Release of some endogenous trace amines from rat striatal slices in the presence and absence of a monoamine oxidase inhibitor

The basal and 50 mM K+-stimulated release of m-tyramine (mTA), p-tyramine (pTA), tryptamine (TR) and phenylethylamine (PE) from striatal slices obtained from rats pretreated with a monoamine oxidase inhibitor (MAOI) was investigated. A K+-stimulated release of mTA and pTA was observed, but K+ did not stimulate either TR or PE release. The latter two amines, therefore, are unlikely to be conventional neurotransmitters in the rat striatum. The release of endogenous striatal pTA from control rats was also investigated. Veratridine stimulated endogenous pTA release, but 50 mM K+ did not. It is possible, therefore, that endogenous pTA can be released in a transmitter-like fashion.     

Changes in Levels of Dopamine and Tyramine in the Rat Caudate Nucleus Following Alterations of Impulse Flow in the Nigrostriatal Pathway

Following electric stimulation of the substantia nigra for 1 h there was a substantial increase in dopamine (DA) turnover in the rat caudate nucleus evidenced by an increase in its acid metabolite homovanillic acid (HVA). Concurrently there was an increase in striatal m-tyramine (mTA) and a substantial decrease in p-tyramine (pTA). Lesioning the substantia nigra to decrease impulse flow resulted in a buildup of striatal DA and mTA, but again a decrease in pTA. Following pretreatment with a tyrosine hydroxylase inhibitor, the effects of stimulation of the nigra on mTA were reversed, there being a significant decrease in this amine. The decrease of pTA in response was partially prevented by tyrosine hydroxylase inhibition. The effects of stimulation or substantia nigra lesions on pTA levels were reversed, however, by tyrosine hydroxylase inhibition, a significant increase in this amine being recorded. mTA and DA levels were largely unaffected by a combination of lesion and tyrosine hydroxylase inhibition. The results provide insight into the possible biosynthetic interrelationships between DA and the tyramine isomers in the rat caudate nucleus.     

Release of endogenous dopamine, 3,4-dihydroxyphenylacetic acid,and amino acid transmitters from rat striatal slices

The release of endogenous dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) was measured in superfused striatal slices of the rat and the results compared with data obtained for the release of endogenous (a) DA and DOPAC in the cerebral cortex, nucleus accumbens and thalamus; (b) 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), GABA, and glutamate in the striatum; and (c) GABA, glutamate and 5-HT in the cerebral cortex. In superfused slices of all four CNS regions, there appeared to be a Ca²⁺-dependent, K⁺-stimulated release of endogenous DA. In addition, in slices of the striatum and nucleus accumbens there also appeared to be a Ca²⁺-dependent, 60 mM K⁺ stimulated release of endogenous DOPAC. In the striatum, 16 mM Mg²⁺ was as effective as 2.5 mM Ca²⁺ in promoting the 60 mM K⁺-stimulated release of DOPAC. In addition, 16 mM Mg²⁺ appeared to function as a weak Ca²⁺ agonist since it also promoted the release of DA to approximately 40% of the level attained with Ca²⁺ in the presence of 60 mM K⁺. On the other hand, in the striatum, 16 mM Mg²⁺ inhibited the Ca²⁺-dependent, 60 mM K⁺-stimulated release of GABA and glutamate. Similar Mg²⁺-inhibition was observed in the cerebral cortex not only for GABA and glutamate but also for DA and 5-HT. With the use of -methyl -tyrosine (tyrosine hydroxylase inhibitor), cocaine (uptake inhibitor) and pargyline (monoamine oxidase inhibitor), it was determined that (a) most of the released DA and DOPAC was synthesized in the slices during the superfusion; (b) DOPAC was not formed from DA which had been released and taken up; and (c) DA and DOPAC were released from DA nerve terminals. In addition, the data indicate a difference in the release process between the amino acids and the monoamines from striatal slices since Mg²⁺ inhibited the Ca²⁺-dependent, K⁺-stimulated release of GABA and glutamate and appeared to promote the release of DA and 5-HT.     

