Optical studies of the biphasic modulatory effects of glycine on excitatory postsynaptic potentials in the chick brainstem and their embryogenesis

doi:10.1016/0306-4522(95)00581-1

Neuroscience

Volume 72, Issue 3, June 1996, Pages 833-846

https://doi.org/10.1016/0306-4522(95)00581-1 Get rights and content

Abstract

Multiple-site optical recording of transmembrane potential activity, using a voltage-sensitive dye, was employed to monitor neural activity from the nucleus tractus solitarius of the chick embryo. Optical signals related to glutamate-mediated excitatory postsynaptic potentials were evoked by a brief square current pulse applied with a microsuction electrode to the vagus nerve, and were recorded simultaneously from many sites in the brainstem slice preparation. We have found that glycine has biphasic modulatory effects on the glutamate-mediated excitatory postsynaptic potentials: at lower concentrations, glycine enhances the glutamate-mediated excitatory postsynaptic potential-related optical signal, and at higher concentrations, it reduces the glutamate-mediated excitatory postsynaptic potential-signal. The enhancing effect was insensitive to strychnine, but the reducing effect was blocked by strychnine, suggesting that the former effect was induced by glycine which increased glutamate binding to N-methyl-d-aspartate receptors, and the latter resulted from an increase in chloride conductance through the strychnine-sensitive inhibitory glycine receptors in postsynaptic neurons. The inhibitory effect of glycine was first observed in the brainstem preparations at the seven-day-old embryonic stage, and the enhancing effect was first observed in the nine-day-old preparations.

We determined regional distributions of the biphasic effects of glycine in the seven- to nine-day old embryonic preparations. The spatial distribution of the enhancing effect appeared to be concentrated on the ventral side of the nucleus tractus solitarius, and the inhibitory effect was relatively concentrated in the medial portion. Furthermore, we compared the glycine effect with the effect of Mg²⁺ on N-methyl-d-aspartate receptors, and we have found that the Mg²⁺ site is functionally organized prior to the glycine site during embryonic development.

