Jørn Hounsgaard

Bistability of alpha‐motoneurones in the decerebrate cat and in the acute spinal cat after intravenous 5‐hydroxytryptophan.

1. In the preceding paper (Crone, Hultborn, Kiehn, Mazieres & Wigström, 1988) it was shown that a short‐lasting synaptic excitation (‘on’ stimulus) of extensor motoneurones (primarily triceps surae) in the decerebrate cat often resulted in a maintained excitability increase, which could be reset by a short‐lasting inhibitory stimulus train (‘off’ stimulus). In the present experiments intracellular recording from triceps surae motoneurones and the electroneurogram (ENG activity) from triceps surae nerve branches were performed in parallel. 2. Sustained firing of individual triceps surae motoneurones was most often recorded in parallel with the maintained ENG activity following a synaptic ‘on’ stimulus. When the motoneurone was silenced, by a hyperpolarizing current through the microelectrode, there was no sign of on‐going synaptic excitation during the maintained ENG activity following an ‘on’ stimulus. It was therefore suggested that voltage‐dependent intrinsic properties of the motoneurones themselves could be responsible for the maintained firing. 3. In confirmation of this hypothesis it was found that short‐lasting depolarizing current pulses through the recording microelectrode could trigger a self‐sustained firing in the motoneurone provided that the bias current (i.e. the holding potential) was kept within certain limits. Hyperpolarizing current pulses terminated the firing. When the spike‐generating mechanism was inactivated (by long‐lasting excessive depolarization) similar depolarizing and hyperpolarizing current pulses could initiate and terminate plateau potentials in the motoneurones. By grading the depolarizing current pulses it was found that the plateau potentials were of all‐or‐none character, typically around 10 mV in amplitude. The two levels of excitability which can be triggered by short‐lasting excitation and inhibition of the motoneurones is referred to as ‘bistable’ behaviour of the motoneurones. 4. After an acute spinal transection, in the unanaesthetized cat, the bistable behaviour of the motoneurones disappeared. However, it reappears following intravenous injection of the serotonin precursor 5‐hydroxytryptophan (50‐120 mg/kg). 5. Individual triceps surae motor units were recorded by selective EMG electrodes during tonic stretch reflexes in the decerebrate preparations. Based on an analysis of their firing pattern during lengthening and shortening (or vibration) of the muscle it is suggested that plateau potentials in motoneurones are recruited during the tonic stretch reflex. Furthermore, it is argued that a quantitatively important part of the depolarization of motoneurones during the tonic stretch reflex indeed originates from these plateau potentials.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonin‐induced bistability of turtle motoneurones caused by a nifedipine‐sensitive calcium plateau potential.

1. The effect of serotonin on the firing properties of motoneurones was studied in transverse sections of the adult turtle spinal cord in vitro with intracellular recording techniques. 2. In normal medium, turtle motoneurones adapt from an initial high frequency to a low steady firing during a depolarizing current pulse. In the presence of serotonin (4‐100 microM) motoneurones responded with accelerated firing and a frequency jump during a depolarizing current pulse followed by an after‐depolarization outlasting the stimulus. From a depolarized holding potential motoneuronal activity was shifted between two stable states by brief depolarizing and hyperpolarizing current pulses. As an expression of this bistable firing behaviour, the frequency‐current relation in response to a triangular current injection was counter‐clockwise in serotonin while clockwise in normal medium. 3. The delay to onset of the frequency jump was shortened as the amplitude of the activation pulse was increased. From a positive holding potential the after‐depolarization exceeded spike threshold and its duration increased with an increase in steady bias current. The effect of serotonin on turtle motoneurones could be blocked by methysergide (10 microM). 4. When action potentials were depressed by tetrodotoxin, a voltage‐dependent, non‐inactivating plateau potential, intrinsic to the motoneurone, was revealed. Activation of this voltage plateau provides the motoneurones with two stable states of firing. The apparent input resistance was 2‐4‐fold lower during the plateau than at rest. 5. The serotonin‐induced plateau potential was Ca2+‐dependent and was blocked when Ca2+ was replaced by either Co2+ (3 mM) or Mn2+ (3 mM). 6. The Ca2+ plateau was blocked by nifedipine (1‐15 microM). 7. Serotonin reduced the slow after‐hyperpolarization following action potentials. The change in balance between inward and outward currents seems to be sufficient to reveal the plateau response. 8. Although a small plateau response was induced by Bay K 8644 (1‐15 microM), this L‐channel agonist could not reproduce the pronounced effect of serotonin. 9. It is concluded that serotonin induces a Ca2+‐dependent and nifedipine‐sensitive plateau potential in turtle motoneurones primarily by reducing a K+‐current responsible for the slow after‐hyperpolarization.

Presynaptic inhibitory action of acetylcholine in area CA1 of the hippocampus.

