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L Cazin

Publications and source records attributed to L Cazin.

At least 37 records · Page 2Linked to original sources
PubMed1997

[Permanent percutaneous electric connection. General principles].

The Swedes for more than twenty years, and the Germans for over five years have been able to maintain inert or active prostheses with permanent percutaneous connections, thanks to the dependable and proven material and techniques of extraoral implants. The significant improvement extra-oral implants have brought about is not only in a new therapeutic approach to the treatment of important facial defects or transmission deafness; it is also because for some twenty old years now, the few millimeter wide cylinders of Titanium, the < > affixed on the implants, have crossed the cutaneous barrier for extended periods without complications. The percutaneous abutment thus creates a permanent communication between the interior and the exterior of the organism. If the abutment, instead of simply carrying a Maxillo-Facial Prosthesis or an auditive prosthesis, is modified by placing an electric conductor inside it, the simple "percutaneous peg" will turn out to be, in a way, a "percutaneous electric plug". By adapting classic "mechanical" abutments and implants, authors have created a Permanent Percutaneous Electric Connection (PPEC) which has been successfully experimented on rabbits to record EEG. Clinical applications on humans would make it possible either to receive "bio-electrical information" coming from within the organism, or to send electrical energy into the organism. This last application opens vast perspectives of improvement both in diagnosis and therapy in many fields.

Humans↗
PubMed1996

Adenosine inhibits L- and N-type calcium channels in pituitary melanotrophs. Evidence for the involvement of a G protein in calcium channel gating.

It has been previously demonstrated that activation of A1 adenosine receptors in frog melanotrophs causes inhibition of spontaneous action potential discharges and alpha-melanocyte-stimulating hormone secretion. In the present study, we have investigated the effect of adenosine on high-voltage-activated (HVA) calcium currents in cultured melanotrophs, using the whole-cell variant of the patch-clamp technique with barium as a charge carrier. Adenosine and the specific A1 adenosine receptor agonist R-PIA (50 microM each) produced a decrease of the amplitude of the barium current, while the selective A2 adenosine receptor agonist CGS 21680 did not affect the current. The inhibitory effect of R-PIA was observed throughout the activation range of the current, with stronger responses at more positive potentials. R-PIA inhibited both the L- and N-type components of the current, the effect on the N-component being two-fold higher than on the L-component. The inhibitory effect of R-PIA was rendered irreversible by addition of GTP gamma S (100 microM) to the intracellular solution. Pre-treatment of the cells with pertussis toxin (1 microgram/ml; 12 h) totally abolished the effect of R-PIA on the HVA calcium channels. Conversely, addition of a high concentration of cAMP (100 microM) together with the phosphodiesterase inhibitor IBMX (100 microM) to the intracellular solution did not modify the effect of R-PIA on the current. It is concluded that, in frog melanotrophs, adenosine induces inhibition of L- and N-calcium currents and that this effect is mediated by a pertussis toxin-sensitive G protein. Our data also indicate that the inhibitory effect of adenosine on the calcium currents is not mediated by inhibition of adenylyl cyclase.

1-Methyl-3-isobutylxanthine↗
PubMed1996

Patch clamp study on mechanism of adenosine-induced inhibitory effects in frog pituitary melanotrophs.

Our laboratory demonstrated that adenosine inhibits the activation of adenylyl cyclase and the secretion of the alpha-melanocyte-stimulating hormone (alpha-MSH) from the intermediate lobe of the frog pituitary. This paper showed the bioelectric effects induced by adenosine, the ionic conductances modulated by adenosine, and the possible involvement of intracellular messengers, indicated the mechanism by which adenosine controls the secretion of alpha-MSH. The results show that adenosine acting on A1 adenosine receptor subtype reduced the Ca2+ influx necessary for the secretion, through 4 distinct mechanisms: 1) a hyperpolarization resulting from the activation of a voltage-insensitive K+ conductance, 2) a reduction of the duration of spontaneous action potentials due to an increase of the outward delayed rectifyer K+ current (lk), 3) a diminution of the cellular excitability by an activation of the transient outward K+ current (lA), and 4) an inhibition of the L- and N-type Ca2+ currents, with a predominant action on the N-type component. Cell dialysis with GTP gamma S rendered irreversible the effects of adenosine on the K+ conductances and Ca2+ channels, whereas PTX pretreatment totally abolished the response to adenosine, suggesting all bioelectric effects of adenosine were mediated by pertussis toxin-sensitive G proteins. Whether the implicated G proteins regulate the K+ and Ca2+ channels by tight-coupling or via a second-messenger system remains to be solved. With our results, the involvement of adenylyl cyclase can be excluded because addition of cAMP and IBMX, an inhibitor of phosphodiesterases, in the intracellular solution, or application of dibutyryl cAMP in the extracellular solution did not modify the adenosine-induced responses.

