral GABA neurons to the amount of [3H]GABA release from the substantia nigra, is likely linked to their high spontaneous firing rate.

Regulation of neurotensin-containing neurons in the rat striatum. Effects of unilateral striatal lesions with quinolinic acid and ibotenic acid on neurotensin content and its binding site density

Recently, we reported bilateral increases in striatal neurotensin (NT) levels following unilateral 6-hydroxydopamine lesion of the nigrostriatal dopaminergic pathway. In the present study, the effect of unilateral striatal lesions with quinolinic acid (QA, 300 nmol) or ibotenic acid (IBO, 130 nmol) on striatal NT levels and binding site densities were analyzed in order to investigate other possible regulations of NT systems. QA and IBO injection decreased gamma-aminobutyric acid (GABA) levels and [125I]iodosulpride (a specific D2 receptor antagonist) binding site densities in the lesioned striatum, indicating degeneration of striatal intrinsic neurons. Striatal dopaminergic terminals were not altered by QA as shown by the lack of changes in [3H]dihydrotetrabenazine [( 3H]TBZOH, a specific ligand of the vesicular monoamine transporter) binding site densities. Moreover, QA lesion induced an increase in NT levels and a decrease in NT binding sites in the lesioned striatum without any change in the contralateral structure. In contrast to QA, IBO might destroy a certain proportion of dopaminergic terminals in the lesioned striatum, as shown by a 54% decrease in [3H]TBZOH binding. Furthermore, IBO lesion enhanced striatal NT levels bilaterally, while NT binding sites decreased in the lesioned striatum and increased in the contralateral side. The present results suggest that not only dopaminergic neurons but also striatal intrinsic neurons may control NT systems in the striatum.

Evoked release of endogenous amino acids from rat striatal slices and its modulation

The release of endogenous gamma-aminobutyric acid (GABA), glutamate and aspartate stimulated by high K+ was studied by superfusing rat striatal slices. Stimulation with 5 min of 30 mM K+ was applied twice (S1 and S2) at a 20 min interval. The maximum release of GABA following stimulation was 40 (at S1) and 26 (at S2) times greater than the basal release. S1 and S2 each produced a maximum release of almost same magnitude for both glutamate and aspartate (about 2.5 times basal release). The removal of Ca2+ from the perfusion medium reduced the maximum release of these amino acids by more than 80% without affecting basal release significantly. Striatal slices were next stimulated in the same way after the addition of apomorphine or haloperidol to the perfusion medium. Apomorphine, 10 or 100 microM, reduced the K+-evoked release of GABA (by 24% at S1 and 35% at S2 with 10 microM; by 37% at S1 and 47% at S2 with 100 microM) but failed to affect the simultaneous release of glutamate and aspartate. Haloperidol (1 microM) reduced the S1-induced release of GABA but had no significant effect on the subsequent S2-induced release. The evoked release of aspartate or glutamate did not respond significantly to haloperidol, but there was a tendency to a decrease in aspartate release similar to the decrease seen for GABA release, particularly with S1. Based on these results, we discuss the property of GABA, aspartate and glutamate as neurotransmitters and the possible dopaminergic regulation of the release of these amino acids.

Compartmental distribution of endogenous amino acids in the substantia nigra of the rat

Using a push-pull cannula we have monitored the spontaneous efflux of 8 endogenous amino acids into perfusates of the substantia nigra of the rat. The extracellular concentrations of the amino acids were estimated and compared to their respective tissue levels. High intra-/extracellular concentration ratios were found for gamma-aminobutyric acid (GABA), aspartate, glutamate and taurine. Much lower values were found for glycine, alanine, serine and glutamine. These results provide evidence for possible neurotransmitter roles for aspartate, glutamate and taurine in the substantia nigra in addition to that, long recognized, for GABA.

Suppression of preoptic GABA release caused by push-pull-perfusion with sodium valproate

The in vivo-effects of various concentrations of the anticonvulsant drug sodium valproate--within and above the therapeutic range for humans (40-100 micrograms/ml)--on the release of gamma-aminobutyric acid (GABA) were studied perfusing the preoptic area of unanaesthetized, freely moving ovarectomized rats through push-pull-cannulae at a flow rate of 20 microliters/min with a fraction period of 5 and 15 min, respectively. Local treatment with 40, 80, 100, and 200 micrograms valproate/ml perfusion medium induced a highly significant decrease in preoptic GABA release. After return to valproate-free medium this effect was reversible. A rapid onset and termination of the valproate effect within 5 min could be observed. Going higher with valproate concentrations the suppressive effect became less and at supratherapeutic valproate levels of 1600 micrograms/ml CSF an increase in GABA release could be observed in 4 out of 8 animals. This does response relationship points to a biphasic effect of valproate on the available amount of GABA in the synaptic cleft, which may be produced by at least two different dose-dependent mechanisms of action. The present results indicate that the action of therapeutic concentrations of valproate involves an alteration of GABAergic transmission different from increasing synaptic GABA release. Nevertheless, the data suggest that valproate action, at least at the level of the preoptic area, involves an enhancement of GABAergic transmission causing--via a negative feedback mechanism--the observed suppression of GABA release into the synaptic cleft.

