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

Stimulation of otolith irregular fibers produces a rostro-caudal gradient in activity in the vestibular nuclear complex (VNC), but not the vestibulocerebellum (VeCb)

The vestibular system is important for posture, balance, motor control, and spatial orientation. Each of the vestibular end organs have specialized neuroepithelia with both regular and irregular afferents. In otolith organs, the utricle and saccule, afferents most responsive to linear jerk (jerk - derivative of acceleration) are located in the striola and project centrally to the vestibular nuclear complex (VNC) as well as the uvula and nodulus of the vestibulocerebellum (VeCb). The pattern of central neuronal activation attributed to otolith irregular afferents is relatively unknown. To address this gap, c-Fos was used as a marker of neuronal activity to map the distribution of active neurons throughout the rostro-caudal extent of the VNC and VeCb. Immunohistochemistry for c-Fos was performed to assess activation of VNC and VeCb neurons in response to a linear jerk stimulus delivered in the naso-occipital plane. Activated neurons were distributed throughout the VNC, including the lateral vestibular nucleus (LVe), magnocellular medial vestibular nucleus (MVeMC), parvocellular medial vestibular nucleus (MVePC), spinal vestibular nucleus (SpVe), and superior vestibular nucleus (SuVe). Notably, after stimulation, the MVePC exhibited the greatest number of c-Fos labeled nuclei. Significant increases in c-Fos labeling were found in mid-rostrocaudal and caudal regions of the VNC in the LVe, MVe, and SpVe. Additionally, c-Fos labeling was observed across all regions of the VeCb after jerk stimulation. Significant increases in the number of labeled nuclei were found throughout the rostro-caudal extent of the nodulus and uvula. However, jerk stimulated increases in activity for the paraflocculus were restricted to the caudal VeCb. The distribution of neuronal activity suggests that regions receiving the greatest direct otolith input exhibit the most substantial changes in response to otolith derived, irregular fiber stimulation.

Highlights

Nuclei with descending projections (LVe, MVePC, and SpVe) demonstrated the greatest change in activity after naso-occipital jerk stimulation.Naso-occipital jerk stimulation preferentially activates caudal VNC neuronsNaso-occipital jerk stimulation activates neurons throughout the VeCbJerk stimulation in the naso-occipital plane has the greatest effects on activity in VNC and VeCb regions with the greatest inputs from afferents originating in gravity receptors.

Sensorineural correlates of failed functional recovery after natural regeneration of vestibular hair cells in adult mice

Vestibular hair cells (HCs) are mechanoreceptors that sense head motions by modulating the firing rate of vestibular ganglion neurons (VGNs), whose central processes project to vestibular nucleus neurons (VNNs) and cerebellar neurons. We explored vestibular function after HC destruction in adult Pou4f3+/DTR (DTR) mice, in which injections of high-dose (50 ng/g) diphtheria toxin (DT) destroyed most vestibular HCs within 2 weeks. At that time, DTR mice had lost the horizontal vestibulo-ocular reflex (aVORH), and their VNNs failed to upregulate nuclear cFos expression in response to a vestibular stimulus (centrifugation). Five months later, 21 and 14% of HCs were regenerated in utricles and horizontal ampullae, respectively. The vast majority of HCs present were type II. This degree of HC regeneration did not restore the aVORH or centrifugation-evoked cFos expression in VNNs. The failure to regain vestibular pathway function was not due to degeneration of VGNs or VNNs because normal neuron numbers were maintained after HC destruction. Furthermore, sinusoidal galvanic stimulation at the mastoid process evoked cFos protein expression in VNNs, indicating that VGNs were able to regulate VNN activity after HC loss. aVORH and cFos responses in VNNs were robust after low-dose (25 ng/g) DT, which compared to high-dose DT resulted in a similar degree of type II HC death and regeneration but spared more type I HCs in both organs. These findings demonstrate that having more type I HCs is correlated with stronger responses to vestibular stimulation and suggest that regenerating type I HCs may improve vestibular function after HC loss.

