Functional Development of the Auditory Brainstem Nuclei During Embryogenesis of the Mouse Revealed by Optical Recording With a Voltage-Sensitive Dye

A fundamental question in auditory neuroscience is when and how complex connections between the nuclei/centers are organized during ontogenesis. In the present study, we applied a multiple-site optical recording technique with a voltage-sensitive dye and surveyed the developmental organization of the auditory system in the mouse embryo. Stimulation of the cochlear (auditory) nerve elicited excitatory postsynaptic potential (EPSP)-related optical responses in the lateral brainstem, which corresponded to the auditory sensory nucleus, namely, the cochlear nucleus. The EPSP was mediated by glutamate and mainly dependent on N-methyl-d-aspartate (NMDA) receptors. The EPSP was first detected at E (embryonic day) 12, indicating that functional connections between the periphery and neurons in the cochlear nucleus are established at this stage. At later developmental stages, cochlear nerve stimulation elicited additional postsynaptic responses in the ipsilateral and contralateral ventral brainstem, which corresponded to the higher center of the auditory pathway, the superior olivary complex (SOC). The EPSP in the SOC was detected from E15 in normal physiological solution and E14 in a Mg2+-free solution. Thus, the synaptic connections between the cochlear nucleus and the SOC are established by E14, but they are suppressed by the Mg2+ block on the NMDA receptors before E15. Overall, our results suggest that postsynaptic responses in the cochlear nucleus and the SOC exhibit much earlier than previously reported, and functional synapses are generated soon after the arrival of afferent fibers and before morphological differentiation of the brainstem nuclei has been completed.

Advances in physiological and clinical relevance of hiPSC-derived brain models for precision medicine pipelines

Precision, or personalized, medicine aims to stratify patients based on variable pathogenic signatures to optimize the effectiveness of disease prevention and treatment. This approach is favorable in the context of brain disorders, which are often heterogeneous in their pathophysiological features, patterns of disease progression and treatment response, resulting in limited therapeutic standard-of-care. Here we highlight the transformative role that human induced pluripotent stem cell (hiPSC)-derived neural models are poised to play in advancing precision medicine for brain disorders, particularly emerging innovations that improve the relevance of hiPSC models to human physiology. hiPSCs derived from accessible patient somatic cells can produce various neural cell types and tissues; current efforts to increase the complexity of these models, incorporating region-specific neural tissues and non-neural cell types of the brain microenvironment, are providing increasingly relevant insights into human-specific neurobiology. Continued advances in tissue engineering combined with innovations in genomics, high-throughput screening and imaging strengthen the physiological relevance of hiPSC models and thus their ability to uncover disease mechanisms, therapeutic vulnerabilities, and tissue and fluid-based biomarkers that will have real impact on neurological disease treatment. True physiological understanding, however, necessitates integration of hiPSC-neural models with patient biophysical data, including quantitative neuroimaging representations. We discuss recent innovations in cellular neuroscience that can provide these direct connections through generative AI modeling. Our focus is to highlight the great potential of synergy between these emerging innovations to pave the way for personalized medicine becoming a viable option for patients suffering from neuropathologies, particularly rare epileptic and neurodegenerative disorders.

Optical recording of oscillatory activity in the absence of external Ca2+ in the embryonic chick olfactory bulb

We applied 464/1020-site optical recording systems with a voltage-sensitive dye (NK2761) to the embryonic chick olfactory system and detected oscillatory activity in the olfactory bulb (OB) in the absence of synaptic transmission. In embryonic day 8-10 (E8-E10) chick olfactory nerve (N.I)-OB-forebrain preparations, the removal of Ca2+ from the external solution completely blocked the glutamatergic excitatory postsynaptic potential (EPSP) from the N.I to the OB as well as oscillations following the EPSP. However, a novel type of oscillatory activity was detected in the OB with the long-term perfusion of a Ca2+-free solution. The characteristics of oscillatory activity in the Ca2+-free solution differed from those in normal physiological solution. The present results suggest the existence of a neural communication system in the absence of synaptic transmission at the early stage of embryonic development.

