Release Of Tritiated Dopamine Research Articles

P2X7 Receptor-Dependent Layer-Specific Changes in Neuron-Microglia Reactivity in the Prefrontal Cortex of a Phencyclidine Induced Mouse Model of Schizophrenia.

Background: It has been consistently reported that the deficiency of the adenosine triphosphate (ATP) sensitive purinergic receptor P2X7 (P2X7R) ameliorates symptoms in animal models of brain diseases.Objective: This study aimed to investigate the role of P2X7R in rodent models of acute and subchronic schizophrenia based on phencyclidine (PCP) delivery in animals lacking or overexpressing P2X7R, and to identify the underlying mechanisms involved.Methods: The psychotomimetic effects of acute i.p. PCP administration in C57Bl/6J wild-type, P2X7R knockout (P2rx7−/−) and overexpressing (P2X7-EGFP) young adult mice were quantified. The medial prefrontal cortex (mPFC) of P2rx7−/− and heterozygous P2X7-EGFP acutely treated animals was characterized through immunohistochemical staining. The prefrontal cortices of young adult P2rx7−/− and P2rx7tg/+ mice were examined with tritiated dopamine release experiments and the functional properties of the mPFC pyramidal neurons in layer V from P2rx7−/− mice were assessed by patch-clamp recordings. P2rx7−/− animals were subjected to a 7 days subchronic systemic PCP treatment. The animals working memory performance and PFC cytokine levels were assessed.Results: Our data strengthen the hypothesis that P2X7R modulates schizophrenia-like positive and cognitive symptoms in NMDA receptor antagonist models in a receptor expression level-dependent manner. P2X7R expression leads to higher medial PFC susceptibility to PCP-induced circuit hyperactivity. The mPFC of P2X7R knockout animals displayed distinct alterations in the neuronal activation pattern, microglial organization, specifically around hyperactive neurons, and were associated with lower intrinsic excitability of mPFC neurons.Conclusions: P2X7R expression exacerbated PCP-related effects in C57Bl/6J mice. Our findings suggest a pleiotropic role of P2X7R in the mPFC, consistent with the observed behavioral phenotype, regulating basal dopamine concentration, layer-specific neuronal activation, intrinsic excitability of neurons in the mPFC, and the interaction of microglia with hyperactive neurons. Direct measurements of P2X7R activity concerning microglial ramifications and dynamics could help to further elucidate the molecular mechanisms involved.

Access to palliative care varies widely across Canada

Repeated cocaine exposure upregulates κ opioids and their receptors in the mesocorticolimbic system; the ensuing κ-mediated dysphoria appears to contribute to addiction and withdrawal. As a potential rehabilitation strategy to reverse cocaine-induced κ sensitization, the present study used tritiated dopamine release assays to examine the induction of κ-opioid tolerance in cultured mesencephalic neurons. Administration of the κ agonist U69,593 inhibited tetrodotoxin-sensitive, spontaneous (EC₅₀ = 1.5 nm), and potassium-stimulated (EC₅₀ = 10 nm) release. These effects were blocked by pertussis toxin and by the κ antagonist nor-binaltorphimine. The 2 d agonist exposure (1 μm) caused a shift in the U69,593 dose–response curve that was greater in the potassium-stimulated paradigm (140-fold) than in the spontaneous release assay (sixfold). These results were attributable to the attenuation of κ-receptor signaling mechanisms and to dependence. In the stimulated release assay, attenuation of κ signaling caused by 4 hr of U69,593 exposure recovered with a half-life of 1.1 hr, whereas attenuation after 144 hr of exposure recovered slowly (<i>t</i>_½ = 20 hr). In the spontaneous release assay, attenuation of κ-opioid signaling occurred slowly (<i>t</i>_½ = 22 hr), and resensitization after a 144 hr exposure was rapid (<i>t</i>_½ &lt; 1 hr). κ-Opioid dependence was observed after 144 hr of U69,593 exposure. Thus multiple mechanisms of adaptation to κ-opioid exposure occur in mesocorticolimbic neurons. These data support the idea that the administration of κ opioids might facilitate drug rehabilitation.

