Neurotransmitters in vestibular pathways

Pre- and Postsynaptic Effects of Glutamate in the Frog Labyrinth

The role of glutamate in quantal release at the cytoneural junction was examined by measuring mEPSPs and afferent spikes at the posterior canal in the intact frog labyrinth. Release was enhanced by exogenous glutamate, or dl-TBOA, a blocker of glutamate reuptake. Conversely, drugs acting on ionotropic glutamate receptors did not affect release; the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R) blocker CNQX decreased mEPSP size in a dose-dependent manner; the NMDA-R blocker d-AP5 at concentrations <200 µM did not affect mEPSP size, either in the presence or absence of Mg and glycine. In isolated hair cells, glutamate did not modify Ca currents. Instead, it systematically reduced the compound delayed potassium current, IKD, whereas the metabotropic glutamate receptor (mGluR)-II inverse agonist, (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid (LY341495), increased it. Given mGluR-II decrease cAMP production, these finding are consistent with the reported sensitivity of IKD to protein kinase A (PKA)-mediated phosphorylation. LY341495 also enhanced transmitter release, presumably through phosphorylation-mediated facilitation of the release machinery. The observed enhancement of release by glutamate confirms previous literature data, and can be attributed to activation of mGluR-I that promotes Ca release from intracellular stores. Glutamate-induced reduction in the repolarizing IKD may contribute to facilitation of release. Overall, glutamate exerts both a positive feedback action on mGluR-I, through activation of the phospholipase C (PLC)/IP₃ path, and the negative feedback, by interfering with substrate phosphorylation through G_i/0-coupled mGluRs-II/III. The positive feedback prevails, which may explain the increase in overall rates of release observed during mechanical stimulation (symmetrical in the excitatory and inhibitory directions). The negative feedback may protect the junction from over-activation.

Transient alteration of the vestibular calyceal junction and synapse in response to chronic ototoxic insult in rats

Ototoxicity is known to cause permanent loss of vestibule function through degeneration of sensory hair cells (HCs). However, functional recovery has been reported during washout after chronic ototoxicity, although the mechanisms underlying this reversible dysfunction are unknown. Here, we study this question in rats chronically exposed to the ototoxic compound 3,3′-iminodipropionitrile (IDPN). Pronounced alterations in vestibular function appeared before significant loss of HCs or stereociliary coalescence became evident by ultrastructural analyses. This early dysfunction was fully reversible if the exposure was terminated promptly. In cristae and utricles, the distinct junctions formed between type I HCs (HCI) and calyx endings were completely dismantled at these early stages of reversible dysfunction, and completely rebuilt during washout. Immunohistochemical observations revealed loss and recovery of the junction proteins CASPR1 and tenascin-C and RT-PCR indicated that their loss was not due to decreased gene expression. KCNQ4 was mislocalized during intoxication and recovered control-like localization after washout. At early stages of the intoxication, the calyces could be classified as showing intact or lost junctions, indicating that calyceal junction dismantlement is triggered on a calyx-by-calyx basis. Chronic toxicity also altered the presence of ribeye, PSD-95 and GluA2 puncta in the calyces. These synaptic alterations varied between the two types of calyx endings (formed by calyx-only or dimorphic afferents) and some persisted at the end of the washout period. The present data reveal new forms of plasticity of the calyx endings in adult mammals, including a robust capacity for rebuilding the calyceal junction. These findings contribute to a better understanding of the phenomena involved in progressive vestibular dysfunction and its potential recovery during and after ototoxic exposure.

Summary: New forms of damage and repair have been identified in the vestibular sensory epithelium using a rat model of chronic ototoxicity and recovery that causes reversible vestibular dysfunction.

Vestibular damage in chronic ototoxicity: a mini-review

Ototoxicity is a major cause of the loss of hearing and balance in humans. Ototoxic compounds include pharmaceuticals such as aminoglycoside antibiotics, anti-malarial drugs, loop diuretics and chemotherapeutic platinum agents, and industrial chemicals including several solvents and nitriles. Human and rodent data indicate that the main target of toxicity is hair cells (HCs), which are the mechanosensory cells responsible for sensory transduction in both the auditory and the vestibular system. Nevertheless, the compounds may also affect the auditory and vestibular ganglion neurons. Exposure to ototoxic compounds has been found to cause HC apoptosis, HC necrosis, and damage to the afferent terminals, of differing severity depending on the ototoxicity model. One major pathway frequently involved in HC apoptosis is the c-jun N-terminal kinase (JNK) signaling pathway activated by reactive oxygen species, but other apoptotic pathways can also play a role in ototoxicity. Moreover, little is known about the effects of chronic low-dose exposure. In rodent vestibular epithelia, extrusion of live HCs from the sensory epithelium may be the predominant form of cell demise during chronic ototoxicity. In addition, greater involvement of the afferent terminals may occur, particularly the calyx units contacting type I vestibular HCs. As glutamate is the neurotransmitter in this synapse, excitotoxic phenomena may participate in afferent and ganglion neuron damage. Better knowledge of the events that take place in chronic ototoxicity is of great interest, as it will increase understanding of the sensory loss associated with chronic exposure and aging.

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.

