Slow Tail Research Articles

On the association of terrestrial gamma‐ray bursts with lightning and implications for sprites

Measurements of ELF/VLF radio atmospherics (sferics) at Palmer Station, Antarctica, provide evidence of active thunderstorms near the inferred source regions of two different gamma‐ray bursts of terrestrial origin [Fishman et al., 1994]. In one case, a relatively intense sferic occurring within ±1.5 ms of the time of the gamma‐ray burst provides the first indication of a direct association of this burst with a lightning discharge. This sferic and many others launched by positive cloud‐to‐ground (CG) discharges and observed at Palmer during the periods studied exhibit ‘slow tail’ waveforms, indicative of continuing currents in the causative lightning discharges. The slow tails of these sferics are similar to those of sferics originating in positive CG discharges that are associated with sprites.

Ca2+ clearance mechanisms in isolated rat adrenal chromaffin cells.

1. Intracellular Ca2+ clearance mechanisms were studied in rat adrenal chromaffin cells, by measuring slow tail currents through small-conductance Ca(2+)-activated K+ channels and using indo-1 photometry following depolarization-induced Ca2+ loading. 2. Following several-hundred millisecond depolarizations, [Ca2+]i decayed in three phases. An initial fast decay was followed by a long-lasting, low plateau, then [Ca2+]i returned to the resting level slowly. 3. Replacement of external Na+ moderately slowed [Ca2+]i decay, indicating a contribution of plasma membrane Na(+)-Ca2+ exchange. 4. Raising external pH or application of extracellular Eosin of La3+ prolonged slow tail currents, indicating a contribution of plasma membrane Ca(2+)-ATPase to Ca2+ clearance. 5. Ca(2+)-induced Ca2+ release from caffeine-sensitive stores occurred during depolarization. 6. Inhibition of endoplasmic reticulum Ca(2+)-ATPase had little effect on Ca2+ clearance. 7. Slow tail currents and [Ca2+]i decay following 0.2 - 2 s depolarizations were much prolonged by mitochondrial inhibition with carbonyl cyanide m-chlorophenylhydrazone (CCCP) or Ruthenium Red, which abolished the initial rapid decay and plateau of [Ca2+]i. 8. In conclusion, mitochondrial Ca2+ uptake plays a major role in Ca2+ clearance by rapidly and reversibly sequestering Ca2+ during depolarization-evoked Ca2+ loads.

Dantrolene depolarizes hippocampal neurons in slices from rats

In intracellular recordings by current clamp, dantrolene (sodium salt, 20 microM) regularly depolarized CA1 neurons (by approximately 13 mV), even in the presence of tetrodotoxin. Dantrolene almost abolished slow (but not medium) afterhyperpolarizations (AHP) (down by approximately 80%). As in previous experiments, dantrolene strongly depressed the hyperpolarization and resistance drop caused by hypoxia. Under voltage clamp (at approximately -70 mV), dantrolene accordingly elicited an inward current of approximately -100 pA and suppressed a voltage-dependent slow outward current and tail current, as well as the outward current generated by hypoxia. In addition, dantrolene enhanced Q-current (IQ) inward relaxations; but it had no effect on high voltage activated Ca currents and did not prevent their suppression by hypoxia. We conclude that depression of slow AHP current and increase in IQ probably account for the depolarizing action of dantrolene.

Patch-clamp study of neurones in rat olfactory cortex slices: properties of a slow post-stimulus afterdepolarizing current (IADP).

Whole-cell recordings were made from neurones in slices of rat olfactory cortex (10-19 days old), using potassium methylsulphate-filled pipettes. Positive commands applied from -60 mV in the presence of oxotremorine-M (10-20 microgramM, 20 of 30 cells) or trans-ACPD (10-50 microgramM, 4 of 9 cells) evoked a slow inward tail current similar to the K+ -mediated tail current (I K,ADP) recorded with sharp electrodes. I ADP was reduced by hyperpolarization, showed a 27% decrease in input conductance at its peak and was depressed by Cd(2+), 4AP or high K+ but unaffected by Cs+, Ba(2+) or TEA (5 mM). IADP was significantly larger in cells between postnatal days 13 and 15 than in younger neurones (10-12 days). These data show that stable whole-cell recordings of I ADP can be made from olfactory neurones in cortical slices, and support our hypothesis that IADP is a slowly reactivating, Ca(2+) -sensitive K+ conductance.

