Endogenous Dopamine Increases Extracellular Concentrations of Glutamate and GABA in Striatum of the Freely Moving Rat: Involvement of D1 and D2 Dopamine Receptors

Interactions between endogenous dopamine, glutamate, GABA, and taurine were investigated in striatum of the freely moving rat by using microdialysis. Intrastriatal infusions of the selective dopamine uptake inhibitor nomifensine (NMF) were used to increase the endogenous extracellular dopamine. NMF produced a dose-related increase in extracellular dopamine and also increased extracellular concentrations of glutamate, GABA, and taurine. Extracellular increases of dopamine were significantly correlated with extracellular increases of glutamate and GABA, but not taurine. To investigate whether the increased extracelular dopamine produced by NMF was responsible for the concomitant increase of glutamate and GABA, D1, and D2 receptor antagonists were used. Dopamine receptor antagonists D1 (SCH23390) and D2 (sulpiride) significantly attenuated the increases of glutamate and GABA produced by NMF. These data suggest that endogenous dopamine, through both D1 and D2 dopamine receptors, plays a role in releasing glutamate and GABA in striatum of the freely moving rat.     

Effects of Endogenous Glutamate on Extracellular Concentrations of GABA, Dopamine, and Dopamine Metabolites in the Prefrontal Cortex of the Freely Moving Rat: Involvement of NMDA and AMPA/KA Receptors

Using microdialysis, interactions between endogenous glutamate, dopamine, and GABA were investigated in the medial prefrontal cortex of the freely moving rat. Interactions between glutamate and other neurotransmitters in the prefrontal cortex had already been studied using pharmacological agonists or antagonists of glutamate receptors. This research investigated whether glutamate itself, through the increase of its endogenous extracellular concentration, is able to modulate the extracellular concentrations of GABA and dopamine in the prefrontal cortex. Intracortical infusions of the selective glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) were used to increase the endogenous extracellular glutamate. PDC (0.5, 2, 8, 16 and 32 mM) produced a dose-related increase in dialysate glutamate in a range of 1–36 M. At the dose of 16 mM, PDC increased dialysate glutamate from 1.25 to 28 M. PDC also increased extracellular GABA and taurine, but not dopamine; and decreased extracellular concentrations of the dopamine metabolites DOPAC and HVA. NMDA and AMPA/KA receptor antagonists were used to investigate whether the increases of extracellular glutamate were responsible for the changes in the release of GABA, and dopamine metabolites. The NMDA antagonist had no effect on the increase of extracellular GABA, but blocked the decreases of extracellular DOPAC and HVA, produced by PDC. In contrast, the AMPA/KA antagonist blocked the increases of extracellular GABA without affecting the decreases of extracellular DOPAC and HVA produced by PDC. These results suggest that endogenous glutamate acts preferentially through NMDA receptors to decrease dopamine metabolism, and through AMPA/KA receptors to increase GABAergic activity in the medial prefrontal cortex of the awake rat.     

Role of Endogenous Taurine on the Glutamate Analogue-Induced Neurotoxicity in the Rat Hippocampus In Vivo

The glutamate analogues N-methyl-D-aspartate (NMDA), kainic acid (KA), and quisqualic acid (QA), prepared in different hypertonic media, were perfused in vivo in the hippocampal CA1 field of rats using a microdialysis technique. Extracellular taurine levels, estimated after analysis of the taurine content of dialysates, increased during perfusion of all three agonists but varied according to the osmolarity of the medium. The NMDA-induced increase in extracellular taurine content was only slightly inhibited by perfusion of 150 and 300 mM sucrose. The KA-evoked increase was partially dependent on extracellular osmolarity, because addition of 50 and 150 mM sucrose caused a dose-dependent inhibition that was not augmented using higher sucrose concentrations. QA caused a taurine increase that was totally abolished by addition of 50 mM sucrose. These results indicate that the rise in extracellular taurine level elicited by QA and part of the increase elicited by KA are probably due to a release caused by the cellular swelling that these substances evoke, a finding substantiating the previously proposed osmoregulatory role of taurine. However, almost all the increase in extracellular taurine content caused by NMDA and all the osmotically insensitive part of the KA-evoked rise cannot be explained as release triggered by cell swelling and may reflect a function of taurine other than osmoregulation.     