Effect of ethanol on [3H]Dopamine release in rat nucleus accumbens and striatal slices

Ethanol (10–200 mM) transiently increased tritium overflow from superfused rat nucleus accumbens slices previously incubated with [³H]dopamine (DA) and [¹⁴C]choline. The effect was greater in striatal tissue and did not appear to be a non-specific membrane effect since [¹⁴C]acetylcholine (ACh) release was not affected. Lack of antagonism by picrotoxin suggested that -aminobutyric acid (GABA) receptors were not involved. Calcium was not a requirement and the DA uptake blocker, nomifensine, was without effect. Ethanol appeared to be causing [³H]DA release into the cytoplasm. K⁺-stimulated release of [³H]DA and [¹⁴C]ACh from nucleus accumbens and striatal slices was not affected. Clonidine-mediated inhibition of the K⁺-evoked release of [³H]DA remained unaltered. Ethanol attenuated the isoproterenol-induced enhancement of [³H]DA release. Ethanol therefore appeared to interact with components of the DA terminal causing a transient increase in the release of neurotransmitter without impairing K⁺-evoked release but apparently interfering with the isoproterenol-induced effect.     

Long-Term Survival of Intrastriatal Dopaminergic Grafts: Modulation of Acetylcholine Release by Graft-Derived Dopamine

The nigrostriatal dopaminergic system of rats was unilaterally lesioned with 6-hydroxydopamine. Part of the animals was grafted 2 weeks later with fetal dopaminergic cells on the lesioned side; untreated rats of the same strain served as controls. Both 3 and 12-14 months after surgery the striatal dopamine (DA) content and the in vivo rotational response following injection of D-amphetamine showed significant changes in grafted as compared to lesioned animals. At 12-14 months after transplantation, the electrically evoked release of tritiated DA and acetylcholine (ACh) in slices (preincubated with [3H]DA or [3H]choline, respectively) of striata of intact, lesioned, or grafted animals was also investigated. Electrical field stimulation of striatal slices of the lesioned side did not evoke any significant [3H]DA overflow, whereas a marked [3H]DA release was observed in slices of grafted and control striata. Moreover, both DL-amphetamine (3 microM) and nomifensine (10 microM) strongly enhanced basal 3H outflow in these slices. Electrically evoked [3H]ACh release was significantly reduced in slices from all striatal tissues by 0.01 microM apomorphine. In slices from denervated striata a clearcut hypersensitivity for this action of apomorphine was present, indicating supersensitivity of DA receptors on cholinergic terminals; this hypersensitivity was significantly reduced in graft-bearing striata. Furthermore, because this hypersensitivity was unchanged in slices of lesioned striata under stimulation conditions (four pulses/100 Hz) avoiding inhibition by endogenously released DA, it is concluded that lesion-induced DA receptor supersensitivity is caused by an increase in receptor density or efficacy rather than by a decreased competition between endogenous and exogenous agonists. Both reuptake blockade of DA with nomifensine (10 microM) and release of endogenous DA by DL-amphetamine (3 microM) potently reduced [3H]ACh release only in control and grafted but not in lesioned tissue. In experiments using potassium-evoked [3H]ACh release, tetrodotoxin had no effect on the inhibitory activity of amphetamine and nomifensine, indicating that the DA receptors involved in their indirect inhibitory action are located directly on the cholinergic terminals.     

Transmitter-Like Release of Endogenous 3,4-Dihydroxyphenylalanine from Rat Striatal Slices

Biphasic electrical field stimulation (0.5-5 Hz, 2 ms, 25 V, 3 min) and high K+ (10-30 mM, 5 min) released endogenous 3,4-dihydroxyphenylalanine (DOPA) from superfused rat striatal slices. Characteristics of the DOPA release were compared with those of 3,4-dihydroxyphenylethylamine (dopamine, DA). Electrical stimulation at 2 Hz evoked DOPA and DA over similar time courses. alpha-Methyl-p-tyrosine (0.2 mM) markedly reduced release of DOPA but not of DA. Maximal release (0.3 pmol) of DOPA was obtained at 2 Hz and at 15 mM K+. The impulse-evoked release of DOPA and DA was completely tetrodotoxin (0.3 microM) sensitive and Ca2+ dependent and the 15 mM K+-evoked release was also Ca2+ dependent. On L-[3,5-3H]tyrosine (1 microM) superfusion, high K+ (15 and 60 mM) released DOPA and DA together with concentration-dependent decreases in tyrosine 3-monooxygenase (EC 1.14.16.2) activity as indicated by [3H]H2O formation, followed by concentration-dependent increases after DOPA and DA release ended. These findings suggest that striatal DOPA is released by a Ca2+-dependent excitation-secretion coupling process similar to that involved in transmitter release.     

