In Vivo Mechanisms Underlying Dopamine Release from Rat Nigrostriatal Terminals: I. Studies Using Veratrine and Ouabain

The in vivo mechanisms underlying the dopamine (DA)-releasing actions of veratrine and ouabain in the striatum of halothane-anaesthetised rats have been investigated using brain microdialysis. Relevant catecholamines and indoleamines were separated and quantified using HPLC combined with an electrochemical detection system. Veratrine (10 micrograms/ml-1 mg/ml) and ouabain (10 microM-1 mM) were added to the medium perfusing the dialysis probes. Both compounds increased dialysate DA content in a dose-related manner. Dialysate levels of the DA metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid and the serotonin metabolite 5-hydroxyindoleacetic acid were reduced by both veratrine and ouabain. Veratrine-induced DA efflux was maximal in the first 20-min sample collected after drug infusion began, whereas the maximal effect of ouabain was not observed until 20-40 min after administration began. Veratrine-induced DA efflux was unaffected by systemic injection of the DA uptake inhibitor nomifensine but was inhibited by either coperfusion of tetrodotoxin (TTX) or removal of calcium from the perfusing buffer. These data suggest that veratrine induces release of DA via a carrier-independent mechanism, perhaps involving an exocytotic release process. In contrast, ouabain-induced DA release was reduced by nomifensine but was inhibited to a lesser degree by calcium depletion and TTX. Detailed analyses of these data suggest that although ouabain initially induces release of DA via a carrier-dependent mechanism, an exocytotic process may also be involved. The finding that ouabain-induced DA efflux exhibits a degree of TTX and calcium sensitivity suggests that membrane depolarisation caused by Na+,K(+)-ATPase blockade opens voltage-gated sodium channels and initiates an exocytotic release of DA. The intracellular pools of DA involved in the release of DA induced by veratrine and ouabain were also examined. Depletion of vesicular pools of DA by pretreatment with reserpine reduced the amount of DA release induced by both agents, although this effect was only significant in the case of veratrine. However, in reserpinised animals the residual amount of DA release induced by veratrine was inhibited by nomifensine, a result suggesting that DA may be released via a carrier-dependent process in the absence of vesicular DA. Newly synthesised pools of DA were also depleted by pretreatment with the DA synthesis inhibitor alpha-methyl-p-tyrosine. Under these conditions, both veratrine- and ouabain-induced DA efflux was reduced.(ABSTRACT TRUNCATED AT 400 WORDS)     

Mechanism of action of methylmercury on in vivo striatal dopamine release. Possible involvement of dopamine transporter

Methylmercury (MeHg) produces significant increases in the spontaneous output of dopamine (DA) from rat striatal tissue. The mechanism through MeHg produces such increase in the extracellular DA levels could be due to increased DA release or decreased DA uptake into DA terminals. One of the aims of this study was to investigate the role of DA transporter (DAT) in the MeHg-induced DA release. Coinfusion of 400 microM MeHg and nomifensine (50 microM) or amphetamine (50 microM) produced increases in the release of DA similar to those produced by nomifensine and amphetamine alone. In the same way, MeHg-induced DA release was not attenuated under Ca(2+)-free conditions or after pretreatment with reserpine (10 mg/kg i.p.) or tetrodotoxin (TTX), suggesting that the DA release was independent of calcium and vesicular stores, as well as it was not affected by the blockade of voltage sensitive sodium channels. Thus, to investigate whether depolarization of dopaminergic terminal was able to affect MeHg-induced DA release, we infused 75 mM KCl through the dialysis membrane. Our results clearly showed a decrease induced by MeHg in the KCl-evoked DA release. Taken together, these results suggest that MeHg induces release of DA via transporter-dependent, calcium- and vesicular-independent mechanism and it decreases the KCl-evoked DA release.     