N-methyl-d-Aspartic Acid Differentially Regulates Extracellular Dopamine, GABA, and Glutamate Levels in the Dorsolateral Neostriatum of the Halothane-Anesthetized Rat: An In Vivo Microdialysis Study

Abstract: The effects of local perfusion with the glutamate receptor agonist NMDA and the noncompetitive NMDA receptor antagonist dizolcipine (MK-801) on extracellular dopamine (DA), GABA, and glutamate (Glu) levels in the dorsolateral striatum were monitored using in vivo microdialysis in the halothane-anesthetized rat. In addition, the sensitivity of both the basal and NMDA-induced increases in levels of these neurotransmitter substances to perfusion with tetrodotoxin (TTX; 10^?5 M) and a low Ca²⁺ concentration (0.1 mM) was studied. The results show that the local perfusion (10 min) with both the 10^?3 and 10^?4 M dose of NMDA increased striatal DA and GABA outflow, whereas only the (10^?3 M) dose of NMDA was associated with a small and delayed increase in extracellular Glu levels. The NMDA-induced effects were dose-dependently counteracted by simultaneous perfusion with MK-801 (10^?6 and 10^?5 M). Both the basal and NMDA (10^?3 M)-induced increase in extracellular striatal DA content was reduced in the presence of TTX and a low Ca²⁺ concentration, whereas both basal and NMDA-stimulated GABA levels were unaffected by these treatments. Both the basal and NMDA-stimulated Glu levels were enhanced following TTX treatment, whereas perfusion with a low Ca²⁺ concentration reduced basal Glu levels and enhanced and prolonged the NMDA-induced stimulation. These data support the view that NMDA receptor stimulation plays a role in the regulation of extracellular DA, GABA, and Glu levels in the dorsolateral neostriatum and provide evidence for a differential effect of NMDA receptor stimulation on these three striatal neurotransmitter systems, possibly reflecting direct and indirect actions mediated via striatal NMDA receptors.     

Muscarinic Modulation of Striatal Dopamine, Glutamate, and GABA Release, as Measured with In Vivo Microdialysis

Abstract: Intrastriatal microdialysis was used to administer muscarinic drugs in freely moving rats for 40 min at a flow rate of 2 µl/min. Administration of the nonselective agonist pilocarpine at 10 m M increased striatal dopamine release and decreased extracellular GABA and glutamate overflow. Perfusion with the muscarinic M₂ antagonist methoctramine at 75 µ M increased extracellular dopamine and glutamate concentrations but exerted no changes on extracellular GABA levels. Intrastriatal administration of the M₁ antagonist pirenzepine at 0.05 µ M decreased extracellular dopamine overflow. Application of pirenzepine (0.05 and 5 µ M ) exerted no effects on the measured GABA or glutamate levels. There are thus important differences in applied doses of muscarinic drugs needed to obtain modulatory effects. High doses of agonists are probably needed to superimpose on the background of tonic influences of striatal acetylcholine, whereas antagonists can block the receptors in small doses. We further suggest that M₁ receptors might tonically facilitate striatal dopamine release, that M₂ receptors might tonically inhibit striatal glutamate efflux, and that acetylcholine does not exert tonic effects on striatal GABA release. The link with the pilocarpine animal model for temporal lobe epilepsy will be discussed.     

The Effect of Colchicine and Cytochalasin B on the Release of Taurine from the Chick Retina

Abstract: The effect of colchicine (0.5 mM) and of cytochalasin B (10⁻⁴ M) on the release of [³⁵S]taurine from the isolated chick retina, upon stimulation by 68.5 mM-KCl, 10⁻⁵ M-veratridine and 10 mM-glutamate, was studied. Cytochalasin and colchicine effects on taurine release were compared with those on K⁺-stimulated release of [³H]dopamine and [³H]GABA. Colchicine caused a marked decrease of the [³⁵S]taurine release evoked by the three stimulatory agents; it also decreased [³H]dopamine release without affecting that of [³H]GABA. Cytochalasin B significantly decreased the efflux of [³⁵S]taurine stimulated by glutamate or veratridine without altering that evoked by 68.5 mM-KCl. Cytochalasin practically suppressed the [³H]dopamine-stimulated release and slightly decreased that of [³H]GABA. This drug produced a high increase in the spontaneous release of labeled GABA and taurine. These results suggest that the release of taurine and GABA from the chick retina probably occurs through different mechanisms. It is suggested that taurine release may be related to a process involving contractile proteins.     

