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

Molecular, Morphological and Electrophysiological Differences between Alpha and Gamma Motoneurons with Special Reference to the Trigeminal Motor Nucleus of Rat

The muscle contraction during voluntary movement is controlled by activities of alpha- and gamma-motoneurons (αMNs and γMNs, respectively). In spite of the recent advances in research on molecular markers that can distinguish between αMNs and γMNs, electrophysiological membrane properties and firing patterns of γMNs have remained unknown, while those of αMNs have been clarified in detail. Because of the larger size of αMNs compared to γMNs, blindly or even visually recorded MNs were mostly αMNs, as demonstrated with molecular markers recently. Subsequently, the research on αMNs has made great progress in classifying their subtypes based on the molecular markers and electrophysiological membrane properties, whereas only a few studies demonstrated the electrophysiological membrane properties of γMNs. In this review article, we provide an overview of the recent advances in research on the classification of αMNs and γMNs based on molecular markers and electrophysiological membrane properties, and discuss their functional implication and significance in motor control.

Kisspeptin-13 prevented the electrophysiological alterations induced by Amyloid-Beta pathology in rat: Possible involvement of stromal interaction molecules and pCREB

Alzheimer's disease (AD) is a progressive neurological disease that slowly causing memory impairments with no effective treatment. We have recently reported that kisspeptin-13 (KP-13) ameliorates Aβ toxicity-induced memory deficit in rats. Here, the possible cellular impact of kisspeptin receptor activation in a rat model of the early stage AD was assessed using whole-cell patch-clamp recording from CA1 pyramidal neurons and molecular approaches. Compared to neurons from the control group, cells from the Aβ-treated group displayed spontaneous and evoked hyperexcitability with lower spike frequency adaptation. These cells had also a lower sag ratio in response to hyperpolarizing prepulse current delivered before a depolarizing current injection. Neurons from the Aβ-treated group exhibited short spike onset latency, lower rheobase and short utilization time compared with those in the control group. Furthermore, phase plot analysis of action potential showed that Aβ treatment affected the action potential features. These electrophysiological changes induced by Aβ were associated with increased expression of stromal interaction molecules (STIMs), particularly (STIM2) and decreased pCREB/CREB ratio. Treatment with KP-13 following Aβ injection into the entorhinal cortex, however, prevented the excitatory effect of Aβ on spontaneous and evoked neuronal activity, increased the latency of onset, enhanced the sag ratio, increased the rheobase and utilization time, and prevented the changes induced Aβ on spike parameters. In addition, the KP-13 application after Aβ treatment reduced the expression of STIMs and increased the pCREB/CREB ratio compared to those receiving Aβ treatment alone. In summary, these results provide evidence that activation of kisspeptin receptor may be effective against pathology of Aβ.

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.

Electrical and Morphological Properties of Developing Motoneurons in Postnatal Mice and Early Abnormalities in SOD1 Transgenic Mice

In this chapter, we review electrical and morphological properties of lumbar motoneurons during postnatal development in wild-type (WT) and transgenic superoxide dismutase 1 (SOD1) mice, models of amyotrophic lateral sclerosis. First we showed that sensorimotor reflexes do not develop normally in transgenic SOD1^G85R pups. Fictive locomotor activity recorded in in vitro whole brainstem/spinal cord preparations was not induced in these transgenic SOD1^G85R mice using NMDA and 5HT in contrast to WT mice. Further, abnormal electrical properties were detected as early as the second postnatal week in lumbar motoneurons of SOD1 mice while they develop clinical symptoms several months after birth. We compared two different strains of mice (G85R and G93A) at the same postnatal period using intracellular recordings and patch clamp recordings of WT and SOD1 motoneurons. We defined three types of motoneurons according to their discharge firing pattern (transient, sustained and delayed onset firing) when motor units are not yet mature. The delayed-onset firing motoneurons had the higher rheobase compared to the transient and sustained firing groups in the WT mice. We demonstrated hypoexcitability in the delayed onset-firing motoneurons of SOD1 mice. Intracellular staining of motoneurons revealed dendritic overbranching in SOD1 lumbar motoneurons that was more pronounced in the sustained firing motoneurons. We suggested that motoneuronal hypoexcitability is an early pathological sign affecting a subset of lumbar motoneurons in the spinal cord of SOD1 mice.

Diversity of Mammalian Motoneurons and Motor Units

Although they share the common function of controlling muscle fiber contraction, spinal motoneurons display a remarkable diversity. Alpha-motoneurons are the "final common pathway", which relay all the information from spinal and supraspinal centers and allow the organism to interact with the outside world by controlling the contraction of muscle fibers in the muscles. On the other hand, gamma-motoneurons are specialized motoneurons that do not generate force and instead specifically innervate muscle fibers inside muscle spindles, which are proprioceptive organs embedded in the muscles. Beta-motoneurons are hybrid motoneurons that innervate both extrafusal and intrafusal muscle fibers. Even among alpha-motoneurons, there exists an exquisite diversity in terms of motoneuron electrical and molecular properties, physiological and structural properties of their neuromuscular junctions, and molecular and contractile properties of the innervated muscle fibers. This diversity, across species, across muscles, and across muscle fibers in a given muscle, underlie the vast repertoire of movements that one individual can perform.