Changes of amino acid concentrations in the rat vestibular nuclei after inferior cerebellar peduncle transection

Although there is a close relationship between the vestibular nuclear complex (VNC) and the cerebellum, little is known about the contribution of cerebellar inputs to amino acid neurotransmission in the VNC. Microdissection of freeze-dried brain sections and high-performance liquid chromatography (HPLC) were combined to measure changes of amino acid concentrations within the VNC of rats following transection of the cerebellovestibular connections in the inferior cerebellar peduncle. Distributions of 12 amino acids within the VNC at 2, 4, 7, and 30 days after surgery were compared with those for control and sham-lesioned rats. Concentrations of gamma-aminobutyric acid (GABA) decreased by 2 days after unilateral peduncle transection in nearly all VNC regions on the lesioned side and to lesser extents on the unlesioned side and showed partial recovery up to 30 days postsurgery. Asymmetries between the two sides of the VNC were maintained through 30 days. Glutamate concentrations were reduced bilaterally in virtually all regions of the VNC by 2 days and showed complete recovery in most VNC regions by 30 days. Glutamine concentrations increased, starting 2 days after surgery, especially on the lesioned side, so that there was asymmetry generally opposite that of glutamate. Concentrations of taurine, aspartate, and glycine also underwent partially reversible changes after peduncle transection. The results suggest that GABA and glutamate are prominent neurotransmitters in bilateral projections from the cerebellum to the VNC and that amino acid metabolism in the VNC is strongly influenced by its cerebellar connections.

Cellular distribution of d-serine, serine racemase and d-amino acid oxidase in the rat vestibular sensory epithelia

Glutamate is the main neurotransmitter at the synapses between sensory cells and primary afferents in the peripheral vestibular system. Evidence has recently been obtained demonstrating that the atypical amino acid D-serine is the main endogenous co-agonist of the N-methyl-D-aspartate receptors in the CNS. We studied the distribution of D-serine and its synthesizing and degrading enzymes, serine racemase and d-amino acid oxidase in the rat vestibular sensory epithelium using immunocytochemistry. D-serine, serine racemase and D-amino acid oxidase were localized in the transitional cells, which are parasensory cells located between the sensory epithelium and the dark cells. The dark cells expressed only serine racemase. D-Serine was also detected in the supporting cells of the sensory epithelium. These cells, which are in close contact with glutamatergic synapses, express GLAST, a glial specific transporter for glutamate. They may have similar functions to glial cells in the CNS and thus expression of D-serine suggests a neuromodulator role for D-serine at the glutamatergic synapses in the peripheral vestibular system. Our data also indicate that the metabolism of D-serine is not restricted to glial cells suggesting that the amino acid may play an additional role in the peripheral nervous system.

Gamma-aminobutyric acid is present in a spatially discrete subpopulation of hair cells in the crista ampullaris of the toadfish Opsanus tau

Although gamma-aminobutyric acid (GABA) and glutamate are known to be present in the vestibular sensory epithelia of a variety of species, the functional relationship between these two transmitters is not clear. The present study addresses the three-dimensional spatial distribution of GABA and glutamate immunoreactivity in the vestibular labyrinth of the oyster toadfish by using whole end organs labeled by immunofluorescence with monoclonal anti-GABA and/or antiglutamate antibodies and visualized as whole mounts by multiphoton confocal microscopy. We find glutamate-immunoreactive hair cells present throughout the sensory epithelium. In contrast, prominent GABA immunoreactivity is restricted to a small population of hair cells located in the central region of the crista. Double immunofluorescence reveals two distinct staining patterns in GABA-labeled hair cells. Most ( approximately 80%) GABA-labeled cells show trace levels of glutamate, appropriate for the metabolic/synthetic role of cytoplasmic glutamate. The remainder of the GABA-stained cells contain substantial levels of both GABA and glutamate, suggesting transmitter colocalization. In the toadfish utricle, glutamatergic hair cells are present throughout the macula. GABA-immunoreactive hair cells follow the arc of the striola, and most GABA-labeled receptor cells coexpress glutamate. The localization of GABA was explored in other species as well. In the pigeon, GABAergic hair cells are present throughout the crista ampullaris. Our findings demonstrate that multiple, neurochemically distinct types of hair cells are present in vestibular sensory epithelia. These observations, together with the excitatory activity generally associated with 8th nerve afferent fibers, strongly suggest that GABA serves an important, specific, and complex role in determining primary afferent response dynamics.

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.

Exercise and drug therapy alter recovery from labyrinth lesion in humans

Acute unilateral vestibular failure is characterized by rotatory vertigo, horizontal-rotatory nystagmus, and postural imbalance, all of which last from days to weeks. These signs and symptoms are caused by a vestibular tone imbalance between the two labyrinths. Recovery results from a combination of peripheral restoration of labyrinthine function (usually incomplete) and central vestibular compensation (CVC) of the vestibular tone imbalance. Acute unilateral failure is most often caused by vestibular neuritis, which is most likely due to the reactivation of a latent HSV-1 infection. Therefore, therapeutic strategies to improve the outcome of VN are theoretically based on two principles: (a) vestibular exercises and drugs to improve CVC and (b) drug treatment of the assumed viral inflammation. The following conclusions can be drawn from studies in animals and/or humans: (1) There is strong evidence that vestibular exercises may improve vestibulo-spinal compensation. These exercises should begin as early as possible after symptom onset. Moreover, slower exercises are likely to be more effective than faster exercises because slower ones seem to depend more on the vestibular system. (2) Despite extensive data from animal experiments indicating that drugs have a favorable effect on CVC, this has not been clinically proven and thus cannot be recommended yet. (3) Preliminary results of an interim analysis from an ongoing randomized, prospective study showed that methylprednisolone (plus an antiviral agent?) may be useful for improving peripheral vestibular function in vestibular neuritis.

