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A C Dolphin

Publications and source records attributed to A C Dolphin.

At least 109 records · Page 6Linked to original sources
PubMed1987

Adenosine antagonists increase spontaneous and evoked transmitter release from neuronal cells in culture.

To examine the role played by endogenous adenosine in the modulation of transmitter release in the CNS, the effect of adenosine antagonists has been studied. Two systems have been used: EPSPs recorded from pyramidal cells in organotypic hippocampal cultures; and release of newly synthesized [3H]glutamate from cerebellar granule cells in dissociated culture. Bath application of 0.1-1 microM 8-phenyltheophylline (8-PT) reversibly increased both the number and size of spontaneous EPSPs and caused bursting activity in some cells. This effect was blocked by the glutamate antagonist gamma-D-glutamylglycine (DGG) (1 mM) but not by atropine (10 microM) or bicuculline (100 microM). Another adenosine antagonist isobutylmethylxanthine (IBMX, 10 microM) had a similar effect to 8-PT. Spontaneous activity in pyramidal cells and that induced by adenosine antagonists was blocked by the adenosine agonist 2-chloroadenosine (2-CA) (0.2-20 microM). 8-PT (10 microM) markedly potentiated K+-stimulated release of newly synthesized glutamate, and also enhanced basal glutamate release. The agonist (-)-phenylisopropyladenosine ((-)-PIA, 2 microM) which is relatively selective for A1 receptors, reduced by 19 +/- 5% the 8-PT-induced enhancement, and reduced K+-stimulated glutamate release in the absence of 8-PT to a similar extent. In contrast 5'-N-ethylcarboxamido adenosine (NECA, 2 microM), which is a relatively selective A2 agonist, slightly enhanced glutamate release. From these results it is likely that 8-PT potentiates glutamate release in both systems by blocking the effect of endogenous adenosine on presynaptic A1 receptors.

Adenosine↗
PubMed1987

Calcium channel currents and their inhibition by (-)-baclofen in rat sensory neurones: modulation by guanine nucleotides.

1. The effect of intracellular application of the hydrolysis-resistant GTP and GDP analogues, guanosine 5'-O-3-thiotriphosphate (GTP-gamma-S), and guanosine 5'-O-2-thiodiphosphate (GDP-beta-S) has been examined on voltage-activated calcium-channel currents and the ability of the gamma-aminobutyric acid B agonist baclofen to inhibit them, in cultured rat dorsal root ganglion (d.r.g.) neurones. 2. Under control conditions, the calcium-channel current, recorded using the whole-cell patch technique with Ba2+ rather than Ca2+ as the permeant divalent cation, consists of an inactivating and a sustained current. In the presence of 500 microM-GTP-gamma-S included in the patch pipette, the calcium-channel current was activated more slowly and was largely non-inactivating during the 100 ms depolarization voltage step. The effects of GTP-gamma-S were abolished by pre-treatment of cells with pertussis toxin. 3. The calcium-channel current recorded in the presence of 500 microM-GDP-beta-S had a more marked transient component than the control calcium-channel current. The proportion of transient calcium-channel current in the presence of GDP-beta-S was not reduced in Na+-free medium. 4. No statistically significant effects of GTP-gamma-S and GDP-beta-S were observed on the calcium-activated potassium current IK(Ca), the transient outward potassium current activated in Ca2+-free medium, or on the inwardly rectifying current (Ih) activated by hyperpolarization. 5. GTP-gamma-S increased the ability of baclofen to inhibit calcium-channel currents, whereas this was decreased by GDP-beta-S and by pre-treatment of cells with pertussis toxin. The half-maximal effective dose (EC50) for baclofen was 2 microM in the presence of GTP-gamma-S, 15 microM for control and 50 microM in the presence of GDP-beta-S. Comparable results were obtained using a single concentration of the adenosine agonist 2-chloroadenosine (2-CA, 0.05 microM) to inhibit calcium-channel currents; its effect was significantly increased by GTP-gamma-S and reduced by GDP-beta-S. 6. The ability of baclofen to inhibit calcium-channel currents was not affected by 1 microM-forskolin or 50 microM-intracellular cyclic AMP. 7. It is concluded that calcium channels in d.r.g.s are associated with a nucleotide binding protein, and that this mediates the effect of baclofen and 2-CA on calcium-channel currents. The ability of GTP-gamma-S to inhibit the transient component of calcium-channel currents in the absence of agonist may represent a means of differentially regulating calcium-channel activity.

Action Potentials↗
PubMed1986

Regulation of calcium currents by a GTP analogue: potentiation of (-)-baclofen-mediated inhibition.