Functional development of olfactory nerve-related neural circuits in the embryonic chick forebrain revealed by voltage-sensitive dye imaging

Multiple-site optical recordings with NK2761, a voltage-sensitive absorption dye, were applied to the embryonic chick olfactory system, and the functional development of olfactory nerve (N.I)-related neural circuits was examined in the forebrain. The stimulation of the N. I elicited neural responses in N.I-olfactory bulb (OB)-forebrain preparations at the embryonic 8-12 day (E8-E12) stages. At the E11 stage, we functionally identified two circuits projecting from the OB to the forebrain. The first circuit passed through the ventral side of the forebrain and spread in the dorso-caudal direction, while the second circuit passed through the dorsal side to the first circuit. Pharmacological experiments showed that NMDA receptor function was more significant for the transfer of sensory information in these circuits. The functional development of N.I-related circuits was investigated, and the results obtained revealed that the ventral circuit was generated earlier than the dorsal circuit. Neural responses in the ventral circuit were detected from the E9 stage in normal physiological solution and the E8 stage in Mg²⁺ -free solution, which activated NMDA receptor function. At the E10 stage, neural responses in the dorsal circuit were clearly recognized in addition to ventral responses. We attempted to identify possible candidates for relay nuclei in the forebrain by comparing contour line maps of the optical signal amplitude with previously reported neuroanatomical data. The present results suggest that N.I-related neural circuits from the periphery to the subpallium functionally mature earlier than those to the pallium during ontogenesis.

The Brainstem-Informed Autism Framework: Early Life Neurobehavioral Markers

Autism spectrum disorders (ASD) have long-term implications on functioning at multiple levels. In this perspective, we offer a brainstem-informed autism framework (BIAF) that traces the protracted neurobehavioral manifestations of ASD to early life brainstem dysfunctions. Early life brainstem-mediated markers involving functions of autonomic/arousal regulation, sleep-wake homeostasis, and sensorimotor integration are delineated. Their possible contributions to the early identification of susceptible infants are discussed. We suggest that the BIAF expands our multidimensional understanding of ASD by focusing on the early involvement of brainstem systems. Importantly, we propose an integrated BIAF screener that brings about the prospect of a sensitive and reliable early life diagnostic scheme for weighing the risk for ASD. The BIAF screener could provide clinicians substantial gains in the future and may carve customized interventions long before the current DSM ASD phenotype is manifested using dyadic co-regulation of brainstem-informed autism markers.

Prenatal exposure to nicotine disrupts synaptic network formation by inhibiting spontaneous correlated wave activity

Correlated spontaneous activity propagating over a wide region of the central nervous system is expressed during a specific period of embryonic development. We previously demonstrated using an optical imaging technique with a voltage-sensitive dye that this wave-like activity, which we referred to as the depolarization wave, is fundamentally involved in the early process of synaptic network formation. We found that the in ovo application of bicuculline/strychnine or d-tubocurarine, which blocked the neurotransmitters mediating the wave, significantly reduced functional synaptic expression in the brainstem sensory nucleus. This result, particularly for d-tubocurarine, an antagonist of nicotinic acetylcholine receptors, suggested that prenatal nicotine exposure associated with maternal smoking affects the development of neural circuit formation by interfering with the correlated wave. In the present study, we tested this hypothesis by examining the effects of nicotine on the correlated activity and assessing the chronic action of nicotine in ovo on functional synaptic expression along the vagal sensory pathway. In ovo observations of chick embryo behavior and electrical recording using in vitro preparations showed that the application of nicotine transiently increased embryonic movements and electrical bursts associated with the wave, but subsequently inhibited these activities, suggesting that the dominant action of the drug was to inhibit the wave. Optical imaging with the voltage-sensitive dye showed that the chronic exposure to nicotine in ovo markedly reduced functional synaptic expression in the higher-order sensory nucleus of the vagus nerve, the parabrachial nucleus. The results suggest that prenatal nicotine exposure disrupts the initial formation of the neural circuitry by inhibiting correlated spontaneous wave activity.

Voltage-sensitive dye recording of glossopharyngeal nerve-related synaptic networks in the embryonic mouse brainstem

