Tonic activity of rat medial vestibular nucleus neurones in vitro and its inhibition by GABA

Parvalbumin-positive neurons in the medial vestibular nucleus contribute to vestibular compensation through commissural inhibition

Background

The commissural inhibitory system between the bilateral medial vestibular nucleus (MVN) plays a key role in vestibular compensation. Calcium-binding protein parvalbumin (PV) is expressed in MVN GABAergic neurons. Whether these neurons are involved in vestibular compensation is still unknown.

Methods

After unilateral labyrinthectomy (UL), we measured the activity of MVN PV neurons by in vivo calcium imaging, and observed the projection of MVN PV neurons by retrograde neural tracing. After regulating PV neurons' activity by chemogenetic technique, the effects on vestibular compensation were evaluated by behavior analysis.

Results

We found PV expression and the activity of PV neurons in contralateral but not ipsilateral MVN increased 6 h following UL. ErbB4 is required to maintain GABA release for PV neurons, conditional knockout ErbB4 from PV neurons promoted vestibular compensation. Further investigation showed that vestibular compensation could be promoted by chemogenetic inhibition of contralateral MVN or activation of ipsilateral MVN PV neurons. Additional neural tracing study revealed that considerable MVN PV neurons were projecting to the opposite side of MVN, and that activating the ipsilateral MVN PV neurons projecting to contralateral MVN can promote vestibular compensation.

Conclusion

Contralateral MVN PV neuron activation after UL is detrimental to vestibular compensation, and rebalancing bilateral MVN PV neuron activity can promote vestibular compensation, via commissural inhibition from the ipsilateral MVN PV neurons. Our findings provide a new understanding of vestibular compensation at the neural circuitry level and a novel potential therapeutic target for vestibular disorders.

Long-Lasting Visuo-Vestibular Mismatch in Freely-Behaving Mice Reduces the Vestibulo-Ocular Reflex and Leads to Neural Changes in the Direct Vestibular Pathway

Calibration of the vestibulo-ocular reflex (VOR) depends on the presence of visual feedback. However, the cellular mechanisms associated with VOR modifications at the level of the brainstem remain largely unknown. A new protocol was designed to expose freely behaving mice to a visuo-vestibular mismatch during a 2-week period. This protocol induced a 50% reduction of the VOR. In vivo pharmacological experiments demonstrated that the VOR reduction depends on changes located outside the flocculus/paraflocculus complex. The cellular mechanisms associated with the VOR reduction were then studied in vitro on brainstem slices through a combination of vestibular afferent stimulation and patch-clamp recordings of central vestibular neurons. The evoked synaptic activity demonstrated that the efficacy of the synapses between vestibular afferents and central vestibular neurons was decreased. In addition, a long-term depression protocol failed to further decrease the synapse efficacy, suggesting that the VOR reduction might have occurred through depression-like mechanisms. Analysis of the intrinsic membrane properties of central vestibular neurons revealed that the synaptic changes were supplemented by a decrease in the spontaneous discharge and excitability of a subpopulation of neurons. Our results provide evidence that a long-lasting visuo-vestibular mismatch leads to changes in synaptic transmission and intrinsic properties of central vestibular neurons in the direct VOR pathway. Overall, these results open new avenues for future studies on visual and vestibular interactions conducted in vivo and in vitro.

The role of GABAB receptors in the vestibular oculomotor system in mice

Systemic administration of a gamma-amino butyric acid type B (GABAB) receptor agonist, baclofen, affects various physiological and psychological processes. To date, the effects on oculomotor system have been well characterized in primates, however those in mice have not been explored. In this study, we investigated the effects of baclofen focusing on vestibular-related eye movements. Two rotational paradigms, i.e. sinusoidal rotation and counter rotation were employed to stimulate semicircular canals and otolith organs in the inner ear. Experimental conditions (dosage, routes and onset of recording) were determined based on the prior studies exploring the behavioral effects of baclofen in mice. With an increase in dosage, both canal and otolith induced ocular responses were gradually affected. There was a clear distinction in the drug sensitivity showing that eye movements derived from direct vestibulo-ocular reflex pathways were relatively unaltered, while the responses through higher-order neural networks in the vestibular system were substantially decreased. These findings were consistent with those observed in primates suggesting a well-conserved role of GABAB receptors in the oculomotor system across frontal-eyed and lateral-eyed animals. We showed here a previously unrecognized effect of baclofen on the vestibular oculomotor function in mice. When interpreting general animal performance under the drug, the potential contribution of altered balance system should be taken into consideration.

