Biomedical subjects

C Allgaier

Publications and source records attributed to C Allgaier.

At least 37 records · Page 2Linked to original sources

PubMed1999

NMDA receptor characterization and subunit expression in rat cultured mesencephalic neurones.

1. NMDA-induced changes in free intracellular Ca2+ concentration ([Ca2+]i) were determined in individual cultured rat mesencephalic neurones by the fura-2 method. mRNA expression encoding NMDA receptor subunits (NR1, NR2A-D) was examined by RT-PCR. 2. NMDA (1-100 microM, plus 10 microM glycine) induced a concentration-dependent increase in [Ca2+]i (EC50 = 5.7 microM). The effect of NMDA was virtually insensitive to tetrodotoxin (0.3 microM) and nitrendipine (1 microM), but dependent on extracellular Ca2+. 5,7-Dichlorokynurenic acid (10 microM), a specific antagonist at the glycine binding site on the NMDA receptor, abolished the NMDA response. 3. Memantine, an open-channel blocker, and ifenprodil, a preferential non-competitive NR1/NR2B receptor antagonist diminished the NMDA effect with an IC50 value of 0.17 and 1 microM, respectively. Ethanol at 50 and 100 mM caused about 25 and 45%-inhibition, respectively. 4. Agarose gel analysis of the PCR products followed by ethidium bromide fluorescence or CSPD chemiluminescence detection revealed an almost exclusive expression of the NR1 splice variants lacking exon (E) 5 and E22. The 3' splice form without both E21 and E22 exceeded that containing E21 by approximately 4 fold. The relative amounts of NR2A, NR2B, NR2C corresponded to approximately 1:2:1. NR2D mRNA was also detectable. 5. In conclusion, mesencephalic neurones bear ethanol-sensitive NMDA receptors which might be involved in the development of ethanol dependence and withdrawal. The high affinity of NMDA to this receptor, its sensitivity to ifenprodil and memantine may suggest that the mesencephalic NMDA receptor comprises the NR1 splice variant lacking E5, NR2B, and NR2C, respectively.

2-Amino-5-phosphonovalerate↗

PubMed1998

Postnatal development of muscarinic autoreceptors modulating acetylcholine release in the septohippocampal cholinergic system. I. Axon terminal region: hippocampus.

We studied the postnatal development of the release of acetylcholine (ACh) and of presynaptic, release-inhibiting muscarinic autoreceptors in the rat hippocampus. To this end, hippocampal slices (350 microns thick) from rats of various postnatal ages (postnatal day 3 [P3] to P16) were preincubated with [3H]choline and stimulated twice (S1, S2: 360 pulses, 2 ms, 3 Hz, 60 mA) during superfusion with physiological buffer containing hemicholinium-3 (10 microM). In parallel, the activities of hemicholinium-sensitive high-affinity choline uptake (HACU, in synaptosomes) and of choline acetyltransferase (ChAT, in crude homogenates) were determined as markers for the cholinergic ingrowth. In hippocampal slices preincubated with [3H]choline, the electrically evoked overflow of 3H at S1 increased from 0.11 (P3) to 0.81% of tissue 3H (P16), the latter value being still much lower than that of hippocampal slices from adult rats (2.89% of tissue 3H). Already at P3 the evoked overflow of 3H was Ca(2+)-dependent and sensitive to tetrodotoxin, indicating an action potential-evoked exocytotic mechanism of ACh release. The muscarinic agonist oxotremorine (1 microM) significantly inhibited the evoked ACh release in hippocampal slices with increasing effectivity from P4 to P16; no significant effect was detectable at P3. The ACh esterase inhibitor physostigmine and the muscarinic antagonist atropine (1 microM, each) exhibited significant inhibitory and facilitatory effects, respectively, only at P15-16. The specific activities of both hippocampal HACU (pmoles/mg protein/min) and ChAT (nmoles/mg protein/min) continuously increased from P3 to P16. It is concluded (1) that cholinergic nerve terminals arriving at the hippocampal formation during postnatal ingrowth are already endowed with the apparatus for action potential-induced, Ca(2+)-sensitive (exocytotic) ACh release; (2) that, in contrast, the expression of presynaptic muscarinic autoreceptors on these cholinergic axon terminals is delayed; and (3) that autoinhibition due to endogenous ACh develops even later, probably when the density of presynaptic terminals in the hippocampus and hence, the concentration of released ACh has reached a suprathreshold value.

