Development and regulation of response properties in spinal cord motoneurons

Multistability of bursting rhythms in a half-center oscillator and the protective effects of synaptic inhibition

For animals to meet environmental challenges, the activity patterns of specialized oscillatory neural circuits, central pattern generators (CPGs), controlling rhythmic movements like breathing and locomotion, are adjusted by neuromodulation. As a representative example, the leech heartbeat is controlled by a CPG driven by two pairs of mutually inhibitory interneurons, heart interneuron (HN) half-center oscillators (HCO). Experiments and modeling indicate that neuromodulation of HCO navigates this CPG between dysfunctional regimes by employing a co-regulating inverted relation; reducing Na+/K+ pump current and increasing hyperpolarization-activated (h-) current. Simply reducing pump activity or increasing h-current leads to either seizure-like bursting or an asymmetric bursting dysfunctional regime, respectively. Here, we demonstrate through modeling that, alongside this coregulation path, a new bursting regime emerges. Both regimes fulfill the criteria for functional bursting activity. Although the cycle periods and burst durations of these patterns are roughly the same, the new one exhibits an intra-burst spike frequency that is twice as high as the other. This finding suggests that neuromodulation could introduce additional functional regimes with higher spike frequency, and thus more effective synaptic transmission to motor neurons. We found that this new regime co-exists with the original bursting. The HCO can be switched between them by a short pulse of excitatory or inhibitory conductance. In this domain of coexisting functional patterns, an isolated cell model exhibits only one regime, a severely dysfunctional plateau-containing, seizure-like activity. This aligns with widely reported notion that deficiency of inhibition can cause seizures and other dysfunctional neural activities. We show that along the coregulation path of neuromodulation, the high excitability of the single HNs induced by myomodulin is harnessed by mutually inhibitory synaptic interactions of the HCO into the functional bursting pattern.

Changes in Sensorimotor Connectivity to dI3 Interneurons in Relation to the Postnatal Maturation of Grasping

Primitive reflexes are evident shortly after birth. Many of these reflexes disappear during postnatal development as part of the maturation of motor control. This study investigates the changes of connectivity related to sensory integration by spinal dI3 interneurons during the time in which the palmar grasp reflex gradually disappears in postnatal mice pups. Our results reveal an increase in GAD65/67-labeled terminals to perisomatic Vglut1-labeled sensory inputs contacting cervical and lumbar dI3 interneurons between postnatal day 3 and day 25. In contrast, there were no changes in the number of perisomatic Vglut1-labeled sensory inputs to lumbar and cervical dI3 interneurons other than a decrease between postnatal day 15 and day 25. Changes in postsynaptic GAD65/67-labeled inputs to dI3 interneurons were inconsistent with a role in the sustained loss of the grasp reflex. These results suggest a possible link between the maturation of hand grasp during postnatal development and increased presynaptic inhibition of sensory inputs to dI3 interneurons.

Normal Development and Pathology of Motoneurons: Anatomy, Electrophysiological Properties, Firing Patterns and Circuit Connectivity

This chapter will provide an introduction into motoneuron anatomy, electrophysiological properties, firing patterns focusing on development and also describing several pathological conditions that affect mononeurons. It starts with a historical retrospective describing the early landmark work into motoneurons. The next section lays out the various types of motoneurons (alpha, beta, and gamma) and their subclasses (fast-twitch fatigable, fast-twitch fatigue-resistant, and slow-twitch fatigue resistant), highlighting the functional relevance of this classification scheme. The third section describes the development of motoneurons' passive and active electrophysiological properties. This section also defines the major terms one uses in describing how a neuron functions electrophysiologically. The electrophysiological aspects of a neuron is critical to understanding how it behaves within a circuit and contributes to behavior since the firing of an action potential is how neurons communicate with each other and with muscles. The electrophysiological changes of motoneurons over development underlies how their function changes over the lifetime of an organism. After describing the properties of individual motoneurons, the chapter then turns to revealing how motoneurons interact within complex neural circuits, with other motoneurons as well as sensory neurons, and how these circuits change over development. Finally, this chapter ends with highlighting some recent advances made in motoneuron pathology, focusing on spinal muscular atrophy, amyotrophic lateral sclerosis, and axotomy.

M-wave and H-reflex recruitment curves in boys and men

The aim of the present study was to check whether the M-wave and H-reflex recruitment curves differ between prepubertal boys and men. Eleven boys (9-11 yr) and eleven men (18-35 yr) were magnetically stimulated at the tibial nerve in a prone position. M-wave and H-reflex maximal amplitudes (H_max; M_max ; H_max /M_max ), thresholds, regression slopes (H_slp ; M_slp ; H_slp /M_slp ) were extracted from M-wave and H-reflex recruitment curves and compared between the two age groups. Overall, no significant difference in M-wave and H-reflex recruitment curve parameters was found between the two populations. Nevertheless, the size of the M-wave associated with maximal H-reflex amplitude was lower in boys as compared to men when expressed relative to maximal M-wave amplitude (M_Hmax /M_max : 0.18 ± 0.06 vs. 0.31 ± 0.13; p < .05). This result suggests that the development of peripheral nerve was completed in 9 to 11-year-old boys and did not affect the M-wave and H-reflex recruitment curves parameters. In neuromuscular function studies, it implies that H_max /M_max and H_slp /M_slp could be used indifferently to compare spinal motoneuron excitability between 9-11-year-old boys and men. Conversely, evoking H-reflexes at a given percentage of M_max may bias the comparison between boys and men.

Postnatal development of persistent inward currents in rat XII motoneurons and their modulation by serotonin, muscarine and noradrenaline

KEY POINTS

Persistent inward currents (PICs) in spinal motoneurons are long-lasting, voltage-dependent currents that increase excitability; they are dramatically potentiated by serotonin, muscarine, and noradrenaline (norepinephrine). Loss of these modulators (and the PIC) during sleep is hypothesized as a major contributor to REM sleep atonia. Reduced excitability of XII motoneurons that drive airway muscles and maintain airway patency is causally implicated in obstructive sleep apnoea (OSA), but whether XII motoneurons possess a modulator-sensitive PIC that could be a factor in the reduced airway tone of sleep is unknown. Whole-cell recordings from rat XII motoneurons in brain slices indicate that PIC amplitude increases ∼50% between 1 and 23 days of age, when potentiation of the PIC by 5HT₂ , muscarinic, or α₁ noradrenergic agonists peaks at <50%, manyfold lower than the potentiation observed in spinal motoneurons. α₁ noradrenergic receptor activation produced changes in XII motoneuron firing behaviour consistent with PIC involvement, but indicators of strong PIC activation were never observed; in vivo experiments are needed to determine the role of the modulator-sensitive PIC in sleep-dependent reductions in airway tone.

ABSTRACT

Hypoglossal (XII) motoneurons play a key role in maintaining airway patency; reductions in their excitability during sleep through inhibition and disfacilitation, i.e. loss of excitatory modulation, is implicated in obstructive sleep apnoea. In spinal motoneurons, 5HT₂ , muscarinic and α₁ noradrenergic modulatory systems potentiate persistent inward currents (PICs) severalfold, dramatically increasing excitability. If the PICs in XII and spinal motoneurons are equally sensitive to modulation, loss of the PIC secondary to reduced modulatory tone during sleep could contribute to airway atonia. Modulatory systems also change developmentally. We therefore characterized developmental changes in magnitude of the XII motoneuron PIC and its sensitivity to modulation by comparing, in neonatal (P1-4) and juvenile (P14-23) rat brainstem slices, the PIC elicited by slow voltage ramps in the absence and presence of agonists for 5HT₂ , muscarinic, and α₁ noradrenergic receptors. XII motoneuron PIC amplitude increased developmentally (from -195 ± 12 to -304 ± 19 pA). In neonatal XII motoneurons, the PIC was only potentiated by α₁ receptor activation (5 ± 4%). In contrast, all modulators potentiated the juvenile XII motoneurons PIC (5HT₂ , 5 ± 5%; muscarine, 22 ± 11%; α₁ , 18 ± 5%). These data suggest that the influence of the PIC and its modulation on XII motoneuron excitability will increase with postnatal development. Notably, the modulator-induced potentiation of the PIC in XII motoneurons was dramatically smaller than the 2- to 6-fold potentiation reported for spinal motoneurons. In vivo measurements are required to determine if the modulator-sensitive, XII motoneuron PIC is an important factor in sleep-state dependent reductions in airway tone.