The involvement of dopamine and nitric oxide in the endocrine and behavioural action of endomorphin-1

Previous publications have demonstrated a prominent central and corticotropin releasing hormone-mediated action of the endomorphins (EMs) on both open-field behaviour and the hypothalamo–pituitary–adrenal (HPA) axis. In the present experiments, the direct action of endomorphin-1 (EM1) on pituitary adrenocorticotropic hormone (ACTH) release, adrenal corticosterone secretion and the roles of nitric oxide (NO) and dopamine (DA) in the HPA and behavioural responses elicited by EM1 were investigated in mice. In vitro perifusion studies indicated that the action of EM1 on the HPA system appears to be confined to the hypothalamus, as EM1 did not influence the corticosterone secretion from adrenal slices and moderately attenuated the ACTH release from anterior pituitary slices. In in vivo experiments, N G-nitro- l-arginine (L-NNArg) pretreatment brought about a profound inhibition of both the endocrine and the behavioural responses. On the other hand, haloperidol completely abolished the increases in square crossing and rearing, without affecting corticosterone release. The direct action of EM1 on striatal DA release was therefore also investigated in an in vitro superfusion system. Although EM1 did not influence the basal release of tritiated DA, it significantly enhanced the transmitter release evoked by electric impulses and pretreatment with L-NNArg resulted in a considerable inhibition of the release elicited by EM1. In conclusion, our endocrine studies suggest an important role of NO in the mediation of the EM1-evoked corticosterone secretion. They also indicate that EM1 activates the HPA axis at a hypothalamic level and dopamine is not involved in this process. In contrast, the behavioural experiments reflect that the locomotor activation induced by EM1 is mediated by NO and dopamine, and the superfusion studies demonstrate that NO transmits the dopamine release enhancing effect of EM1.

Formula omitted]-DA release evoked by low pH medium and internal H + accumulation in rat hypothalamic synaptosomes: involvement of calcium ions

The pH fluctuations have been often interpreted as an insufficient regulation or as a consequence of the onset of pathological events, such as ischemia, in which a significant decrease in pH levels occurs. Neurotransmitter release appears to be affected by pH drop significantly. In this study, we investigated the effect of an extracellular and an intracellular acidification on tritiated dopamine release ( [ 3 H] -DA release), from superfused rat hypothalamic synaptosomes. When compared to basal release, extracellular acidification, due to a reduction in the external pH of the nominally carbonic-free superfusion media, provoked a significant increase in [ 3 H] -DA release that showed a sensitiveness to calcium omission. Intraterminal acidification, obtained blocking the Na +/H + exchanger by 5-( N-ethyl- N-isopropyl)-amiloride (EIPA) and 5-( N, N-dimethyl)-amiloride (DMA), induced a significant increase in [ 3 H] -DA outflow which occurred in a calcium-dependent manner (80% inhibition in absence of calcium from superfusion media). To further promote an intraterminal acidification through a H + inner accumulation, the proton ionophore nigericin was used. At every dose employed (10 μM), this compound induced a significant increase in [ 3 H] -DA outflow, compared to basal release. Nigericin-evoked [ 3 H] -DA release showed a 50% decrease when calcium was omitted from superfusion media. When BAPTA-AM, a chelator of intracellular calcium, was added, nigericin-evoked [ 3 H] -DA was completely abolished. These data indicate that [ 3 H] -DA release can be induced by extracellular acidification due to a lowering of external pH and by an intraterminal acidification due to an internal proton accumulation. The mechanism that can trigger this exocytotic process appears to depend on calcium presence, and in particular, on an increased intraterminal calcium availability.

Alterations in mesolimbic dopamine function during the abstinence period following chronic ethanol consumption

Previous work demonstrated that the locomotor stimulant actions of amphetamine, cocaine and nicotine were increased when these drugs were given during the abstinence phase after chronic ethanol consumption. These changes were seen at 6 days and at 2 months after cessation of alcohol. The present study examined neuronal alterations which might be related to these changes in behaviour. Markedly reduced spontaneous firing rates of dopaminergic cells in the ventral tegmental area (VTA) in midbrain slices were seen 6 days into the abstinence period after cessation of chronic ethanol consumption, but by 2 months the firing rates had returned to control values. Increased affinity of striatal receptors for the D1-like receptor ligand 3 H -SCH23390, but no change in the receptor density, was found both at the 6 day and the 2 month intervals. The binding properties of striatal D2-like receptors, of D1-like and D2-like receptors in the frontal cerebral cortex, and the release of tritiated dopamine from slices of striatum or frontal cerebral cortex, were unchanged at 6 days and 2 months. It is suggested that the decreased neuronal firing leads to a persistent increase in sensitivity of D1-like receptors and that these changes could explain the increased effects of the other drugs of abuse.