GABA(A) receptor agonist and antagonist alter vestibular compensation and different steps of reactive neurogenesis in deafferented vestibular nuclei of adult cats

Strong reactive cell proliferation occurs in the vestibular nuclei after unilateral vestibular neurectomy (UVN). Most of the newborn cells survive, differentiate into glial cells and neurons with GABAergic phenotype, and have been reported to contribute to recovery of the posturo-locomotor functions in adult cats. Because the GABAergic system modulates vestibular function recovery and the different steps of neurogenesis in mammals, we aimed to examine in our UVN animal model the effect of chronic infusion of GABA(A) receptor (R) agonist and antagonist in the vestibular nuclei. After UVN and one-month intracerebroventricular infusions of saline, GABA(A)R agonist (muscimol) or antagonist (gabazine), cell proliferation and differentiation into astrocytes, microglial cells, and neurons were revealed using immunohistochemical methods. We also determined the effects of these drug infusions on the recovery of posturo-locomotor and oculomotor functions through behavioral tests. Our results showed that surprisingly, one month after UVN, newborn cells did not survive in the UVN-muscimol group whereas the number of GABAergic pre-existent neurons increased, and the long-term behavioral recovery of the animals was drastically impaired. Conversely, a significant number of newborn cells survived up to 1 month in the UVN-gabazine group whereas the astroglial population increased, and these animals showed the fastest recovery in behavioral functions. This study reports for the first time that GABA plays multiple roles, ranging from beneficial to detrimental on the different steps of a functional postlesion neurogenesis and further, strongly influences the time course of vestibular function recovery.

5-HT2A receptor-mediated excitation on cerebellar fastigial nucleus neurons and promotion of motor behaviors in rats

It has long been known that serotonergic afferent inputs are the third largest afferent population in the cerebellum after mossy fibers and climbing fibers. However, the role of serotonergic inputs in cerebellar-mediated motor behaviors is still largely unknown. Here, we show that only 5-HT2A receptors among the 5-HT2 receptor subfamily are expressed and localized in the rat cerebellar fastigial nucleus (FN), one of the ultimate outputs of the spinocerebellum precisely regulating trunk and limb movements. Remarkably, selective activation of 5-HT2A receptors evokes a postsynaptic excitatory effect on FN neurons in a concentration-dependent manner in vitro, which is in accord with the 5-HT-elicited excitation on the same tested neurons. Furthermore, selective 5-HT2A receptor antagonist M100907 concentration-dependently blocks the excitatory effects of 5-HT and TCB-2, a 5-HT2A receptor agonist, on FN neurons. Consequently, microinjection of 5-HT into bilateral FNs significantly promotes rat motor performances on accelerating rota-rod and balance beam and narrows stride width rather than stride length in locomotion gait. All these motor behavioral effects are highly consistent with those of selective activation of 5-HT2A receptors in FNs, and blockage of the component of 5-HT2A receptor-mediated endogenous serotonergic inputs in FNs markedly attenuates these motor performances. All these results demonstrate that postsynaptic 5-HT2A receptors greatly contribute to the 5-HT-mediated excitatory effect on cerebellar FN neurons and promotion of the FN-related motor behaviors, suggesting that serotonergic afferent inputs may actively participate in cerebellar motor control through their direct modulation on the final output of the spinocerebellum.

Histamine excites neurons of the inferior vestibular nucleus in rats by activation of H1 and H2 receptors

By using brain slice preparations and extracellular recordings, the effect of histamine on spontaneous firing activities of neurons in the inferior vestibular nucleus (IVN), a key structure responsible for integration of vestibular, multisensory, and cerebellar inputs, in rats was investigated. Perfusing slices with histamine (1-10μM) elicited an excitatory response on IVN neurons. The responses were not blocked by low Ca(2+)/high Mg(2+) medium, indicating a direct postsynaptic effect of the amine. Furthermore, the histamine-induced excitation was partially blocked by selective histamine H1 receptor antagonist mepyramine (1μM) and H2 receptor antagonist ranitidine (1μM), respectively. Co-application of mepyramine and ranitidine nearly totally antagonized the histamine-induced excitation. Additionally, both selective H1 receptor agonist 2-pyridylethylamine (30-300μM) and H2 receptor agonist dimaprit (10-100μM) effectively mimicked the excitatory action of histamine on IVN neurons. Moreover, selective H4 antagonist JNJ7777120 (10μM) and agonist VUF8430 (30-300μM) had no effect on IVN neurons. These results demonstrate that histamine excites IVN neurons via postsynaptic H1 and H2 rather than H4 receptors, and suggest that the central histaminergic system actively modulate all four major vestibular nuclei including the IVN and may subsequently influence the vestibular nuclei-related reflexes and functions.