In vivo analysis of cortical amino acid neurotransmitters collected in the rat by a new double lumen push-pull catheter system

The release of both endogenous and newly synthesized amino acid neurotransmitters was examined simultaneously in different areas of the cerebral cortex in the freely moving rat. An array of push-pull guide tubes was implanted permanently to rest above the frontal, parietal, temporal and occipital areas of the cortex of each rat. Then a new double-lumen catheter system, specially adapted for localized push-pull perfusion of the conscious animal, was used to perfuse an artificial cerebrospinal fluid at each cortical site. For the new synthesis experiments, 0.5 microCi of [14C]glucose in a volume of 2.0 microliter was first microinjected into the perfusion site as a precursor to label amino acids. After the site was perfused at a rate of 12.0 microliter/min, each of the samples was assayed by two-dimensional thin-layer chromatography. In a second analysis, the content of six endogenous amino acids present in unlabeled samples of push-pull perfusate was quantified by high-performance liquid chromatography analysis with electrochemical detection. The results showed a notable homogeneity among each of the four cortical areas in the content of four of the six amino acids examined. Endogenous glutamine exhibited the highest proportional content in the cortical perfusates, whereas glutamic acid was proportionally higher in terms of new synthesis. An anatomical analysis revealed that the level of endogenous glutamic acid in the frontal area was significantly lower than that found in the occipital or temporal regions of the rat's cortex. An opposite result was obtained when the proportional synthesis of glutamic acid from [14C] glucose was compared in different cortical regions in that a statistically higher release occurred in the frontal than in the occipital cortex.

Striatal grafts in rats with unilateral neostriatal lesions--II. In vivo monitoring of GABA release in globus pallidus and substantia nigra

GABA release was recorded in vivo by push-pull perfusion from the globus pallidus and substantia nigra of control rats, rats with unilateral ibotenic acid lesions of the neostriatum, and rats with embryonic striatal tissue grafts implanted in the lesioned striatum. The lesions reduced baseline levels of GABA release to 5% of control levels in the globus pallidus and to 13% of control levels in the substantia nigra pars reticulata. GABA release was substantially restored in both the globus pallidus and substantia nigra of the grafted rats, to 34 and 60%, respectively. Peripheral injection of the dopaminergic stimulant methamphetamine induced a short (lasting approximately 20 min) 4-5 fold increase in GABA release in the intact globus pallidus and a longer (lasting longer than 80 min) increase in the substantia nigra. The stimulatory effect of methamphetamine on GABA release was completely abolished in both sites by the strial lesions, suggesting that the effect was mediated via a direct or indirect dopaminergic action on striatal output neurons. The grafts reinstated methamphetamine-induced stimulation of GABA release in striatal output targets to a level (as a proportion of baseline) that was similar to that seen in the control rats. The results support the view that activation of the dopaminergic inputs to the striatum is functionally excitatory on the major striatal output projections to the globus pallidus and substantia nigra pars reticulata. The results also support the hypothesis that striatal grafts have the capacity to become functionally incorporated by reciprocal graft-host connections into the neural circuitry of the host brain.

Release of endogenous amino acids, dopamine, and cyclic AMP from the rat brain: methodological aspects and mutual interferences

The release from the rat brain of endogenous dopamine, its metabolites, cyclic AMP, transmitter, and other amino acids was studied with push-pull cannula perfusion or dialysis. The amino acid output from the striatum with either perfusion technique was virtually identical. Relative to the tissue content, the egress of the dopamine metabolites and of the non-transmitter (metabolic) amino acids was higher than that of the transmitter substances. The intracellular second messenger cyclic AMP was also released into the perfusate, and its content was enhanced by local application of various biogenic amines. During and after electroconvulsive shocks the release (and presumably the formation) of alanine was enhanced, which may reflect increased glycolysis and transamination in the brain. Following local application of tricyclic antidepressants, the content of transmitter amino acids in push-pull perfusates of the rat thalamus and striatum was enhanced several fold. Such an increase was not found following systemic injection of high doses of mianserin and desmethylimipramine. In contrast, the effect of amphetamine on dopamine release was seen after both local and systemic applications. Locally applied dopamine produced a specific decrease in the release of gamma-aminobutyric acid in both the substantia nigra and in the striatum. Muscimol decreased, whereas picrotoxine enhanced the release of GABA from the rat striatum. Our observations emphasize that transmitter release and brain metabolism can be monitored by perfusion techniques and that the effects of drugs may depend on the route of administration. As has frequently been shown with in vitro techniques, our data indicate the importance of local control of transmitter release in vivo by both autoreceptors and alloreceptors in addition to firing activity.

