Chylomicron components activate duodenal vagal afferents via a cholecystokinin A receptor-mediated pathway to inhibit gastric motor function in the rat

The relationship between the electrophysiological properties of motoneurones and their muscle units has been established in animal models. A functionally significant relationship exists whereby motoneurones with long post-spike afterhyperpolarizations (AHPs) innervate slow contracting muscle units. The purpose of this study was to determine whether the time course of the AHP as measured by its time constant is associated with the contractile properties of its muscle unit in humans. Using an intramuscular fine wire electrode, 46 motor units were recorded in eight subjects as they held a low force contraction of the first dorsal interosseus muscle for approximately 10 min. By applying a recently validated transform to the interspike interval histogram, the mean voltage versus time trajectory of the motoneurone AHP was determined. Spike-triggered averaging was used to extract the muscle unit twitch from the whole muscle force with strict control over force variability and motor unit discharge rate (interspike intervals between 120 and 200 ms). The AHP time constant was positively correlated to the time to half-force decay (ρ= 0.36, P < 0.05) and twitch duration (ρ= 0.57, P < 0.001); however, time to peak force failed to reach significance (ρ= 0.27, P < 0.07). These results suggest that a similar functional relationship exists in humans between the motoneurone AHP and the muscle unit contractile properties.     

Afterhyperpolarization in neurones of the red nucleus

Summary Afterhyperpolarization (AHP) following single or short trains of spikes in rubrospinal neurones (RN neurones) of the cat has been studied with intracellular recording techniques. The AHP amplitude was potential dependent; it increased with depolarization and decreased with hyperpolarization and had an extrapolated reversal potential about 20 mV below resting membrane potential. The AHP was associated with an increase in the membrane conductance and it was concluded that the AHP is primarily caused by an increase in membrane conductance to potassium ions. The time course of the conductance change underlying the AHP was measured with short current pulses and calculated from the AHP voltage. The AHP following a single spike was conditioned at different interspike intervals by a preceding spike (or several spikes). In many RN neurones the AHP (conductance) following a spike added approximately linear to that generated by a preceding spike. In most cells, however, the AHP following a spike was instead depressed by a preceding spike. The summation of AHPs increased progressively, while the depression appeared to be already maximal with one preceding spike. The depression was then approximately constant for interspike intervals less than the AHP duration. It will be shown in a following paper that these properties of the AHP are reflected in the behaviour of the repetitive discharge evoked by constant current pulses in the same neurones.Dr. H. Hultborn was supported by the Japan Society for the Promotion of Science.     

How shunting inhibition affects the discharge of lumbar motoneurones: a dynamic clamp study in anaesthetized cats

In the present work, dynamic clamp was used to inject a current that mimicked tonic synaptic activity in the soma of cat lumbar motoneurones with a microelectrode. The reversal potential of this current could be set at the resting potential so as to prevent membrane depolarization or hyperpolarization. The only effect of the dynamic clamp was then to elicit a constant and calibrated increase of the motoneurone input conductance. The effect of the resulting shunt was investigated on repetitive discharges elicited by current pulses. Shunting inhibition reduced very substantially the firing frequency in the primary range without changing the slope of the current–frequency curves. The shift of the I–f curve was proportional to the conductance increase imposed by the dynamic clamp and depended on an intrinsic property of the motoneurone that we called the shunt potential. The shunt potential ranged between 11 and 37 mV above the resting potential, indicating that the sensitivity of motoneurones to shunting inhibition was quite variable. The shunt potential was always near or above the action potential voltage threshold. A theoretical model allowed us to interpret these experimental results. The shunt potential was shown to be a weighted time average of membrane voltage. The weighting factor is the phase response function of the neurone that peaks at the end of the interspike interval. The shunt potential indicates whether mixed synaptic inputs have an excitatory or inhibitory effect on the ongoing discharge of the motoneurone.     