Extracellular adenosine 5'-ATP-induced calcium signaling in isolated vestibular ganglion cells of the guinea pig

Extracellular adenosine 5'-triphosphate (ATP)-induced intracellular calcium concentration ([Ca2+]i) changes in acutely isolated vestibular ganglion cells (VGCs) of the guinea pig were investigated using the Ca2+ -sensitive dye Fura-2. Extracellular ATP induced an increase in [Ca2+]i in VGCs in a dose-dependent manner. ATP induced an increase in [Ca2+]i even in the absence of extracellular Ca2+ (1 mM Ethylene Glycol-bis (beta-aminoethyl Ether) N,N,N',N'-Tetraacetic Acid (EGTA)), thus suggesting that ATP induces Ca2+ release from the intracellular stores. The P2-receptor antagonists suramin and reactive blue 2 inhibited the ATP-induced [Ca2+]i increase in a dose-dependent manner. The P1-receptor agonist adenosine did not induce any changes in [Ca2+]i. These results suggest that VGCs may possess a P2-purinergic receptor but not a P1-purinergic receptor. La3+, a receptor-mediated calcium channel blocker, inhibited the ATP-induced [Ca2+]i increase but, in contrast, nifedipine, a L-type calcium channel blocker, did not. These results suggest that ATP induces both a Ca2+ -release from the intracellular stores and a Ca2+ influx from the extracellular space through La3+ -sensitive and nifedipine-insensitive Ca2+ channels in VGCs. Our results also suggest that extracellular ATP may act as a neurotransmitter or neuromodulator of the vestibular peripheral system in the guinea pig.

Optical recording of membrane potential in dissociated mouse vestibular ganglion cells using a voltage-sensitive dye

We investigated membrane electrophysiological features of dissociated vestibular ganglion neurons, using a voltage-sensitive dye and a multiple site optical imaging system. The neuronal nature of the cultured vestibular ganglion cells was confirmed by positive staining with the anti-neurofilament 200 kDa antibody, using immunocytochemical methods. Optical absorption of the dye which binds to the external surface of neuron membranes increased while the cells were depolarized during perfusion with 150 mM potassium solution. The relative ratio (deltaI/I) of optical absorption change was 0.23 +/- 0.08% (means +/- S.D., n = 16). These optical responses were wavelength dependent, therefore, the optical response apparently originated from the voltage-sensitive dye. Under our experimental conditions, photodynamic damage and pharmacological effects of the dye were either absent or insignificant. We therefore concluded that optical recording is a new, practical and non-invasive method to simultaneously monitor changes in membrane potential from cultured vestibular ganglion cells. Optical recording is expected to provide further insight into mechanisms of information processing by vestibular ganglion neurons.

Immunoelectron microscopy of AMPA receptor subunits reveals three types of putative glutamatergic synapse in the rat vestibular end organs

To characterize the synapses between hair cells and afferent nerve endings in the rat vestibular end organs, the ultrastructural localization of AMPA receptor subunits (GluR1-4) was examined by postembedding immunogold cytochemistry. Immunoreactivities for GluR2/3 and GluR4 were associated with the synapses between type I hair cells and the surrounding chaliceal nerve endings and with the bouton type nerve endings contacting type II hair cells. There was no detectable immunoreactivity for GluR1. A third type of immunoreactive synapse was found between the outer face of chalices and type II hair cells. While the linear densities of gold particles (particles per micrometer postsynaptic specialization) of bouton type endings and chaliceal nerve endings were the same, the former type of ending showed larger postsynaptic specializations and, hence, a higher number of receptor molecules. These data indicate that there are three types of putative glutamatergic synapse in the vestibular end organ.

Central vestibular networks in the guinea-pig: functional characterization in the isolated whole brain in vitro