Rapid method for micro-analysis of endogenous amino acid neurotransmitters in brain perfusates in the rat by isocratic HPLC-EC

A simplified method is described for the isocratic analysis of endogenous amino acid neurotransmitters contained in brain perfusates by high performance liquid chromatography (HPLC) with electrochemical detection (EC). Pre-column o-phthalaldehyde (OPA) tert-butylthiol derivatives of the amino acids were injected into a C18 3 microns column. After linear concentration curves for standard solutions were obtained, the content of 6 amino acid neurotransmitters was analyzed in push-pull perfusates obtained from the hypothalamus and cerebral cortex of the unrestrained rat. Each analysis which included the simultaneous quantification of aspartic acid, glutamic acid, glutamine, glycine, taurine and gamma-aminobutyric acid (GABA), was completed in less than 15 min. The sensitivity of the assay ranged from 1.0 to 5.0 pmol of each amino acid contained within a 20 microliters aliquot of each perfusion sample.

Effects of KCl-induced depolarization on the GABA concentration in the corpus striatum and in the substantia nigra

The effects of potassium ion depolarization on the concentration, the synthesis and the utilization of GABA as well as the effects on the concentrations of glutamine, dopamine (DA) and DOPAC in the substantia nigra and in the corpus striatum of the rat were investigated. An intranigral KCl injection did not influence the GABA concentration or the synthesis and utilization of GABA in the substantia nigra. Following an intrastriatal KCl injection the GABA concentration increased and the glutamine concentration decreased in the corpus striatum. A marked increase in the DNA concentration in the corpus striatum was seen following an intranigral KCl injection, whereas the DA concentration decreased and the DOPAC concentration increased following an intrastriatal KCl injection. It is concluded that the GABA in the substantia nigra and in the corpus striatum is affected differently by potassium ions, perhaps due to the different cellular localization of GABA in the two structures. The increase of GABA in the corpus striatum was accompanied by a decreased glutamine concentration indicating that glutamine might be a precursor of GABA.

Amino acids in rat striatal dialysates: methodological aspects and changes after electroconvulsive shock

Extracellular levels of amino acids were estimated in dialysates of the rat striatum that were collected 1, 2, and/or more than 5 days after surgery, before (resting release) and during exposure to high K concentrations (50 mM) or electroconvulsive shocks. The resting release of several amino acids (Glu, Asn, Thr, Tau, Tyr, Gly, and Ala) was higher 9 days as compared to 1 day after surgery. In the 1-day preparation the resting release correlated highly with that observed with push-pull cannulas. The correlation with the tissue content of the amino acids was high only when they were divided into two groups (putative transmitters and metabolic intermediates). High K exposure produced increased output of Ala, ethanolamine (Eam), Asp, Glu, Tau, and Gly and a decrease in the egress of Gln 1 or 2 days after surgery. The effects on Asp and Glu had disappeared, and that on Gln reversed after 4-9 days. Electrically induced convulsions produced increased output of Ala, Gln, and Eam 1 or 2 days and 2 weeks after implantation of the probe. Changes were seen not only during but also (and some cases even more prominent) after the seizure. This study shows the usefulness of dialysis to monitor extracellular transmitter amino acids in the striatum of conscious rats (also bilateral dialysis was possible) for only a limited time after implantation of the probe. The dialysis method is suitable for longer time, when metabolic changes in amino acids are to be followed. In addition to transmitter release, glycolysis can be monitored by the measurement of Ala in the dialysate.

Improved method for isolating synaptosomes from 11 regions of one rat brain: electron microscopic and biochemical characterization and use in the study of drug effects on nerve terminal gamma-aminobutyric acid in vivo

