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

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.

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.

Optical recording of vagal pathway formation in the embryonic brainstem

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.

Cardiovascular response to a group III mGluR agonist in NTS requires NMDA receptors

Previous studies have demonstrated that microinjection of the putative group III metabotropic glutamate receptor (mGluR) agonist, l(+)-2-amino-4-phosphonobutyric acid (L-AP4), into the nucleus tractus solitarius (NTS) produces depressor and sympathoinhibitory responses. These responses are significantly attenuated by a group III mGluR antagonist and may involve ionotropic glutamatergic transmission. Alternatively, a previous report in vitro suggests that preparations of L-AP4 may nonspecifically activate NMDA channels due to glycine contamination (Contractor A, Gereau RW, Green T, and Heinemann SF. Proc Natl Acad Sci USA 95: 8969-8974, 1998). Therefore, the present study tested whether responses to L-AP4 specifically require the N-methyl-D-aspartate (NMDA) receptor and whether they are due to actions at the glycine site on the NMDA channel. To test these possibilities in vivo, we performed unilateral microinjections of L-AP4, glycine, and selective antagonists into the NTS of urethane-anesthetized rats. L-AP4 (10 mM, 30 nl) produced sympathoinhibitory responses that were abolished by the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (AP-5, 10 mM) but were unaffected by the non-NMDA antagonist 6-nitro-7-sulfamobenzoquinoxaline-2,3-dione (NBQX, 2 mM). Microinjection of glycine (0.02-20 mM) failed to mimic sympathoinhibitory responses to L-AP4, even in the presence of the inhibitory glycine antagonist, strychnine (3 mM). Strychnine blocked pressor and sympathoexcitatory actions of glycine (20 mM) but failed to reveal a sympathoinhibitory component due to presumed activation of NMDA receptors. The results of these experiments suggest that responses to L-AP4 require NMDA receptors and are independent of non-NMDA receptors. Furthermore, although it is possible that glycine contamination or other nonspecific actions are responsible for the sympathoinhibitory actions of L-AP4, our data and data in the literature argue against this possibility. Thus we conclude that responses to L-AP4 in the NTS are mediated by an interaction between group III mGluRs and NMDA receptors. Finally, we also caution that nonselective actions of L-AP4 should be considered in future studies.

Spreading depolarization waves triggered by vagal stimulation in the embryonic chick brain: optical evidence for intercellular communication in the developing central nervous system

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 18beta-glycyrrhetinic acid suggests that the depolarization wave depends on functions of gap junctions. The profile obtained with Ca(2+)-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

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

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 Ca(2+)-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.

Spatial and temporal patterns of evoked neural activity from auditory nuclei in chick brainstem detected by optical recording

In order to detect the spatial patterning of the auditory projection of the embryonic chick brainstem, anatomical methods such as orthograde transport of horseradish peroxidase have been used. However, these methods do not provide the continuous information required about the absolute value and time-course of varying neural excitement. Furthermore, the use of conventional electrophysiological methods makes it difficult or impossible to detect the transmembrane voltage change because of the small size and fragility of the cells of the young chick brainstem. We thus believe that optical measurement of membrane potential might be beneficial in circumstances where electrodes are difficult to use for reasons of cell size, complexity, or membrane topology. In the present work, we therefore examined the feasibility of an optical method for delineating the synaptic transmission of afferent input in the auditory nuclei in the chick brainstem. We used embryonic chick brainstem slice preparations featuring an intact eighth nerve, and loaded depolarizing square current pulses from tungsten microelectrodes into the auditory nerve for stimulation of these preparations. In this approach, we used a multiple-site optical recording system comprising a 16 x 16-element photodiode array and a voltage sensitive dye (NK-2761). Neural excitation evoked by stimulation to the left auditory nerve was propagated to the dorsal side of the brainstem. This area in which the optical signal was detected is located on the auditory nuclei. Since the physiological spatial patterning of the auditory nerve projection could be roughly estimated by the optical technique, the technique is considered useful for examining the electrical activity generated from auditory nuclei in the brainstem. This is the first report of spatial patterning of auditory neurons in the embryonic chick brainstem generated through optical recording.

Optical mapping reveals the functional organization of the trigeminal nuclei in the chick embryo

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.

Optical identification of calcium-dependent action potentials transiently expressed in the embryonic rat brainstem

