Thyrotrophin-Releasing Hormone Analogues Increase Dopamine Release from Slices of Rat Brain

Abstract: Rat brain slices were incubated with a high concentration of K⁺, thyrotrophin-releasing hormone (TRH), or one of two biologically stable TRH analogues (CG 3509 or RX 77368). Basal release of endogenous dopamine, measured by electrochemical detection, was increased by K⁺ (30 m M ) from slices of hypothalamus, septum, nucleus accumbens, and striatum. CG 3509 (10⁵–10⁻³ M ) increased the release of dopamine from slices of nucleus accumbens, septum, and hypothalamus in a dose-dependent fashion, whereas RX 77368 (10⁻⁴ M ) increased the release of dopamine from the septum only. Neither analogue increased the release of striatal dopamine. The results provide further evidence for specific regional interactions between TRH and dopamine in rat brain.     

Dopamine Inhibition of the Release of Endogenous Acetylcholine from Corpus Striatum and Cerebral Cortex in Tissue Slices and Synaptosomes: A Presynaptic Response?

Abstract: The effect of dopamine on the release of endogenous acetylcholine from striatal slices and synaptosomes and from cerebral cortex synaptosomes was studied. K⁺ (56 m M ) and veratrine (75 μM ) increased the release of acetylcholine from striatal slices by 3.7 and 3.3 times the resting release, respectively. The effect of veratrine was completely abolished by tetrodotoxin (1 μM ). Dopamine (10⁻⁶ to 10⁻³ M ) reduced the K⁺-evoked release of acetylcholine from striatal slices in a dose-dependent manner. The resting release of acetylcholine was also significantly reduced by dopamine. Apomorphine (20 μM ) significantly reduced the K⁺-evoked release of acetylcholine, and both this effect and the inhibition due to dopamine (1 m M ) were significantly antagonised by chlorpromazine (20 μM ). Dopamine had a similar effect on the release of acetylcholine from striatal synaptosome beds; the resting release was depressed 32% by the presence of dopamine (1 m M ). A greater effect of dopamine was seen on the release of acetylcholine from cerebral cortex synaptosome beds, the resting release being reduced by 54% and the K⁺-evoked release by 29%. These results are discussed in terms of the possible role of presynaptic dopamine receptors in controlling the release of acetylcholine and the magnitude of their contribution compared with that of the postsynaptic dopamine receptor.     

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

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

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

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

Induction of nitrate reductase in needles of Scots pine seedlings by NOX and NO3−

The possibility to induce nitrate reductase (NR; EC 1.6.6.2) in needles of Scots pine ( Pinus sylvestris L.) seedlings was studied. The NR activity was measured by an in vivo assay. Although increased NR activities were found in the roots after application of NO₃⁻, no such increase could be detected in the needles. Detached seedlings placed in NO₃⁻ solution showed increasing NR activities with increasing NO₃⁻ concentrations. Exposure of seedlings to NO_x (70–80 ppb NO₂ and 8–12ppb NO) resulted in an increase of the NR activity from 10–20 nmol NO₂⁻ (g fresh weight)⁻¹ h⁻¹ to about 400 nmol NO₂⁻ (g fresh weight)⁻¹ h⁻¹. This level was reached after 2–4 days of exposure, thereafter the NR activity decreased to about 200 nmol NO₂⁻ (g fresh weight)⁻¹ h⁻¹. Analyses of free amino acids showed low concentrations of arginine and glutamine in NO_x-fumigated seedlings compared to corresponding controls.     