Hindlimb unweighting for 2 weeks alters physiological properties of rat hindlimb motoneurones

We sought to determine whether decreased neuromuscular use in the form of hindlimb unweighting (HU) would affect the properties of innervating motoneurones. Hindlimb weight-bearing was removed in rats for a period of 2 weeks via hindlimb suspension by the tail. Following this the electrophysiological properties of tibial motoneurones were recorded under anaesthesia in situ . After HU, motoneurones had significantly ( P < 0.05) elevated rheobase currents, lower antidromic spike amplitudes, lower afterhyperpolarization (AHP) amplitudes, faster membrane time constants, lower cell capacitances, and depolarized spike thresholds. Frequency–current ( f – I ) relationships were shifted significantly to the right (i.e. more current required to obtain a given firing frequency), although there was no change in f – I slopes. 'Slow' motoneurones (AHP half-decay times, > 20 ms) were unchanged in proportions in HU compared to weight-bearing rats. Slow motoneurones had significantly lower minimum firing frequencies and minimum currents necessary for rhythmic firing than 'fast' motoneurones in weight-bearing rats; these differences were lost in HU rats, where slow motoneurones resembled fast motoneurones in these properties. In a five-compartment motoneurone model with ion conductances incorporated to resemble firing behaviour in vivo , most of the changes in passive and rhythmic firing properties could be reproduced by reducing sodium conductance by 25% and 15% in the initial segment and soma, respectively, or by increasing potassium conductance by 55% and 42%, respectively. This supports previous conclusions that changes in chronic neuromuscular activity, either an increase or decrease, may result in physiological adaptations in motoneurones due to chronic changes in ion conductances.     

Double stimulation modulates afterhyperpolarization phase following action potentials evoked in rat motoneurones

The influence of a pair of stimuli running in time sequence between 5-10 ms (a doublet) on the basic parameters of antidromic action potentials was studied in rat motoneurones. Electrophysiological experiments were based on stimulation of axons in the sciatic nerve and intracellular recording of antidromic action potentials from individual motoneurones located in L4-L5 segments of the spinal cord. The following parameters were analyzed after application of a single stimulus and a doublet: amplitude and duration of the antidromic spike, amplitude, total duration, time to minimum, half-decay time of the afterhyperpolarization (AHP). It was demonstrated that application of a pair of stimuli resulted in: (1) a prolongation of action potentials, (2) a prolongation of the total duration and half-decay time of the AHP, (3) a decline of the time to minimum of the AHP, (4) an increase of the AHP amplitude of the spike evoked by the second stimulus. Significant differences in AHP parameters were found either in fast or slow motoneurones. We suppose that doublet-evoked changes in the AHP amplitude and duration are linked to intrinsic properties of individual motoneurones and may lead to the prolongation of the time interval to subsequent motoneuronal discharges during voluntary activity.     

Doublet of action potentials evoked by intracellular injection of rectangular depolarization current into rat motoneurones

A doublet of action potentials is frequently observed at the beginning of motoneuronal discharge patterns and its appearance leads to a considerable increase in the motor unit force. The aims of this study were (1) to determine the relationship between the intensity of rectangular depolarization currents injected into motoneurones and their ability to generate doublets and (2) to evaluate the influence of the initial doublets on changes in motoneuronal firing frequency. Experiments were performed on anesthetized rats, and recordings were taken from motoneurones located in the L4–L5 segments of the spinal cord. The depolarization current necessary to evoke the initial doublet of action potentials was measured and expressed in multiples of the rheobase. A gradual increase in the intensity of current injected into motoneurones resulted in initial doublets in 80% of the cases studied, at doublet threshold ranges between 1.25 and 4.0 times the rheobase. This suggests that doublets are an effect of strong synaptic excitation of motoneurones that may precede a sudden change in force during a movement. Moreover, in the great majority of the studied motoneurones, this initial doublet caused changes in the subsequent firing rate by the prolongation of the first interspike interval.     

Concomitant changes in afterhyperpolarization and twitch following repetitive stimulation of fast motoneurones and motor units

The study aimed at determining changes in a course of motoneuronal afterhyperpolarization (AHP) and in contractile twitches of motor units (MUs) during activity evoked by increasing number of stimuli (from 1 to 5), at short interspike intervals (5 ms). The stimulation was applied antidromically to spinal motoneurones or to isolated axons of MUs of the medial gastrocnemius muscle within two separate series of experiments on anesthetized rats. Alterations in the amplitude and time parameters of the AHP of successive spikes were compared to changes in force and time course of successive twitches obtained by mathematical subtraction of tetanic contractions evoked by one to five stimuli. The extent of changes of the studied parameters depended on a number of applied stimuli. The maximal modulation of the AHP and twitch parameters (a prolongation and an increase in the AHP and twitch amplitudes) was typically observed after the second pulse, while higher number of pulses at the same frequency did not induce so prominent changes. One may conclude that changes observed in parameters of action potentials of motoneurons are concomitant to changes in contractile properties of MU twitches. This suggests that both modulations of the AHP and twitch parameters reflect mechanisms leading to force development at the beginning of MU activity.     

