Contribution of potassium conductances to a time-dependent transition in electrical properties of a cockroach motoneuron soma

Bursting reverberation as a multiscale neuronal network process driven by synaptic depression-facilitation

Neuronal networks can generate complex patterns of activity that depend on membrane properties of individual neurons as well as on functional synapses. To decipher the impact of synaptic properties and connectivity on neuronal network behavior, we investigate the responses of neuronal ensembles from small (5-30 cells in a restricted sphere) and large (acute hippocampal slice) networks to single electrical stimulation: in both cases, a single stimulus generated a synchronous long-lasting bursting activity. While an initial spike triggered a reverberating network activity that lasted 2-5 seconds for small networks, we found here that it lasted only up to 300 milliseconds in slices. To explain this phenomena present at different scales, we generalize the depression-facilitation model and extracted the network time constants. The model predicts that the reverberation time has a bell shaped relation with the synaptic density, revealing that the bursting time cannot exceed a maximum value. Furthermore, before reaching its maximum, the reverberation time increases sub-linearly with the synaptic density of the network. We conclude that synaptic dynamics and connectivity shape the mean burst duration, a property present at various scales of the networks. Thus bursting reverberation is a property of sufficiently connected neural networks, and can be generated by collective depression and facilitation of underlying functional synapses.

Paraoxon suppresses Ca2+ spike and afterhyperpolarization in snail neurons: Relevance to the hyperexcitability induction

The effects of organophosphate (OP) paraoxon, active metabolite of parathion, were studied on the Ca(2+) and Ba(2+) spikes and on the excitability of the neuronal soma membranes of land snail (Caucasotachea atrolabiata). Paraoxon (0.3 muM) reversibly decreased the duration and amplitude of Ca(2+) and Ba(2+) spikes. It also reduced the duration and the amplitude of the afterhyperpolarization (AHP) that follows spikes, leading to a significant increase in the frequency of Ca(2+) spikes. Pretreatment with atropine and hexamethonium, selective blockers of muscarinic and nicotinic receptors, respectively, did not prevent the effects of paraoxon on Ca(2+) spikes. Intracellular injection of the calcium chelator BAPTA dramatically decreased the duration and amplitude of AHP and increased the duration and frequency of Ca(2+) spikes. In the presence of BAPTA, paraoxon decreased the duration of the Ca(2+) spikes without affecting their frequency. Apamin, a neurotoxin from bee venom, known to selectively block small conductance of calcium-activated potassium channels (SK), significantly decreased the duration and amplitude of the AHP, an effect that was associated with an increase in spike frequency. In the presence of apamin, bath application of paraoxon reduced the duration of Ca(2+) spike and AHP and increased the firing frequency of nerve cells. In summary, these data suggest that exposure to submicromolar concentration of paraoxon may directly affect membrane excitability. Suppression of Ca(2+) entry during the action potential would down regulate Ca(2+)-activated K(+) channels leading to a reduction of the AHP and an increase in cell firing.

Ion currents and spiking properties of identified subtypes of locust octopaminergic dorsal unpaired median neurons

Efferent dorsal unpaired median (DUM) neurons are key elements of an insect neuromodulatory system. In locusts, subpopulations of DUM neurons mediate octopaminergic modulation at specific targets depending on their activity during different behaviours. This study investigates whether in addition to synaptic inputs, activity in DUM neurons depends on intrinsic membrane properties. Intracellular in situ recordings and whole-cell patch-clamp recordings from freshly isolated somata characterize somatic voltage signals and the underlying ion currents of individual subtypes of DUM neurons identified beforehand by a vital retrograde tracing technique. Na(+), Ca(2+), K(+) currents and a hyperpolarization-activated (I(h)) current are described in detail for their (in-)activation properties and subtype-specific current densities. In addition, a Ca(2+)-dependent K(+) current is demonstrated by its sensitivity to cadmium and charybdotoxin. This complex current composition determines somatic excitability similar in all subtypes of DUM neurons. Both Na(+) and Ca(2+) currents generate overshooting somatic action potentials. Repolarizing K(+) currents, in particular transient, subthreshold-activating A-currents, regulate the firing frequency and cause delayed excitation by shunting depolarizing input. An opposing hyperpolarization-activated (I(h)) current contributes to the resting membrane potential and induces rebound activity after prolonged inhibition phases. A quantitative analysis reveals subtype-specific differences in current densities with more inhibitory I(K) but less depolarizing I(Na) and I(h) - at least in DUM3 neurons promoting a reliable suppression of their activity as observed during behaviour. In contrast, DUM neurons that are easily activated during behaviour (DUM3,4,5 and DUMETi) express less I(K) and a pronounced depolarizing I(h) promoting excitability.