Amino Acid Neurotransmitters and Dopamine in Brain and Pituitary of the Goldfish: Involvement in the Regulation of Gonadotropin Secretion

An isocratic high-performance liquid chromatographic technique was developed to measure levels of gamma-aminobutyric acid (GABA), glutamate, and taurine in the brain and pituitary of goldfish. Accuracy of this procedure for quantification of these compounds was established by evaluating anesthetic and postmortem effects and by selectively manipulating GABA concentrations by intraperitoneal administration of the glutamic acid decarboxylase (GAD) inhibitor 3-mercaptopropionic acid or the GABA transaminase inhibitor gamma-vinyl GABA. The technique provided a simple, rapid, and reliable method for evaluating the concentrations of these amino acids without the use of complex gradient chromatographic systems. To investigate the relationship between neurotransmitter amino acids and the control of pituitary secretion of gonadotropin, the effects of injection of taurine, GABA, or monosodium glutamate on GABA, glutamate, taurine, and, in some instances, monoamine concentrations in the brain and pituitary were evaluated and related to serum gonadotropin levels. Injection of taurine caused an elevation in serum gonadotropin concentrations. In addition, injection of the taurine precursor hypotaurine but not the taurine catabolite isethionic acid elevated serum gonadotropin levels. Intracerebroventricular injection of either GABA or taurine also elevated serum gonadotropin concentrations. Pretreatment of recrudescent fish with alpha-methyl-p-tyrosine reduced pituitary dopamine concentrations and also potentiated the serum gonadotropin response to taurine. Injection of monosodium glutamate caused an increase of glutamate content in the pituitary at 24 h; this was followed by a decrease at 72 h after administration. Pituitary GABA, taurine, and dopamine concentrations underwent a transient depletion after monosodium glutamate administration, and this was associated with an elevation of serum gonadotropin content.(ABSTRACT TRUNCATED AT 250 WORDS)     

Taurine fails to protect against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced striatal dopamine depletion in mice

Taurine, a known antioxidant and neuroprotector has been investigated for its free radical scavenging action in vitro in isolated mitochondria, and tested whether it protects against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopaminergic neurodegeneration in mice. Taurine (0.1-10 mM) did not affect 1-methyl-4-phenyl pyridinium-induced hydroxyl radical production in isolated mitochondria. Systemic administration of taurine (250 mg/kg, i.p.) caused a small, but significant loss of dopamine levels in the striatum of mice. Taurine failed to reverse MPTP-induced striatal dopamine depletion, but caused significant increase in dopamine turnover in these animals. In the light of the present study it may be suggested that consumption of taurine may neither help in scavenging of neurotoxic hydroxyl radicals in the brain mitochondria, nor would it help in blocking the process of neurodegeneration.     

Differential Control by N-Methyl-D-Aspartate and Kainate of Striatal Dopamine Release In Vivo: A Trans-Striatal Dialysis Study

Using the technique of trans-striatal dialysis in halothane-anesthetized rats, we have studied the effects of intrastriatally infused N-methyl-D-aspartate (NMDA), kainate, and quisqualate on the liberation of endogenous striatal dopamine. The striatal infusion of NMDA (10(-3)-10(-2) M) or kainate (10(-4)-10(-2) M) but not of quisqualate (up to 10(-2) M) for one 20-min fraction provoked a dramatic increase in striatal dopamine efflux up to a maximum of 1,200 and 3,400% of basal levels for NMDA and kainate, respectively. NMDA (10(-3) M) evoked liberation of striatal dopamine was totally blocked by coinfusion of 2-amino-5-phosphonovalerate (2-APV; 5 X 10(-4) M) and by the systemic injection of phencyclidine (3 mg/kg i.p.). The effects of NMDA (10(-3) M) were also totally antagonized in a dose-dependent manner by the striatal coinfusion of atropine (10(-7)-10(-4) M), and abolished in rats that had received bilateral striatal ibotenate lesions (10 micrograms/1 microliter) 1 week prior to implantation of the dialysis fiber. The striatal infusion of tetrodotoxin (10(-6) M) reduced basal dopamine efflux by 60-70% and abolished the NMDA (10(-3) M)-evoked liberation of striatal dopamine. The effects of kainate (10(-3) M) on striatal dopamine efflux were only partially reduced by doses of 2-APV or atropine that totally blocked the NMDA response, and were also partially resistant to tetrodotoxin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nitric oxide can differentially modulate striatal neurotransmitter concentrations via soluble guanylate cyclase and peroxynitrite formation