Adenosine A2a Receptor-Mediated Modulation of Striatal [3H]GABA and [3H]Acetylcholine Release

Abstract: The ability of adenosine agonists to modulate K⁺-evoked 4D†-[³H]aminobutyric acid ([³H]GABA) and acetylcholine (ACh) release from rat striatal synaptosomes was investigated. The A_2a receptor-selective agonist CGS 21680 inhibited Ca²⁺-dependent [³H]GABA release evoked by 15 m M KCI with a maximal inhibition of 29 ± 4% (IC₅₀ of ∼4 ± 10 ⁻¹² M ). The relative order of potency of three agonists was CGS 21680 ± 5'- N -ethylcarboxamidoadenosine > R-phenylisopropyladenosine (R-PIA), with the inhibition being blocked by A_2a receptor-selective antagonists (CP 66,713 and CGS 15943A) but not by the A₁-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). When release of [³H]GABA was evoked by 30 mM KCI, no significant inhibition was observed. In contrast, CGS 21680 stimulated the release of [³H]ACh evoked by 30 m M KCI, with a maximal stimulation of 26 ± 5% (IC₅₀ of ∼10⁻¹¹ M ). This effect was blocked by CP 66,713 but not by DPCPX. The A₁ agonist R -PIA inhibited [³H]ACh release, an effect blocked by DPCPX. It is concluded that adenosine A_2a receptors are present on both GABAergic and cholinergic striatal nerve terminals where they inhibit and stimulate transmitter release, respectively. Key Words : GABA—Acetylcholine—Adenosine receptors—Striatum.     

Characterisation of Inhibitory 5-Hydroxytryptamine Receptors That Modulate Dopamine Release in the Striatum

Abstract: The effect of a series of indoleamines on the potassium-evoked tritium release of previously accumulated [³H]dopamine from rat striatal slices has been investigated. The indoleamines 5-hydroxytryptamine, 5-methoxy-tryptamine, 5-methoxy- N, N' -dimethyltryptamine and tryptamine (10⁻⁷ to 10⁻³ M) all reduced potassium-evoked release of tritium, to a maximum of 50%. The uptake of [³H]dopamine was unaffected by these compounds. A series of 5-hydroxytryptamine antagonists were examined for their ability to reduce the inhibition of potassium-evoked tritium release induced by 5-methoxytryptamine. The relative order of antagonist potency obtained was methysergide > metergoline > methiothepin > cinanserin > cyproheptadine > mianserin, and was consistent with an action on 5-hydroxytryptamine receptors. It is concluded that there are inhibitory 5-hydroxytryptamine receptors located on the terminals of dopaminergic neurones in the striatum.     

THE RELEASE OF COPPER-COMPLEXING LIGANDS BY THE BROWN ALGA FUCUS VESICULOSUS (PHAEOPHYCEAE) IN RESPONSE TO INCREASING TOTAL COPPER LEVELS

The growth of Fucus vesiculosus L. germlings in chemically defined culture media containing a range of Cu concentrations (20–1000 nM) was monitored simultaneously with measurement of the Cu speciation in the media by competitive equilibrium-adsorptive cathodic stripping voltammetry. Fucus vesiculosus germlings were found to exude Cu-complexing ligands with conditional stability constants of the order of 1.6 × 10¹¹. Ligand concentrations increased with increasing total dissolved Cu concentrations (Cu_T) until a concentration of 500–800 neq Cu·L⁻¹ was reached. Concentrations of the ligand exceeded Cu_T in treatments containing 20 and 100 nM Cu, were similar to Cu_T in the 500-nM Cu treatment, but were less than Cu_T in the 1000-nM treatment. Therefore, [Cu²⁺] were calculated to be at concentrations of 10⁻¹¹− 10⁻¹⁰ M in the 20- and 100-nM treatments, 10⁻⁹ M in the 500-nM treatment, and 10⁻⁷ M in the 1000-nM treatment. Growth rates were lowest at Cu²⁺ concentration > 10⁻⁹. These results are discussed within the context of the potential roles for exuded copper-complexing ligands.     

The interactions of water hardness and pH with the acute toxicity of zinc to the brown trout, Salmo trutta L.