A procedure is described for the rapid preparation of nerve ending particles (synaptosomes) from 11 regions of one rat brain. The synaptosomal fractions have been characterized by electron microscopy and determination of four marker enzymes, i.e., glutamate decarboxylase (GAD), acetylcholinesterase, succinate dehydrogenase, and glycerol 3-phosphate dehydrogenase. Comparison with a much lengthier standard (Ficoll-sucrose) preparation showed that the synaptosomal yield of the new procedure was substantially better as judged by both morphological evaluation and protein recovery. The improved synaptosome preparation was used for determination of regional gamma-aminobutyric acid (GABA) levels in synaptosomal fractions. The postmortem increase in GABA level during removal and dissection of brain tissue and homogenization and fractionation procedures could be minimized by rapid processing of the tissue at low temperatures and inclusion of the GAD inhibitor 3-mercaptopropionic acid (3-MP; 1 mM) in the homogenizing medium. The addition of GABA (0.2 mM) to the homogenizing medium did not alter the GABA levels in the synaptosomes, indicating that no significant redistribution of GABA occurred during subcellular fractionation in sodium-free media. Synaptosomal GABA levels determined in the 11 rat brain areas showed the same regional distribution as the GABA-synthesizing enzyme GAD. On the basis of these findings, it was suggested that the synaptosome preparation could be used to evaluate the in vivo effects of drugs on nerve terminal GABA. Treatment of rats with a convulsant dose of 3-MP (50 mg/kg i.p.) 3 min before decapitation significantly lowered synaptosomal GABA levels in olfactory bulb, hippocampus, thalamus, tectum, and cerebellum. The 3-MP-induced seizures and reduction of GABA levels could be prevented by administration of valproic acid (200 mg/kg i.p.) 15 min before the 3-MP injection. The data indicate that the improved synaptosome preparation offers a convenient method of preparing highly purified synaptosomes from a large number of small tissue samples and can provide useful information on the in vivo effects of drugs on regional GABA levels in nerve terminals.

Potassium-induced release of endogenous amino acids in the guinea pig cochlea

Guinea pig cochleae were perfused with high-potassium solutions to depolarize hair cells artificially and induce the release of afferent neurotransmitter. Sequential injections of artificial perilymph containing 5 mM KCl, then 50 mM KCl, and finally 5 mM KCl were made into the scala tympani. This injection sequence was conducted under either normal divalent-cation conditions (2.0 mM CaCl2, 1.0 mM MgCl2) or calcium-deficient conditions intended to antagonize evoked transmitter release (0.1 mM CaCl2, 20.0 mM MgCl2). The levels of 21 endogenous primary amines in effluent collected from the scala vestibuli were determined by gradient-elution, reverse-phase HPLC using o-phthaldialdehyde-thiol adducts with fluorescence detection. Analyses indicated effluent concentrations of glutamate, taurine, and a coeluting taurine-gamma-aminobutyrate (GABA) fraction (but not GABA alone) increased significantly after exposure to 50 mM KC1 and returned to baseline levels after reintroduction of 5 mM KC1 under normal divalent-cation conditions. Correspondent changes in the release of these constituents were significantly attenuated under calcium-deficient conditions. This was not the case for potassium-induced changes in the release of arginine, aspartate, and isoleucine. These data are consistent with the hypothesis that the receptoneuronal transmitter is glutamate and further suggest a calcium-dependent mechanism involving taurine.

Role of the ventromedial nucleus of the thalamus in motor behaviour--I. Effects of focal injections of drugs

An assortment of drugs was injected into one or both ventromedial nuclei of the thalamus, to see how these influenced stereotypy, locomotion and posture in spontaneously behaving and actively rotating rats. Unilateral intrathalamic muscimol promoted weak ipsiversive circling, while bilateral treatment gave catalepsy. Similar injections of 4-amino-hex-5-enoic acid, which inhibits gamma-aminobutyrate metabolism, raised gamma-aminobutyrate levels in the ventromedial nuclei more than three-fold yet had none of these behavioural effects. The indirectly acting gamma-aminobutyrate agonists flurazepam and cis-1,3-aminocyclohexane carboxylic acid had little effect on posture and locomotion and, like muscimol and 4-amino-hex-5-enoic acid, elicited only very weak stereotypies. Procaine behaved like the gamma-aminobutyrate antagonist bicuculline, provoking vigorous locomotor hyperactivity and teeth chattering if given uni- or bilaterally. Pretreatment of one ventromedial nucleus with muscimol or 4-amino-hex-5-enoic acid, and to a lesser extent flurazepam or cis- 1,3-aminocyclohexane carboxylic acid, gave rise to pronounced ipsilateral asymmetries when combined with a large systemic dose of apomorphine. Contraversive rotations were initiated by unilateral stereotaxic injection of muscimol into the substantia nigra pars reticulata, or with apomorphine from the supersensitive striatum in unilaterally 6-hydroxydopamine lesioned rats. Drug treatments in the ipsilateral ventromedial nucleus showed a similar rank order of potency at inhibiting these circling behaviours, seemingly by reducing apomorphine-induced posture and muscimol-induced hypermotility. The suppression of circling by muscimol in these tests was highlighted by introducing the compound into the ventromedial nucleus at the height of circling activity. Both types of circling stimulus lost the capacity to increase locomotion, but still caused head turning and stereotypy in rats made cataleptic with bilateral ventromedial muscimol. Treating one ventromedial thalamus with muscimol greatly intensified any pre-existing posture directed towards that side, and vice versa. These data suggest that the ventromedial nucleus is not involved with the expression of stereotyped behaviours, but can profoundly influence posture and locomotion, especially in the presence of some other motor stimulus. The recovery of circus movements in rats with impaired ventromedial nucleus function implies this nucleus is not essential for the execution of circling in these models.