On the regulation of repetitive firing in lumbar motoneurones during fictive locomotion in the cat

Summary Repetitive firing of motoneurones was examined in decerebrate, unanaesthetised, paralysed cats in which fictive locomotion was induced by stimulation of the mesencephalic locomotor region. Repetitive firing produced by sustained intracellular current injection was compared with repetitive firing observed during fictive locomotion in 17 motoneurones. During similar interspike intervals, the afterhyperpolarisations (AHPs) during fictive locomotion were decreased in amplitude compared to the AHPs following action potentials produced by sustained depolarising current injections. Action potentials were evoked in 10 motoneurones by the injection of short duration pulses of depolarising current throughout the step cycles. When compared to the AHPs evoked at rest, the AHPs during fictive locomotion were reduced in amplitude at similar membrane potentials. The post-spike trajectories were also compared in different phases of the step cycle. The AHPs following these spikes were reduced in amplitude particularly in the depolarised phases of the step cycles. The frequency-current (f-I) relations of 7 motoneurones were examined in the presence and absence of fictive locomotion. Primary ranges of firing were observed in all cells in the absence of fictive locomotion. In most cells (6/7), however, there was no relation between the amount of current injected and the frequency of repetitive firing during fictive locomotion. In one cell, there was a large increase in the slope of the f-I relation. It is suggested that this increase in slope resulted from a reduction in the AHP conductance; furthermore, the usual elimination of the relation is consistent with the suggestions that the repetitive firing in motoneurones during fictive locomotion is not produced by somatic depolarisation alone, and that motoneurones do not behave as simple input-output devices during this behaviour. The correlation of firing level with increasing firing frequency which has previously been demonstrated during repetitive firing produced by afferent stimulation or by somatic current injection is not present during fictive locomotion. This lends further support to the suggestion that motoneurone repetitive firing during fictive locomotion is not produced or regulated by somatic depolarisation. It is suggested that although motoneurones possess the intrinsic ability to fire repetitively in response to somatic depolarisation, the nervous system need not rely on this ability in order to produce repetitive firing during motor acts. This capability to modify or bypass specific motoneuronal properties may lend the nervous system a high degree of control over its motor output.     

Visual information gain and the regulation of constant force levels

Visual information is essential in human motor control, and especially in the continuous modulation of isometric force. The gain of visual feedback, that is, the amount of space used to represent change in force, has been shown to affect both the magnitude and time-dependent properties of variability in the force output. However, little is known regarding the interacting effects of visual gain and target force level on force variability and whether the effects of force level can be mediated by a gain that is adjusted to force level. We examined the effect of different types and levels of visual feedback gain and target force level (1, 2, 4, 8, and 12 N) on the magnitude (standard deviation, SD) and regularity (approximate entropy, ApEn) of isometric force variability. Young adults performed an isometric force task with high and low levels of constant (same gain level for all forces) and scaled (proportional to force level) gain. The magnitude of force variability increased exponentially as a function of force level once the SD was corrected for the limits of the display area. The time-dependent properties of force variability remained constant across force levels when gain was adjusted to force level. These findings suggest that the time-dependent properties of force variability are the result an interaction between visual feedback and task force level demands, while the increases in SD over force levels are primarily due to the invariant properties of human muscle and the motor system.     

Effects of daily spontaneous running on the electrophysiological properties of hindlimb motoneurones in rats

No evidence currently exists that motoneurone adaptations in electrophysiological properties can result from changes in the chronic level of neuromuscular activity. We examined, in anaesthetized (ketamine/xylazine) rats, the properties of motoneurones with axons in the tibial nerve, from rats performing daily spontaneous running exercise for 12 weeks in exercise wheels ('runners') and from rats confined to plastic cages ('controls'). Motoneurones innervating the hindlimb via the tibial nerve were impaled with sharp glass microelectrodes, and the properties of resting membrane potential, spike threshold, rheobase, input resistance, and the amplitude and time-course of the afterhyperpolarization (AHP) were measured. AHP half-decay time was used to separate motoneurones into 'fast' (AHP half-decay time < 20 ms) and 'slow' (AHP half-decay time ≥ 20 ms), the proportions of which were not significantly different between controls (58 % fast) and runners (65 % fast). Two-way ANOVA and ANCOVA revealed differences between motoneurones of runners and controls which were confined to the 'slow' motoneurones. Specifically, runners had slow motoneurones with more negative resting membrane potentials and spike thresholds, larger rheobasic spike amplitudes, and larger amplitude AHPs compared to slow motoneurones of controls. These adaptations were not evident in comparing fast motoneurones from runners and controls. This is the first demonstration that physiological modifications in neuromuscular activity can influence basic motoneurone biophysical properties. The results suggest that adaptations occur in the density, localization, and/or modulation of ionic membrane channels that control these properties. These changes might help offset the depolarization of spike threshold that occurs during rhythmic firing.     