Adenosine↗
PubMed1995

Osmo- and mechanosensitivity of the transient outward K+ current in a mammalian neuronal cell line.

1. The transient outward current in NG108-15 cells was investigated with the whole-cell patch-clamp technique. The current was inhibited by external 4-aminopyridine or tetra-ethylammonium. The reversal potential shifted rightward with increased external K+ concentrations. 2. Current inactivation was markedly accelerated in hyperosmotic media (+30 mosmol l-1) and after nearby ejection of isosmotic solution with maximal acceleration occurring after 15-20 s and full recovery within 2-4 min, thus demonstrating an osmo- and mechanosensitivity of this current. Voltage-dependent Na+ and Ca2+ currents were unaffected. 3. Hyperosmotic solution shifted the voltage dependence of inactivation leftward. Inactivation was sensitive to reducing and oxidizing intracellular conditions. Reduction blocked the acceleration of current inactivation induced by hyperosmotic media, while oxidation did not hamper the response. 4. Action potentials had a decreased amplitude and a slower repolarization after hyperosmotic ejections. 5. It is concluded that the transient K+ current is osmo- and mechanosensitive, thus providing a mechanism for extracellular osmolarity to modulate neuronal excitability. The response appeared to be mediated through a changed sensitivity of the inactivating principle to the membrane electric field and was dependent on the redox state of the cell.

4-Aminopyridine↗
PubMed1995

A-type potassium current modulated by A1 adenosine receptor in frog melanotrophs.

1. Transient outward current was recorded in cultured frog melanotrophs with the whole-cell configuration of the patch-clamp technique. The ionic dependence, kinetics and pharmacological properties of the current were studied. The effects of the A1 adenosine receptor agonist R-N6-phenylisopropyl-adenosine (R-PIA) on this current were also investigated. 2. In tetrodotoxin- and cobalt-containing solution, depolarization from -120 mV elicited both transient and delayed outward currents. Pulses from -60 mV activated only a sustained late current. 3. 4-Aminopyridine (4 mM) reduced the transient outward current much more than the delayed outward current. In contrast, tetraethylammonium (10-20 mM) selectively reduced the delayed current. 4. Tail current measurements showed a positive shift in the reversal potential when external K+ concentration was increased, indicating that K+ was the predominant charge carrier. 5. Steady-state inactivation was complete at potentials positive to -10 mV and removed by hyperpolarization. 6. Inactivation of the transient current was slowed and accelerated in oxidizing and reducing conditions, respectively, confirming the involvement of an inactivating 'ball and chain' peptide. 7. R-PIA increased the transient current. The steady-state inactivation curve was shifted towards more positive potentials without changing the activation kinetics. Pretreatment with pertussis toxin (1 microgram ml-1) blocked the response to R-PIA. 8. It is concluded that frog melanotrophs possess an A-type current that is likely to play an important role in excitability. This current, which is directly modulated by A1 adenosine receptors through a Gi/G(o) protein, appears to be responsible for the inhibitory effects of adenosine on electrical activity.

4-Aminopyridine↗
PubMed1995

Activation of dopamine D4 receptor inhibits an L-type calcium current in cerebellar granule cells.

The functions of the D4 receptor, a newly cloned D2-like receptor, as well as the identity of cells expressing it, are still poorly defined. Using quantitative polymerase chain reaction we detected the messenger RNA of the D4, but not other D2-like receptor, in cultured granule cells from neonatal rat cerebellum. In these neurons, dopamine reduced high-voltage-activated calcium current, with a pharmacology corresponding to that of the D4 receptor. The response declined from one to three days, when calcium currents were mostly sensitive to nifedipine, to 15 days, when nifedipine-insensitive calcium currents were also present and D4 receptor messenger RNA had declined. The dopamine response was abolished after pretreatment of the cells by pertussis toxin, was potentiated and made irreversible by infusion of guanosine 5'-O-(3-thiotriphosphate) but persisted in the presence of cyclic AMP and isobutylmethylxanthine. These results indicate the presence in the neonatal cerebellum of a functional D4 receptor inhibiting an L-type calcium current, an action involving a Gi/Go protein but independent from adenylate cyclase inhibition.