Influence of the vestibular system on the neonatal motor behaviors in the gray short-tailed opossum (Monodelphis domestica)

Marsupials are born very immature yet must be sufficiently autonomous to crawl on the mother's belly, find a teat and attach to it to pursue their development. Sensory inputs are necessary to guide the newborn to a teat and induce attachment. The vestibular system, which perceives gravity and head movements, is one of the senses proposed to guide newborns towards the teats but there are conflicting observations about its functionality at birth (postnatal day (P) 0). To test if the vestibular system of opossum newborns is functional and can influence locomotion, we used two approaches. First, we stimulated the vestibular apparatus in in vitro preparations from opossums aged from P1 to P12 and recorded motor responses: at all ages studied, mechanical pressures applied on the vestibular organs induced spinal roots activity whereas head tilts did not induce forelimb muscle contractions. Second, using immunofluorescence, we assessed the presence of Piezo2, a protein involved in mechanotransduction in vestibular hair cells. Piezo2 labeling was scant in the utricular macula at birth, but observed in all vestibular organs at P7, its intensity increasing up to P14; it seemed to stay the same at P21. Our results indicate that neural pathways from the labyrinth to the spinal cord are already in place around birth but that the vestibular organs are too immature to influence motor activity before the end of the second postnatal week in the opossum. It may be the rule in marsupial species that the vestibular system becomes functional only after birth.

Timely insertion of AMPA receptor in developing vestibular circuits is required for manifestation of righting reflexes and effective navigation

Vestibular information processed first by the brainstem vestibular nucleus (VN), and further by cerebellum and thalamus, underlies diverse brain function. These include the righting reflexes and spatial cognitive behaviour. While the cerebellar and thalamic circuits that decode vestibular information are known, the importance of VN neurons and the temporal requirements for their maturation that allow developmental consolidation of the aforementioned circuits remains unclear. We show that timely unsilencing of glutamatergic circuits in the VN by NMDA receptor-mediated insertion of AMPAR receptor type 1 (GluA1) subunits is critical for maturation of VN and successful consolidation of higher circuits that process vestibular information. Delayed unsilencing of NMDA receptor-only synapses of neonatal VN neurons permanently decreased their functional connectivity with inferior olive circuits. This was accompanied by delayed pruning of the inferior olive inputs to Purkinje cells and permanent reduction in their plasticity. These derangements led to deficits in associated vestibular righting reflexes and motor co-ordination during voluntary movement. Vestibular-dependent recruitment of thalamic neurons was similarly reduced, resulting in permanently decreased efficiency of spatial navigation. The findings thus show that well-choreographed maturation of the nascent vestibular circuitry is prerequisite for functional integration of vestibular signals into ascending pathways for diverse vestibular-related behaviours.

A New Vestibular Stimulation Mode for Motion Sickness With Emphatic Analysis of Pica

Motion sickness (MS) was frequently introduced for rodents in research work through passive motion that disturbed vestibular signals in the presence of visual and aleatory, proprioceptive inputs. Inducement of MS in this way causes conflicting signals that activate intermixed neural circuits representing multimodal stimulation. From reductionism, a lab setup to elicit rat MS via vestibular stimulation was configured in the present study for MS study in connection with dissection of the central vestibular component causally underlying MS. The individual animal was blinded to light with a custom-made restrainer, and positioned at an inclination of 30° for otolith organs to receive unusual actions by gravitoinertial vector. Following a 2-h double-axis (earth-vertical) rotation involving angular acceleration/deceleration, a suit of behaviors characterizing the MS was observed to be significantly changed including pica (eating non-nutritive substance like kaolin), conditioned taste avoidance and locomotion (p < 0.05). Notably, for the statistical hypothesis testing, the utility of net increased amount of kaolin consumption as independent variables in data processing was expounded. In addition, Fos-immunostained neurons in vestibular nucleus complex were significantly increased in number, suggesting the rotation-induced MS was closely related to the vestibular activation. In conclusion, our work indicated that the present setup could effectively elicit the MS by disturbing vestibular signals in rat in the context of well-controlled proprioceptive inputs and lack of visual afference.