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Citation Excerpt :
The inhibitory action of glycine was weaker than that induced by GABA, suggesting that GABA rather than glycine is the main inhibitory modulator in the embryonic NTS. The enhancing effect of glycine can first be observed at day 9, while the inhibitory effect of glycine is already generated by day 7 (Sato et al., 1996) (Fig. 2). This suggests a difference in the developmental expression of positive and negative effects of glycine.
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.
Optical recording of vagal pathway formation in the embryonic brainstem
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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.
Spreading depolarization waves triggered by vagal stimulation in the embryonic chick brain: Optical evidence for intercellular communication in the developing central nervous system
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Throughout experiments on multiple-site voltage-sensitive dye recordings of neural activity in embryonic chick brain preparations, we have found a novel type of depolarization waves which spread widely from the brainstem to the whole brain region at a rapid rate (mm/s). This depolarization wave was triggered by glutamate-mediated postsynaptic potentials and was especially correlated to N-methyl-d-aspartate receptor function. Evidence that the spreading depolarization wave is eliminated by octanol or 18β-glycyrrhetinic acid suggests that the depolarization wave depends on functions of gap junctions. The profile obtained with Ca²⁺-imaging experiments also suggests that the propagation of the depolarization wave is accompanied by a calcium wave.
These results provide new evidence for intercellular functional communication between neural cells in the vertebrate central nervous system during embryonic development.
Optical approaches to embryonic development of neural functions in the brainstem
2001, Progress in Neurobiology
Citation Excerpt :
Glycine also serves an allosteric regulator of excitatory neurotransmission mediated by NMDA receptors (Johnson and Ascher, 1987; Thomson, 1989). To determine whether and how glycine modulates the embryonic glutamatergic EPSP, the early expression and spatiotemporal characteristics of glycinergic action were examined in E7 to E9 chick NTS (Sato et al., 1996). The results show that glycine has biphasic modulatory effects on the glutamate-mediated EPSPs evoked by vagal stimulation: at lower concentrations (1–10 μM), glycine enhances the EPSP, and at higher concentrations (100 μM–1 mM), it reduces the EPSP.
The ontogenetic approach to physiological events is a useful strategy for understanding the functional organization/architecture of the vertebrate brainstem. However, conventional electrophysiological techniques are difficult or impossible to employ in the early embryonic central nervous system. Optical techniques using voltage-sensitive dyes have made it possible to monitor neural activities from multiple regions of living systems, and have proven to be a useful tool for analyzing the embryogenetic expression of brainstem neural function. This review describes recent progress in optical studies made on embryonic chick and rat brainstems. Several technical issues concerning optical recording from the embryonic brainstem preparations are discussed, and characteristics of the optical signals evoked by cranial nerve stimulation or occurring spontaneously are described. Special attention is paid to the chronological analyses of embryogenetic expression of brainstem function and to the spatial patterning of the functional organization/architecture of the brainstem nuclei. In addition, optical analyses of glutamate, GABA, and glycine receptor functions during embryogenesis are described in detail for the chick nucleus tractus solitarius. This review also discusses intrinsic optical signals associated with neuronal depolarization. Some emphases are also placed on the physiological properties of embryonic brainstem neurons, which may be of interest from the viewpoint of developmental neurobiology.
Multiple-site optical recording of mouse brainstem evoked by vestibulocochlear nerve stimulation
2000, Brain Research
We used optical imaging to investigate the mouse cochlear and vestibular nucleus in brainstem slices using a voltage-sensitive dye, RH 155. As a result, the spatiotemporal patterns of excitatory propagation were shown. These optical signals consisted of two components consisting of a spike-like fast signal and a long-lasting slow signal. All responses were abolished by tetrodotoxin. The slow signals were eliminated under a Ca²⁺-free solution. In addition, synaptic fatigue was also observed. The present study indicated the feasibility of optical recording for visually revealing the synaptic transmission in both the vestibular and cochlear nucleus.
Optical mapping reveals the functional organization of the trigeminal nuclei in the chick embryo
1999, Neuroscience
The functional organization of the trigeminal nuclei during embryogenesis was investigated using multiple-site optical recording with a fast voltage-sensitive dye. Brainstem preparations with three classified trigeminal nerve afferents, the ophthalmic, maxillary and mandibular nerves, together with motor nerve fibers, were dissected from five- to eight-day-old chick embryos. Electrical responses evoked by trigeminal nerve stimulations were optically recorded simultaneously from many loci of the stained preparations. We identified three response areas related to the trigeminal nerve: area I, located cephalic to the level of the trigeminal ganglion; area II, located caudal to the level of the trigeminal ganglion; and area III, located at the level of the trigeminal root. The neural responses in areas I and II were evoked by ophthalmic, maxillary or mandibular nerve stimulation, while the responses in area III were detected when the stimulation was applied to the trigeminal motor nerve. In comparison with the morphology indicated by DiI labeling, the results suggest that areas I, II and III correspond to the principal sensory nucleus of the trigeminal nerve, the spinal sensory nucleus of the trigeminal nerve and the trigeminal motor nucleus, respectively. We identified two components of the optical response: a fast and a slow signal. In five-day-old preparations, fast spike-like signals related to action potentials were recorded from the three response areas. In six-day-old preparations, slow optical signals which reflect glutamate-mediated excitatory postsynaptic potentials were detected from area II only when the ophthalmic nerve was stimulated: no slow signal was evoked by maxillary or mandibular nerve stimulation. In seven- and eight-day-old preparations, slow signals were detected from both areas I and II with every nerve stimulation.
These results suggest that synaptic function is first generated in the spinal trigeminal nucleus by the six-day embryonic stage, and the developmental organization of synaptic function is not the same in the three trigeminal nerves or in the two sensory nuclei. Contour line maps of the signal amplitude revealed that the size and the area of the neural responses within the trigeminal nuclei changed dramatically with development. We compared the spatial distribution and temporal dynamics of the optical signals between the ophthalmic, maxillary and mandibular nerve stimulations, and we found that somatotopic organization is less clear in a rostrocaudal/mediolateral X–Y plane, although the areas of the maxillary and mandibular nerves appeared to separate in the lateral direction.

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^∗: Present address: Department of Physiology, Shimane Medical School, Izumo-shi, Shimane 693, Japan.

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Optical studies of the biphasic modulatory effects of glycine on excitatory postsynaptic potentials in the chick brainstem and their embryogenesis

Abstract

Neurosci. Res.

Neurosci. Lett.

Eur. J. Pharmac.

Adv. Biophys.

Neurosci. Res.

Neurosci. Lett.

Neurosci. Lett.

Brain Res.

Neurosci. Res. Suppl.

Trends Neurosci.

Heterogeneity of glycine receptors and their messenger RNAs in rat brain and spinal cord

Science

Biochemical aspects of transmission at inhibitory synapses: the role of glycine

Optical measurement of membrane potential

Rev. Physiol. Biochem. Pharmac.

Synaptic activation of N-methyl-d-aspartate receptors in the Schaffer collateral-commissural pathway of rat hippocampus

J. Physiol., Lond.

Excitatory amino acid receptors in the vertebrate central nervous system

Pharmac. Rev.

Excitatory amino acid neurotransmission: NMDA receptors and Hebb-type synaptic plasticity

A. Rev. Neurosci.

Selective antagonism by Mg2+ of amino acid-induced depolarizations of spinal neurons

Experimentia

Selective antagonism by Mg²⁺ of amino acid-induced depolarizations of spinal neurons