The isolated, in vitro whole brain of guinea-pig was used to assess some of the main physiological and pharmacological properties of the vestibulo-ocular pathways in this species. Extracellular and intracellular recordings were obtained from the vestibular, abducens and oculomotor nuclei, as well as from the abducens and oculomotor nerves, while inputs from the vestibular afferents, the visual pathways and the spinal cord were activated. The three main types of medial vestibular nucleus neurons (A, B and B+LTS), previously described on slices, were also identified in the isolated brain. They had similar membrane properties in both preparations. Eighty-five per cent of cells recorded in the vestibular nucleus responded with monosynaptic, excitatory postsynaptic potentials (latency 1.05-1.9 ms) to stimulation of the ipsilateral vestibular nerve, and were thus identified as second-order vestibular neurons. In addition, stimulation of the contralateral vestibular afferents revealed in most cases a disynaptic or trisynaptic, commissural inhibition. Second-order vestibular neurons displayed in the isolated brain a high degree of variability of their spontaneous activity, as in alert guinea-pigs. Type A neurons always exhibited a regular firing, while type B and B+LTS cells could have very irregular patterns of spontaneous discharge. Thus, type A and type B neurons might correspond, respectively, to the tonic and phasic vestibular neurons described in vivo. The regularity of spontaneous discharge was positively correlated with the amplitude of spike after hyperpolarization, and there was a trend for irregular neurons to be excited from ipsilateral vestibular afferents at shorter latencies than regular units. Synaptic activation could trigger subthreshold plateau potentials and low-threshold spikes in some of the second-order vestibular neurons. As a second step, the pharmacology of the synaptic transmission between primary vestibular afferents and second-order neurons was assessed using specific antagonists of the glutamatergic receptors. Both the synaptic field potentials and excitatory postsynaptic potentials elicited in the medial vestibular nucleus by single shock stimulation of the ipsilateral vestibular nerve were largely or, sometimes, totally blocked by 6-cyano-7-nitroquinoxaline-2,3-dione, indicating a dominating role of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated glutamatergic transmission. The remaining component of the responses was completely or partially suppressed by DL-2-amino-5-phosphonovaleric acid in 35% of the cases, suggesting a concomitant, moderate involvement of N-methyl-D-asparate receptors. In addition, a synaptic response resistant to both antagonists, but sensitive to a zero Ca2+/high Mg(2+)-containing solution, was often observed. Finally, recordings from abducens and oculomotor complexes confirmed the existence in the guinea-pig of strong bilateral, disynaptic excitatory and inhibitory inputs from vestibular afferents to motoneurons of extraocular muscles, which contribute to generation of the vestibulo-ocular reflex. The functional integrity of vestibular-related pathways in the isolated brain was additionally checked by stimulation of the spinal cord and optic tract. Stimulation of the spinal cord evoked, in addition to antidromic responses in the vestibular nucleus, short-latency synaptic responses in both the vestibular nucleus and abducens motoneurons, suggesting possible recruitment of spinal afferents. Activation of visual pathways at the level of the optic chiasm often induced long latency responses in the various structures under study. These results demonstrate that the in vitro isolated brain can be readily used for detailed, functional studies of the neuronal networks underlying gaze and posture control.

Glutamate as a primary afferent neurotransmitter in the medial vestibular nucleus as detected by in vivo microdialysis

An in vivo microdialysis study using alpha-chloralose-anesthetized cats was performed to elucidate whether glutamate is actually released from the vestibular nerve terminals in the medial vestibular nucleus (MVN) with electrical stimulation of the vestibular nerve. When repetitive stimuli composed of rectangular pulses (200 micros in duration, 0.5 mA, and 0.1-50 Hz) were applied to the vestibular nerve for 10 min, a significant frequency-dependent increase in the release of glutamate was observed in the MVN. However, the levels of other amino acids such as aspartate, glycine and GABA remained unaltered with the stimuli. These findings indicate that glutamate is the primary afferent neurotransmitter from the vestibular nerve to the MVN neurons.

Multiple voltage-dependent calcium currents in acutely isolated mouse vestibular neurons

We investigated the presence of voltage-gated calcium currents in vestibular neurons acutely isolated from postnatal mice vestibular ganglions using the whole-cell patch-clamp technique. The neuronal origin of the recorded cells was confirmed by immunohistochemical detection of neurofilaments and calretinin. High and low voltage-activated calcium currents were recorded. High voltage-activated currents were present in all investigated neurons. Low voltage-activated currents were recorded in only a few large vestibular neurons. High and low voltage-activated currents were distinguished by their thresholds of activation and their ability to run-up during early recordings. Among high voltage-activated currents. L-, N- and P-type currents were identified by their sensitivity to, respectively, the dihydropyridines agonist Bay K 8644 (3 microM) and antagonist nitrendipine (3 microM), the co-conotoxin GVIA (3 microM) and the omega-agatoxin IVA at low concentration (50 nM). An inactivating current sensitive to 1 microM omega-agatoxin IVA with characteristics similar to those of the Q-type current was also recorded in vestibular neurons. When L-, N-, P-, Q-type barium currents were blocked, a residual high voltage-activated current defined by its resistance to saturating concentrations of all above blockers was detected. This residual current was completely blocked by 0.5 mM nickel and cadmium. Our results reveal that primary vestibular neurons express a variety of voltage-activated calcium currents with distinct physiological and pharmacological properties. This diversity could be related both with their functional synaptic characteristic, and with the intrinsic physiological properties of each class of vestibular afferents.