Basic Concepts in Understanding Recovery of Function in Vestibular Reflex Networks during Vestibular Compensation

Unilateral peripheral vestibular lesions produce a syndrome of oculomotor and postural deficits with the symptoms at rest, the static symptoms, partially or completely normalizing shortly after the lesion due to a process known as vestibular compensation. The symptoms are thought to result from changes in the activity of vestibular sensorimotor reflexes. Since the vestibular nuclei must be intact for recovery to occur, many investigations have focused on studying these neurons after lesions. At present, the neuronal plasticity underlying early recovery from the static symptoms is not fully understood. Here we propose that knowledge of the reflex identity and input–output connections of the recorded neurons is essential to link the responses to animal behavior. We further propose that the cellular mechanisms underlying vestibular compensation can be sorted out by characterizing the synaptic responses and time course for change in morphologically defined subsets of vestibular reflex projection neurons. Accordingly, this review focuses on the perspective gained by performing electrophysiological and immunolabeling studies on a specific subset of morphologically defined, glutamatergic vestibular reflex projection neurons, the principal cells of the chick tangential nucleus. Reference is made to pertinent findings from other studies on vestibular nuclei neurons, but no comprehensive review of the literature is intended since broad reviews already exist. From recording excitatory and inhibitory spontaneous synaptic activity in principal cells, we find that the rebalancing of excitatory synaptic drive bilaterally is essential for vestibular compensation to proceed. This work is important for it defines for the first time the excitatory and inhibitory nature of the changing synaptic inputs and the time course for changes in a morphologically defined subset of vestibular reflex projection neurons during early stages of vestibular compensation.

No effects of anti-motion sickness drugs on vestibular evoked myogenic potentials outcome parameters

OBJECTIVE

To investigate the effects of meclizine (50 mg), baclofen (10 mg), cinnarizine (20 mg) + dimenhydrinate (40 mg), and promethazine (25 mg) + dextro-amphetamine (5 mg) on the parameters of the vestibular evoked myogenic potential (VEMP) test.

STUDY DESIGN

Double-blind placebo-controlled prospective randomized trial.

SETTING

University hospital.

SUBJECTS

Twenty-four (first block: baclofen versus placebo) and 20 healthy male subjects (second block: meclizine, cinnarizine + dimenhydrinate and promethazine + dextro-amphetamine versus placebo).

INTERVENTIONS

VEMP test.

MAIN OUTCOME MEASURES

Threshold, p13 and n23 latencies, p13-n23 latency difference, p13-n23 peak-to-peak amplitude, mean rectified voltage of the sternocleidomastoid muscle contraction and the corrected amplitude.

RESULTS

There were no clinically significant pharmacologic effects on the VEMP outcome parameters. However, there was a statistically significant left-right asymmetry after intake of the combination promethazine + d-amphetamine for the parameters p13 and latency difference.

CONCLUSION

The absence of clinically significant effects can be explained by the predominant presence of the target receptors for the applied drugs in the medial vestibular nucleus, which receives the lowest grade of saccular projections. It also can be hypothesized that the VEMP methodology and techniques in general do not allow determining pharmacologic effects in a healthy group of subjects because of a too small discriminative power. The left-right asymmetry can be explained by a depressive action of the drugs on the central compensation mechanisms. Because there were no significant differences between the VEMP parameters obtained after intake of the placebos of both blocks, we concluded that there were no training effects.