Calcium-dependent plateau potentials in a crab stomatogastric ganglion motor neuron. II. Calcium-activated slow inward current

1. Using intracellular recording and voltage-clamp techniques, we examined the biophysical properties of a Ca(2+)-activated slow inward current and its physiological role in plateau potential generation in the dorsal gastric (DG) motor neuron of the stomatogastric ganglion in the crab, Cancer borealis. 2. As shown in the accompanying paper, a brief puff of serotonin (5-HT) evoked a plateau potential in the DG neuron. Intracellular loading of the Ca2+ chelator ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) prevented 5-HT from evoking a plateau potential. On the contrary, rapid increase of intracellular Ca2+ by photolysis of caged-Ca2+ (bound to DM-nitrophen) evoked a plateau response in DG bathed in normal saline. 3. Extracellular tetrodotoxin (TTX), tetraethylammonium (TEA), 4-aminopyridine (4-AP), and Cs+ and intracellular iontophoresis of Cs+ were used to block voltage-dependent INa, IK, and Ih. Under these conditions we voltage clamped DG using two electrodes and isolated a long-lasting tail current after a short depolarization of the cell. 4. The reversal potential of the slow tail current was extrapolated to be -27 +/- 3.5 (SE) mV. Na+ substitutions with choline+, tris(hydroxymethyl)aminomethane+ (Tris+) or n-methyl-glucamine+ (NMG+) did not significantly affect the reversal potential or the amplitude. 5. The slow tail current was Ca2+ dependent. It was reduced or abolished by the Ca2+ channel blocker Co2+, intracellular injection of EGTA, and by Ba2+ replacement of Ca2+ as the charge carrier. The activation and deactivation of this current do not show an apparent dependence on voltage. 6. When the voltage-dependent Na+, K+, and Ca2+ channels were blocked, a brief puff of caffeine evoked a slow depolarization. In voltage clamp, caffeine evoked a slow inward current with an apparent conductance increase. This current was reduced by intracellular EGTA. The current-voltage (I-V) relationship of the caffeine-evoked current was linear with a reversal potential of -25 +/- 4.8 mV. This was not statistically different from the reversal potential of the depolarization-evoked tail current. 7. 5-HT enhanced the depolarization-evoked slow tail current but had no effect on the caffeine-evoked slow inward current. 8. We conclude that the slow tail current is a Ca(2+)-activated nonselective current, similar to the Ca(2+)-activated nonspecific cation currents described in other preparations. This current appears to play an important role in plateau generation and maintenance in DG. 5-HT has no direct effect on the properties of this current, but it indirectly enhances the current through an increase of voltage-dependent Ca2+ current.

Characteristic features of inhibitory junction potentials evoked by single stimuli in the guinea‐pig isolated taenia caeci.

1. Changes in membrane potential of the guinea-pig isolated taenia caeci evoked by single stimuli have been investigated using intracellular recording techniques. Nifedipine (10 microM) was used to arrest spontaneous muscle action potentials. Single stimuli elicited complex junction potentials which consisted of both excitatory and inhibitory components. 2. The excitatory component of the compound junction potential was unaffected by hexamethonium (100 microM) but abolished by atropine (1 microM) and omega-conotoxin GVIA (10-100 nM). 3. In the presence of atropine, single stimuli elicited fast inhibitory junction potentials (IJPs). IJPs were sometimes biphasic during repolarization with a noticeable 'slow tail'. Apamin (30-100 nM) potently inhibited the fast IJP and revealed an underlying slow IJP. 4. The fast IJP was also abolished by omega-conotoxin GVIA (100 nM). However, the slow IJP was insensitive to omega-conotoxin GVIA but was abolished by cadmium (30 microM). 5. Guanethidine (3 microM) and N omega-nitro-L-arginine (10-100 microM) had no detectable effects on either of the IJPs. The dye Reactive Blue 2 reduced the amplitude of the fast IJP but this reduction was associated with a membrane hyperpolarization. 6. The existence of two distinct IJPs in the guinea-pig taenia caeci has been demonstrated. The ability of omega-conotoxin GVIA to selectively abolish the fast IJP leaving the slow IJP intact suggests that separate nerves are involved in mediating these responses.