Propensity for Bistability of Bursting and Silence in the Leech Heart Interneuron

The coexistence of neuronal activity regimes has been reported under normal and pathological conditions. Such multistability could enhance the flexibility of the nervous system and has many implications for motor control, memory, and decision making. Multistability is commonly promoted by neuromodulation targeting specific membrane ionic currents. Here, we investigated how modulation of different ionic currents could affect the neuronal propensity for bistability. We considered a leech heart interneuron model. It exhibits bistability of bursting and silence in a narrow range of the leak current parameters, conductance (g_leak) and reversal potential (E_leak). We assessed the propensity for bistability of the model by using bifurcation diagrams. On the diagram (g_leak, E_leak), we mapped bursting and silent regimes. For the canonical value of E_leak we determined the range of g_leak which supported the bistability. We use this range as an index of propensity for bistability. We investigated how this index was affected by alterations of ionic currents. We systematically changed their conductances, one at a time, and built corresponding bifurcation diagrams in parameter planes of the maximal conductance of a given current and the leak conductance. We found that conductance of only one current substantially affected the index of propensity; the increase of the maximal conductance of the hyperpolarization-activated cationic current increased the propensity index. The second conductance with the strongest effect was the conductance of the low-threshold fast Ca²⁺ current; its reduction increased the propensity index although the effect was about two times smaller in magnitude. Analyzing the model with both changes applied simultaneously, we found that the diagram (g_leak, E_leak) showed a progressively expanded area of bistability of bursting and silence.

Motor Neurons

Motor neurons translate synaptic input from widely distributed premotor networks into patterns of action potentials that orchestrate motor unit force and motor behavior. Intercalated between the CNS and muscles, motor neurons add to and adjust the final motor command. The identity and functional properties of this facility in the path from synaptic sites to the motor axon is reviewed with emphasis on voltage sensitive ion channels and regulatory metabotropic transmitter pathways. The catalog of the intrinsic response properties, their underlying mechanisms, and regulation obtained from motoneurons in in vitro preparations is far from complete. Nevertheless, a foundation has been provided for pursuing functional significance of intrinsic response properties in motoneurons in vivo during motor behavior at levels from molecules to systems. © 2017 American Physiological Society. Compr Physiol 7:463-484, 2017.

Modulation of Rhythmic Activity in Mammalian Spinal Networks Is Dependent on Excitability State

Neuromodulators play an important role in activating rhythmically active motor networks; however, what remains unclear are the network interactions whereby neuromodulators recruit spinal motor networks to produce rhythmic activity. Evidence from invertebrate systems has demonstrated that the effect of neuromodulators depends on the pre-existing state of the network. We explored how network excitation state affects the ability of dopamine to evoke rhythmic locomotor activity in the neonatal mouse isolated spinal cord. We found that dopamine can evoke unique patterns of motor activity that are dependent on the excitability state of motor networks. Different patterns of motor activity ranging from tonic, nonrhythmic activity to multirhythmic, nonlocomotor activity to locomotor activity were produced by altering global motor network excitability through manipulations of the extracellular potassium and bath NMDA concentration. A similar effect was observed when network excitation was manipulated during an unstable multirhythm evoked by a low concentration (15 µm) of 5-HT, suggesting that our results are not neuromodulator specific. Our data show in vertebrate systems that modulation is a two-way street and that modulatory actions are largely influenced by the network state. The level of network excitation can account for variability between preparations and is an additional factor to be considered when circuit elements are removed from the network.

Elevated intracellular Na+ concentrations in developing spinal neurons

Over 25 years ago it was first reported that intracellular chloride levels (Cl^-_in ) were higher in developing neurons than in maturity. This finding has had significant implications for understanding the excitability of developing networks and recognizing the underlying causes of hyperexcitability associated with disease and neural injury. While there is some evidence that intracellular sodium levels (Na⁺_in ) change during the development of non-neural cells, it has largely been assumed that Na⁺_in is the same in developing and mature neurons. Here, using the sodium indicator SBFI, we test this idea and find that Na⁺_in is significantly higher in embryonic spinal motoneurons and interneurons than in maturity. We find that Na⁺_in reaches ~ 60 mM in mid-embryonic development and is then reduced to ~ 30 mM in late embryonic development. By retrogradely labeling motoneurons with SBFI we can reliably follow Na⁺_in levels in vitro for hours. Bursts of spiking activity, and blocking voltage-gated sodium channels did not influence observed motoneuron sodium levels. On the other hand, Na⁺_in was reduced by blocking the Na⁺ -K⁺ -2Cl^- cotransporter NKCC1, and was highly sensitive to changes in external Na⁺ and a blocker of the Na⁺ /K⁺ ATPase. Our findings suggest that the Na⁺ gradient is weaker in embryonic neuronal development and strengthens in maturity in a manner similar to that of Cl^- .

The influence of increased membrane conductance on response properties of spinal motoneurons

During functional spinal neural network activity motoneurons receive massive synaptic excitation and inhibition, and their membrane conductance increases considerably - they are switched to a high-conductance state. High-conductance states can substantially alter response properties of motoneurons. In the present study we investigated how an increase in membrane conductance affects spike frequency adaptation, the gain (i.e., the slope of the frequency-current relationship) and the threshold for action potential generation. We used intracellular recordings from adult turtle motoneurons in spinal cord slices. Membrane conductance was increased pharmacologically by extracellular application of the GABAA receptor agonist muscimol. Our findings suggest that an increase in membrane conductance of about 40-50% increases the magnitude of spike frequency adaptation, but does not change the threshold for action potential generation. Increased conductance causes a subtractive rather than a divisive effect on the initial and the early frequency-current relationships and may have not only a subtractive but also a divisive effect on the steady-state frequency-current relationship.

Developing electrical properties of postnatal mouse lumbar motoneurons

We studied the rapid changes in electrical properties of lumbar motoneurons between postnatal days 3 and 9 just before mice weight-bear and walk. The input conductance and rheobase significantly increased up to P8. A negative correlation exists between the input resistance (Rin) and rheobase. Both parameters are significantly correlated with the total dendritic surface area of motoneurons, the largest motoneurons having the lowest Rin and the highest rheobase. We classified the motoneurons into three groups according to their discharge firing patterns during current pulse injection (transient, delayed onset, sustained). The delayed onset firing type has the highest rheobase and the fastest action potential (AP) whereas the transient firing group has the lowest rheobase and the less mature AP. We found 32 and 10% of motoneurons with a transient firing at P3-P5 and P8, respectively. About 20% of motoneurons with delayed onset firing were detected at P8. At P9, all motoneurons exhibit a sustained firing. We defined five groups of motoneurons according to their discharge firing patterns in response to ascending and descending current ramps. In addition to the four classical types, we defined a fifth type called transient for the quasi-absence of discharge during the descending phase of the ramp. This transient type represents about 40% between P3-P5 and tends to disappear with age. Types 1 and 2 (linear and clockwise hysteresis) are the most preponderant at P6-P7. Types 3 and 4 (prolonged sustained and counter clockwise hysteresis) emerge at P8-P9. The emergence of types 3 and 4 probably depends on the maturation of L type calcium channels in the dendrites of motoneurons. No correlation was found between groups defined by step or triangular ramp of currents with the exception of transient firing patterns. Our data support the idea that a switch in the electrical properties of lumbar motoneurons might exist in the second postnatal week of life in mice.

Potassium currents dynamically set the recruitment and firing properties of F-type motoneurons in neonatal mice

In neonatal mice, fast- and slow-type motoneurons display different patterns of discharge. In response to a long liminal current pulse, the discharge is delayed up to several seconds in fast-type motoneurons and their firing frequency accelerates. In contrast, slow-type motoneurons discharge immediately, and their firing frequency decreases at the beginning of the pulse. Here, we identify the ionic currents that underlie the delayed firing of fast-type motoneurons. We find that the firing delay is caused by a combination of an A-like potassium current that transiently suppresses firing on a short time scale and a slowly-inactivating potassium current that inhibits the discharge over a much longer time scale. We then show how these intrinsic currents dynamically shape the discharge threshold and the frequency-input function of fast-type motoneurons. These currents contribute to the orderly recruitment of motoneurons in neonates and might play a role in the postnatal maturation of motor units.

Divisive gain modulation of motoneurons by inhibition optimizes muscular control

When using muscles, the precision with which force is delivered is as important as the delivery of force itself. Force is regulated by both the number of recruited motoneurons and their spike frequency. While it is known that the recruitment is ordered to reduce variability in force, it remains unclear whether the motoneuron gain, i.e., the slope of the transformation between synaptic input and spiking output, is also modulated to reduce variability in force. To address this issue, we use turtle hindlimb scratching as a model for fine motor control, since this behavior involves precise limb movement to rub the location of somatic nuisance touch. We recorded intracellularly from motoneurons in a reduced preparation where the limbs were removed to increase mechanical stability and the motor nerve activity served as a surrogate for muscle force. We found that not only is the gain of motoneurons regulated on a subsecond timescale, it is also adjusted to minimize variability. The modulation is likely achieved via an expansive nonlinearity between spike rate and membrane potential with inhibition having a divisive influence. These findings reveal a versatile mechanism of modulating neuronal sensitivity and suggest that such modulation is fundamentally linked to optimization.

