Slow Component Of Recovery Research Articles

Fully-primed slowly-recovering vesicles mediate presynaptic LTP at neocortical neurons.

Pre- and postsynaptic forms of long-term potentiation (LTP) are candidate synaptic mechanisms underlying learning and memory. At layer 5 pyramidal neurons, LTP increases the initial synaptic strength but also short-term depression during high-frequency transmission. This classical form of presynaptic LTP has been referred to as redistribution of synaptic efficacy. However, the underlying mechanisms remain unclear. We therefore performed whole-cell recordings from layer 5 pyramidal neurons in acute cortical slices of rats and analyzed presynaptic function before and after LTP induction by paired pre- and postsynaptic neuronal activity. LTP was successfully induced in about half of the synaptic connections tested and resulted in increased synaptic short-term depression during high-frequency transmission and a decelerated recovery from short-term depression due to an increased fraction of a slow recovery component. Analysis with a recently established sequential two-step vesicle priming model indicates an increase in the abundance of fully-primed and slowly-recovering vesicles. A systematic analysis of short-term plasticity and synapse-to-synapse variability of synaptic strength at various types of synapses revealed that stronger synapses generally recover more slowly from synaptic short-term depression. Finally, pharmacological stimulation of the cyclic adenosine monophosphate and diacylglycerol signaling pathways, which are both known to promote synaptic vesicle priming, mimicked LTP and slowed the recovery from short-term depression. Our data thus demonstrate that LTP at layer 5 pyramidal neurons increases synaptic strength primarily by enlarging a subpool of fully-primed slowly-recovering vesicles.

Nav1.3 and FGF14 are primary determinants of the TTX-sensitive sodium current in mouse adrenal chromaffin cells.

Adrenal chromaffin cells (CCs) in rodents express rapidly inactivating, tetrodotoxin (TTX)-sensitive sodium channels. The resulting current has generally been attributed to Nav1.7, although a possible role for Nav1.3 has also been suggested. Nav channels in rat CCs rapidly inactivate via two independent pathways which differ in their time course of recovery. One subpopulation recovers with time constants similar to traditional fast inactivation and the other ∼10-fold slower, but both pathways can act within a single homogenous population of channels. Here, we use Nav1.3 KO mice to probe the properties and molecular components of Nav current in CCs. We find that the absence of Nav1.3 abolishes all Nav current in about half of CCs examined, while a small, fast inactivating Nav current is still observed in the rest. To probe possible molecular components underlying slow recovery from inactivation, we used mice null for fibroblast growth factor homology factor 14 (FGF14). In these cells, the slow component of recovery from fast inactivation is completely absent in most CCs, with no change in the time constant of fast recovery. The use dependence of Nav current reduction during trains of stimuli in WT cells is completely abolished in FGF14 KO mice, directly demonstrating a role for slow recovery from inactivation in determining Nav current availability. Our results indicate that FGF14-mediated inactivation is the major determinant defining use-dependent changes in Nav availability in CCs. These results establish that Nav1.3, like other Nav isoforms, can also partner with FGF subunits, strongly regulating Nav channel function.

Microtubule and Actin Differentially Regulate Synaptic Vesicle Cycling to Maintain High-Frequency Neurotransmission.

Cytoskeletal filaments such as microtubules (MTs) and filamentous actin (F-actin) dynamically support cell structure and functions. In central presynaptic terminals, F-actin is expressed along the release edge and reportedly plays diverse functional roles, but whether axonal MTs extend deep into terminals and play any physiological role remains controversial. At the calyx of Held in rats of either sex, confocal and high-resolution microscopy revealed that MTs enter deep into presynaptic terminal swellings and partially colocalize with a subset of synaptic vesicles (SVs). Electrophysiological analysis demonstrated that depolymerization of MTs specifically prolonged the slow-recovery time component of EPSCs from short-term depression induced by a train of high-frequency stimulation, whereas depolymerization of F-actin specifically prolonged the fast-recovery component. In simultaneous presynaptic and postsynaptic action potential recordings, depolymerization of MTs or F-actin significantly impaired the fidelity of high-frequency neurotransmission. We conclude that MTs and F-actin differentially contribute to slow and fast SV replenishment, thereby maintaining high-frequency neurotransmission.SIGNIFICANCE STATEMENT The presence and functional role of MTs in the presynaptic terminal are controversial. Here, we demonstrate that MTs are present near SVs in calyceal presynaptic terminals and that MT depolymerization specifically prolongs the slow-recovery component of EPSCs from short-term depression. In contrast, F-actin depolymerization specifically prolongs fast-recovery component. Depolymerization of MT or F-actin has no direct effect on SV exocytosis/endocytosis or basal transmission, but significantly impairs the fidelity of high-frequency transmission, suggesting that presynaptic cytoskeletal filaments play essential roles in SV replenishment for the maintenance of high-frequency neurotransmission.