Modulation of Membrane Potential in Mesothoracic Moto- and Interneurons During Stick Insect Front-Leg Walking

During walking, maintenance and coordination of activity in leg motoneurons requires intersegmental signal transfer. In a semi-intact preparation of the stick insect, we studied membrane potential modulations in mesothoracic (middle leg) motoneurons and local premotor nonspiking interneurons that were induced by stepping of a front leg on a treadmill. The activity in motoneurons ipsi- and contralateral to the stepping front leg was recorded from neuropilar processes. Motoneurons usually exhibited a tonic depolarization of ≤5 mV throughout stepping sequences. This tonic depolarization depended on membrane potential and was found to reverse in the range of −32 to −47 mV. It was accompanied by a mean membrane resistance decrease of ∼12%. During front-leg stepping, an increased spike activity to depolarizing current pulses was observed in 73% of contralateral flexor motoneurons that were tested. Motoneurons ipsilateral to the walking front leg exhibited phasic membrane potential modulations coupled to steps in accordance with previously published results. Coupling patterns were typical for a given motoneuron pool. Local nonspiking mesothoracic interneurons that provide synaptic drive to tibial motoneurons also contribute to the modulation of membrane potential of tibial motoneurons during front-leg walking. We hypothesize that the tonic depolarization of motoneurons during walking is a cellular correlate of arousal that usually increases effectiveness of phasic excitation in supporting motoneuron firing.

Differential expression of voltage-sensitive K+ and Ca2+ currents in neurons of the honeybee olfactory pathway

In order to understand the neuronal processes underlying olfactory learning, biophysical properties such as ion channel activity need to be analysed within neurons of the olfactory pathway. This study analyses voltage-sensitive ionic currents of cultured antennal lobe projection neurons and mushroom body Kenyon cells in the brain of the honeybee Apis mellifera. Rhodamine-labelled neurons were identified in vitro prior to recording, and whole-cell K(+) and Ca(2+) currents were measured. All neurons expressed transient and sustained outward K(+) currents, but Kenyon cells expressed higher relative amounts of transient A-type K(+) (I(K,A)) currents than sustained delayed rectifier K(+) current (I(K,V)). The current density of the I(K,V) was significantly higher in projection neurons than in Kenyon cells. The voltage-dependency of K(+) currents at positive membrane potentials was linear in Kenyon cells, but N-shaped in projection neurons. Blocking of voltage-sensitive Ca(2+) currents transformed the N-shaped voltage-dependency into a linear one, indicating activation of calcium-dependent K(+) currents (I(K,Ca)). The densities of currents through voltage-sensitive Ca(2+) channels did not differ between the two neuron classes and the voltage-dependency of current activation was similar. Projection neurons thus express higher calcium-dependent K(+) currents. These analyses revealed that the various neurons of the honeybee olfactory pathway in vitro have different current phenotypes, which may reflect functional differences between the neuron types in vivo.

Anoxia induces thermotolerance in the locust flight system

Heat shock and anoxia are environmental stresses that are known to trigger similar cellular responses. In this study, we used the locust to examine stress cross-tolerance by investigating the consequences of a prior anoxic stress on the effects of a subsequent high-temperature stress. Anoxic stress and heat shock induced thermotolerance by increasing the ability of intact locusts to survive normally lethal temperatures. To determine whether induced thermotolerance observed in the intact animal was correlated with electrophysiological changes, we measured whole-cell K+ currents and action potentials from locust neurons. K+ currents recorded from thoracic neuron somata were reduced after anoxic stress and decreased with increases in temperature. Prior anoxic stress and heat shock increased the upper temperature limit for generation of an action potential during a subsequent heat stress. Although anoxia induced thermotolerance in the locust flight system, a prior heat shock did not protect locusts from a subsequent anoxic stress. To determine whether changes in bioenergetic status were implicated in whole-animal cross-tolerance, phosphagen levels and rates of mitochondrial respiration were assayed. Heat shock alone had no effect on bioenergetic status. Prior heat shock allowed rapid recovery after normally lethal heat stress but afforded no protection after a subsequent anoxic stress. Heat shock also afforded no protection against disruption of bioenergetic status after a subsequent exercise stress. These metabolite studies are consistent with the electrophysiological data that demonstrate that a prior exposure to anoxia can have protective effects against high-temperature stress but that heat shock does not induce tolerance to anoxia.

