Co-localization of NMDA receptors and AMPA receptors in neurons of the vestibular nuclei of rats

Pigeon as a model to study peripheral projections from the horizontal semicircular canal vestibular apparatus to a brainstem target immunoreactive for AMPA

Purpose:

To evaluate whether the pigeon (Columba livia) is a good model for evaluating the vestibular system involved with postural maintenance during movement.

Methods:

This study maps the brainstem targets of the horizontal ampullary inputs from the vestibular periphery of the pigeon. We used biotin dextran amine (BDA) injection in horizontal semicircular canal (HSCC), immunohistochemistry for GluR2/3 and GluR4 AMPA and computerized histomorphology reconstruction.

Results:

Our results show the same distribution pattern with ipsilateral projections to vestibular nuclear complex (VNC) from the HSCC, with the majority of labeled fibers being, long, thin, with few varicosities and many ramifications. Horizontal semicircular canal projections achieve neurons belonging to all nuclei of the VNC with exception of dorsal portion of lateral vestibular nucleus and this area express GluR2/3 and GluR4 AMPA receptors reinforcing the idea of glutamate participation in these connections.

Conclusions:

Pigeon is an appropriated experimental model to study of projections of HSCC and reinforcing the information that the vestibular system has strong relation with the fast responses necessary for postural control. Moreover, its phylogenetic organization apparently conservation, also seems to be a fundamental characteristic for vertebrates.

Hypergravity-induced malfunction was moderated by the regulation of NMDA receptors in the vestibular nucleus

Gravity alteration is one of the critical environmental factors in the space, causing various abnormal behaviors related with the malfunctioned vestibular system. Due to the high plastic responses in the central vestibular system, the behavioral failures were resolved in a short period of time (in approx. 72 h). However, the plastic neurotransmission underlying the functional recovery is still elusive. To understand the neurotransmitter-induced plasticity under hypergravity, the extracellular single neuronal recording and the immunohistochemistry were conducted in the vestibular nucleus (VN). The animals were grouped as control, 24-h, 72-h, and 15-day exposing to 4G-hypergravity, and each group had two subgroups based on the origins of neuronal responses, such as canal and otolith. The averaged firing rates in VN showed no significant difference in the subgroups (canal-related: p > 0.105, otolith-related: p > 0.138). Meanwhile, the number of NMDAr was significantly changed by the exposing duration to hypergravity. The NMDAr decreased in 24 h (p = 1.048 × 10^–9), and it was retrieved in 72 h and 15 days (p < 4.245 × 10^–5). Apparently, the reduction and the retrieval in the number of NMDAr were synchronized with the generation and recovery of the abnormal behaviors. Thus, the plasticity to resolve the hypergravity-induced malfunctional behaviors was conducted by regulating the number of NMDAr.

Repeated Galvanic Vestibular Stimulation Modified the Neuronal Potential in the Vestibular Nucleus

Vestibular nucleus (VN) and cerebellar flocculus are known as the core candidates for the neuroplasticity of vestibular system. However, it has been still elusive how to induce the artificial neuroplasticity, especially caused by an electrical stimulation, and assess the neuronal information related with the plasticity. To understand the electrically induced neuroplasticity, the neuronal potentials in VN responding to the repeated electrical stimuli were examined. Galvanic vestibular stimulation (GVS) was applied to excite the neurons in VN, and their activities were measured by an extracellular neural recording technique. Thirty-eight neuronal responses (17 for the regular and 21 for irregular neurons) were recorded and examined the potentials before and after stimulation. Two-third of the population (63.2%, 24/38) modified the potentials under the GVS repetition before stimulation (p = 0.037), and more than half of the population (21/38, 55.3%) changed the potentials after stimulation (p = 0.209). On the other hand, the plasticity-related neuronal modulation was hardly observed in the temporal responses of the neurons. The modification of the active glutamate receptors was also investigated to see if the repeated stimulation changed the number of both types of glutamate receptors, and the results showed that AMPA and NMDA receptors decreased after the repeated stimuli by 28.32 and 16.09%, respectively, implying the modification in the neuronal amplitudes.