Chromatographic analysis of glutamic acid decarboxylase in biological samples

A number of non-chromatographic and chromatographic methods for analysis of GAD activity in biological tissues have been described. The majority of these chromatographic methods utilize the analysis of GABA formed from incubation homogenates. Depending upon the analytical technique selected, limits of detection range from nanogram to picogram levels of GABA. Also discussed have been some of the commonly associated problems and their resolution with sample collection, postmortem changes and alternative pathways of CO2 and GABA production which can lead to errors in accurate determination of GAD activity in biological samples.

Automated precolumn derivatization device to determine neurotransmitter and other amino acids by reversed-phase high-performance liquid chromatography

A device to derivatize amino acids with o-phthaldialdehyde, which is directly connected to high-performance liquid chromatographic equipment is described. Its principle is that a sample (10-500 microliters) is mixed with a reagent (containing o-phthaldialdehyde, 2-mercaptoethanol and sodium hydrogen carbonate buffer), using a peristaltic pump. This mixture is pumped into a loop of a pneumatically controlled injection valve at atmospheric pressure. When the derivatization is complete the valve switches, so that the sample is applied to a column and the amino acid derivatives are separated with a gradient of methanol-phosphate buffers. The reproducibility is such that brain perfusates or tissue extracts can be analyzed for the amino acid transmitter content and no internal standard is necessary. The major advantages of the present device are that it produces thorough mixing of reagent and sample, so that a high and constant degree of derivatization occurs (thus producing high sensitivity; less than 0.1 pmol can be detected) and its low cost.

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

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

The release of [3H]GABA formed from [3H]glutamate in rat hippocampal slices: comparison with endogenous and exogenous labeled GABA

to compare the storage and release of endogenous GABA, of [3H]GABA formed endogenously from glutamate, and of exogenous [14C]GABA, hippocampal slices were incubated with 5 microCi/ml [3,4-3H]1-glutamate and 0.5 microCi/ml [U-14C]GABA and then were superfused in the presence or absence of Ca+ with either 50 mM K+ or 50 microM veratridine. Endogenous GABA was determined by high performance liquid chromatography which separated labeled GABA from its precursors and metabolites. Exogenous [14C]GABA content of the slices declined spontaneously while endogenous GABA and endogenously formed [3H]GABA stayed constant over a 48 min period. In the presence of Ca+ 50 mM K+ and in the presence or absence of Ca2+ veratridine released exogenous [14C]GABA more rapidly than endogenous or endogenously formed [3H]GABA, the release of the latter two occurring always in parallel. The initial specific activity of released exogenous [14C]GABA was three times, while that of endogenously formed [3H]GABA was only 50% higher than that in the slices. There was an excess of endogenous GABA content following superfusion with 50 mM K+ and Ca2+, which did not occur in the absence of Ca2+ or after veratridine. The observation that endogenous GABA and [3H]GABA formed endogenously from glutamate are stored and released in parallel but differently from exogenous labelled GABA, suggests that exogenous [3H] glutamate can enter a glutamate pool that normally serves as precursor of GABA.

Interactions between dopamine and gamma-aminobutyrate in the substantia nigra: implications for the striatonigral output hypothesis

Experiments employing a rodent circling model were conducted to test the predictive capacity of the theory which states that striatonigral gamma-aminobutyrate neurones transmit striatal information influencing the animal's locomotion and orientation. In agreement with this proposal, blocking nerve conduction in one substantia nigra with procaine, or nigral gamma-aminobutyrate receptors with bicuculline administered stereotaxically, frequently forced rats to move ipsiversively to systemic apomorphine, as though the treatment had impaired striatonigral transmission on that side of the brain. Attempts to reverse the direction of apomorphine circling by stimulating gamma-aminobutyrate receptors with muscimol, by facilitating the amino acid's action with flurazepam, or by increasing its synaptic concentration either with a breakdown inhibitor (ethanolamine O-sulphate or 4-amino-hex-5-enoic acid) or an uptake blocker (cis-1,3-aminocyclohexane carboxylic acid) in one nigra, proved unsuccessful. In fact, ethanolamine O-sulphate, flurazepam and muscimol all gave the appearance of hindering rather than enhancing the passage of striatal-derived motor information through the nigra. Broadly speaking, these drugs gave predictable behavioral responses from the ventromedial thalamus, suggesting they were acting in accordance with known mechanisms. The anomalous behaviour with ethanolamine O-sulphate may be attributed to its elevating gamma-aminobutyrate levels in other brain areas, since similar ipsiversive rotations occurred if gamma-aminobutyrate catabolism was prevented at a wide variety of extranigral sites. A simple explanation for the paradoxical ipsiversive behaviours produced by intranigral flurazepam or muscimol in combination with systemic or intracerebral injection of dopamine agonists, is that they act via presynaptic receptors to inhibit the release of endogenous gamma-aminobutyrate and thereby impede striatonigral outflow ipsilaterally.