Voltage-activated calcium currents were recorded in cultured rat dorsal root ganglion neurones using the whole cell clamp technique. The effect of inclusion in the patch pipette of the non-hydrolysable guanine nucleotide analogues, guanosine 5'-O-3-thiotriphosphate (GTP-gamma-S) and guanosine 5'-O-2-thiodiphosphate (GDP-beta-S) was examined both on ICa itself and on the ability of the GABAB agonist baclofen to inhibit ICa. In the absence of either guanine nucleotide, the maximum ICa recorded at between -20 and 0 mV consists of two components, a rapid transient and a non- or slowly inactivating current. Baclofen (50 microM) inhibited peak ICa by 37.5 +/- 4.7%. In the presence of 500 microM GTP-gamma-S the inactivating calcium current was largely absent and the effect of 50 microM baclofen was increased to a 78.1 +/- 2.3% inhibition. GDP-beta-S had opposite effects to GTP-gamma-S. The results suggest that the calcium channels in these neurones are associated with a GTP binding protein which regulates the calcium currents and mediates the inhibitory effect of neurotransmitters.

Animals↗
PubMed1986

Inhibition of calcium currents in cultured rat dorsal root ganglion neurones by (-)-baclofen.

Voltage-dependent inward calcium currents (ICa) activated in cultured rat dorsal root ganglion neurones were reversibly reduced in a dose-dependent manner by (-)-baclofen (10 microM to 100 microM). Baclofen (100 microM) reduced the calcium-dependent slow outward potassium current (IK(Ca)). This current was abolished in calcium-free medium and by 300 microM cadmium chloride. The action of baclofen on IK(Ca) was reduced when the calcium concentration in the medium was increased from 5 mM to 30 mM. The calcium independent fast transient voltage-dependent outward current (IK(Vt] was also reduced by baclofen; this effect remained present when Ca2+-free medium was used to prevent contamination by IK(Ca). 4-Aminopyridine (500 microM) reduced IK(Vt) and induced a small increase in ICa. The action of baclofen on ICa was partially antagonized by 4-aminopyridine. GABAB receptor-mediated inhibition of ICa in cultured rat dorsal root ganglion neurones involves a direct mechanism rather than resulting indirectly from an increase in the residual outward potassium currents activated by depolarization. The reduction in ICa by baclofen was variable and dependent on the amplitude of control ICa, larger currents being more resistant to the baclofen-induced inhibition.

4-Aminopyridine↗
PubMed1986

Calcium-dependent currents in cultured rat dorsal root ganglion neurones are inhibited by an adenosine analogue.

Dorsal root ganglion neurones from 2-day-old rats were grown in dissociated culture. The effect of the adenosine analogue 2-chloroadenosine (2-CA) was investigated on action potential duration and on Ca2+ current (ICa) activation. 2-CA (0.5 microM) shortened both control action potentials and those prolonged by tetraethylammonium (TEA), Ba2+, or intracellular Cs+. This effect was prevented by two adenosine antagonists isobutylmethylxanthine (IBMX, 1-2 mM) and 8-phenyltheophylline (8-PT, 2.5 microM). The inward current, ICa, recorded using the 'whole-cell' patch-clamp technique in medium containing 2.5 microM-tetrodotoxin, 25 mM-TEA and 2.5 mM-Ba2+ was reduced by 2-CA (0.05 microM). The activation of ICa was decreased, but its reversal potential was unchanged. The effect of 2-CA was antagonized by IBMX (1 mM) and 8-PT (1 microM). 2-CA also reduced the large inward tail currents which occurred at the termination of the depolarizing voltage step command in a proportion of neurones. Brief application of 2-CA (0.05 microM) did not affect the inward holding current required to maintain the cells at -80 mV. In the presence of TTX (2.5 microM) and Ca2+ (5 mM), 2-CA decreased the activation of outward K+ currents caused by 5 s depolarizing voltage commands from -80 mV or -40 mV. The GABAB agonist (-)-baclofen (50-100 microM) also shortened the action potential duration and reduced ICa. 8-PT (1 microM) did not prevent the effect of baclofen on ICa. It is concluded that in cultured rat dorsal root ganglion neurones 2-CA reduces ICa activation by a direct effect on an A1 adenosine receptor.

1-Methyl-3-isobutylxanthine↗
PubMed1983

An adenosine agonist inhibits and a cyclic AMP analogue enhances the release of glutamate but not GABA from slices of rat dentate gyrus.