The glossopharyngeal nerve (N.IX) transfers motor and sensory information related to visceral and somatic functions, such as salivary secretion, gustation and the control of blood pressure. N.IX-related neural circuits are indispensable for these essential functions. Compared with the strenuous analysis of morphogenesis, we are only just starting to elucidate the functiogenesis of these neural circuits during ontogenesis. In the present study, we applied voltage-sensitive dye recording to the embryonic mouse brainstem, and examined the functional development of the N.IX-related neural circuits. First, we optically identified the motor nucleus (the inferior salivatory nucleus (ISN)) and the first-order sensory nucleus (the nucleus of the tractus solitarius (NTS)). We also succeeded in recording optical responses in the second/higher-order sensory nuclei via the NTS, including the parabrachial nucleus. Second, we pursued neuronal excitability and the onset of synaptic function in the N.IX-related nuclei. The neurons in the ISN were excitable at least at E11, and functional synaptic transmission in the NTS was first expressed at E12. In the second/higher-order sensory nuclei, synaptic function emerged at around E12-13. Third, by mapping optical responses to N.IX and vagus nerve (N.X) stimulation, we showed that the distribution patterns of neural activity in the NTS were different between the N.IX and the N.X from the early stage of ontogenesis. We discuss N.IX-related neural circuit formation in the brainstem, in comparison with our previous results obtained from chick and rat embryos.

Advances in stem cell therapy for amyotrophic lateral sclerosis

INTRODUCTION

Amyotrophic Lateral Sclerosis (ALS) is a progressive, incurable neurodegenerative disease that targets motoneurons. Cell-based therapies have generated widespread interest as a potential therapeutic approach but no conclusive results have yet been reported either from pre-clinical or clinical studies.

AREAS COVERED

This is an integrated review of pre-clinical and clinical studies focused on the development of cell-based therapies for ALS. We analyze the biology of stem cell treatments and results obtained from pre-clinical models of ALS and examine the methods and the results obtained to date from clinical trials. We discuss scientific, clinical, and ethical issues and propose some directions for future studies.

EXPERT OPINION

While data from individual studies are encouraging, stem-cell-based therapies do not yet represent a satisfactory, reliable clinical option. The field will critically benefit from the introduction of well-designed, randomized and reproducible, powered clinical trials. Comparative studies addressing key issues such as the nature, properties, and number of donor cells, the delivery mode and the selection of proper patient populations that may benefit the most from cell-based therapies are now of the essence. Multidisciplinary networks of experts should be established to empower effective translation of research into the clinic.

Optical analysis of functional development of the facial motor nucleus in the embryonic rat brainstem

Facial motor neurons of the rat embryo are first generated in rhombomere 4 and then migrate in the caudo-ventral direction. This migration forms a unique axonal trajectory called the genu, a loop of facial motor axons around the abducens nucleus. It is still unclear when and how this unique structure is functionally established during ontogenesis. Using voltage-sensitive dye (VSD) recording and the DiI staining method, we identified neural responses evoked by facial nerve (N.VII) stimulation and examined developmental processes of the facial motor nucleus in E12-E17 rat brainstems. We identified two types of fast spike-like signals; a long-duration signal, which corresponded to the action potential in the N.VII soma, and a short-duration signal, which reflected the action potential in the N.VII axons. The long-duration signal was detected as early as E13, suggesting that the N.VII motor neuron is already excitable at the beginning of cell migration. The response area of the long-duration signal extended caudally at E13-E14, and shifted in a ventral direction at E15. At E16-E17, the long-duration signal was concentrated in the caudo-ventral area, which was comparable to the location of the facial motor nucleus in the adult rat brainstem. These results demonstrate that developmental processes of cell migration and nuclear organization can be visualized and identified functionally with the VSD recording. We discuss the results by comparing functiogenesis and morphogenesis of the N.VII pathway.

Developmental roles of the spontaneous depolarization wave in synaptic network formation in the embryonic brainstem

One of the earliest activities expressed within the developing central nervous system is a widely propagating wave-like activity, which we referred to as the depolarization wave. Despite considerable consensus concerning the global features of the activity, its physiological role is yet to be clarified. The depolarization wave is expressed during a specific period of functional synaptogenesis, and this developmental profile has led to the hypothesis that the wave plays some roles in synaptic network organization. In the present study, we tested this hypothesis by inhibiting the depolarization wave in ovo and examining its effects on the development of functional synapses in vagus nerve-related brainstem nuclei of the chick embryo. Chronic inhibition of the depolarization wave had no significant effect on the developmental time course, amplitude, and spatial distribution of monosynaptic excitatory postsynaptic potentials in the first-order nuclei of the vagal sensory pathway (the nucleus of the tractus solitarius (NTS) and the contralateral non-NTS region), but reduced polysynaptic responses in the higher-order nucleus (the parabrachial nucleus). These results suggest that the depolarization wave plays an important role in the initial process of functional synaptic expression in the brainstem, especially in the higher-order nucleus of the cranial sensory pathway.