Release of [3H]L-glutamine acid (L-Glu) and [3H]D-aspartic acid (D-Asp) in the area of nucleus tractus solitarius in vivo produced by stimulation of the vagus nerve

We have investigated the effects of bilateral electrical stimulation of the vagus nerves in anesthetized, paralyzed rats on the release of exogenously administered [3H]L-glutamic acid ([3H]L-Glu) or [3H]D-aspartic acid ([3H]D-Asp) from the intermediate portion of the nucleus tractus solitarius (NTS). Electrical stimulation of afferent fibers with the frequency, pulse, duration, and intensity required to activate C-fibers, elicited hypotension and bradycardia. Such stimuli induced the release of [3H]L-Glu, or its stable analogue [3H]D-Asp, from the NTS into perfusate collected through push-pull cannulae. The release of radioactive materials, calculated as a percent of increase in radioactivity above the prestimulation level, was for [3H]L-Glu 114.4 +/- 25.1% (n = 20) during bilateral vagal stimulation, and 45.6 +/- 11.3% (n = 9) (P less than 0.001) during unilateral stimulation. The release of [3H]D-Asp induced by bilateral vagal stimulation was 100.4 +/- 31.9%. The release, which was anatomically specific and restricted to the NTS, was directly related to stimulus (and hence reflex) intensity. Overflow of the inert substances [14C]urea OR [14C]sucrose, co-administered with the [3H] amino acids, did not increase at the same time. Local depolarization of the cells in the NTS by K+ (53mM) increased the overflow of [3H]L-Glu, as well as [14C]urea, and was able to induce the release of [3H]L-Glu when electrical stimulation failed to have an effect. The results are consistent with the hypothesis that L-Glu is a neurotransmitter of neurons in the NTS mediating vasodepressor response from vagal afferents, including those from systemic baroreceptors.

Bilateral asymmetrical changes in the nigral release of [3H]GABA induced by unilateral application of acetylcholine in the cat caudate nucleus

In halothane anaesthetized cats, a push-pull cannula was implanted into the right caudate nucleus (CN) and in each substantia nigra (SN). The release of [3H]GABA continuously formed from [3H]glutamine was estimated in each structure. Acetylcholine (ACh, 5 x 10(-5) M) added in presence of eserine (5 x 10(-5) M) for 50 min in the right caudate nucleus 2 h after the onset of superfusion with [3H]glutamine, stimulated the [3H]GABA release locally. The effect was biphasic when ACh application was made in the median two-thirds of the structure and it was monophasic and transient when the ACh application was restricted to the lateral part. ACh application in the right caudate nucleus also induced changes in [3H]GABA released in the anterior (pars reticulata) and posterior (pars compacta) parts of both SN. While [3H]GABA release was enhanced in the ipsilateral anterior SN, it was reduced in the contralateral anterior SN. Respective opposite effects were observed in the posterior parts of the ipsi- and contralateral SN. These bilateral asymmetrical changes in [3H]GABA release were not dependent on the site of ACh application in the right caudate nucleus. These results indicate that the facilitation of cholinergic transmission in one caudate nucleus influences in an opposite way the striato-nigral GABA neurones on both sides of the brain.

Measurement of release of endogenous GABA and catabolites of [3H]GABA from synaptosomal preparations using ion-exchange chromatography

Picomole quantities of endogenous GABA in acidified superfusates of synaptosomal preparations have been measured using micro-bore ion-exchange chromatography and post-column formation of the fluorescent iso-indole derivative. Using this technique superfusates have been analyzed directly, without further manipulations, to investigate the release of endogenous GABA. Spontaneous release of GABA was 2-5 pmol/200 microliters superfusate increasing to 20 pmol/200 microliters with potassium stimulation. When gamma-vinyl GABA (RMI 71754), an inhibitor of GABA-T was injected into rats (750 mg/kg) and synaptosomes prepared the potassium-evoked release of GABA was increased 3-fold compared to controls. Chromatographic separations and measurement of release of endogenous and radiolabeled GABA allowed the real specific activity of released GABA to be calculated. Only when 500 microM amino-oxyacetic acid was added during isolation of synaptosomes was the specific activity of released GABA the same as the initial specific activity.