Morphological evidence for local microcircuits in rat vestibular maculae

Previous studies suggested that intramacular, unmyelinated segments of vestibular afferent nerve fibers and their large afferent endings (calyces) on type I hair cells branch. Many of the branches (processes) contain vesicles and are presynaptic to type II hair cells, other processes, intramacular nerve fibers, and calyces. This study used serial section transmission electron microscopy and three-dimensional reconstruction methods to document the origins and distributions of presynaptic processes of afferents in the medial part of the adult rat utricular macula. The ultrastructural research focused on presynaptic processes whose origin and termination could be observed in a single micrograph. Results showed that calyces had 1) vesiculated, spine-like processes that invaginated type I cells and 2) other, elongate processes that ended on type II cells pre- as well as postsynaptically. Intramacular, unmyelinated segments of afferent nerve fibers gave origin to branches that were presynaptic to type II cells, calyces, calyceal processes, and other nerve fibers in the macula. Synapses with type II cells occurred opposite subsynaptic cisternae (C synapses); all other synapses were asymmetric. Vesicles were pleomorphic but were differentially distributed according to process origin. Small, clear-centered vesicles, approximately 40-60 nm in diameter, predominated in processes originating from afferent nerve fibers and basal parts of calyces. Larger vesicles approximately 70-120 nm in diameter having approximately 40-80 nm electron-opaque cores were dominant in processes originating from the necks of calyces. Results are interpreted to indicate the existence of a complex system of intrinsic feedforward (postsynaptic)-feedback (presynaptic) connections in a network of direct and local microcircuits. The morphological findings support the concept that maculae dynamically preprocess linear acceleratory information before its transmission to the central nervous system.

The vestibular system as a model of sensorimotor transformations. A combined in vivo and in vitro approach to study the cellular mechanisms of gaze and posture stabilization in mammals

To understand the cellular mechanisms underlying behaviours in mammals, the respective contributions of the individual properties characterizing each neuron, as opposed to the properties emerging from the organization of these neurons in functional networks, have to be evaluated. This requires the use, in the same species, of various in vivo and in vitro experimental preparations. The present review is meant to illustrate how such a combined in vivo in vitro approach can be used to investigate the vestibular-related neuronal networks involved in gaze and posture stabilization, together with their plasticity, in the adult guinea-pig. Following first a general introduction on the vestibular system, the second section describes various in vivo experiments aimed at characterizing gaze and posture stabilization in that species. The third and fourth parts of the review deal with the combined in vivo-in vitro investigations undertaken to unravel the physiological and pharmacological properties of vestibulo-ocular and vestibulo-spinal networks, together with their functional implications. In particular, we have tried to use the central vestibular neurons as examples to illustrate how the preparation of isolated whole brain can be used to bridge the gap between the results obtained through in vitro, intracellular recordings on slices and those collected in vivo, in the behaving animal.

Expression of the AMPA-selective receptor subunits in the vestibular nuclei of the chinchilla

The distribution of the AMPA type glutamate receptor has been investigated throughout the central nervous system; however, no detailed description of its distribution is available in the vestibular nuclei. In the present study, in situ hybridization histochemistry and immunohistochemistry were used to localize the messenger RNAs and proteins of the AMPA-selective receptor subunits GluR1, GluR2, GluR3 and GluR4 in the vestibular nuclei of the chinchilla. Immunohistochemistry with subunits specific antisera showed differential distribution of the subunits in the vestibular nuclei. GluR2/3 antiserum labeled the most neurons, suggesting that many if not all vestibular neurons receive glutamatergic input. GluR1-positive neurons were fewer than GluR2/3 immunoreactive neurons and GluR4 immunoreactivity was found in the fewest number of neurons. GluR1 and GluR4 immunoreactivity was also found in astrocyte-like structures. In situ hybridization with 35S-labeled complementary RNA probes confirmed the distribution of the AMPA receptor subunits obtained by immunohistochemistry. Quantitative analysis of the levels of hybridization showed a high degree of diversity in the levels of expression of the GluR2 subunit mRNA, with the highest levels of expression in the giant Deiter's cells of the lateral vestibular nuclei and the lowest levels in the small neurons throughout the vestibular nuclei. The subunit compositions of the AMPA receptors determine their physiological properties. Differential distribution and levels of expression of the receptor subunits in the vestibular nuclei may be related to the characteristics of information processing through the vestibular system.

Effects of substance P on medial vestibular nucleus neurons in guinea-pig brainstem slices

The undecapeptide substance P (SP) has been recently implicated in the control of vestibular function. In particular, it seems to be co-localized with glutamate in approximately half of the primary vestibular afferents in mammals. Using intracellular recordings in guinea-pig brainstem slices, we have investigated the effects of SP and of several agonists of the three known tachykinin receptor subtypes (NK1, NK2 and NK3) on the three main types (A, B and B+LTS) of guinea-pig medial vestibular nucleus neurons (MVNn) that we had previously described. SP could induce two distinct kinds of effects on all types of MVNn. Whereas around half of them were depolarized and had their membrane resistance increased by SP, approximately 10% of all MVNn were in contrast hyperpolarized and inhibited while their membrane resistance was decreased. Both responses persisted under conditions of blockade of synaptic transmission, and were thus due to the activation of postsynaptic binding sites. The SP-induced membrane depolarization could not be reproduced with any one of the specific agonists of the three tachykinin receptor subtypes, nor was it blocked by the specific NK1 receptor antagonists GR 82664 and CP 99994. This effect might therefore be due to the activation of a new, pharmacologically distinct, 'NK1-like' receptor. Only the hyperpolarizing effects, which were in contrast mimicked by the specific NK1 receptor agonists GR 73632 and [Sar9, Met (O2)11]-SP, would be mediated by the few typical NK1 receptors which have been demonstrated in the medial vestibular nucleus.