Postsynaptic GluN2B-containing NMDA receptors contribute to long-term depression induction in medial vestibular nucleus neurons of juvenile rats

Medial vestibular nucleus (MVN) neurons are involved in the regulation of eye movements to endure the stability of the image during head movement, and play a critical role in plasticity of the vestibulo-ocular reflex (VOR) during the juvenile period. We have previously shown that the long-term depression (LTD) of synaptic transmission was induced by high frequency stimulation (HFS) and blocked by N-methyl-D-aspartate (NMDA) receptor antagonist D-APV at the vestibular afferent synapses of type-B MVN neurons. In the present study, we used whole-cell patch-clamp recordings in vitro to investigate the subunit composition of these NMDA receptors in the induction of LTD in MVN slices from postnatal 13-16 day rats. We found that LTD induced in type-B neurons of the rat MVN with HFS was blocked by Ro 25-6981, a specific antagonist for GluN2B-containing NMDA receptors. Moreover, the other selective GluN2B-containing NMDA receptor antagonist (ifenprodil) also prevented the induction of LTD. However, bath application of the GluN2A-containing NMDA receptor antagonists (Zn²⁺ and TCN 201) had no influence on the induction of LTD. Similar results were obtained by exogenously applied two GluN2C/GluN2D-preferring NMDA receptor antagonists (PPDA and UBP 141). Furthermore, presynaptic NMDA receptor subunits are not necessary for vestibular LTD. These results suggest that the induction of LTD by HFS in vestibular afferent synapses of type-B MVN neurons requires postsynaptic GluN2B-containing NMDA receptors, but not GluN2A-containing NMDA receptors or GluN2C/GluN2D-containing NMDA receptors.

Noradrenergic modulation of neuronal responses to n-methyl-d-aspartate in the vestibular nuclei: an electrophysiological and immunohistochemical study

Excitatory responses evoked by N-methyl-d-aspartate (NMDA) in the vestibular nuclei (VN) of the rat were studied in vivo during microiontophoretic application of noradrenaline (NA) and/or its agonists and antagonists. Ejection of NA-modified excitatory responses mediated by NMDA receptors (NMDAR) in all neurons tested; the effect was enhancement in 59% of cases and depression in the remaining 41%. Enhancements prevailed in all VN with the exception of the lateral vestibular nucleus, where both effects were recorded in an equal number of cases. The enhancing action of NA on NMDAR-mediated responses was mimicked by the noradrenergic beta-receptor agonist isoproterenol, the beta1 specific agonist denopamine and the alpha2 agonist clonidine. These effects were blocked respectively by the generic beta-receptor antagonist timolol, the beta1 antagonist atenolol and the alpha2 antagonist yohimbine. In contrast, application of the alpha1 receptor agonist cirazoline and the specific alpha1 antagonist prazosin respectively mimicked and partially antagonized the depression of NMDAR-mediated excitations induced by NA. Double-labeling immunohistochemical techniques demonstrated broad colocalization of NMDAR (specifically NR1 and NR2 subunits) with noradrenergic receptors (alpha1, alpha2 and beta1) in many VN neurons; only minor differences were found between nuclei. These results indicate that NA can produce generalized modulation of NMDAR-mediated excitatory neurotransmission in VN, which may in turn modify synaptic plasticity within the nuclei.

Neuronal detection thresholds during vestibular compensation: contributions of response variability and sensory substitution

The vestibular system is responsible for processing self-motion, allowing normal subjects to discriminate the direction of rotational movements as slow as 1-2 deg s(-1). After unilateral vestibular injury patients' direction-discrimination thresholds worsen to ∼20 deg s(-1), and despite some improvement thresholds remain substantially elevated following compensation. To date, however, the underlying neural mechanisms of this recovery have not been addressed. Here, we recorded from first-order central neurons in the macaque monkey that provide vestibular information to higher brain areas for self-motion perception. Immediately following unilateral labyrinthectomy, neuronal detection thresholds increased by more than two-fold (from 14 to 30 deg s(-1)). While thresholds showed slight improvement by week 3 (25 deg s(-1)), they never recovered to control values - a trend mirroring the time course of perceptual thresholds in patients. We further discovered that changes in neuronal response variability paralleled changes in sensitivity for vestibular stimulation during compensation, thereby causing detection thresholds to remain elevated over time. However, we found that in a subset of neurons, the emergence of neck proprioceptive responses combined with residual vestibular modulation during head-on-body motion led to better neuronal detection thresholds. Taken together, our results emphasize that increases in response variability to vestibular inputs ultimately constrain neural thresholds and provide evidence that sensory substitution with extravestibular (i.e. proprioceptive) inputs at the first central stage of vestibular processing is a neural substrate for improvements in self-motion perception following vestibular loss. Thus, our results provide a neural correlate for the patient benefits provided by rehabilitative strategies that take advantage of the convergence of these multisensory cues.