Effects of Action Potential Duration on Excitation-Contraction Coupling in Rat Ventricular Myocytes

Although each of the fundamental processes involved in excitation-contraction coupling in mammalian heart has been identified, many quantitative details remain unclear. The initial goal of our experiments was to measure both the transmembrane Ca2+ current, which triggers contraction, and the Ca2+ extrusion due to Na(+)-Ca2+ exchange in a single ventricular myocyte. An action potential waveform was used as the command for the voltage-clamp circuit, and the membrane potential, membrane current, [Ca2+]i, and contraction (unloaded cell shortening) were monitored simultaneously. Ca(2+)-dependent membrane current during an action potential consists of two components: (1) Ca2+ influx through L-type Ca2+ channels (ICa-L) during the plateau of the action potential and (2) a slow inward tail current that develops during repolarization negative to approximately -25 mV and continues during diastole. This slow inward tail current can be abolished completely by replacement of extracellular Na+ with Li+, suggesting that it is due to electrogenic Na(+)-Ca2+ exchange. In agreement with this, the net charge movement corresponding to the inward component of the Ca(2+)-dependent current (ICa-L) was approximately twice that during the slow inward tail current, a finding that is predicted by a scheme in which the Ca2+ that enters during ICa is extruded during diastole by a 3 Na(+)-1 Ca2+ electrogenic exchanger. Action potential duration is known to be a significant inotropic variable, but the quantitative relation between changes in Ca2+ current, action potential duration, and developed tension has not been described in a single myocyte. We used the action potential voltage-clamp technique on ventricular myocytes loaded with indo 1 or rhod 2, both Ca2+ indicators, to study the relation between action potential duration, ICa-L, and cell shortening (inotropic effect). A rapid change from a "short" to a "long" action potential command waveform resulted in an immediate decrease in peak ICa-L and a marked slowing of its decline (inactivation). Prolongation of the action potential also resulted in slowly developing increases in the magnitude of Ca2+ transients (145 +/- 2%) and unloaded cell shortening (4.0 +/- 0.4 to 7.6 +/- 0.4 microns). The time-dependent nature of these effects suggests that a change in Ca2+ content (loading) of the sarcoplasmic reticulum is responsible. Measurement of [Ca2+]i by use of rhod 2 showed that changes in the rate of rise of the [Ca2+]i transient (which in rat ventricle is due to the rate of Ca2+ release from the sarcoplasmic reticulum) were closely correlated with changes in the magnitude and the time course of ICa-L.(ABSTRACT TRUNCATED AT 400 WORDS)

Indirect stimulation of Ca(2+)-activated Cl- current by Na+/Ca2+ exchange in rabbit portal vein smooth muscle.