Functiogenesis of the embryonic central nervous system revealed by optical recording with a voltage-sensitive dye

Clarification of the functiogenesis of the embryonic central nervous system (CNS) has long been problematic, because conventional electrophysiological techniques have several limitations. First, early embryonic neurons are small and fragile, and the application of microelectrodes is challenging. Second, the simultaneous monitoring of electrical activity from multiple sites is limited, and as a consequence, spatiotemporal response patterns of neural networks cannot be assessed. We have applied multiple-site optical recording with a voltage-sensitive dye to the embryonic CNS and paved a new way to analyze the functiogenesis of the CNS. In this review, we discuss key points of optical recording in the embryonic CNS and introduce recent progress in optical investigations on the embryonic CNS with special emphasis on the development of the chick olfactory system. The studies clearly demonstrate the usefulness of voltage-sensitive dye recording as a powerful tool for elucidating the functional organization of the vertebrate embryonic CNS.

Modeling psychiatric disorders: from genomic findings to cellular phenotypes

Major programs in psychiatric genetics have identified >150 risk loci for psychiatric disorders. These loci converge on a small number of functional pathways, which span conventional diagnostic criteria, suggesting a partly common biology underlying schizophrenia, autism and other psychiatric disorders. Nevertheless, the cellular phenotypes that capture the fundamental features of psychiatric disorders have not yet been determined. Recent advances in genetics and stem cell biology offer new prospects for cell-based modeling of psychiatric disorders. The advent of cell reprogramming and induced pluripotent stem cells (iPSC) provides an opportunity to translate genetic findings into patient-specific in vitro models. iPSC technology is less than a decade old but holds great promise for bridging the gaps between patients, genetics and biology. Despite many obvious advantages, iPSC studies still present multiple challenges. In this expert review, we critically review the challenges for modeling of psychiatric disorders, potential solutions and how iPSC technology can be used to develop an analytical framework for the evaluation and therapeutic manipulation of fundamental disease processes.

Development of Spontaneous Activity in the Avian Hindbrain

Spontaneous activity in the developing central nervous system occurs before the brain responds to external sensory inputs, and appears in the hindbrain and spinal cord as rhythmic electrical discharges of cranial and spinal nerves. This spontaneous activity recruits a large population of neurons and propagates like a wave over a wide region of the central nervous system. Here, we review spontaneous activity in the chick hindbrain by focusing on this large-scale synchronized activity. Asynchronous activity that is expressed earlier than the above mentioned synchronized activity and activity originating in midline serotonergic neurons are also briefly mentioned.

Development of synaptic networks in the mouse vagal pathway revealed by optical mapping with a voltage-sensitive dye

The central issue in developmental neuroscience is when and how neural synaptic networks are established and become functional within the central nervous system (CNS). Investigations of the neural network organization have been hampered because conventional electrophysiological means have some technical limitations. In this study, the multiple-site optical recording technique with a voltage-sensitive dye was employed to survey the developmental organization of the vagal system in the mouse embryo. Stimulation of the vagus nerve in E11-E14 mouse embryos elicited optical responses in areas corresponding to the vagal sensory and motor nuclei. Postsynaptic responses in the first-order sensory nucleus, the nucleus of the tractus solitarius (NTS), were identified from E11, suggesting that sensory information becomes transferred to the brain at this stage. In addition to the NTS, optical responses were identified in the rostral and contralateral brainstem regions, which corresponded to second/higher order nuclei of the vagus nerve including the parabrachial nucleus (PBN). Postsynaptic responses in the second/higher-order nuclei were detected from E12, suggesting that polysynaptic networks were functional at this stage. We discuss the results of our optical mapping, comparing them with previous findings obtained in the chick and rat embryos, and suggest some fundamental principles in the functional organization of synaptic networks in the embryonic brain.