Regulatory roles of perineuronal nets and semaphorin 3A in the postnatal maturation of the central vestibular circuitry for graviceptive reflex

Perineuronal nets (PN) restrict neuronal plasticity in the adult brain. We hypothesize that activity-dependent consolidation of PN is required for functional maturation of behavioral circuits. Using the postnatal maturation of brainstem vestibular nucleus (VN) circuits as a model system, we report a neonatal period in which consolidation of central vestibular circuitry for graviception is accompanied by activity-dependent consolidation of chondroitin sulfate (CS)-rich PN around GABAergic neurons in the VN. Postnatal onset of negative geotaxis was used as an indicator for functional maturation of vestibular circuits. Rats display negative geotaxis from postnatal day (P) 9, coinciding with the condensation of CS-rich PN around GABAergic interneurons in the VN. Delaying PN formation, by removal of primordial CS moieties on VN with chondroitinase ABC (ChABC) treatment at P6, postponed emergence of negative geotaxis to P13. Similar postponement was observed following inhibition of GABAergic transmission with bicuculline, in line with the reported role of PN in increasing excitability of parvalbumin neurons. We further reasoned that PN-CS restricts bioavailability of plasticity-inducing factors such as semaphorin 3A (Sema3A) to bring about circuit maturation. Treatment of VN explants with ChABC to liberate PN-bound Sema3A resulted in dendritic growth and arborization, implicating structural plasticity that delays synapse formation. Evidence is thus provided for the role of PN-CS-Sema3A in regulating structural and circuit plasticity at VN interneurons with impacts on the development of graviceptive postural control.

Differential Gene Expression Profile in the Rat Caudal Vestibular Nucleus is Associated with Individual Differences in Motion Sickness Susceptibility

OBJECTIVE

To identify differentially expressed genes associated with motion sickness (MS) susceptibility in the rat caudal vestibular nucleus.

METHODS

We identified MS susceptible (MSS) and insusceptible (inMSS) rats by quantifying rotation-induced MS symptoms: defecation and spontaneous locomotion activity. Microarray analysis was used to screen differentially expressed genes in the caudal vestibular nucleus (CVN) after rotation. Plasma stress hormones were identified by radioimmunoassay. Candidate genes were selected by bioinformatics analysis and the microarray results were verified by real-time quantitative-PCR (RT-qPCR) methods. By using Elvax implantation, receptor antagonists or recombinant adenovirus targeting the candidate genes were applied to the CVN to evaluate their contribution to MS susceptibility variability. Validity of gene expression manipulation was verified by RT-qPCR and western blot analysis.

RESULTS

A total of 304 transcripts were differentially expressed in the MSS group compared with the inMSS group. RT-qPCR analysis verified the expression pattern of candidate genes, including nicotinic cholinergic receptor (nAchR) α3 subunit, 5-hydroxytryptamine receptor 4 (5-HT4R), tachykinin neurokinin-1 (NK1R), γ-aminobutyric acid A receptor (GABAAR) α6 subunit, olfactory receptor 81 (Olr81) and homology 2 domain-containing transforming protein 1 (Shc1). In MSS animals, the nAchR antagonist mecamylamine significantly alleviated rotation-induced MS symptoms and the plasma β-endorphin response. The NK1R antagonist CP99994 and Olr81 knock-down were effective for the defecation response, while the 5-HT4R antagonist RS39604 and Shc1 over-expression showed no therapeutic effect. In inMSS animals, rotation-induced changes in spontaneous locomotion activity and the plasma β-endorphin level occurred in the presence of the GABAAR antagonist gabazine.