HP 749 enhances calcium-independent release of [3H]norepinephrine from rat cortical slices and synaptosomes

Previous studies have shown that, at concentrations of 1 M and 10 M, HP 749 increased electrically-stimulated release of [³H]norepinephrine (NE) from rat cortical slices. These effects were Ca²⁺-dependent, indicating an effect on release from vesicular stores. At 100 M, HP 749 had two effects. In addition to enhancing the Ca²⁺-dependent electrically-evoked release, it also induced a rise in the basal efflux (spontaneous release) of [³H]NE, which was observed in both cortical slices and synaptosomes. The spontaneous release effect was (1) not blocked by the reuptake inhibitor nomifensine, (2) not affected by removal of external calcium, (3) not blocked by vesicular depletion with reserpine, and (4) not inhibited by the sodium channel blocker tetrodotoxin (TTX). As would be expected, the spontaneous [³H]NE release induced by the cytoplasmic releaser tyramine and the sodium channel activator veratridine were blocked by nomifensine and TTX, respectively. Notably, however, the Ca²⁺-independent veratridine-induced release was completely blocked by 100 M HP 749. The mechanism of spontaneous release of [³H]NE caused by 100 M HP 749 is unresolved at present; however, the data are consistent with this release originating from a cytoplasmic source.     

Anoxia-induced dopamine release from rat striatal slices: involvement of reverse transport mechanism

Incubation of rat striatal slices in the absence of oxygen (anoxia), glucose (aglycemia), or oxygen plus glucose (ischemia) caused significant increases in dopamine (DA) release. Whereas anoxia decreased extracellular 3,4-dihydroxyphenylacetic acid levels by 50%, aglycemia doubled it, and ischemia returned this aglycemia-induced enhancement to its control level. Although nomifensine, a DA uptake blocker, completely protected the slices against anoxia-induced DA depletion, aglycemia- and ischemia-induced increases were not altered. Moreover, hypothermia differentially affected DA release stimulated by anoxia, aglycemia, and ischemia. Involvement of glutamate in DA release induced by each experimental condition was tested by using MK-801 and also by comparing the glutamate-induced DA release with that during anoxia, aglycemia, or ischemia. MK-801 decreased the anoxia-induced DA depletion in a dose-dependent manner. This treatment, however, showed a partial protection in aglycemic conditions but failed to improve ischemia-induced DA depletion. Like anoxia, DA release induced by exogenous glutamate was also sensitive to nomifensine and hypothermia. These results indicate that anoxia enhances DA release by a mechanism involving both the reversed DA transporter and endogenous glutamate. Partial or complete lack of effect of nomifensine, hypothermia, or MK-801 in the absence of glucose or oxygen plus glucose also suggests that experimental conditions, such as the degree of anoxia/ischemia, may alter the mechanism(s) involved in DA depletion.     

Effects of Selective Monoamine Oxidase Inhibitors on the In Vivo Release and Metabolism of Dopamine in the Rat Striatum

Brain microdialysis was used to examine the in vivo efflux and metabolism of dopamine (DA) in the rat striatum following monoamine oxidase (MAO) inhibition. Relevant catecholamines and indoleamines were quantified by HPLC coupled with a electrochemical detection system. The MAO-B inhibitor selegiline only affected DA deamination at a dose shown to inhibit partially type A MAO. Alterations in DA and metabolite efflux were not observed when using the MAO-B-selective dose of 1 mg/kg of selegiline. At 10 mg/kg, selegiline reduced the efflux of DA metabolites to approximately 70% of basal values without affecting DA efflux. K(+)- and veratrine-stimulated DA efflux was not affected by selegiline. Experiments using amphetamine and the DA uptake inhibitor nomifensine demonstrated that the effect of selegiline on DA metabolism was unlikely to be mediated either by inhibition of DA uptake or by an indirect effect of its metabolite amphetamine. The possibility that the effect of selegiline is mediated via a nonspecific inhibition of MAO is discussed. In contrast, the MAO-A inhibitor clorgyline inhibited basal DA metabolism and increased basal and depolarisation-induced DA efflux. A 1 mg/kg dose of clorgyline reduced basal DA metabolite efflux (40-60% of control values) without affecting DA efflux. At 10 mg/kg of clorgyline, DA efflux increased to 253 +/- 19% of basal values, whereas efflux of DA metabolites was reduced to between 15 and 26% of control values. The release of DA induced by K+ and veratrine was not affected by 1 mg/kg of clorgyline but was increased by approximately 200% following pretreatment with 10 mg/kg of clorgyline. The nonselective MAO inhibitor pargyline caused similar but more pronounced alterations in these parameters.(ABSTRACT TRUNCATED AT 250 WORDS)     