Maturation of persistent and hyperpolarization-activated inward currents shapes the differential activation of motoneuron subtypes during postnatal development

The size principle underlies the orderly recruitment of motor units; however, motoneuron size is a poor predictor of recruitment amongst functionally defined motoneuron subtypes. Whilst intrinsic properties are key regulators of motoneuron recruitment, the underlying currents involved are not well defined. Whole-cell patch-clamp electrophysiology was deployed to study intrinsic properties, and the underlying currents, that contribute to the differential activation of delayed and immediate firing motoneuron subtypes. Motoneurons were studied during the first three postnatal weeks in mice to identify key properties that contribute to rheobase and may be important to establish orderly recruitment. We find that delayed and immediate firing motoneurons are functionally homogeneous during the first postnatal week and are activated based on size, irrespective of subtype. The rheobase of motoneuron subtypes becomes staggered during the second postnatal week, which coincides with the differential maturation of passive and active properties, particularly persistent inward currents. Rheobase of delayed firing motoneurons increases further in the third postnatal week due to the development of a prominent resting hyperpolarization-activated inward current. Our results suggest that motoneuron recruitment is multifactorial, with recruitment order established during postnatal development through the differential maturation of passive properties and sequential integration of persistent and hyperpolarization-activated inward currents.

Functional Organization of Extraocular Motoneurons and Eye Muscles

Eye movements are indispensable for visual image stabilization during self-generated and passive head and body motion and for visual orientation. Eye muscles and neuronal control elements are evolutionarily conserved, with novel behavioral repertoires emerging during the evolution of frontal eyes and foveae. The precise execution of eye movements with different dynamics is ensured by morphologically diverse yet complementary sets of extraocular muscle fibers and associated motoneurons. Singly and multiply innervated muscle fibers are controlled by motoneuronal subpopulations with largely selective premotor inputs from task-specific ocular motor control centers. The morphological duality of the neuromuscular interface is matched by complementary biochemical and molecular features that collectively assign different physiological properties to the motor entities. In contrast, the functionality represents a continuum where most motor elements contribute to any type of eye movement, although within preferential dynamic ranges, suggesting that signal transmission and muscle contractions occur within bands of frequency-selective pathways.

Fast A-type currents shape a rapidly adapting form of delayed short latency firing of excitatory superficial dorsal horn neurons that express the neuropeptide Y Y1 receptor

KEY POINTS

Neuropeptide Y Y1 receptor-expressing neurons in the dorsal horn of the spinal cord contribute to chronic pain. For the first time, we characterized the firing patterns of Y1-expressing neurons in Y1eGFP reporter mice. Under hyperpolarized conditions, most Y1eGFP neurons exhibited fast A-type potassium currents and delayed, short-latency firing (DSLF). Y1eGFP DSLF neurons were almost always rapidly adapting and often exhibited rebound spiking, characteristics of spinal pain neurons under the control of T-type calcium channels. These results will inspire future studies to determine whether tissue or nerve injury downregulates the channels that underlie A-currents, thus unmasking membrane hyperexcitability in Y1-expressing dorsal horn neurons, leading to persistent pain.

ABSTRACT

Neuroanatomical and behavioural evidence indicates that neuropeptide Y Y1 receptor-expressing interneurons (Y1-INs) in the superficial dorsal horn (SDH) are predominantly excitatory and contribute to chronic pain. Using an adult ex vivo spinal cord slice preparation from Y1eGFP reporter mice, we characterized firing patterns in response to steady state depolarizing current injection of GFP-positive cells in lamina II, the great majority of which expressed Y1 mRNA (88%). Randomly sampled (RS) and Y1eGFP neurons exhibited five firing patterns: tonic, initial burst, phasic, delayed short-latency <180 ms (DSLF) and delayed long-latency >180 ms (DLLF). When studied at resting membrane potential, most RS neurons exhibited delayed firing, while most Y1eGFP neurons exhibited phasic firing. A preconditioning membrane hyperpolarization produced only subtle changes in the firing patterns of RS neurons, but dramatically shifted Y1eGFP neurons to DSLF (46%) and DLLF (24%). In contrast to RS DSLF neurons, which rarely exhibited spike frequency adaptation, Y1eGFP DSLF neurons were almost always rapidly adapting, a characteristic of nociceptive-responsive SDH neurons. Rebound spiking was more prevalent in Y1eGFP neurons (6% RS vs. 32% Y1eGFP), indicating enrichment of T-type calcium currents. Y1eGFP DSLF neurons exhibited fast A-type potassium currents that are known to delay or limit action potential firing and exhibited smaller current density as compared to RS DSLF neurons. Our results will inspire future studies to determine whether tissue or nerve injury downregulates channels that contribute to A-currents, thus potentially unmasking T-type calcium channel activity and membrane hyperexcitability in Y1-INs, leading to persistent pain.

Ca2+ Channels Involvement in Low-Frequency Stimulation-Mediated Suppression of Intrinsic Excitability of Hippocampal CA1 Pyramidal Cells in a Rat Amygdala Kindling Model

Low-frequency stimulation has demonstrated promising seizure suppression in animal models of epilepsy, while the mechanism of the effect is still debated. Changes in intrinsic properties have been recognized as a prominent pathophysiologically relevant feature of numerous neurological disorders including epilepsy. Here, it was evaluated whether LFS can preserve the intrinsic neuronal electrophysiological properties in a rat model of epilepsy, focusing on the possible involvement of voltage-gated Ca²⁺ channels. The amygdala kindling model was induced by 3 s monophasic square wave pulses (50 Hz, 1 ms duration, 12times/day at 5 min intervals). Both LFS alone and kindled plus LFS (KLFS) groups received four packages of LFS (each consisting of 200 monophasic square pulses, 0.1 ms pulse duration at 1 Hz with the after discharge threshold intensity), which in KLFS rats was applied immediately after kindling induction. Whole-cell patch-clamp recordings were made in the presence of fast synaptic blockers 24 h after the last kindling stimulations or following kindling stimulations plus LFS application. In the KLFS group, both the rebound excitation and kindling-induced intrinsic hyperexcitability were decreased, associated with a regular intrinsic firing as indicated by a lower coefficient of variation. The amplitude of afterdepolarization (ADP) and its area under the curve were both decreased in the KLFS group compared to the kindled group. LFS prevented the increasing effect of kindling on Ca²⁺ currents in the KLFS group. Findings provided evidence for a novel form of epileptiform activity suppression by LFS in the presence of synaptic blockade possibly by decreasing Ca²⁺ currents.