Neural correlates of motor learning in the vestibulo-ocular reflex: dynamic regulation of multimodal integration in the macaque vestibular system

Motor learning is required for the reacquisition of skills that have been compromised as a result of brain lesion or disease, as well as for the acquisition of new skills. Behaviors with well characterized anatomy and physiology are required to yield significant insight into changes that occur in the brain during motor learning. The vestibulo-ocular reflex (VOR) is well suited to establish connections between neurons, neural circuits, and motor performance during learning. Here, we examined the linkage between neuronal and behavioral VOR responses in alert behaving monkeys (Macaca mulatta) during the impressive recovery that occurs after unilateral vestibular loss. We show, for the first time, that motor learning is characterized by the dynamic reweighting of inputs from different modalities (i.e., vestibular vs extravestibular) at the level of the single neurons that constitute the first central stage of vestibular processing. Specifically, two types of information, which did not influence neuronal responses before the lesion, had an important role during compensation. First, unmasked neck proprioceptive inputs played a critical role in the early stages of this process demonstrated by faster and more substantial recovery of vestibular responses in proprioceptive sensitive neurons. Second, neuronal and VOR responses were significantly enhanced during active relative to passive head motion later in the compensation process (>3 weeks). Together, our findings provide evidence linking the dynamic regulation of multimodal integration at the level of single neurons and behavioral recovery, suggesting a role for homeostatic mechanisms in VOR motor learning.

Excitatory pathways from the vestibular nuclei to the NTS and the PBN and indirect vestibulo-cardiovascular pathway from the vestibular nuclei to the RVLM relayed by the NTS

Previous studies have confirmed the existence of vestibulo-sympathetic pathways in the central nervous system. However, the exact pathways and neurotransmitters underlying this reflex are unclear. The present study was undertaken to investigate whether the vestibulo-cardiovascular responses are a result of activated glutamate receptors in the caudal vestibular nucleus. We also attempt to verify the indirect excitatory pathways from the vestibular nucleus (VN) to the rostral ventrolateral medulla (RVLM) using a tracing method combined with a vesicular glutamate transporter (VGluTs) immunofluorescence. In anesthetized rats, unilateral injection of l-glutamate (5 nmol) into the medial vestibular nucleus (MVe) and spinal vestibular nucleus (SpVe) slightly increased the mean arterial pressure (MVe: 93.29+/-11.58 to 96.30+/-11.66, SpVe: 91.72+/-15.20 to 95.48+/-17.16). Local pretreatment with the N-methyl-D-aspartate (NMDA)-receptor antagonist MK-801 (2 nmol) significantly attenuated the pressor effect of L-glutamate injected into the MVe compared to the contralateral self-control. After injection of biotinylated dextran amine (BDA) into the MVe and SpVe, and fluorogold (FG) into the RVLM, some BDA-labeled fibres and terminals in the nucleus of solitary tract (NTS) and the parabrachial nucleus (PBN) were immunoreactive for VGluT1 and VGluT2. Several BDA-labeled fibres were closely apposed to FG-labeled neurons in the NTS. These results suggested that activation of caudal vestibular nucleus neurons could induce pressor response and NMDA receptors might contribute to this response in the MVe. The glutamatergic VN-NTS and VN-PBN pathways might exist, and the projections from the VN to the RVLM relayed by the NTS comprise an indirect vestibulo-cardiovascular pathway in the brain stem.

Developmental expression of NMDA and AMPA receptor subunits in vestibular nuclear neurons that encode gravity-related horizontal orientations

We examined the expression profile of subunits of ionotropic glutamate receptors [N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionate (AMPA)] during postnatal development of connectivity in the rat vestibular nucleus. Vestibular nuclear neurons were functionally activated by constant velocity off-vertical axis rotation, a strategy to stimulate otolith organs in the inner ear. These neurons indicated Fos expression as a result. By immunodetection for Fos, otolith-related neurons that expressed NMDA/AMPA receptor subunits were identified as early as P7, and these neurons were found to increase progressively up to adulthood. Although there was developmental invariance in the percentage of Fos-immunoreactive neurons expressing the NR1, NR2A, GluR1, or GluR2/3 subunits, those expressing the NR2B subunit decreased from P14 onward, and those expressing the GluR4 subunit decreased in adults. These double-immunohistochemical data were corroborated by combined immuno-/hybridization histochemical data obtained from Fos-immunoreactive neurons expressing NR2B mRNA or GluR4 mRNA. The staining of both NR2B and GluR4 in the cytoplasm of these neurons decreased upon maturation. The percentage of Fos-immunoreactive neurons expressing the other ionotropic glutamate receptor subunits (viz. NR1, NR2A, GluR1, and GluR2/3) remained relatively constant throughout postnatal maturation. Triple immunofluorescence further demonstrated coexpression of NR1 and NR2 subunits in Fos-immunoreactive neurons. Coexpression of NR1 subunit with each of the GluR subunits was also observed among the Fos-immunoreactive neurons. Taken together, the different expression profiles of ionotropic glutamate receptor subunits constitute the histological basis for glutamatergic neurotransmission in the maturation of central vestibular connectivity for the coding of gravity-related horizontal head movements.

