Increased electrotonic coupling in spinal motoneurons after transient botulinum neurotoxin paralysis in the neonatal rat

Eye Movements But Not Vision Drive the Development of Palisade Endings

Purpose

To test whether visual experience and/or eye movements drive the postnatal development of palisade endings in extraocular muscles.

Methods

In three newborn cats, the right eye was covered until 30 days from postnatal (P) day 7 (before opening their eyes), and in three cats both eyes were covered until 45 days, also from P7. To block eye movements, another seven cats received a retrobulbar injection of botulinum neurotoxin A (BoNT-A) into the left orbit at birth and survived for 45 days (three cats) and 95 days (four cats). The distal third of the rectus muscles containing the palisade endings was used for whole-mount preparation and triple-fluorescence labeling with anti-neurofilament along with (1) anti-synaptophysin and phalloidin or (2) anti-growth associated protein 43 (GAP43) and phalloidin. Immunolabeled specimens were analyzed in the confocal laser scanning microscope.

Results

After unilateral and bilateral dark rearing, palisade endings were qualitatively and quantitatively equal to those from age-matched controls. After BoNT-A induced eye immobilization for 45 or 95 days, palisade endings were absent in the superior rectus and lateral rectus muscles and only present in the inferior rectus and medial rectus muscle. These BoNT-A-treated palisade endings were rudimentary and reduced in number, and the expression of the neuronal developmental protein GAP43 was significantly reduced.

Conclusions

This study demonstrates that eye immobilization, but not visual deprivation, affects palisade ending development. Palisade endings develop in the first month of life, and the present findings indicate that, during this time window, palisade endings are prone to oculomotor perturbations.

On the Diverse Functions of Electrical Synapses

Electrical synapses are the neurophysiological product of gap junctional pores between neurons that allow bidirectional flow of current between neurons. They are expressed throughout the mammalian nervous system, including cortex, hippocampus, thalamus, retina, cerebellum, and inferior olive. Classically, the function of electrical synapses has been associated with synchrony, logically following that continuous conductance provided by gap junctions facilitates the reduction of voltage differences between coupled neurons. Indeed, electrical synapses promote synchrony at many anatomical and frequency ranges across the brain. However, a growing body of literature shows there is greater complexity to the computational function of electrical synapses. The paired membranes that embed electrical synapses act as low-pass filters, and as such, electrical synapses can preferentially transfer spike after hyperpolarizations, effectively providing spike-dependent inhibition. Other functions include driving asynchronous firing, improving signal to noise ratio, aiding in discrimination of dissimilar inputs, or dampening signals by shunting current. The diverse ways by which electrical synapses contribute to neuronal integration merits furthers study. Here we review how functions of electrical synapses vary across circuits and brain regions and depend critically on the context of the neurons and brain circuits involved. Computational modeling of electrical synapses embedded in multi-cellular models and experiments utilizing optical control and measurement of cellular activity will be essential in determining the specific roles performed by electrical synapses in varying contexts.

Control of mammalian locomotion by ventral spinocerebellar tract neurons

Locomotion is a complex behavior required for animal survival. Vertebrate locomotion depends on spinal interneurons termed the central pattern generator (CPG), which generates activity responsible for the alternation of flexor and extensor muscles and the left and right side of the body. It is unknown whether multiple or a single neuronal type is responsible for the control of mammalian locomotion. Here, we show that ventral spinocerebellar tract neurons (VSCTs) drive generation and maintenance of locomotor behavior in neonatal and adult mice. Using mouse genetics, physiological, anatomical, and behavioral assays, we demonstrate that VSCTs exhibit rhythmogenic properties and neuronal circuit connectivity consistent with their essential role in the locomotor CPG. Importantly, optogenetic activation and chemogenetic silencing reveals that VSCTs are necessary and sufficient for locomotion. These findings identify VSCTs as critical components for mammalian locomotion and provide a paradigm shift in our understanding of neural control of complex behaviors.