Orexinergic Modulation of Spinal Motor Activity in the Neonatal Mouse Spinal Cord

The role of orexin during development, and especially in terms of spinal cord function, is not well understood. It is for this reason that we focused on the network actions of orexin during the first week of development. We found that orexinergic fibers were present in the lumbar spinal cord of postnatal day 0 (P0) to P3 mice. The fibers were expressed mainly in the dorsal horn, but occasional fibers were observed in the ventral horn. Both orexin (OX) A and OXB increased the motoneurons (MNs) tonic neurogram discharge. However, only OXA was found to significantly increase spontaneous bursting activity and the frequency of fictive locomotor bursts. We show that OXA is able to act directly on MNs. To test the contribution of the recurrent MN collaterals, we blocked the nicotinic cholinergic drive and observed that OXA retained its ability to increase fictive locomotor activity. Additionally, we recorded neurograms from ventral lateral funiculi, where OXA had no effect on population discharge. These effects were also confirmed by recording from descending commissural interneurons via patch recordings. The loci of the effects of OXA were further investigated in a dorsal horn-removed preparation where OXA also shows an increase in the discharge from ventral root neurograms but no increase in the frequency of spontaneous or fictive locomotion burst activity. In summary, multiple lines of evidence from our work demonstrate the robust effects of orexins on spinal cord networks and MNs at the time of birth.

Kv1.2 Channels Promote Nonlinear Spiking Motoneurons for Powering Up Locomotion

Spinal motoneurons are endowed with nonlinear spiking behaviors manifested by a spike acceleration whose functional significance remains uncertain. Here, we show in rodent lumbar motoneurons that these nonlinear spiking properties do not rely only on activation of dendritic nifedipine-sensitive L-type Ca2+ channels, as assumed for decades, but also on the slow inactivation of a nifedipine-sensitive K+ current mediated by Kv1.2 channels that are highly expressed in axon initial segments. Specifically, the pharmacological and computational inhibition of Kv1.2 channels occluded the spike acceleration of rhythmically active motoneurons and the correlated slow buildup of rhythmic motor output recorded at the onset of locomotor-like activity. This study demonstrates that slow inactivation of Kv1.2 channels provides a potent gain control mechanism in mammalian spinal motoneurons and has a behavioral role in enhancing locomotor drive during the transition from immobility to steady-state locomotion.

Electrophysiological and Morphological Properties of α and γ Motoneurons in the Rat Trigeminal Motor Nucleus

The muscle contraction during voluntary movement is regulated by activities of α- and γ-motoneurons (αMNs and γMNs, respectively). The tension of jaw-closing muscles can be finely tuned over a wide range. This excellent function is likely to be achieved by the specific populations of αMNs innervating jaw-closing muscles. Indeed, we have recently demonstrated that in the rat dorsolateral trigeminal motor nucleus (dl-TMN), the size distribution of αMNs was bimodal and the population of smaller αMNs showed a size distribution similar to that of γMNs, by immunohistochemically identifying αMNs and γMNs based on the expressions of estrogen-related receptor gamma (Err3) and neuronal DNA binding protein NeuN together with ChAT. This finding suggests the presence of αMNs as small as γMNs. However, differences in the electrophysiological membrane properties between αMNs and γMNs remain unknown also in the dl-TMN. Therefore, in the present study, we studied the electrophysiological membrane properties of MNs in the dl-TMN of infant rats at postnatal days 7–12 together with their morphological properties using whole-cell current-clamp recordings followed by immunohistochemical staining with an anti-NeuN and anti-ChAT antibodies. We found that the ChAT-positive and NeuN-positive αMNs were divided into two subclasses: the first one had a larger cell body and displayed a 4-aminopyridine (4-AP)-sensitive current while the second one had a smaller cell body and displayed a less prominent 4-AP-sensitive current and a low-threshold spike, suitable for their orderly recruitment. We finally found that γMNs showing ChAT-positive and NeuN-negative immunoreactivities had smaller cell bodies and displayed an afterdepolarization mediated by flufenamate-sensitive cation current. It is suggested that these electrophysiological and morphological features of MNs in the dl-TMN are well correlated with the precise control of occlusion.

The Preparation of Oblique Spinal Cord Slices for Ventral Root Stimulation

Electrophysiological recordings from spinal cord slices have proven to be a valuable technique to investigate a wide range of questions, from cellular to network properties. We show how to prepare viable oblique slices of the spinal cord of young mice (P2 - P11). In this preparation, the motoneurons retain their axons coming out from the ventral roots of the spinal cord. Stimulation of these axons elicits back-propagating action potentials invading the motoneuron somas and exciting the motoneuron collaterals within the spinal cord. Recording of antidromic action potentials is an immediate, definitive and elegant way to characterize motoneuron identity, which surpasses other identification methods. Furthermore, stimulating the motoneuron collaterals is a simple and reliable way to excite the collateral targets of the motoneurons within the spinal cord, such as other motoneurons or Renshaw cells. In this protocol, we present antidromic recordings from the motoneuron somas as well as Renshaw cell excitation, resulting from ventral root stimulation.