YM-09151-2 but Not l-Sulpiride Induces Transient Dopamine Release in Rat Striatum via a Tetrodotoxin-Insensitive Mechanism

Abstract: The effects of the selective dopamine D₂ receptor antagonists YM-09151-2 and l -sulpiride on the in vivo release of dopamine (DA), l -3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) in rat striatum were investigated. The drugs were injected into the striatum through a microinjection needle attached to a dialysis probe. YM-09151-2 (0.1 or 1.0 μg/0.5 μl) injected into the striatum produced a dramatic rapid-onset transient increase in striatal DA release in a dose-dependent manner. However, the DA increase induced by l -sulpiride (15 or 75 ng/0.5 μl) was small and of slower onset. An increase of DOPAC levels by YM-09151-2 was biphasic: The first peak occurred at 40 min, followed by a delayed-onset gradual increase. Slower-onset gradual increases were also found in DOPAC levels after l -sulpiride injection and in HVA levels after injections of both YM-09151-2 and l -sulpiride. The infusion of tetrodotoxin (TTX; 2 μM) revealed two different types of DA release mechanisms: The rapid-onset transient DA release induced by YM-09151-2 was TTX insensitive, whereas the slower-onset DA release induced by l -sulpiride was TTX sensitive. Moreover, the rapid-onset transient DA release was Ca²⁺ independent and was not affected by pre-treatment with l -sulpiride or nomifensine. Therefore, it is concluded that YM-09151-2 injected into the striatum produced a transient striatal DA release that is independent of D₂ receptors and the action potential.     

Dihydropyridine Modulation of Voltage-Activated Calcium Channels in PC12 Cells: Effect of Pertussis Toxin Pretreatment

In this study, we report the effect of pertussis toxin pretreatment on dihydropyridine modulation of voltage-sensitive calcium channels in PC12 cells. The rise in intracellular calcium concentration caused by potassium depolarization is not affected significantly by pertussis toxin pretreatment. Nicardipine, a dihydropyridine derivative, added either before or after potassium-induced depolarization, reduces the resultant elevation in cytosolic calcium level both in control and in pertussis toxin-treated cells. The dihydropyridine agonist Bay K 8644, when added before potassium, is able to enhance the potassium-induced spike of cytosolic calcium levels, an effect significantly reduced by pertussis toxin pretreatment. Moreover, the addition of Bay K 8644 after potassium holds the intracellular calcium concentration at a cytosolic sustained level during the slow inactivating phase of depolarization. This effect of Bay K 8644 is inhibited by nicardipine. Pertussis toxin pretreatment slightly weakens the effect of Bay K 8644 when added after potassium-induced depolarization, whereas it significantly reduces the nicardipine inhibition of cytosolic calcium rise stimulated by potassium and Bay K 8644, but not by potassium alone. In conclusion, our findings suggest that a pertussis toxin-sensitive guanine nucleotide regulatory protein could be involved in the interaction between dihydropyridine derivatives and voltage-dependent calcium channels.     

Osmotic strength differentiates between two types of calcium transport pathways regulating catecholamine secretion from cultured bovine chromaffin cells

Calcium transport and catecholamine secretion was measured in cultured bovine chromaffin cells. Calcium ions which entered the cells following stimulation with either nicotine or 50 mM KCl (high potassium) triggered catecholamine release, but then inactivated the secretory process. The nicotine and the high potassium-induced calcium transport mechanisms were mechanistically distinct, but functionally dependent on each other. The specific evidence is that whereas the high potassium-induced Ca2+ influx was found to be inhibited by hyperosmotic medium, the nicotine-stimulated calcium influx was unaffected under these conditions. High potassium and nicotine-stimulated catecholamine release were also differently affected by hyperosmotic medium. While potassium-stimulated catecholamine release was profoundly inhibited by hyperosmolarity, nicotine-stimulated release was only moderately inhibited. Sequential treatments of cells with nicotine and high potassium, under isotonic physiological conditions, indicate that there is a functional, biochemical communication between the otherwise mechanistically distinct calcium channels. Calcium ions which were found to inactivate these channels may be the basis for such communication.     