Influence of testosterone on synaptic transmission in the rat medial vestibular nuclei: estrogenic and androgenic effects

In brainstem slices of young male rat, we investigated the influence of the neuroactive steroid testosterone (T) on the synaptic responses by analyzing the field potential evoked in the medial vestibular nucleus (MVN) by vestibular afferent stimulation. T induced three distinct and independent long-term synaptic changes: fast long-lasting potentiation (fLP), slow long-lasting potentiation (sLP) and long-lasting depression (LD). The fLP was mediated by 17β-estradiol (E(2)) since it was abolished by blocking the estrogen receptors (ERs) or the enzyme converting T to E(2). Conversely, sLP and LD were mediated by 5α-dihydrotestosterone (DHT) since they were prevented by blocking the androgen receptors (ARs) or the enzyme converting T to DHT. Therefore, the synaptic effects of T were mediated by its androgenic or estrogenic metabolites. The pathways leading to estrogenic and androgenic conversion of T might be co-localized since, the occurrence of fLP under block of androgenic pathway, and that of sLP and LD under estrogenic block, were higher than those observed without blocks. In case of co-localization, the effect on synaptic transmission should depend on the prevailing enzymatic activity. We conclude that circulating and neuronal T can remarkably influence synaptic responses of the vestibular neurons in different and opposite ways, depending on its conversion to estrogenic or androgenic metabolites.

Orexins excite neurons of the rat cerebellar nucleus interpositus via orexin 2 receptors in vitro

Orexins are newfound hypothalamic neuropeptides implicated in the regulation of feeding behavior, sleep-wakefulness cycle, nociception, addiction, emotions, as well as narcolepsy. However, little is known about roles of orexins in motor control. Therefore, the present study was designed to investigate the effect of orexins on neuronal activity in the cerebellum, an important subcortical center for motor control. In this study, perfusing slices with orexin A (100 nM-1 microM) or orexin B (100 nM-1 microM) both produced neurons in the rat cerebellar interpositus nucleus (IN) a concentration-dependent excitatory response (96/143, 67.1%). Furthermore, both of the excitations induced by orexin A and B were not blocked by the low-Ca(2+)/high-Mg(2+) medium (n = 8), supporting a direct postsynaptic action of the peptides. Highly selective orexin 1 receptor antagonist SB-334867 did not block the excitatory response of cerebellar IN neurons to orexins (n = 22), but [Ala(11), D-Leu(15)] orexin B, a highly selective orexin 2 receptor (OX(2)R) agonist, mimicked the excitatory effect of orexins on the cerebellar neurons (n = 18). These results demonstrate that orexins excite the cerebellar IN neurons through OX(2)R and suggest that the central orexinergic nervous system may actively participate in motor control through its modulation on one of the final outputs of the spinocerebellum.

Effects of 17beta-estradiol on glutamate synaptic transmission and neuronal excitability in the rat medial vestibular nuclei

We investigated the effects of the neurosteroid 17beta-estradiol (E(2)) on the evoked and spontaneous activity of rat medial vestibular nucleus (MVN) neurons in brainstem slices. E(2) enhances the synaptic response to vestibular nerve stimulation in type B neurons and depresses the spontaneous discharge in both type A and B neurons. The amplitude of the field potential, as well as the excitatory post-synaptic potential (EPSP) and current (EPSC), in type B neurons, are enhanced by E(2). Both effects are long-term phenomena since they outlast the drug washout. The enhancement of synaptic response is mainly due to facilitation of glutamate release mediated by pre-synaptic N-methyl-D-aspartate receptors (NMDARs), since the reduction of paired pulse ratio (PPR) and the increase of miniature EPSC frequency after E(2) are abolished under D-(-)-2-amino-5-phosphonopentanoic acid (AP-5). E(2) also facilitates post-synaptic NMDARs, but it does not affect directly alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and group I-metabotropic glutamate receptors (mGluRs-I). In contrast, the depression of the spontaneous discharge of type A and type B neurons appears to depend on E(2) modulation of intrinsic ion conductances, as the effect remains after blockade of glutamate, GABA and glycine receptors (GlyRs). The net effect of E(2) is to enhance the signal-to-noise ratio of the synaptic response in type B neurons, relative to resting activity of all MVN neurons. These findings provide evidence for a novel potential mechanism to modulate the responsiveness of vestibular neurons to afferent inputs, and so regulate vestibular function in vivo.