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.

Could electrical coupling contribute to the formation of cell assemblies?

Cell assemblies and central pattern generators (CPGs) are related types of neuronal networks: both consist of interacting groups of neurons whose collective activities lead to defined functional outputs. In the case of a cell assembly, the functional output may be interpreted as a representation of something in the world, external or internal; for a CPG, the output ‘drives’ an observable (i.e. motor) behavior. Electrical coupling, via gap junctions, is critical for the development of CPGs, as well as for their actual operation in the adult animal. Electrical coupling is also known to be important in the development of hippocampal and neocortical principal cell networks. We here argue that electrical coupling – in addition to chemical synapses – may therefore contribute to the formation of at least some cell assemblies in adult animals.

Gap junctions and hemichannels: communicating cell death in neurodevelopment and disease

Gap junctions are unique membrane channels that play a significant role in intercellular communication in the developing and mature central nervous system (CNS). These channels are composed of connexin proteins that oligomerize into hexamers to form connexons or hemichannels. Many different connexins are expressed in the CNS, with some specificity with regard to the cell types in which distinct connexins are found, as well as the timepoints when they are expressed in the developing and mature CNS. Both the main neuronal Cx36 and glial Cx43 play critical roles in neurodevelopment. These connexins also mediate distinct aspects of the CNS response to pathological conditions. An imbalance in the expression, translation, trafficking and turnover of connexins, as well as mutations of connexins, can impact their function in the context of cell death in neurodevelopment and disease. With the ever-increasing understanding of connexins in the brain, therapeutic strategies could be developed to target these membrane channels in various neurological disorders.

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.

Retrograde response in axotomized motoneurons: nitric oxide as a key player in triggering reversion toward a dedifferentiated phenotype

The adult brain retains a considerable capacity to functionally reorganize its circuits, which mainly relies on the prevalence of three basic processes that confer plastic potential: synaptic plasticity, plastic changes in intrinsic excitability and, in certain central nervous system (CNS) regions, also neurogenesis. Experimental models of peripheral nerve injury have provided a useful paradigm for studying injury-induced mechanisms of central plasticity. In particular, axotomy of somatic motoneurons triggers a robust retrograde reaction in the CNS, characterized by the expression of plastic changes affecting motoneurons, their synaptic inputs and surrounding glia. Axotomized motoneurons undergo a reprograming of their gene expression and biosynthetic machineries which produce cell components required for axonal regrowth and lead them to resume a functionally dedifferentiated phenotype characterized by the removal of afferent synaptic contacts, atrophy of dendritic arbors and an enhanced somato-dendritic excitability. Although experimental research has provided valuable clues to unravel many basic aspects of this central response, we are still lacking detailed information on the cellular/molecular mechanisms underlying its expression. It becomes clear, however, that the state-switch must be orchestrated by motoneuron-derived signals produced under the direction of the re-activated growth program. Our group has identified the highly reactive gas nitric oxide (NO) as one of these signals, by providing robust evidence for its key role to induce synapse elimination and increases in intrinsic excitability following motor axon damage. We have elucidated operational principles of the NO-triggered downstream transduction pathways mediating each of these changes. Our findings further demonstrate that de novo NO synthesis is not only "necessary" but also "sufficient" to promote the expression of at least some of the features that reflect reversion toward a dedifferentiated state in axotomized adult motoneurons.

Variety of alternative stable phase-locking in networks of electrically coupled relaxation oscillators

We studied the dynamics of a large-scale model network comprised of oscillating electrically coupled neurons. Cells are modeled as relaxation oscillators with short duty cycle, so they can be considered either as models of pacemaker cells, spiking cells with fast regenerative and slow recovery variables or firing rate models of excitatory cells with synaptic depression or cellular adaptation.