In Vivo Microdialysis Evidence for Transient Dopamine Release by Benzazepines in Rat Striatum

Abstract: The effects of benzazepine derivatives on extracellular levels of dopamine (DA) and l -3,4-dihydroxyphenylacetic acid (DOPAC) in the dorsal striatum of freely moving rats were studied using in vivo microdialysis. Direct injection of SKF-38393 (0.5 or 1.5 µg/0.5 µl), a selective D₁ receptor agonist, into the striatum through a cannula secured alongside a microdialysis probe produced a rapid dose-dependent transient increase in striatal DA efflux and a more gradual reduction in efflux of DOPAC. The rapid increase in DA efflux was not affected by infusion of tetrodotoxin (TTX; 2 µ M ) or Ca²⁺-free Ringer's solution and occurred after either enantiomer of SKF-38393. A TTX-insensitive increase in DA level similar to that induced by SKF-38393 was also seen after other benzazepines acting as agonists (SKF-75670 and SKF-82958, each 1.5 µg in 0.5 µl) and antagonists (SCH-23390, 1.5 µg in 0.5 µl) at the D₁ receptor and after (+)-amphetamine. These effects were inhibited by infusion of nomifensine (100 µ M ). It is concluded that the transient increases in striatal DA efflux seen after intrastriatal injection of SKF-38393 and other benzazepines are not mediated by presynaptic D₁ receptors but by an amphetamine-like action on the dopamine transporter.     

EFFECT OF IONS ON STIMULUS-INDUCED RELEASE OF AMINO ACIDS FROM MAMMALIAN BRAIN SLICES

Slices of mammalian brain accumulate amino acids contained in physiological medium. When such tissues were subjected to mild electrical stimulation of short duraation capable of depolarizing neural membranes, there occurred a striking increase in the efflux of exogenous amino acids. The effects on representative acidic, neutral, and basic amino acids were similar. Elevated levels of potassium chloride evoked release of amino acids comparable to electrical stimulation. Electrically stimulated release of [³H]γ-aminobutyric acid was not inhibited by the presence of reduced concentrations of calcium ions. Although amino acids are actively accumulated by liver and kidney slices, electrical stimulation of these tissues failed to release these compounds. Stimulation-induced release was significantly diminished by the presence of small amounts of lithium in the perfusing medium.     

Characterization and Pharmacological Responsiveness of Dopamine Release Recorded by Microdialysis in the Substantia Nigra of Conscious Rats

The extracellular concentration of dopamine (DA) and 3,4-dihydroxyphenylacetic acid in the substantia nigra (SN) and striatum was estimated by microdialysis. The dialysate content of DA from the SN was recorded during infusion of a DA uptake blocker (nomifensine; 5 mumol/L) dissolved in the perfusion fluid. Perfusion of tetrodotoxin (1 mumol/L) produced a virtually complete disappearance of nigral and striatal DA release. Dendritic as well as terminal release of DA was inhibited for several hours when the nerve impulse flow in dopaminergic neurons was blocked by systemic administration of gamma-butyrolactone (750 mg/kg, i.p.). The systemic administration (0.3 mg/kg, i.p.) as well as infusion (1 mumol/L) of the D2 agonist (-)-N-0437 [2-(n-propyl-N-2-thienylethylamino)-5-hydroxytetralin] produced a significant decrease in the release of DA in both the striatum and the SN. DA levels were recorded in the striatum both with and without addition of nomifensine to the perfusion fluid. The decrease in the striatum after (-)-N-0437 was suppressed in the presence of nomifensine. Infusion (1 mumol/L) as well as systemic administration (40 mg/kg) of sulpiride caused a similar increase in the release of striatal DA; this increase was, in both experiments, potentiated by nomifensine coinfusion. Sulpiride administration induced a small increase in the release of nigral DA. Infusion of (-)-N-0437 or (-)-sulpiride into the nigra caused a moderate decrease and increase, respectively, of striatal DA level.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of a carrier transport process on in vivo release and metabolism of dopamine: dependence on extracellular Na+