κ-Opioid Tolerance and Dependence in Cultures of Dopaminergic Midbrain Neurons

Repeated cocaine exposure upregulates kappa opioids and their receptors in the mesocorticolimbic system; the ensuing kappa-mediated dysphoria appears to contribute to addiction and withdrawal. As a potential rehabilitation strategy to reverse cocaine-induced kappa sensitization, the present study used tritiated dopamine release assays to examine the induction of kappa-opioid tolerance in cultured mesencephalic neurons. Administration of the kappa agonist U69,593 inhibited tetrodotoxin-sensitive, spontaneous (EC(50) = 1.5 nM), and potassium-stimulated (EC(50) = 10 nM) release. These effects were blocked by pertussis toxin and by the kappa antagonist nor-binaltorphimine. The 2 d agonist exposure (1 microM) caused a shift in the U69,593 dose-response curve that was greater in the potassium-stimulated paradigm (140-fold) than in the spontaneous release assay (sixfold). These results were attributable to the attenuation of kappa-receptor signaling mechanisms and to dependence. In the stimulated release assay, attenuation of kappa signaling caused by 4 hr of U69,593 exposure recovered with a half-life of 1.1 hr, whereas attenuation after 144 hr of exposure recovered slowly (t(1/2) = 20 hr). In the spontaneous release assay, attenuation of kappa-opioid signaling occurred slowly (t(1/2) = 22 hr), and resensitization after a 144 hr exposure was rapid (t(1/2) < 1 hr). kappa-Opioid dependence was observed after 144 hr of U69,593 exposure. Thus multiple mechanisms of adaptation to kappa-opioid exposure occur in mesocorticolimbic neurons. These data support the idea that the administration of kappa opioids might facilitate drug rehabilitation.

Chronic ethanol administration alters activity in ventral tegmental area neurons after cessation of withdrawal hyperexcitability

The present study investigated the activity of neurons in the mesolimbic dopamine system after the end of the acute phase of the behavioural signs of ethanol withdrawal in mice. This was designed to provide a comparison with earlier behavioural studies, in which greater development of sensitisation to amphetamine and cocaine, but no change in the initial effects of these compounds, or in the behaviour in the absence of drug treatment, was seen when repeated injection of these psychostimulants were given after chronic ethanol consumption. In the present study, single unit recordings were made from dopamine-sensitive neurons in the ventral tegmental area in perfused midbrain slices prepared 24 h after cessation of chronic ethanol consumption. Profound decreases in firing of the ventral tegmental area (VTA) neurons were seen in slices prepared after the ethanol treatment. Firing rates increased after application of N-methyl- dl-aspartate, but still remained lower and more variable after the ethanol treatment. Application of dopamine or amphetamine, following stimulation of firing with a low concentration of N-methyl- dl-aspartate, also resulted in lower firing rates in slices from ethanol-treated mice. No changes were seen in release of tritiated dopamine, in response to applied KCl or amphetamine, from slices of striatum or cerebral cortex, prepared 24 h after cessation of the chronic ethanol consumption, compared with control values. The results demonstrate that very substantial decreases in firing rate, and in the number of active cells, occur in VTA neurons at a time when withdrawal hyperexcitability was no longer apparent and overt changes in behaviour were not seen.

Pathophysiology of schizophrenia: dopamine hypothesis.