The effect of iontophoretically applied acetylcholin (ACh) was investigated in area CA1 of transverse hippocampal slices maintained in vitro. In synaptically activated regions of the dendritic field, ACh reduced the amplitude of the population spike recorded from the pyramidal layer. In dendritic regions which were not synaptically activated, ACh increased the amplitude of the population spike or it had no effect. The depressing effect of ACh was abolished in denervated dendritic regions. The intracellularly recorded excitatory postsynaptic potential (EPSP) decreased in amplitude when ACh was applied at the synaptic site. The resting membrane potential, the time course of the EPSP, and the membrane resistance were unaffected. ACh increased the excitability of afferent fibers and this was independent of synaptic transmission. We conclude that ACh in addition to its postsynaptic effects has a presynaptic site of action.

Influence of Phasic and Tonic Dopamine Release on Receptor Activation

Tonic and phasic dopamine release is implicated in learning, motivation, and motor functions. However, the relationship between spike patterns in dopaminergic neurons, the extracellular concentration of dopamine, and activation of dopamine receptors remains unresolved. In the present study, we develop a computational model of dopamine signaling that give insight into the relationship between the dynamics of release and occupancy of D1 and D2 receptors. The model is derived from first principles using experimental data. It has no free parameters and offers unbiased estimation of the boundaries of dopaminergic volume transmission. Bursts primarily increase occupancy of D1 receptors, whereas pauses translate into low occupancy of D1 and D2 receptors. Phasic firing patterns, composed of bursts and pauses, reduce the average D2 receptor occupancy and increase average D1 receptor occupancy compared with equivalent tonic firing. Receptor occupancy is crucially dependent on synchrony and the balance between tonic and phasic firing modes. Our results provide quantitative insight in the dynamics of volume transmission and complement experimental data obtained with electrophysiology, positron emission tomography, microdialysis, amperometry, and voltammetry.

Intrinsic membrane properties causing a bistable behaviour of α-motoneurones

SummaryIn decerebrate cats a train of impulses in Ia afferents may lead to a sustained increase in excitability of α-motoneurones of homonymous and heteronymous muscles. It was previously suggested that this long-lasting excitability increase reflects a maintained synaptic input to the motoneurones from excitatory interneurones. With intracellular recording from motoneurones we here demonstrate that the sustained increase of α-motoneurone activity is due to an all-or-none plateau depolarization. This plateau can be induced by a short train of excitatory synaptic potentials or a brief, intracellularly injected depolarizing current pulse and is terminated by a short train of inhibitory synaptic potentials or a hyperpolarizing current pulse. It is concluded that maintained motor unit firing triggered by a brief train of impulses in Ia afferent reflects an intrinsic bistable behaviour of α-motoneurones.

Ca++ dependent bistability induced by serotonin in spinal motoneurons

SummaryThe plateau potential, responsible for the bistable state of spinal motoneurons, recently described in the decerebrate cat, was suggested to depend on serotonin (Hounsgaard et al. 1984). In an in vitro preparation of the spinal cord of the turtle we now show that serotonin, applied directly to the bath, transforms the intrinsic response properties of motoneurons, uncovering a plateau potential and voltage sensitive bistability. The changes induced by serotonin were blocked by Mn++, while the plateau potential and the bistability remained after application of tetrodotoxin. We conclude that serotonin controls the expression of a Ca++ dependent plateau potential in motoneurons.

Calcium spikes and calcium plateaux evoked by differential polarization in dendrites of turtle motoneurones in vitro.

1. The ability of dendrites in turtle motoneurones to support calcium spikes and calcium plateaux was investigated using differential polarization by applied electric fields. 2. Electric fields were generated by passing current through transverse slices of the turtle spinal cord between two plate electrodes. The linear extracellular voltage gradient generated by the field implied that the tissue was ohmic and homogeneous. 3. The transmembrane potential at the cell body of motoneurones was measured as the voltage difference between an intracellular and an extracellular microelectrode. 4. In normal medium an applied field induced synaptic activity as well as intrinsic polarization of motoneurones. Synaptic activity was suppressed by tetrodotoxin (TTX, 1 microM). 5. In the presence of TTX and tetraethylammonium (TEA, 1‐5 mM), applied fields evoked multicomponent Ca2+ spikes in both the soma‐hyperpolarizing and soma‐depolarizing direction of the field. The different components of Ca2+ spikes were discrete and additive. High amplitude components had higher threshold and faster time course and were followed by larger after‐hyperpolarizations, than low amplitude components. The frequency of field‐evoked regenerative responses was relatively insensitive to somatic bias current. 6. TTX‐resistant Ca(2+)‐mediated plateau potentials promoted by apamin were evoked by differential polarization in both the soma‐depolarizing and soma‐hyperpolarizing direction. 7. It is concluded that Ca2+ channels responsible for Ca2+ spikes and Ca2+ plateaux are present in dendrites of spinal motoneurones of the turtle.