In vivo microdialysis was utilized to evaluate the role of extracellular Na+ in regulating dopamine (DA) neurotransmission in the caudate-putamen of halothane-anaesthetized rats. Reduction of the extracellular Na+ concentration by introduction of a perfusion media containing 50mM Na+ (with choline replacement) produced an excessive release of DA that could be effectively blocked by nomifensine and Lu 19-005, potent inhibitors of an amine transport carrier. These results substantiate reports of a carrier-mediated efflux of DA from presynaptic terminals. Pretreatment with amphetamine, considered both a DA uptake inhibitor and releaser, did not, however, influence the efflux of DA induced by the low extracellular Na+ environment. Although the release of DA from an apparent non-granular cytosolic pool was greatly enhanced by the low extracellular Na+ environment, 3,4-dihydrophenylacetic acid (DOPAC) levels, which supposedly reflect metabolism of non-vesicular DA, were minimally effected. In contrast, homovanillic acid (HVA) was sensitive to extracellular Na+ and not directly related to extracellular levels of either DA or DOPAC, suggesting the possibility of a Na+-sensitive (carrier-mediated?) process involved in the formation of HVA. Overall, the results of this paper cannot be completely reconciled with the traditional concept of intracellular organization of DA pools and suggests the possibility of various non-granular pools being differentially sensitive to efflux and metabolism.     

Dopaminergic reduction of intracellular calcium: the role of calcium influx

The effects of dopamine (DA) on 45Ca2+ ion movement and prolactin release in dispersed female rat anterior pituitary cells were studied to elucidate the mechanism for DA reduction of intracellular calcium levels. In 45Ca2+ prelabeled cells, DA inhibited fractional calcium efflux and prolactin release simultaneously and continuously in a concentration-dependent manner (IC50 20 nM DA). We then studied unidirectional calcium influx and observed haloperidol-reversible, concentration-dependent DA suppression of calcium influx into unlabeled cells. These data complement and extend reported fluorescent dye studies and suggest that dopamine primarily inhibits calcium influx, thereby reducing intracellular calcium levels, which leads to suppression of prolactin release and is manifest secondarily as a reduction in fractional 45Ca2+ efflux.     

Release of endogenous dopamine from tuberoinfundibular neurons

Release of endogenous dopamine (DA) from arcuate-periventricular nucleus-median eminence fragments has been analyzed in an $\text{in vitro}$ static incubation system.Exposure of these hypothalamic fragments to increasing concentrations of K⁺ ions produced a dose-dependent release of endogenous DA. The highest rate of K⁺-stimulated DA efflux occurred in the first 10 minutes, thereafter it progressively decline reaching prestimulated levels at 30 minutes. If two consecutive depolarizing stimuli of 40 mM KCl were applied to the same hypothalamic fragment, after a 40 minutes rest period, an equivalent release of endogenous DA occurred. Removal of Ca⁺⁺ ions from the incubation medium containing the Ca⁺⁺ chelator EGTA caused a decrease of basal DA efflux and completely prevented the K⁺-induced release of DA.Furthermore when verapamil, a blocker of Ca⁺⁺ entrance, was added to the incubation medium in a concentration of 50 μM, the K⁺-induced DA efflux was completely counteracted, whereas spontaneous release was unmodified.Finally nomifensine, a potent blocker of DA uptake, added $\text{in vitro}$ in a final concentration of 10 μM, significantly reinforced K⁺-induced release of endogenous DA. Since nomifensine did not modify basal DA release, this study confirmed its prevalent uptake blocking property rather than its releasing action on DA.     

Halothane Increases Non-vesicular [<Superscript>3</Superscript>H]dopamine Release from Brain Cortical Slices