The adenosine agonist 2-chloroadenosine inhibited the K+-induced release of endogenously synthesized [3H]glutamate but not [3H]GABA from slices of rat dentate gyrus. In contrast, the K+-stimulated release of [3H]glutamate was augmented by the adenosine antagonist theophylline and was further enhanced by the cyclic AMP analogue 8-bromo-cyclic AMP in the presence of theophylline.

2-Chloroadenosine↗
PubMed1983

The adenosine agonist 2-chloroadenosine inhibits the induction of long-term potentiation of the perforant path.

The effect of 2-chloroadenosine (5 and 10 microM) on long-term potentiation of the perforant path has been investigated. Experiments were performed in vivo using a push-pull cannula implanted in the dentate gyrus. A high intensity, high frequency train applied to the perforant path during perfusion with the adenosine agonist 2-chloroadenosine, was markedly attenuated in its ability to produce long-term potentiation. Inhibitory effects on pre- and post-synaptic calcium calcium influx are discussed.

2-Chloroadenosine↗
PubMed1983

The excitatory amino-acid antagonist gamma-D-glutamylglycine masks rather than prevents long term potentiation of the perforant path.

The effect of the glutamate antagonist gamma-D-glutamylglycine on the induction of long-term potentiation in the dentate gyrus has been investigated in vivo. gamma-D-glutamyglycine (10(-3) M) perfused through a push-pull cannula into the dentate gyrus, rapidly reduced perforant path evoked potentials. Application to the perforant path of a high-frequency train (250 Hz, 500 ms), which in control animals reliably produced long term potentiation, had no effect on the evoked potentials when applied during blockade by gamma-D-glutamylglycine. This result was obtained despite the inability of gamma-D-glutamylglycine completely to inhibit the evoked potentials. However, when standard medium was reintroduced, potentiation was revealed in animals that had received the high-frequency train, whereas in animals that had received no high-frequency train during gamma-D-glutamylglycine inhibition the potentials returned only to pre-drug levels. In additional experiments, in which the dentate gyrus was continuously perfused with [3H]glutamine, and the steady state outflow of [3H]glutamate was measured, it was observed that gamma-D-glutamylglycine (10(-3) M) increased the steady state release of [3H]glutamate into the perfusate. From this result it is likely that gamma-D-glutamylglycine does not have any presynaptic inhibitory activity at the perforant path-granule cell synapse. The results indicate that a high frequency train applied to the perforant path during a period of inhibition by gamma-D-glutamylglycine was able to induce long term potentiation, whose expression was, however, masked until the glutamate antagonist was removed.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗
PubMed1983

Cyclic nucleotide-dependent protein kinases and some major substrates in the rat cerebellum after neonatal X-irradiation.

The levels of cAMP-dependent protein kinase (type I), or cGMP-dependent protein kinase, or protein I, and of a 23,000 MW substrate for the cGMP-dependent protein kinase were measured in cerebella from normal rats and in the cerebella from rats in which a selective loss of interneurons in the cerebellar cortex had been produced by X-irradiation. A decrease was observed in the concentrations of cAMP-dependent protein kinase and of protein I, whereas an increase was observed in the concentrations of cGMP-dependent protein kinase and of the 23,000 MW substrate. The data, taken together with the results of other studies, support the interpretation that cAMP-dependent protein kinase and protein I are distributed throughout the cerebellum, but that cGMP-dependent protein kinase and the 23,000 MW substrate are highly concentrated in Purkinje cells.

Animals↗
PubMed1982

Noradrenergic modulation of glutamate release in the cerebellum.

The effect of alpha- and beta-adrenergic agonists has been studied on the release of newly synthesized [3H]glutamate and [3H]GABA from slices of rat cerebellum. The beta 2-adrenergic agonist salbutamol, and also noradrenaline in the presence of the alpha-adrenergic antagonist phentolamine, both potentiated the K+-evoked release of [3H]glutamate. This potentiation appears to be mediated by adenylate cyclase activation. No effects of beta-adrenergic stimulation were observed on [3H]GABA release. The alpha-adrenergic agonist clonidine inhibited both the [3H]glutamate and the [3H]GABA release evoked by K+. The results suggest that noradrenergic modulation of cerebellar activity may have a presynaptic as well as postsynaptic origin.

Animals↗
PubMed1981

Serotonin stimulates phosphorylation of protein I in the facial motor nucleus of rat brain.