Morphometric analysis of the AMPA-type neurons in the Deiter's vestibular complex of the chick brain

Chicken (Gallus gallus) brains were used to investigate the typology and the immunolabel pattern for the subunits composing the AMPA-type glutamate receptors (GluR) of hindbrain neurons of the dorsal (dND) and ventral nuclei (vND) of the Deiter's vestibular complex (CD), which is the avian correspondent of the lateral vestibular nucleus (LVN) of mammals. Our results revealed that neurons of both divisions were poor in GluR1. The vND, the GluR2/3+ and GluR4+ label presented no area or neuronal size preference, although most neurons were around 75%. The dND neurons expressing GluR2/3 are primarily around 85%, medium to large-sized 85%, and predominantly 60% located in the medial portion of the rostral pole and in the lateral portion of the caudal pole. The majority of dND neurons containing GluR4 are also around 75%, larger (70% are large and giant), exhibiting a distribution that seems to be complementary to that of GluR2/3+ neurons. This distinct arrangement indicates functional differences into and between the DC nuclei, also signaling that such variation could be attributed to the diverse nature of the subunit composition of the GluRs. Discussion addresses the morphological and functional correlation of the avian DC with the LVN of mammals in addition to the high morphological correspondence, To include these data into the modern comparative approach we propose to adopt a similar nomenclature for the avian divisions dND and vND that could be referred as dLVN and vLVN.

Maturation of otolith-related brainstem neurons in the detection of vertical linear acceleration in rats

To investigate the critical maturation time of otolith-related neurons in processing vertical orientations, rats (postnatal day 4 to adults) were studied for functional activation of c-fos expression in brainstem neurons by immuno-/hybridization histochemistry. Conscious rats were subjected to sinusoidal linear acceleration along the vertical plane. Labyrinthectomized and/or stationary controls showed only sporadically scattered Fos-labeled neurons in the vestibular nuclei, confirming an otolithic origin of c-fos expression. Functionally activated Fos expression in neurons of the medial and spinal vestibular nuclei and group x were identifiable by P7 and those in group y by P9. A small number of Fos-labeled neurons characterized by small soma size were found in the ventral part of lateral vestibular nucleus by P9. Other vestibular-related areas such as prepostitus hypoglossal nucleus, gigantocellular reticular nucleus and locus coeruleus of normal experimental rats showed functionally activated c-fos expression at P7. Neurons in dorsal medial cell column and beta subnucleus of the inferior olive only showed functionally activated c-fos expression by the second postnatal week. These findings revealed a unique critical maturation time for each of the vestibular-related brainstem areas in the recognition of gravity-related vertical head orientations. By mapping the three-dimensional distribution of Fos-immunoreactive neurons, we found an even distribution of otolith-related neurons within the spinal vestibular nucleus in groups x and y but a clustered distribution in the middle-lateral-ventral part of the medial vestibular nucleus. Taken together, our findings reveal the developmental profile of neuronal subpopulations within the vertical otolith system, thereby providing an anatomical basis for postnatal coding of gravity-related vertical head movements.

Spatial coding capacity of central otolith neurons

This review focuses on recent approaches to unravel the capacity of otolith-related brainstem neurons for coding head orientations. In the first section, the spatiotemporal features of central vestibular neurons in response to natural otolithic stimulation are reviewed. Experiments that reveal convergent inputs from bilateral vestibular end organs bear important implications on the processing of spatiotemporal signals and integration of head orientational signals within central otolith neurons. Another section covers the maturation profile of central otolith neurons in the recognition of spatial information. Postnatal changes in the distribution pattern of neuronal subpopulations that subserve the horizontal and vertical otolith systems are highlighted. Lastly, the expression pattern of glutamate receptor subunits and neurotrophin receptors in otolith-related neurons within the vestibular nuclear complex are reviewed in relation to the potential roles of these receptors in the development of vestibular function.