CONCLUSION

Our findings suggested that the variability of the CVN gene expression profile after motion stimulation might be a putative molecular basis for individual differences in MS susceptibility and provide information for the development of new therapeutic strategies for MSS individuals.

Projection neurons of the vestibulo-sympathetic reflex pathway

Changes in head position and posture are detected by the vestibular system and are normally followed by rapid modifications in blood pressure. These compensatory adjustments, which allow humans to stand up without fainting, are mediated by integration of vestibular system pathways with blood pressure control centers in the ventrolateral medulla. Orthostatic hypotension can reflect altered activity of this neural circuitry. Vestibular sensory input to the vestibulo-sympathetic pathway terminates on cells in the vestibular nuclear complex, which in turn project to brainstem sites involved in the regulation of cardiovascular activity, including the rostral and caudal ventrolateral medullary regions (RVLM and CVLM, respectively). In the present study, sinusoidal galvanic vestibular stimulation was used to activate this pathway, and activated neurons were identified through detection of c-Fos protein. The retrograde tracer Fluoro-Gold was injected into the RVLM or CVLM of these animals, and immunofluorescence studies of vestibular neurons were conducted to visualize c-Fos protein and Fluoro-Gold concomitantly. We observed activated projection neurons of the vestibulo-sympathetic reflex pathway in the caudal half of the spinal, medial, and parvocellular medial vestibular nuclei. Approximately two-thirds of the cells were ipsilateral to Fluoro-Gold injection sites in both the RVLM and CVLM, and the remainder were contralateral. As a group, cells projecting to the RVLM were located slightly rostral to those with terminals in the CVLM. Individual activated projection neurons were multipolar, globular, or fusiform in shape. This study provides the first direct demonstration of the central vestibular neurons that mediate the vestibulo-sympathetic reflex.

Identification of neural networks that contribute to motion sickness through principal components analysis of fos labeling induced by galvanic vestibular stimulation

Motion sickness is a complex condition that includes both overt signs (e.g., vomiting) and more covert symptoms (e.g., anxiety and foreboding). The neural pathways that mediate these signs and symptoms are yet to identified. This study mapped the distribution of c-fos protein (Fos)-like immunoreactivity elicited during a galvanic vestibular stimulation paradigm that is known to induce motion sickness in felines. A principal components analysis was used to identify networks of neurons activated during this stimulus paradigm from functional correlations between Fos labeling in different nuclei. This analysis identified five principal components (neural networks) that accounted for greater than 95% of the variance in Fos labeling. Two of the components were correlated with the severity of motion sickness symptoms, and likely participated in generating the overt signs of the condition. One of these networks included neurons in locus coeruleus, medial, inferior and lateral vestibular nuclei, lateral nucleus tractus solitarius, medial parabrachial nucleus and periaqueductal gray. The second included neurons in the superior vestibular nucleus, precerebellar nuclei, periaqueductal gray, and parabrachial nuclei, with weaker associations of raphe nuclei. Three additional components (networks) were also identified that were not correlated with the severity of motion sickness symptoms. These networks likely mediated the covert aspects of motion sickness, such as affective components. The identification of five statistically independent component networks associated with the development of motion sickness provides an opportunity to consider, in network activation dimensions, the complex progression of signs and symptoms that are precipitated in provocative environments. Similar methodology can be used to parse the neural networks that mediate other complex responses to environmental stimuli.

Maturation profile of inferior olivary neurons expressing ionotropic glutamate receptors in rats: role in coding linear accelerations