Short-term synchronization of intercostal motoneurone activity.

1. The hypothesis is advanced that the joint occurrence of unitary excitatory post-synaptic potentials e.p.s.p.s) evoked in motoneurones by branches of common stem pre-synaptic fibres causes short-term synchronization of their discharge during the rising phases of the unitary e.p.s.p.s. 2. This hypothesis was tested using the pre- and post-stimulus time (PPST) histogram to detect synchronized firing among groups of intercostal motoneurones discharging in response to their natural synaptic drives. 3. Motor nerve action potentials were recorded monophasically from nerve filaments of the external intercostal muscles of anaesthetized, paralysed cats maintained on artificial ventilation. 4. Computer methods were used to measure peak spike amplitude, spike amplitude, spike interval and filament identification for simultaneous recordings from four filaments. The spike amplitude histograms were derived for each filament and groups of spikes were selected for analysis. 5. With spikes of one group designated as 'stimuli' (occurring at zero time) and those of a second as 'response' the PPST histogram was computed with different time bin widths. 6. With bin widths of 100 and 10 msec the central respiratory periodicity was apparent in the PPST histogram. With 1.0 msec bins the PPST histogram showed a narrow central peak extending to +/- 3.0 msec at its base. This 'short-term synchronization' supports the hypothesis of joint firing due to common presynaptic connectivity. 7. It was shown that detection of short-term synchronization was critically dependent on a sufficient quantity of data but that provided a simple criterion of adequate counts per bin in the PPST histogram was met, short-term synchronization could be detected between intercostal motoneurones of the same and adjacent segments.     

Separation of two voltage-sensitive potassium currents, and demonstration of a tetrodotoxin-resistant calcium current in frog motoneurones.

1. Depolarization-induced voltage and conductance changes were studied in frog montoneurones in isolated, perfused spinal cord slices. Two types of afterhyperpolarization are observed following action potentials in normal Ringer, a fast afterhyperpolarization lastin 5-10 msec and a slow afterhyperpolarization lasting 60-200 msec. Both afterhyperpolarizations are mediated by an increased K+ conductance. 2. The slow afterhyperpolarization and conductance increase underlying it are selectively and reversibly inhibited by perfusion with solutions containing low [Ca2+] (less than or equal to 0-2 nM) or the Ca2+ antagonists Mn2+ (1mM) or Co2+ (5 mM), and are enhanced by perfusion with high [Ca2+]. 3. Addition of 2-5 mM tetraethylammonium ion (TEA+) to the perfusing solution prolongs the falling phase of the action potential and abolishes the fast afterhyperpolarization, but does not inhibit the slow afterhyperolarization. 4. When the voltage-dependent Na+ current is blocked by perfusion with TTX (10-5 M), intracellularly applied depolarizing current steps evoke fast and slow hyperpolarizations with kinetics and pharmacological sensitivities similar to those of the fast and slow afterhyperpolarizations, respectively. The fast hyperpolarization is maximally activated by brief, intense depolarizations, the slow hyperpolarization by prolonged, less intense depolarizations. 5. These pharmacological and kinetic data demonstrate that in frog motoneurones the repolarization-fast afterhyperpolarization sequence and the slow afterhyperpolarization are produced by different K+ conductance systems. The fast K+ conductance activates rapidly on depolarization, decays rapidly on repolarization, and is TEA+ sensitive, while the slow K+ conducatance activates and decays more slowly and is Ca2+-dependent. 6. Motoneurones perfused with TEA+ and TEA often show a slow, regenerative depolarizing response to applied depolarizing currents. These regenerative depolarizations are probably produced by an influx of Ca2+, because they persist in isotonic CaCl2 and are blocked by Mn2+ or low [Ca2+]. The Ca2+-dependence of the slow afterhyperpolarization and the increase in slow afterhyperpolarization magnitude observed following the slow Ca2+ potentials suggest that a depolarization-evoked Ca2+ influx activates the K+ conductance underlying the slow afterhyperpolarization. 7. Motoneurones in which the slow Ca2+ and K+ conductance systems have been enhanced by high [Ca2+] or blocked by Mn2+ show altered discharge patterns in response to intracellularly applied depolarizing current steps. Perfusion with twice normal [Ca2+] (4 mM) causes montoneurones to discharge more slowly at all current intensities, and reduces the slope of the 'steady-state' frequency-current relationship. Mn2+-perfused motoneurones exhibit fairly normal high-frequency discharge at the onset of the current step, but unlike normal motoneurones, do not discharge at frequencies below 60/sec...     