In vivo GABA release from the medial preoptic area of diestrous and ovariectomized rats

The push-pull cannula technique was used to examine the endogenous release of GABA from the medial preoptic area (MPO) of unanesthetized rats. In diestrous females the mean resting release of GABA was 27.1 +/- 2.0 pmol/min. GABA release was significantly elevated by increasing the potassium concentration in the perfusion solution to 50 mM, whereas it was dramatically inhibited by mercaptoproprionic acid (1.0 mM), a glutamic acid decarboxylase inhibitor. A comparison between diestrous females and chronically castrated animals indicated that endogenous GABA release in OVX animals was only 60-70% of that in diestrous animals. A model for the presynaptic inhibition of NE by estrogen receptive GABAergic neurons in the MPO is proposed.

Gas chromatographic method for the determination of trace amounts of putative amino acid neurotransmitters from brain perfusates collected in vivo

Glass capillary gas chromatography in combination with thermionic detection has been developed for the measurement of trace amounts of glycine, beta-alanine, aspartic acid, gamma-aminobutyric acid and glutamic acid from brain perfusates collected in vivo by push-pull cannula techniques. Amino acids extracted from the dried perfusate residues are converted to the corresponding N-pentafluoropropionyl hexafluoroisopropyl esters (PFP-HFIP derivatives) and separated by gas chromatography on a glass capillary column coated with methylsiloxane. The detection limit of the amino acids (signal-to-noise ratio 3:1) ranges between 200 fmol and 2.0 pmol. The assay was applied to the measurement of amino acids released in vivo in the pigeon optic tectum into perfusates collected by push-pull techniques.

Determination of gamma-aminobutyric acid by liquid chromatography with electrochemical detection

gamma-Aminobutyric acid (GABA) has been determined in rat brain by derivatization with 2,4,6-trinitrobenzenesulfonic acid. The derivative and an internal standard, 2,4,6-trinitrophenyl-delta-aminovaleric acid, are extracted into toluene and separated by reversed-phase chromatography. Electrochemical reduction of these derivatives permits picomole measurements of GABA in microgram amounts of brain tissue.

Mass fragmentographic method for the determination of trace amounts of putative amino acid neurotransmitters and related compounds from brain perfusates collected in vivo

A mass fragmentographic method for the determination of trace amounts of amino acid neurotransmitter candidates from brain perfusates is described. The analytical procedure includes the measurements of glycine, beta-alanine, gamma-aminobutyric acid, proline, aspartic acid, and glutamic acid; alpha-alanine, leucine, and sarcosine, undergoing gas chromatographic coelution, are detected simultaneously. Amino acids extracted from dried perfusate residues are converted to the corresponding N-pentafluoropropionyl hexafluoroisopropyl esters by a single-step procedure. Gas chromatographic separation of the amino acid derivatives is achieved on a packed glass column filled with trifluoropropylsilicone as stationary phase. The limit of detection for the different derivatives (signal-to-noise, 3:1) ranges from 50 femtomol to 1 picomol. Deuterium-labeled amino acid analogues are used as internal standards for quantitative measurements. The mass spectral characteristics of the derivatives are compared and discussed. The technique has been applied to the assay of amino acids released in vivo within the pigeon optic tectum, demonstrating the capabilities of the present analytical approach.

Compartments of labeled and endogenous gamma-aminobutyric acid giving rise to release evoked by potassium or veratridine in rat cortical slices

To establish compartments involved in depolarization-induced release of gamma-aminobutyric acid (GABA) in rat brain slices, the amount of exogenous labeled and endogenous GABA released and retained was followed during 48 min exposure to 50 mM-K+ or to 50 microM-veratridine. Endogenous GABA was measured with high performance liquid chromatography. The presence of 10 microM-aminooxyacetic acid throughout prevented both the metabolism of GABA and the formation of endogenous GABA due to depolarization. During superfusion with 50 mM-K+ and 2.6 nM-Ca2+ the efflux of labeled and endogenous GABA after an initial large increase declined to 10% of the highest value with constant and identical rates. Kinetic analysis of efflux showed that 10% of endogenous and 25% of labeled GABA present is available for release by high K+ and Ca2+. In the absence of Ca2+, release by high K+ of both labeled and endogenous GABA was nearly suppressed. Veratridine, unlike high K+, caused an efflux which declined with an initial fast and late very slow phase. The slow efflux by veratridine was doubled in the absence of Ca2+. Exposure to veratridine in the absence of Ca2+ during 120 min released nearly 70% of labeled and endogenous GABA present. Results suggest that only about 0.25 mumol . g-1 endogenous GABA is the source of physiological Ca2+-dependent release, while much of the remaining GABA present is released only under unphysiological conditions.