Differential expression of NMDA and AMPA/KA receptor subunits in the inferior olive of postnatal rats

We have employed immunohistochemistry to determine the expression patterns of receptor subunits of N-methyl-d-aspartate (NMDA-NR1 and NR2A/B) and alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid/kainic acid (AMPA/KA-GluR1, GluR2, GluR2/3, GluR4, and GluR5/6/7) in the inferior olive of postnatal rats up to adulthood. Immunoreactivity for distinct receptor subunits was predominantly localized in the soma and dendrites of neurons. Semi-quantification showed that the overall immunoreactivity in the inferior olive of adults was intense for GluR1, moderate for NR1 and NR2A/B, and low for GluR2, GluR2/3, GluR4, and GluR5/6/7. At P7, GluR1 was restricted to the dorsomedial cell column, subnucleus beta, principal nucleus and ventrolateral protrusion while the other subunits were found in all subnuclei of the inferior olive. The immunoreactivities for all glutamate receptor subunits ranged from low to moderate. As the rats matured, the immunoreactivity of GluR4 decreased after the second postnatal week, while those of the other subunits showed a general trend of increase, reaching adult level during the third postnatal week. Double immunofluorescence revealed that all NR1-containing neurons exhibited NR2A/B immunoreactivity, indicating that native NMDA receptors comprise of hetero-oligomeric combinations of NR1 and NR2A/B. Furthermore, co-localization of NMDA and AMPA/KA receptor subunits was demonstrated in individual neurons of the inferior olive. All NR1-containing neurons exhibited GluR1 immunoreactivity, and all NR2A/B-containing neurons showed GluR5/6/7 immunoreactivity. Our data suggest that NMDA and AMPA/KA receptors are involved in glutamate-mediated neurotransmission, contributing to synaptic plasticity and reorganization of circuitry in the inferior olive during postnatal development.

The anatomy of the vestibular nuclei

The vestibular portion of the eighth cranial nerve informs the brain about the linear and angular movements of the head in space and the position of the head with respect to gravity. The termination sites of these eighth nerve afferents define the territory of the vestibular nuclei in the brainstem. (There is also a subset of afferents that project directly to the cerebellum.) This chapter reviews the anatomical organization of the vestibular nuclei, and the anatomy of the pathways from the nuclei to various target areas in the brain. The cytoarchitectonics of the vestibular brainstem are discussed, since these features have been used to distinguish the individual nuclei. The neurochemical phenotype of vestibular neurons and pathways are also summarized because the chemical anatomy of the system contributes to its signal-processing capabilities. Similarly, the morphologic features of short-axon local circuit neurons and long-axon cells with extrinsic projections are described in detail, since these structural attributes of the neurons are critical to their functional potential. Finally, the composition and hodology of the afferent and efferent pathways of the vestibular nuclei are discussed. In sum, this chapter reviews the morphology, chemoanatomy, connectivity, and synaptology of the vestibular nuclei.

Differential expression of AMPA receptor subunits in substance P receptor-containing neurons of the caudate-putamen of rats

Previous evidence has suggested that glutamate-driving neurotransmission and glutamate-excitotoxicity are modulated by substance P in the basal ganglia, but the assembly of glutamate receptors mediating this process remains to be delineated. By using a double immunofluorescence, cellular expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunits (GluR1-4) in substance P receptor (SPR)-containing neurons was examined in the striatum of rats. It revealed that distribution of SPR-immunoreactive neurons completely overlapped with that of GluR1, 2, 3 or 4-immunoreactive neurons in the caudate-putamen. Neurons showing both SPR and AMPA receptor subunits (except of GluR3)-immunoreactivity were observed: all (100%) of SPR-positive neurons displayed GluR1-, GluR2- or GluR4-immunoreactivity, and the double-labeled neurons constituted about 33, 3 or 29% of total GluR-positive ones. In contrast, the neurons exhibiting both SPR- and GluR3-immunoreactivity were not detected, though numerous GluR3-positive neurons were still distributed in the caudate-putamen regions. Co-localization of SPR and distinct AMPA receptor subunits in the striatal neurons has provided a basis for functional modulation of neuronal APMA receptors by substance P in the caudate-putamen of rodents. Taken together with previous observations, this study has also suggested that, through interaction with AMPA receptors composed of subunits 1, 2 and 4, substance P or neurokinin peptides may play important roles in regulating neuronal properties and protecting neurons from excitotoxicity in the basal ganglia of mammals.