Macroscopic currents were recorded in freshly dissociated smooth muscle cells from the rabbit portal vein using the tight seal whole cell recording mode (22 degrees C). In some experiments, the indo 1 fluorescence technique was used to simultaneously monitor the changes in the concentration of free intracellular Ca2+ ([Ca2+]i; indo 1 ratio, 400/500 nm). In cells exposed to tetraethylammonium chloride (TEA) to inhibit K+ channels and 1-10 microM nifedipine or nicardipine to inhibit L-type Ca2+ channels, cell dialysis with 30 mM Na+ increased [Ca2+]i and induced membrane current consistent with the activation of Ca(2+)-activated Cl- channels [ICl(Ca)]. From holding potential (HP) of -60 mV, high intracellular Na+ concentration ([Na+]i)-mediated current was instantaneous in response to 0.5- to 10-s voltage clamp pulses from -80 to +20 mV; steps ranging from +20 to +80 mV evoked slow time-dependent outward current (I(t); superimposed on the instantaneous current) and voltage-dependent Ca2+ transient; on return to HP, slow inward tail current appeared that reflected deactivation of I(t). Both current components 1) exhibited outward rectifying properties, 2) reversed near the predicted equilibrium potential for Cl-, 3) were stimulated by elevation of extracellular Ca2+ concentration, 4) were abolished when the cells were dialyzed with 5 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, and 5) were inhibited by extracellular application of niflumic acid (50 microM). Complete replacement of extracellular Na+ concentration with tetramethylammonium increased both the instantaneous and time-dependent components of ICl(Ca), resting [Ca2+]i at -60 mV and Ca2+ transient at +40 mV. Cell dialysis with Na(+)-free pipette solution prevented these effects. Our results are consistent with an indirect mechanism of stimulation of ICl(Ca), which involves intracellular Ca2+ accumulation via reverse-mode electrogenic Na+/Ca2+ exchange activity.

Electrogenic Na-Ca exchange clears Ca2+ loads from retinal amacrine cells in culture

Calcium influx into cultured retinal amacrine cells is followed by a small, slow, inward current that we show here results from the operation of electrogenic Na-Ca exchange. The activity of the exchanger is shown to correlate with the magnitude of the Ca2+ load and to depend on both the Ca2+ and Na+ gradients. Li+ is unable to substitute for Na+ and in the absence of Na+, slow tail currents are almost entirely suppressed. A rapid change in [K+]o does not affect the activity of the exchanger, suggesting that only Na+ and Ca2+ are transported. The ratio of charge entering as Ca2+ current to the charge entering as exchange current is highly variable between cells. We suggest that variability results from a variable fraction of Ca2+ load, we estimate typically 40%, being removed by a process other than Na-Ca exchange. This process is likely to involve internal buffering or sequestration since inhibition of the plasmalemmal Ca(2+)-ATPase does not increase the fraction of Ca2+ expelled by the exchanger. Ca2+ loading performed in the absence of Na+o generates smaller exchange charge the longer the delay in returning Na+o to the neuron. About 30% of exchange charge is lost for a delay of 1 sec.

Pharmacological types of calcium channels and their modulation by baclofen in cerebellar granules

Voltage-dependent calcium currents were measured by whole-cell recording technique in cultured cerebellar granule neurons from 8 d old rats, in 10 mM BaCl2 and with a holding potential of -80 mV. A saturating dose (10 microM) of the dihydropyridine nimodipine reversibly inhibited the maximum current by 25% and the dose dependence showed IC50 close to 50 nM. omega-Conotoxin GVIA (cgtx, 5 microM) and omega-agatoxin IVA (agatx, 200 nM) irreversibly inhibited the current by 17% and by 47%, respectively. The effect of nimodipine was additive with that of the toxins. The GABAB agonist (+/-)baclofen, or (-)baclofen (100 microM), reduced the calcium current by 30 +/- 5%, with a IC50 4 microM. The effect was mediated by a pertussis toxin-sensitive G-protein. In cells treated with cgtx during the experiment or preincubated with the toxin for 30 min, the effect of baclofen was significantly reduced. However, the action of baclofen was not confined to cgtx-sensitive channels: application of nimodipine or agatx resulted in a 50% reduction of the baclofen effect as well. In contrast, baclofen inhibited approximately the same amount of current both before and after the increase caused by the dihydropyridine agonist BayK 8644 and did not modify the slow BayK-induced tail current. These results indicate (1) the modulation through GABAB receptors does not clearly discriminate between pharmacologically distinct calcium channels and (2) L-type calcium channels represent an heterogeneous population in these neurons.

