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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. [ABSTRACT FROM AUTHOR]

Highlights: [•] We examined the initial expression of synaptic function in the embryonic chick CNS. [•] We applied voltage-sensitive dye imaging to the N.V-brainstem preparation. [•] We optically identified N.V-related sensory and motor nuclei in the brainstem. [•] Synaptic function in the sensory nuclei emerged earlier than previously considered. [•] Development of synaptic function differs among nerves and between sensory nuclei. [ABSTRACT FROM AUTHOR]

Abstract: The multiple-site optical recording technique with a voltage-sensitive dye, NK2761, was used to survey the functional organization of neuronal networks related to the vagus nerve (N.X) in the E16-stage rat brainstem. When we stimulated N.X, in addition to the responses in the vagal sensory nucleus (nucleus of the tractus solitarius (NTS)) on the stimulated side, other response areas were bilaterally detected. Characteristics of the optical signals in these areas suggested that they correspond to neural activity in the second/higher-ordered nucleus of the vagal pathway. The first area was located at the level of the pons. Based upon morphological information, we suggest that this area corresponds to the parabrachial nucleus (PBN), which receives inputs from the NTS. The second area was located between the NTS and the PBN. We suggest that this area is the A5 noradrenergic group. These results suggest that the N.X-related neural networks are established similarly to the adult pattern from an early developmental stage. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: We examined the intracellular Ca2+ ([Ca2+]i) elevation evoked by GABA in an 8-day embryonic chick brainstem slice using a Ca imaging technique with Ca green-1 AM. When small quantities of GABA were pressure-ejected on the surface of the slice, the [Ca2+]i elevation was clearly detected. The GABA-induced [Ca2+]i elevation was eliminated in a Ca2+-free solution, whereas the previously reported GABA-induced light-scattering change was independent of extracellular Ca2+. Although, micro-application of glycine or glutamate also induced [Ca2+]i elevation, these changes were smaller than that by GABA. These results suggest that the GABA-induced [Ca2+]i elevation is due to Ca2+ entry resulting from membrane depolarization and may play an important role in the development of the central nervous system (CNS). [Copyright &y& Elsevier]

Abstract: Multiple-site optical recording with a fast voltage-sensitive dye, absorption dye NK2761, was used to study the developmental organization of functional synaptic networks in the vagal pathway. Glutamatergic excitatory postsynaptic potentials (EPSPs) evoked by vagus nerve stimulation was first detected from the nucleus of the tractus solitarius (NTS) at embryonic day 7 (E7) in chick embryos and E15 in rat embryos, when morphological differentiation of pre- and postsynaptic neurons is incomplete. When extracellular Mg2+ was removed, small EPSPs were elicited at E6 in chick embryos and E14 in rat embryos. These results suggest that synaptic function mediated by N-methyl-d-aspartate (NMDA) receptors is latently generated 1 day before the expression of glutamatergic EPSP. Functional synapses related to the glossophyaryngeal nerve appear to be generated at the same time as the vagus nerve, but their spatial distribution was different from that of the vagus nerve. We further investigated the development of second synaptic pathways from the NTS to higher centers, and found that neuronal circuits from the NTS are already generated when the primary afferents form functional synapses with NTS neurons. [Copyright &y& Elsevier]

We traced developmental changes in the ventro-dorsal distribution pattern of glossopharyngeal nerve (N. IX) responses by applying an optical sectioning method to thick slice preparations dissected from E4 to E8 chick embryos. We identified the motor and sensory nuclei related to the glossopharyngeal nerve in the rostral and caudal focal planes, respectively. In the E4 and E5 preparations, the motoneuronal responses appeared on the central part of the stimulated side of the brainstem. As development proceeded to E6, the response area became localized on the dorsal region. The change in the ventro-dorsal distribution pattern was similar to that observed in the vagus nerve-related nuclei, suggesting that there might be an essential process underlying functional organization of the brainstem nuclei. [Copyright &y& Elsevier]