Preferential release of newly synthesized [3H]GABA from striatal slices to preloaded [3H]GABA

The release of [³H]GABA which is newly synthesized from [³H]l-glutamic acid (GLU) has been examined using striatal slices obtained from the rat brain. It was found that 8–10% of [³H]GLU transported was converted to [³H]GABA during the incubation of striatal slices in the presence of nipecotic acid (5 × 10^?5 M). Nipecotic acid was added to the medium in order to prevent possible reuptake of [³H]GABA released during its synthesis, and it was found to have no significant effect on the formation of [³H]GABA from [³H]GLU as well as on the uptake of [³H]GLU. The application of high potassium (60 mM) stimulation exhibited a significant enhancement of the release of this newly synthesized [³H]GABA in a Ca²⁺ dependent manner. Kinetic analysis revealed that the evoked release of newly synthesized [³H]GABA was approximately two times greater than that of previously-loaded [³H]GABA, whereas no significant difference was observed in the spontaneous release. An immobilization stress in water failed to affect the release of newly synthesized [³H]GABA from striatal slices despite the occurrence of a significant enhancement of GABA formation in this structure.These results suggest that newly synthesized GABA may be preferentially released from its nerve terminals in response to the excitation of neurons at least in the striatum as compared with previously accumulated GABA.     

Increased dopamine D2 receptor-mediated inhibition of [14C]acetylcholine release in the dorsomedial part of the nucleus accumbens

Dopamine (DA) D2 receptor-mediated inhibition of the K⁺-stimulated release of [¹⁴C]acetylcholine (ACh) from prelabeled rat dorsomedial nucleus accumbens slices was found to be 1.7 times greater than that observed in dorsorostral and ventromedial slices. This observation is consistent with the 1.9 fold higher DA D2 receptor density found in the dorsomedial area. In contrast, there were no differences in the DA D2 receptor-mediated effects on [³H]DA release in these areas. In addition, DA D2 receptor-mediated effects on [³H]DA and [¹⁴C]ACh release could not be demonstrated in the ventrorostral part of the nucleus accumbens consistent with the fact that DA D2 receptors were barely detectable in this area. The results suggest that cholinergic terminals in the dorsomedial part of the nucleus accumbens are under greater inhibitory DA control than in other areas of the nucleus accumbens.     

Regional Differences in Striatal Dopamine Uptake and Release Associated with Recovery from MPTP-Induced Parkinsonism

Abstract : This study directly assessed striatal dopamine (DA) uptake rates and peak release in response to KCl in normal, symptomatic, and recovered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated cats using in vivo electrochemistry. DA uptake rates measured after direct application of known concentrations of DA to the striatum were slowed significantly in both dorsal and ventral striatum in symptomatic cats compared with rates recorded in normal animals. DA uptake rates remained significantly slowed in recovered cats and were not significantly different from the rates recorded in symptomatic animals. In symptomatic cats, both DA uptake rates and the signal recorded in response to KCl stimulation were significantly decreased from normal in all dorsal and ventral striatal regions sampled. Reduction/oxidation (redox) ratios recorded in response to KCl stimulation suggested DA to be the predominant electroactive species. In spontaneously recovered MPTP-treated cats, recordings in the ventral striatum subsequent to KCl stimulation again suggested DA to be the predominant electroactive species released, and peak levels were significantly higher than those recorded in symptomatic animals. In the dorsal striatum of recovered cats, redox ratios recorded subsequent to KCl stimulation suggested serotonin rather than DA to be the predominant electroactive species released. Peak levels of release in the dorsal striatum were not significantly greater than those recorded in symptomatic animals. These results suggest that in spontaneously recovered MPTP-treated cats, there is partial recovery of ventral striatal DAergic terminals, persistent loss of dorsal striatal DAergic terminals, and a down-regulation of DA transporter number/function throughout the striatum. These processes may contribute to volume transmission of DA in the striatum and promote functional recovery.     