Adenylyl Cyclases↗
PubMed1994

Inhibitory effect of adenosine on electrical activity of frog melanotrophs mediated through A1 purinergic receptors.

1. The effects of adenosine were studied in cultured frog melanotrophs by the patch-clamp technique. 2. In cell-attached experiments, most cells responded to adenosine (50 microM) by a reversible inhibition of action current discharges without any apparent desensitization. 3. In whole-cell experiments, adenosine provoked a hyperpolarization accompanied by a depression of spontaneous action potentials and a decrease in membrane resistance. When adenosine was repeatedly applied, tachyphylaxis was observed. Addition of GTP (100 microM) in the intracellular solution augmented the percentage of cells hyperpolarized by adenosine, and the duration and amplitude of the hyperpolarization, and prevented the tachyphylaxis. 4. Pretreatment with pertussis toxin (1 microgram ml-1) blocked adenosine-induced inhibition. 5. In cells dialysed with the non-hydrolysable GTP analogue GTP gamma S (100 microM), adenosine caused a sustained, strong hyperpolarization and an irreversible inhibition of spikes. 6. The effect of adenosine was mimicked by the A1 receptor agonist R-PIA (R-N6-phenylisopropyl-adenosine; 50 microM) and blocked by the A1 receptor antagonist CPDPX (8-cyclopentyl-1,3-dipropylxanthine, 50 microM). The A2 receptor antagonist CGS15943 (9-chloro-2-(2-furanyl)-5,6-dihydro-1,2,4-triazolo[1,5-c] quinazoline-5-imine; 50 microM) did not affect the adenosine-induced response. 7. The results suggest that, in frog melanotrophs, adenosine exerts a direct hyperpolarizing effect accompanied by blockage of spontaneous action potentials. The effect of adenosine is mediated through A1 receptors coupled to a Gi/o protein.

Action Potentials↗
PubMed1994

Melanostatin (NPY) inhibited electrical activity in frog melanotrophs through modulation of K+, Na+ and Ca2+ currents.

1. Melanostatin, a thirty-six amino acid peptide recently isolated from the frog brain due to its ability to inhibit alpha-melanocyte-stimulating hormone (alpha-MSH) release, is the amphibian counterpart of mammalian neuropeptide Y (NPY). The effect of synthetic melanostatin on the bioelectrical activity of cultured frog melanotrophs was studied in 124 cells by using the whole-cell patch-clamp technique. 2. In current-clamp experiments, melanostatin (1 microM) provoked a reversible hyperpolarization and a suppression of spontaneous action potentials. In some cells the hyperpolarizing response was absent, but an arrest of spike firing still occurred. 3. Melanostatin-induced hyperpolarization was associated with a decrease in membrane resistance. In voltage-clamp experiments, melanostatin induced an outward current at a constant command potential. This hyperpolarizing outward current appeared to be carried by potassium ions. 4. Cell dialysis with the non-hydrolysable GTP analogue guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) sustained the outward current produced by melanostatin. Dopamine (1 microM), which generates a similar hyperpolarizing outward current in frog melanotrophs, was not capable of increasing the current provoked by melanostatin and sustained by GTP gamma S. 5. Melanostatin also modulated voltage-operated currents. The amplitude of voltage-activated potassium current was increased by 30%. 6. Melanostatin reduced the fast sodium current. This inhibitory effect was rather persistent compared to the other modulated currents. 7. Melanostatin markedly scaled down high voltage-activated N- and L-like calcium currents. The activation kinetics of these two calcium currents were not altered by the peptide. 8. Pretreatment of melanotrophs with pertussis toxin (1 microgram ml-1) blocked melanostatin-induced inhibition of N- and L-like calcium currents. 9. It is concluded that the NPY-related peptide melanostatin generates a very complex pattern of electrical responses in frog melanotrophs, including hyperpolarization and modulation of voltage-activated currents underlying action potentials. G proteins appear to mediate at least part of these effects.

Action Potentials↗
PubMed1994

Characterization of the GABA-induced current in frog pituitary melanotrophs.