Dynamics of photoexcited carriers in poly( p-phenylenevinylene) and its soluble derivative

The dynamics of photoexcited carriers in poly( p-phenylene vinylene) (PPV) and its soluble derivative, poly[2-methoxy-5-(2′-ethylhexyloxy)- p-phenylenevinylene] (MEH-PPV) are studied using the measurement of subnanosecond transient photoconductivity (PC). An initial transient photocurrent decays in the subnanosecond time regime, followed by a thermally-activated slow tail. The magnitude of the peak transient PC is almost temperature independent and proportional to the light intensity and electric field. The results are interpreted as a free carrier generation via a π-π ∗ interband transition. The transient PC in MEH-PPV is an order of magnitude smaller than that in PPV, indicating that the decrease of the interchain coupling due to the bulky side-chains results in a smaller quantum efficiency for carrier generation.

Ion selectivity and gating of small conductance Ca(2+)‐activated K+ channels in cultured rat adrenal chromaffin cells.

1. The ion selectivity and gating of apamin-sensitive, small conductance Ca(2+)-activated K+ (SK) channels were studied in cultured rat adrenal chromaffin cells using patch clamp techniques. 2. The amplitude of slow tail currents showed a bell-shaped dependence on depolarization potentials. Slow tail currents were abolished in a Ca(2+)-free external solution or by adding 100 microM Cd2+ to the external solution. Reversal potentials followed the predictions of the Nernst equation for a K+ electrode. 3. Slow tail currents were largely blocked by external application of apamin (dissociation constant, Kd, 4.4 nM), (+)-tubocurarine (Kd, 20 microM), and tetraethylammonium (Kd, 5.4 mM). 4. The relative permeability (PX/PK, where X may be any one of the ions listed) of SK channels was: Tl+ (1.87) > K+ (1.0) > Rb+ (0.81) > Cs+ (0.16) > NH4+ (0.11). Na+, Li+ and methylamine were not measurably permeant (PX/PK < 0.005). Open SK channels seem to have an effective pore diameter of 0.34-0.38 nm. The relative conductance (gX/gK) was: Tl+ (1.29) > K+ (1.0) > Rb+ (0.85) > Cs+ (0.45) approximately NH4+ (0.44). 5. With mixtures of Tl+ and K+, SK channels showed anomalous mole-fraction behaviour. 6. Ca2+ dependence of SK channel gating was studied using inside-out macropatches. The [Ca2+] required for half-maximal activation and the Hill coefficient were 0.69 microM and 1.7, respectively, and independent of membrane potentials. 7. Single-channel conductance was 13-14 pS (160 mM K+).

Coupling of ELF/ULF energy from lightning and MeV particles to the middle atmosphere, ionosphere, and global circuit

In an attempt to explain numerous atmospheric electrical phenomena, the elements of the global electrical circuit are reexamined. In addition to being a “quasistatic “DC” generator” and source of radiated energy at VLF and higher, the thunderstorm is found to be a pulse generator, with most of the external energy contained in ELF and ULF pulse currents to the ionosphere (and Earth). The pulse energy is found to deposit largely in the middle atmosphere above the thunderstorm. The VLF and above components are well understood, as are the ULF components due to the conductivity gradient. However, a previously poorly understood ELF component on the millisecond timescale, or “slow tail,” contains a large fraction of the electrical energy. This component couples strongly to the ionosphere and also launches a unipolar transverse electromagnetic (TEM) wavelet in the radial Earth‐ionosphere transmission line. The increase in charge with distance associated with such wavelets, and their ensemble sum at a point, may explain some large mesospheric “DC” fields but there are still difficulties explaining other than rare occurences, except for antipodal reconvergence. These millisecond duration unipolar wavelets also couple to the ionosphere and may trigger other lightning at a distance. A schema is elucidated by which the charge of MeV particles deposited in the middle atmosphere persists for much longer than the local relaxation time. This also gives rise to unipolar waves of global extent which may explain lower‐latitude field perturbations associated with solar/geomagnetic events.