Immunocytochemical localization of aspartate and glutamate in the peripheral vestibular system

Controversy exists concerning the identity of the neurotransmitter in the mammalian peripheral vestibular system. Several candidates have been proposed, including the excitatory amino acids glutamate and aspartate and the inhibitory amino acid gamma-aminobutyric acid (GABA). Previous studies have demonstrated vestibuloneural electrophysiological activity associated with glutamate and aspartate. Paraffin sections of rat vestibular ganglia and end-organs were examined for the presence of glutamate-like and aspartate-like immunoreactivity. Our results demonstrate the presence of both aspartate-like and glutamate-like immunoreactivity in vestibular hair cells, peripheral vestibular nerve fibers, and vestibular ganglion cells. Minimal immunoreactivity was noted in the tissues surrounding these cells. These data add support to the hypothesis that the excitatory amino acids glutamate and aspartate are involved in vestibular neurotransmission.

NMDA (NMDAR1) and AMPA-type (GluR2/3) receptor subunits are expressed in the inner ear

Using receptor subunit-specific antibodies, the cellular localization of NMDA and AMPA type glutamate receptor subunits was studied within the rodent (rat, guinea pig) and non-human primate (monkey) inner ear. In the spiral and vestibular ganglion, almost all cells were immunoreactive for the NMDAR1 subunit and the AMPA type receptor subunit GluR2/3. This indicates that both NMDA and non-NMDA type glutamate receptors may be co-distributed in the primary afferent neuronal components, and are possibly involved in neurotransmission in the primary auditory and vestibular systems. This study also indicated the possible localizations of glutamate receptors in the nonneuronal cells in the inner ear, suggesting that some nonneuronal cells may also have the ability to mediate glutamate signalling.

Differential cellular distribution of glutamate and glutamine in the rat vestibular endorgans: an immunocytochemical study

The cellular and subcellular localization of glutamate and glutamine in the rat vestibular endorgans was studied by means of postembedding immunocytochemistry. Glutamate immunoreactivity was preferentially distributed in the hair cells, whereas glutamine immunoreactivity was enriched in supporting cells. This points to a metabolic compartmentation similar to that found in glutamatergic nerve terminals and adjacent glial processes in the central nervous system. The present immunocytochemical results are consistent with the existence of a glutamate-glutamine cycle in the vestibular sensory epithelium. Our data are also in agreement with a transmitter role of glutamate in both types of hair cell, although a vesicular enrichment of glutamate in these cells remains to be demonstrated.

Effects of baclofen on medial vestibular nucleus neurones in guinea-pig brainstem slices

Using intracellular recordings of medial vestibular nucleus neurones (MVNn) in guinea-pig brainstem slices, the effects of baclofen, a specific agonist of the metabotropic GABAB receptors, were tested on the three main types of MVNn (A, B and B + LTS MVNn) that were previously identified in this nucleus. Regardless of their type, almost all MVNn were hyperpolarized and inhibited by baclofen. These hyperpolarizing effects persisted following either the addition of tetrodotoxin (TTX) in the perfusion medium, or in the presence of a high Mg2+/low Ca2+ solution known to block synaptic transmission. These results demonstrate that all types of MVNn are endowed with postsynaptic GABAB receptors.

Expression of substance P, CGRP, and GABA in the vestibular periphery, with special reference to species differences

The present study was conducted to elucidate species differences in the distribution of neuroactive substances, including substance P (SP), calcitonin gene-related peptide (CGRP) and gamma-aminobutyric acid (GABA), in the vestibular periphery of various animals (chicken, pigeon, rat, guinea pig and squirrel monkey). SP-like immunoreactivity was found in a subset of the primary vestibular afferents with no marked species differences in the staining pattern. In contrast, the localization of CGRP- and GABA-like immunoreactivities in the efferent nerve fibers varied according to species. This difference in the distribution pattern of neuroactive substances, found in the efferent system, may indicate that each species has a chemically (and probably functionally) distinct efferent system which is related to its specific environment and/or evolution.

Distribution of neurotransmitters, neuropeptides, and receptors in the vestibular nuclei complex of the rat: an immunocytochemical, in situ hybridization and quantitative receptor autoradiographic study

This article investigates the distribution of neurotransmitters, neuropeptides, and related receptors in the vestibular nuclei complex (VNC) of the adult rat by means of immunohistochemistry, in situ hybridization, and quantitative receptor autoradiography. The entire complex proves to be rich in muscarinic receptors and it shows a high density of imipramine and benzodiazepine binding sites. Peptidergic neurons and a few positive fibers are described in the caudal part of the VNC. In particular, the medial vestibular nucleus contains a number of neurons expressing both the enkephalin mRNA and peptide. This nucleus and the lateral vestibular nucleus are also rich in opiate receptors. Substance P, thyrotropin releasing hormone, and neurotensin receptors are also found in the medial and in the spinal vestibular nuclei. In spite of the presence of alpha 2 catecholaminergic receptors, no thyrosine-hydroxylase-immuno-reactive elements are seen in the caudal VNC. The possible functional meaning of these data is discussed.

Neuroactive substances in the human vestibular end organs

In order to evaluate the involvement of neuroactive substances in the human vestibular periphery, the immunocytochemical distribution of substance P (SP), calcitonin gene-related peptide (CGRP), and choline acetyltransferase (ChAT) was examined. SP-like immunoreactivity (LI) was present around and beneath sensory hair cells, probably corresponding to their afferent nerve endings. SP-LI was found predominantly in subpopulations of the primary afferents distributed in the peripheral region of the end organs. ChAT-LI and CGRP-LI were found throughout as small puncta below the hair cell layer, probably corresponding to efferent endings. The present results indicate that these neuroactive substances, previously described in animals, are also distributed in the human vestibular periphery, and almost certainly contribute to human vestibular function.

