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A Constanti

Publications and source records attributed to A Constanti.

At least 55 records · Page 3Linked to original sources
PubMed1985

Calcium-dependent inward currents in voltage-clamped guinea-pig olfactory cortex neurones.

Guinea-pig olfactory cortex neurones in vitro (23 degrees C--25 degrees C) were voltage clamped by means of a single microelectrode sample-and-hold technique. In most Cs+-loaded neurones (in the presence of tetrodotoxin), membrane depolarization beyond -60 mV elicited inward currents, which had rapid activation kinetics. The steady-state current-voltage relationship was N-shaped with a region of negative slope conductance between - 50 mV and - 20 mV. The rate of inactivation varied according to the holding potential and the command potential. The inward currents were maintained when external Ca2+ was replaced by Ba2+, and were blocked by Cd2+, suggesting that Ca2+ was the principal charge carrier. The results demonstrate the existence of calcium current in olfactory cortex neurones.

Animals↗
PubMed1984

A system for the translation of receptor messenger-RNA and the study of the assembly of functional receptors.

The Xenopus oocyte translation system has been developed for the recognition of receptor messenger-RNA molecules. Observations with the nicotinic acetylcholine, GABA, glycine, glutamate and serotonin receptors show that the mRNAs coding for their subunits can be faithfully translated in the oocyte and the products processed, glycosylated, assembled to a receptor structure, inserted correctly in the cell membrane and organised to form the functional ion channel. The specific regulatory interactions between different sites on one receptor, as exemplified in the GABAA receptor, are established in this process. This system is of value for the detection of mRNAs for receptor gene cloning and for the study of receptor assembly and of the expression of multiple receptor genes. The receptor ion channels formed can also be studied in their own right in a highly accessible and controlled situation.

Acetylcholine↗
PubMed1983

Pre- and postsynaptic effects of zinc on in vitro prepyriform neurones.

Intracellular recordings from guinea-pig olfactory cortex neurones revealed a dual effect of zinc: firstly (at 100-500 microM), the responses to bath-applied GABA, muscimol and 3-aminopropanesulphonate were reversibly enhanced, and secondly (at 25-500 microM), the excitatory postsynaptic potential was dramatically prolonged. At Zn2+ doses higher than 50 microM, transmission was eventually blocked irreversibly. These effects of zinc were not produced by 4-aminopyridine, or other divalent cations. The GABA-enhancement is suggested to occur via an interaction of Zn2+ with the post-synaptic GABA receptor, and the prolonged transmitter release probably via blockade of an outward current in nerve terminals. The latter effect may be a contributory factor in the epileptogenic activity of zinc.

Animals↗
PubMed1983

Synthesis of functional chick brain GABA-benzodiazepine-barbiturate/receptor complexes in mRNA-injected Xenopus oocytes.

Membrane conductance changes evoked by bath-applied GABA (2-640 microM) and some related agonists were recorded intracellularly in Xenopus oocytes previously injected with chick brain mRNA. Muscimol and 3-aminopropanesulphonate were approximately 4 and 0.25 times as potent as GABA in producing a conductance increase. GABA responses were antagonized by bicuculline (10 microM) or picrotoxinin (10-100 microM) but were clearly enhanced by the benzodiazepine (BZ) receptor ligand chlorazepate or by pentobarbitone. We conclude that an appropriate fraction of brain mRNA was capable of directing the synthesis and correct insertion of functional GABA-BZ-barbiturate/receptor complexes in the oocyte membrane.

Animals↗
PubMed1983

M-current in voltage-clamped olfactory cortex neurones.

The soma of olfactory cortex neurones in vitro was voltage-clamped by means of a single microelectrode sample-and-hold technique. In most neurones, hyperpolarizing voltage commands from relatively positive holding potentials (-40 to -50 mV) elicited a slow inward current relaxation with voltage-dependent and kinetic properties similar to the non-inactivating K+-current (M-current; IM), first described in amphibian sympathetic neurons. Deactivation of IM at negative potentials probably accounts for the slow sag of the hyperpolarizing electrotonic potential measured during current-clamp experiments. IM was inhibited by the cholinergic agonist muscarine or barium ions.