Time windows for postnatal changes in morphology and membrane excitability of genioglossal and oculomotor motoneurons

Time windows for postnatal changes in morphology and membrane excitability of genioglossal (GG) and oculomotor (OCM) motoneurons (MNs) are yet to be fully described. Analysis of data on brain slices in vitro of the 2 populations of MNs point to a well-defined developmental program that progresses with common age-related changes characterized by: (1) increase of dendritic surface along with length and reshaping of dendritic tree complexity; (2) disappearance of gap junctions early in development; (3) decrease of membrane passive properties, such as input resistance and time constant, together with an increase in the number of cells displaying sag, and modifications in rheobase; (4) action potential shortening and afterhyperpolarization; and (5) an increase in gain and maximum firing frequency. These modifications take place at different time windows for each motoneuronal population. In GG MNs, active membrane properties change mainly during the first postnatal week, passive membrane properties in the second week, and dendritic increasing length and size in the third week of development. In OCM MNs, changes in passive membrane properties and growth of dendritic size take place during the first postnatal week, while active membrane properties and rheobase change during the second and third weeks of development. The sequential order of changes is inverted between active and passive membrane properties, and growth in size does not temporally coincide for both motoneuron populations. These findings are discussed on the basis of environmental cues related to maturation of the respiratory and OCM systems.

Optimization of neuronal cultures from rat superior cervical ganglia for dual patch recording

Superior cervical ganglion neurons (SCGN) are often used to investigate neurotransmitter release mechanisms. In this study, we optimized the dissociation and culture conditions of rat SCGN cultures for dual patch clamp recordings. Two weeks in vitro are sufficient to achieve a significant CNTF-induced cholinergic switch and to develop mature and healthy neuronal profiles suited for detailed patch clamp analysis. One single pup provides sufficient material to prepare what was formerly obtained from 12 to 15 animals. The suitability of these cultures to study neurotransmitter release mechanisms was validated by presynaptically perturbing the interaction of the v-SNARE VAMP2 with the vesicular V-ATPase V0c subunit.

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.

Early intrinsic hyperexcitability does not contribute to motoneuron degeneration in amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS) the large motoneurons that innervate the fast-contracting muscle fibers (F-type motoneurons) are vulnerable and degenerate in adulthood. In contrast, the small motoneurons that innervate the slow-contracting fibers (S-type motoneurons) are resistant and do not degenerate. Intrinsic hyperexcitability of F-type motoneurons during early postnatal development has long been hypothesized to contribute to neural degeneration in the adult. Here, we performed a critical test of this hypothesis by recording from identified F- and S-type motoneurons in the superoxide dismutase-1 mutant G93A (mSOD1), a mouse model of ALS at a neonatal age when early pathophysiological changes are observed. Contrary to the standard hypothesis, excitability of F-type motoneurons was unchanged in the mutant mice. Surprisingly, the S-type motoneurons of mSDO1 mice did display intrinsic hyperexcitability (lower rheobase, hyperpolarized spiking threshold). As S-type motoneurons are resistant in ALS, we conclude that early intrinsic hyperexcitability does not contribute to motoneuron degeneration.

DOI:http://dx.doi.org/10.7554/eLife.04046.001

Amyotrophic lateral sclerosis (ALS), which is also known as Lou Gherig's disease or motoneuron disease, is a neurodegenerative disorder in which muscles throughout the body gradually waste away due to the death of the neurons that control their activity. The disease often begins with weakness of the arms or legs, but progresses to include difficulties with movements such as swallowing and breathing. Around half of those affected die within 3 or 4 years of diagnosis.

Although the causes of the disease are unclear, one leading theory is that the neurons that control muscle activity—motoneurons—are hyperexcitable during early development, and therefore fire too frequently. This causes too much calcium to enter the neurons and, because calcium is toxic to cells in high quantities, leads ultimately to the death of the neurons. But despite the popularity of this idea, and the fact that many therapeutic assays for ALS are based on attempts to reverse this process, there is no direct evidence that early hyperexcitability of motoneurons causes their death in ALS.

Leroy et al. have now tested this theory directly by taking advantage of the fact that not all motoneurons are affected by ALS. The large ‘F-type’ motoneurons that control fast-contracting muscle fibres degenerate in ALS, whereas the small ‘S-type’ motoneurons that control slow-contracting muscle fibres do not. A comparison of F-type and S-type motoneurons in a mouse model of ALS revealed that, surprisingly, S-type motoneurons are hyperexcitable in young ALS mice, whereas F-type motoneurons are not.

Given that S-type motoneurons are resistant to the effects of ALS, this indicates that early hyperexcitability cannot be the cause of motoneuron degeneration. Previous studies have tended to pool different types of motoneurons together, which might explain why this difference has not been seen before. Further experiments are now required to determine whether the hyperexcitability of S-type motoneurons persists into adulthood, and whether it might even contribute to their survival in ALS.

DOI:http://dx.doi.org/10.7554/eLife.04046.002

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.