Ca2+ cooperativity in neurosecretion measured using photolabile Ca2+ chelators

1. The photolabile Ca2+ chelator DM-nitrophen was injected into crayfish motor neuron terminals and photolyzed with light flashes of different intensity to determine the cooperativity of Ca2+ action in releasing neurotransmitter. 2. Each flash elicited a phasic postsynaptic response resembling an excitatory junctional potential, apparently due to a presynaptic "spike" in intracellular calcium concentration ([Ca2+]i). 3. When postsynaptic currents were measured under voltage clamp, a Ca2+ cooperativity of approximately 3-4 was inferred from a supralinear dependence of responses on changes in peak [Ca2+]i caused by flashes differing in intensity by 32-46%. 4. A similar Ca2+ cooperativity was inferred from postsynaptic potentials in response to flashes of varying intensity. 5. The time course of transmitter release indicated by flash responses had slightly slower rising and falling phases than excitatory postsynaptic potentials. There was also a slow tail of transmitter release lasting for approximately 200 ms after a flash. 6. This time course was explained quantitatively by simulations of DM-nitrophen photolysis and binding reactions and a model of Ca2+ activation of transmitter release.

Brief increases in intracellular Ca 2+ activate K + current and non-selective cation current in rat nucleus basalis neurons

Neurons were acutely dissociated from the rat nucleus basalis, and membrane currents (whole-cell patch-clamp) and intracellular free Ca 2+ concentrations (Fura-2) were measured simultaneously from large neurons (approximately 25 μm in diameter). A brief depolarization from −60 to 0 mV for 200ms evoked an increase in intracellular free calcium and a slow outward tail current (72 ± 8pA, n = 30). The outward current reversed polarity at −75.5 ± 2.7mV ( n = 14). The tail current declined and the intracellular calcium recovered its resting level exponentially with time-constants of 1.0 ± 0.1s and 2.5 ± 0.2s, respectively ( n = 17). In neurons loaded with Cs-gluconate, a similar depolarizing pulse evoked a similar increase in intracellular free calcium, but this was now followed by an inward tail current ( 118 ± 8pA, n = 44). The inward tail current reversed polarity at −27.8 ± 3.8mV (n = 7), and was suppressed by removal of external sodium ions. Neither outward nor inward tail currents were observed, when the external solution was calcium-free or when the pipette solution contained EGTA (10 mM). These results indicate that a depolarization causes a calcium entry and that this consequently increases both K + conductance and non-selective cation conductance.

Modification of sodium channel gating and kinetics by versutoxin from the Australian funnel-web spider Hadronyche versuta.

The effects of a neurotoxin (versutoxin VTX), purified from the venom of the Australian Blue Mountains funnel-web spider Hadronyche versuta, on the ionic currents in rat dorsal root ganglion cells were investigated under voltage-clamp conditions using the whole-cell patch-clamp technique. VTX had no effect on tetrodotoxin-resistant (TTX-R) sodium currents or potassium currents. In contrast VTX produced a dose-dependent slowing or removal of tetrodotoxin-sensitive (TTX-S) sodium current inactivation, a reduction in peak TTX-S sodium current but did not markedly slow tail current kinetics of TTX-S sodium currents. This steady-state sodium current was maintained during prolonged depolarizations at all test potentials and the reduction in sodium current amplitude produced by VTX had an apparent Ki of 37 nM. In the presence of 32 nM VTX the voltage dependence of steady-state sodium channel inactivation (h infinity) also showed a significant 7 mV shift in the voltage midpoint in the hyperpolarizing direction, with no change in the slope factor. In addition there was a steady-state or non-inactivating component present (14 +/- 2% of maximal sodium current) at prepulse potentials more depolarized than -40 mV, potentials which normally inactivate all TTX-S sodium channels. Finally, there was an observed increase in the rate of recovery from inactivation in the presence of VTX. These selective actions of VTX on sodium channel gating and kinetics are similar to those of alpha-scorpion and sea anemone toxins.