It was already shown that electrically coupled relaxation oscillators exhibit not only synchrony but also anti-phase behavior if electrical coupling is weak. We show that a much wider spectrum of spatiotemporal patterns of activity can emerge in a network of electrically coupled cells as a result of switching from synchrony, produced by short external signals of different spatial profiles. The variety of patterns increases with decreasing rate of neuronal firing (or duty cycle) and with decreasing strength of electrical coupling. We study also the effect of network topology - from all-to-all – to pure ring connectivity, where only the closest neighbors are coupled.

We show that the ring topology promotes anti-phase behavior as compared to all-to-all coupling. It also gives rise to a hierarchical organization of activity: during each of the main phases of a given pattern cells fire in a particular sequence determined by the local connectivity. We have analyzed the behavior of the network using geometric phase plane methods and we give heuristic explanations of our findings.

Our results show that complex spatiotemporal activity patterns can emerge due to the action of stochastic or sensory stimuli in neural networks without chemical synapses, where each cell is equally coupled to others via gap junctions. This suggests that in developing nervous systems where only electrical coupling is present such a mechanism can lead to the establishment of proto-networks generating premature multiphase oscillations whereas the subsequent emergence of chemical synapses would later stabilize generated patterns.

Re-evaluation of connexins associated with motoneurons in rodent spinal cord, sexually dimorphic motor nuclei and trigeminal motor nucleus

Electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36) are a common feature in mammalian brain circuitry, but less is known about their deployment in spinal cord. It has been reported based on connexin mRNA and/or protein detection that developing and/or mature motoneurons express a variety of connexins, including Cx26, Cx32, Cx36 and Cx43 in trigeminal motoneurons, Cx36, Cx37, Cx40, Cx43 and Cx45 in spinal motoneurons, and Cx32 in sexually dimorphic motoneurons. We re-examined the localization of these connexins during postnatal development and in adult rat and mouse using immunofluorescence labeling for each connexin. We found Cx26 in association only with leptomeninges in the trigeminal motor nucleus (Mo5), Cx32 only with oligodendrocytes and myelinated fibers among motoneurons in this nucleus and in the spinal cord, and Cx37, Cx40 and Cx45 only with blood vessels in the ventral horn of spinal cord, including those among motoneurons. By freeze-fracture replica immunolabeling, > 100 astrocyte gap junctions but no neuronal gap junctions were found based on immunogold labeling for Cx43, whereas 16 neuronal gap junctions at postnatal day (P)4, P7 and P18 were detected based on Cx36 labeling. Punctate labeling for Cx36 was localized to the somatic and dendritic surfaces of peripherin-positive motoneurons in the Mo5, motoneurons throughout the spinal cord, and sexually dimorphic motoneurons at lower lumbar levels. In studies of electrical synapses and electrical transmission between developing and between adult motoneurons, our results serve to focus attention on mediation of this transmission by gap junctions composed of Cx36.

Neuronal gap junction coupling as the primary determinant of the extent of glutamate-mediated excitotoxicity

In the mammalian central nervous system (CNS), coupling of neurons by gap junctions (electrical synapses) increases during early postnatal development, then decreases, but increases in the mature CNS following neuronal injury, such as ischemia, traumatic brain injury and epilepsy. Glutamate-dependent neuronal death also occurs in the CNS during development and neuronal injury, i.e., at the time when neuronal gap junction coupling is increased. Here, we review our recent studies on regulation of neuronal gap junction coupling by glutamate in developing and injured neurons and on the role of gap junctions in neuronal cell death. A modified model of the mechanisms of glutamate-dependent neuronal death is discussed, which includes neuronal gap junction coupling as a critical part of these mechanisms.