Differential expression of NMDA and AMPA receptor subunits in DARPP-32-containing neurons of the cerebral cortex, hippocampus and neostriatum of rats

Dopamine and cyclic adenosine 3',5'-monophosphate-regulated phosphoprotein, 32 kDa (DARPP-32) is a key element of dopamine/D1/DARPP-32/protein phosphatase-1 (PP-1) signaling cascades of mammalian brain. We are interested in the expression patterns of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in DARPP-32-containing neurons, which may constitute morphological basis for interaction between dopamine and ionotropic glutamate receptors in dopaminoceptive cells. Double immunofluorescence was performed to visualize neurons showing coexpression of DARPP-32 with NMDA or AMPA receptor subunits (i.e., NR1, NR2a/b, glutamate receptor subunit 1 [GluR1], GluR2/3, and GluR4) in the forebrains of rats. Distribution of DARPP-32-positive neurons completely or partially overlapped with that of NMDA receptor- or AMPA receptor-immunoreactive ones in the frontal and parietal cortex, hippocampus and neostriatum, and neurons double-labeled with DARPP-32/NR1, DARPP-32/NR2a/b, DARPP-32/GluR1, DARPP-32/GluR2/3, or DARPP-32/GluR4 immunoreactivity were numerously observed. Semiquantification analysis indicated that most of DARPP-32-containing neurons (86-98%) expressed NR1, NR2a/b and GluR2/3, while less of them (14-90%) expressed GluR1 and GluR4. Although high rates (90-98%) of DARPP-32-positive cells expressed NMDA receptors in all regions above, variant percentages of them expressing AMPA receptor subunits were observed among the cortex (54-90%), hippocampus (59-97%) and neostriatum (14-97%). The study presents differential expression patterns of NMDA and AMPA receptors in DARPP-32-postive neurons in these forebrain regions. Taken together with previous reports, the present data suggest that interaction between dopamine and glutamate receptors may occur in the dopaminoceptive neurons with distinct receptor compositions and may be involved in modulating neuronal properties and excitotoxicity in mammalian forebrain.

Fos expression in otolith-related brainstem neurons of postnatal rats following off-vertical axis rotation

To determine the critical time of responsiveness of developing otolith organ-related brainstem neurons and their distribution, Fos protein expression in response to off-vertical axis rotations (OVAR) was mapped in conscious Sprague Dawley rats from P5 to adulthood. OVAR was used to activate sequentially all utricular hair cells per 360 degrees revolution. We detected the coding of horizontal head positions in otolith organ-related neurons within the vestibular nucleus as early as P7. In the vestibular nuclear complex and its subgroups, the density of Fos-immunoreactive (Fos-ir) neurons increased steadily with age and reached the adult level by P21. In both labyrinthectomized rats subjected to OVAR and normal rats kept stationary, labeled neurons were found sporadically in the aforementioned brain regions in each age group, confirming that Fos labeling observed in neurons of normal experimental rats subjected to OVAR was due to otolith organ stimulation. Whereas OVAR-induced Fos-ir neurons were also first observed in vestibular-related brain areas, such as the prepositus hypoglossal nucleus, gigantocellular reticular nucleus, and locus coeruleus, of normal experimental rats at P7, those in the inferior olive were observed only from P14 onward. This indicates the unique maturation time of inferior olivary neurons in gravity-related spatial coding. In general, age-dependent increase in OVAR-induced Fos-ir neurons was observed in brain areas that received otolith inputs. The locus coeruleus was exceptional in that prominent OVAR-induced Fos-ir neuronal number did not change with maturation, and this was well above the low but significant number of Fos-ir neurons in control preparations. Taken together, our results suggest that neuronal subpopulations within the developing network of the horizontal otolith system provide an anatomical basis for the postnatal development of otolith organ-related sensorimotor functions. J. Comp. Neurol. 470:282-296, 2004.

Neurotrophin receptor immunostaining in the vestibular nuclei of rats

The distribution of high-affinity neurotrophin receptors in cells of the vestibular nuclear complex and its subnuclei of adult rats was examined. We noted a high density of tyrosine kinase (Trk) A- and B- and a lower density of TrkC-immunostained cells. In particular, long, intensely labelled immunostained-TrkB fibres formed networks in the neuropil. Both TrkA- and TrkB-immunostained cells were widely distributed in the lateral, medial and spinal vestibular nuclei, and were less frequently seen in the superior vestibular nucleus, x and y subnuclei. However, immunostaining for TrkC was weak in many cells within the vestibular nuclei. The widespread and abundant neuronal distribution of Trk receptors predicts that their associated neurotrophins exert significant effects on individual cells within the vestibular nuclei.