Resolving the excited state relaxation dynamics of guanosine monomers and hydrogen-bonded homodimers in chloroform solution

The relaxation pathways of silyl-modified guanosine nucleoside monomers (G) and double-hydrogen-bonded homodimers (GG1) are compared in chloroform solution after 260-nm ultraviolet excitation. Transient absorption spectra support two previously reported relaxation pathways for the monomer with time constants of 210 ± 20 fs and 2.6 ± 0.1 ps. These pathways are associated with bifurcated approach to a seam of conical intersections between the excited 1ππ* 1La state and the ground electronic state. In the homodimer, an increase in the larger time constant to 18 ± 2 ps is attributed to slower passage through the minimum energy region of the 1ππ* state. A further time constant of 70 ± 10 fs indicates wavepacket evolution out of the 1ππ* state Franck-Condon region. A slow component of recovery of ground-state GG1 is proposed to result either from relaxation of the product of inter-base electron-driven proton transfer, or from the lowest triplet state (3ππ* , T1).

The very low-frequency band of heart rate variability represents the slow recovery component after a mental stress task.

The very low-frequency (VLF) band of heart rate variability (HRV) has different characteristics compared with other HRV components. Here we investigated differences in HRV changes after a mental stress task. After the task, the high-frequency (HF) band and ratio of high- to low-frequency bands (LF/HF) immediately returned to baseline. We evaluated the characteristics of VLF band changes after a mental stress task. We hypothesized that the VLF band decreases during the Stroop color word task and there would be a delayed recovery for 2 h after the task (i.e., the VLF change would exhibit a “slow recovery”).Nineteen healthy, young subjects were instructed to rest for 10 min, followed by a Stroop color word task for 20 min. After the task, the subjects were instructed to rest for 120 min. For all subjects, R-R interval data were collected; analysis was performed for VLF, HF, and LF/HF ratio. HRV during the rest time and each 15-min interval of the recovery time were compared. An analysis of the covariance was performed to adjust for the HF band and LF/HF ratio as confounding variables of the VLF component.HF and VLF bands significantly decreased and the LF/HF ratio significantly increased during the task compared with those during rest time. During recovery, the VLF band was significantly decreased compared with the rest time. After the task, the HF band and LF/HF ratio immediately returned to baseline and were not significantly different from the resting values. After adjusting for HF and LF/HF ratio, the VLF band had significantly decreased compared with that during rest.The VLF band is the “slow recovery” component and the HF band and LF/HF ratio are the “quick recovery” components of HRV. This VLF characteristic may clarify the unexplained association of the VLF band in cardiovascular disease prevention.

Lattice-Strain-Induced Slow Electron Cooling Due to Quasi-Type-II Behavior in Type-I CdTe/ZnS Nanocrystals

Detailed analysis on charge separation energetics and dynamics for CdTe/ZnS nanocrystals have been carried out with varying shell thickness to elucidate quasi-type-II behavior in a standard type-I system. Redshift in the absorption–photoluminescence spectra and increase of the excited state lifetime in the core/shell nanocrystals with a thick ZnS shell (2 and 4 ML of ZnS) indicate quasi-type-II behavior caused by charge separation. Separation of charge arises as the lattice strain at the core/shell interface alters the conduction band energy levels for both the core and the shell in an opposite way, extending the electronic wave function toward the shell. To find out the energetics of the charge separation, the steady-state spectra were analyzed in the realm of Marcus theory to reveal charge separation occurring in the inverted region with −ΔG°ET > λ. Slow electron cooling as observed from ultrafast transient absorption measurements with increasing shell thickness also confirms electron being decoupled from the hole as the electronic wave function spreads out to the shell. Consistent with the Marcus theory analysis, the separation of charge is clearly exhibited in the nanocrystal with the highest ZnS shell thickness because the excitonic bleach shows a slower electron cooling rate and increased amplitude of a slow recovery component in the red region of transient absorption spectrum.