Nitric oxide in the rat vestibular system

Nitric oxide is known to function as a neurotransmitter in the central nervous system. It is also known to be involved in the central nervous system excitatory amino acid neurotransmission cascade. Activation of excitatory amino acid receptors causes an influx of calcium, which activates nitric oxide synthase. The resulting increase in intracellular nitric oxide activates soluble guanylate cyclase, leading to a rise in cyclic guanosine monophosphate. The excitatory amino acids glutamate and aspartate are found in the vestibular system and have been postulated to function as vestibular system neurotransmitters. Although nitric oxide has been investigated as a neurotransmitter in other tissues, no published studies have examined the role of nitric oxide in the vestibular system. Neuronal NADPH-diaphorase has been characterized as a nitric oxide synthase. This enzyme catalyzes the conversion of L-arginine to L-citrulline, producing nitric oxide during the reaction. We used a histochemical stain characterized by Hope et al. (Proc Natl Acad Sci 1991;88:2811) as specific for neuronal nitric oxide synthase to localize the enzyme in the rat vestibular system. An immunocytochemical stain was used to examine rat inner ear tissue for the presence of the enzyme's end product, L-citrulline, thereby demonstrating nitric oxide synthase activity. Staining of vestibular ganglion sections showed nitric oxide synthase presence and activity in ganglion cells and nerve fibers. These results indicate the presence of active nitric oxide synthase in these tissues and suggest modulation of vestibular neurotransmission by nitric oxide.

Treatment of abnormal eye movements that impair vision: strategies based on current concepts of physiology and pharmacology

Certain abnormal eye movements, especially pathological nystagmus, degrade vision and cause illusory motion of the seen environment. These symptoms are due to excessive movement of images of stationary objects on the retina. Recently, the pathophysiology underlying several types of nystagmus and saccadic oscillations was better defined by the development of animal models and by experimental pharmacological studies. Despite this, few reliable therapies are currently available for these abnormal eye movements. In clinical studies, a number of drugs reportedly helped individual patients, but few drugs have been subjected to double-blind trials. An alternative approach to pharmacological suppression of abnormal eye movements is optical stabilization of images on the retina, which is helpful in selected patients. Weakening of the extraocular muscles, using botulinum toxin or surgery, is prone to cause diplopia and may induce plastic-adaptive changes that render the effect temporary. In some patients, treatment of an underlying condition, such as the Arnold-Chiari malformation, reduces nystagmus and improves vision. There is a need for multicenter trials to evaluate systematically potential treatments of abnormal eye movements that impair vision.

Do the organs of the labyrinth differentially influence the sympathetic and parasympathetic systems?

It has long been recognized that the vestibular system plays a major role in autonomic control. The nature of this control remains in dispute, however, as some evidence points to a vestibularly mediated parasympathetic activation, whereas other evidence points to a sympatho-excitatory role for labyrinthine outputs. A theoretical explanation is offered that attempts to resolve this issue by postulating that the utricles exert a predominantly sympatho-excitatory influence via their interactions with brain noradrenergic pathways, while the semicircular canals (and possibly saccules) increase parasympathetic tone via their cholinergic brain stem and cerebellar projections. This explanation is relevant for understanding the vestibular role in orthostatic regulation, motion sickness, oculomotor control, and in many disorders or situations associated with neurochemical or autonomic imbalances.

Effects of efferent neurotransmitters on intracellular Ca2+ concentration in vestibular hair cells of the guinea pig

The effects of putative efferent neurotransmitters on intracellular Ca2+ concentration ([Ca2+]i) in isolated vestibular hair cells (VHCs) of the guinea pig were determined using the Ca2+ sensitive dye Fura-2 and digital imaging microscopy. In the presence of 1 mM acetylcholine (ACh) there was a gradual increase in [Ca2+]i. In the presence of 10 microM ATP there was a rapid rise in [Ca2+]i. Thus, both ACh and ATP seem to be efferent neurotransmitters in VHCs of the guinea pig. When 100 microM Gamma-aminobutyric acid (GABA) was added there was a rapid increase in [Ca2+]i; thus GABA also seems to be an efferent neurotransmitter in VHCs. With the independent addition of 10 microM calcitonin gene-related peptide and 10 microM M- and L-enkephalin there were no significant increase in [Ca2+]i. We presume that these neuropeptides were functioning as efferent neuromodulators rather than as neurotransmitters.

Different calcium-binding proteins identify subpopulations of vestibular ganglion neurons in the rat

Vestibular neurons were studied by cytochrome oxidase (CO) histochemistry and by immunocytochemistry using antibodies against parvalbumin (PV), calbindin (CaBP), calretinin (CaR) and 160 KD neurofilament protein (NF). All the neurons present a high level of CO activity and a high content of PV. CaBP and CaR are restricted to a specific population of about 16% of the neurons and are among the largest ones. The latter neurons also have a high density of NF 160 KD protein. In conclusion the biochemical characteristics of the vestibular ganglion neurons are discussed in relation to their morphological and physiological properties.