Functional expression of T-type Ca2+ channels in spinal motoneurons of the adult turtle

Voltage-gated Ca²⁺ (Ca_V) channels are transmembrane proteins comprising three subfamilies named Ca_V1, Ca_V2 and Ca_V3. The Ca_V3 channel subfamily groups the low-voltage activated Ca²⁺ channels (LVA or T-type) a significant role in regulating neuronal excitability. Ca_V3 channel activity may lead to the generation of complex patterns of action potential firing such as the postinhibitory rebound (PIR). In the adult spinal cord, these channels have been found in dorsal horn interneurons where they control physiological events near the resting potential and participate in determining excitability. In motoneurons, Ca_V3 channels have been found during development, but their functional expression has not yet been reported in adult animals. Here, we show evidence for the presence of Ca_V3 channel-mediated PIR in motoneurons of the adult turtle spinal cord. Our results indicate that Ni²⁺ and NNC55-0396, two antagonists of Ca_V3 channel activity, inhibited PIR in the adult turtle spinal cord. Molecular biology and biochemical assays revealed the expression of the Ca_V3.1 channel isotype and its localization in motoneurons. Together, these results provide evidence for the expression of Ca_V3.1 channels in the spinal cord of adult animals and show also that these channels may contribute to determine the excitability of motoneurons.

Sequential acquisition of cacophony calcium currents, sodium channels and voltage-dependent potassium currents affects spike shape and dendrite growth during postembryonic maturation of an identified Drosophila motoneuron

During metamorphosis the CNS undergoes profound changes to accommodate the switch from larval to adult behaviors. In Drosophila and other holometabolous insects, adult neurons differentiate either from respecified larval neurons, newly born neurons, or are born embryonically but remain developmentally arrested until differentiation during pupal life. This study addresses the latter in the identified Drosophila flight motoneuron 5. In situ patch-clamp recordings, intracellular dye fills and immunocytochemistry address the interplay between dendritic shape, excitability and ionic current development. During pupal life, changes in excitability and spike shape correspond to a stereotyped, progressive appearance of voltage-gated ion channels. High-voltage-activated calcium current is the first current to appear at pupal stage P4, prior to the onset of dendrite growth. This is followed by voltage-gated sodium as well as transient potassium channel expression, when first dendrites grow, and sodium-dependent action potentials can be evoked by somatic current injection. Sustained potassium current appears later than transient potassium current. During the early stages of rapid dendritic growth, sodium-dependent action potentials are broadened by a calcium component. Narrowing of spike shape coincides with sequential increases in transient and sustained potassium currents during stages when dendritic growth ceases. Targeted RNAi knockdown of pupal calcium current significantly reduces dendritic growth. These data indicate that the stereotyped sequential acquisition of different voltage-gated ion channels affects spike shape and excitability such that activity-dependent calcium influx serves as a partner of genetic programs during critical stages of motoneuron dendrite growth.

Sodium-mediated plateau potentials in lumbar motoneurons of neonatal rats

The development and the ionic nature of bistable behavior in lumbar motoneurons were investigated in rats. One week after birth, almost all (∼80%) ankle extensor motoneurons recorded in whole-cell configuration displayed self-sustained spiking in response to a brief depolarization that emerged when the temperature was raised >30°C. The effect of L-type Ca(2+) channel blockers on self-sustained spiking was variable, whereas blockade of the persistent sodium current (I(NaP)) abolished them. When hyperpolarized, bistable motoneurons displayed a characteristic slow afterdepolarization (sADP). The sADPs generated by repeated depolarizing pulses summed to promote a plateau potential. The sADP was tightly associated with the emergence of Ca(2+) spikes. Substitution of extracellular Na(+) or chelation of intracellular Ca(2+) abolished both sADP and the plateau potential without affecting Ca(2+) spikes. These data suggest a key role of a Ca(2+)-activated nonselective cation conductance ((CaN)) in generating the plateau potential. In line with this, the blockade of (CaN) by flufenamate abolished both sADP and plateau potentials. Furthermore, 2-aminoethoxydiphenyl borate (2-APB), a common activator of thermo-sensitive vanilloid transient receptor potential (TRPV) cation channels, promoted the sADP. Among TRPV channels, only the selective activation of TRPV2 channels by probenecid promoted the sADP to generate a plateau potential. To conclude, bistable behaviors are, to a large extent, determined by the interplay between three currents: L-type I(Ca), I(NaP), and a Na(+)-mediated I(CaN) flowing through putative TRPV2 channels.

Bistability of bursting and silence regimes in a model of a leech heart interneuron

Bursting is one of the primary activity regimes of neurons. Our study is focused on determining a generic biophysical mechanism underlying the coexistence of the bursting and silent regimes observed in a neuron model. We show that the main ingredient for this mechanism is a saddle periodic orbit. The stable manifold of the orbit sets a threshold between the regimes of activity. Thus, the range of the controlling parameters, where the coexistence is observed, is limited by the bifurcations' values at which the saddle orbit appears and disappears. We show that it appears through the subcritical Andronov-Hopf bifurcation, where the equilibrium representing the silent regime loses stability, and disappears at the homoclinic bifurcation. Correspondingly, the bursting regime disappears in close proximity to the homoclinic bifurcation.

Six types of multistability in a neuronal model based on slow calcium current

BACKGROUND

Multistability of oscillatory and silent regimes is a ubiquitous phenomenon exhibited by excitable systems such as neurons and cardiac cells. Multistability can play functional roles in short-term memory and maintaining posture. It seems to pose an evolutionary advantage for neurons which are part of multifunctional Central Pattern Generators to possess multistability. The mechanisms supporting multistability of bursting regimes are not well understood or classified.

METHODOLOGY/PRINCIPAL FINDINGS

Our study is focused on determining the bio-physical mechanisms underlying different types of co-existence of the oscillatory and silent regimes observed in a neuronal model. We develop a low-dimensional model typifying the dynamics of a single leech heart interneuron. We carry out a bifurcation analysis of the model and show that it possesses six different types of multistability of dynamical regimes. These types are the co-existence of 1) bursting and silence, 2) tonic spiking and silence, 3) tonic spiking and subthreshold oscillations, 4) bursting and subthreshold oscillations, 5) bursting, subthreshold oscillations and silence, and 6) bursting and tonic spiking. These first five types of multistability occur due to the presence of a separating regime that is either a saddle periodic orbit or a saddle equilibrium. We found that the parameter range wherein multistability is observed is limited by the parameter values at which the separating regimes emerge and terminate.

CONCLUSIONS

We developed a neuronal model which exhibits a rich variety of different types of multistability. We described a novel mechanism supporting the bistability of bursting and silence. This neuronal model provides a unique opportunity to study the dynamics of networks with neurons possessing different types of multistability.

Diversification of Intrinsic Motoneuron Electrical Properties During Normal Development and Botulinum Toxin–Induced Muscle Paralysis in Early Postnatal Mice

During early postnatal development, between birth and postnatal days 8–11, mice start to achieve weight-bearing locomotion. In association with the progression of weight-bearing locomotion there are presumed developmental changes in the intrinsic electrical properties of spinal α-motoneurons. However, these developmental changes in the properties of α-motoneuron properties have not been systematically explored in mice. Here, data are presented documenting the developmental changes of selected intrinsic motoneuron electrical properties, including statistically significant changes in action potential half-width, intrinsic excitability and diversity (quantified as coefficient of variation) of rheobase current, afterhyperpolarization half-decay time, and input resistance. In various adult mammalian preparations, the maintenance of intrinsic motoneuron electrical properties is dependent on activity and/or transmission-sensitive motoneuron–muscle interactions. In this study, we show that botulinum toxin–induced muscle paralysis led to statistically significant changes in the normal development of intrinsic motoneuron electrical properties in the postnatal mouse. This suggests that muscle activity during early neonatal life contributes to the development of normal motoneuron electrical properties.