Experimental data suggest that halothane anesthesia is associated with significant changes in dopamine (DA) concentration in some brain regions but the mechanism of this effect is not well known. Rat brain cortical slices were labeled with [³H]DA to further characterize the effects of halothane on the release of this neurotransmitter from the central nervous system. Halothane induced an increase on the release of [³H]DA that was dependent on incubation time and anesthetic concentration (0.012, 0.024, 0.048, 0.072 and 0.096 mM). This effect was independent of extracellular or intracellular calcium. In addition, [³H]DA release evoked by halothane was not affected by TTX (blocker of voltage-dependent Na⁺ channels) or reserpine (a blocker of vesicular monoamine transporter). These data suggest that [³H]DA release induced by halothane is non-vesicular and would be mediated by the dopamine transporter (DAT) and norepinephrine transporter (NET). GBR 12909 and nomifensine, inhibitors of DAT, decreased the release of [³H]DA evoked by halothane. Nisoxetine, a blocker of NET, reduced the release of [³H]DA induced by halothane. In addition, GBR 12909, nisoxetine and, halothane decrease the uptake of [³H]DA into rat brain cortical slices. A decrease on halothane-induced release of [³H]DA was also observed when the brain cortical slices were incubated at low temperature and low extracellular sodium, which are known to interfere with the carrier-mediated release of the neurotransmitter. Ouabain, a Na⁺/K⁺ ATPase pump inhibitor, which induces DA release through reverse transport, decreased [³H]DA release induced by halothane. It is suggested that halothane increases [³H]DA release in brain cortical slices that is mediated by DAT and NET present in the plasma membrane.     

Hyperforin depletes synaptic vesicles content and induces compartmental redistribution of nerve ending monoamines

Hyperforin, a phloroglucinol derivative found in Hypericum perforatum (St. John's wort) extracts has antidepressant properties in depressed patients. Hyperforin has a unique pharmacological profile and it inhibits uptake of biogenic monoamines as well as amino acid transmitters. We have recently showed that the monoamines uptake inhibition exerted by hyperforin is related to its ability to dissipate the pH gradient across the synaptic vesicle membrane thereby interfering with vesicular monoamines storage. In the present study we demonstrate that hyperforin induces dose-dependent efflux of preloaded [3H]5HT and [3H]DA from rat brain slices. Moreover, we show that hyperforin attenuates depolarization- dependent release of monoamines, while increasing monoamine release by amphetamine or fenfluramine. It is also demonstrated that preincubation of brain slices with reserpine is associated with dose- dependent blunting of efflux due to hyperforin. Our data indicate that hyperforin-induced efflux of [3H]5HT and [3H]DA reflect elevated cytoplasmic concentrations of the two monoamines secondary to the depletion of the synaptic vesicle content and the compartmental redistribution of nerve ending monoamines.     

Dopamine Transporter-Dependent and -Independent Endogenous Dopamine Release from Weaver Mouse Striatum In Vitro

Abstract: The weaver mutant mouse (wv/wv) has an ～70% loss of nigrostriatal dopamine (DA) neurons, but the fractional DA release evoked by amphetamine (but not a high potassium level) has been shown to be greater from striatal slices of the weaver compared with +/+ mice. In the present work we tested the hypothesis that fractional DA release from weaver striatum would be greater when release was mediated by the DA transporter. Serotonin (5-HT)-stimulated fractional DA release was greater from weaver than from +/+ striatum. The release evoked by 5-HT in the presence of 10 µM nomifensine (an antagonist of the DA transporter) was less than in its absence, but the difference between weaver and +/+ striatum remained. In the presence of nomifensine, 1-(m-chlorophenyl)biguanide, classified as a 5-HT₃ agonist, also induced a greater fractional release from weaver compared with +/+ striatum. When veratridine was used at a low concentration (1 µM), the fractional evoked release of DA was higher from the weaver in the presence and absence of nomifensine. These findings suggest that the reason for the difference in the responsiveness of the two genotypes to these release-inducing agents is not related to DA transporter function.     

Potassium-Stimulated Release of [3HJTaurine from Cultured GABAergic and Glutamatergic Neurons