Salviae Miltiorrhizae BGE Radix Increases Rat Striatal K+-Stimulated Dopamine Release and Activates the Dopamine Release with Protection Against Hydrogen Peroxide-Induced Injury in Rat Pheochromocytoma PC12 Cells

The present study investigated the effect of the medicinal plant Salviae miltiorrhizae radix (SMR) on dopaminergic neurotransmission in comparison with amphetamine. The effect of SM (0.1 g/ml) on K⁺ (20 mM)-stimulated dopamine (DA) release from rat striatal slices was compared with amphetamine (10⁻⁴ M). Amphetamine and SMR significantly increased K⁺-stimulated DA release (P<0.001) from rat striatal slices when compared with K⁺-stimulated alone. On the other hand, to examine whether in vitro SMR treatment induces DA release in PC12 cells, the role of protein kinases has been investigated in the induction of the SMR-mediated events by using inhibitors of protein kinase C (PKC), mitogen activated protein kinase (MAP kinase) or protein kinase A (PKA). PKC inhibitors chelerythrine (50 and 100 nM), Ro31-8220 (100 nM) and the MAP kinase inhibitor, PD98059 (20 μM) inhibited the ability of SMR to elicit the SMR-stimulated DA release. The direct-acting PKC activator, 12-O-tetradecanoyl phorbol 13-acetate (TPA, 100 nM) mimicked the ability of SMR to elicit DA release. On the contrary, a selective PKA inhibitor, 50 μM Rp-8-Br-cAMP, blocked the development of SMR-stimulated DA release. The results demonstrated that SMR may stimulate DA release and that SMR-induced increases in MAP kinase and PKC are important for induction of the enhancement in transporter-mediated DA release and PKA was also required for the enhancement in SMR-stimulated DA release. SMR treatment (0.1–10 μg/ml) to the hydrogen peroxide (H₂O₂)-treated PC12 cells activated the enzyme activities such as catalase, superoxide dismutase and glutathione peroxidase, and decreased the malondialdehyde level, indicating that SMR has also protective effects against free radical-induced cell toxicity. Therefore, the mechanism by which SMR induces the enhancement in SMR-stimulated DA release is apparent. It remains to be determined whether the effect of SMR on DA function is important in its therapeutic use in the treatment of drug addiction.     

Influence of peptides on 3H-dopamine release from superfused rat striatal slices

The effects of nine neuropeptides on the uptake and release of ³H-dopamine (DA) were studied in slices of rat striatum. Superfusion of preloaded slices at room temperature gave a steady release of ³H-DA after 30 min. Repetitive depolarisation with 20 mM K⁺ evoked pronounced, calcium-dependent releases of ³H-DA which declined exponentially. Substance P (SP), neurotensin and somatostatin all actively released ³H-DA, whilst cholecystokinin octapeptide (CCK), luteinising hormone releasing hormone (LHRH), thyrotrophin releasing hormone (TRH), adrenocorticotrophin (ACTH), vasopressin and oxytocin were without effect on spontaneous ³H-DA efflux. The effects of the peptides on K⁺-induced ³H-DA overflow were more varied; neurotensin, TRH and ACTH had no effect, whereas CCK, somatostatin and LHRH potentiated and SP, vasopressin and oxytocin attenuated ³H-DA release. None of the peptides altered the uptake of ³H-DA at the concentrations which modified its release (0.1–10 μM). It is suggested that modulating DA release may be fundamental to the central actions of these substances in the intact animal.     

Chronic Nicotine Administration Differentially Affects Neurotransmitter Release from Rat Striatal Slices