Abstract 18703: Fluorescence Tracking of Domain IV Voltage Sensor Motion Shows a Progressively Varying Molecular Phenotype in Long QT Syndrome Type 3 Mutants

Background: Long QT Syndrome type 3 (LQT3) is caused by mutations in SCN5a, which encodes the hNa V 1.5 α subunit; each contains 4 domains (DI-DIV) with 6 transmembrane segments (S1-S6). S1-S4 form a voltage sensing domain (VSD) that transduces membrane potential into channel gating. The molecular mechanisms that lead to LQT3 and the consequences of mutation location remain unclear, and better definition will lead to improved phenotypic precision and therapeutic targeting. We use a recent technique, voltage-clamp fluorometry (VCF), to evaluate three LQT3 mutations in the DIV VSD: R1623Q, R1626P, and previously uncharacterized, S1609W. Methods: To track the S4, a cysteine was introduced into the DIV S3-S4 linker, and conjugated to a TAMRA-MTS label. mRNA was injected into Xenopus oocytes and currents and fluorescence were recorded after 3-5 days with cut-open VCF. Results: Activation, inactivation, and recovery were similar in WT and S1609W. R1626P activation and inactivation recovery was not significantly different from WT while inactivation was left shifted. Both mutants exhibited a pro-arrhythmic delay of inactivation onset at -30mV - S1609W by 44% (0.8 ms) and R1626P by 36% (0.7 ms), when compared to onset in WT (1.9 ± 0.0 ms). DIV S4 deactivation also differed significantly among WT, S1609W, and R1626P. Exponential fitting revealed two time constants for the WT DIV S4 to return to rest of 0.23 ms and 17.44 ms with magnitudes of 0.3 and 0.7 ± 0.1, respectively. In contrast, S1609W showed a significant increase in the fast component (0.8 and 0.2 ± 0.1). A single signal from R1626P showed an intermediate phenotype (0.7 and 0.3). Further, a delay in sensor deactivation onset observed in the WT was absent from both mutants. No fluorescence was detected from R1623Q, perhaps indicating an especially severe phenotype with respect to the DIV S4 motion. Conclusion: Fluorescent kinetics from two LQT3 mutants, S1609W and R1626P showed a progressive molecular phenotype in which a slow component of recovery to the resting state was impaired by the mutation. Comparison to ionic current inactivation revealed that the reduction in this slow component corresponded to an increase in the time to peak, implying that the onset of fast inactivation is delayed by these mutations.

Crumbs Affects Protein Dynamics In Anterior Regions Of The Developing Drosophila Embryo

Maintenance of apico-basal polarity is essential for epithelial integrity and requires particular reinforcement during tissue morphogenesis, when cells are reorganised, undergo shape changes and remodel their junctions. It is well established that epithelial integrity during morphogenetic processes depends on the dynamic exchange of adherens junction components, but our knowledge on the dynamics of other proteins and their dynamics during these processes is still limited. The early Drosophila embryo is an ideal system to study membrane dynamics during morphogenesis. Here, morphogenetic activities differ along the anterior-posterior axis, with the extending germband showing a high degree of epithelial remodelling. We developed a Fluorescence Recovery After Photobleaching (FRAP) assay with a higher temporal resolution, which allowed the distinction between a fast and a slow component of recovery of membrane proteins during the germband extension stage. We show for the first time that the recovery kinetics of a general membrane marker, SpiderGFP, differs in the anterior and posterior parts of the embryo, which correlates well with the different morphogenetic activities of the respective embryonic regions. Interestingly, absence of crumbs, a polarity regulator essential for epithelial integrity in the Drosophila embryo, decreases the fast component of SpiderGFP and of the apical marker Stranded at Second-Venus specifically in the anterior region. We suggest that the defects in kinetics observed in crumbs mutant embryos are the first signs of tissue instability in this region, explaining the earlier breakdown of the head epidermis in comparison to that of the trunk, and that diffusion in the plasma membrane is affected by the absence of Crumbs.

Gain recovery in a quantum dot semiconductor optical amplifier and corresponding pattern effects in amplified optical signals at 15 μm

Fast gain recovery observed in quantum-dot semiconductor-optical-amplifiers (QDSOAs) is useful for amplifying high-speed optical signals. The small but finite slow recovery component can deteriorate the signal amplification due to the accumulation of gain saturation during 10 Gb/s operation. A study of the gain recoveries and pattern effects in signals amplified using a 1.5 μm InAs/InGaAsP QDSOA reveals that the gain recovery is always fast, and pattern-effect-free amplification is observed at the ground state. However, at the excited state, the slow component increases with the current, and significant pattern effects are observed. Simulations of the pattern effects agreed with the observed experimental trends.