The effect of depolarizing concentrations of potassium (56 mM) on the release of [3H]taurine was examined in two types of cultured neurons from mouse brain: cerebral cortex neurons, which are largely GABAergic, and cerebellar neurons, which after treatment with kainate consist almost entirely of glutamatergic granule cells. The release of [3H]taurine was compared to that of gamma-[3H]aminobutyric acid [( 3H]GABA) in cortical neurons and to that of D-[3H]aspartate in granule cells. Cortical neurons responded to potassium stimulation (1 min or continuously) by an immediate increase in [3H]GABA efflux of more than six times over the basal efflux, followed by a sharp decline despite the persistence of the stimulatory agent. The potassium-induced release of [3H]GABA was largely calcium-dependent. The release of [3H]taurine was considerably less in magnitude, only doubling after the stimulus, with a time course delayed in both onset and decline. The release of [3H]taurine was partially calcium-dependent and was also decreased in low-chloride solutions. In cerebellar granule cells, exposure to potassium resulted in a large (sixfold) and prompt release of D-[3H]aspartate, largely calcium-dependent. A totally different pattern was observed for the release of [3H]taurine. A stimulatory effect occurred only when cells were exposed continuously to potassium. Taurine efflux was very delayed, with a broad stimulus plateau reached after 15-20 min of stimulation. Taurine release was unaffected by omission of calcium, but it was abolished in a low-chloride medium. These results suggest that taurine is released from cells handling other neuroactive amino acids as neurotransmitters.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regional release of [3H]dopamine from rat brain in vitro: effects of opioids on release induced by potassium, nicotine, and L-glutamic acid

Previous studies have suggested that the release of dopamine (DA) in the rat brain may be sensitive to modulation by opioid agents, including the endogenous opioid peptides (enkephalins and endorphins). The present study examined the effects of morphine and the enkephalin analogue D-Ala2-Met5-enkephalinamide (DALA) on the release of radiolabeled DA from superfused slices of rat brain regions. The release of preloaded [3H]DA was evoked from slices of the caudate-putamen (CP) by application of potassium (K+), nicotine (NIC), or L-glutamic acid (L-GLU). The release of [3H]DA from slices of the nucleus accumbens (NA), olfactory tubercle (OT), and substantia nigra (SN) was evoked by L-GLU. Both K+ and NIC evoked a concentration-related release of [3H]DA from CP slices. K+-induced release was only partially dependent on calcium (Ca2+), while NIC-evoked release was completely Ca2+ independent. Neither morphine nor DALA influenced the release of [3H]DA evoked by K+ or NIC. L-GLU produced a concentration-dependent release of [3H]DA from slices of CP, NA, OT, and SN. In all four brain regions, this release was (a) Ca2+-dependent, (b) strongly inhibited by low concentrations of magnesium (Mg2+), (c) greater than the release evoked by D-GLU, (d) attenuated by the putative L-GLU receptor antagonist glutamic acid diethylester (GDEE), and (e) insensitive to tetrodotoxin (TTX) except in the SN. Morphine produced a significant inhibition of L-GLU-evoked [3H]DA release from all four regions. Naloxone, which by itself had no significant effect on the L-GLU-evoked release of [3H]DA, blocked the inhibitory effect of morphine on this release in the CP but not in the other regions. Levorphanol and dextrorphan were equipotent in reducing the glutamate-stimulated release of [3H]DA from CP slices. DALA had no effect on L-GLU-induced release in any of the brain regions examined. The results indicate that L-GLU provokes regional release of DA by acting at a Mg2+-sensitive glutamate receptor. This release is selectively modified by morphine through a mechanism which is insensitive to naloxone.     

Use of Calcium Antagonism for the Characterization of Drug-Evoked Dopamine Release from the Brain of Conscious Rats Determined by Microdialysis

Dopamine was determined by microdialysis of the striatum of conscious rats. We investigated whether the release of dopamine, induced by nine different pharmacological treatments, was sensitive to calcium antagonism. Calcium antagonism was determined by Mg2+ or Cd2+ infusion. The following conditions were investigated: haloperidol, haloperidol plus GBR 12909, nomifensine, (+)-amphetamine (all administered intraperitoneally), KCl, 1-methyl-4-phenyl-pyridinium ion (MPP+), glutamate, ouabain, and 120 mmol/L magnesium (all applied by infusion through the dialysis membrane). The results on calcium antagonism were combined with data on tetrodotoxin (TTX) sensitivity. With the combined data, three different types of dopamine release were characterized. First, action potential-dependent dopamine release was observed in animals treated with saline, haloperidol, haloperidol plus GBR 12909, nomifensine, and ouabain. Second, action potential-independent release was established in the case of (+)-amphetamine, glutamate, MPP+, and 120 mmol/L Mg2+. Finally, K+-induced dopamine release was classified as TTX independent and calcium dependent. It is concluded that brain dialysis is a powerful method for differentiating between different types of neurotransmitter release.