Oscillations in the embryonic chick olfactory bulb: initial expression and development revealed by optical imaging with a voltage-sensitive dye

In a previous study, we applied a multiple-site optical recording technique with a voltage-sensitive dye to the embryonic chick olfactory system and showed that functional synaptic transmission in the olfactory bulb was expressed at embryonic 6-7-day stages. It is known that oscillations, i.e. stereotyped sinusoidal neural activity, appear in the olfactory system of various species. The focus of the present study is to determine whether the oscillation is also generated in the embryonic chick olfactory bulb and, if this is the case, when the oscillation appears and how its profiles change during embryogenesis. At the early stages of development (embryonic 6- to 8-day stages), postsynaptic response-related optical signals evoked by olfactory nerve stimulation exhibited a simple monophasic waveform that lasted for a few seconds. At embryonic 9-day stage, the optical signal became multi-phasic, and the oscillatory event was detected in some preparations. The oscillation was restricted to the distal half of the olfactory bulb. As development proceeded, the incidence and duration of the oscillation gradually increased, and the waveform became complicated. In some cases at embryonic 12-day stage, the oscillation lasted for nearly a minute. The frequency of the oscillation increased slightly with development, but it remained in the range of theta oscillation during the 9- to 12-day stages. We discuss the ontogenetic dynamics of the oscillation and the significance of this activity in the developing olfactory bulb.

Design and Use of Organic Voltage Sensitive Dyes

The chemistry and the physics of voltage sensitive dyes (VSDs) should be understood and appreciated as a prerequisite for their optimal application to problems in neuroscience cardiology. This chapter provides a basic understanding of the properties of the large variety of available organic VSDs. The mechanisms by which the dyes respond to voltage guides the best set up of the optics for recording or imaging electrophysiological activity. The physical and chemical properties of the dyes can be tuned to optimize delivery to and staining of the cells in different experimental preparations. The aim of this chapter is to arm the experimentalists who use the dyes with enough information and data to be able to intelligently choose the best dye for their specific requirements.

Optical survey of initial expression of synaptic function in the embryonic chick trigeminal sensory nucleus

We examined the initial expression of synaptic function in the embryonic chick trigeminal nucleus using voltage-sensitive dye recording. Brainstem preparations with three trigeminal nerve afferents, the ophthalmic nerve (N.V1), maxillary nerve (N.V2) and mandibular nerve (N.V3), were dissected from 5.5- to 6.5-day-old chick embryos. In our previous study [Sato et al., 1999], we detected slow signals corresponding to glutamatergic excitatory postsynaptic potentials and identified the principal sensory nucleus of the trigeminal nerve (Pr5), spinal sensory nucleus of the trigeminal nerve (Sp5) and trigeminal motor nucleus. In this study, we examined the effects of removing Mg(2+) from the physiological solution, which enhanced N-methyl-d-aspartate receptor function in the sensory nuclei. In 6.5-day-old (St 29) embryos, the slow signal was observed in Pr5 and Sp5 only when N.V1 was stimulated, whereas it appeared in Mg(2+)-free solution with every nerve stimulation. In 6-day-old (St 28) embryos, the slow signal was observed in Sp5 with N.V1 stimulation, and the appearance of synaptic function in Mg(2+)-free solution varied, depending on the nerves and preparations used. In 5.5-day-old (St 27) embryos, synaptic function was not detected even when external Mg(2+) was removed. These results indicate that the initial expression of synaptic function in the trigeminal system occurs earlier than previously considered, and that the developmental organization of synaptic function differs among the three trigeminal nerves and between the two sensory nuclei.

Evaluation of voltage-sensitive fluorescence dyes for monitoring neuronal activity in the embryonic central nervous system

Using an optical imaging technique with voltage-sensitive dyes (VSDs), we investigated the functional organization and architecture of the central nervous system (CNS) during embryogenesis. In the embryonic nervous system, a merocyanine-rhodanine dye, NK2761, has proved to be the most useful absorption dye for detecting neuronal activity because of its high signal-to-noise ratio (S/N), low toxicity and small dye bleaching. In the present study, we evaluated the suitability of fluorescence VSDs for optical recording in the embryonic CNS. We screened eight styryl (hemicyanine) dyes in isolated brainstem-spinal cord preparations from 7-day-old chick embryos. Measurements of voltage-related optical signals were made using a multiple-site optical recording system. The signal size, S/N, photobleaching, effects of perfusion and recovery of neural responses after staining were compared. We also evaluated optical responses with various magnifications. Although the S/N was lower than with the absorption dye, clear optical responses were detected with several fluorescence dyes, including di-2-ANEPEQ, di-4-ANEPPS, di-3-ANEPPDHQ, di-4-AN(F)EPPTEA, di-2-AN(F)EPPTEA and di-2-ANEPPTEA. Di-2-ANEPEQ showed the largest S/N, whereas its photobleaching was faster and the recovery of neural responses after staining was slower. Di-4-ANEPPS and di-3-ANEPPDHQ also exhibited a large S/N but required a relatively long time for recovery of neural activity. Di-4-AN(F)EPPTEA, di-2-AN(F)EPPTEA and di-2-ANEPPTEA showed smaller S/Ns than di-2-ANEPEQ, di-4-ANEPPS and di-3-ANEPPDHQ; but the recovery of neural responses after staining was faster. This study demonstrates the potential utility of these styryl dyes in optical monitoring of voltage changes in the embryonic CNS.