Mechanisms of Kv2.1 channel inhibition by celecoxib – modification of gating and channel block

Selective cyclooxygenase-2 (COX-2) inhibitors such as rofecoxib (Vioxx) and celecoxib (Celebrex) were developed as NSAIDs with reduced gastric side effects. Celecoxib has now been shown to affect cellular physiology via an unexpected, COX-independent, pathway - by inhibiting K(v)2.1 and other ion channels. In this study, we investigated the mechanism of the action of celecoxib on K(v)2.1 channels. The mode of action of celecoxib on rat K(v)2.1 channels was studied by whole-cell patch-clamping to record currents from channels expressed in HEK-293 cells. Celecoxib reduced current through K(v)2.1 channels when applied from the extracellular side. At low concentrations (<or=3 microM), celecoxib accelerated kinetics of activation, deactivation and inactivation. Recovery of rat K(v)2.1 channels from inactivation could be characterized by two components, with celecoxib selectively accelerating the slow component of recovery at <or=10 microM. At >3 microM, celecoxib led to closed-channel block with relative slowing of activation. At 30 microM, it additionally induced open-channel block that manifested in use-dependent inhibition and slower recovery from inactivation. Celecoxib reduced current through K(v)2.1 channels by modifying gating and inducing closed- and open-channel block, with the three effects manifesting at different concentrations. These data will help to elucidate the mechanisms of action of this widely prescribed drug on ion channels and those underlying its neurological, cardiovascular and other effects.

The Relationship between Agonist Potency and AMPA Receptor Kinetics

AMPA-type glutamate receptors are tetrameric ion channels that mediate fast excitatory synaptic transmission in the mammalian brain. When agonists occupy the binding domain of individual receptor subunits, this domain closes, triggering rearrangements that couple agonist binding to channel opening. Here we compare the kinetic behavior of GluR2 channels activated by four different ligands, glutamate, AMPA, quisqualate, and 2-Me-Tet-AMPA, full agonists that vary in potency by up to two orders of magnitude. After reduction of desensitization with cyclothiazide, deactivation decays were strongly agonist dependent. The time constants of decay increased with potency, and slow components in the multiexponential decays became more prominent. The desensitization decays of agonist-activated currents also contained multiple exponential components, but they were similar for the four agonists. The time course of recovery from desensitization produced by each agonist was described by two sigmoid components, and the speed of recovery varied substantially. Recovery was fastest for glutamate and slowest for 2-Me-Tet-AMPA, and the amplitude of the slow component of recovery increased with agonist potency. The multiple kinetic components appear to arise from closed-state transitions that precede channel gating. Stargazin increases the slow kinetic components, and they likely contribute to the biexponential decay of excitatory postsynaptic currents.

Transient outward K+ currents in rat dissociated subfornical organ neurones and angiotensin II effects

Although angiotensin II inhibits transient outward K+ currents (I(A)s) in subfornical organ neurones, there is no evidence concerning which Kv channels are involved. We investigated I(A)-generating Kv channels in dissociated rat subfornical organ neurones, using molecular, electrophysiological and pharmacological techniques, and studied the effects of angiotensin II. Conventional RT-PCR showed the presence of mRNAs for channels of the Kv3.4, Kv1.4 and Kv4 families, which are capable of generating I(A)s. Tetraethylammonium at 1 mm, which blocks Kv3 channel-derived currents, and blood-depressing substance-I, a Kv3.4-specific blocker, at 2 microm suppressed the I(A)-like component of whole-cell outward currents in some neurones. 4-Aminopyridine at 5 mm inhibited I(A)s in the presence of tetraethylammonium at 1 mm. Cd2+ at 300 microm shifted the activation and inactivation curves of the 4-aminopyridine-sensitive and tetraethylammonium-resistant I(A)s positively. The tetraethylammonium-resistant I(A)s showed fast and slow components during the process of recovery from inactivation, but the slow component was not seen in all neurones. The time constant of the fast recovery component was less than 200 ms, while that of the slow recovery component was around 1 s. Using single-cell RT-PCR, mRNAs for Kv4.2 and Kv4.3L were detected frequently, but those for Kv1.4 and Kv3.4 were seen only rarely. Angiotensin II at 30 nm inhibited the fast recovery component of tetraethylammonium-resistant I(A)s in many neurones. These results suggest that the fast recovery component of the tetraethylammonium-resistant I(A) in subfornical organ neurones depends upon Kv4, and that it can be modulated by angiotensin II.