Using multiple-site optical recording of transmembrane potential changes, we have found a new type of calcium-dependent action potential expressed transiently in the embryonic rat dorsal motor nucleus of the vagus nerve. Slice preparations with vagus nerve fibers attached were dissected from 12- to 16-day-old embryonic (E12-E16) rat brainstems, and they were stained with a voltage-sensitive merocyanine-rhodanine dye (NK2761). Electrical activities in response to vagal stimuli were optically recorded simultaneously from many sites using 1020- or 128-element photodiode array measuring systems. In brainstem preparations, two types of action potential-related optical signals were identified. One was detected from the dorsolateral region, and was related to sensory nerve activity (Type I). The other was detected from the dorsomedial region, and corresponded to the action potential in the dorsal motor nucleus of the vagus nerve (Type II). We found a difference in the ionic basis of the Type I vs Type II signals. The Type I signal was not altered in Ca2+-free bathing solution and was eliminated by tetrodotoxin, suggesting that the sensory nerve activity is mediated by Na+ currents. The Type II signal at early developmental stages (E12-E13, and some preparations in E14) was also independent of Ca2+. However, the Type II signal in later developmental stages (E15-E16, and some preparations in E14) did depend upon Ca2+: it was eliminated in Ca2+-free Ringer's solution, blocked by Cd2+, Ni2+ or Mn2+, and elicited in Sr2+-containing Ringer's solution, where CaCl2 was replaced with SrCl2. These results suggest that the cation which dominates the motoneuron action potential changes from Na+ to Ca2+ during development, and this change occurs around E14. With pharmacological analysis using Ca2+ channel blockers, we show that the Ca2+ channel mediating the motoneuron action potential is distinct from T-, L-, N-, P- or Q-type channels. Because the vagal action potential in adult mammals is mainly mediated by Na+, we suggest that a Ca2+ action potential mediated by a new type of Ca2+ channel is expressed transiently in the rat dorsal motor nucleus of the vagus nerve at particular stages of development.

Optical imaging of the spatiotemporal patterning of neural responses in the embryonic chick superior cervical ganglion

Multiple-site optical recording of transmembrane potential changes with a voltage-sensitive dye was used to reveal the functional expression and developmental changes of the postsynaptic potentials in the early embryonic chick superior cervical ganglion. The ganglia were isolated from five- to 12-day-old chick embryos with preganglionic nerve fibres (vertebral and/or cervical carotic nerves) attached. The preparations were stained with a voltage-sensitive merocyanine-rhodanine dye (NK2761). Voltage-related optical (absorbance) changes were recorded simultaneously from 127 contiguous loci in the preparation, using a 12 x 12-element photodiode array. Optical changes having two components were evoked by preganglionic nerve stimulation. One component was the fast spike-like signal and another the delayed slow signal. The amplitude of the slow signal was decreased by repetitive stimulation, reduced by low external calcium ion concentrations and eliminated in the presence of manganese or cadmium ions. The slow signals were also eliminated in the presence of D-tubocurarine. Accordingly, we concluded that the slow signal corresponds to cholinergic excitatory postsynaptic potentials. In the five- and six-day-old superior cervical ganglia, only the fast optical signals (referred to as the action potentials) were recorded. Slow optical signals (referred to as the excitatory postsynaptic potentials) were detected from preparations older than seven days. The amplitude of the slow optical signal gradually increased, together with an expansion of the response area, as the developmental stage proceeded from seven to 10 days. To compare the distribution patterns of the neural responses evoked by stimuli applied to the cervical carotic and vertebral nerves, we have mapped and imaged the spatial patterning of the synaptic responses. In the maps, the positions of the peak size regions of the slow signals were assessed, and we found that there were differences in the location of these areas for the cervical carotic vs vertebral nerves. From these experimental results, we conclude that synaptic function within the chick superior cervical ganglion is initiated at the seven-day-old embryonic stage, and reaches a maximum level at 10 days. Synaptic transmission at these stages is mediated solely by nicotinic acetylcholine receptors. The spatial mapping of the synaptic responses reveals that the neural populations related synaptically to the cervical carotic and vertebral nerves are located separately within the ganglion, even at an early developmental stage.

Optical mapping of neural responses in the embryonic rat brainstem with reference to the early functional organization of vagal nuclei

We examined the functional organization of the vagal nuclei of the rat embryo during morphogenesis, using multiple-site optical recording with a voltage-sensitive dye. Slice preparations with vagus nerve fibers were dissected from 13- to 16-d-old embryonic (E13-E16) rat brainstems, and they were stained with the dye. Electrical activity in response to vagal stimulation was recorded optically from many sites. In the E13-E14 preparations, two types of spike-like optical signals were recorded: one was a narrow signal (type I), and the other was a broader signal (type II). Comparison with the morphology revealed by DiI labeling suggests that the type I signal response area corresponds to the nucleus of the tractus solitarius, and the type II signal response area corresponds to the dorsal motor nucleus of the vagus nerve. In the E15-E16 preparations, type I signals were followed by a slow signal related to glutamate-mediated excitatory postsynaptic potentials, suggesting that synaptic function is organized in the nucleus of the tractus solitarius by the 15-d-old embryonic stage. In the E14 preparation, a small, slow signal was evoked only in Mg2+-free solution, implying that postsynaptic function related to NMDA receptors emerges, in latent form, at the 14-d-old embryonic stage. In the E15 and E16 preparations, although the nucleus ambiguus is identified morphologically, no neural response-related optical signal was observed there, indicating that the embryonic organization of morphology and physiological function is not necessarily temporally coincident. We have mapped the dynamic spatiotemporal patterns of the evoked optical signals and have outlined the early phase of the functional organization of the cranial nuclei related to the vagus.