T he following artide is the second in our series to highlight the role of psychopharmacology in contributing to our theories of the etiologic basis of mental disorders. In parallel to the catecholamine hypothesis of depression (presented in the fall 1995 issue of the Journal, pp. 523533), the dopamine hypothesis for schizophrenia developed and became influential in the 1960s and 1970s. The origins of this hypothesis can be traced back to 1938, when Shube1 first reported on amphetamine-induced psychosis. This phenomenon was not widely accepted until 1958, when Connell1 published a monograph on a study of 42 cases. Carlsson and Lindqvist in 19632 were the first to suggest that the antipsychotic properties of neuroleptic medication were related to monoaminergic receptor blockade in the brain. The notion that dopamine had a specific role in the pathogenesis of psychosis gained support when it was shown that normal subjects and nonschizophrenic psychiatric patients could become psychotic when administered high-dose amphetamine and L-dopa.6 In 1975, Seeman and Lee7 reported that the dosages of 25 neuroleptic drugs required to inhibit electrically stimulated release of tritiated dopamine correlated closely with the average clinical dose required to control psychosis. They suggested that neuroleptics acted at presynaptic dopamine terminals. The following year, Creese, Burt, and Snyder reported in the article reprinted below that the ability of a wide array of antipsychotic medications to compete for tritiated haloperidol binding (at what is now referred to as the D2 receptor) predicted the dosage required to inhibit dopamine-mediated behavior in animals and to control psychosis in humans. Seeman et al. independently reported similar findings in a report that was published in Nature two months later.8 In a telephone interview, Dr. Creese described the historical context in which he conducted his study. Dr. Creese completed his Ph.D. at Cambridge University with Susan Iversen, with whom he had conducted behavior studies in 6-hydroxydopamine-lesioned rats. This crystallized his interest in the molecular mechanisms of dopamine-mediated behaviors. Dr. Creese took a postdoctoral position at Johns Hopkins with Professor Solomon Snyder, who, along with Candace Pert and Gavril Pasternak, had just identified opiate receptors. Dr. Creese described feeling exhilarated and privileged to work at Johns Hopkins, where many exciting discoveries were made in opiate, dopamine, GABA, glycine, muscarinic, adrenergic, and serotonin receptor identification. Dr. Creese used techniques for studying receptor pharmacology pioneered by Snyder to demonstrate definitively that the antipsychotic properties of multiple neuroleptic medications were related to their ability to bind to 1)2 dopamine receptors. Dr. Creese commented that current research in dopamine receptors continues to implicate dopamine in psychotic disorders, but the nature of that interaction now appears more complex. Postmortem studies of schizophrenic brains have found increased D2 receptors; however, the role of neuroleptic exposure has not been eliminated as a possible confounding factor. PET studies have variably found increased D2 binding in drug-naive patients; findings vary depending on the ligand and the mathematical model used for the analysis. A current hypothesis is that schizophrenic subjects may have an increase in the newly discovered D4 receptor, which has a structure and pharmacology similar to those of the 1)2 receptor. Dr. Creese reported that every year there are new discoveries about the complex nature of dopamine receptors, all of which may contribute to more specific and safer treatment of psychosis.

Differential effects of nitric oxide gas and nitric oxide donors on depolarization-induced release of [ 3H]norepinephrine from rat hippocampal slices

The NO-generating compounds sodium nitroprusside (NP), nitroglycerin (NTG), and isosorbide dinitrate (ISDN) all significantly inhibited N- methyl- d-aspartate (NMDA)-stimulated release of tritiated norepinephrine ([ 3H]NA) from preloaded hippocampal slices of adult male Sprague-Dawley rats with IC 50 's of 114 μM, 1.2 mM, and 1.7 mM respectively. NTG and ISDN also inhibited KCl-stimulated release, while NP had no significant effect on KCl-stimulated release. Although these results suggest that the inhibitory effects of these compounds were mediated by release of NO, NTG and ISDN did not generate detectable levels of NO, and iron-cyanide complexes similar in structure to NP but lacking NO also inhibited release. In contrast, both S- nitroso-N- acetyl- d , l-penicillamine (SNAP) and authentic NO gas significantly enhanced NMDA-stimulated release of [ 3H]NA (EC 50's: 331 and 3.4 μ M respectively). This enhancement was not selective for NMDA-stimulated release, since both SNAP and NO potentiated KCl-stimulated release as well. In addition, NO gas significantly enhanced NMDA-stimulated release of tritiated dopamine ([ 3H]DA) from striatal slices and [ 3H]NA from cortical and cerebellar slices. Analogs of cyclic guanosine monophosphate (cGMP) had no significant effect on NMDA-stimulated transmitter release, suggesting that the observed increase in release is via a cGMP-independent mechanism. While exogenous NO enhanced both NMDA- and KCl-stimulated neurotransmitter release, it appears that endogenous NO does not play a role in this depolarization-induced release since NO synthase inhibitors did not significantly reduce NMDA-stimulated [ 3H]NA release. The possibility remains that endogenous NO could modulate neurotransmitter release in other circumstances.