In vivo microdialysis was used to investigate whether nitric oxide (NO) modulates striatal neurotransmitter release in the rat through inducing cyclic GMP formation via soluble guanylate cyclase or formation of peroxynitrite (ONOO(-)). When NO donors, S-nitroso-N-acetyl-DL-penicillamine (SNAP; 1 mM) or (Z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1- ium-1, 2-diolate (NOC-18; 1 mM), were retrodialysed for 15 min, acetylcholine (ACh), serotonin (5-HT), glutamate (Glu), gamma-aminobutyric acid (GABA), and taurine levels were significantly increased, whereas those of dopamine (DA), dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) were decreased. Only effects on ACh, 5-HT, and GABA showed calcium dependency. Inhibition of soluble guanylate cyclase by 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ; 100 and 200 microM) dose-dependently reduced NO donor-evoked increases in ACh, 5-HT, Glu, and GABA levels. Coperfusion of SNAP or NOC-18 with an ONOO(-) scavenger, L-cysteine (10 mM) resulted in enhanced concentrations of Glu and GABA. On the other hand, DA concentrations increased rather than decreased, and no reductions in DOPAC and 5-HIAA occurred. This increase in DA and the potentiation of Glu and GABA were calcium-dependent and prevented by ODQ. Similar to NO, infusions of ONOO(-) (10 or 100 microM) decreased DA, DOPAC, and 5-HIAA. Overall, these results demonstrate that NO increases ACh, 5-HT, Glu, and GABA levels primarily through a cyclic GMP-dependent mechanism. For DA, DOPAC, and 5-HIAA, effects are determined by levels of ONOO(-) stimulated by NO donors. When these are high, they effectively reduce extracellular concentrations through oxidation. When they are low, DA concentrations are increased in a cyclic GMP-dependent manner and may act to facilitate Glu and GABA release further. Thus, changes in brain levels of antioxidants, and the altered ability of NO to stimulate cyclic GMP formation during ageing, or neurodegenerative pathologies, may particularly impact on the functional consequences of NO on striatal dopaminergic and glutamatergic function.     

Displacement of [H]GABA binding to bovine brain receptors by sulfur-containing analogues

The displacement of [³H]GABA binding to GABA receptors of bovine brain cortical membranes by some sulfur-containing compounds (homothiotaurine, thiotaurine and carboxymethylcysteamine) was investigated and their potency was compared to that of other known sulfur-containing analogues of GABA, such as homotaurine, homohypotaurine and taurine. Displacement studies showed homotaurine to be more effective as a GABA displacer than homohypotaurine and homothiotaurine (IC₅₀: 3.9 × 10⁻⁸, 6.7 × 10⁻⁷ and 6.8 × 10⁻⁷ M, respectively). Saturation experiments showed that the effect of taurine, homothiotaurine, homotaurine and homohypotaurine was due to a loss of high-affinity GABA sites (K_d = 10.7 nM). Homotaurine seems also to interact with low-affinity sites, decreasing the affinity constant, whereas the number of binding sites remains unchanged.     

Maitotoxin-Evoked γ-Aminobutyric Acid Release Is Due Not Only to the Opening of Calcium Channels

The effects of maitotoxin (MTX) on endogenous amino acid release were tested on highly purified striatal neurons differentiated in primary culture. MTX induced a large and concentration-dependent release of gamma-aminobutyric acid (GABA). This effect was abolished when experiments were performed in the absence of external Ca2+, and restored when Ca2+ ions were added after removing the MTX-containing Ca2+-free solution. MTX-induced amino acid release was not affected by 1 microM nifedipine and only slightly inhibited by 1 mM Co2+. MTX also induced a massive accumulation of 45Ca2+ in the neurons which, in contrast to the MTX-evoked GABA release, was totally blocked in the presence of 1 mM Co2+. Whereas 500 nM tetrodotoxin was without significant effect, MTX-evoked GABA release was dependent on the presence of external Na+ and sensitive to nipecotic acid, a GABA uptake inhibitor. It is concluded that, on striatal neurons, MTX induced Na+ influx only in the presence of external Ca2+. The increase in cytoplasmic Na+ ions then triggers the release of GABA.     