Acetylcholine↗

PubMed1995

Cultured chick sympathetic neurons: modulation of electrically evoked noradrenaline release by P2-purinoceptors.

The present study investigates the pharmacological profile of P2-purinoceptors modulating noradrenaline release from cultured chick sympathetic neurons. ATP (30 microM-3 mM) and 2-methylthio-ATP (3-100 microM), but not alpha, beta-methylene-ATP (up to 100 microM), caused a significant facilitation of electrically evoked [3H]-noradrenaline release when added 2 min before depolarization. The facilitation declined with time of exposure suggesting receptor desensitization. The facilitatory effect was markedly diminished by the P2-purinoceptor antagonists reactive blue 2 (3 microM) and suramin (300 microM), but not changed by mecamylamine (10 microM), a nicotinic receptor antagonist. At 1 mM and higher concentrations, ATP added for 12 min, inhibited noradrenaline release; release was virtually abolished by 6 mM ATP. The inhibitory effect of ATP was slightly diminished by suramin but not affected by reactive blue 2. Electrically evoked [3H]-noradrenaline release remained unaffected in the presence of the adenosine (P1)-receptor agonists R(-)N6-(2-phenylisopropyl)adenosine (R-PIA), 2-[p-(2-carboxyethyl) phenylethylamino]-5'-N-ethylcarboxamidoadenosine (NECA), and N6-2-(4-aminophenyl)ethyladenosine (APNEA), used up to 1 microM. The present results confirm the existence of two P2-purinoceptors affecting noradrenaline release: 1) a facilitatory receptor which is activated by 2-methylthio-ATP as well as ATP, and blocked by suramin as well as reactive blue 2, and 2) an inhibitory receptor which is activated by ATP, only slightly affected by suramin but not at all by reactive blue 2 and does not belong to the established P2-purinoceptor subtypes.

Adenosine Triphosphate↗

PubMed1995

Cultured chick sympathetic neurons: ATP-induced noradrenaline release and its blockade by nicotinic receptor antagonists.

The ATP-induced increase in tritium outflow from cultured chick sympathetic neurons prelabelled with [3H]-noradrenaline was investigated. Seven days-old dissociated cell cultures of embryonic paravertebral ganglia, loaded with [3H]-noradrenaline (0.05 microM), were superfused in the presence of (+)-oxaprotiline and exposed to ATP, ATP-analogues, or 1,1-dimethyl-4-piperazinium (DMPP) for 2 min. ATP (3 microM-3 mM), 2-methylthio-ATP (3-100 microM), as well as DMPP (10 and 100 microM) induced a significant overflow of tritium. The EC50-value of ATP was 20 microM. Both the ATP-induced and the DMPP-induced tritium overflow was Ca(2+)-dependent and sensitive to tetrodotoxin (0.3 microM) and omega-conotoxin (0.1 microM); in addition, it was inhibited by the alpha 2-adrenoceptor agonist 5-bromo-6-(2-imidazoline-2-ylamino)-quinoxaline (UK-14,304; 1 microM). The effects of ATP and DMPP were not additive. The ATP-induced as well as the DMPP-induced overflow of tritium was diminished by the P2-purinoceptor antagonists suramin (300 microM) and reactive blue 2 (3 microM); in all 4 cases, the inhibition amouted to approximately 40%. The tritium overflow induced by ATP or DMPP was almost abolished by the nicotinic receptor antagonist mecamylamine (10 microM) and markedly inhibited by hexamethonium (100 microM). Neither ATP nor electrical stimulation caused an overflow of tritium from cultures loaded with [3H]-choline. The results suggest that ATP at mumolar concentrations induces noradrenaline release from cultured chick sympathetic neurons via an action on a subclass of the nicotinic cholinoceptor.

Adenine Nucleotides↗

PubMed1995

Cultured chick sympathetic neurons: prostanoid EP1 receptor-mediated facilitation of noradrenaline release.