Neuronal gap junctions: making and breaking connections during development and injury

In the mammalian central nervous system (CNS), coupling of neurons by gap junctions (i.e., electrical synapses) and the expression of the neuronal gap junction protein, connexin 36 (Cx36), transiently increase during early postnatal development. The levels of both subsequently decline and remain low in the adult, confined to specific subsets of neurons. However, following neuronal injury [such as ischemia, traumatic brain injury (TBI), and epilepsy], the coupling and expression of Cx36 rise. Here we summarize new findings on the mechanisms of regulation of Cx36-containing gap junctions in the developing and mature CNS and following injury. We also review recent studies suggesting various roles for neuronal gap junctions and in particular their role in glutamate-mediated neuronal death.

Novel model for the mechanisms of glutamate-dependent excitotoxicity: role of neuronal gap junctions

In the mammalian central nervous system (CNS), coupling of neurons by gap junctions (electrical synapses) increases during early post-natal development, then decreases, but increases in the mature CNS following neuronal injury, such as ischemia, traumatic brain injury and epilepsy. Glutamate-dependent neuronal death also occurs in the CNS during development and neuronal injury, i.e., at the time when neuronal gap junction coupling is increased. Here, we review our recent studies on the regulation of neuronal gap junction coupling by glutamate during development and injury and on the role of gap junctions in neuronal cell death. A novel model of the mechanisms of glutamate-dependent neuronal death is discussed, which includes neuronal gap junction coupling as a critical part of these mechanisms.

Gap junction-mediated intercellular communication in the adrenal medulla: an additional ingredient of stimulus-secretion coupling regulation

The traditional understanding of stimulus-secretion coupling in adrenal neuroendocrine chromaffin cells states that catecholamines are released upon trans-synaptic sympathetic stimulation mediated by acetylcholine released from the splanchnic nerve terminals. Although this statement remains largely true, it deserves to be tempered. In addition to its neurogenic control, catecholamine secretion also depends on a local gap junction-mediated communication between chromaffin cells. We review here the insights gained since the first description of gap junctions in the adrenal medullary tissue. Adrenal stimulus-secretion coupling now appears far more intricate than was previously envisioned and its deciphering represents a challenge for neurobiologists engaged in the study of the regulation of neuroendocrine secretion. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

The use of PRV-Bartha to define premotor inputs to lumbar motoneurons in the neonatal spinal cord of the mouse

Background

The neonatal mouse has become a model system for studying the locomotor function of the lumbar spinal cord. However, information about the synaptic connectivity within the governing neural network remains scarce. A neurotropic pseudorabies virus (PRV) Bartha has been used to map neuronal connectivity in other parts of the nervous system, due to its ability to travel trans-neuronally. Its use in spinal circuits regulating locomotion has been limited and no study has defined the time course of labelling for neurons known to project monosynaptically to motoneurons.

Methodology/Principal Findings

Here we investigated the ability of PRV Bartha, expressing green and/or red fluorescence, to label spinal neurons projecting monosynaptically to motoneurons of two principal hindlimb muscles, the tibialis anterior (TA) and gastrocnemius (GC). As revealed by combined immunocytochemistry and confocal microscopy, 24–32 h after the viral muscle injection the label was restricted to the motoneuron pool while at 32–40 h the fluorescence was seen in interneurons throughout the medial and lateral ventral grey matter. Two classes of ipsilateral interneurons known to project monosynaptically to motoneurons (Renshaw cells and cells of origin of C-terminals) were consistently labeled at 40 h post-injection but also a group in the ventral grey matter contralaterally. Our results suggest that the labeling of last order interneurons occurred 8–12 h after motoneuron labeling and we presume this is the time taken by the virus to cross one synapse, to travel retrogradely and to replicate in the labeled cells.

Conclusions/Significance

The study establishes the time window for virally - labelling monosynaptic projections to lumbar motoneurons following viral injection into hindlimb muscles. Moreover, it provides a good foundation for intracellular targeting of the labeled neurons in future physiological studies and better understanding the functional organization of the lumbar neural networks.