Inhibition of cardiac Na+ current by primaquine.

The electrophysiological effects of the anti-malarial drug primaquine on cardiac Na(+) channels were examined in isolated rat ventricular muscle and myocytes. In isolated ventricular muscle, primaquine produced a dose-dependent and reversible depression of dV/dt during the upstroke of the action potential. In ventricular myocytes, primaquine blocked I(Na)(+) in a dose-dependent manner, with a K(d) of 8.2 microM. Primaquine (i) increased the time to peak current, (ii) depressed the slow time constant of I(Na)(+) inactivation, and (iii) slowed the fast component for recovery of I(Na)(+) from inactivation. Primaquine had no effect on: (i) the shape of the I - V curve, (ii) the reversal potential for Na(+), (iii) the steady-state inactivation and g(Na)(+) curves, (iv) the fast time constant of inactivation of I(Na)(+), and (v) the slow component of recovery from inactivation. Block of I(Na)(+) by primaquine was use-dependent. Data obtained using a post-rest stimulation protocol suggested that there was no closed channel block of Na(+) channels by primaquine. These results suggest that primaquine blocks cardiac Na(+) channels by binding to open channels and unbinding either when channels move between inactivated states or from an inactivated state to a closed state. Cardiotoxicity observed in patients undergoing malaria therapy with aminoquinolines may therefore be due to block of Na(+) channels, with subsequent disturbances of impulse conductance and contractility.

Integration of calcium signals by calmodulin in rat sensory neurons.

We have used the fluorescently labelled calmodulin TA-CaM to follow calmodulin activation during depolarization of adult rat sensory neurons. Calcium concentration was measured simultaneously using the low affinity indicator Oregon Green BAPTA 5N. TA-CaM fluorescence increased during a 200-ms depolarization but then continued to increase during the subsequent 500 ms, even though total cell calcium was falling at this time. In the next few seconds TA-CaM fluorescence fell, but to a new elevated level that was then maintained for several tens of seconds. During a train of depolarizations that evoked a series of largely independent calcium changes TA-CaM fluorescence was in contrast raised for the duration of the train and for many tens of seconds afterwards. The presence of a peptide corresponding to the calmodulin binding domain of myosin light chain kinase significantly increased the depolarization-induced TA-CaM fluorescence increase and slowed the subsequent fall of fluorescence. We interpret the slow recovery component of the TA-CaM signal as reflecting the slow dissociation of calcium--calmodulin--calmodulin binding protein complexes. Our results show that after brief electrical activity calmodulin's interaction with calmodulin binding proteins persists for approximately one minute.

Habituation and desensitization of the Hering-Breuer reflex in rat.

1. Many processes in mammalian and invertebrate central nervous systems exhibit habituation and/or sensitization of their responses to repetitive stimuli. Here, we studied the adaptive behaviours of the respiratory pattern generator in rat on repetitive vagal-afferent stimulation and compared these behaviours obtained in vivo with the reported effects of such stimuli on synaptic transmission in the corresponding signal pathway in vitro. 2. Sustained (1 min) electrical pulsed stimulation of the vagus nerve elicited the classic Hering-Breuer (HB) reflex slowing of the respiratory rhythm followed by a bi-exponential recovery, and a post-stimulus rebound (PR). The recovery from the HB reflex satisfied the classic criteria of habituation. 3. The fast component of the recovery and the PR were abolished by systemic administration of an NMDA receptor antagonist or electrolytic lesioning of the pontine Kolliker-Fuse nucleus. The characteristics of the fast recovery and PR suggest a vagally induced desensitization of the NMDA receptor-dependent pontine input to the respiratory pattern generator. 4. The slow component of recovery persist after both experimental interventions and accounted for the habituation to the vagal input. The characteristics of the slow recovery in vivo were reminiscent of the reported synaptic accommodation in vitro in the medullary region where vagal afferents terminate. 5. The habituation of vagal input and desensitization of pontine input act in concert to offset the HB reflex. Such simultaneous habituation-desensitization in parallel neural pathways with differing sensitivities to NMDA receptor activation represent a hitherto unknown pairing of dual non-associative learning processes in the mammalian brain.