Prostanoid EP receptor-mediated modulation of noradrenaline release from cultured chick sympathetic neurons was investigated. Transmitter release from dissociated cell cultures of embryonic paravertebral ganglia, loaded with [3H]-noradrenaline, was elicited either by electrical field stimulation (36 pulses/3 Hz) or by elevating the extracellular concentration of K+ (to 30 mM; for 2 min). Prostaglandin E2 (PGE2; 0.01-3 microM) enhanced electrically evoked [3H]-noradrenaline release in a concentration-dependent manner with a maximal increase by about 50% at 1 microM. Also iloprost (0.1-3 microM) increased transmitter release concentration- dependently, whereas misoprostol (0.1-3 microM) had no effect. Indometacin (10 microM) influenced neither evoked release per se nor the enhancement caused by PGE2- AH6809 (3 microM), a selective EP1 receptor antagonist, blocked the enhancement caused by both PGE2 and iloprost K(+)-evoked noradrenaline release, which was virtually insensitive to tetrodotoxin (0.3 microM), was increased by PGE2 to an extent comparable to that observed after electrical stimulation. In summary, the present data indicate that PGE2 facilitates noradrenaline release from cultured chick sympathetic neurons by a receptor which shows the pharmacological profile of the EP1 subtype and is probably located at the processes of the neuron.

Animals↗

PubMed1995

Effects of 5-HT receptor agonists on depolarization-induced [3H]-noradrenaline release in rabbit hippocampus and human neocortex.

1. The present study attempted to determine whether noradrenaline (NA) release in rabbit hippocampus and human neocortex is modulated by presynaptic 5-hydroxytryptamine (5-HT) receptors. 2. Slices of rabbit hippocampus and human neocortex, loaded with [3H]-noradrenaline ([3H]-NA) were superfused and the effects of 5-hydroxytryptamine (5-HT) receptor ligands on electrically evoked [3H]-NA release were investigated. 3. In rabbit hippocampus, 5-HT, 5-carboxamidotryptamine (5-CT; 32 microM) and 2-CH3-5-HT (32 microM) increased [3H]-NA release elicited with 360 pulses/3 Hz. Facilitation of transmitter release was not influenced by the 5-HT3 receptor antagonist, tropisetron but was prevented by the alpha 2-adrenoceptor antagonist, rauwolscine. When autoinhibition was avoided by stimulating the tissue with 4 pulses/100 Hz (pseudo-one pulse-(POP) stimulation), 2-CH3-5-HT decreased evoked transmitter release, whereas 5-HT and 5-CT had no effect. Inhibition caused by 2-CH3-5-HT was not affected by tropisetron but counteracted by the alpha 2-adrenoceptor ligands, clonidine and rauwolscine. Inhibition caused by clonidine was diminished in the presence of 5-CT or 2-CH3-5-HT. 4. In human neocortex, [3H]-NA release elicited with 360 pulses/3 Hz was increased by 10 microM 5-HT and 32 microM 5-CT, whereas 2-CH3-5-HT was ineffective. [3H]-NA release evoked with a modified POP stimulation (2 bursts of 4 pulses/100 Hz, 3.5 min apart) was not affected by 2-CH3-5-HT or 5-CT. 5. The present results indicate that 5-HT, 2-CH3-5-HT and 5-CT can act on presynaptic alpha 2-autoreceptors as partial agonists (2-CH3-5-HT; in rabbit hippocampal tissue) or antagonists (5-HT and 5-CT; in tissue of rabbit hippocampus and human neocortex). Furthermore the existence of autoinhibition dictates whether these drugs cause facilitation of release, inhibition or have no effect.

Age Distribution↗

PubMed1994

P2 purinoceptors modulating noradrenaline release from sympathetic neurons in culture.

ATP (1 mM) inhibited, whereas 2-methylthio-ATP (30 microM), a P2Y-selective purinoceptor agonist, increased electrically evoked release of [3H]noradrenaline from chick sympathetic neurons. The P2X-selective purinoceptor agonist alpha,beta-methylene-ATP (30 microM) had no effect. The ATP-induced inhibition of release as well as the facilitation caused by 2-methylthio-ATP was not affected by the selective adenosine (P1) receptor antagonist 8-(p-sulfophenyl)-theophylline (8-PST; 100 microM), but completely prevented by the non-selective P2 antagonist suramin (300 microM). The present data reveal a dual regulation of noradrenaline release from sympathetic neurons. Facilitation seems to be mediated by a P2Y purinoceptor, whereas inhibition is caused by a P2 purinoceptor which needs further subtype characterization.

Adenosine Triphosphate↗

PubMed1994

Presynaptic opioid receptors on dopaminergic nerves in the rabbit caudate nucleus: coupling to pertussis toxin-sensitive G-proteins and interaction with D2 autoreceptors?