The molecular mechanisms regulating GABAA receptor activity in cultured frog melanotrophs were studied using the patch-clamp technique. In the whole-cell configuration, application of GABA evoked a dose-related increase of inward chloride currents. The ED50 value, estimated from the sigmoidal dose-response curve was 2 x 10(-6) M and the Hill coefficient was 1.55. The amplitude of the GABA-induced current decayed with time. Kinetics analysis of the desensitization revealed that the time-course of the current decrement was fitted by one exponential. Graded doses of GABA or association of GABA with the benzodiazepine receptor agonist flunitrazepam accelerated the desensitization process. In contrast, the time-course of the current did not significantly vary at different holding potentials. In the outside-out configuration, GABA was found to activate channels which displayed three unitary conductance levels (8, 15 and 30 pS). The channel openings of the more frequent conductance level (30 pS) exhibited short and long lasting open states (1.2 and 28.3 ms at -60 mV). Altogether these data reveal that frog melanotrophs possess a single population of GABAA receptors which interconvert into a higher affinity state in the presence of benzodiazepine receptor agonists. Two GABA molecules must bind to the receptor to trigger long lasting channel openings. In addition, the activity of the GABAA receptor appears to be independent of the accumulation of intracellular chloride ions.

Animals↗
PubMed1994

Adrenaline induces hyperpolarization in frog pituitary melanotrophs through activation of potassium channels.

A patch-clamp study was conducted on cultured frog pituitary melanotrophs, in order to investigate the effects of adrenaline on the electrical activity of these cells. In the whole-cell configuration, adrenaline (1 microM) caused hyperpolarization that was accompanied by a fall in membrane input resistance and a blockage of spontaneous action potentials. Under voltage clamp, adrenaline elicited a net-outward current. The hyperpolarization became undetectable at a command voltage of -100 mV which corresponded to the equilibrium potential of potassium ions. The effect of adrenaline on membrane potential and spontaneous activity was blocked by the alpha 2-adrenergic receptor antagonist yohimbine (1-10 microM) but could not be mimicked by the alpha 2-adrenergic agonist clonidine (1-10 microM). In the cell-attached configuration, exposure of the extra-patch membrane to adrenaline increased the occurrence of single-channel currents with a slope conductance of 100 pS. The deduced reversal potential of these currents corresponded to the equilibrium potential of potassium ions. These results suggest that frog melanotrophs display an alpha 2-adrenergic receptor subtype coupled to potassium channels involved in hyperpolarization.

Adrenergic alpha-Antagonists↗
PubMed1992

Voltage-dependent modulation of calcium current by GTP gamma S and dopamine in cultured frog pituitary melanotrophs.

Dopamine (1 microM) reversibly scaled down barium current through high-voltage activated (HVA) calcium channels but had little effect on the time course of current activation in cultured frog melanotrophs. Intracellular perfusion with guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S; 100 microM) sustained the effect of dopamine. Moreover, GTP gamma S drastically slowed down the current activation kinetics. The latter effect was in part reversed by dopamine. A conditioning prepulse to +70 mV facilitated the current in GTP gamma S-dialyzed cells but not in cells exposed to dopamine. These results suggest the existence of a dual G protein-mediated mechanism for reducing HVA calcium current.

Animals↗
PubMed1992

Central-type benzodiazepines modulate GABAA receptor chloride channels in cultured pituitary melanotrophs.

The effects of gamma-aminobutyric acid (GABA) and benzodiazepines on the electrical activity of cultured frog melanotrophs were studied using the patch-clamp technique. In the cell-attached configuration, the exposure to GABA caused a blockage of the spontaneous firing. In the whole-cell configuration, with physiological chloride concentrations, GABA evoked a hyperpolarization associated with a decrease of membrane resistance, generating an inward chloride current. Clonazepam, a central-type benzodiazepine agonist, potentiated the GABA-induced current and the resulting hyperpolarization. In addition, the benzodiazepine inverse agonist Ro 19-4603 totally abolished GABA-induced hyperpolarizing chloride current. Since the pars intermedia of the frog pituitary is composed of a 'pure' population of endocrine cells enriched with GABAA receptors, our results indicate that these cells represent a valuable model in which to investigate the electrophysiological effects of ligands for the GABAA benzodiazepine receptor complex.

Animals↗
PubMed1991

Involvement of non-selective cationic channels in the generation of pacemaker depolarizations and firing behaviour in cultured frog melanotrophs.