Persistent muscarinic excitation in guinea-pig olfactory cortex neurons: Involvement of a slow post-stimulus afterdepolarizing current

The persistent excitatory effects of the muscarinic agonist oxotremorine-M were investigated in guinea-pig olfactory cortex neurons in vitro (28–30°C) using a single-microelectrode current-clamp/voltageclamp technique. In 40% of recorded cells (type 1), bath-application of oxotremorine-M (2–10μM; 1–2 min) induced a strong membrane depolarization, an increase in input resistance and a sustained neuronal discharge lasting over 30 min following agonist washout. A large depolarizing stimulus applied during the action of oxotremorine-M, evoked a slow post-stimulus afterdepolarization (~ 10–15 mV) lasting ~30s. Injection of steady negative current at the peak of this response produced a slow repolarization of the membrane potential (half-time ~0.6 min) towards a plateau level (“hyperpolarization recovery”); these effects of oxotremorine-M were slowly reversed on washout or by application of atropine (l μ(M). In a second population of neurons (type 2; 39% of total), oxotremorine-M produced a large depolarization, a resistance increase and repetitive firing that did not persist after agonist washout; these neurons failed to generate a prominent slow afterdepolarization on stimulation, and showed no hyperpolarization recovery effect. Their resting membrane properties were not significantly different from those of type 1 cells. The remaining proportion of cells (type 3) elicited little or no muscarinic response to oxotremorine-M and no slow afterdepolarization; these cells showed characteristic spike fractionation (pre-potentials) during an evoked train of action potentials. In type 1 cells, the inward tail current underlying the slow afterdepolarization was revealed under voltage-clamp; the afterdepolarization current had a slow time to peak, an exponential decay, and its amplitude was reduced (but not reversed) by hyperpolarization between −60 and − 100mV, or by raising the extracellular K + concentration from 3 to 9mM. The afterdepolarization current was suppressed by Cd2 + (100 μM) or by removal of external Ca 2+, but not by adding Ba 2+ (500μM) or La 3+ (50–100μM). In oxotremorine-M, voltage-clamp commands from −70 to − 40 mV, evoked slowly-decaying outward current relaxations followed by slow inward tail currents. A slowly-relaxing outward current underlying the hyperpolarization recovery phenomenon was also revealed on stepping from −40 to−85 mV for 2 min. These slow relaxations were reduced by Cd2 + (100μM) or in Ca 2+-free medium, and were thought to represent the slow Ca 2+-induced de-activation and re-activation, respectively, of the afterdepolarization current conductance mechanism. Intracellular loading with the Ca 2+ chelators EGTA or BAPTA failed to reduce the afterdepolarization current; however, the tail current evoked under “hybrid” voltage-clamp, was dramatically enhanced after pre-treatment with tetrabutylammonium (500 μM), most likely due to increased Ca 2+ entry. These results add further support to the view that the afterdepolarization tail current and slow outward current relaxations revealed under voltage-clamp during muscarinic receptor stimulation reflect the slow re-activation of a novel K + conductance that is de-activated by Ca 2+ entry during a depolarizing command. We suggest that the de-activation of this conductance by maintained Ca 2+ influx at depolarized membrane potentials (e.g. during repetitive neuronal firing) could contribute to the prolonged muscarinic excitation of olfactory and perhaps other mammalian cortical neurons.