Slices of the rabbit caudate nucleus, preincubated with [3H]dopamine and subjected to electrical field stimulation, were used (1) to investigate the involvement of G-proteins in the signal transduction of presynaptic D2 (auto)receptors and kappa-opioid receptors on dopaminergic axon terminals in this tissue and (2) to study a possible mutual interaction of these two presynaptic receptors. Pretreatment of the slices with either pertussis toxin (8 micrograms/ml; 18 h), or N-ethylmaleimide (30 microM, 30 min) significantly reduced the inhibitory effects of both the D2 agonist quinpirole and the kappa-opioid receptor agonist U-50488H on the [3H]overflow evoked by 36 pulses (2 ms, 24 mA, 0.3 Hz), suggesting the coupling of both receptors to G-proteins. Experiments designed to study possible interactions of these two presynaptic receptors were carried out under stimulation conditions (only 1 pulse), which strongly diminish interference of endogenous transmitters released in the tissue with modulatory effects of exogenous drugs. For instance, due to the presence of endogenous dopamine, quinpirole was much less potent during 36-pulse-than during 1-pulse field stimulation, whereas the D2 antagonist domperidone was almost without effect in the latter case. Using the 1-pulse stimulation paradigm, the concentration/response curve of quinpirole was unaffected in the presence of the half-maximal inhibitory concentration of U-50,488 H (0.1 microM). On the other hand, also quinpirole at its half-maximal inhibitory concentration (0.1 microM), hardly affected the concentration/response curve of U-50,488 H: only high concentrations of U-50,488 H (above 1 microM) seemed to be slightly less effective in the presence than in the absence of the D2 agonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Modulation of electrically evoked [3H]-noradrenaline release from cultured chick sympathetic neurons.

In the present study we attempted a comprehensive characterization of modulation of noradrenaline release from chick sympathetic neurons. To this purpose sympathetic neurons derived from chick lumbosacral paravertebral ganglia and kept in culture for 7 days were loaded with 0.05 mumol/l [3H]-noradrenaline and subjected to electrical field stimulation (36 pulses/3 Hz). Since the released transmitter was partially recaptured, superfusion was usually performed in the presence of (+)-oxaprotiline, an inhibitor of noradrenaline re-uptake. [3H]-Noradrenaline was released in a manner which was dependent on extracellular Ca2+ and sensitive to tetrodotoxin (TTX). omega-Conotoxin (omega-CTX; 100 nmol/l) abolished [3H]-noradrenaline release indicating that influx through omega-CTX-sensitive Ca(2+)-channels was essential for transmitter release. 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2- (trifluoromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester ((+/-) Bay K 8644) and 4-(4-benzofurazanyl)-1,4-dihydro-2,6- dimethyl-3-nitro-5-pyridinecarboxylic acid isopropyl ester ((+)-202-791), agonists at L-type voltage sensitive Ca(2+)-channels (VSCCs), increased noradrenaline release and induced, in addition, an overflow of tritium which was Ca(2+)-dependent and prevented by the presence of TTX. The L-type VSCC antagonists (-)-202-791 and (+)-4-(4-benzofurazanyl)-1,4-dihydro-2,6-dimethyl-3,5- pyridinedicar boxylic acid methyl, isopropyl ester) ((+)-PN 200-110) diminished [3H]-noradrenaline release. These data suggest that L-type VSCCs, probably located on the cell body of the neuron, play an additional role in modulation of release.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic alpha-Agonists↗

PubMed1994

New insights into receptor theory, as provided by an artificial partial agonist made-to-measure.