Specific vulnerability of mouse spinal cord motoneurons to membrane depolarization

Intracellular calcium (Ca(2+)) concentration determines neuronal dependence on neurotrophic factors (NTFs) and susceptibility to cell death. Ca(2+) overload induces neuronal death and the consequences are thought to be a probable cause of motoneuron (MN) degeneration in neurodegenerative diseases. In the present study, we show that membrane depolarization with elevated extracellular potassium (K(+)) was toxic to cultured embryonic mouse spinal cord MNs even in the presence of NTFs. Membrane depolarization induced an intracellular Ca(2+) increase. Depolarization-induced toxicity and increased intracellular Ca(2+) were blocked by treatment with antagonists to some of the voltage-gated Ca(2+) channels (VGCCs), indicating that Ca(2+) influx through these channels contributed to the toxic effect of depolarization. Ca(2+) activates the calpains, cysteine proteases that degrade a variety of substrates, causing cell death. We investigated the functional involvement of calpain using a calpain inhibitor and calpain gene silencing. Pre-treatment of MNs with calpeptin (a cell-permeable calpain inhibitor) rescued MNs survival; calpain RNA interference had the same protective effect, indicating that endogenous calpain contributes to the cell death caused by membrane depolarization. These findings suggest that MNs are especially vulnerable to extracellular K(+) concentration, which induces cell death by causing both intracellular Ca(2+) increase and calpain activation.

A neurocomputational model of tonic and phasic dopamine in action selection: a comparison with cognitive deficits in Parkinson's disease

The striatal dopamine signal has multiple facets; tonic level, phasic rise and fall, and variation of the phasic rise/fall depending on the expectation of reward/punishment. We have developed a network model of the striatal direct pathway using an ionic current level model of the medium spiny neuron that incorporates currents sensitive to changes in the tonic level of dopamine. The model neurons in the network learn action selection based on a novel set of mathematical rules that incorporate the phasic change in the dopamine signal. This network model is capable of learning to perform a sequence learning task that in humans is thought to be dependent on the basal ganglia. When both tonic and phasic levels of dopamine are decreased, as would be expected in unmedicated Parkinson's disease (PD), the model reproduces the deficits seen in a human PD group off medication. When the tonic level is increased to normal, but with reduced phasic increases and decreases in response to reward and punishment, respectively, as would be expected in PD medicated with L-Dopa, the model again reproduces the human data. These findings support the view that the cognitive dysfunctions seen in Parkinson's disease are not solely either due to the decreased tonic level of dopamine or to the decreased responsiveness of the phasic dopamine signal to reward and punishment, but to a combination of the two factors that varies dependent on disease stage and medication status.

Muscarinic control of the excitability of hindlimb motoneurons in chronic spinal-transected salamanders

The excitability of spinal motoneurons (MNs) is regulated by acetylcholine via the activation of muscarinic receptors. The objective of the present study was to determine whether this cholinergic modulation of MN excitability is altered following a chronic spinal cord transection. Juvenile salamanders (Pleurodeles waltlii) were spinally transected at the mid-trunk level, and patch-clamp recordings from hindlimb MNs in spinal cord slices were performed 9-30 days after transection, with and without bath application of muscarine (20 mum). Our results showed that the input-output relationship was larger in MNs recorded 2 weeks after spinal transection than in MNs recorded 3-4 weeks after spinal transection. They further revealed that muscarine increased both the gain of MNs and the proportion of MNs that could exhibit plateau potentials and afterdischarges, whereas it decreased the amplitude of the medium afterhypolarizing potential. Moreover, muscarine had no effect on the hyperpolarization-activated cation current (I(h)), whereas it increased the inward rectifying K(+) current (I(Kir)) in MNs recorded > or = 2 weeks after spinal transection. We conclude that following chronic spinal cord injury, the muscarinic modulation of some intrinsic properties of MNs previously reported in acute spinal-transected animals [S. Chevallier et al. (2006)The Journal of Physiology, 570, 525-540] was preserved, whereas that of other intrinsic properties of MNs was suppressed, either transiently (I(Kir)) or definitively (I(h)). These alterations in muscarinic modulation of MN excitability may contribute to the spontaneous recovery of locomotion displayed in long-term chronic spinal-transected salamanders.

Postnatal Changes in the Inactivation Properties of Voltage-Gated Sodium Channels Contribute to the Mature Firing Pattern of Spinal Motoneurons

Most mammals are born with the necessary spinal circuitry to produce a locomotor-like pattern of neural activity. However, rodents seldom demonstrate weight-supported locomotor behavior until the second or third postnatal week, possibly due to the inability of the neuromuscular system to produce sufficient force during this early postnatal period. As spinal motoneurons mature they are seen to fire an increasing number of action potentials at an increasing rate, which is a necessary component of greater force production. The mechanisms responsible for this enhanced ability of motoneurons are not completely defined. In the present study we assessed the biophysical properties of the developing voltage-gated sodium current to determine their role in the maturing firing pattern. Using dissociated postnatal lumbar motoneurons in short-term culture (18–24 h) we demonstrate that currents recorded from the most mature postnatal age group (P10–P12) were significantly better able to maintain channels in an available state during repetitive stimulation than were the younger age groups (P1–P3, P4–P6, P7–P9). This ability correlated with the ability of channels to recover more quickly and more completely from an inactivated state. These age-related differences were seen in the absence of changes in the voltage dependence of channel gating. Differences in both closed-state inactivation and slow inactivation were also noted between the age groups. The results indicate that changes in the inactivation properties of voltage-gated sodium channels are important for the development of a mature firing pattern in spinal motoneurons.

Action-based sensory encoding in spinal sensorimotor circuits

The concept of a modular organisation of the spinal withdrawal reflex circuits has proven to be fundamental for the understanding of how the spinal cord is organised and how the sensorimotor circuits translate sensory information into adequate movement corrections. Recent studies indicate that a task-related body representation is engraved at the network level through learning-dependent mechanisms involving an active probing procedure termed 'somatosensory imprinting' during development. It was found that somatosensory imprinting depends on the tactile input that is associated with spontaneous movements that occur during sleep and results in elimination of erroneous connections and establishment of correct connections. In parallel studies it was found that the strength of the first order tactile synapses in rostrocaudally elongated zones in the adult dorsal horn in the lower lumbar cord is related to the modular organisation of the withdrawal reflexes. Hence, the topographical organisation of the tactile input to this spinal area seems to be action-based rather than a simple body map as previously thought. Far from being innate and adult like at birth, the adult organisation seems to emerge from an initial 'floating' and diffuse body representation with many inappropriate connections through profound activity-dependent rearrangements of afferent synaptic connections. It is suggested that somatosensory imprinting plays a key role in the self-organisation of the spinal cord during development.

Pudendal nerve stimulation, interneurons post-discharge and delayed depolarization in hind limb motoneurons of the female cat

The present experiments were done in the spinal female cat. In a prior work in the decerebrate female cat, stimulation of the sensory pudendal nerve (SPN) induced a depolarizing wave (LD) in hind limb motoneurons that outlasted the stimulus by up to 6 s. LD triggered self-sustained motoneuron firing (bistability). An intrinsic potential underlies bistable firing, which, in the cat, depends on two main factors; first, the integrity of pathways descending from the brain stem to the spinal cord and, second, the membrane potential of the motoneuron just before the stimulus; at high resting potential, excitatory short-lasting inputs induce transient but no sustained firing. Thus, no bistability occurs in the spinal cat or in hyperpolarized motoneurons of the decerebrate cat. LD might be an intrinsic potential that could also be absent in the spinal cat, or an extrinsic (synaptic) potential induced by spinal interneurons. In the latter case, the interneurons generating LD should show post-discharge as prolonged as LD. LD was produced in spinal cats and its amplitude did not change or increase slightly during hyperpolarizing pulses, which suggests that LD might be a synaptic response. Interneurons showing post-discharge to train of stimulation to SPN were located 100-200 microm above the pools of hind limb motoneurons. Some post-discharges were as prolonged as LD. We conclude that LD might be a synaptic response to local interneurons.

Expression of L-type calcium channel alpha(1)-1.2 and alpha(1)-1.3 subunits on rat sacral motoneurons following chronic spinal cord injury

In the presence of the monoamines serotonin and norepinephrine, motoneurons readily generate large persistent inward currents (PICs). The resulting plateau potentials amplify and sustain motor output. Monoaminergic input to the cord originates in the brainstem and the sharp reduction in monoamine levels that occurs following acute spinal cord injury greatly decreases motoneuron excitability. However, recent studies in the adult sacral cord of the rat have shown that motoneurons reacquire the ability to generate PICs and plateau potentials within 1-2 months following spinal transection. Ca(v)1.3 L-type calcium channels are involved in generating PICs in both healthy and injured animals. Additionally, expression of Ca(v)1.2 and Ca(v)1.3 L-type calcium channels is altered in several pathological conditions. Therefore, in this paper we analyzed the expression of L-type calcium channel alpha(1) subunits within the motoneuron pool following a complete transection of the spinal cord at the level of the sacral vertebra (S)2 segment. The analysis was done both caudally (S4 segment) and rostrally [thoracic vertebra (T)6 segment] from the injury site. The S4 segment was significantly reduced in diameter when compared with control animals, and this reduction was more evident in the white matter. Ca(v)1.2 alpha(1) subunit expression significantly increased (26%) in the motoneuron pool located caudally but not rostrally from the injury site. In contrast, the expression of Ca(v)1.3 alpha(1) subunit remained unchanged in both S4 and T6 segments. The differential expression of the two alpha(1) subunits in spinal injury suggests that Ca(v)1.2 and Ca(v)1.3 channels have different functions in neuronal adaptation following spinal cord injury.