Afterhyperpolarization time-course and minimal discharge rate in low threshold motor units in humans

The alpha-motoneurone afterhyperpolarization (AHP) duration correlates with a number of its muscle unit properties in animal preparations. In humans, the interval death rate (IDR) analysis has been used to estimate the time course of human motoneurone AHP based on the pattern of motor unit firing. The purpose of this experiment was first, to examine the relationship between estimated AHP time course and the minimal firing rate of the motor unit and second, to examine the relationship between the AHP and motor unit contractile properties in the tibialis anterior (TA) muscle. Motor unit data were obtained from the TA muscle during low force isometric contractions lasting 600 s. Muscle unit twitch characteristics were determined using spike-triggered averaging (STA) and the motoneurone AHP time course was estimated using the IDR analysis. Minimal discharge rate and derecruitment threshold torque were determined for 2 s preceding motor unit derecruitment. The AHP time constant and minimal discharge rate were negatively correlated, whereas the derecruitment threshold torque was not associated with the AHP time constant. The estimated AHP duration, however, is considerably shorter than the mean ISI of the minimal discharge rate suggesting that synaptic noise and AHP duration are important factors in dictating the minimal discharge rate in low force voluntary contractions in humans. The AHP time constant did not vary significantly with motor unit twitch amplitude; however, significant positive relationships were found between the AHP time constant and the temporal properties of the motor unit twitch. The calculated AHP time course using the IDR analysis, therefore, is a reasonable estimate and coupled with motor unit properties attained with STA, it provides a powerful method to describe low-threshold motor units.     

Differential sensitivity of spinal neurones to amino acids: an intracellular study on the frog spinal cord

Intracellular recordings from in vitro neurones of the frog spinal cord slice preparation were performed in order to examine the mechanism of action of gamma-aminobutyrate and glutamate on two distinct neuronal populations in the same region of the central nervous system. Amino acids were superfused at fast rate and low temperature (7 degrees C) to reduce their uptake process. On interneurones, the inhibitory action of gamma-aminobutyrate was characterized by a large input conductance increase while on motoneurones the conductance change was much smaller. Glutamate excited interneurones which greatly increased their input conductance and showed burst firing; motoneurones were also excited by glutamate but usually did not fire repeatedly nor showed large conductance changes. In spite of these differences the amplitude of depolarization in the presence of the same concentration of glutamate was similar for motoneurones and interneurones. It is suggested that amino acids (particularly glutamate) may act through different membrane mechanisms on two neuronal populations in the same region of the spinal cord.     

Motoneurone properties and motor fatigue

Summary Gastrocnemius motoneurones with different types of muscle unit were compared with respect to their repetitive discharges during 4 min periods of steady intracellular stimulation. The cells were activated by a constant injected current of 5 nA above threshold. Among neurones capable of discharging for 10 s or more, the discharge duration showed no significant correlation to the contraction time or amplitude of the muscle unit twitch. Neither was there any obvious correlation between discharge duration and the sensitivity to contractile fatigue. The slow drop in discharge rate, as measured from the 2nd to the 26th s of firing, was more pronounced for fast-twitch units than for the ones with a slower twitch. Among fast-twitch neurones with about the same initial discharge rate, no difference in the extent of slow frequency drop was found between cells with fatigue-resistant and fatigue-sensitive muscle units. For fast-twitch neurones, measurements and calculations showed that, if the effects of peripheral potentiation and fatigue were disregarded, the drop in firing rate was great enough to cause a decrease in force by more than 60% during the first minute of constant stimulation. Among the fast-twitch units studied, the mean recorded fall in contractile force was initially less than expected (potentiation dominating) and it had become about equal to the expected one at 1 min after the onset of the discharge. It is concluded that, particularly with respect to fast-twitch motoneurones, the late adaptation is likely to be a significant factor for the development of central fatigue in voluntary or reflex contractions. Thanks to their small amount of late adaptation, slow-twitch motoneurones are par ticularly suitable for producing a steady postural contraction.Supported by NIHNS 11574 during this work     