Heat shock-induced thermoprotection of action potentials in the locust flight system

There is increasing evidence that heat shock (HS) has long-term effects on electrophysiological properties of neurons and synapses. Prior HS protects neural circuitry from a subsequent heat stress but little is known about the mechanisms that mediate this plasticity and induce thermotolerance. Exposure of Locusta migratoria to HS conditions of 45 degrees C for 3 h results in thermotolerance to hitherto lethal temperatures. Locust flight motor patterns were recorded during tethered flight at room temperature, before and after HS. In addition, intracellular action potentials (APs) were recorded from control and HS motoneurons in a semi-intact preparation during a heat stress. HS did not alter the timing of representative depressor or elevator muscle activity, nor did it affect the ability of the locust to generate a steering motor pattern in response to a stimulus. However, HS did increase the duration of APs recorded from neuropil segments of depressor motoneurons. Increases in AP duration were associated with protection of AP generation against failure at subsequent elevated temperatures. Failure of AP generation at high temperatures was preceded by a concomitant burst of APs and depolarization of the membrane. The protective effects of HS were mimicked by pharmacological blockade of I(K+) with tetraethylammonium (TEA). Taken together, these findings are consistent with a hypothesis that HS protects neuronal survival and function via K+ channel modulation.

Myokymia and neonatal epilepsy caused by a mutation in the voltage sensor of the KCNQ2 K+ channel

KCNQ2 and KCNQ3 are two homologous K(+) channel subunits that can combine to form heterotetrameric channels with properties of neuronal M channels. Loss-of-function mutations in either subunit can lead to benign familial neonatal convulsions (BFNC), a generalized, idiopathic epilepsy of the newborn. We now describe a syndrome in which BFNC is followed later in life by myokymia, involuntary contractions of skeletal muscles. All affected members of the myokymia/BFNC family carried a mutation (R207W) that neutralized a charged amino acid in the S4 voltage-sensor segment of KCNQ2. This substitution led to a shift of voltage-dependent activation of KCNQ2 and a dramatic slowing of activation upon depolarization. Myokymia is thought to result from hyperexcitability of the lower motoneuron, and indeed both KCNQ2 and KCNQ3 mRNAs were detected in the anterior horn of the spinal cord where the cells of the lower motoneurons arise. We propose that a difference in firing patterns between motoneurons and central neurons, combined with the drastically slowed voltage activation of the R207W mutant, explains why this particular KCNQ2 mutant causes myokymia in addition to BFNC.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

Pattern generation for walking and searching movements of a stick insect leg. II. Control of motoneuronal activity

In the stick insect, Cuniculina impigra, intracellular recordings from mesothoracic motoneurons that control flexion and extension of the tibia and depression and levation of the trochantero-femur were made while the leg performed walking-like movements on a treadband or stereotyped rhythmic searching movements. We were interested in how synaptic input and intrinsic properties contribute to form the activity pattern of motoneurons during rhythmic leg movements without sensory feedback from other legs. During searching and walking, motoneurons expressed a rhythmic bursting pattern that was formed by a depolarizing input followed by a hyperpolarizing input in the inter-burst interval. This basic pattern was similar in all fast, semi-fast, and slow motoneurons that were recorded. Hyperpolarizations were in synchrony with activity in the antagonistic motoneurons. De- and hyperpolarizations were associated with a decrease in input resistance. All motoneurons showed spike frequency adaptation when depolarized by current injection to a membrane potential similar to that observed during walking. In the hyperpolarizing phase of fast flexor motoneurons, the initial maximum hyperpolarization was followed by a sag in potential toward more depolarized values. Consistent with this observation, only fast flexor motoneurons developed a depolarizing sag potential when hyperpolarized by injection of constant negative current.