In the present study a mixture of a full agonist (noradrenaline) and a full antagonist (yohimbine) was used to mimic the effects of a partial agonist (clonidine) on alpha 2-autoreceptor-mediated regulation of noradrenaline release in order to learn more about the shape of concentration-response curves in the absence and presence of spare receptors. The sigmoidal shape of the cloud of single experimental data points may be reflected by different curve fits based on either descriptive or mechanistic mathematical models. Only mechanistic models allow the interpretation of the relationship between occupancy of receptors and induced response. The experiments were performed in rat neocortex and in rabbit hippocampus tissue where electrical field stimulation with 4 pulses/100 Hz of slices prelabelled with [3H]noradrenaline elicited the release of noradrenaline. A receptor reserve was found in the rabbit hippocampus and quantified from the concentration-response curve of noradrenaline in this tissue using a mechanistic general response function, developed to reflect the condition of spare receptors. The mixture of noradrenaline and yohimbine, NA-Yoh, (three parts to one part), corrected by the different affinities to the alpha 2-autoreceptors, was designed to mirror the quantified proportion of 75% non-spare and 25% spare alpha 2-autoreceptors. In the spare receptor-free rat cortex NA-Yoh acted like a typical partial agonist, as clonidine, with nearly the same EC50 (= Kd in this case) as the full agonist noradrenaline, but with a maximum effect significantly lower than that of noradrenaline. In the rabbit hippocampus, however, the same maximum effect was obtained with NA-Yoh, noradrenaline and clonidine.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Co-release of noradrenaline and ATP from cultured sympathetic neurons.

The vesicles of postganglionic sympathetic axons store both noradrenaline and ATP. The theory of noradrenaline-ATP co-transmission implies that both compounds are released by nerve action potentials and elicit postjunctional effects. Many properties of postjunctional responses support the theory. However, neural release of ATP has been difficult to detect biochemically: by far the major part of the overflow of ATP from intact tissues upon sympathetic nerve stimulation comes from non-neural elements, especially smooth muscle and endothelial cells. Here we describe a parallel electrically evoked overflow of [3H]noradrenaline and endogenous ATP from cultured chick sympathetic neurons. The overflow was abolished by tetrodotoxin, omega-conotoxin and withdrawal of Ca2+, was increased by tetraethylammonium and 4-aminopyridine, and was not changed by prazosin or suramin. The results demonstrate directly the action potential-evoked, Ca(2+)-dependent and presumably vesicular and exocytotic release of ATP from postganglionic sympathetic neurons. They support the co-transmitter theory and suggest that cultured sympathetic neurons are a preparation in which noradrenaline-ATP co-release can be examined free from postjunctional components.

4-Aminopyridine↗

PubMed1993

Effect of K+ channel blockers on the alpha 2-adrenoceptor-coupled regulation of electrically evoked noradrenaline release in hippocampus.

The question whether presynaptic alpha 2-adrenoceptors regulating noradrenaline release in hippocampus directly couple to tetraethylammonium chloride (TEA) or alpha-dendrotoxin (alpha-DTX)-sensitive K+ channels was investigated. Hippocampal slices, prelabelled with [3H] noradrenaline, were superfused in the presence of (+)-oxaprotiline and electrically stimulated with 4 pulses delivered at 100 Hz, in order to avoid autoinhibition due to released noradrenaline. TEA enhanced the evoked [3H]noradrenaline release in rabbit hippocampus in a concentration-dependent manner, yielding an approximately 4-fold increase at 30 mmol/l, whereas the spontaneous outflow of tritium was only slightly affected at this concentration. The alpha 2-adrenoceptor agonist clonidine, at 10-100 nmol/l inhibited the evoked [3H]noradrenaline release between 77% and 96%. The inhibitory effect of the alpha 2-agonist was distinctly diminished in the presence of 30 mmol/l TEA but was restored in low Ca2+/high Mg2+ buffer. Therefore, the diminution of the alpha 2-agonist effect by TEA observed in experiments with normal Ca2+ can be explained by an increase of the Ca2+ availability for the release process due to the prolongation of action potentials. In rabbit hippocampus alpha-DTX (10-200 nmol/l) did neither affect the evoked release of [3H]noradrenaline nor its alpha 2-agonist-induced modulation. However, in rat hippocampus alpha-DTX significantly increased the evoked transmitter release and diminished the effect of clonidine. Taken together, the present data for the rabbit hippocampus exclude the possibility that activation of presynaptic alpha 2-adrenoceptors inhibits depolarization-evoked [3H]noradrenaline release by inducing an outward K+ current through TEA- or alpha-DTX-sensitive K+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Mathematical modelling and quantification of the autoinhibitory feedback control of noradrenaline release in brain slices.