Four subtypes of motor neurons exhibiting mutually exclusive firing patterns in the spinal ventral horn

Motor neurons (MNs) communications are thought to occur primarily through spike bursts and regularly firing action potential trains. Reports of both burst and nonburst firing MNs suggest that these neurons may regularly fire in a variety of controlled output patterns with unique characteristics. Based on the cellular response to somatic current injection in these neurons, four distinct MN subtypes are identified from the spinal ventral horn. Approximately 42% of MNs exhibited regular firing, with minimal current injection (rheobase) exhibited a short latency, and with stronger current intensities exhibited significant spike frequency adaptation (SFA). Another 30% of MNs exhibited delayed onset at rheobase with a weakly-adapting firing pattern as stimulation increased. The remaining 18% and 10% of MNs exhibited transient firing patterns or exhibited irregular firing patterns when strongly depolarized, respectively. Our results provide a basis for improvement in the classification and study of MNs.

Spike-timing precision and neuronal synchrony are enhanced by an interaction between synaptic inhibition and membrane oscillations in the amygdala

The basolateral complex of the amygdala (BLA) is a critical component of the neural circuit regulating fear learning. During fear learning and recall, the amygdala and other brain regions, including the hippocampus and prefrontal cortex, exhibit phase-locked oscillations in the high delta/low theta frequency band (∼2-6 Hz) that have been shown to contribute to the learning process. Network oscillations are commonly generated by inhibitory synaptic input that coordinates action potentials in groups of neurons. In the rat BLA, principal neurons spontaneously receive synchronized, inhibitory input in the form of compound, rhythmic, inhibitory postsynaptic potentials (IPSPs), likely originating from burst-firing parvalbumin interneurons. Here we investigated the role of compound IPSPs in the rat and rhesus macaque BLA in regulating action potential synchrony and spike-timing precision. Furthermore, because principal neurons exhibit intrinsic oscillatory properties and resonance between 4 and 5 Hz, in the same frequency band observed during fear, we investigated whether compound IPSPs and intrinsic oscillations interact to promote rhythmic activity in the BLA at this frequency. Using whole-cell patch clamp in brain slices, we demonstrate that compound IPSPs, which occur spontaneously and are synchronized across principal neurons in both the rat and primate BLA, significantly improve spike-timing precision in BLA principal neurons for a window of ∼300 ms following each IPSP. We also show that compound IPSPs coordinate the firing of pairs of BLA principal neurons, and significantly improve spike synchrony for a window of ∼130 ms. Compound IPSPs enhance a 5 Hz calcium-dependent membrane potential oscillation (MPO) in these neurons, likely contributing to the improvement in spike-timing precision and synchronization of spiking. Activation of the cAMP-PKA signaling cascade enhanced the MPO, and inhibition of this cascade blocked the MPO. We discuss these results in the context of spike-timing dependent plasticity and modulation by neurotransmitters important for fear learning, such as dopamine.

A multi-compartment model for interneurons in the dorsal lateral geniculate nucleus

GABAergic interneurons (INs) in the dorsal lateral geniculate nucleus (dLGN) shape the information flow from retina to cortex, presumably by controlling the number of visually evoked spikes in geniculate thalamocortical (TC) neurons, and refining their receptive field. The INs exhibit a rich variety of firing patterns: Depolarizing current injections to the soma may induce tonic firing, periodic bursting or an initial burst followed by tonic spiking, sometimes with prominent spike-time adaptation. When released from hyperpolarization, some INs elicit rebound bursts, while others return more passively to the resting potential. A full mechanistic understanding that explains the function of the dLGN on the basis of neuronal morphology, physiology and circuitry is currently lacking. One way to approach such an understanding is by developing a detailed mathematical model of the involved cells and their interactions. Limitations of the previous models for the INs of the dLGN region prevent an accurate representation of the conceptual framework needed to understand the computational properties of this region. We here present a detailed compartmental model of INs using, for the first time, a morphological reconstruction and a set of active dendritic conductances constrained by experimental somatic recordings from INs under several different current-clamp conditions. The model makes a number of experimentally testable predictions about the role of specific mechanisms for the firing properties observed in these neurons. In addition to accounting for the significant features of all experimental traces, it quantitatively reproduces the experimental recordings of the action-potential- firing frequency as a function of injected current. We show how and why relative differences in conductance values, rather than differences in ion channel composition, could account for the distinct differences between the responses observed in two different neurons, suggesting that INs may be individually tuned to optimize network operation under different input conditions.

Spike-time precision and network synchrony are controlled by the homeostatic regulation of the D-type potassium current

Homeostatic plasticity of neuronal intrinsic excitability (HPIE) operates to maintain networks within physiological bounds in response to chronic changes in activity. Classically, this form of plasticity adjusts the output firing level of the neuron through the regulation of voltage-gated ion channels. Ion channels also determine spike timing in individual neurons by shaping subthreshold synaptic and intrinsic potentials. Thus, an intriguing hypothesis is that HPIE can also regulate network synchronization. We show here that the dendrotoxin-sensitive D-type K+ current (ID) disrupts the precision of AP generation in CA3 pyramidal neurons and may, in turn, limit network synchronization. The reduced precision is mediated by the sequence of outward ID followed by inward Na+ current. The homeostatic downregulation of ID increases both spike-time precision and the propensity for synchronization in iteratively constructed networks in vitro. Thus, network synchronization is adjusted in area CA3 through activity-dependent remodeling of ID.