Using sinusoidal oscillations of linear acceleration along both the horizontal and vertical planes to stimulate otolith organs in the inner ear, we charted the postnatal time at which responsive neurons in the rat inferior olive (IO) first showed Fos expression, an indicator of neuronal recruitment into the otolith circuit. Neurons in subnucleus dorsomedial cell column (DMCC) were activated by vertical stimulation as early as P9 and by horizontal (interaural) stimulation as early as P11. By P13, neurons in the β subnucleus of IO (IOβ) became responsive to horizontal stimulation along the interaural and antero-posterior directions. By P21, neurons in the rostral IOβ became also responsive to vertical stimulation, but those in the caudal IOβ remained responsive only to horizontal stimulation. Nearly all functionally activated neurons in DMCC and IOβ were immunopositive for the NR1 subunit of the NMDA receptor and the GluR2/3 subunit of the AMPA receptor. In situ hybridization studies further indicated abundant mRNA signals of the glutamate receptor subunits by the end of the second postnatal week. This is reinforced by whole-cell patch-clamp data in which glutamate receptor-mediated miniature excitatory postsynaptic currents of rostral IOβ neurons showed postnatal increase in amplitude, reaching the adult level by P14. Further, these neurons exhibited subthreshold oscillations in membrane potential as from P14. Taken together, our results support that ionotropic glutamate receptors in the IO enable postnatal coding of gravity-related information and that the rostral IOβ is the only IO subnucleus that encodes spatial orientations in 3-D.

Postnatal expression of TrkB receptor in rat vestibular nuclear neurons responsive to horizontal and vertical linear accelerations

We examined the maturation expression profile of tyrosine kinase B (TrkB) receptor in rat vestibular nuclear neurons that were activated by sinusoidal linear acceleration along the horizontal or vertical axis. The otolithic origin of Fos expression in these neurons was confirmed with labyrinthectomized controls and normal controls, which showed only sporadically scattered Fos-labeled neurons in the vestibular nucleus. In P4-6 test rats, no Fos-labeled neurons were found in the vestibular nucleus, but the medial and spinal vestibular neurons showed weak immunoreactivity for TrkB. The intensity of TrkB immunoreactivity in vestibular nuclear neurons progressively increased in the second postnatal week but remained low in adults. From P7 onward, TrkB-expressing neurons responded to horizontal or vertical otolithic stimulation with Fos expression. The number of Fos-labeled vestibular nuclear neurons expressing TrkB increased with age, from 13-43% in P7 rats to 85-90% in adult rats. Our results therefore suggest that TrkB/neurotrophin signaling plays a dominant role in modulating vestibular nuclear neurons for the coding of gravity-related horizontal head movements and for the regulation of vestibular-related behavior during postnatal development.

VL, Martinelli GP, Ogorodnikov D, Yakushin SB, Cohen B. Fos expression in neurons of the rat vestibulo-autonomic pathway activated by sinusoidal galvanic vestibular stimulation

The vestibular system sends projections to brainstem autonomic nuclei that modulate heart rate and blood pressure in response to changes in head and body position with regard to gravity. Consistent with this, binaural sinusoidally modulated galvanic vestibular stimulation (sGVS) in humans causes vasoconstriction in the legs, while low frequency (0.02-0.04 Hz) sGVS causes a rapid drop in heart rate and blood pressure in anesthetized rats. We have hypothesized that these responses occur through activation of vestibulo-sympathetic pathways. In the present study, c-Fos protein expression was examined in neurons of the vestibular nuclei and rostral ventrolateral medullary region (RVLM) that were activated by low frequency sGVS. We found c-Fos-labeled neurons in the spinal, medial, and superior vestibular nuclei (SpVN, MVN, and SVN, respectively) and the parasolitary nucleus. The highest density of c-Fos-positive vestibular nuclear neurons was observed in MVN, where immunolabeled cells were present throughout the rostro-caudal extent of the nucleus. c-Fos expression was concentrated in the parvocellular region and largely absent from magnocellular MVN. c-Fos-labeled cells were scattered throughout caudal SpVN, and the immunostained neurons in SVN were restricted to a discrete wedge-shaped area immediately lateral to the IVth ventricle. Immunofluorescence localization of c-Fos and glutamate revealed that approximately one third of the c-Fos-labeled vestibular neurons showed intense glutamate-like immunofluorescence, far in excess of the stain reflecting the metabolic pool of cytoplasmic glutamate. In the RVLM, which receives a direct projection from the vestibular nuclei and sends efferents to preganglionic sympathetic neurons in the spinal cord, we observed an approximately threefold increase in c-Fos labeling in the sGVS-activated rats. We conclude that localization of c-Fos protein following sGVS is a reliable marker for sGVS-activated neurons of the vestibulo-sympathetic pathway.