ERG voltage-gated K+ channels regulate excitability and discharge dynamics of the medial vestibular nucleus neurones

The discharge properties of the medial vestibular nucleus neurones (MVNn) critically depend on the activity of several ion channel types. In this study we show, immunohistochemically, that the voltage-gated K(+) channels ERG1A, ERG1B, ERG2 and ERG3 are highly expressed within the vestibular nuclei of P10 and P60 mice. The role played by these channels in the spike-generating mechanisms of the MVNn and in temporal information processing was investigated electrophysiologically from mouse brain slices, in vitro, by analysing the spontaneous discharge and the response to square-, ramp- and sinusoid-like intracellular DC current injections in extracellular and whole-cell patch-clamp studies. We show that more than half of the recorded MVNn were responsive to ERG channel block (WAY-123,398, E4031), displaying an increase in spontaneous activity and discharge irregularity. The response to step and ramp current injection was also modified by ERG block showing a reduction of first spike latency, enhancement of discharge rate and reduction of the slow spike-frequency adaptation process. ERG channels influence the interspike slope without affecting the spike shape. Moreover, in response to sinusoid-like current, ERG channel block caused frequency-dependent gain enhancement and phase-lead shift. Taken together, the data demonstrate that ERG channels control the excitability of MVNn, their discharge regularity and probably their resonance properties.

Retrograde GABA signaling adjusts sound localization by balancing excitation and inhibition in the brainstem

Central processing of acoustic cues is critically dependent on the balance between excitation and inhibition. This balance is particularly important for auditory neurons in the lateral superior olive, because these compare excitatory inputs from one ear and inhibitory inputs from the other ear to compute sound source location. By applying GABA(B) receptor antagonists during sound stimulation in vivo, it was revealed that these neurons adjust their binaural sensitivity through GABA(B) receptors. Using an in vitro approach, we then demonstrate that these neurons release GABA during spiking activity. Consequently, GABA differentially regulates transmitter release from the excitatory and inhibitory terminals via feedback to presynaptic GABA(B) receptors. Modulation of the synaptic input strength, by putative retrograde release of neurotransmitter, may enable these auditory neurons to rapidly adjust the balance between excitation and inhibition, and thus their binaural sensitivity, which could play an important role as an adaptation to various listening situations.

Role of the commissural inhibitory system in vestibular compensation in the rat

We investigated the role of the vestibular commissural inhibitory system in vestibular compensation (VC, the behavioural recovery that follows unilateral vestibular loss), using in vivo microdialysis to measure GABA levels in the bilateral medial vestibular nucleus (MVN) at various times after unilateral labyrinthectomy (UL). Immediately after UL, in close correlation with the appearance of the characteristic oculomotor and postural symptoms, there is a marked increase in GABA release in the ipsi-lesional MVN. This is not prevented by bilateral flocculectomy, indicating that it is due to hyperactivity of vestibular commissural inhibitory neurones. Over the following 96 h, as VC occurs and the behavioural symptoms ameliorate, the ipsi-lesional GABA levels return to near-normal. Contra-lesional GABA levels do not change significantly in the initial stages of VC, but decrease at late stages so that when static symptoms have abated there remains a significant difference between the MVNs of the two sides. We also investigated the role of the commissural inhibition in Bechterew's phenomenon, by reversibly inactivating the intact contra-lesional labyrinth in compensating animals through superfusion of local anaesthetic on the round window. Transient inactivation of the intact labyrinth elicited the lateralized behaviour described by Bechterew, but did not alter the GABA levels in either MVN, suggesting the involvement of distinct cellular mechanisms. These findings indicate that an imbalanced commissural inhibitory system is a root cause of the severe oculomotor and postural symptoms of unilateral vestibular loss, and that re-balancing of commissural inhibition occurs in parallel with the subsequent behavioural recovery during VC.