A neonatal mouse spinal cord injury model for assessing post-injury adaptive plasticity and human stem cell integration

Despite limited regeneration capacity, partial injuries to the adult mammalian spinal cord can elicit variable degrees of functional recovery, mediated at least in part by reorganization of neuronal circuitry. Underlying mechanisms are believed to include synaptic plasticity and collateral sprouting of spared axons. Because plasticity is higher in young animals, we developed a spinal cord compression (SCC) injury model in the neonatal mouse to gain insight into the potential for reorganization during early life. The model provides a platform for high-throughput assessment of functional synaptic connectivity that is also suitable for testing the functional integration of human stem and progenitor cell-derived neurons being considered for clinical cell replacement strategies. SCC was generated at T9–T11 and functional recovery was assessed using an integrated approach including video kinematics, histology, tract tracing, electrophysiology, and high-throughput optical recording of descending inputs to identified spinal neurons. Dramatic degeneration of axons and synaptic contacts was evident within 24 hours of SCC, and loss of neurons in the injured segment was evident for at least a month thereafter. Initial hindlimb paralysis was paralleled by a loss of descending inputs to lumbar motoneurons. Within 4 days of SCC and progressively thereafter, hindlimb motility began to be restored and descending inputs reappeared, but with examples of atypical synaptic connections indicating a reorganization of circuitry. One to two weeks after SCC, hindlimb motility approached sham control levels, and weight-bearing locomotion was virtually indistinguishable in SCC and sham control mice. Genetically labeled human fetal neural progenitor cells injected into the injured spinal cord survived for at least a month, integrated into the host tissue and began to differentiate morphologically. This integrative neonatal mouse model provides opportunities to explore early adaptive plasticity mechanisms underlying functional recovery as well as the capacity for human stem cell-derived neurons to integrate functionally into spinal circuits.

Optical analysis of developmental changes in synaptic potentiation in the neonatal rat corticostriatal projection

We applied voltage-sensitive dye imaging to neonatal rat cortical slice preparations and analyzed developmental changes in synaptic plasticity, long-term potentiation (LTP), in the corticostriatal projection. Coronal slice preparations were dissected from postnatal 1- to 21-day (P1-P21) rats, and the transmembrane voltage-related optical signals evoked by cortical stimulation were recorded using a 464ch optical recording system with the voltage-sensitive absorption dye. In the striatum, the optical signal was composed of a fast spike-like signal followed by a slow signal, which corresponded to an action potential and an excitatory postsynaptic potential (EPSP), respectively. The slow signal could be detected at the P1 stage, suggesting that the EPSP is already expressed in the corticostriatal projection at least at early stages after birth. On the other hand, the slow signal was potentiated with a single shot of tetanic stimulation and the potentiation lasted at least 1 h, which is considered to correspond to long-term potentiation. With ontogenetic examinations, we found that (1) the EPSP could be potentiated with tetanic stimulation from the P9 stage and that (2) after the LTP induction, the potentiation was maintained for a longer time in the postnatal 3W stage than in the 2W stage. These results suggest that characteristics of LTP change dynamically during postnatal development.

The embryonic brain and development of vagal pathways

To regulate the autonomic function, the vagus nerve transfers various sensory information from peripheral organs, and appropriate motor reflexes are produced in the neural circuit. The functional development of the vagal pathway during the early phase of embryonic development has long been unclear. Optical recording with voltage-sensitive dyes has provided a new approach to the analysis of the functional development of the embryonic central nervous system. In this review, we present recent progress in optical studies on the vagal pathway in the embryonic chick and rat brainstems. The topics include how neural excitability is initially expressed in the motor and sensory nuclei [e.g. the dorsal motor nucleus of the vagus nerve (DMNV) and the nucleus of the tractus solitarius (NTS)] and how synapse networks are formed in the primary and higher-ordered sensory nuclei [e.g. the parabrachial nucleus (PBN)]. We also refer to the functional development of the glossopharyngeal nuclei and compare the developmental steps with those of the vagal nuclei.