Mesencephalic dopaminergic neurons in primary cultures express functional neurotensin receptors

The cellular distribution and functional aspects of neurotensin (NT) binding sites in rat mesencephalic cells in primary culture were investigated by an original approach combining anatomical and biochemical studies. Using a double-labeling protocol combining 125I-NT receptor radioautography and tyrosine hydroxylase (TH) immunocytochemistry, we obtained the first direct visualization of NT binding sites on TH-immunoreactive neurons. Eighty percent of the TH neurons were endowed with NT binding sites, which can be observed on both cell bodies and processes. TH-immunoreactive neurons were characterized as dopaminergic neurons by their ability to take up dopamine in a benztropine- and nomifensine-sensitive manner. In the mesencephalic cultures, NT increased potassium-evoked release of tritiated dopamine, and the relative potencies of various NT-related peptides to increase dopamine release were in good agreement with their abilities to bind to NT sites. These results show for the first time that cultured rat mesencephalic dopaminergic cells express functional NT receptors. Finally, the specificity and distribution of NT receptors on dopaminergic neurons in primary culture are quite similar to what was observed in the adult rat brain using pharmacological and radioautographic approaches. These data indicate that NT can influence the activity of dopaminergic neurons at very early stages of the rat brain development.

The putative polyamine antagonists ifenprodil and SL 82.0715 enhance dopamine efflux from rat striatal slices independent of NMDA receptor activation

NMDA stimulated the release of endogenous or tritiated dopamine from rat striatal slices and tritiated norepinephrine from cortical and hippocampal slices. The putative polyamine antagonists ifenprodil and SL 82.0715 inhibited [ 3H]norepinephrine release from cortical and hippocampal slices but enhanced the basal efflux of endogenous and tritiated dopamine from striatal slices. Incubation of striatal slices in a calcium-free buffer did not ameliorate these effects suggesting that the increase in dopamine efflux was not due to a calcium-dependent release process. Superfusion of striatal slices with 10 μM of either ifenprodil, cocaine, or amphetamine resulted in a significant release of tritiated dopamine which was reversed when the slices were again superfused with non-drug-containing buffer. The release observed in the presence of 10 μM ifenprodil (7-fold increase over basal) was intermediate between that observed for cocaine (3-fold increase) and amphetamine (12-fold increase). Both ifenprodil and SL 82.0715 also blocked the uptake of [ 3H]dopamine into striatal synaptosomes with IC 50 values of approximately 1.5 μM. This was again intermediate between the values obtained for cocaine (0.43 μM) and amphetamine (4.2 μM). These results suggest that ifenprodil and its analog SL 82.0715 may act as indirect dopamine agonists by both blocking presynaptic dopamine uptake and by directly increasing the basal efflux of dopamine.

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.

Release of [ 3H]dopamine from rat prefrontal cortex: Modulation through presynaptic cholinergic heteroreceptors

The release of tritiated dopamine ([ 3H]DA) from slices of the rat prefrontal cortex was studied using a superfusion technique. Release appeared to be voltage-dependent and also dependent on external Ca 2+, suggesting the presence of a specific neurotransmitter release mechanism. γ-Aminobutyric acid (GABA), aspartate, glutamate and serotonin had no effect on either basal DA release or K +-stimulated release. Carbachol, a cholinergic agonist, inhibited K +-stimulated DA release. The results demonstrate that cholinergic heteroreceptors on dopaminergic terminals of the prefrontal cortex modulate DA release.