Abstract: The objective of these experiments was to determine whether the chronic administration of nicotine, at a dose regimen that increases the density of nicotine binding sites, alters the nicotine-induced release of [³H]dopamine ([³H]DA), [³H]norepinephrine ([³H]NE), [³H]serotonin ([³H]5-HT), or [³H]acetylcholine ([³H]ACh) from rat striatal slices. For these experiments, rats received subcutaneous injections of either saline or nicotine bitartrate [1.76 mg (3.6 µmol)/kg, dissolved in saline] twice daily for 10 days, and neurotransmitter release was measured following preloading of the tissues with [³H]DA, [³H]NE, [³H]5-HT, or [³H]choline. Chronic nicotine administration did not affect the accumulation of tritium by striatal slices, the basal release of radioactivity, or the 25 mM KCl-evoked release of neurotransmitter. Superfusion of striatal slices with 1, 10, and 100 µM nicotine increased [³H]DA release in a concentration-dependent manner, and release from slices from nicotine-injected animals was significantly (p < 0.05) greater than release from saline-injected controls; release from the former increased to 132, 191, and 172% of release from the controls following superfusion with 1, 10, and 100 µM nicotine, respectively. Similarly, [³H]5-HT release increased in a concentration-related manner following superfusion with nicotine, and release from slices from nicotine-injected rats was significantly (p < 0.05) greater than that from controls. [³H]5-HT release from slices from nicotine-injected rats evoked by superfusion with 1 and 10 µM nicotine increased to 453 and 217%, respectively, of release from slices from saline-injected animals. The nicotine-induced release of [³H]NE from striatal slices was also concentration dependent but was unaffected by chronic nicotine administration. [³H]ACh release from striatal slices could not be detected when samples were superfused with nicotine but was measurable when tissues were incubated with nicotine. The release of [³H]ACh from slices from nicotine-injected rats was significantly (p < 0.05) less than release from controls and decreased to 36, 83, and 77% of control values following incubation with 1, 10, or 100 µM nicotine, respectively. This decreased [³H]ACh release could not be attributed to methodological differences because slices from nicotine-injected rats incubated with nicotine exhibited an increased [³H]DA release, similar to results from superfusion studies. In addition, it is unlikely that the decreased release of [³H]ACh from striatal slices from nicotine-injected rats was secondary to increased DA release because [³H]ACh release from slices from hippocampus, which is not tonically inhibited by DA, also decreased significantly (p < 0.05) in response to nicotine; hippocampal slices from nicotine-injected rats incubated with 1 and 10 µM nicotine decreased to 42 and 70%, respectively, of release from slices from saline-injected animals. Results indicate that the chronic administration of nicotine increases the ability of nicotine to induce the release of [³H]DA and [³H]5-HT and decreases the ability of nicotine to evoke the release of [³H]ACh but does not alter the nicotine-induced release of [³H]NE from brain slices.     

Effect of muscarinic agonists on the release of 3H-norepinephrine and 3H-dopamine by potassium and electrical stimulation from rat brain slices

The effect of acetylcholine (ACh) on the release of ³H-norepinephrine (NE) from the cerebellum and ³H-dopamine (DA) from the striatum following the administration of potassium chloride or electrical field stimulation was studied in superfused brain slices. ACh in conc. of 10^?6 and 10^?5M significantly inhibited the release of ³H-NE from cerebellar slices and ³H-DA from striatal slices following 2 min infusion of 50mM potassium chloride. In addition ACh produced a dose-dependent inhibition of the release of ³H-DA from striatal slices following electrical stimulation. The results obtained in the present study are quite consistent with the concept that a muscarinic inhibitory mechanism may be operative on noradrenergic and dopaminergic neurons in the brain.     

Studies on the specificity of uptake and release of radiolabelled histamine in rat brain slices

Previously it has been shown that radiolabelled histamine is taken up by brain slices and may subsequently be released by depolarizing stimuli in a calcium-dependent manner, indicating the involvement of neurons in uptake and release of histamine.The present study demonstrates that after incubation of brain slices with low (nM) concentrations of [³H]histamine the amine may be taken up by (and released from) dopaminergic and serotonergic neurons (nerve terminals). Thus 6-hydroxydopamine- and 5,7-dihydroxytryptamine-induced lesions not only reduced the uptake of [³H]dopamine (in striatal slices) and [³H]serotonin (in hippocampal slices), but also, though to a lesser extent, that of [³H]histamine. Immunocytochemical findings revealed that the neurotoxins did not visibly affect histaminergic neurons. Lesioning of noradrenergic neurons appeared not to alter significantly the uptake of [³H]histamine. Further, various drugs acting on either catecholamine-, serotonin- or opioid-receptors and known to cause presynaptic inhibition of the release of [³H]dopamine or [³H]wrotonin from striatal or hippocampal slices also inhibited [³H]histamine release.It is concluded that incubation of brain slices with low concentrations of [³H]histamine does not result in a selective labelling of histaminergic neurons. The possibility that, unlike other monoamines, histamine is not subject to high-affinity uptake by the nerve terminals from which it was released, is discussed.     