Expression of doublecortin, a neuronal migration protein, in unipolar brush cells of the vestibulocerebellum and dorsal cochlear nucleus of the adult rat

Doublecortin (DCX) is a microtubule-associated protein that is critical for neuronal migration and the development of the cerebral cortex. In the adult, it is expressed in newborn neurons in the subventricular and subgranular zones, but not in the mature neurons of the cerebral cortex. By contrast, neurogenesis and neuronal migration of cells in the cerebellum continue into early postnatal life; migration of one class of cerebellar interneuron, unipolar brush cells (UBCs), may continue into adulthood. To explore the possibility of continued neuronal migration in the adult cerebellum, closely spaced sections through the brainstem and cerebellum of adult (3-16 months old) Sprague-Dawley rats were immunolabeled for DCX. Neurons immunoreactive (ir) to DCX were present in the granular cell layer of the vestibulocerebellum, most densely in the transition zone (tz), the region between the flocculus (FL) and ventral paraflocculus (PFL), as well as in the dorsal cochlear nucleus (DCN). These DCX-ir cells had the morphological appearance of UBCs with oval somata and a single dendrite ending in a brush. There were many examples of colocalization of DCX with Eps8 or calretinin, UBC markers. We also identified DCX-ir elements along the fourth ventricle and its lateral recess that had labeled somata but lacked the dendritic structure characteristic of UBCs. Labeled UBCs were seen in nearby white matter. These results suggest that there may be continued neurogenesis and/or migration of UBCs in the adult. Another possibility is that UBCs maintain DCX expression even after migration and maturation, reflecting a role of DCX in adult neuronal plasticity in addition to a developmental role in migration.

Decreased Fos protein expression in rat caudal vestibular nucleus is associated with motion sickness habituation

We investigated the temporal change of Fos protein expression in the caudal vestibular nucleus of rats exposed to daily 2-h Ferris-wheel like (FWL) rotation. Repeated rotation (2h daily for 14 consecutive days) caused an initial increase in defecation, followed by a gradual decline back to the baseline level after 8 rotation sessions. Unlike defecation, the Kaolin consumption of rats showed a bitonic function during the daily rotation sessions (2h daily for 33 consecutive days) and finally recovered to the baseline after about 31 sessions. Immunohistochemistry study revealed increased Fos immunolabeled (Fos-LI) neurons in the medial vestibular nucleus and spinal vestibular nucleus during the initial 7 rotation sessions, and it decreased to the baseline level after 10 rotation sessions. There was a strong linear relationship between the amount of Fos-LI neurons and rat defecation level throughout the whole rotation sessions. These results suggest that the change of neuronal plasticity in the caudal vestibular nucleus might contribute to attenuation of gastrointestinal symptoms during motion sickness habituation process.

Otolith stimulation induces c-Fos expression in vestibular and precerebellar nuclei in cats and squirrel monkeys

Vestibular information is critical for the control of balance, posture, and eye movements. Signals from the receptors, the semicircular canals and otoliths, are carried by the eighth nerve and distributed to the four nuclei of the vestibular nuclear complex, the VNC. However, anatomical and physiological data suggest that many additional brainstem nuclei are engaged in the processing of vestibular signals and generation of motor responses. To assess the role of these structures in vestibular functions, we have used the expression of the immediate early gene c-Fos as a marker for neurons activated by stimulation of the otoliths or the semicircular canals. Excitation of the otolith organs resulted in widespread c-Fos expression in the VNC, but also in other nuclei, including the external cuneate nucleus, the postpyramidal nucleus of the raphé, the nucleus prepositus hypoglossi, the subtrigeminal nucleus, the pontine nuclei, the dorsal tegmental nucleus, the locus coeruleus, and the reticular formation. Rotations that excited the semicircular canals were much less effective in inducing c-Fos expression. The large number of brainstem nuclei that showed c-Fos expression may reflect the multiple functions of the vestibular system. Some of these neurons may be involved in sensory processing of the vestibular signals, while others provide input to the vestibulo-ocular, vestibulocollic, and vestibulospinal reflexes or mediate changes in autonomic function. The data show that otolith stimulation engages brainstem structures both within and outside of the VNC, many of which project to the cerebellum.