Distribution of alpha-amino-3-hydroxy-5-methyl-4 isoazolepropionic acid and N-methyl-D-aspartate receptor subunits in the vestibular and spiral ganglia of the mouse during early development

We investigated the distribution of the glutamate receptor subunits, alpha-amino-3-hydroxy-5-methyl-4 isoazolepropionic acid (AMPA) GluR2 and GluR2/R3, and N-methyl-D-aspartate (NMDA) NR1, and the timing of their appearance during early development of the mouse vestibular and spiral ganglia. NMDA NR1 was the first to be expressed, in the statoacoustic ganglion neurons on E11. GluR2/R3 immunoreactivity was detected in these neurons on E12. This signal probably corresponded exclusively to GluR3, as no signal was obtained for GluR2 alone at this stage. The appearance of these proteins began much earlier than previously reported. GluR2 staining was observed later, on E14 in the vestibular neurons and on E17 in the spiral neurons. The sequence in which these three glutamate receptors appeared suggested possible differences in their roles in the establishment of neuronal circuitry in the inner ear sensory epithelia. The production of NR1 and GluR2/R3 began during the early period of neuron growth and fasciculation. GluR2 appeared later and its expression paralleled synaptogenesis in the vestibular sensory epithelia and in the organ of Corti.

Activation of different vestibular subnuclei evokes differential respiratory and pressor responses in the rat

Activation of the vestibular system can either increase or decrease ventilation. The objectives of the present study were to clarify whether these different responses are the result of activating different vestibular subnuclei, by addressing three questions. Do neurones within the medial, lateral and spinal vestibular nuclei (VN(M), VN(L) and VN(S), respectively) function differently in respiratory modulation? Is the ventral medullary nucleus gigantocellularis (NGC) required to fully express the VN-mediated respiratory responses? Is glutamate, by acting on N-methyl-D-aspartic acid (NMDA) receptors in the vestibular subnuclei, capable of modulating respiration? In anaesthetized, tracheotomized and spontaneously breathing rats, electrical stimuli (< 10 s) applied in the VN(L) and VN(S) significantly elevated ventilation by 35 % and 30 % (P < 0.05), respectively. However, VN(M) stimulation produced statistically significant (P < 0.05) changes that differed depending upon the stimulation site: either ventilatory inhibition (by 40 % in 57 % of the trials) or excitation (by 55 % in 43 % of trials), and which were often accompanied by a pressor response. These electrical-stimulation-evoked cardiorespiratory responses were almost eliminated following microinjection of ibotenic acid into the stimulation sites (P < 0.05) or bilaterally into the NGC (P < 0.05). As compared to vehicle, microinjection of NMDA into the unilateral VN(M), VN(L) and VN(S) significantly increased ventilation to 74 %, 58 % and 60 % (P < 0.05), respectively, with no effect on arterial blood pressure. These data suggest that neurones within the vestibular subnuclei play different roles in cardiorespiratory modulation, and that the integrity of the NGC is essential for the full expression of these VN-mediated responses. The evoked respiratory excitatory responses are probably mediated by glutamate acting on NMDA receptors, whereas the neurotransmitters involved in VN(M)-mediated respiratory inhibition and hypertension remain unknown.

A general framework for neurobiological modeling: an application to the vestibular system

The otolith organs in the vestibular system are excellent detectors of linear accelerations. However, any measurement of linear acceleration is ambiguous between a tilt in a gravitational field and an inertial acceleration. Angelaki et al. have put forward a general hypothesis about how inertial accelerations can be computed based on vestibular signals (J. Neurosci. 19 (1999) 316). We have constructed a realistic, detailed model of the relevant systems to test this hypothesis. The model produces useful predictions about what kinds of neurons should be found in the vestibular nucleus if such a computation is actually performed in the vestibular system. The model is constructed using general principles of neurobiological simulation (J. Neurophys. 84 (2000) 2113).

Differential expression of AMPA receptor subunits in dopamine neurons of the rat brain: a double immunocytochemical study

We have examined the distribution of dopamine neurons expressing alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunits (glutamate receptors 1, 2/3 and 4) in the A8-A15 regions of the rat brain using double immunofluorescence. The distribution of glutamate receptor 1- or 2/3-like immunoreactive neurons completely overlapped that of tyrosine hydroxylase-like immunoreactive neurons in dopamine cell groups in the retrorubral field (A8), the substantia nigra (A9), the ventral tegmental area and the nucleus raphe linealis (A10), and the rostral hypothalamic periventricular nucleus (A14, A15). In the caudal hypothalamic periventricular nucleus (A11), arcuate nucleus (A12) and zona incerta (A13), the distribution was partially overlapping. Neurons double-labeled for tyrosine hydroxylase and glutamate receptor 1 or 2/3 immunoreactivities were, however, exclusively found in certain dopamine cell regions: in areas A14-A15, 85-88% of tyrosine hydroxylase-containing neurons expressed glutamate receptor 1 and 22-25% expressed glutamate receptor 2/3, while in areas A8-A10, 20-43% expressed glutamate receptor 1 and 63-84% expressed glutamate receptor 2/3. In contrast, the double-labeled neurons were hardly detected in the A11-A13 regions. No tyrosine hydroxylase-positive neurons displayed glutamate receptor 4 immunoreactivity, though a partially overlapping distribution of tyrosine hydroxylase- and glutamate receptor 4-immunopositive neurons was also seen in regions A8-10, A11 and A13. The present study has demonstrated the morphological evidence for direct modulation of dopamine neurons via AMPA receptors in rat mesencephalon and hypothalamus. This distribution may provide the basis for a selective dopamine neuron loss in neurodegenerative disorders, such as Parkinson's disease.

