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During the early development of the nervous system, synchronized activity is observed in a variety of structures, and is considered to play a fundamental role in neural development. One of the most striking examples of such activity is the depolarization wave reported in chick and rat embryos. In the accompanying paper (Momose-Sato et al., 2012), we have demonstrated that a depolarization wave is also present in the mouse embryo by showing large-scale optical waves, which spread remarkably over the central nervous system, including the spinal cord, hindbrain, cerebellum, midbrain, and forebrain. In the present study, we examined the pharmacological nature of the mouse depolarization wave and its developmental changes. We show here that two types of switching in pharmacological characteristics occur during development. One is that the depolarization wave is strongly dependent on nicotinic acetylcholine receptors during the early developmental stage [embryonic day (E)11-12], but is dominated by glutamate at the later stage (E13 onwards). The second is that γ-aminobutyric acid (GABA), which acts as an excitatory mediator of the depolarization wave during the early phase, becomes an inhibitory modulator by E14. These changes seemed to occur earlier in the hindbrain than in the spinal cord. Furthermore, we show that the second switch causes the loss of synchronization over the network, resulting in the disappearance of the depolarization wave and segregation of the activity into discrete regions of the medulla and spinal cord. We suggest that pharmacological switching is a possible mechanism underlying replacement of the primordial correlated network by a mature neuronal circuit., (© 2012 The Authors. European Journal of Neuroscience © 2012 Federation of European Neuroscience Societies and Blackwell Publishing Ltd.)

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. Appling voltage‐sensitive dye recording and the DiI staining method to the embryonic rat brainstem, we examined functiogenesis/morphogenesis of the facial motor system. Stimulation of the facial nerve elicited a long‐duration signal, which reflected the activity in the soma, and a short‐duration signal, which was associated with the axonal pathway. By examining the spatiotemporal patterns of these signals, we elucidated the developmental dynamics and functional organization of the facial motor nucleus. [ABSTRACT FROM AUTHOR]

The multiple-site optical recording technique with a voltage-sensitive dye, NK2761, was used to survey functional organization of neural circuits related to the vagus nerve in the embryonic chick brainstem. When we stimulated the vagus nerve, in addition to the responses in the vagal sensory nucleus (nucleus of the tractus solitarius (NTS)) and motor nucleus (dorsal motor nucleus of the vagus nerve (DMNV)) on the stimulated side, another response area was discriminated at the level of the pons/rostral medulla on the contralateral side. Characteristics of the contralateral optical signals suggested that they correspond to the neural activity in the second/higher-ordered nucleus of the vagal pathway, possibly the parabrachial nucleus, which receives inputs from the NTS. Blockade of non-N-methyl- d-aspartate (NMDA) receptors abolished the responses on the contralateral side, together with the postsynaptic firing in the NTS, suggesting the significance of non-NMDA receptor function in sensory information transfer via the NTS. The responses on the contralateral side were first detected from the 7-day-old embryonic stage, when the glutamatergic excitatory postsynaptic potentials were first expressed in the NTS. The results suggest that the synaptic pathway from the NTS to the contralateral nucleus is already generated when the primary vagal afferents make functional synapses on NTS neurons. [ABSTRACT FROM AUTHOR]