Time gain influences adaptive visual-motor isometric force control

This study examined the influence of time gain on the visual-motor control of isometric force. Time gain denotes the spatial length on the computer screen representing the unit of elapsed time of the force output, through which the time properties of the visually perceived force output can be compressed or extended. Five time gains and three force target waveforms (sinewave, brown noise, and straight line) with different time-dependent properties were tested in the experiment. The results revealed that time gain influenced task performance nonlinearly in a way that was dependent on the predictability of the target waveforms. In the sinewave target condition, there was a U-shaped modulation of time gain on the mean and variability of force error, and an inverted U-shaped modulation on the time-dependent structure of force variability. The time gain modulation effect was weaker in the brown noise target condition and absent in the constant force target condition. The results extend the effect of visual information gain regulation from force gain to time gain. The interaction between the time gain and target waveform supports the general proposition that the control of motor output is influenced by the interaction of different categories of constraints where the influence of visual information is dependent on the temporal properties and predictability of the force output and the task requirement.     

Classical conditioning reduces amplitude and duration of calcium-dependent afterhyperpolarization in rabbit hippocampal pyramidal cells

1. The afterhyperpolarization (AHP) that follows action potentials was studied in CA1 hippocampal pyramidal cells from classically conditioned and control rabbits. Measurements of the AHP were obtained with intracellular recordings from CA1 cells within hippocampal slices. 2. The AHP of rabbit CA1 pyramidal cells was found to be accompanied by a conductance increase. The AHP was reduced by bath applications of the calcium channel blockers, cadmium and cobalt, by bath application of the cholinergic agonist, carbamylcholine chloride, and intracellular injection of the calcium chelator, ethylene glycol-bis(B-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). 3. The AHP was markedly reduced in cells from rabbits that were well-trained with the nictitating membrane conditioning procedure, as compared with cells from pseudoconditioned or naive control animals. The difference in AHP amplitudes between conditioned and control groups increased as the number of spikes elicited by the stimulation pulse increased from one to four. Both the duration (measured as the time constant of AHP decay) and amplitude of the AHP were reduced in cells from conditioned animals. 4. The reduced AHP in cells from conditioned animals remained reduced in a medium that contained 0.5 microM tetrodotoxin (TTX) and 5.0 mM tetraethylammonium chloride (TEA); the AHP following calcium spikes was measured under these conditions. Since this medium eliminated synaptic transmission elicited by Schaeffer collateral stimulation, the AHP reduction in pyramidal cells from conditioned animals was not due to a modification in synaptic properties. There were no significant differences in the mean voltage thresholds, amplitudes, or durations of calcium spikes between cells from animals in the three groups. Thus the AHP reduction appears to be due to a modification of a Ca2+ -dependent K+ conductance and was not due to a secondary effect of reductions in calcium conductances underlying the spike. 5. In medium containing TTX and TEA, the amount of injected current required to elicit a calcium spike (current threshold) was significantly greater in cells from conditioned animals than in cells from control animals. This increase in current threshold persisted in 4-aminopyridine (4-AP)-containing medium and so cannot be attributed entirely to conditioning-specific increases in the A-current. 6. The conditioning-specific AHP reduction resulted in increased excitability in cells from conditioned animals versus pseudoconditioned control animals. Cells from conditioned animals fired more spikes to trains of 100-ms depolarizing current pulses than did cells from controls.     

Increased persistent Na+ current and its effect on excitability in motoneurones cultured from mutant SOD1 mice

Mutations in the enzyme superoxide dismutase 1 (SOD1) initiate a progressive motoneurone degeneration in amyotrophic lateral sclerosis (ALS). Transgenic mice overexpressing this mutation develop a similar progressive motoneurone degeneration. In spinal motoneurones cultured from presymptomatic mice expressing the glycine to alanine mutation at base pair 93 (G93A) SOD1 mutation, a marked increase in the persistent component of the Na⁺ current was observed, without changes in passive properties. This increase only enhanced neuronal excitability in high input conductance cells, as low input conductance cells exhibited a compensatory outward shift in the current remaining after Na⁺ blockade. High input conductance motoneurones tend to be large, so these results may explain the tendency of large motoneurones to degenerate first in ALS. Riluzole, at the therapeutic concentration used to treat ALS, decreased neuronal excitability and persistent Na⁺ current in G93A motoneurones to levels observed in the control motoneurones. Aberrations in the intrinsic electrical properties may be among the first symptoms to emerge in SOD1-linked ALS.     