Experience-dependent maturation of the glomerular microcircuit

Spontaneous and patterned activity, largely attributed to chemical transmission, shape the development of virtually all neural circuits. However, electrical transmission also has an important role in coordinated activity in the brain. In the olfactory bulb, gap junctions between apical dendrites of mitral cells increase excitability and synchronize firing within each glomerulus. We report here that the development of the glomerular microcircuit requires both sensory experience and connexin (Cx)36-mediated gap junctions. Coupling coefficients, which measure electrical coupling between mitral cell dendrites, were high in young mice, but decreased after postnatal day (P)10 because of a maturational increase in membrane conductance. Sensory deprivation, induced by unilateral naris occlusion at birth, slowed the morphological development of mitral cells and arrested the maturational changes in membrane conductance and coupling coefficients. As the coupling coefficients decreased in normal mice, a glutamate-mediated excitatory postsynaptic current (EPSC) between mitral cells emerged by P30. Although mitral-mitral EPSCs were generally unidirectional, they were not present in young adult Cx36(-/-) mice, suggesting that gap junctions are required for the development and/or function of the mature circuit. The experience-dependent transition from electrical transmission to combined chemical and electrical transmission provides a previously unappreciated mechanism that may tune the response properties of the glomerular microcircuit.

BOTULINUM TOXIN: FROM SPASTICITY RELIEVER TO A NEUROMOTOR RE-LEARNING TOOL

Botulinum toxin is becoming increasingly popular as the drug of choice for relief of spasticity in a wide range of conditions, from stroke to strabismus to vaginismus. Besides this role as spasticity reliever, several recent clinical reports claim that in stroke, cerebral palsy, spinal cord injury (SCI), and dystonias, BoTx brings about significant improvement in function--attributed to synaptic plasticity of the muscular afferents. The authors' research had shown that BoTx also generates synaptic plasticity in spinal alpha-motoneurons-interneurons. The article describes how BoTx facilitates relearning by Hebbian and Contrastive Hebbian modes and how it can be used as a neuro-relearning tool to enhance and hasten motor recovery in the aforementioned disorders.

A chronically implantable device for the controlled delivery of substances, and stimulation and recording of activity in severed nerves

We describe the use of an implantable device for peripheral nerves that allows chronic simultaneous delivery of small volumes of solution, recording of both field and multiunit potentials, and electrical stimulation. This custom-made multifunctional device was attached to the cut end of the abducens (VIth) nerve for stimulation, recording and injection purposes. Our device consists of a polyethylene chamber with two electrodes that can be used for stimulation and recording and two Teflon tubes that serve as inlet and outlet for administering chemicals to the nerve fitted inside. Since the device is implanted in a retro-orbital position, we herein will refer to it as an intraorbitary device (IOD). The applicability of the IOD is demonstrated with an electrophysiological and anatomical account of the properties of the abducens nerve. Furthermore, it is shown that certain neuronal discharge properties can be inferred from the nerve recordings. The IOD can also be efficiently used for the delivery of small volume of pharmacological substances or conventional retrograde markers.

Transduction of motor neurons and muscle fibers by intramuscular injection of HIV-1-based vectors pseudotyped with select rabies virus glycoproteins

For studies of motor neuron function or for therapeutic purposes, novel pseudotype HIV-1-based vectors were developed that are capable of expressing transgenes in motor neurons following injection into mouse hind limb muscles. To specifically target motor neurons, glycoproteins from two rabies virus (RV) isolates, the mouse-brain adapted challenge virus 24 (CVS-24) variants, CVS-N2c and CVS-B2c were evaluated for pseudotype formation with an HIV-1-based vector. Both RV glycoproteins incorporated into vector envelopes, and both pseudotypes yielded high titers with Hek293T and cortical plate neuron cultures. Increased neuronotropism by the CVS-N2c pseudotype was not observed, suggesting that vector tropism is not solely determined by the fusogenic viral glycoprotein. Vector injection into hind limb muscles resulted in EYFP reporter gene expression in the injected muscle fibers and in spinal cord motor neurons innervating the same muscle, indicating retrograde vector transport. Intramuscular vector injections into the soleus and tibialis anterior muscles transduced 26% and 16% of all motor neurons in each motor nucleus, respectively. These transduction efficiencies may allow novel approaches to functional studies of the motor system and the treatment of neuromuscular disease.