Control of stepping velocity in a single insect leg during walking

In the single middle leg preparation of the stick insect walking on a treadmill, the activity of flexor and extensor tibiae motor neurons and muscles, which are responsible for the movement of the tibia in stance and swing phases, respectively, was investigated with respect to changes in stepping velocity. Changes in stepping velocity were correlated with cycle period. There was a close correlation of flexor motor neuron activity (stance phase) with stepping velocity, but the duration and activation of extensor motor neurons (swing phase) was not altered. The depolarization of flexor motor neurons showed two components. At all step velocities, a stereotypic initial depolarization was generated at the beginning of stance phase activity. A subsequent larger depolarization and activation was tightly linked to belt velocity, i.e. it occurred earlier and with larger amplitude during fast steps compared with slow steps. Alterations in a tonic background excitation appear not to play a role in controlling the motor neuron activity for changes in stepping velocity. Our results indicate that in the single insect leg during walking, mechanisms for altering stepping velocity become effective only during an already ongoing stance phase motor output. We discuss the putative mechanisms involved.

Long-term and immediate effect of testosterone on single T-type calcium channel in neonatal rat cardiomyocytes

In the cardiovascular system, T-type calcium channels play an important role for the intracellular calcium homeostasis and spontaneous pacemaker activity and are involved in the progression of structural heart diseases. Androgens influence the cardiovascular physiology and pathophysiology. However, their effect on native T-type calcium currents (I(Ca,T)) remains unclear. To test the chronic effect of testosterone on the cardiac I(Ca,T), cultured neonatal rat ventricular cardiomyocytes were treated with testosterone (1 nM-10 microM) for 24-30 h. Current measurements were performed after testosterone washout to exclude any acute testosterone effects. Testosterone (100 nm) pretreatment significantly increased whole-cell I(Ca,T) density from 1.26 +/- 0.48 pA/pF (n = 8) to 5.06 +/- 1.75 pA/pF (n = 7; P < 0.05) and accelerated beating rate. This was attributed to both increased expression levels of the pore-forming subunits Ca(v)3.1 and Ca(v)3.2 and increased T-type single-channel activity. On single-channel level, the increase of the ensemble average current by testosterone vs. time-matched controls was due to an increased availability (58.1 +/- 4.2 vs. 21.5 +/- 4.0%, P < 0.01) and open probability (2.78 +/- 0.29 vs. 0.85 +/- 0.23%, P < 0.01). Cotreatment with the selective testosterone receptor antagonist flutamide (10 mum) prevented these chronic testosterone-induced effects. Conversely, acute application of testosterone (10 microM) decreased T-type single-channel activity in testosterone pretreated cells by reducing the open probability (0.78 +/- 0.13 vs. 2.91 +/- 0.38%, P < 0.01), availability (23.6 +/- 3.3 vs. 57.6 +/- 4.5%, P < 0.01), and peak current (-20 +/- 4 vs. -58 +/- 4 fA, P < 0.01). Flutamide (10 microM) did not abolish the testosterone-induced acute block of T-type calcium channels. Our results indicate that long-term testosterone treatment increases, whereas acute testosterone decreases neonatal rat T-type calcium currents. These effects seem to be mediated by a genomic chronic stimulation and a nongenomic acute inhibitory action.

Intrinsic Properties Shape the Firing Pattern of Ventral Horn Interneurons From the Spinal Cord of the Adult Turtle

Interneurons in the ventral horn of the spinal cord play a central role in motor control. In adult vertebrates, their intrinsic properties are poorly described because of the lack of in vitro preparations from the spinal cord of mature mammals. Taking advantage of the high resistance to anoxia in the adult turtle, we used a slice preparation from the spinal cord. We used the whole cell blind patch-clamp technique to record from ventral horn interneurons. We characterized their firing patterns in response to depolarizing current pulses and found that all the interneurons fired repetitively. They displayed bursting, adapting, delayed, accelerating, or oscillating firing patterns. By combining electrophysiological and pharmacological tests, we showed that interneurons expressed slow inward rectification, plateau potential, voltage-sensitive transient outward rectification, and low-threshold spikes. These results demonstrate a diversity of intrinsic properties that may enable a rich repertoire of activity patterns in the network of ventral horn interneurons.

Transient expression of A-type K channel alpha subunits Kv4.2 and Kv4.3 in rat spinal neurons during development

A-type K(+) currents (I(A)s) have been detected from the ventral horn neurons in rat spinal cord during embryonic day (E) 14 to postnatal day (P) 8 but not in adulthood. It is not known which types of neurons and which A-type K(+) channel alpha subunits express the I(A)s and what the possible function might be. Here, we examined the expression of two A-type K(+) channel alpha subunits, Kv4.2 and Kv4.3, in rat spinal cord at various developmental stages by immunohistochemistry. We found a transient expression of Kv4.2 in somatic motoneurons during E13.5-P8 with a peak around E17.5, which coincides temporally with the natural selection of motoneurons. Transient expression of Kv4.2 and Kv4.3 was also observed in the intermediate gray (IG) interneurons. During E19.5-P14, some IG interneurons express Kv4.2, some express Kv4.3 and a subset co-express Kv4.2 and Kv4.3. Peak expression of Kv4.2 and Kv4.3 in the IG interneurons was detected around P1, which coincides temporally with the developmental selection of IG interneurons. In contrast to the I(A)-expressing subunits Kv4.2 and Kv4.3, a delayed-rectifier K(+) channel alpha subunit Kv1.6 is persistently expressed in somatic motoneurons and IG interneurons. Together, these data support the hypothesis that expression of I(A)s may protect I(A)-expressing somatic motoneurons, and possibly also IG interneurons, from naturally occurring cell death during developmental selection.

Dynamic sensorimotor interactions in locomotion

Locomotion results from intricate dynamic interactions between a central program and feedback mechanisms. The central program relies fundamentally on a genetically determined spinal circuitry (central pattern generator) capable of generating the basic locomotor pattern and on various descending pathways that can trigger, stop, and steer locomotion. The feedback originates from muscles and skin afferents as well as from special senses (vision, audition, vestibular) and dynamically adapts the locomotor pattern to the requirements of the environment. The dynamic interactions are ensured by modulating transmission in locomotor pathways in a state- and phase-dependent manner. For instance, proprioceptive inputs from extensors can, during stance, adjust the timing and amplitude of muscle activities of the limbs to the speed of locomotion but be silenced during the opposite phase of the cycle. Similarly, skin afferents participate predominantly in the correction of limb and foot placement during stance on uneven terrain, but skin stimuli can evoke different types of responses depending on when they occur within the step cycle. Similarly, stimulation of descending pathways may affect the locomotor pattern in only certain phases of the step cycle. Section ii reviews dynamic sensorimotor interactions mainly through spinal pathways. Section iii describes how similar sensory inputs from the spinal or supraspinal levels can modify locomotion through descending pathways. The sensorimotor interactions occur obviously at several levels of the nervous system. Section iv summarizes presynaptic, interneuronal, and motoneuronal mechanisms that are common at these various levels. Together these mechanisms contribute to the continuous dynamic adjustment of sensorimotor interactions, ensuring that the central program and feedback mechanisms are congruous during locomotion.