Response properties of motoneurones in a slice preparation of the turtle spinal cord.

1. Motoneurones in transverse sections of the turtle spinal cord were investigated in vitro with intracellular recording techniques. 2. Turtle motoneurones had a resting membrane potential of ‐60 to ‐80 mV, spike height of 90‐110 mV and were able to maintain rhythmic firing during depolarization. In agreement with the size variation of the cells the input resistance and time constant ranged from 18 M omega and 12 ms to 55 M omega and 61 ms. 3. The active response properties of motoneurones included time‐dependent inward rectification in response to hyperpolarizing current pulses. The action potential had an initial segment (IS) and a soma‐dendritic (SD) component and was followed by a fast and a slow after‐hyperpolarization (AHP) with different sensitivity to membrane potential. 4. The relation between firing rate and injected current was sigmoid when determined for the first few interspike intervals during depolarizing current pulses. Adaptation was biphasic with an early phase lasting 0.5‐1 s and a late phase lasting 10‐20 s. 5. The ionic conductances responsible for the active membrane properties included a tetrodotoxin (TTX)‐sensitive Na+ conductance generating the action potential and a Ca2+ conductance transiently activated during the action potential. A tetraethylamonium (TEA)‐sensitive K+ conductance was responsible for spike repolarization and the fast AHP. A Ca2+‐dependent K+ conductance, blocked by Mn2+ and apamin, accounted for only part of the slow AHP. The time‐dependent inward rectification was selectively blocked by extracellular Cs+ at concentrations below 1 mM. 6. The results show that the response properties of spinal motoneurones of the turtle are closely similar to those known from mammals in vivo. The experiments confirm and extend the identification of the ionic conductances underlying the active response properties of spinal motoneurones.

Calcium conductance and firing properties of spinal motoneurones in the turtle.

1. The contribution of Ca2+ conductance to the firing properties of motoneurones was investigated in transverse slices of the turtle spinal cord. 2. In the presence of tetrodotoxin (TTX), tetraethylamonium (TEA) in low extracellular concentration (less than 5 mM) promoted Ca2+ spikes. In higher concentrations of TEA, a suprathreshold depolarizing current pulse was followed by an after‐discharge of Ca2+ spikes riding on a Ca2+ plateau potential. 3. The Ca2+‐dependent plateau was also promoted by Cs+, 4‐aminopyridine (4‐AP) and apamin. However, Ca2+ spikes during plateaux were an order of magnitude faster when promoted by Cs+ or 4‐AP rather than TEA, and apamin did not promote Ca2+ spikes at all. 4. Ca2+ plateaux but not Ca2+ spikes were blocked by nifedipine. 5. In normal medium all effects of the transient Ca2+ influx during action potentials were attributable to its influence on the slow after‐hyperpolarization. The nifedipine‐sensitive, sustained Ca2+ influx was expressed exclusively as plateau potentials and only under conditions of reduced K+ current. 6. It is concluded that the transient and the sustained Ca2+ fluxes in spinal motoneurones are curtailed by different K+ conductances. The two Ca2+ responses are suggested as being mediated by two different types of Ca2+ channels.

Plateau-generating neurones in the dorsal horn in an in vitro preparation of the turtle spinal cord.

1. In transverse slices of the spinal cord of the turtle, intracellular recordings were used to characterize and analyse the responses to injected current and activation of primary afferents in dorsal horn neurones. 2. A subpopulation of neurones, with cell bodies located laterally in the deep dorsal horn and dendrites radiating towards the pial surface, was distinguished by the ability to generate plateau potentials. Activation of the plateau potential by a suprathreshold depolarizing current pulse produced an increasing firing frequency during the first few seconds and a sustained after‐discharge. 3. The plateau potential was assumed to be mediated by L‐type Ca2+ channels since it was blocked by Co2+ (3 mM) and nifedipine (10 microM) and enhanced by Bay K 8644 (0.5‐2 microM). 4. The threshold for activating the plateau potential declined during the first few seconds of depolarization. The decline in threshold gradually subsided over 3‐10 s after repolarization. 5. Frequency potentiation of the plateau potential contributed to wind‐up of the response to depolarizing current pulses and primary afferent stimuli repeated at frequencies higher than 0.1‐0.3 Hz. 6. The sustained after‐discharge mediated by the plateau potential was curtailed by a slow after‐hyperpolarization (sAHP) evoked by strong depolarizations. The relative strength of the plateau potential and sAHP varied among cells. In some cells the plateau potential and sAHP interacted to produce damped oscillations upon depolarization. The sAHP was mediated by both apamin and tetraethylammonium (TEA)‐sensitive K+ channels. 7. Our findings suggest that basic properties of sensory integration may reside with the specialized intrinsic response properties of particular subtypes of neurones in the dorsal horn.