Muscarinic modulation of conductances underlying the afterhyperpolarization in neurons of the rat basolateral amygdala

The excitability level of pyramidal neurons in the basolateral amygdala (BLA) is greatly increased following muscarinic receptor activation, an effect associated with an increased rate of action potential firing and reduction of the afterhyperpolarization (AHP). We impaled BLA pyramidal neurons in slices of rat ventral forebrain with a single microelectrode to examine the currents underlying the AHP and spike frequency accomodation and determine their sensitivities to muscarinic modulation. In voltage-clamp, depolarizing steps were followed by biphasic outward tail currents, consisting of rapidly decaying ( I Fast) and slowly decaying ( I Slow) current components. These corresponded temporally with the medium and slow portions of the AHP, respectively. The reversal potential or the I Fast component of the AHP tail current shifted in the depolarizing direction with increases in the extracellular K + concentration. The amplitude of I Fast was reduced during perfusion of 0-Ca 2+ medium or by superfusion of TEA (1–5 mM) or carbachol (10–40 μM). It is suggested that I Fast was produced by the rapidly decaying Ca 2+-activated K + current ( I C) and the muscarinic-sensitive M-current ( I M). The I Slow tail current component reversed at the estimated values for E K in medium containing either normal or elevated K + levels. This component was eliminated by perfusion of 0-Ca 2+ medium or inclusion of cyclic-AMP in the recording electrode. It was not blocked by TEA (5 mM) or apamin (50–500 nM), but was reduced by carbachol in a dose-dependent manner ( IC 50=0.5 μM). Electrical stimulation cholinergic afferent pathways to the BLA produced inhibition of I Slow, an effect which was enhanced by eserine and prevented by atropine. Loss of the I Slow component was always accompanied by similar reductions in accomodation and the slow AHP. It was concluded that this tail current component resulted from the slowly decaying Ca 2+-activated K + current, I AHP. Thus, the muscarinic inhibition of I AHP contributes to the enhanced excitability exhibited by BLA pyramidal neurons following cholinergic stimulation.

Single-photon avalanche diode with ultrafast pulse response free from slow tails

A single-photon avalanche diode (SPAD) with a novel structure is presented. The slow tail in the pulse response, which plagued the performance of previous ultrahigh-speed SPADs, has been eliminated. The pulse response is cleaner and sharper than that of the fastest microchannel-plate photomultipliers, making the new devices almost ideal for photon timing measurements on fast optical waveforms. The typical photon detection efficiencies are 16% at 630 nm and 4% at 850 nm, and the timing resolution of the 8- mu m devices is 45 ps full width at half maximum (FWHM). Unprecedented performance is demonstrated: just 300 ps after a light pulse, a signal three orders of magnitude weaker can be easily measured. >

Ionic dependence of a slow inward tail current in rat dorsal raphe neurones.

1. The mechanism underlying a large slow inward tail current was studied in serotonergic dorsal raphe (DR) neurones. The tail current is most easily observed under conditions of suppressed K+ channel outward currents and follows the activation of a calcium current. This current may underlie a slow after-depolarizing potential (ADP) which follows action potentials observed in acutely isolated DR neurones. 2. The after-hyperpolarizing potential (AHP) following action potentials which should reverse at EK (the reversal potential for potassium) becomes an ADP at less negative potentials than expected due to contamination by the slow inward tail current. 3. DR neurones were acutely isolated enzymatically; the ADP in current clamp and the tail current underlying it in voltage clamp were studied using the patch clamp method. When the external Na+ was replaced with TEA or choline the slow inward tail current was completely abolished. Blocking K+ channels from the inside of the cell membrane with 40 mM TEACl or large concentrations of internal Cs+ also blocked the slow inward tail current. 4. The tail current proved to be independent of calcium influx or intracellular calcium release as it was not affected by inorganic calcium channel blockers or caffeine. 5. The tail grew exponentially upon lengthening the depolarizing test pulse and appeared to reverse close to 0 mV indicating that the current was carried by a nonselective cation conductance. Removal of external Na+ and replacement with Li+ ions reversibly blocked the tail current by 77%. 6. The data rule out several mechanisms for the generation of the current, namely: a calcium-activated chloride conductance, a calcium-activated non-selective cation conductance, a Na(+)-Ca2+ exchange pump current or a sodium-activated K+ conductance. 7. The slow tail current may be explained by postulating an inward movement of Na+ through a channel which is blocked by high concentrations of external TEA and Li+ or internal Cs+ or 40 mM TEA.