Electrical coupling between locomotor-related excitatory interneurons in the mammalian spinal cord

Locomotor rhythm generation is a fundamental characteristic of neural networks in the spinal cord. Identifying the synaptic interactions between neurons in the locomotor circuitry is key to our understanding of the mechanisms that underlie the production of rhythmic motor outputs. Using transgenic mice in which the homeobox gene HB9 drives the reporter green fluorescent protein (GFP), we have demonstrated that a genetically distinct cluster of Hb9/GFP-expressing interneurons (Hb9 INs) can generate locomotor-like rhythms in the newborn mouse spinal cord (Hinckley et al., 2005b). Processes of Hb9 INs are in close apposition to adjacent Hb9 INs, raising the possibility that the interneurons are synaptically interconnected. To test this hypothesis, whole-cell paired recordings were performed from visually identified Hb9 INs. High-incidence bidirectional electrical coupling was evident between Hb9 INs in spinal cords of newborn and juvenile mice. The coupling strength varied from 2 to 32% with an average of 12%. Our data suggested that the variability was not correlated with the distribution of electrical synapses at different electronic distances. Electrical synapses behaved as low-pass filters, reducing currents passing at frequencies >3 Hz. Episodes of spontaneous bursts of EPSCs were synchronous in coupled Hb9 INs, indicating that common synaptic inputs coordinated their activity. However, non-NMDA receptor-mediated synaptic transmission was not required to synchronize neurochemically induced membrane oscillations between electrically coupled interneurons. The finding that electrical transmission persists in mice that can walk is indicative of its importance in coordinating the activity of this neuronal population in functionally mature spinal networks.

Flipping the switch from electrical to chemical communication

Immature neurons in many brain regions are electrically coupled through gap junctions, which are lost as chemical synaptic transmission matures. This developmental uncoupling is now shown to require NMDA receptor activation.

Regulation of gephyrin cluster size and inhibitory synaptic currents on Renshaw cells by motor axon excitatory inputs

Renshaw cells receive a high density of inhibitory synapses characterized by large postsynaptic gephyrin clusters and mixed glycinergic/GABAergic inhibitory currents with large peak amplitudes and long decays. These properties appear adapted to increase inhibitory efficacy over Renshaw cells and mature postnatally by mechanisms that are unknown. We tested the hypothesis that heterosynaptic influences from excitatory motor axon inputs modulate the development of inhibitory synapses on Renshaw cells. Thus, tetanus (TeNT) and botulinum neurotoxin A (BoNT-A) were injected intramuscularly at postnatal day 5 (P5) to, respectively, elevate or reduce motor axon firing activity for approximately 2 weeks. After TeNT injections, the average gephyrin cluster areas on Renshaw cells increased by 18.4% at P15 and 28.4% at P20 and decreased after BoNT-A injections by 17.7% at P15 and 19.9% at P20. The average size differences resulted from changes in the proportions of small and large gephyrin clusters. Whole-cell recordings in P9-P15 Renshaw cells after P5 TeNT injections showed increases in the peak amplitude of glycinergic miniature postsynaptic currents (mPSCs) and the fast component of mixed (glycinergic/GABAergic) mPSCs compared with controls (60.9% and 78.9%, respectively). GABAergic mPSCs increased in peak amplitude to a smaller extent (45.8%). However, because of the comparatively longer decays of synaptic GABAergic currents, total current transfer changes after TeNT were similar for synaptic glycine and GABA(A) receptors (56 vs 48.9% increases, respectively). We concluded that motor axon excitatory synaptic activity modulates the development of inhibitory synapse properties on Renshaw cells, influencing recruitment of postsynaptic gephyrin and glycine receptors and, to lesser extent, GABA(A) receptors.