Early excitability changes in lumbar motoneurons of transgenic SOD1G85R and SOD1G(93A-Low) mice

This work characterizes the properties of wild-type (WT) mouse motoneurons in the second postnatal week and compares these at the same age and in the same conditions to those of two different SOD1 mutant lines used as models of human amyotrophic lateral sclerosis (ALS), the SOD1(G93A) low expressor line and SOD1(G85R) line, to describe any changes in the functional properties of mutant motoneurons (Mns) that may be related to the pathogenesis of human ALS. We show that very early changes in excitability occur in SOD1 mutant Mns that have different properties from those of WT animals. The SOD1(G93A-Low) low expressor line displays specific differences that are not found in other mutant lines including a more depolarized membrane potential, larger spike width, and slower spike rise slope. With current pulses SOD1(G93A-Low) were hyperexcitable, but both mutants had a lower gain with current ramps stimulation. Changes in the threshold and intensities of Na(+) and Ca(2+) persistent inward currents were also observed. Low expressor mutants show reduced total persistant inward currents compared with WT motoneurons in the same recording conditions and give arguments toward modifications of the balance between Na(+) and Ca(2+) persistent inward currents. During the second week postnatal, SOD1(G93A-Low) lumbar motoneurons appear more immature than those of SOD1(G85R) compared with WT and we propose that different time course of the disease, possibly linked with different toxic properties of the mutated protein in each model, may explain the discrepancies between excitability changes described in the different models.

Evidence for a critical period in the development of excitability and potassium currents in mouse lumbar superficial dorsal horn neurons

The output of superficial dorsal horn (SDH; laminae I-II) neurons is critical for processing nociceptive, thermal, and tactile information. Like other neurons, the combined effects of synaptic inputs and intrinsic membrane properties determine their output. It is well established that peripheral synaptic inputs to SDH neurons undergo extensive reorganization during pre- and postnatal development. It is unclear, however, how membrane properties or the subthreshold whole cell currents that shape SDH neuron output change during this period. Here we assess the intrinsic membrane properties and whole cell currents in mouse SDH neurons during late embryonic and early postnatal development (E15-P25). Transverse slices were prepared from lumbar spinal cord and whole cell recordings were obtained at 32 degrees C. During this developmental period resting membrane potential (RMP) became more hyperpolarized (by approximately 10 mV, E15-E17 vs. P21-P25) and input resistance decreased (1,074 +/- 78 vs. 420 +/- 27 MOmega). In addition, action potential (AP) amplitude and AP afterhyperpolarization increased, whereas AP half-width decreased. Before and after birth (E15-P10), AP discharge evoked by intracellular current injection was limited to a single AP at depolarization onset in many neurons (>41%). In older animals (P11-P25) this changed, with AP discharge consisting of brief bursts at current onset ( approximately 46% of neurons). Investigation of major subthreshold whole cell currents showed the rapid A-type potassium current (I(Ar)) dominated at all ages examined (90% of neurons at E15-E17, decreasing to >50% after P10). I(Ar) expression levels, based on peak current amplitude, increased during development. Steady-state inactivation and activation for I(Ar) were slightly less potent in E15-E17 versus P21-P25 neurons at potentials near RMP (-55 mV). Together, our data indicate that intrinsic properties and I(Ar) expression change dramatically in SDH neurons during development, with the greatest alterations occurring on either side of a critical period, P6-P10.

Recording Temperature Affects the Excitability of Mouse Superficial Dorsal Horn Neurons, In Vitro

Superficial dorsal horn (SDH) neurons in laminae I–II of the spinal cord play an important role in processing noxious stimuli. These neurons represent a heterogeneous population and are divided into various categories according to their action potential (AP) discharge during depolarizing current injection. We recently developed an in vivo mouse preparation to examine functional aspects of nociceptive processing and AP discharge in SDH neurons and to extend investigation of pain mechanisms to the genetic level of analysis. Not surprisingly, some in vivo data obtained at body temperature (37°C) differed from those generated at room temperature (22°C) in spinal cord slices. In the current study we examine how temperature influences SDH neuron properties by making recordings at 22 and 32°C in transverse spinal cord slices prepared from L3–L5 segments of adult mice (C57Bl/6). Patch-clamp recordings (KCH3SO4 internal) were made from visualized SDH neurons. At elevated temperature all SDH neurons had reduced input resistance and smaller, briefer APs. Resting membrane potential and AP afterhyperpolarization amplitude were temperature sensitive only in subsets of the SDH population. Notably, elevated temperature increased the prevalence of neurons that did not discharge APs during current injection. These reluctant firing neurons expressed a rapid A-type potassium current, which is enhanced at higher temperatures and thus restrains AP discharge. When compared with previously published whole cell recordings obtained in vivo (37°C) our results suggest that, on balance, in vitro data collected at elevated temperature more closely resemble data collected under in vivo conditions.

Morphology, Intrinsic Membrane Properties, and Rotation-Evoked Responses of Trochlear Motoneurons in the Turtle

Intrinsic properties and rotation-evoked responses of trochlear motoneurons were investigated in the turtle using an in vitro preparation consisting of the brain stem with attached temporal bones that retain functional semicircular canals. Motoneurons were divided into two classes based on intrinsic properties. The first class exhibited higher impedance (123.0 ± 11.0 MΩ), wider spikes (0.99 ± 0.05 ms), a single spike afterhyperpolarization (AHP), little or no spike frequency adaptation (SFA), and anomalous rectification, characterized by an initial “sag” in membrane potential in response to hyperpolarizing current injection. The second class exhibited lower impedance (21.8 ± 2.5 MΩ), narrower spikes (0.74 ± 0.03 ms), a double AHP, substantial SFA, and little or no rectification. Vestibular responses were evoked by horizontal sinusoidal rotation (1/12-1/3 Hz; peak velocity: 30–100°/s). Spiking in higher-impedance cells was recruited earlier in the response and exhibited a more limited dynamic range relative to that of lower impedance cells. Spiking evoked by injecting depolarizing current during rotation was blocked during contraversive motion and was consistent with a shunting inhibition. No morphological features were identified in neurobiotin-filled cells that correlated with the two physiological classes. Recovered motoneurons were multipolar but exhibited a less-complex dendritic morphology than ocular motoneurons of similarly sized mammals. The two physiologically defined cell classes have homologues in other vertebrates, suggesting that intrinsic membrane properties play an important role in oculomotor processing.