Functional role of cyclic nucleotide-gated channels in rat medial vestibular nucleus neurons

Although cyclic nucleotide-gated (CNG) channels are expressed in numerous brain areas, little information is available on their functions in CNS neurons. The aim of the present study was to define the distribution of CNG channels in the rat medial vestibular nucleus (MVN) and their possible involvement in regulating MVN neuron (MVNn) excitability. The majority of MVNn expressed both CNG1 and CNG2 A subunits. In whole-cell current-clamp experiments carried out on brainstem slices containing the MVNn, the membrane-permeant analogues of cyclic nucleotides, 8-Br-cGMP and 8-Br-cAMP (1 mM), induced membrane depolarizations (8.9 +/- 0.8 and 9.2 +/- 1.0 mV, respectively) that were protein kinase independent. The cGMP-induced depolarization was associated with a significant decrease in the membrane input resistance. The effects of cGMP on membrane potential were almost completely abolished by the CNG channel blockers, Cd(2+) and L-cis-diltiazem, but they were unaffected by blockade of hyperpolarization-activated cyclic nucleotide-gated channels. In voltage-clamp experiments, 8-Br-cGMP induced non-inactivating inward currents (-22.2 +/- 3.9 pA) with an estimated reversal potential near 0 mV, which were markedly inhibited by reduction of extracellular Na(+) and Ca(2+) concentrations. Membrane depolarization induced by CNG channel activation increased the firing rate of MVNn without changing the action potential shape. Collectively, these findings provide novel evidence that CNG channels affect membrane potential and excitability of MVNn. Such action should have a significant impact on the function of these neurons in sensory-motor integration processes. More generally, it might represent a broad mechanism for regulating the excitability of different CNS neurons.

Neurosteroid modulation of neuronal excitability and synaptic transmission in the rat medial vestibular nuclei

In rat brainstem slices, we investigated the influence of the neurosteroids tetrahydrodeoxycorticosterone (THDOC) and allopregnanolone (ALLO) on the synaptically driven and spontaneous activity of vestibular neurons, by analysing their effects on the amplitude of the field potentials evoked in the medial vestibular nuclei (MVN) by vestibular afferent stimulation and on the spontaneous firing rate of MVN neurons. Furthermore, the interaction with gamma-aminobutyric acid (GABA) and glutamate receptors was analysed by using specific antagonists for GABA(A) (bicuculline), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/ kainate [2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo(f)quinoxaline-7-sulphonamide disodium salt (NBQX)], N-methyl-D-aspartate (NMDA) [D-(-)-2-amino-5-phosphonopentanoic acid (AP-5)] and group I metabotropic glutamate receptors (mGlu-I) [(R,S)-1-aminoindan-1,5-dicarboxylic acid (AIDA)] receptors. THDOC and ALLO evoked two opposite long-lasting effects, consisting of either a potentiation or a reduction of field potential and firing rate, which showed early and late components, occurring in conjunction or separately after neurosteroid application. The depressions depended on GABA(A) receptors, as they were abolished by bicuculline, while early potentiation involved glutamate AMPA/kainate receptors, as NBQX markedly reduced the incidence of early firing rate enhancement and, in the case of ALLO, even provoked depression. This suggests that THDOC and ALLO enhance the GABA(A) inhibitory influence on the MVN neurons and facilitate the AMPA/kainate facilitatory one. Conversely, a late potentiation effect, which was still induced after glutamate and GABA(A) receptor blockade, might involve a different mechanism. We conclude that the modulation of neuronal activity in the MVN by THDOC and ALLO, through their actions on GABA(A) and AMPA/kainate receptors, may have a physiological role in regulating the vestibular system function under normal conditions and during the stress response that accompanies many forms of vestibular dysfunction.

Inhibitory Synaptic Transmission Differs in Mouse Type A and B Medial Vestibular Nucleus Neurons In Vitro

Fast inhibitory synaptic transmission in the medial vestibular nucleus (MVN) is mediated by GABAA receptors (GABAARs) and glycine receptors (GlyRs). To assess their relative contribution to inhibition in the MVN, we recorded miniature inhibitory postsynaptic currents (mIPSCs) in physiologically characterized type A and type B MVN neurons. Transverse brain stem slices were prepared from mice (3–8 wk old), and whole cell patch-clamp recordings were obtained from visualized MVN neurons (CsCl internal; Vm = –70 mV; 23°C). In 81 MVN neurons, 69% received exclusively GABAAergic inputs, 6% exclusively glycinergic inputs, and 25% received both types of mIPSCs. The mean amplitude of GABAAR-mediated mIPSCs was smaller than those mediated by GlyRs (22.6 ± 1.8 vs. 35.3 ± 5.3 pA). The rise time and decay time constants of GABAAR- versus GlyR-mediated mIPSCs were slower (1.3 ± 0.1 vs. 0.9 ± 0.1 ms and 10.5 ± 0.3 vs. 4.7 ± 0.3 ms, respectively). Comparison of type A ( n = 20) and type B ( n = 32) neurons showed that type A neurons received almost exclusively GABAAergic inhibitory inputs, whereas type B neurons received GABAAergic inputs, glycinergic inputs, or both. Intracellular labeling in a subset of MVN neurons showed that morphology was not related to a MVN neuron's inhibitory profile ( n = 15), or whether it was classified as type A or B ( n = 29). Together, these findings indicate that both GABA and glycine contribute to inhibitory synaptic processing in MVN neurons, although GABA dominates and there is a difference in the distribution of GABAA and Gly receptors between type A and type B MVN neurons.