Relationship between K+ channel down-regulation and [Ca2+]i in rat ventricular myocytes following myocardial infarction.

1. Cardiac hypertrophy and prolongation of the cardiac action potential are hallmark features of heart disease. We examined the molecular mechanisms and the functional consequences of this action potential prolongation on calcium handling in right ventricular myocytes obtained from rats 8 weeks following ligation of the left anterior descending coronary artery (post-myocardial infarction (MI) myocytes). 2. Compared with myocytes from sham-operated rats (sham myocytes), post-MI myocytes showed significant reductions in transient outward K+ current (Ito) density (sham 19.7 +/- 1.1 pA pF-1 versus post-MI 11.0 +/- 1.3 pA pF-1; means +/- s.e.m.), inward rectifier K+ current density (sham -13.7 +/- 0.6 pA pF-1 versus post-MI -10.3 +/- 0.9 pA pF-1) and resting membrane potential (sham -84.4 +/- 1.3 mV versus post-MI -74.1 +/- 2.6 mV). Depressed Ito amplitude correlated with significant reductions in Kv4.2 and Kv4.3 mRNA and Kv4.2 protein levels. Kv1.4 mRNA and protein levels were increased and coincided with the appearance of a slow component of recovery from inactivation for Ito. 3. In current-clamp recordings, post-MI myocytes showed a significant increase in [Ca2+]i transient amplitude compared with sham myocytes. Using voltage-clamp depolarizations, no intrinsic differences in Ca2+ handling by the sarcoplasmic reticulum or in L-type Ca2+ channel density (ICa,L) were detected between the groups. 4. Stimulation of post-MI myocytes with an action potential derived from a sham myocyte reduced the [Ca2+] transient amplitude to the sham level and vice versa. 5. The net Ca2+ influx per beat via ICa,L was increased about 2-fold in myocytes stimulated with post-MI action potentials compared with sham action potentials. 6. Our findings demonstrate that reductions in K+ channel expression in post-MI myocytes prolong action potential duration resulting in elevated Ca2+ influx and [Ca2+]i transients.

Latent addition in motor and sensory fibres of human peripheral nerve.

1. The time constants of motor and sensory nerve fibres were studied in normal human ulnar nerves by the method of latent addition, using threshold tracking to follow the recovery of excitability after brief conditioning current pulses. The 60 microseconds test and conditioning stimuli were applied at the wrist, and the conditioning stimuli were set to 90, 60, 30, -30, -60 and -90% of the control threshold current. Compound muscle action potentials were recorded from abductor digiti minimi, and sensory nerve action potentials from the little finger. 2. Recovery from depolarizing conditioning pulses was slower than recovery from hyperpolarizing pulses and strongly dependent on conditioning pulse amplitude. The voltage dependence of latent addition was attributed to subthreshold activation of sodium channels (local response). 3. Motor and sensory nerve excitability generally recovered from -90% hyperpolarizing pulses as the sum of two exponential components, although the slow component was negligible in some motor nerves. The fast component (time constant 43.3 +/- 2.0 microseconds, mean +/- S.E.M., n = 9) was similar between motor and sensory fibres in the same subject. It showed no consistent voltage dependence, and was attributed to a passive input time constant of the fibres. The slow component of recovery from hyperpolarizing pulses was greater in sensory than in motor fibres and was voltage dependent: it could be greatly increased in motor and sensory fibres by steady depolarization. It was attributed to a regenerative membrane current, active at the resting potential in sensory and at least some motor nerves. 4. The latent addition responses were compared with the computed responses of four theoretical models. Both motor and sensory responses were well fitted by a model in which a fraction of the sodium channels (less in motor than in sensory fibres) were activated at potentials 20 mV more negative than normal and at half the normal rate, and did not inactivate. 5. It is concluded that the differences in latent addition between motor and sensory fibres are primarily due to differences in non-classical, voltage-dependent ion channels, active close to the resting potential. These "threshold channels' may help to account for the longer strength-duration time constant of sensory fibres, for their lower rheobase, and for their greater tendency to fire repetitively.

Na(+)-Ca2+ exchange in rat dorsal root ganglion neurons.