Uniform action potential repolarization within the sarcolemma of in situ ventricular cardiomyocytes

Previous studies have speculated, based on indirect evidence, that the action potential at the transverse (t)-tubules is longer than at the surface membrane in mammalian ventricular cardiomyocytes. To date, no technique has enabled recording of electrical activity selectively at the t-tubules to directly examine this hypothesis. We used confocal line-scan imaging in conjunction with the fast response voltage-sensitive dyes ANNINE-6 and ANNINE-6plus to resolve action potential-related changes in fractional dye fluorescence (DeltaF/F) at the t-tubule and surface membranes of in situ mouse ventricular cardiomyocytes. Peak DeltaF/F during action potential phase 0 depolarization averaged -21% for both dyes. The shape and time course of optical action potentials measured with the water-soluble ANNINE-6plus were indistinguishable from those of action potentials recorded with intracellular microelectrodes in the absence of the dye. In contrast, optical action potentials measured with the water-insoluble ANNINE-6 were significantly prolonged compared to the electrical recordings obtained from dye-free hearts, suggesting electrophysiological effects of ANNINE-6 and/or its solvents. With either dye, the kinetics of action potential-dependent changes in DeltaF/F during repolarization were found to be similar at the t-tubular and surface membranes. This study provides what to our knowledge are the first direct measurements of t-tubule electrical activity in ventricular cardiomyocytes, which support the concept that action potential duration is uniform throughout the sarcolemma of individual cells.

Imaging activity of neuronal populations with new long-wavelength voltage-sensitive dyes

We have assessed the utility of five new long-wavelength fluorescent voltage-sensitive dyes (VSD) for imaging the activity of populations of neurons in mouse brain slices. Although all the five were capable of detecting activity resulting from activation of the Schaffer collateral-CA1 pyramidal cell synapse, they differed significantly in their properties, most notably in the signal-to-noise ratio of the changes in dye fluorescence associated with neuronal activity. Two of these dyes, Di-2-ANBDQPQ and Di-1-APEFEQPQ, should prove particularly useful for imaging activity in brain tissue and for combining VSD imaging with the control of neuronal activity via light-activated proteins such as channelrhodopsin-2 and halorhodopsin.

Switching of the transmitters that mediate hindbrain correlated activity in the chick embryo

Widely propagating correlated neuronal activity is a hallmark of the developing nervous system. The activity is usually mediated by multiple transmitters, and the contribution of gap junctions has also been suggested in several systems. In some structures, such as the retina and spinal cord, it has been shown that the dominant transmitter mediating the correlated wave switches from acetylcholine to glutamate during development, although the functional significance of this phenomenon has not been clarified. An important question is whether such a transmitter switch occurs in other systems, especially in the brain. In the present study, we demonstrate that the major transmitter mediating correlated wave activity in the embryonic chick hindbrain changes from acetylcholine/gamma-aminobutyric acid (GABA)/glycine to glutamate/GABA as development proceeds. The results show for the first time that the dominant transmitter switches from acetylcholine to glutamate in a region other than the retina and spinal cord. This finding sheds more light on the role of nicotinic acetylcholine receptors in the generation of correlated wave activity, which is considered to regulate the development of the nervous system.

Origin of the earliest correlated neuronal activity in the chick embryo revealed by optical imaging with voltage-sensitive dyes

Spontaneous correlated neuronal activity during early development spreads like a wave by recruiting a large number of neurons, and is considered to play a fundamental role in neural development. One important and as yet unresolved question is where the activity originates, especially at the earliest stage of wave expression. In other words, which part of the brain differentiates first as a source of the correlated activity, and how does it change as development proceeds? We assessed this issue by examining the spatiotemporal patterns of the depolarization wave, the optically identified primordial correlated activity, using the optical imaging technique with voltage-sensitive dyes. We surveyed the region responsible for the induction of the evoked and spontaneous depolarization waves in chick embryos, and traced its developmental changes. The results showed that the wave initially originated in a restricted area near the obex and was generated by multiple regions at later stages. We suggest that the upper cervical cord/lower medulla near the obex is the kernel that differentiates first as the source of the correlated activity, and that regional and temporal differences in neuronal excitability might underlie the developmental profile of wave generation in early chick embryos.