Role of excitatory amino acids in the direct and indirect presynaptic regulation of dopamine release from nerve terminals of nigrostriatal dopaminergic neurons

Summary In vivo experiments carried out in halothane-anaesthetized cats implanted with push-pull cannulae demonstrated that glutamate (GLU) released from corticostriatal fibers triggers the release of dopamine (DA), even in the absence of activity in nigral DA cells. As shown in vitro, using rat striatal slices or synaptosomes or in vivo in the cat, both NMDA and AMPA receptors subtypes are involved in the GLU-induced release of DA. Beside this direct regulation, GLU also exert several indirect facilitatory and inhibitory controls on DA release, particularly through cholinergic and GABAergic striatal neurons. Indeed, as shown by numerous authors, the GLU-evoked release of DA is markedly reduced in the presence of tetrodotoxin, bicuculline or atropine or by previous kainate- or ibotenate-induced lesion of striatum. Differences in the presynaptic regulation of DA release in striosomal and matrix compartments have also been found with NMDA and acetylcholine. The effect of acetylcholine was of shorter duration in the matrix than in the striosomal-enriched areas. Two opposite indirect regulations of DA release could be demonstrated: one is facilitatory and involves nicotinic receptors, the other is inhibitory, involves muscarinic receptors and mediated, at least in the matrix by dynorphin containing neurons. The NMDA-evoked responses are of larger amplitude and more sensitive to tetrodotoxin in the matrix than in the striosomes. In conclusion, GLU released from corticostriatal fibers, is able to control the release of DA from terminals of nigrostriatal neurons through direct facilitatory mechanisms (NMDA and AMPA receptors), but also through indirect facilitatory and inhibitory local circuits involving cholinergic and GABAergic neurons.     

Catecholamine exocytosis is diminished in R6/2 Huntington's disease model mice

In this work, the mechanisms responsible for dopamine (DA) release impairments observed previously in Huntington's disease model R6/2 mice were evaluated. Voltammetrically measured DA release evoked in striatal brain slices from 12-week old R6/2 mice by a single electrical stimulus pulse was only 19% of wild-type (WT) control mice. Iontophoresis experiments suggest that the concentration of released DA is not diluted by a larger striatal extracellular volume arising from brain atrophy, but, rather, that striatal dopaminergic terminals have a decreased capacity for DA release. This decreased capacity was not due to an altered requirement for extracellular Ca²⁺, and, as in WT mice, the release in R6/2 mice required functioning vesicular transporters. Catecholamine secretion from individual vesicles was measured during exocytosis from adrenal chromaffin cells harvested from R6/2 and WT mice. While the number of exocytotic events was unchanged, the amounts released per vesicle were significantly diminished in R6/2 mice, indicating that vesicular catecholamines are present in decreased amounts. Treatment of chromaffin cells with 3-nitropropionic acid decreased the vesicular release amount from WT cells by 50%, mimicking the release observed from untreated R6/2 cells. Thus, catecholamine release from tissues isolated from R6/2 mice is diminished because of impaired vesicle loading.     

The Role of GlycineB Binding Site and Glycine Transporter (GlyT1) in the Regulation of [3H]GABA and [3H]Glycine Release in the Rat Brain

The effect of N-methyl-D-aspartic acid (NMDA), a selective glutamate receptor agonist, on the release of previously incorporated [³H]-aminobutyric acid(GABA) was examined in superfused striatal slices of the rat. NMDA (0.01 to 1.0 mM) increased [³H]GABA overflow with an EC₅₀ value of 0.09 mM. The [³H]GABA releasing effect of NMDA was an external Ca²⁺-dependent process and the GABA uptake inhibitor nipecotic acid (0.1 mM) potentiated this effect. These findings support the view that NMDA evokes GABA release from vesicular pool in striatal GABAergic neurons. Addition of glycine (1 mM), a cotransmitter for NMDA receptor, did not influence the NMDA-induced [³H]GABA overflow. Kynurenic acid (1 mM), an antagonist of glycine_B site, decreased the [³H]GABA-releasing effect of NMDA and this reduction was suspended by addition of 1 mM glycine. Neither glycine nor kynurenic acid exerted effects on resting [³H]GABA outflow. These data suggest that glycine_B binding site at NMDA receptor may be saturated by glycine released from neighboring cells. Glycyldodecylamide (GDA) and N-dodecylsarcosine, inhibitors of glycineT1 transporter, inhibited the uptake of [³H]glycine (IC₅₀ 33 and 16 M) in synaptosomes prepared from rat hippocampus. When hippocampal slices were loaded with [³H]glycine, resting efflux was detected whereas electrical stimulation failed to evoke [³H]glycine overflow. Neither GDA (0.1 mM) nor N-dodecylsarcosine (0.3 mM) influenced [³H]glycine efflux. Using Krebs-bicarbonate buffer with reduced Na⁺ for superfusion of hippocampal slices produced an increased [³H]glycine outflow and electrical stimulation further enhanced this release. These experiments speak for glial and neuronal [³H]glycine release in hippocampus with a dominant role of the former one. GDA, however, did not influence resting or stimulated [³H]glycine efflux even when buffer with low Na⁺ concentration was applied.     