Animals↗
PubMed1983

Ca-activated potassium current in vertebrate sympathetic neurons.

Ca-activated K-currents (IC) in sympathetic neurones have been triggered by intracellular Ca-injection or by activating ICa. IC is strongly voltage-dependent, with a peak slope of 11 mV/e-fold depolarization above -50 mV. Relaxation, fluctuation and single channel analysis suggests this to result from voltage-dependent opening and closing rates. Time-constants for channel opening and closing are about 15 msec near zero mV. Single channel conductance is about 100 pS. Currents can be blocked by TEA. IC is activated very rapidly (less than or equal to 5 msec) and sometimes transiently by a depolarizing voltage-step. It is suggested that IC contributes to both spike repolarization and spike after-hyperpolarization. Spontaneous miniature ICs have also been recorded, probably activated by the release of packets of intracellular Ca.

Animals↗
PubMed1983

Fast inward-rectifying current accounts for anomalous rectification in olfactory cortex neurones.

The somatic membrane of guinea-pig olfactory cortex neurones in vitro (23 degrees C) was voltage clamped by means of a single-micro-electrode sample-and-hold technique. In most cells the current-voltage (I-V) relationship showed inward (anomalous) rectification with increasing hyperpolarization beyond the resting potential (ca. -80 mV). Under current-clamp conditions a time-dependent 'sag' of the hyperpolarizing electrotonic potentials was observed following an initial overshoot. No depolarizing after-potential was seen on return to the resting potential. Inward rectification was activated between -100 and -110 mV (irrespective of pre-set resting potential) and increased the membrane input conductance by up to three-fold. The rectification was unaffected by tetrodotoxin or Cd2+. Under somatic voltage clamp, hyperpolarization beyond -110 mV activated a rapid inward relaxation fitted by a single exponential. The relaxation time constant (tau on) decreased e-fold for a 40 mV hyperpolarization. (Typical values: 28 ms at -110 mV declining to 13 ms at -140 mV; external K+ concentration 3 mM, 23 degrees C). More extreme hyperpolarizations evoked a slower 'inactivation' phase (tau = 40-60 ms). A transient outward-decaying 'tail' current reflecting deactivation of inward rectification was seen on stepping from -140 mV to more positive potentials. tau off became slower with hyperpolarization. The tail current disappeared at a potential close to the expected VK but was rarely inverted to an inward-decaying tail. It is proposed that the fast inward-rectifying current of olfactory neurones (If.i.r.) is a K+ current analogous to the anomalous K+ rectifier of marine egg and frog muscle membranes. The behaviour of the inward rectifier was dependent on external K+ concentration in accordance with the unique 'V--VK' dependence of classical anomalous rectification; however, of several agents tested (external Cs+, Ba2+, Rb+, Tl+ or tetraethylammonium), only Cs+ and Ba2+ blocked If.i.r. in a time- and voltage-dependent manner. The effect of tetraethylammonium resembled that of an increase in external K+. The possible contribution of the inward rectifier to the passive cell membrane properties is discussed.

Action Potentials↗
PubMed1982

A novel effect of zinc on the lobster muscle GABA receptor.

The effect of zinc (and copper) was investigated on the lobster muscle gamma-aminobutyric acid (GABA) receptor. Zinc (10 microns-1 mM) depressed the GABA-evoked conductance increase in a fully reversible manner by possibly binding to an imidazole group, suggested from pH titration studies on the evoked-chloride conductance. Other transition metal (period 4) divalent cations (up to 500 microM) were inactive in antagonizing GABA responses. Variation of external chloride or anion substitution did not perturb the zinc antagonism; however, decreasing the pH markedly decreased the potency of zinc. A possible explanation for these results is discussed. Although the zinc antagonism resembled that produced by picrotoxinin, combination of these two agents depressed the GABA dose--conductance curve in a manner expected for two antagonists acting on independent sites. The zinc binding site was also discrete from the GABA recognition site; the results are interpreted in terms of a distinct binding site for zinc and H+. The distortion of an agonist dose--response curve by formation of an inactive agonist-divalent cation complex is discussed; however, complexation of GABA did not explain the observed antagonism by zinc. By comparison, zinc had no effect on the GABA responses of rat ganglionic neurons. It is concluded that the zinc binding site, on lobster muscle, may be an important modulatory site for the GABA-evoked chloride conductance.