Local and distal effects induced by unilateral striatal application of opiates in the absence or in the presence of naloxone on the release of dopamine in both caudate nuclei and substantiae nigrae of the cat

Halothane-anesthetized cats implanted with push-pull cannulae in both caudate nuclei (CN) and substantiae nigrae (SN) were used to study the effects of naloxone and various opiates when applied into the left CN on the release of newly synthetized tritiated dopamine (DA) from nerve terminals and dendrites of the two nigro-striatal dopaminergic pathways. In all cases, the drugs (naloxone, opiates alone or in the presence of naloxone) were applied for 30 min into the left CN. When applied alone, naloxone (10 −6 M) induced a delayed reduction in tritiated DA release both in the ipsilateral and contralateral CN. These effects were seen after removal of the drug from the superfusion fluid. Complementary experiments made with tritiated naloxone (10 −6 M) revealed that the contralateral effect on DA release was not due to a diffusion of the opiate antagonist from its application site. Locally, d-Ala 2, Met-enkephalinamide ( d-Ala 2, Met-Enk, 10 −6 M) and the potent δ agonist Tyr- d-Ser-Gly-Phe-Leu-Thr (DSThr, 5 × 10 −8 M) induced a biphasic increase in tritiated DA release. The local changes in tritiated DA release evoked by morphine (10 −6 M) and μ agonists such as Tyr- d-Ala-Gly-NH-C 6H 13 (10 −8 M) and fentanyl (10 −8 M) differed from those of δ agonists and furthermore differed from each other. For instance, morphine induced a delayed increase in tritiated DA release whereas a biphasic increase followed by a delayed inhibition occurred with fentanyl. Among all the opiates tested d-Ala 2-Met-Enk was the only one which elicited a distal effect, that is a reduction of tritiated DA release in the ipsilateral SN. Marked differences in these opiates' effects on tritiated DA release occurred both locally and in distal structures when opiates were applied simultaneously with naloxone (10 −6 M). Locally, the changes induced by μ agonists were particularly altered since during morphine's application with naloxone a reduction of tritiated DA release occurred. In addition, the opiate antagonist prevented the second increase and the delayed inhibition of tritiated DA release evoked by fentanyl (10 −8 M). Interestingly, the combined application of naloxone with either d-Ala 2,Met-Enk (10 −6 M) DSThr (5 × 10 −8 M) or morphine (10 −6 M) resulted in the appearance of changes in tritiated DA release in contralateral structures. The most striking effect was seen with d-Ala 2,Met-Enk which enhanced tritiated DA release in the contralateral CN and SN. These results are discussed in the light of the involvement of several types of opiate receptors and of the polysynaptic pathways responsible for the distal changes in dopaminergic transmission.

Somatostatin regulates dopamine release in rat striatal slices and cat caudate nuclei

The effects of somatostatin on the release of tritiated dopamine (DA) formed continuously from tritiated tyrosine were studied in vitro in superfused striatal slices and in vivo in both caudate nuclei and both substantiae nigrae of halothane-anesthetized cats using a push-pull cannula technique. Somatostatin (3 X 10(-10) to 3 X 10(-7) M) increased the spontaneous tritiated dopamine release from rat striatal slices. This effect was dose dependent and was completely prevented by tetrodotoxin (5 X 10(-7) M). When applied for 30 min in one cat caudate nucleus, somatostatin (10(-7) M) immediately increased the local release of tritiated DA, while a gradual inhibition of the tritiated amine's efflux was observed in the contralateral caudate nucleus. No changes in tritiated dopamine were seen in either substantia nigra during or after the peptide's application in the caudate nucleus. These results suggest that somatostatin in the striatum may play a role in the local and the distal control of dopamine release from the terminals of dopaminergic nigrostriatal neurons.