Plateau Potentials in Developing Antennal-Lobe Neurons of the Moth,Manduca sexta

Using whole cell recordings from antennal-lobe (AL) neurons in vitro and in situ, in semi-intact brain preparations, we examined membrane properties that contribute to electrical activity exhibited by developing neurons in primary olfactory centers of the brain of the sphinx moth, Manduca sexta. This activity is characterized by prolonged periods of membrane depolarization that resemble plateau potentials. The presence of plateau potential–generating mechanisms was confirmed using a series of tests established earlier. Brief depolarizing current pulses could be used to trigger a plateau state. Once triggered, plateau potentials could be terminated by brief pulses of hyperpolarizing current. Both triggering and terminating of firing states were threshold phenomena, and both conditions resulted in all-or-none responses. Rebound excitation from prolonged hyperpolarizing pulses could also be used to generate plateau potentials in some cells. These neurons were found to express a hyperpolarization-activated inward current. Neither the generation nor the maintenance of plateau potentials was affected by removal of Na+ions from the extracellular medium or by blockade of Na+currents with TTX. However, blocking of Ca2+currents with Cd2+(5 × 10−4M) inhibited the generation of plateau potentials, indicating that, in Manduca AL neurons, plateau potentials depend on Ca2+. Examining Ca2+currents in isolation revealed that activation of these currents occurs in the absence of experimentally applied depolarizing stimuli. Our results suggest that this activity underlies the generation of plateau potentials and characteristic bursts of electrical activity in developing AL neurons of M. sexta.

Changes during the postnatal development in physiological and anatomical characteristics of rat motoneurons studied in vitro

The postnatal maturation of rat brainstem (oculomotor and hypoglossal nuclei) and spinal motoneurons, based on data collected from in vitro studies, is reviewed here. Membrane input resistance diminishes with age, but to a greater extent for hypoglossal than for oculomotor motoneurons. The time constant of the membrane diminishes with age in a similar fashion for both oculomotor and hypoglossal motoneurons. The current required to reach threshold (rheobase) decreases in oculomotor motoneurons, in contrast with the increase observed in hypoglossal motoneurons. The depolarization voltage required to generate an action potential also diminishes in oculomotor motoneurons, whereas it remains constant in hypoglossal motoneurons. A membrane potential rectification (sag) appears in response to negative current steps, hyperpolarizing brainstem motoneurons more than 20 mV relative to the rest. This membrane response is more frequent in adult motoneurons. The durations of the action potential and its medium afterhyperpolarization (mAHP) decrease with postnatal development in all motoneurons studied, although the shortening of mAHP is more evident in oculomotor motoneurons. A rise in firing rate for all motoneurons with age is universal; this trend is also more pronounced in oculomotor motoneurons. Developing motoneurons exhibit a postinhibitory rebound depolarization that is capable of triggering an action potential or a short burst of spikes. This phenomenon is voltage-dependent and requires less of a membrane hyperpolarization to elicit an action potential in adult than in neonatal cells. In all developing brainstem and spinal motoneurons, the adult somal size is reached within the newborn period, although their dendrites continue to elongate. In summary, input resistance, time constant, and durations of action potential and mAHP decrease, while the frequency of sag and postinhibitory rebound, as well as the motoneuron firing rate and dendritic length, increase with postnatal age. These trends are universal to all the motoneuronal populations studied; however, the extent of these changes differs for each motoneuronal pool. A further distinction is evident in the inconsistent age-dependent change in rheobase and depolarization voltage for the two brainstem motoneuron nuclei.

Recruitment of motor neuronal persistent inward currents shapes withdrawal reflexes in the frog

The details of behaviour are determined by the interplay of synaptic connectivity within neuronal circuitry and the intrinsic membrane properties of individual neurones. One particularly dramatic intrinsic property displayed by neurones in many regions of the nervous system is membrane potential bistability, in which transient excitation of a neurone results in a persistent depolarization outlasting the initial excitation. Here we characterize the contribution of such intrinsic bistability, also referred to as plateau properties and mediated by persistent inward currents (PICs), in spinal motor neurones to the production of withdrawal behaviours in the frog. We performed experiments on the isolated frog spinal cord with attached hindlimb. This preparation allowed the simultaneous monitoring of muscle activations during motor behaviour and intracellular neuronal recordings. We found that PICs, following their potentiation by serotonin (5-HT), are recruited and contribute to the production of withdrawal behaviours. These properties conferred a voltage-dependent prolongation to the duration of motor neuronal activity. Consistent with this potentiation of motor neuronal PICs, 5-HT also increased the duration of evoked muscle activations. This behavioural potentiation, as well as the expression of PICs in individual neurones, was reduced following antagonism of L-type Ca(2+) channels. These results demonstrate that PICs in motor neurones can be recruited during the production of behaviour and play a role in specifying the temporal details of motor output.

Differential expression of voltage-activated calcium channels in III and XII motoneurones during development in the rat

To further our understanding of the role that voltage-activated Ca2+ channels play in the development, physiology and pathophysiology of motoneurones (MNs), we used whole-cell patch-clamp recording to compare voltage-activated Ca2+ currents in oculomotor (III) and hypoglossal (XII) MNs of neonatal [postnatal day (P)1-5] and juvenile (P14-19) rats. In contrast to III MNs that innervate extraocular muscles, XII MNs that innervate tongue muscles mature more rapidly, fire bursts of low frequency action potentials and are vulnerable to degeneration in amyotrophic lateral sclerosis. In neonates, low voltage-activated (LVA) Ca2+ current densities are similar in XII and III MNs but high voltage-activated (HVA) Ca2+ current densities are twofold higher in XII MNs. The HVA Ca2+ channel antagonists (nimodipine and nifedipine for L-type, omega-agatoxin-TK for P/Q-type and omega-conotoxin-GVIA for N-type) revealed that, while N- and P/Q-type HVA Ca2+ channels are present in both MN pools, a 3.5-fold greater P/Q-type Ca2+ current in XII MNs accounts for their greater HVA Ca2+ currents. Developmentally, LVA and HVA Ca2+ current densities decrease in III MNs but remain unchanged in XII MNs. Thus, the differences between these MN pools increase developmentally so that, in juveniles, the LVA Ca2+ current density is twofold greater and the HVA Ca2+ current density is threefold greater in XII compared with III MNs. We propose that this differential expression of LVA and HVA Ca2+ channels in XII and III MNs during development contributes to their distinct physiology and may also be a factor contributing to the greater susceptibility of XII MNs to degeneration as seen in amyotrophic lateral sclerosis.

Steps during the development of the zebrafish locomotor network

This review summarizes recent data from our lab concerning the development of motor activities in the developing zebrafish. The zebrafish is a leading model for studies of vertebrate development because one can obtain a large number of transparent, externally and rapidly developing embryos with motor behaviors that are easy to assess (e.g. for mutagenic screens). The emergence of embryonic motility was studied behaviorally and at the cellular level. The embryonic behaviors appear sequentially and include an early, transient period of spontaneous, alternating tail coilings, followed by responses to touch, and swimming. Patch clamp recording in vivo revealed that an electrically coupled network of a subset of spinal neurons generates spontaneous tail coiling, whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming and requires input from the hindbrain. Swimming becomes sustained in larvae once serotonergic neuromodulatory effects are integrated. We end with a brief overview of the genetic tools available for the study of the molecular determinants implicated in locomotor network development in the zebrafish. Combining genetic, behavioral and cellular experimental approaches will advance our understanding of the general principles of locomotor network assembly and function.

Roles of ryanodine and inositol triphosphate receptors in regulation of plateau potentials in turtle spinal motoneurons

Generation of plateau potentials in spinal motoneurons depends on activation of voltage sensitive L-type Ca(2+) channels. These channels are facilitated by metabotropic receptors known to promote release of Ca(2+) from intracellular stores. The aim of this study is to determine if Ca(2+)-release receptors in the endoplasmic reticulum (ER) that are sensitive to ryanodine (RyRs) and to inositol triphosphate receptors (IP(3)Rs) contribute to the generation of plateau potentials. The effects of antagonists to RyRs, IP(3)Rs and phospholipase C (PLC) were tested on discharge patterns associated with plateau potentials in motoneurons in slices from the spinal cord of the turtle. Plateau-related discharge patterns, un-facilitated or facilitated by agonists for group I glutamate metabotropic receptors, muscarine-sensitive cholinergic receptors or L-type Ca(2+) channels were inhibited by blockade of RyRs. In contrast, antagonists of IP(3)Rs or PLC preferentially inhibited plateau-related discharge patterns when facilitated by activation of metabotropic receptors but in only half of the cells when promoted in the absence of metabotropic facilitators. Our findings show that RyRs and IP(3)Rs regulate the generation of plateau potentials in motoneurons and suggest that RyRs may be directly involved with activation of the plateau potential.