Animals↗
PubMed1982

Measurement of GABA-evoked conductance changes of lobster muscle fibres by a three-microelectrode voltage clamp technique.

The effective membrane conductance and capacity of lobster muscle fibres was measured by a three-intracellular-microelectrode voltage clamp technique. Conductance values agreed well with those determined under current clamp, by means of the 'short' cable equations. Reversible increases in conductance evoked by gamma-aminobutyric acid (GABA) were reflected by differences (delta V) in electrotonic potential amplitude recorded at the centre, and midway between the centre and fibre end respectively. GABA dose--conductance curves derived from cable theory or from delta V measurements were virtually identical. The effective capacity (ceff), determined from the area beneath the 'on' delta V capacity transient, yielded values of the membrane time constant consistently lower than those obtained by the graphical method of E. Stefani & A.B. Steinbach (J. Physiol., London. 203, 383-401 (1969)); one possible explanation for this discrepancy is discussed. In the presence of GABA, the effective capacity was reduced in a dose-related manner. The results were interpreted in terms of an equivalent circuit in which surface membrane was arranged in parallel with cleft-tubular membrane of finite conductance, charged through an access resistance. GABA was though to be decreasing ceff by selectively increasing the conductance of the cleft-tubular membranes.

Animals↗
PubMed1982

Voltage-sensitive K-currents in sympathetic neurons and their modulation by neurotransmitters.

A description is given of methods for voltage clamping and for studies of the several varieties of K-currents occurring in sympathetic neurons. Conclusions were derived chiefly from experiments conducted on bullfrog lumbar paravertebral sympathetic ganglia but observations made were compared with results obtained from studies of neurons of the rat superior cervical ganglion. Discussion is given of means used for revealing and identifying these currents and for studying actions of drugs, calcium, etc. and circumstances such as membrane state on the four quite distinct time- and voltage-dependent K-currents: IK, IC, IA and IM. The latter (IM) is under direct presynaptic control and the major focus of the paper is on this current and its role. However, considerable information is given concerning the characteristic of the other three (IK, IC and IA).

Animals↗
PubMed1982

M-currents and other potassium currents in bullfrog sympathetic neurones.

1. Bullfrog lumbar sympathetic neurones were voltage-clamped in vitro through twin micro-electrodes. Four different outward (K(+)) currents could be identified: (i) a large sustained voltage-sensitive delayed rectifier current (I(K)) activated at membrane potentials more positive than -25 mV; (ii) a calcium-dependent sustained outward current (I(C)) activated at similar positive potentials and peaking at +20 to +60 mV; (iii) a transient current (I(A)) activated at membrane potentials more positive than -60 mV after a hyperpolarizing pre-pulse, but which was rapidly and totally inactivated at all potentials within its activation range; and (iv) a new K(+) current, the M-current (I(M)).2. I(M) was detected as a non-inactivating current with a threshold at -60 mV. The underlying conductance G(M) showed a sigmoidal activation curve between -60 and -10 mV, with half-activation at -35 mV and a maximal value (G(M)) of 84+/-14 (S.E.M.) nS per neurone. The voltage sensitivity of G(M) could be expressed in terms of a simple Boltzmann distribution for a single multivalent gating particle.3. I(M) activated and de-activated along an exponential time course with a time constant uniquely dependent upon voltage, maximizing at approximately 150 ms at -35 mV at 22 degrees C.4. Instantaneous current-voltage (I/V) curves were approximately linear in the presence of I(M), suggesting that the M-channels do not show appreciable rectification. However, the time- and voltage-dependent opening of the M-channels induced considerable rectification in the steady-state I/V curves recorded under both voltage-clamp and current-clamp modes between -60 and -25 mV. Both time- and voltage-dependent rectification in the voltage responses to current injection over this range could be predicted from the kinetic properties of I(M).5. It is suggested that I(M) exerts a strong potential-clamping effect on the behaviour of these neurones at membrane potentials subthreshold to excitation.