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

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

Effect of aging on 24-hour changes in dopamine and serotonin turnover and amino acid and somatostatin contents of rat corpus striatum

This study examined the 24-hour changes in a number of transmitters in the corpus striatum of young and middle-aged male Wistar rats. The contents of excitatory amino acids (glutamate, aspartate) and inhibitory amino acids (gamma-aminobutyric acid, GABA; taurine, glycine) and of somatostatin were measured in 2-month- and 18- to 20-month-old rats killed at six different time points along the 24-hour cycle. The striatal serotonin and dopamine turnover was also measured. Both young and middle-aged rats showed significant 24-hour variations in striatal glutamate and aspartate contents; only in young rats these variations fitted a cosine function, with acrophase during the first part of rest span. Mesor values of striatal excitatory amino acid contents were lowest in middle-aged rats. Significant 24-hour variations in striatal contents of GABA, taurine, and glycine occurred in young rats, while only striatal GABA exhibited 24-hour changes in middle- aged rats (acrophases during the first part of rest span). For every inhibitory transmitter, the mesor values in middle-aged rats were significantly lower than in young rats. The 24-hour variation of the striatal somatostatin content showed acrophase during the first part of rest span, mesor values and amplitude being lowest in middle-aged rats. Aging rats exhibited significantly higher mesor values of striatal serotonin turnover (34% increase) and lower mesor values of dopamine turnover (69% decrease) than their younger counterparts. Some of the circadian modifications of motor function seen in aging rats could be related to the striatal transmitter changes reported herein.     

MPP+ toxicity in rat striatal slices: Relationship between non-selective effects and free radical production

Incubations of rat striatal slices have been used to assay MPP⁺ neurotoxicity. MPP⁺, at concentrations of 1 mM or higher, caused a marked increase in hydroxyl radicals, measured as malondialdehyde (MDA) accumulation, but not in nitric oxide production. At these doses, MPP⁺ showed an effect on dopamine terminals, causing a massive dopamine decrease, and on non-neuronal glial cells, where a marked reduction in glutamine synthetase activity was detected. At lower concentrations (25 μM), the toxic effect on dopaminergic endings was maintained without increasing malondialdehyde concentrations or inhibiting glutamine synthetase activity. The effect on glutamine synthetase was prevented by the addition to the medium of 0.5% dimethyl sulfoxide, a hydroxyl-radical scavenger, but this did not protect the effect of dopamine depletion. We propose that non-selective effects of MPP⁺, at doses of 1 mM or higher, are mediated by extracellular overproduction of hydroxyl radicals. The main factor responsible for this overproduction would not be the released dopamine but rather the MPP⁺ itself, through non selective inhibition of the mitochondrial respiratory chain or through a redox cycling that can trigger oxygen radical production.     

Role of External and Internal Calcium on Heterocarrier-Mediated Transmitter Release

Abstract: Release-regulating heterocarriers exist on brain nerve endings. We have investigated in this study the mechanisms involved in the neurotransmitter release evoked by GABA heterocarrier activation. GABA increased the basal release of [³H]acetylcholine and [³H]noradrenaline from rat hippocampal synaptosomes and of [³H]dopamine from striatal synaptosomes. These GABA effects, insensitive to GABA receptor antagonists, were prevented by inhibiting GABA uptake but not by blocking noradrenaline, choline, or dopamine transport. Lack of extracellular Ca²⁺ or addition of tetrodotoxin selectively abolished the GABA-evoked release of [³H]noradrenaline, leaving unaffected that of [³H]acetylcholine or [³H]dopamine. 1,2-Bis(2-aminophenoxy)-ethane-N,N,N′,N′-tetraacetic acid acetoxymethyl ester (BAPTA-AM) or vesamicol attenuated the release of [³H]acetylcholine elicited by GABA. Reserpine, but not BAPTA-AM, prevented the effect of GABA on [³H]dopamine release. Autoreceptor activation inhibited the GABA-evoked release of [³H]noradrenaline but not that of [³H]acetylcholine or [³H]dopamine. It is concluded that (a) the release of [³H]noradrenaline consequent to activation of GABA heterocarriers sited on noradrenergic terminals meets the criteria of a conventional exocytotic process, (b) the extracellular [Ca²⁺]-independent releases of [³H]acetylcholine and [³H]dopamine appear to occur from vesicles possibly through involvement of intraterminal Ca²⁺, and (c) autoreceptor activation only affects heterocarrier-mediated vesicular release linked to entry of extracellular Ca²⁺.     

A Search for Receptors Modulating the Release of γ-[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.     