Protein I is one of the best candidates for a neuronal protein whose phosphorylation may have a functional role in synaptic activity. It is a substrate for both cyclic AMP-dependent and protein kinases, and these kinases show differential specificity for its multiple phosphorylation sites. Protein I is found exclusively in the central and peripheral nervous systems, and immunohistochemical and subcellular fractionation studies suggest an association primarily with synaptic vesicles. Using slices of rat cerebral cortex incubated in vitro, Protein I was phosphorylated both by agents which increase intracellular cyclic AMP and by agents causing Ca2+ influx, although not by any putative neurotransmitters or neuromodulators. We have now examined the facial motor nucleus and report here that serotonin produces a phosphorylation of Protein I when incubated with facial nucleus slices. Demonstration of a neurotransmitter-dependent alteration in the state of phosphorylation of a synapse-specific protein may be due to the relatively simple neuronal circuitry within the facial motor nucleus.

Animals↗
PubMed1981

Presence of protein I, a phosphoprotein associated with synaptic vesicles, in cerebellar granule cells.

The cerebellar levels of Protein I, a synapse-specific neuronal phosphoprotein, have been investigated in the cerebellar mouse mutants stagger (sg), weaver (wv), nervous (nr), and Purkinje cell degeneration (pcd). The Protein I concentration was reduced by about 66% in sg and wv mutants, representing a 90% loss of Protein I per cerebellum. A heterozygote effect was observed in the wv mutant. These results indicate that a great majority of Protein I in the normal cerebellum may be present in the granule cells. In nr mutants the cerebellar Protein I concentration was reduced by only 12% in 62-day-old mice, suggesting that Purkinje cells contribute little to cerebellar Protein I. However, a greater reduction was observed in pcd mutants, which may reflect on the nature of the pcd mutation.

Animals↗
PubMed1981

Neurotransmitter- and neuromodulator-dependent alterations in phosphorylation of protein I in slices of rat facial nucleus.

Protein I is a neuronal phosphoprotein associated primarily with synaptic vesicles. Regulation of its state of phosphorylation has been investigated in slices of rat facial nucleus. This brainstem motor nucleus has a facilitatory serotonergic input and contains no interneurons. Serotonin (5-hydroxytryptamine, 5-HT, 10(-4) M), in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX, 4 x 10(-5) M), converted approximately 26% of Protein I in these slices from the dephospho-form to the phospho-form. This effect was partially inhibited using two classical 5-HT antagonists, mianserin added to the slices during in vitro incubation and metergoline administered in vivo. The effect of 5-HT appeared to be Ca2+-dependent, unlike that of IBMX (10(-3) M). Adenosine, its analog 2-chloroadenosine, and ATP also increased the phosphorylation of Protein I in facial nucleus slices. 2-Chloroadenosine (5 x 10(-4) M) caused a 29% phosphorylation of Protein I, and this effect was not dependent on extracellular Ca2+. The phosphorylation of Protein I caused both by 2-chloroadenosine and by ATP was inhibited by the adenosine antagonist 2'-deoxyadenosine. Results of additional experiments suggest that the great majority of the Protein I in the facial nucleus is present in presynaptic terminals other than the serotonergic afferents. It is concluded that the stimulation by 5-HT and adenosine of Protein I phosphorylation results largely from a direct action of these compounds on those Protein I-containing terminals.

1-Methyl-3-isobutylxanthine↗
PubMed1980

Neuronal protein phosphorylation: recent studies concerning protein I, a synapse-specific phosphoprotein.

Recent studies are described concerning the physiological role and tissue distribution of Protein I, a synapse-specific phosphoprotein. This protein is present in both the central and the peripheral nervous system and appears to be enriched in synaptic vesicles. It is a major substrate for both Ca2(-)- and cyclic AMP-dependent phosphorylation in the brain. In slices of the facial motor nucleus from rat brainstem, serotonin (5-HT) added to the incubation medium stimulates the phosphorylation of Protein I. This effect is potentiated by low concentrations of isobutylmethylxanthine and is prevented by the serotonin antagonist mianserin.

Animals↗
PubMed1979

Regulation of the state of phosphorylation of specific neuronal proteins in mouse brain by in vivo administration of anesthetic and convulsant agents.

The effect of drug treatment in vivo on the state of phosphorylation of two specific neuronal proteins, proteins Ia and Ib, has been studied in mouse brain. For this purpose, animals were killed by immersion into liquid nitrogen, and proteins Ia and Ib were extracted by a procedure designed to prevent alterations in their state of phosphorylation. Several anesthetic agents (pentobarbital, chloral hydrate, and urethane) each caused a decrease in the state of phosphorylation of these proteins. Conversely, the convulsant agents pentylenetetrazol and picrotoxin each caused an increase in the state of phosphorylation of these proteins. Neither the anesthetic nor the convulsant agents affected the total amount of these proteins. The results are compatible with a role for proteins Ia and Ib in neuronal function.

Anesthetics↗