Developmental maturation of ionotropic glutamate receptor subunits in rat vestibular nuclear neurons responsive to vertical linear acceleration

We investigated the maturation profile of subunits of ionotropic glutamate receptors in vestibular nuclear neurons that were activated by sinusoidal linear acceleration along the vertical plane. The otolithic origin of Fos expression in these neurons was confirmed as a marker of functional activation when labyrinthectomized and/or stationary control rats contrasted by showing sporadically scattered Fos-labeled neurons in the vestibular nuclei. By double immunohistochemistry for Fos and one of the receptor subunits, otolith-related neurons that expressed either alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate or N-methyl-d-aspartate subunits were first identified in the medial vestibular nucleus, spinal vestibular nucleus and Group x by postnatal day (P)7, and in the lateral vestibular nucleus and Group y by P9. No double-labeled neurons were found in the superior vestibular nucleus. Within each vestibular subnucleus, these double-labeled neurons constituted approximately 90% of the total Fos-labeled neurons. The percentage of Fos-labeled neurons expressing the GluR1 or NR2A subunit showed developmental invariance in all subnuclei. For Fos-labeled neurons expressing the NR1 subunit, similar invariance was observed except that, in Group y, these neurons decreased from P14 onwards. For Fos-labeled neurons expressing the GluR2, GluR2/3, GluR4 or NR2B subunit, a significant decrease was found by the adult stage. In particular, those expressing the GluR4 subunit showed a two- to threefold decrease in the medial vestibular nucleus, spinal vestibular nucleus and Group y. Also, those expressing the NR2B subunit showed a twofold decrease in Group y. Taken together, the postsynaptic expression of ionotropic glutamate receptor subunits in different vestibular subnuclei suggests that glutamatergic transmission within subregions plays differential developmental roles in the coding of gravity-related vertical spatial information.

Time course of cortically induced fos expression in auditory thalamus and midbrain after bilateral cochlear ablation

Expression of c-fos in the medial geniculate body (MGB) and the inferior colliculus (IC) in response to bicuculline-induced corticofugal activation was examined in rats at different time points after bilateral cochlear ablation (4 h-30 days). Corticofugal activation was crucial in eliciting Fos expression in the MGB after cochlear ablation. The pars ovoidea (OV) of the medial geniculate body ventral division (MGv) showed dense Fos expression 4 h after cochlear ablation; the expression declined to very low levels at 24 h and thereafter. In turn, strong Fos expression was found in the pars lateralis (LV) of the MGv 24 h after cochlear ablation and dropped dramatically at 14 days. The dorsal division of the MGB (MGd) showed high Fos expression 7 days after cochlear ablation, which persisted for a period of time. Using multi-electrode recordings, neuronal activity of different MGB subnuclei was found to correlate well with Fos expressions. The temporal changes in cortically activated Fos expression in different MGB subnuclei after bilateral cochlear ablation indicate differential denervation hypersensitivities of these MGB neurons and likely point to differential dependence of these nuclei on both auditory ascending and corticofugal descending inputs. After bilateral cochlear ablation, significant increases in Fos-positive neurons were detected unilaterally in all IC subnuclei, ipsilateral to the bicuculline injection.

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.