Modification of membrane excitability of neurons in the rat's dorsal cortex of the inferior colliculus by preceding hyperpolarization

The inferior colliculus (IC) is among the largest nuclei in the central auditory system and is considered to be a major integration center in the auditory pathway. To understand how IC contributes to auditory processing, we investigated the effects of preceding hyperpolarization on membrane excitability and firing behavior of neurons located in the dorsal cortex of the inferior colliculus (ICD). We made whole-cell patch clamp recordings from ICD neurons (n=96) in rat brain slices. We classified ICD neurons into three types, i.e. sustained-regular, sustained-adapting and buildup, according to their responses to depolarizing current injection. Nearly 91% of the neurons had sustained firing throughout the period of current injection, showing either regular or adapting pattern. About 9% of the neurons exhibited a buildup pattern, in which sustained firing started after a long delay. Rebound depolarization and spikes after hyperpolarization were seen in 51.7% of the sustained neurons, but were not seen in buildup neurons. When depolarizing current was preceded by a hyperpolarizing current, various forms of the modification on membrane excitability were observed. For non-rebound neurons, the membrane excitability was either suppressed or unchanged after pre-hyperpolarization. The first spike latency lengthened in neurons whose firing changed to a buildup pattern, shortened in those whose firing changed to a pauser pattern, and remained unchanged in those whose discharge pattern remained sustained. For rebound neurons, the firing rate decreased in neurons whose firing pattern was changed to onset or pauser, increased in neurons whose firing was changed to adapting, or remained unchanged in neurons whose firing became irregular. The first spike latency was shortened in all the rebound cells. The results suggest that intrinsic membrane properties can play an important role in integration of excitatory and inhibitory inputs and thereby in determination of the output of ICD neurons.

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.

Phasic and tonic firing properties in rat oculomotor nucleus motoneurons, studied in vitro

Alert-chronic studies show that ocular motoneurons (Mns) exhibit a phasic and tonic firing correlated with eye saccade-velocity and position (fixation), respectively. Differences in the phasic and tonic firing among Mns depend on synaptic inputs and/or the intrinsic membrane properties. We have used in vitro slice preparation to investigate the contribution of membrane properties to firing properties of Wistar rat oculomotor nucleus Mns. We recorded different discharge patterns and focused on Mns with sustained discharge (type I) because they were the most abundant, and their firing pattern resembles that reported in alert preparations. Various differences divided these Mns into types I(A) and I(B); the afterhyperpolarization (AHP) phase of the spike was monophasic in I(A) and biphasic in I(B); I(A) Mns showed tonic or phasic-tonic firing depending on the current intensity, while I(B) Mns showed phasic-tonic discharge; the phasic firing was higher in I(B) than in I(A) Mns; I(A) Mns fired in a narrower range than did I(B) Mns; and I(A) Mns showed lower maximum frequency than did I(B) Mns. In conclusion, I(A) and I(B) Mns show different phasic firing properties and dynamic range, supported by intrinsic membrane properties. We suggest that I(A) and I(B) Mns innervate fast-twitch muscle fibres with different contraction speeds, and could contribute to generating a fine phasic signal for a graded muscle contraction. Finally, we have demonstrated an inverse relationship between Mn thresholds and tonic firing gain, concluding that intrinsic membrane properties could not support the covariation between tonic firing gain and recruitment thresholds reported in alert studies.

Oscillatory and Intrinsic Membrane Properties of Guinea Pig Nucleus Prepositus Hypoglossi Neurons In Vitro

Numerous models of the oculomotor neuronal integrator located in the prepositus hypoglossi nucleus (PHN) involve both highly tuned recurrent networks and intrinsic neuronal properties; however, there is little experimental evidence for the relative role of these two mechanisms. The experiments reported here show that all PHN neurons (PHNn) show marked phasic behavior, which is highly oscillatory in ∼25% of the population. The behavior of this subset of PHNn, referred to as type D PHNn, is clearly different from that of the medial vestibular nucleus neurons, which transmit the bulk of head velocity-related sensory vestibular inputs without integrating them. We have investigated the firing and biophysical properties of PHNn and developed data-based realistic neuronal models to quantitatively illustrate that their active conductances can produce the oscillatory behavior. Although some individual type D PHNn are able to show some features of mathematical integration, the lack of robustness of this behavior strongly suggests that additional network interactions, likely involving all types of PHNn, are essential for the neuronal integrator. Furthermore, the relationship between the impulse activity and membrane potential of type D PHNn is highly nonlinear and frequency-dependent, even for relatively small-amplitude responses. These results suggest that some of the synaptic input to type D PHNn is likely to evoke oscillatory responses that will be nonlinearly amplified as the spike discharge rate increases. It would appear that the PHNn have specific intrinsic properties that, in conjunction with network interconnections, enhance the persistent neural activity needed for their function.