Influence of serotonin on the glutamate-induced excitations of secondary vestibular neurons in the rat

The excitatory responses evoked by glutamate and its agonists in secondary vestibular neurons of the rat were studied during microiontophoretic application of 5-hydroxytryptamine (5-HT). Ejection of 5-HT modified neuronal responsiveness to glutamate in 86% of the studied units, the effect being a depression of the excitatory responses in two-thirds of cases and an enhancement in the remaining third. 5-HT was also effective in modifying 94% of the responses evoked by N-methyl-d-aspartate (NMDA), inducing a depressive effect in 76% of cases and an enhancement in the remaining ones. Quisqualate-evoked effects were depressed and enhanced by 5-HT in about the same number of cases; in contrast, kainate-evoked responses were enhanced. The depressive action of 5-HT was mimicked by application of alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT), a 5-HT(2) receptor agonist, whereas the enhancing effect could be evoked by application of 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT(1A) receptor agonist. The 5-HT(2) receptor antagonist ketanserin was able to reduce, but not to block totally, the depressive action of 5-HT on glutamate- or NMDA-evoked responses. No significant difference was detected between neuronal responses in the lateral and the superior vestibular nucleus. These results indicate that 5-HT is able to modulate the responsiveness of secondary vestibular neurons to excitatory amino acids. Its action is mostly depressive, involves 5-HT(2) receptors, and is exerted on NMDA receptors. A minor involvement of other 5-HT receptors (at least 5-HT(1A)) and other glutamate receptors (for quisqualate and kainate) in the modulatory action of 5-HT is plausible.

Spontaneous discharge and response characteristics of central otolith neurons of rats during postnatal development

To study the developmental profile of otolith-related vestibular nuclear neurons, their spontaneous activities and response dynamics were examined in decerebrate rats aged seven, 14, 21 and 84 (adult) days. Extracellular recordings were performed in the lateral and descending vestibular nucleus of animals held at the stationary position in the earth-horizontal or subjected to constant velocity off-vertical axis rotation, which selectively stimulates the otolith receptors. All neurons displayed sinusoidal position-dependent modulation in discharge rate, indicating their capability in coding spatial information during low-frequency head movement. Some neurons showed a full-cycle response to off-vertical axis rotation (non-clipped), while other neurons were silenced in discharge during parts of each rotary cycle (clipped). In seven-day-old rats, three-quarters of the responsive neurons sampled were clipped and the proportion progressively decreased to less than one-quarter in adult rats. In each age group, the clipped neurons discharged in approximately 60% of the stimulus cycle. Response gains of the neurons increased with age, reaching a plateau from 21 days of age for clipped neurons and 14 days for non-clipped neurons. The clipped neurons demonstrated higher response gains than the non-clipped neurons at or beyond 21 days of age. Spontaneous activities of the neurons at the stationary and earth-horizontal positions were analysed in relation to their response gains; a positive correlation was observed from 14 days of age onwards. Both types of neurons showed progressive increase in spontaneous activity as the rats matured, though the clipped neurons exhibited significantly lower resting rates than the non-clipped neurons at each of the age groups studied. Some neurons that responded to off-vertical axis rotation were not spontaneously active at the stationary position, but the proportion of these decreased significantly with age. The coefficient of variation of each age group showed a bimodal distribution, thereby allowing spontaneously active neurons to be assigned as regular or irregular. Though the vast majority of both the clipped and non-clipped neurons showed irregular discharge patterns at seven days of age, the overall population became more regular as the rats matured. Irregular neurons of young rats exhibited phase-stable and phase-shift responses, while those of older rats showed only the phase-stable response. This distinction was not observed amongst regular neurons over the ages studied. Our results reveal features of central otolith neurons that can be taken as signs of maturation during the first three postnatal weeks. These neuronal features provide the framework for the analysis of behaviours mediated by the otolith system during postnatal maturation.