Functional and biochemical characteristics of a putative quisqualate-type receptor in rat striatum: Effect of brain lesions

Excitatory amino acids such asl-glutamate (Glu) and quisqualate (QUIS) markedly potentiated K⁺-evoked release of exogeneous [³H]dopamine (DA) from rat striatal slices. Intranstriatal kainic acid injections resulted in a total disappearance of the stimulatory effects of Glu on evoked-release of [³H]DA as well as in a parallel reduction in the maximal number (B_max) of ad-aspartate-insensitivel-[³H]Glu binding site in striatal particulate fractions. Following cortical ablation, the potentiating effect of Glu on [³H]DA release in decorticated striatal slices lasted longer, compared to normal slices, and occured during the 2nd min following K⁺-depolarization. However, the extent (%) of Glu stimulation on [³H]DA release remained the same in decorticated and normal striatal slices. Cortical ablation produced also a significant decrease in the B_max and in theK _d of thed-aspartateinsensitive binding site towardsl[³H]Glu. These results support the proposal that thed-aspartate-insensitive Glu binding site is somehow related to an amino acid receptor-mediated modulation of dopaminergic transmission in the rat corpus striatum.     

Ethanol-Induced Increase in Endogenous Dopamine Release May Involve Endogenous Opiates

The effect of opiate peptides on basal and potassium-stimulated endogenous dopamine (DA) release from striatal slices was studied in vitro. Dual stimulation of the striatal slices gave a reproducible increase in DA release that was calcium dependent. Addition of the delta-opiate receptor agonists Met5-enkephalin, [D-Ala2,D-Leu5]enkephalin (DADLE), and [D-Ser2]Leu-enkephalin-Thr (DSLET), increased the basal DA release without affecting potassium-stimulated release in a dose-dependent manner. The effect of DADLE was antagonized by the addition of naloxone. In contrast, the mu-opioid receptor agonist [D-Ala2,N-MePhe4,Gly-ol5]enkephalin (DAGO) and the epsilon-opioid agonist beta-endorphin inhibited the stimulated DA release without changing the basal release. The inhibitory effect of DAGO on potassium-stimulated release was antagonized by naloxone. The addition of ethanol (75 mM) to the incubation media produced a delayed increase of both the basal and stimulated DA release. There was no change in stimulated DA release when the change in basal release was subtracted, suggesting that ethanol produced a dose-dependent, selective increase in basal DA release. Naloxone and the selective delta-opiate antagonist ICI 174864 inhibited the ethanol-induced increase in basal DA release. Naloxone and ICI 174864 added alone did not alter either basal or stimulated DA release. We therefore suggest that the ethanol-induced increase in basal DA release is an indirect effect involving an endogenous delta-opiate agonist.     

Intraneuronal Site of Action for Imipramine in Rat Striatal Slices

: The uptake of ³H-labeled imipramine ([³H]IMI) in rat corpus striatum slices was found to be strongly temperature-dependent. The accumulation was shown to be saturable and two apparent K_m's were found: 2.2 × 10^?8 and 3.5 × 10^?7m . Once incorporated, the labeled drug was released from superfused slices by K⁺ (55 mm ) depolarization in the presence of calcium ions. Imipramine was also studied for its ability to induce the release of [³H]dopamine ([³H]DA) which had been previously accumulated by striatal slices. It was found that striatal slices superfused during 1 or 6 min with imipramine (10^?6-10^?4m ) release substantial amounts of radioactive dopamine, independently of the presence of Ca²⁺ in the medium. This release is completely abolished after reserpine pretreatment. It is proposed that imipramine enters the dopaminergic storage vesicles and displaces dopamine. An intraneuronal mechanism of action for imipramine is discussed.