Anaesthetics may change the shape of isolated type I hair cells

Type I hair cells isolated from animals anaesthetised with barbiturates or ether were found to be shorter and to lack a prominent 'neck' region when compared to cells isolated from non-anaesthetised animals. Ketamine did not have this effect. The changes observed could have important implications for the physiology of inner ear receptors. These findings infer that care should be taken in the choice of anaesthetics used in studies on cells from the inner ear.

Excitatory amino acid receptors in normal and abnormal vestibular function

Although excitatory amino acid (EAA) receptors have been investigated extensively in the limbic system and neocortex, less is known of the function of EAA receptors in the brainstem. A number of biochemical and electrophysiological studies suggest that the synapse between the ipsilateral vestibular (VIIIth) nerve and the brainstem vestibular nucleus (VN) is mediated by an EAA acting predominantly on kainate or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. In addition, there is electrophysiological evidence that input from the contralateral vestibular nerve via the contralateral VN is partially mediated by N-methyl-D-aspartate (NMDA) receptors. Input to the VN from the spinal cord may also be partially mediated by NMDA receptors. All of the electrophysiological studies conducted so far have used in vitro preparations, and it is possible that denervation of the VN during the preparation of an explant or slice causes changes in EAA receptor function. Nonetheless, these results suggest that EAA receptors may be important in many different parts of the vestibular reflex pathways. Studies of the peripheral vestibular system have also shown that EAAs are involved in transmission between the receptor hair cells and the vestibular nerve fibers. A number of recent studies in the area of vestibular plasticity have reported that antagonists for the NMDA receptor subtype disrupt the behavioral recovery that occurs following unilateral deafferentation of the vestibular nerve fibers (vestibular compensation). It has been suggested that vestibular compensation may be owing to an upregulation or increased affinity of NMDA receptors in the VN ipsilateral to the peripheral deafferentation; however; at present, there is no clear evidence to support this hypothesis.

Identification and localization of a kainate binding protein in the frog inner ear by electron microscopy immunocytochemistry

A kainate binding protein (KBP) was studied in Rana pipiens inner ear using monoclonal and polyclonal antibodies against affinity purified KBP from frog brain. The KBP identified and analyzed in inner ear tissue homogenates, with one- and two-dimensional immunoblots, was similar to the affinity purified KBP and to the antibody-identified frog brain KBP. As brain KBP, inner ear KBP had 5 main components in the molecular weight dimension, centered at Mr = 48,000; however, inner ear KBP had a greater abundance of the higher molecular weight components. Light and electron microscopy observations showed KBP immunostaining at two locations: (1) in the dendrites of the eight nerve afferent fibers contacting sensory hair cells, with the postsynaptic density being more intensely stained; and (2) on the cytoplasmic membrane of fibroblasts present in the inner ear connective tissue which displayed intense immunostaining. The presence of kainate (KA) binding sites in the inner ear was assessed using in vitro receptor autoradiography. [3H]KA binding sites were found in connective tissue areas confirming the immunocytochemistry results. The postsynaptic localization of the KBP in afferent endings, strongly supports it as being a component of the KA receptor complex. However, its presence on fibroblasts situated in the inner ear connective tissue makes its function hypothetical. The dual presence of the KBP on non-neuronal cells as well as at postsynaptic membrane sites suggests the existence of a family of proteins involved in KA binding and KA receptors with a complex organization.

Aspartate-like immunoreactivity in vestibulospinal neurons in gerbils

In an effort to further characterize vestibulospinal pathways in the gerbil, immunocytochemistry was combined with retrograde identification of neurons. Vestibulospinal neurons were retrogradely labeled following injections of horseradish peroxidase into the cervical cord of anesthetized gerbils. Sections were reacted with nickel acetate-diaminobenzidine for horseradish peroxidase, giving a black reaction product. Sections were incubated in polyclonal antisera to aspartate, incubated in an avidin-biotin-peroxidase procedure, and reacted to give a brown reaction product. Alternatively, fluoro-gold was used as a retrograde tracer and aspartate-like immunoreactivity was demonstrated with avidin conjugated to Texas red. Cells stained with aspartate-like immunoreactivity, were located in all vestibular nuclei. Double-labeled cells were located in the medial nucleus and in the lateral vestibular nucleus where many of the large cells were double labeled.

GABA immunoreactivity of calyceal nerve endings in the vestibular system of the guinea pig

Neurotransmitters involved in the vestibular system are largely uncharacterized. On the basis of results of earlier electrophysiological and immunohistochemical experiments, glutamate and gamma-amino-butyric acid (GABA) have been proposed in both mammalian and non-mammalian species as afferent transmitters between the sensory cell and the afferent dendrite. GABA is also suspected to act as an efferent neurotransmitter in the cochlea. We describe in this study the immunocytochemical localization of GABA within the vestibular end organs in the guinea pig. GABA immunoreactivity was found in the calyceal nerve endings surrounding type I hair cells of the vestibular epithelia. The most significant labelings were obtained in the crista ampullaris. Labeling was more difficult to observe in the utricular and saccular macula. These results contribute to the recent proposal that the calyx has a secretory function, and suggest that GABA may have a modulatory influence upon the type I hair cells.