Temporal sequence of changes in electrophysiological properties of oculomotor motoneurons during postnatal development

The temporal sequence of changes in electrophysiological properties during postnatal development in different neuronal populations has been the subject of previous studies. Those studies demonstrated major physiological modifications with age, and postnatal periods in which such changes are more pronounced. Until now, no similar systematic study has been performed in motoneurons of the oculomotor nucleus. This work has two main aims: first, to determine whether the physiological changes in oculomotor nucleus motoneurons follow a similar time course for different parameters; and second, to compare the temporal sequence with that in other neuronal populations. We recorded the electrophysiological properties of 134 identified oculomotor nucleus motoneurons from 1 to 40 days postnatal in brain slices of rats. The resting membrane potential did not significantly change with postnatal development, and it had a mean value of -61.8 mV. The input resistance and time constant diminished from 82.9-53.1 M omega and from 9.4-4.9 ms respectively with age. These decrements occurred drastically in a short time after birth (1-5 days postnatally). The motoneurons' rheobase gradually decayed from 0.29-0.11 nA along postnatal development. From birth until postnatal day 15 and postnatal day 20 respectively, the action potential shortened from 2.3-1.2 ms, and the medium afterhyperpolarization from 184.8-94.4 ms. The firing gain and the maximum discharge increased with age. The former rose continuously, while the increase in maximum discharge was most pronounced between postnatal day 16 and postnatal day 20. We conclude that the developmental sequence was not similar for all electrophysiological properties, and was unique for each neuronal population.

A-type and T-type currents interact to produce a novel spike latency-voltage relationship in cerebellar stellate cells

The modification of first-spike latencies by low-threshold and inactivating K+ currents (IA) have important implications in neuronal coding and synaptic integration. To date, cells in which first-spike latency characteristics have been analyzed have shown that increased hyperpolarization results in longer first-spike latencies, producing a monotonic relationship between first-spike latency and membrane voltage. Previous work has established that cerebellar stellate cells express members of the Kv4 potassium channel subfamily, which underlie IA in many central neurons. Spike timing in stellate cells could be particularly important to cerebellar output, because the discharge of even single spikes can significantly delay spike discharge in postsynaptic Purkinje cells. In the present work, we studied the first-spike latency characteristics of stellate cells. We show that first-spike latency is nonmonotonic, such that intermediate levels of prehyperpolarization produce the longest spike latencies, whereas greater hyperpolarization or depolarization reduces spike latency. Moreover, the range of first-spike latency values can be substantial in spanning 20-128 ms with preceding membrane shifts of <10 mV. Using patch clamp and modeling, we illustrate that spike latency characteristics are the product of an interplay between IA and low-threshold calcium current (IT) that requires a steady-state difference in the inactivation parameters of the currents. Furthermore, we show that the unique first-spike latency characteristics of stellate cells have important implications for the integration of coincident IPSPs and EPSPs, such that inhibition can shift first-spike latency to differentially modulate the probability of firing.

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.

Effects of prostaglandin E2 on the electrical properties of thermally classified neurons in the ventromedial preoptic area of the rat hypothalamus

BACKGROUND

Physiological and morphological evidence suggests that activation of the ventromedial preoptic area of the hypothalamus (VMPO) is an essential component of an intravenous LPS-dependent fever. In response to the endogenous pyrogen prostaglandin E2 (PGE2), the majority of temperature insensitive neurons in the VMPO show an increase in firing rate, while warm sensitive neurons are inhibited. We have hypothesized that these PGE2 dependent effects on firing rate are due to changes in the inherent electrical properties of VMPO neurons, which are regulated by the activity of specific ionic currents.

RESULTS

To characterize the electrical properties of VMPO neurons, whole-cell recordings were made in tissue slices from male Sprague-Dawley rats. Our results indicate that PGE2 dependent firing rate responses were not the result of changes in resting membrane potential, action potential amplitude and duration, or local synaptic input. However, PGE2 reduced the input resistance of all VMPO neurons, while increasing the excitability of temperature insensitive neurons and decreasing the excitability of warm sensitive neurons. In addition, the majority of temperature insensitive neurons responded to PGE2 with an increase in the rate of rise of the depolarizing prepotential that precedes each action potential. This response to PGE2 was reversed for warm sensitive neurons, in which the prepotential rate of rise decreased.

CONCLUSION

We would therefore suggest that PGE2 is having an effect on the ionic currents that regulate firing rate by controlling how fast membrane potential rises to threshold during the prepotential phase of the action potential.

Suppression of visually and memory-guided saccades induced by electrical stimulation of the monkey frontal eye field. II. Suppression of bilateral saccades

To understand the neural mechanism of fixation, we investigated effects of electrical stimulation of the frontal eye field (FEF) and its vicinity on visually guided (Vsacs) and memory-guided saccades (Msacs) in trained monkeys and found that there were two types of suppression induced by the electrical stimulation: suppression of ipsilateral saccades and suppression of bilateral saccades. In this report, we characterized the properties of the suppression of bilateral Vsacs and Msacs. Stimulation of the bilateral suppression sites suppressed the initiation of both Vsacs and Msacs in all directions during and approximately 50 ms after stimulation but did not affect the vector of these saccades. The suppression was stronger for ipsiversive larger saccades and contraversive smaller saccades, and saccades with initial eye positions shifted more in the saccadic direction. The most effective stimulation timing for the suppression of ipsilateral and contralateral Vsacs was approximately 40-50 ms before saccade onset, indicating that the suppression occurred most likely in the superior colliculus and/or the paramedian pontine reticular formation. Suppression sites of bilateral saccades were located in the prearcuate gyrus facing the inferior arcuate sulcus where stimulation induced suppression at < or =40 microA but usually did not evoke any saccades at 80 microA and were different from those of ipsilateral saccades where stimulation evoked saccades at < or =50 microA. The bilateral suppression sites contained fixation neurons. The results suggest that fixation neurons in the bilateral suppression area of the FEF may play roles in maintaining fixation by suppressing saccades in all directions.