Influence of afferent synaptic innervation on the discharge variability of cat abducens motoneurones

The discharge variability of abducens motoneurones was studied after blocking inhibitory synaptic inputs or both excitatory and inhibitory inputs by means of an intramuscular (lateral rectus) injection of either a low (0.5 ng kg⁻¹) or a high dose (5 ng kg⁻¹) of tetanus neurotoxin (TeNT), respectively. Motoneuronal firing increased after low-dose TeNT. High-dose treatment, however, produced a firing depression, and in some cells, a total lack of modulation in relation to eye movements. Firing became increasingly more regular with larger TeNT doses as shown by significant reductions in the coefficient of variation after low- and high-dose treatments. Similarly, autocorrelation histograms of interspike intervals increased the number of resolvable peaks twofold in low-dose-treated motoneurones and sevenfold in high-dose-treated motoneurones. The plots of standard deviation versus the mean instantaneous firing frequency showed an upward deflexion with low firing frequencies. The upward deflexion occurred in controls at 39.9 ± 4.9 ms, an interval similar to the mean afterhyperpolarisation (AHP) duration (48.4 ± 8.8 ms). Low-dose TeNT treatment shifted the deflexion point to 20.9 ± 3.9 ms, whereas the high dose increased it to 60.7 ± 6.1 ms, in spite of the fact that no differences in AHP parameters between groups were found. The density of synaptophysin-immunoreactive boutons decreased by 14 % after the low-dose treatment and 40.5 % after the high-dose treatment, indicating that protracted synaptic blockade produces elimination of synaptic boutons. It is concluded that abducens motoneurone spike variability during spontaneous ocular fixations depends largely on the balance between inhibitory and excitatory synaptic innervation.     

Accumulation of cytoplasmic calcium, but not apamin-sensitive afterhyperpolarization current, during high frequency firing in rat subthalamic nucleus cells

The autonomous firing pattern of neurons in the rat subthalamic nucleus (STN) is shaped by action potential afterhyperpolarization currents. One of these is an apamin-sensitive calcium-dependent potassium current (SK). The duration of SK current is usually considered to be limited by the clearance of calcium from the vicinity of the channel. When the cell is driven to fire faster, calcium is expected to accumulate, and this is expected to result in accumulation of calcium-dependent AHP current. We measured the time course of calcium transients in the soma and proximal dendrites of STN neurons during spontaneous firing and their accumulation during driven firing. We compared these to the time course and accumulation of AHP currents using whole-cell and perforated patch recordings. During spontaneous firing, a rise in free cytoplasmic calcium was seen after each action potential, and decayed with a time constant of about 200 ms in the soma, and 80 ms in the dendrites. At rates higher than 10 Hz, calcium transients accumulated as predicted. In addition, there was a slow calcium transient not predicted by summation of action potentials that became more pronounced at high firing frequency. Spike AHP currents were measured in voltage clamp as tail currents after 2 ms voltage pulses that triggered action currents. Apamin-sensitive AHP (SK) current was measured by subtraction of tail currents obtained before and after treatment with apamin. SK current peaked between 10 and 15 ms after an action potential, had a decay time constant of about 30 ms, and showed no accumulation. At frequencies between 5 and 200 spikes s(-1), the maximal SK current remained the same as that evoked by a single action potential. AHP current did not have time to decay between action potentials, so at frequencies above 50 spikes s(-1) the apamin-sensitive current was effectively constant. These results are inconsistent with the view that the decay of SK current is governed by calcium dynamics. They suggest that the calcium is present at the SK channel for a very short time after each action potential, and the current decays at a rate set by the deactivation kinetics of the SK channel. At high rates, repetitive firing was governed by a fast apamin-insensitive AHP current that did not accumulate, but rather showed depression with increases in activation frequency. A slowly accumulating AHP current, also insensitive to apamin, was extremely small at low rates but became significant with higher firing rates.