Exposure of brown trout, Salmo trutta , to zinc under continuous flow conditions over 96 h showed that both water hardness and pH exert major influences on the toxicity of the metal. 96-h LC50 values for total zinc ranged from <0.14mg 1⁻¹ in alkaline soft water (pH 8; lOmg 1⁻¹ as CaCO₃) to 3.20 mg 1⁻¹ in acidic hard water (pH 5; 204 mg 1⁻¹ as CaCO₃). A variable reduction in zinc toxicity in hard water compared with soft water over the pH range 4–9 was attributed to high external calcium. Zinc toxicity was positively correlated with decreasing acidity over the pH range 5–7, the metal being most toxic at pH 8–9 where metal complexes predominate. Below pH 5 metal toxicity also increased, irrespective of hardness. Water hardness and pH interacted with zinc toxicity in a complex manner, apparently dependent on physical and chemical transformations of the metal, and as changes in uptake. detoxification and excretion by the fish.     

Releases of Norepinephrine and Dopamine in Ventriculocisternal Perfusions in Hepatectomized and Laparotomized Rats

Abstract: The completely hepatectomized rat has frequently been used as a model to study changes in the economy of norepinephrine (NE) and dopamine (DA) in hepatic coma. Hypothermia characteristically develops in hepatectomized rats and also occurs in patients in hepatic coma and is associated with improved survival in both. The aims of the present study were to measure both release and uptake of NE and release of DA in brain in warm (37°C) and cool (30–32°C) rats at 3–5 h after laparotomy or hepatectomy. Ventriculocisternal perfusions of the brain were performed on rats under basal conditions and during releases evoked by 40 m M K⁺. Basal releases of NE and DA and evoked release of DA were greater in the warm hepatectomized rats than in all other groups. In some studies, 10⁻⁵ M amitriptyline was added to the perfusates to assess whether neuronal uptake was changed after hepatectomy. Uptake of released NE was equally robust in cool hepatectomized as in cool laparotomized rats but could not be measured in warm hepatectomized rats because of amitriptyline toxicity in these rats. Decreases in NE and increases in DA content were found in most areas of the brain after perfusion. Increased releases of NE and DA may contribute to the pathogenesis of hepatic encephalopathy.     

Statistical analysis of nitrate concentrations from the River Frome (Dorset) for the period 1965–76

SUMMARY. Statistical analysis has been carried out on 11 years (1965–75) weekly data of nitrate concentrations and discharge values from the R. Frome (Dorset). The results have shown a trend over the 11-year period with nitrate concentrations increasing by 0.11 mg NO_3-N 1⁻¹ per year. A predictive model for nitrate concentrations for 1976 was made and even though discharge conditions were extreme, the mean predicted nitrate concentration of 3.37 ± 0.136 mgNO_3-N1⁻¹ was very close to the observed mean nitrate concentration of 3.44mgN0_3-N1⁻¹.     

Some effects on the growth of brown trout from exposure to aluminium at different pH levels

Yearling brown trout, Salmo trutta L., were exposed to various concentrations of inorganic aluminium (0–3.7 μM1⁻¹) over a pH range of 4.3–6.5 in a flow-through bioassay apparatus using synthetic test media. Low pH, in the absence of aluminium, produced little effect on growth or survival except at the lowest pH tested (4.3). At pH less than 5.5, concentrations of total aluminium in excess of 1 μM 1⁻¹ (27μg 1⁻¹) were found to retard growth. The effects of a given aluminium concentration were markedly reduced at pH above 5.5.
The change in aluminium toxicity with pH must be related to changes in aluminium chemistry. When growth rates are correlated with the different aluminium species, calculated using thermodynamic equilibrium constants given in the literature, it appears that the Al(OH)^{2 +} species is the most toxic, with a small contribution also coming from polymeric complexes.     

Acute effects of the sodium salt of 2,4-D on the early developmental stages of bleak, Alburnm alburnus

The acute effects of a herbicide based on 2,4-D on embryos and larvae of bleak, Alburnus alburnus L. exposed to concentrations of 25–3200 mg 1⁻¹ for 12–48 hours were investigated. The 2,4-D delayed or stopped the development of embryos in their early stages, caused behavioural changes and morphological alterations. The L.C.₅₀ values for embryos varied between 12–9 and 15·9–4mg 1⁻¹ and for larvae from 51·6 to 111·2mg 1⁻¹ according to the time of exposure. The level of 2,4-D in Lake Balaton, or similar shallow lakes, without substantial injury of the littoral fish fauna should not surpass 0·5–1·0 mg 1⁻¹.     