Role of GABAB receptors in GABA and baclofen-induced inhibition of adult rat cerebellar interpositus nucleus neurons in vitro

Previous studies suggested that the postsynaptic GABA(B) receptors of deep cerebellar nuclear neurons of adult rats were not activated by selective GABA(B) receptor agonist baclofen or endogenous GABA released by cerebellar cortical Purkinje cells, although the receptors have been demonstrated to exist in the deep cerebellar nuclei. In this study, cerebellar slices of adult rats were prepared for testing effects of GABA, baclofen and muscimol (selective GABA(A) receptor agonist) on cerebellar interpositus nucleus (IN) neurons. Perfusing slices with GABA (10-1000 microM), baclofen (1-30 microM) and muscimol (1-100 microM) respectively produced a dose-dependent inhibitory response on the IN neurons (n = 39, 62 and 50), which was not blocked by low-Ca(2+)/high-Mg(2+) medium (n = 5, 6 and 6), supporting a direct postsynaptic action of these GABAergic agonists. Moreover, both selective GABA(B) receptor antagonist CGP35348 and selective GABA(A) receptor antagonist bicuculline were capable of partially blocking the inhibitory response of IN neurons to GABA (n = 14 and 11), suggesting that the GABA-induced inhibition may contain two components, a GABA(B) receptors-mediated component and a GABA(A) receptors-mediated one. Further experiments revealed that not only muscimol (n = 50) but also baclofen (n = 62) suppressed IN cells' activity. The baclofen-induced inhibition was selectively blocked by CGP35348 (n = 12) but not by bicuculline (n = 8), whereas the muscimol-induced inhibition was selectively antagonized by bicuculline (n = 8) instead of CGP35348 (n = 9). These results indicate that GABA(B) receptors in the IN neurons can be activated not only by GABA but also by baclofen, suggesting that besides GABA(A) receptors, GABA(B) receptors may also be involved in mediating the inhibitory effect of GABA on cerebellar IN neurons of adult rats.

The anatomy of the vestibular nuclei

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

Immunocytochemical and stereological analysis of GABA(B) receptor subunit expression in the rat vestibular nucleus following unilateral vestibular deafferentation

The process of behavioral recovery that occurs following damage to one vestibular labyrinth, vestibular compensation, has been attributed in part to a down-regulation of GABA(B) receptors in the vestibular nucleus complex (VNC) ipsilateral to the lesion, which could potentially reduce commissural inhibition from the contralateral VNC. In this study, we tested the possibility that this occurs through a decrease in the expression of either the GABA(B1) or GABA(B2) subunits of the GABA(B) receptor. We used Western blotting to quantify the expression of these subunits in the VNC at 10 h and 50 h following unilateral vestibular deafferentation (UVD) or sham surgery in rats. We then used immunocytochemistry and stereological counting methods to estimate the number of neurons expressing these subunits in the MVN at 10 h and 2 weeks following UVD or sham surgery. Compared to sham controls, we found no significant changes in either the expression of the two GABA(B) receptor subunits in the VNC or in the number of MVN neurons expressing these GABA(B) receptor subunits post-UVD. These results suggest that GABA(B) receptor expression does not change substantially in the VNC during the process of vestibular compensation.