Release of 3H-gamma-aminobutyric acid (GABA) by inhibitory neurons of the crayfish

Inhibitory neurons innervating the muscle receptor organ (MRO) of crayfish were used to study the uptake and release of tritiated GABA. MROs that have been directly exposed to 3H GABA for 60--75 min release radioactivity during low-frequency electrical stimulation. When ganglia containing the inhibitory cell bodies are exposed to 3H GABA, the isotope travels along a pathway unique to the inhibitory axon, at rates that range between 160 and 240 mm per day. Electrical stimulation of inhibitory axons whose cell bodies have been exposed to 3H GABA for 4--5 hr produces release of isotope from isolated MROs. Low calcium, high magnesium exposure prevents the stimulus-dependent release of radioactivity. Thin layer chromatographic analyses indicate that GABA comprises at least a major fraction of the radioactivity collected from stimulated preparations. A number of unidentified radioactive compounds are usually present with GABA, and it is suggested that most of these are catabolites of GABA.

In vivo release of endogenously synthesized [3H]GABA from the cat substantia nigra and the pallido-entopeduncular nuclei

Halothane anesthetized cats were implanted with push--pull cannulae to study the release of [3H]GABA continuously formed from [3H]glutamine in the substantia nigra (SN) and in the pallido-entopeduncular nuclei (PEP). A spontaneous release of [3H]GABA was observed from both structures and it reached a steady state level 1 h after the beginning of the superfusion with [3H]glutamine. In cats implanted with two push--pull cannulae, the local application of potassium (47 mM) in the PEP stimulated the release of [3H]GABA from the ipsilateral SN. In cats implanted with 4 push--pull cannulae, the unilateral 10 min electrical stimulation of the caudate nucleus evoked the release of [3H]GABA not only from the ipsilateral SN, but also in most cases from the contralateral structure. This stimulus also enhanced the release of [3H]GABA from PEP but the effects were mainly observed in the medio-caudal part of the ipsilateral PEP and in the latero-rostral part of the contralateral structure. In all cases, the changes in [3H]GABA release were observed during and after the electrical stimulation. The ipsilateral effects can be attributed to the direct activation of the caudato-PEP or caudato-SN GABAergic neurons. A polysynaptic neuronal loop must be involved in the symmetric contralateral effects.

Di-n-propylacetate-induced abstinence behaviour as a possible correlate of increased GABA-ergic activity in the rat

Administration of di-n-propylacetate (DPA), an inhibitor of SSA-dehydrogenase, produces in naive rats abstinence behaviour which can be blocked by morphine and bicuculline and may be useful as a behavioural correlate of increased GABA-ergic activity. The usefulness of this model has been demonstrated by studying the effect of bicuculline, picrotoxin, strychnine, morphine, aminooxyacetic acid, 3-mercaptopropionate and thiosemicarbazide on DPA-induced abstinence behaviour. Behaviour was suppressed both by bicuculline or picrotoxin, while the selective glycine antagonist strychnine was ineffective. A comparable syndrome could not be evoked by treatment with aminooxyacetic acid, a GABA-transaminase inhibitor, indicating that the effect of DPA was not caused by inhibition of this enzyme. Instead, aminooxyacetic acid suppressed the DPA-induced abstinence behaviour, suggesting that two GABA-ergic systems with opposite effects on behaviour can be distinguished. The syndrome was also suppressed by convulsant doses of 3-mercaptopropionate, while thiosemicarbazide was ineffective. Abstinence behaviour was further suppressed by morphine with an ED50 of 0.5 mg/kg and this action could be clearly separated from its depressant effect on locomotor activity in non-treated animals. These results suggest that morphine receptors may be involved in DPA-induced abstinence behaviour. Based on these experiments a model has been proposed for GABA-ergic terminals being under the inhibitor influence of GABA-ergic autoreceptors. It is proposed that DPA-induced abstinence behaviour may be useful as a model of increased GABA-ergic activity to aid study of the regulation and properties of the GABA-ergic system in vivo.

In vivo release of endogenous GABA in the cat hypothalamus

The posterior hypothalamus of anaesthetized cats was superfused with artificial cerebrospinal fluid through a push-pull cannula and the release of endogenous GABA from the hypothalamus into the superfusate was studied. The resting release of GABA varied rhythmically, since phases of high rate of release were separated from each other by phases of low rate of release. The time interval between two adjacent phases of high rate of release was about 70 min. Electrical stimulation of the posterior hypothalamus with the tip of the cannula enhanced the rate of release of GABA in a frequency-dependent way. Superfusion of the hypothalamus with CSF which contained a high concentration of potassium and a low concentration of sodium increased the rate of release of GABA; this effect was dependent on the presence of calcium ions in the superfusing fluid. Pretreatment of the cats with reserpine reduced the levels of GABA in hypothalamus and rest of brain and the concentration of GABA in the superfusate as well. Stimulation of the locus coeruleus with a bipolar electrode elicited an increased release of GABA in the hypothalamus.