Loss of the Xeroderma Pigmentosum Group A Gene (XPA) Enhances Apoptosis of Cultured Cerebellar Neurons Induced by UV but Not by Low-K+ Medium

Abstract: To study the involvement of the xeroderma pigmentosum group A gene ( XPA ) in neuronal apoptosis, we cultured cerebellar neurons from mice lacking XPA gene ( XPA ^−/−) and induced apoptosis by exposure to UV irradiation or medium containing a low concentration of potassium (low-K⁺ medium). When cerebellar neurons from postnatal days 15–16 wild-type mice were treated with UV irradiation, apoptotic neuronal death was observed after 24–48 h. About 60% of neurons survived 48 h after UV irradiation at a dose of 5 J/m². On the other hand, neurons from XPA ^−/− mice showed a significantly increased vulnerability to UV irradiation, and >90% of neurons died 48 h after UV irradiation at a dose of 5 J/m². In contrast, low-K⁺ medium induced apoptosis of neurons from mice of each genotype with the same kinetics. These results suggest that the XPA gene is involved in neuronal DNA repair and that it thereby influences apoptosis induced by DNA damage in cultured cerebellar neurons.     

Some effects of low pH and calcium on the growth and tissue mineral content of yearling brown trout, Salmo trutta

Yearling brown trout, Salmo trutta , were exposed to low mineral content water (nominal concentrations of 20μmol 1⁻¹ magnesium, 7.7 μmol 1⁻¹ potassium, 44 μmol 1⁻¹ sodium) over a pH range of 4.0–5.2 with ambient calcium concentrations of 2.5–60 μmol 1⁻¹. All fish died at pH 4.0 and 4.2 irrespective of ambient calcium concentration and also at pH 4.4 with only 2–3 μmol 1 ⁻¹ calcium (that is calcium-free water except for that leached from the diet or excreted by the fish). Good growth rates were obtained over the remaining treatments which extended down to pH 4.4 with as little as 7 μmol 1⁻¹ calcium. When starved, weight loss was inversely correlated with pH. Effects on plasma chloride, percentage dry weight and calcium, potassium sodium, and phosphorus contents of skin, muscle and bone tissue were also investigated. These demonstrated pH effects on mineral metabolism in starved fish, but no effects were detected in fed fish.     

EFFECTS OF CADMIUM TOXICITY ON GROWTH AND ELEMENTAL COMPOSITION OF MARINE PHYTOPLANKTON

Some classes of marine phytoplankton are believed to be more tolerant of high concentrations of trace metals than others, but the results of experimental tests of this hypothesis are ambiguous. Eleven species of phytoplankton representing five classes were grown in Aquil medium containing Cd concentrations between 10⁻⁸ and 10⁻⁵ M ([Cd²⁺]= 10^−9.85 to 10^−6.84 M), and growth rates and intracellular concentrations of Cd, C, N, and S were measured. The mean Cd²⁺ concentration (pCd⁵⁰) that reduced the growth rate of each species to 50% of its maximum varied by 2.5 orders of magnitude, from 10^−6.23 for Emiliania huxleyi to 10^−8.79 for Synechococcus sp. Taxonomic trends in Cd resistance were not apparent in these data. Cadmium quotas (mol Cd·L⁻¹ cell volume) were lowest in species of Bacillariophyceae (ANOVA, P < 0.001), suggesting that they might regulate Cd transport differently than other taxa. Cellular S:C molar ratios increased in four of seven phytoplankton grown at high pCd (7.37–6.84) compared to low Cd ion concentrations (no added Cd), a result of increases in S·L⁻¹ cell volume. Nitrogen:carbon molar ratios were also higher in Cd-exposed phytoplankton, as changes in N and S were highly correlated ( r = 0.98, P < 0.0001). In two species that were examined, S:C ratios increased as a linear function of increasing Cd concentration. The results demonstrate large variability in Cd resistance among phytoplankton that is primarily a function of interspecific differences in Cd detoxification.     