5-HT inhibits N-type but not L-type Ca(2+) channels via 5-HT1A receptors in lamprey spinal neurons

5-HT is a potent modulator of locomotor activity in vertebrates. In the lamprey, 5-HT dramatically slows fictive swimming. At the neuronal level it reduces the postspike slow afterhyperpolarization (sAHP), which is due to apamin-sensitive Ca(2+)-dependent K+ channels (KCa). Indirect evidence in early experiments suggested that the sAHP reduction results from a direct action of 5-HT on KCa channels rather than an effect on the Ca(2+) entry during the action potential. In view of the characterization of different subtypes of Ca(2+) channels with very different properties, we now reinvestigate if there is a selective action of 5-HT on a Ca(2+) channel subtype in dissociated spinal neurons in culture. 5-HT reduced Ca(2+) currents from high voltage activated channels. N-type, but not L-type, Ca(2+) channel blockers abolished this 5-HT-induced reduction. It was also confirmed that 5-HT depresses Ca(2+) currents in neurons, including motoneurons, in the intact spinal cord. 8-OH-DPAT, a 5-HT1A receptor agonist, also inhibited Ca(2+) currents in dissociated neurons. After incubation in pertussis toxin, to block Gi/o proteins, 5-HT did not reduce Ca(2+) currents, further indicating that the effect is caused by an activation of 5-HT1A receptors. As N-type, but not L-type, Ca(2+) channels are known to mediate the activation of KCa channels and presynaptic transmitter release at lamprey synapses, the effects of 5-HT reported here can contribute to a reduction in both actions.

Development of Ca2+-channel and BK-channel expression in embryos and larvae of Xenopus laevis

The patterns and density of channels expressed in neurons critically determine their electrical properties. We have examined developmental regulation of Ca2+-channel expression during the maturation of the spinal motor circuits in Xenopus as it develops from an embryo to a larva. In embryonic neurons approximately 60% of the current is carried by N-type channels, 8% by l-type channels and the remainder by an unidentified channel. As the embryo matures, omega-agatoxin-sensitive P/Q channels are gradually expressed and replace the unidentified HVA channel such that at stage 42 approximately 25% of the current is carried by P/Q channels. We have used fluorescent labelling of selective channel toxins to directly observe the distribution of P/Q, N and BK channels. The P/Q channel distribution was most prevalent on the cell surface proximal to the areas of the soma where processes emerge. Both N and BK channels were distributed throughout the soma but still exhibited concentration around the areas adjacent to the emergence of processes from the soma. The patterns of fluorescence labelling during development mirrored the development of the respective ionic currents. Both N and P/Q channels contribute roughly equally to activation of the BK current, suggesting that overlap in the distribution of the N, P/Q and BK channels is important in their functional interdependence. The newly expressed P/Q channels play a role in spike initiation and repetitive firing in larval spinal neurons and contribute to burst generation during swimming in the larva.

Spontaneous voltage oscillations in striatal projection neurons in a rat corticostriatal slice

In a rat corticostriatal slice, brief, suprathreshold, repetitive cortical stimulation evoked long-lasting plateau potentials in neostriatal neurons. Plateau potentials were often followed by spontaneous voltage transitions between two preferred membrane potentials. While the induction of plateau potentials was disrupted by non-NMDA and NMDA glutamate receptor antagonists, the maintenance of spontaneous voltage transitions was only blocked by NMDA receptor and L-type Ca2+ channel antagonists. The frequency and duration of depolarized events, resembling up-states described in vivo, were increased by NMDA and L-type Ca2+ channel agonists as well as by GABAA receptor and K+ channel antagonists. NMDA created a region of negative slope conductance and a positive slope crossing indicative of membrane bistability in the current-voltage relationship. NMDA-induced bistability was partially blocked by L-type Ca2+ channel antagonists. Although evoked by synaptic stimulation, plateau potentials and voltage oscillations could not be evoked by somatic current injection--suggesting a dendritic origin. These data show that NMDA and L-type Ca2+ conductances of spiny neurons are capable of rendering them bistable. This may help to support prolonged depolarizations and voltage oscillations under certain conditions.

Serotonin patterns locomotor network activity in the developing zebrafish by modulating quiescent periods

Developing neural networks follow common trends such as expression of spontaneous, recurring activity patterns, and appearance of neuromodulation. How these processes integrate to yield mature, behaviorally relevant activity patterns is largely unknown. We examined the integration of serotonergic neuromodulation and its role in the functional organization of the accessible locomotor network in developing zebrafish at behavioral and cellular levels. Locally restricted populations of serotonergic neurons and their projections appeared in the hindbrain and spinal cord of larvae after hatching (approximately day 2). However, 5-HT affected the swimming pattern only from day 4 on, when sustained spontaneous swimming appeared. 5-HT and its agonist quipazine increased motor output by reducing intervals of inactivity, observed behaviorally (by high-speed video) and in recordings from spinal neurons during fictive swimming (by whole-cell current clamp). 5-HT and quipazine had little effect on the properties of the activity periods, such as the duration of swim episodes and swim frequency. Further, neuronal input resistance, rheobasic current, and resting potential were not affected significantly. The 5-HT antagonists methysergide and ketanserin decreased motor output by prolonging the periods of inactivity with little effect on the active swim episode or neuronal properties. Our results suggest that 5-HT neuromodulation is integrated early in development of the locomotor network to increase its output by reducing periods of inactivity with little effect on the activity periods, which in contrast are the main targets of 5-HT neuromodulation in neonatal and adult preparations.

Metabotropic modulation of motoneurons by scratch-like spinal network activity

Glutamate is the main excitatory transmitter in the spinal motor network. The excitation is to a large extent mediated by ionotropic receptors, but glutamate also activates metabotropic receptors. In motoneurons in spinal cord slices the activation of group I metabotropic glutamate (mGlu1) receptors leads to facilitation of CaV1.3 L-type calcium channels. Here we investigate whether this pathway is activated by motor network activity induced by natural sensory stimuli. The lumbar carapace and spinal cord were isolated from adult turtles. In this preparation, mechanical stimulation in the receptive field for the scratch reflex induced episodes of rhythmic motor network activity. During an episode the excitability of coactivated motoneurons increased. This increase was associated with an increased persistent inward current and was abolished by local blockade of either mGlu1 receptors or CaV1.3 L-type calcium channels near the recording site. We conclude that glutamate released during spinal motor network activity excites motoneurons by parallel activation of ionotropic and mGlu1 receptors. The metabotropic facilitation of L-type calcium channels contributes significantly to this excitation. Our findings establish intrinsic modulation as an active component in the spinal motor network for limb movements.

Subcellular distribution of L-type Ca2+ channels responsible for plateau potentials in motoneurons from the lumbar spinal cord of the turtle

L-type calcium channels mediate the persistent inward current underlying plateau potentials in spinal motoneurons. Electrophysiological analysis shows that plateau potentials are generated by a persistent inward current mediated by low threshold L-type calcium channels located in the dendrites. As motoneurons express L-type calcium channels of the CaV1.2 and CaV1.3 subtypes, we have investigated the subcellular distribution of these channels using antibody labelling. The plateau generating a persistent inward current is modulated by the activation of metabotropic receptors. For this reason, we also examined the relationship between CaV1.2 and CaV1.3 subunits in motoneurons and presynaptic terminals labelled with antibodies against synapsin 1a. Motoneurons in the spinal cord of the adult turtle were identified as large neurons, immunopositive for choline acetyltransferase, located in the ventral horn. In these neurons, CaV1.2 subunits were present in the cell bodies and axons. Patches of CaV1.3 subunits were seen in association with the cell membrane of the somata and both the proximal and distal dendrites. Double labelling with an antibody against synapsin 1a showed that CaV1.3 subunits, but not CaV1.2 subunits, were always located at synaptic sites. The distribution of CaV1.2 and CaV1.3 strongly suggests that the persistent inward current underlying plateau potentials in spinal motoneurons is mediated by CaV1.3 and not by CaV1.2. Our findings also show that CaV1.3 may be located in the somatic and dendritic membrane adjacent to particular presynaptic terminals.

A-, T-, and H-type currents shape intrinsic firing of developing rat abducens motoneurons

During postnatal development, profound changes take place in the excitability of nerve cells, including modification in the distribution and properties of receptor-operated channels and changes in the density and nature of voltage-gated channels. We studied here the firing properties of abducens motoneurons (aMns) in transverse brainstem slices from postnatal day (P) 1-13 rats. Recordings were made from aMNs in the whole-cell configuration of the patch-clamp technique. Two main types of aMn could be distinguished according to their firing profile during prolonged depolarizations. Both types were identified as aMns by their fluorescence following retrograde labelling with the lipophilic carbocyanine DiI in the rectus lateralis muscle. The first type (BaMns) exhibited a burst of action potentials (APs) followed by an adaptation of discharge and were encountered in approximately 70 % of aMns. Their discharge profile resembled that of adult aMns and was encountered in all aMns after P9. BaMns exhibited a hyperpolarization-induced rebound potential that was blocked by low concentrations of Ni2+ or by Ca2+-free external solution. This current had the properties of the T-type current. Action potentials of BaMns showed a complex afterhyperpolarization (AHP). An inward rectification was evidenced following hyperpolarization and was blocked by external application of caesium or ZD7288, indicating the presence of the hyperpolarization-activated cationic current (IH). Blocking the IH current almost doubled the input resistance of BaMns. The second class of aMns (DaMns) displayed a delayed excitation that was mediated by A-type K+ currents and was observed only between P4 and P9. DaMns exhibited immature characteristics: an action potential with a simple AHP, a linear current-voltage relation and a large input resistance. The number of aMns remained unchanged when both types were present (P5-P6) and later in development when only BaMns were encountered (P19), suggesting that DaMns mature into BaMns during postnatal development. We conclude that aMns display profound reorganization in their intrinsic excitability during postnatal development.