Agrin mediates a rapid switch from electrical coupling to chemical neurotransmission during synaptogenesis

In contrast to its well-established actions as an organizer of synaptic differentiation at the neuromuscular junction, the proteoglycan agrin is still in search of a function in the nervous system. Here, we report an entirely unanticipated role for agrin in the dual modulation of electrical and chemical intercellular communication that occurs during the critical period of synapse formation. When applied at the developing splanchnic nerve–chromaffin cell cholinergic synapse in rat adrenal acute slices, agrin rapidly modified cell-to-cell communication mechanisms. Specifically, it led to decreased gap junction–mediated electrical coupling that preceded an increase in nicotinic synaptic transmission. This developmental switch from predominantly electrical to chemical communication was fully operational within one hour and depended on the activation of Src family–related tyrosine kinases. Hence, agrin may play a pivotal role in synaptogenesis in promoting a rapid switch between electrical coupling and synaptic neurotransmission.

Electrical synapse formation disrupts calcium-dependent exocytosis, but not vesicle mobilization

Electrical coupling exists prior to the onset of chemical connectivity at many developing and regenerating synapses. At cholinergic synapses in vitro, trophic factors facilitated the formation of electrical synapses and interfered with functional neurotransmitter release in response to photolytic elevations of intracellular calcium. In contrast, neurons lacking trophic factor induction and electrical coupling possessed flash-evoked transmitter release. Changes in cytosolic calcium and postsynaptic responsiveness to acetylcholine were not affected by electrical coupling. These data indicate that transient electrical synapse formation delayed chemical synaptic transmission by imposing a functional block between the accumulation of presynaptic calcium and synchronized, vesicular release. Despite the inability to release neurotransmitter, neurons that had possessed strong electrical coupling recruited secretory vesicles to sites of synaptic contact. These results suggest that the mechanism by which neurotransmission is disrupted during electrical synapse formation is downstream of both calcium influx and synaptic vesicle mobilization. Therefore, electrical synaptogenesis may inhibit synaptic vesicles from acquiring a readily releasable state. We hypothesize that gap junctions might negatively interact with exocytotic processes, thereby diminishing chemical neurotransmission.

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.

Transient electrical coupling delays the onset of chemical neurotransmission at developing synapses

The formation and subsequent elimination of electrical coupling between neurons has been demonstrated in many developing vertebrate and invertebrate nervous systems. The relationship between the disappearance of electrical synaptic connectivity and the appearance of chemical neurotransmission is not well understood. We report here that identified motoneurons from the snail Helisoma formed transient electrical and chemical connections during regeneration both in vivo and in vitro. Electrical connections that formed in vivo were strongest by day 2 and no longer detectable by day 7. During elimination of this electrical connection, an inhibitory chemical connection from 110 onto 19 formed. This sequence of synaptic development was recapitulated in cell culture with a similar time course. The relationship between the appearance of transient electrical coupling and its possible effects on the subsequent chemical synaptogenesis were examined by reducing transient intercellular coupling. Trophic factor-deprived medium resulted in a 66% reduction in coupling coefficient. In these conditions, the unidirectional chemical connection formed readily; in contrast, chemical synaptogenesis was delayed in cell pairs exposed to trophic factors where transient electrical coupling was strong. Dye coupling and synaptic vesicle cycling studies supported electrophysiological results. Exposure to cholinergic antagonists, curare and hexamethonium bromide, which block chemical neurotransmission in these synapses, resulted in prolonged maintenance of the electrical connection. These studies demonstrated an inverse relationship between chemical and electrical connectivity at early stages of synaptic development and suggest a dynamic interaction between these forms of neuronal communication as adult neural networks are constructed or regenerated.