Animals↗
PubMed1982

Pharmacological inhibition of the M-current.

1. The effects of muscarinic agonists, luteinizing hormone-releasing hormone (LHRH) analogues, uridine triphosphate (UTP) and divalent cations on K(+)-currents in voltage-clamped bullfrog sympathetic neurones have been studied.2. Muscarine (1-10 muM), D-ala(6) LHRH (1-5 muM), UTP (50-100 muM) and Ba(2+) (1-4 mM) selectively depressed the M-current (I(M)), without appreciable effect on the delayed rectifier, Ca(2+)-activated or transient outward currents (I(K), I(C) or I(A)).3. I(M)-inhibition was characterized by: (a) elimination of slow current relaxations accompanying voltage jumps in the membrane potential range -30 to -60 mV; (b) reduced voltage-dependent chord conductance over this range with no change in the voltage-independent chord conductance at more negative membrane potentials; (c) suppression of outward rectification in the steady-state current-voltage curve between -70 and -25 mV; and (d) development of an inward current which increased in amplitude between -70 and -20 mV in proportion to the decrease in steady-state I(M). The kinetics and voltage sensitivity of residual I(M) were unchanged.4. The magnitude of the inward current produced by muscarine or LHRH could be accounted for quantitatively by the reduction in steady-state I(M). No increase in leak current could be detected in the range -60 to -30 mV. In two cells muscarine (10 muM) increased the leak current and conductance at -70 to -100 mV, but not at more depolarized levels.5. I(M) was not modified by removing extracellular Ca(2+), adding a selective Ca(2+)-channel blocker (Cd(2+)), adding 1 mM-dibutyryl cyclic AMP or 8'Br cyclic GMP, or by intracellular ionophoresis of Ca(2+), 8'Br cyclic GMP, dibutyryl cyclic AMP, GTP-gamma-S or S-adenosylmethionine.6. It is concluded that the principal effects of these agents in unclamped neurones - depolarization, increased input resistance, reduced outward rectification and increased excitability - are due entirely to a selective inhibition of I(M). The intracellular transduction mechanism for I(M) inhibition is unknown.

Action Potentials↗
PubMed1981

M-Currents in voltage-clamped mammalian sympathetic neurones.

The M-current (IM), a species of time- and voltage-dependent K+-current previously identified in amphibian sympathetic neurones, has been detected in voltage-clamped rat sympathetic neurone somata. IM had an activation threshold of about -70 mV and, since it did not show time-dependent inactivation, contributed an increasing outward component to the steady membrane current between -70 and -20 mV. Selective inhibition of IM by muscarine or by angiotensin-II probably accounts for the membrane depolarization produced by these two agonists on rat ganglia.

Angiotensin II↗
PubMed1981

A re-examination of the GABA-inhibitory action of bicuculline on lobster muscle.

The GABA-inhibitory action of bicuculline on lobster muscle was critically re-examined. Bicuculline (20-100 micrometers) depressed the GABA-evoked conductance increase in a reversible manner, the double reciprocal transformation of the GABA dose/conductance curves remaining linear. If bicuculline was assumed to be a pure non-competitive antagonist, then the observed depression of the GABA curves at high GABA concentrations was underestimated. Also, the action of bicuculline was not in accordance with a dualistic antagonism (competitive and pure non-competitive type). Regarding bicuculline as a "mixed" non-competitive antagonist however, gave a better overall fit to the experimental data. Combinations of bicuculline and picrotoxinin also depressed the GABA response in a manner expected from the combination of two "mixed" non-competitive antagonists. It was concluded that bicuculline (like picrotoxinin and picrotoxin) behaves as a "mixed" non-competitive rather than a pure non-competitive antagonist of GABA on lobster muscle. However bicuculline methochloride proved inactive on the lobster. Moreover, bicuculline itself became less effective in a more acidic solution (pH 5.6, where more of the protonated form would exist). An intracellular or intramembrane site of action of this antagonist was therefore postulated.

Animals↗