5-HT1A receptors increase excitability of spinal motoneurons by inhibiting a TASK-1-like K+ current in the adult turtle

The modulatory effects of serotonin mediated by 5-HT1A receptors in adult spinal motoneurons were investigated by intracellular recordings in a slice preparation from the turtle. In current-clamp mode, activation of 5-HT1A receptors by 8-OH-DPAT led to depolarization and an increase in input resistance in most motoneurons but caused hyperpolarization and a decrease in input resistance in the remaining smaller fraction of cells. When slices were preincubated in medium containing the 5-HT1A receptor antagonist WAY-100635, 8-OH-DPAT had no effect. In voltage-clamp mode, with 1 mM CsCl in the bathing medium, 8-OH-DPAT consistently inhibited a leak current that was sensitive to extracellular acidification and anandamide, a TASK-1 channel blocker. In medium with a low pH, as in the presence of anandamide, 8-OH-DPAT had no effect. Our results show that activation of 5-HT1A receptors contributes to the excitatory effect of serotonin on spinal motoneurons by inhibition of a TASK-1 potassium channel leading to depolarization and increased input resistance.

Mechanisms causing plateau potentials in spinal motoneurones

Plateau potentials are generated by a voltage sensitive persistent inward current. In spinal motoneurones this current is predominantly mediated by influx of Ca2+ through L-type Ca2+ channels of the Ca(v)1.3 subtype. Depolarisation-induced facilitation of L-type Ca2+ channels is thought to be the mechanism for delayed activation (wind-up and warm-up) of the plateau potential and for the hysteresis in firing frequency and I-V relation during triangular depolarisation. L-type Ca2+ channels and plateau potentials in spinal motoneurones are facilitated by activation of metabotropic receptors for glutamate, acetylcholine, noradrenaline and serotonin and down regulated by activation of GABA(B) receptors. The facilitation has been shown to depend on activated calmodulin.

Persistent sodium and calcium currents in rat hypoglossal motoneurons

Voltage-dependent persistent inward currents are thought to make an important contribution to the input-output properties of alpha-motoneurons, influencing both the transfer of synaptic current to the soma and the effects of that current on repetitive discharge. Recent studies have paid particular attention to the contribution of L-type calcium channels, which are thought to be widely distributed on both the somatic and the dendritic membrane. However, the relative contribution of different channel subtypes as well as their somatodendritic distribution may vary among motoneurons of different species, developmental stages, and motoneuron pools. In this study, we have characterized persistent inward currents in juvenile (10- to 24-day-old) rat hypoglossal (HG) motoneurons. Whole-cell, voltage-clamp recordings were made from the somata of visualized rat HG motoneurons in 300-microm brain stem slices. Slow (10 s), triangular voltage-clamp commands from a holding potential of -70 to 0 mV and back elicited whole-cell currents that were dominated by outward, potassium currents, but often showed a region of negative slope resistance on the rising phase of the command. In the presence of potassium channel blockers (internal cesium and external 4-aminopyridine and tetraethylammonium), net inward currents were present on both the rising and falling phases of the voltage-clamp command. A portion of the inward current present on the ascending phase of the command was mediated by TTX-sensitive sodium channels, whereas calcium channels mediated the remainder of the current. We found roughly the same relative contributions of P-, N-, and L-type channels to the calcium currents recorded at the soma that had previously been found in neonatal rat HG motoneurons. In most cells, the somatic voltage thresholds for calcium current onset and offset were similar and the peak current was largest on the ascending phase of the clamp command. However, about one-third of the cells exhibited a substantial clockwise current hysteresis, i.e., inward currents were present at lower voltages on the descending phase of the clamp command. In the same cells, 1-s depolarizing voltage-clamp commands were followed by prolonged tail currents, consistent with a prominent contribution from dendritic channels. In contrast to previous reports on turtle and mouse motoneurons, blocking L-type calcium channels did not eliminate these presumed dendritic currents.

Long-term neuromodulatory regulation of a motor pattern-generating network: maintenance of synaptic efficacy and oscillatory properties

Rhythm generation by the pyloric motor network in the stomatogastric ganglion (STG) of the spiny lobster requires permissive neuromodulatory inputs from other central ganglia. When these inputs to the STG are suppressed by cutting the single, mainly afferent stomatogastric nerve (stn), pyloric neurons cease to burst and the network falls silent. However, as shown previously, if such a decentralized quiescent ganglion is maintained in organ culture, pyloric network rhythmicity returns after 3-4 days and, although slower, is similar to the motor pattern expressed when the stn is intact. Here we use current- and voltage-clamp, primarily of identified pyloric dilator (PD) neurons, to investigate changes in synaptic and cellular properties that underlie this transition in network behavior. Although the efficacy of chemical synapses between pyloric neurons decreases significantly (by <or=50%) after STG decentralization, the fundamental change leading to rhythm recovery occurs in the voltage-dependent properties of the neurons themselves. Whereas pyloric neurons, including the PD, lateral pyloric, and pyloric cell types, are unable to generate burst-producing membrane potential oscillations in the short-term absence of extrinsic modulatory inputs, in long-term decentralized ganglia, the same cells are able to oscillate spontaneously, even after experimental isolation in situ from all other elements in the pyloric network. In PD neurons this reacquisition of rhythmicity is associated with a net reduction in outward tetraethylammonium-sensitive ionic currents that include a delayed-rectifier type potassium current (I(Kd)) and a calcium-dependent K(+) current, I(KCa). By contrast, long-term STG decentralization caused enhancement of a hyperpolarization-activated inward current that resembles I(h). These results are consistent with the hypothesis that modulatory inputs sustain the modulation-dependent rhythmogenic character of the pyloric network by continuously regulating the balance of membrane conductances that underlie neuronal oscillation.

Changes in electrophysiological properties of lamprey spinal motoneurons during fictive swimming

Electrophysiological properties of lamprey spinal motoneurons were measured to determine whether their cellular properties change as the spinal cord goes from a quiescent state to the active state of fictive swimming. Intracellular microelectrode recordings of membrane potential were made from motoneurons in the isolated spinal cord preparation. Electrophysiological properties were first characterized in the quiescent spinal cord, and then fictive swimming was induced by perfusion with D-glutamate and the measurements were repeated. During the depolarizing excitatory phase of fictive swimming, the motoneurons had significantly reduced rheobase and significantly increased input resistance compared with the quiescent state, with no significant changes in these parameters during the repolarizing inhibitory phase of swimming. Spike threshold did not change significantly during fictive swimming compared with the quiescent state. During fictive swimming, the slope of the spike frequency versus injected current (F-I) relationship decreased significantly as did spike-frequency adaptation and the amplitude of the slow after-spike hyperpolarization (sAHP). Serotonin is known to be released endogenously from the spinal cord during fictive swimming and is known to reduce the amplitude of the sAHP. Therefore the effects of serotonin on cellular properties were tested in the quiescent spinal cord. It was found that, in addition to reducing the sAHP amplitude, serotonin also reduced the slope of the F-I relationship and reduced spike-frequency adaptation, reproducing the changes observed in these parameters during fictive swimming. Application of spiperone, a serotonin antagonist, significantly increased the sAHP amplitude during fictive swimming but had no significant effect on F-I slope or adaptation. Because serotonin may act in part through reduction of calcium currents, the effect of calcium-free solution (cobalt substituted for calcium) was tested in the quiescent spinal cord. Similar to fictive swimming and serotonin application, the calcium-free solution significantly reduced the sAHP amplitude, the slope of the F-I relationship, and spike-frequency adaptation. These results suggest that there are significant changes in the firing properties of motoneurons during fictive swimming compared with the quiescent state, and it is possible that these changes may be attributed in part to the endogenous release of serotonin acting via reduction of calcium currents.