Interference of Methyl Mercury with Monoamine Uptake and Release in Rat Brain Synaptosomes

Abstract: Interference of methyl mercury (Met‐Hg), a known neurotoxin, with the high affinity uptake and the release of tritiated dopamine (DA), noradrenaline (NA) and serotonin (5‐HT) was studied in vitro in rat striatal, cortical and hypothalamic synaptosomes respectively. Met‐Hg proved to be a potent inhibitor of uptake; the IC50's were 2.5 μM (DA), 3.2 μM (NA) and 3.4 μM (5‐HT) in 2 min. incubations. The uptake of NA was slightly inhibited already at 0.1–1 μM Met‐Hg whereas DA and 5‐HT were not. The inhibition was rapid, already after 1 min. exposure to 3 μM Met‐Hg the uptake was inhibited totally. The kinetic pattern of the inhibition of DA uptake best agreed with the non‐competitive type. The spontaneous release of monoamines from preloaded synaptosomes was also potently stimulated and was dependent on Met‐Hg concentration. The sensitivity decreased in the order of DA&gt;5‐HT&gt;NA. The apparent inhibition of uptake is probably partly due to the release of the transmitter amine just taken up. Chelation might also contribute to the inhibition of NA and DA uptake. The detailed mechanisms of these phenomena remain to be studied but they may explain some of the direct CNS effects of Met‐Hg.

Effect of Heavy Metals on Dopamine, Noradrenaline and Serotonin Uptake and Release in Rat Brain Synaptosomes

Abstract: The effects of some heavy metals on the initial high affinity uptake and spontaneous release of tritiated dopamine (3H‐DA), noradrenaline (3H‐NA) and 5‐hydroxytryptamine (3H‐5‐HT) were studied in vitro in rat striatal, cortical and hypothalamic synaptosomes, respectively. As uptake inhibitors, metals were quite inactive in these conditions. At 10 μM Cu2+ was most potent, inhibiting 3H‐DA and 3H‐5‐HT uptake nearly completely while inhibition of 3H‐NA uptake varied. 3H‐DA uptake was in addition inhibited slightly by Zn2+, sometimes by Sn2+ but never by Co2+, Hg2+ or Mn2+. Unexpectedly Pb2+ and Cd2+ tended to increase the synaptosomal 3H‐DA uptake. 3H‐5‐HT uptake was affected least while that of 3H‐NA showed some diversity. Zn2+, Pb2+ and Sn2+ induced inhibition of 3H‐NA uptake possibly by direct interference with 3H‐NA. As to the spontaneous release of tritiated amines during short incubation from preloaded synaptosomes, Cd2+ decreased that of 3H‐DA at high concentrations but Hg2+, Pb2+, Sn2+ and Zn2+ were ineffective. The results suggest that in vitro the uptake and the release of 3H‐DA are more affected than those of other amines. The inhibitory mechanisms of monoamine uptake may include both direct effects on synaptosomes and indirect ones by interference with the amines themselves.

Effects of ω-amino acids on tritiated dopamine release from rat striatum: Evidence for a possible glycinergic mechanism

The effects of GABA and glycine on the release of tritiated dopamine from prelabelled slices of rat striatum have been compared. Both GABA (>50 μM) and glycine (>200 μM) released tritiated dopamine, but had no effect on the release of radiolabelled 5-hydroxytryptamine and GABA. The GABA antagonist picrotoxin (50 μM) markedly reduced the ability of GABA to release [ 3H]dopamine, but had no effect on the glycine response. Conversely strychnine (0.5 μM), a specific glycine receptor antagonist at low concentrations, abolished both the GABA and the glycine response on [ 3H]dopamine release. Two other ω-amino acids, β-alanine and taurine, both at 500 μM, had no effect on [ 3H]dopamine release from rat striatal slices. In additional experiments, release of radioactivity was demonstrated from neonatal rat spinal cord and striatal slices after prelabelling with [ 3H]glycine. This release was calcium-dependent. The possibility that glycine may function as a neurotransmitter substance within the striatum is considered, and the hypothesis that GABA may partially exert some of its pharmacological effects through the glycine receptor is discussed.

L-Dopa-induced release of cerebral monoamines.

L-Dopa markedly increased the efflux of tritiated dopamine and tritiated serotonin from rat brain slices. This action appeared contingent on the decarboxylation of L-dopa to dopamine, since it could be blocked by an inhibitor of L-amino acid decarboxylase. Selective destruction of catecholamine-containing nerve terminals by 6-hydroxydopamine significantly decreased the uptake and release of tritiated dopamine but not that of tritiated serotonin. These observations support the hypothesis that a portion of exogenously administered L-dopa may enter central serotonin terminals and undergo decarboxylation to the amine with resultant displacement of the endogenous indoleamine from vesicular stores.