The firing patterns of cultured frog melanotrophs were studied using the patch-clamp technique. In the cell-attached mode, unitary currents were frequently observed as well as biphasic waveforms which were attributed to action potentials 'leaking' through the patch membrane. An inwardly rectifying single-unit current was observed with pipette solutions containing either 100 mM K+ or 100 mM Na+. Under both conditions, these channels displayed an identical I/V relationship, yielding a unitary conductance of 110 pS. The channel opening time was extremely long (50-3000 ms) and single-channel currents showed typical relaxations, which triggered bursts of action currents. In the whole-cell configuration large (2-12 mV) fluctuations in the membrane voltage of current-clamped cells frequently occurred. The deflections appeared to result from single-channel currents. Depolarizing 'events' often led to the discharge of action potentials. Taken together, our data provide evidence for the existence of high-conductance cationic channels in frog pars intermedia cells. These channels may, at least in some cases, be responsible for the generation of pacemaker depolarizations, thereby regulating firing behaviour. It is concluded, that the current traversing a single channel can seriously affect the membrane potential and excitability of frog melanotrophs.

Action Potentials↗
PubMed1991

Dopamine-induced inhibition of action potentials in cultured frog pituitary melanotrophs is mediated through activation of potassium channels and inhibition of calcium and sodium channels.

A patch-clamp study was conducted in order to investigate the effects of dopamine on the ionic currents in cultured frog melanotrophs. Brief applications of dopamine (1 microM) hyperpolarized the cell and inhibited the spontaneous action potentials. The hyperpolarization was accompanied by an increase in membrane conductance. Under voltage clamp, dopamine evoked a net outward current. The dopamine-induced outward current was negligible at the equilibrium potential for potassium ions. It was also observed that dopamine increased the intensity of a voltage-dependent outward potassium current monitored by constant depolarizing pulses. In addition, voltage-dependent L- and N-like calcium currents and sodium current were reduced. In the cell-attached configuration, two distinct channel types were activated and one channel type was blocked by dopamine exposure to the extrapatch membrane, which indicates the involvement of an intracellular factor in the signal transduction pathway. A higher conductance channel (100 pS) was characterized by a very low basal activity which rapidly increased upon dopamine application. A lower conductance channel (30 pS) displayed a basal activity with frequent opening events, and a delayed (30-40 s) increase of activity in response to dopamine. Both currents reversed at a deduced potential corresponding to the equilibrium potential for potassium ions. The channel type inhibited by dopamine had a low conductance of 15 pS. The inhibition of the electrical activity induced by dopamine was totally blocked by the D2 receptor antagonist S(-)-sulpiride (1 microM) but was not affected by the D1 receptor antagonist SKF-83566 (1 microM). It is concluded that dopamine activates potassium channels and inhibits calcium and sodium channels in frog melanotrophs. The results also indicate that stimulus-response coupling is mediated by intracellular messenger system(s).

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗
PubMed1991

Dopamine regulates the electrical activity of frog melanotrophs through a G protein-mediated mechanism.

Recently we have demonstrated that dopamine inhibits action potentials in cultured frog melanotrophs through D2 receptor-mediated activation of hyperpolarizing potassium current and reduction of calcium and sodium currents. Herein, the respective roles of G proteins, guanosine-5'-triphosphate and adenosine-3':5'-cyclic-monophosphate in dopamine-induced electrical responses were investigated using the whole-cell patch-clamp technique. Pretreatment of melanotrophs with pertussis toxin (1 microgram/ml) abolished the hyperpolarization and arrest of action potentials evoked by dopamine (1 microM) in 77% of the cells studied. Addition of guanosine-5'-O-(2-thiodiphosphate) (500 microM) to the intracellular solution did not alter the effects of a first exposure to dopamine, but completely blocked the response of cultured melanotrophs to subsequent pulses of dopamine. In cells which were dialysed with guanosine-5'-O-(3-thiotriphosphate) (100 microM) dopamine caused a sustained hyperpolarization and an irreversible inhibition of spikes. Voltage-clamp recordings with electrodes containing guanosine-5'-O-(3-thiotriphosphate), showed that the increase of potassium current and decrease of calcium and sodium currents caused by dopamine were irreversible. These effects were not modified when the pipette contained, in addition to guanosine-5'-O-(3-thiotriphosphate), a high concentration of adenosine-3':5'-cyclic-monophosphate (100 microM) together with the inhibitor of phosphodiesterases 3-isobutyl-1-methylxanthine (100 microM). It is concluded that, in cultured frog melanotrophs, a pertussis toxin-sensitive G protein is implicated in the coupling of dopamine D2 receptors to activation of potassium channels and inhibition of calcium and sodium channels. Our results also indicate that the G protein-mediated signal transduction does not involve the adenylate cyclase system.

Action Potentials↗