Concentration-response curves, reflecting alpha 2-autoreceptor-mediated inhibition of [3H]-noradrenaline release by exogenous noradrenaline in rat cerebral cortex and rabbit hippocampus slices, were analysed in order to test the usefulness of a mathematical model describing the relation between the independent variable, exogenous noradrenaline, and the dependent variable, inhibition of release. This model was based on the assumption of direct proportionality between receptor occupation and response, implying that there is correspondence between the shape of a concentration-binding curve and a concentration-response curve. The experimental concentration-response curves were obtained by different approaches: noradrenaline release from brain slices prelabelled with [3H]-noradrenaline was elicited electrically either by pseudo-one-pulse (POP) stimulation or by stimulation with 36 pulses applied with a frequency of 3 Hz. POP stimulation avoids autoinhibition by released noradrenaline and, therefore, was a suitable touchstone for the applied mathematical model which evaluates by nonlinear regression analysis two primary parameters: the dissociation constant between noradrenaline and the alpha 2-adrenoceptor and the biophase concentration of noradrenaline which reflects the extent of autoinhibition and should be zero under POP conditions. In rat cerebral cortex tissue, the corresponding biophase concentration of endogenous noradrenaline was indeed estimated to be zero and the dissociation constant was Kd = 10(-7.62 +/- 0.14) mol/l. With 3 Hz stimulation, the biophase concentration was 10(-7.80 +/- 0.05) mol/l, which has to be interpreted with respect to a simultaneously estimated Kd of 10(-7.63 +/- 0.12) mol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Muscarine receptors regulating electrically evoked release of acetylcholine in hippocampus are linked to pertussis toxin-sensitive G proteins but not to adenylate cyclase.

[3H]Acetylcholine release elicited with 360 pulses/3 Hz from slices of rabbit hippocampus is facilitated in the presence of the muscarine (M) receptor antagonist atropine (indicating the existence of autoinhibition) and diminished by the M receptor agonists carbachol and oxotremorine. N-Ethylmaleimide (30 microM) and pertussis toxin (8 micrograms/ml) counteracted antagonist-induced facilitation and agonist-induced inhibition of release, suggesting that a pertussis toxin-sensitive GTP-binding protein is involved in the chain of events mediating activation of M receptors to inhibition of release. Neither 8-bromo-cyclic AMP (300 microM), a membrane analogue of cyclic AMP, nor rolipram (10 microM), a phosphodiesterase inhibitor, affected electrically evoked release of [3H]acetylcholine. They also did not influence the oxotremorine-induced inhibition of transmitter release. In conclusion, no evidence was found for the assumption that activation of M autoreceptors is linked to inhibition of adenylate cyclase.

Acetylcholine↗

PubMed1992

Alpha 2-adrenoceptor mediated inhibition of exocytotic noradrenaline release in the absence of extracellular Ca2+.

The effect of the alpha 2-adrenoceptor agonist clonidine on 3,4-diaminopyridine (3,4-DAP)-evoked [3H]noradrenaline ([32H]NA) release in rat hippocampus slices was studied in the presence or absence (+1 mM EGTA) of extracellular Ca2+. 3H overflow (consisting mainly of unmetabolized [3H]NA) was evoked by addition of 100 microM 3,4-DAP for 10 min to the medium, which always contained 1 microM desipramine. Ligands for L-type voltage-sensitive Ca2+ channels (VSCC) did not affect the evoked [3H]NA release, whereas the preferential N-type VSCC antagonist omega-conotoxin was inhibitory, both in the presence and even more potently in the absence of Ca2+, suggesting an involvement of N-type VSCC in the mechanism of 3,4-DAP-evoked [3H]NA release. In the absence of extracellular Ca2+ the initial Na+ influx, which has been previously proposed to liberate Ca2+ from intracellular stores for the exocytotic process, most probably occurs via N-type VSCC. Clonidine inhibited the 3,4-DAP-evoked [3H]NA release in a concentration-dependent manner, both in the presence and even more potently in the absence of Ca2+; its effects were antagonized by yohimbine. In the presence of extracellular Ca2+ the clonidine effect was not changed by addition of omega-conotoxin. Similar effects of clonidine were found in slices from the rabbit hippocampus. Since the availability of Ca2+ from intracellular stores seems to predominate in the present model, our results lend some support to the suggestion that alpha 2-adrenoceptor activation might affect intracellular mechanisms of Ca2+ homeostasis.(ABSTRACT TRUNCATED AT 250 WORDS)

4-Aminopyridine↗

PubMed1992

Activation of dopamine D1 receptors does not affect D2 receptor-mediated inhibition of acetylcholine release in rabbit striatum.