Corticothalamic synchronization leads to c-fos expression in the auditory thalamus

In this study, we investigated the relationship between c-fos expression in the auditory thalamus and corticofugal activation. The contribution of neurotransmitters and related receptors, the involvement of thalamic reticular nucleus (TRN), and the role of neuronal firing patterns in this process were also examined. The principal nuclei of the medial geniculate body (MGB) showed c-fos expression when the auditory cortex (AC) was activated by direct injection of bicuculline methobromide. However, no expression was detectable with acoustic stimuli alone. This indicated that c-fos expression in the principal nuclei of the MGB was triggered by the corticofugal projection. c-fos expression could be elicited in the MGB by direct injection of glutamate. Direct administration of acetylcholine, alternatively, had no effect. Bicuculline methobromide injection in the AC also triggered synchronized oscillatory activities sequentially in the AC and MGB. Cortically induced c-fos expression in the MGB was not mediated by a pathway involving the TRN because it remained intact after a TRN lesion with kainic acid. The present results also conclude that c-fos expression is not simply associated with firing rate, but also with neuronal firing pattern. Burst firings that are synchronized with the cortical oscillations are proposed to lead to c-fos expression in the principal nuclei of the MGB.

Corticofugal modulation of acoustically induced Fos expression in the rat auditory pathway

To investigate the corticofugal modulation of acoustic information ascending through the auditory pathway of the rat, immunohistochemical techniques were used to study the functional expression of Fos protein in neurons. With auditory stimulation at different frequencies, Fos expression in the medial geniculate body (MGB), inferior colliculus (IC), superior olivary complex, and cochlear nucleus was examined, and the extent of Fos expression on the two sides was compared. Strikingly, we found densely Fos-labeled neurons in all divisions of the MGB after both presentation of an auditory stimulus and administration of a gamma-aminobutyric acid type A (GABA(A)) antagonist (bicuculline methobromide; BIM) to the auditory cortex. The location of Fos-labeled neurons in the ventral division (MGv) after acoustic stimulation at different frequencies was in agreement with the known tonotopic organization. That no Fos-labeled neurons were found in the MGv with acoustic stimuli alone suggests that the transmission of ascending thalamocortical information is critically governed by corticofugal modulation. The dorsal (DCIC) and external cortices (ECIC) of the IC ipsilateral to the BIM-injected cortex showed a significantly higher number of Fos-labeled neurons than the contralateral IC. However, no difference in the number of Fos-labeled neurons was found between the central nucleus of the IC on either side, indicating that direct corticofugal modulation occurs only in the ECIC and DCIC. Further investigations are needed to assess the functional implications of the morphological differences observed between the descending corticofugal projections to the thalamus and the IC.

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.

Expression of Trk receptors in otolith-related neurons in the vestibular nucleus of rats

The expression of the three Trk receptors (TrkA, TrkB, and TrkC) in otolith-related neurons within the vestibular nuclei of adult Sprague-Dawley rats was examined immunohistochemically. Conscious animals were subjected to sinusoidal linear acceleration along either the anterior-posterior (AP) or interaural (IA) axis on the horizontal plane. Neuronal activation was defined by Fos expression in cell nuclei. Control animals, viz labyrinthectomized rats subjected to stimulation and normal rats that remained stationary, showed only a few sporadically scattered Fos-labeled neurons. Among experimental rats, the number of Fos-labeled neurons and their distribution pattern in each vestibular subnucleus in animals stimulated along the antero-posterior axis were similar to those along the interaural axis. No apparent topography was observed among neurons activated along these two directions. Only about one-third of the Trk-immunoreactive neurons in the vestibular nucleus expressed Fos. Double-labeled Fos/TrkA, Fos/TrkB and Fos/TrkC neurons constituted 85-98% of the total number of Fos-labeled neurons in vestibular nuclear complex and its subgroups x and y. Our findings suggest that Trk receptors and their cognate neurotrophins in central otolith neurons may contribute to the modulation of gravity-related spatial information during horizontal head movements.