The role of the Na(+)-Ca2+ exchanger was examined in isolated rat dorsal root ganglion (DRG) neurons. Neurons were dialyzed with the Ca2+ indicator Indo-1. Ca2+ transients were elicited by depolarizing the cells from -80 to 0 mV for 100 ms under voltage clamp conditions. In most cells (45 of 67), the decay of intracellular Ca2+ concentration ([Ca2+]i) could be fitted with a single exponential with a time constant of 2.43 s. In the remaining 22 cells, the decay of [Ca2+]i could be described with a double exponential with time constants of 0.76 and 11.84 s. In cells that displayed a biphasic [Ca2+]i relaxation, Na(+)-free medium caused resting [Ca2+]i to increase from 116 to 186 nM; the slow component of recovery to basal [Ca2+]i was nearly abolished in Na(+)-free medium or by application of 5 mM Ni2+. In 35 of 45 cells displaying a monophasic [Ca2+]i decay, omitting external Na+ increased the time constant of [Ca2+]i decay from 2.02 to 3.63 s. In the remaining 10 cells, Na(+)-free solution did not affect Ca2+ handling. The time constant of [Ca2+]i relaxation was voltage dependent. These findings demonstrate the important role of the Na(+)-Ca2+ exchanger in DRG neurons. Its presence was further confirmed both at the mRNA and the protein level.

Discharge-rate dependence of refractory behavior of cat auditory-nerve fibers

A descriptive model for auditory-nerve (AN) refractory periods is described. The model assumes that interspike intervals consist of a constant-length absolute refractory period (ARP), followed by random-length relative refractory period (RRP) and a random-length waiting time to the next spike. Both the RRP and waiting time are exponentially distributed. This model fits AN hazard functions sufficiently well to provide estimates of the ARP and RRP durations for each fiber. The ARP is found to be constant, independent of discharge rate, with mean value between 0.56 and 0.86 ms in data from 7 experiments. The RRP decreases in duration as discharge rate increases; RRP mean length is less than 2 ms in most cases. There is an additional, slow component of recovery, lasting 20–40 ms, which is not modeled. The variation in RRP with discharge rate is shown to be capable of accounting for the deviation of AN regularity from that predicted for a Poisson process. Finally, properties of peaks seen in hazard functions just at the end of the ARP are described; these peaks are not included in the model, but are shown to be especially prominent in low and medium spontaneous rate fibers.

Multiple kinetic components of sodium channel inactivation in rabbit Schwann cells.

1. We have studied the kinetics of inactivation of sodium currents evoked in Schwann cells from neonatal and adult rabbits with patch-clamp recording techniques. The decay both of whole-cell currents and of ensemble currents obtained from outside-out patches was reasonably well-described by single exponential fits which gave values for the time constant, tau h, similar to those found for such currents in nerve (about 0.5 ms at 0 mV). Although inclusion of an additional exponential component usually improved the fits to the decay of these currents, the relative amplitude of the slower component (time constant 3-6 ms) was always small. 2. The time course of recovery from steady-state inactivation clearly consisted of two exponential components. At -120 mV, recovery from steady-state inactivation at -50 mV consisted of a fast component with a time constant of 2.2 +/- 0.2 ms and a much slower component with a time constant of 1.2 +/- 0.2 s (n = 9). The relative amplitude of the slow component (expressed as a fraction of the sodium current when inactivation was removed completely) was 0.56 +/- 0.03. The corresponding amplitudes of the slow component when similar experiments were done from holding potentials of 0 and -70 mV were 0.80 +/- 0.04 and 0.36 +/- 0.03, respectively (n = 5 and 8). 3. The onset of steady-state inactivation also followed a bi-exponential time course. The time constant of the slower component was similar at each potential examined (0, -50 and -70 mV), being 6-7 s. The relative amplitude of the slow component of onset depended on membrane potential, and it was similar (at each potential examined) to the corresponding amplitude of the slow component of recovery. 4. The inclusion of 40 mM-iodate ions in the pipette solution slowed the decay of whole-cell sodium currents, as did the extracellular application of venom (10 micrograms ml-1) from the scorpion Leiurus quinquestriatus. Prolonged exposure of the cells to Leiurus venom appeared to increase the steady-state amount of slow inactivation. 5. Records of single-channel sodium currents tended to cluster into records which either did, or did not, contain openings. This apparently non-random behaviour depended on membrane potential, and on the frequency at which the test steps were repeated, in the way expected if it resulted from slow inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)