Nitric oxide and potassium chloride-facilitated striatal dopamine efflux in vivo: role of calcium-dependent release mechanisms

Previous studies investigating the calcium-dependency of nitric oxide-facilitated striatal dopamine efflux have produced conflicting results. In the current study, we have investigated the role of extracellular calcium in nitric oxide and potassium chloride-evoked striatal dopamine efflux in vivo using microdialysis. Dialysis probes were implanted in the anterior dorsal striatum of chloral hydrate-anesthetized rats. Intrastriatal infusion (20 min fraction) of the nitric oxide generators sodium nitroprusside (200 μM, 500 μM, or 1 mM) and 3-morpholinosydnonimine (1 mM) increased extracellular dopamine levels. The facilitatory effects of 3-morpholinosydnonimine and potassium chloride on dopamine efflux were attenuated following pretreatment (100 min) and co-infusion of calcium free artificial cerebral spinal fluid containing magnesium chloride. Local potassium chloride infusion (100 mM) administered alone elevated striatal dopamine efflux to a similar degree as potassium chloride (100 mM) delivered 60 min after 3-morpholinosydnonimine infusion. These results demonstrate that like potassium chloride, nitric oxide facilitates striatal dopamine efflux in vivo via a mechanism largely dependent on extracellular calcium. Also, as intrastriatal potassium chloride infusion evoked similar increases in extracellular dopamine levels in controls and subjects receiving pretreatment with the NO-generator 3-morpholinosydnonimine, it is unlikely that the functional integrity of DA nerve terminals is compromised via a neurotoxic disruption of plasma membrane potential following enhanced striatal NO production. © 1999 Elsevier Science Ltd. All rights reserved.     

Effects of Hypoxia on the Activity of the Dopaminergic Neuron System in the Rat Striatum as Studied by In Vivo Brain Microdialysis

The purpose of the present study is to clarify the effects of hypoxia on the activity of the dopaminergic neurons in the brain and its mechanism of action. For this purpose, the effects of hypoxia on the extracellular levels of 3,4-dihy-droxyphenylethylamine (dopamine) were examined in the rat Striatum using in vivo brain microdialysis in the presence or absence of pretreatment with either tetrodotoxin (a blocker of voltage-dependent sodium channels) or nomifensine (a blocker of dopamine reuptake). Exposure to various degrees of hypoxia (15, 10, and 8% O₂ in N₂) increased dopamine levels in striatal dialysates to 200, 400, and 1,100%, respectively, of the control value. On reoxygenation, dopamine levels in the dialysates rapidly returned to the control level. Reexposure to hypoxia increased the dopamine levels to the same extent as during the first exposure. After addition of tetrodotoxin (40 mUM) to the perfusion fluid or pretreatment with nomifensine (100 mg/kg, i.p.), exposure to hypoxia no longer increased the dopamine levels. These results suggest that although hypoxia induces an increase in the extracellular dopamine levels (hence, an apparent increase in the activity of the dopaminergic neurons), this increase is not the result of an increase in dopamine release itself, but rather the result of inhibition of the dopamine reuptake mechanism.     

In Vivo Release of Endogenous γ-Aminobutyric Acid from Rat Striatum: Effects of Muscimol, Oxotremorine, and Morphine

Abstract: A push-pull cannula technique was used to study the in vivo release of endogenous GABA from the striaturn of chloral hydrate anaesthesized rats. The GABA in the perfusate was isolated with hplc and fluorimetrically detected (detection limit 0.6 pmol, signalhoke = 3). The mean resting release of GABA under steady state conditions was 1.62 ± 0.09 pmo/min (n = 180, ± s.E.M.). GABA release was increased after addition of depolarizing amounts of potassium to the perfusion medium. Inhibition of GABA synthesis with 3-mercaptopropionic acid (MPA, 0.5 mM) or blockade of the neuronal activity with tetrodotoxin (TTX, 0.2 μM) diminished the spontaneous release of GABA. MPA, but not TTX, reduced the potassium-induced increase in GABA release. Striatal GABA release was decreased by local application of muscimol (l0 μM) but enhanced by picrotoxin (100 μM); the latter counteracted the effect of muscimol. Intrastriatally applied serotonin (100 μM) did not affect the rate of endogenous GABA release. Oxotremorine (25 μM) added to the perfusion medium slightly increased the striatal GABA release. This effect was blocked both by locally applied atropine (100 μM) and haloperidol (5 μM). The latter two drugs did not themselves affect the rate of GABA release. Perfusion with morphine (100 μM) inhibited striatal GABA release. This effect was not influenced by haloperidol, but was no longer observed in the presence of nalorphine (10 μM) which itself did not alter GABA release. These results indicate that GABA released from the striatum is, at least in part, of neuronal origin and that the spontaneous GABA release can be affected by various neuromodulators (including GABA, enkephalins, and acetylcholine).