The possible involvement of dopamine D1 receptors in the regulation of acetylcholine release in the rabbit caudate nucleus was investigated. Caudate slices, preincubated with [3H]choline, were superfused continuously and subjected to electrical field stimulation with only a single pulse. In agreement with the view that the release of acetylcholine evoked by a single electrical pulse is not influenced by endogenous transmitters, atropine and domperidone failed to increase the evoked release of [3H]acetylcholine, whereas oxotremorine and quinpirole caused a concentration-dependent inhibition of transmitter release. Neither the dopamine D1 receptor antagonist SCH 23390 nor the D1 agonist SKF 38393 in a concentration range of 0.01-1 mumol/l changed the evoked [3H]acetylcholine release. The inhibitory effect of the dopamine D2 receptor agonist quinpirole was virtually abolished in the presence of 0.1 mumol/l domperidone and diminished in the presence of 1 mumol/l SCH 23390. It remained unchanged in the presence of 1 mumol/l SKF 38393. It is concluded that the inhibition of acetylcholine release by dopamine is mediated exclusively via presynaptic dopamine D2 receptors and that the antagonistic effect of SCH 23390 on the inhibition of acetylcholine release by quinpirole is due to its interaction with dopamine D2 rather than D1 receptors located on cholinergic nerve terminals.

Acetylcholine↗

PubMed1992

Estimation of the biophase concentration of noradrenaline at presynaptic alpha 2-adrenoceptors in brain slices.

The aim of the present study was to determine the local concentrations of noradrenaline existing at presynaptic alpha 2-adrenoceptors during electrical pulse train stimulation of brain slices at different frequencies. The experiments are based on the assumption that the concentration of released noradrenaline at the alpha 2-adrenoceptors exerting a certain autoinhibition should be equal to the concentration of exogenous noradrenaline causing the same inhibition under conditions in which any influence of the released transmitter is excluded. In order to avoid autoinhibition, hippocampus and cortex slices of the rabbit and the rat, prelabelled with [3H]noradrenaline and superfused in presence of an uptake inhibitor, were electrically stimulated using 4 pulses delivered at 100 Hz (POP stimulation). Exogenous noradrenaline diminished the overflow of tritium elicited by POP stimulation in a concentration-dependent manner. In rabbit brain tissues the EC50 value and maximum inhibition of noradrenaline release were found to be approximately 6 nmol/l and more than 95%, respectively, whereas in rat tissues the corresponding values were between 20 and 30 nmol/l and approximately 90%. When electrical stimulation was performed with trains of 36 pulses delivered at 0.1, 0.3 or 3 Hz in absence or presence of an uptake inhibitor, the alpha 2-adrenoceptor antagonist yohimbine (1 or 10 mumol/l) enhanced the evoked tritium overflow in a manner which was dependent on the frequency of stimulation and on blockade of the re-uptake mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

N-ethylmaleimide diminishes alpha 2-adrenoceptor-mediated effects on norepinephrine release in rat tail arteries.

Isolated tail arteries from Wistar rats, prelabeled with [3H]norepinephrine (NE) were subjected to electrical field stimulation (24 pulses at 0.4 Hz and 200 mA). Both NE release and vasoconstriction were measured in parallel. The selective alpha 2-adrenoceptor agonist B-HT 933 diminished the evoked NE release in a concentration-dependent manner. This effect of B-HT 933 was counteracted by the selective alpha 2-adrenoceptor antagonist rauwolscine, which given alone enhanced evoked transmitter release, indicating the presence of autoinhibition. N-Ethylmaleimide (NEM) (3 microM), which also in itself increased transmitter release, virtually abolished facilitation of release by 0.1 microM rauwolscine and diminished its inhibition by 10 microM B-HT 933. The diminution of the inhibitory effect of B-HT 933 was even more pronounced when the current strength was decreased from 200 mA to 90 mA to compensate for the NEM-induced increase in transmitter release. Treatment of the arteries with NEM did not affect the perfusion pressure. In contrast, however, the B-HT 933-induced increase in basal perfusion pressure was significantly diminished by NEM. Although 10 microM B-HT 933 given alone did not affect stimulation-evoked vasoconstriction, it caused a significant increase in arteries treated with NEM. In conclusion, the observed NEM-sensitivity of the presynaptic and vascular alpha 2-adrenoceptor mechanisms is compatible with the idea that both pre- and postsynaptic alpha 2-adrenoceptors couple to Pertussis toxin (PTX)-sensitive G proteins.

Animals↗