Effect of Tetanus Toxin on Transmitter Release from the Substantia Nigra and Striatum In Vitro

Abstract: The effect of tetanus toxin on the uptake and release of radiolabelled transmitters from slices prepared from substantia nigra (SN) and striatum of rats has been investigated. Tetanus toxin-500–750 mouse lethal doses (MLD)-injected into the SN 6 h before preparing the slices significantly reduced the calcium-dependent, potassium-evoked release of [³H]GABA. Endogenous GABA levels in the SN and [³H]GABA uptake by nigral slices were unaffected by pretreatment with the toxin. Injections of tetanus toxin (1000–2000 MLD) into the striatum significantly reduced the calcium-dependent, potassium-evoked release of [¹⁴C]GABA and also [³H]dopamine, but had no effect on the K⁺-evoked release of [³H]5-hydroxytryptamine or [¹⁴C]acetylcholine. It is concluded that tetanus toxin inhibits GABA release directly and not by interference with synthesis or inactivation processes.     

The N-Methyl-D-Aspartate Antagonist MK-801 Fails to Protect Dopaminergic Neurons from 1-Methyl-4- Phenylpyridinium Toxicity In Vitro

Abstract: Recent reports suggest that NMDA receptor antagonists when administered in vivo can protect dopaminergic neurons from the toxic actions of MPP⁺. In the present study the possible neuroprotective effects against MPP⁺ toxicity of the noncompetitive NMDA receptor antagonist MK-801 was studied in primary cultures of fetal rat mesencephalic dopamine neurons. MK-801 failed to protect dopaminergic neurons from MPP⁺ toxicity at concentrations that completely block NMDA-induced toxicity of these same neurons. In contrast to work carried out in cerebellar granule cells, MPP⁺ toxicity of mesencephalic dopamine neurons was unaffected by preexposure to subtoxic concentrations of either NMDA or cycloheximide. Our findings suggest that the toxic effects of MPP⁺ on dopaminergic neurons are not mediated through a direct interaction with the NMDA subtype of glutamate receptor.     

Calcium-Dependent and -Independent Acetylcholine Release from Electric Organ Synaptosomes by Pardaxin: Evidence of a Biphasic Action of an Excitatory Neurotoxin

Abstract: The effect of pardaxin, a new excitatory neurotoxin, on neurotransmitter release was tested using purely cholinergic synaptosomes of Torpedo marmorata electric organ. Pardaxin elicited the release of acetylcholine with a biphasic dose dependency. At low concentrations (up to 3 × 10⁻⁷ M ), the release was calcium-dependent and synaptosomal structure was well preserved as revealed by electron microscopy and measurements of occluded lactate dehydrogenase activity. At concentrations from 3 × 10⁻⁷ M to 10⁻⁵ M , the pardaxin-induced release of acetylcholine was independent of extracellular calcium, and occluded synaptosomal lactate dehydrogenase activity was lowered, indicating a synaptosomal membrane perturbation. Electron microscopy of 10⁻⁶ M pardaxin-treated synaptosomes revealed nerve terminals depleted of synaptic vesicles and containing cisternae. At higher toxin concentrations ( 10⁻⁵ M ), there were striking effects on synaptosomal morphology and occluded lactate dehydrogenase activity, suggesting a membrane lytic effect. We conclude that, at low concentrations, this neurotoxin is a promising tool to investigate calcium-dependent mechanisms of neurotransmitter release in the nervous system.     

Ethylenediamine as a Specific Releasing Agent of γ-Aminobutyric Acid in Rat Striatal Slices

Abstract: Following incubation with [¹⁴C]y-aminobutyric acid (GABA) or [³H]dopamine, slices of rat striatum were superfused with media containing 36 mM K⁺ or ethylenediamine (EDA), 1 or 5 mM. Both K⁺ and EDA induced a release of [¹⁴C]GABA, the K⁺-induced release being largely Ca²⁺-dependent, while the EDA-induced release was not. Whereas K⁺ also evoked a Ca²⁺-dependent release of [³H]dopamine, EDA evoked no release of dopamine. EDA may therefore have potential as a specific GABA releasing agent.