Nitric oxide increases the spontaneous firing rate of rat medial vestibular nucleus neurons in vitro via a cyclic GMP-mediated PKG-independent mechanism

The effects of nitric oxide (NO) on the discharge rate of medial vestibular nucleus neurons (MVNn) were investigated in rat brainstem slices. The NO-donor sodium nitroprusside (SNP, 200 microM) caused a marked enhancement (+36.7%) of MVNn spontaneous firing rate, which was prevented by the NO-scavenger, carboxy-PTIO (300 microM). The SNP effects were not modified (+37.4%) by synaptic uncoupling, suggesting that NO influences intrinsic membrane properties of MVNn rather than the synaptic input they receive. The excitatory action of SNP was virtually abolished by slice pretreatment with the soluble guanylyl cyclase inhibitor, ODQ (10 microM), and it was mimicked (+33.1%) by the cGMP analogue 8-Br-cGMP (400 microM). Protein kinase G (PKG) and cAMP/protein kinase A (PKA) were both excluded as downstream effectors of the NO/cGMP-induced excitation. However, the cyclic nucleotide-gated (CNG) channel blockers, L-cis-diltiazem (LCD, 100 microM) and Sp-8-Br-PET-cGMPS (100 microM), significantly reduced the firing rate increase produced by 8-Br-cGMP. Moreover, LCD alone decreased spontaneous MVNn firing (-19.7%), suggesting that putative CNG channels may contribute to the tonic control of resting MVNn discharge. 8-Br-cAMP (1 mM) also elicited excitatory effects in MVNn (+40.8%), which occluded those induced by 8-Br-cGMP, indicating that the two nucleotides share a common target. Finally, nested-polymerase chain reaction assay revealed the expression of CNG channel alpha subunit transcript in MVNn. Our data provide the first demonstration that NO/cGMP signalling modulates MVNn spontaneous firing through a mechanism that is independent of PKG or PKA and probably involves activation of CNG channels.

Spontaneous activity of rat pretectal nuclear complex neurons in vitro

Background

Neurons in the mammalian pretectum are involved in the control of various visual and oculomotor tasks. Because functionally independent pretectal cell populations show a wide variation of response types to visual stimulation in vivo, they may also differ in their intrinsic properties when recorded in vitro. We therefore performed whole-cell patch clamp recordings from neurons in the caudal third of the pretectal nuclear complex in frontal brain slices obtained from 3 to 6 week old hooded rats and tried to classify pretectal neurons electrophysiologically.

Results

Pretectal neurons showed various response types to intracellular depolarizations, including bursting and regular firing behavior. One population of pretectal nuclear complex neurons could be particularly distinguished from others because they displayed spontaneous activity in vitro. These cells had more positive resting potentials and higher input resistances than cells that were not spontaneously active. The maintained firing of spontaneously active pretectal cells was characterized by only small variances in interspike intervals and thus showed a regular temporal patterning. The firing rate was directly correlated to the membrane potential. Removing excitatory inputs by blockade of AMPA and/or NMDA receptors did not change the spontaneous activity. Simultaneous blockade of excitatory and inhibitory synaptic input by a substitution of extracellular calcium with cobalt neither changed the firing rate nor its temporal patterning. Each action potential was preceeded by a depolarizing inward current which was insensitive to calcium removal but which disappeared in the presence of tetrodotoxin.

Conclusions

Our results indicate that a specific subpopulation of pretectal neurons is capable of generating maintained activity in the absence of any external synaptic input. This maintained activity depends on a sodium conductance and is independent from calcium currents.

Vestibulo-oculomotor behaviour in rats following a transient unilateral vestibular loss induced by lidocaine

The effects of a transient vestibular nerve blockade, achieved by intra-tympanic instillation of lidocaine, were studied in rats by recording horizontal eye movements in darkness. Evaluation of the dose-response relationship showed that a maximal effect was attained with a concentration of 4% lidocaine. Within 15 min of lidocaine instillation, a vigorous spontaneous nystagmus was observed which reached maximal frequency and velocity of the slow phase after about 20 min. Subsequently, the nystagmus failed for approximately half an hour before it reappeared. This could be avoided by providing visual feedback in between the recordings in darkness or by a contralateral instillation of 2.5% lidocaine. It is suggested that the failure reflects an overload of the vestibulo-oculomotor circuits. After recovery from the nerve blockade, when the gaze was stable, dynamic vestibular tests were performed. They revealed that a decrease of the slow phase velocity gain and the dominant time constant during, respectively, sinusoidal- and step stimulation toward the unanaesthetised side, had developed with the nerve blockade. These modulations were impaired by a nodulo-uvulectomy but not by bilateral flocculectomy, which is consistent with the concept of vestibular habituation. A GABA(B) receptor antagonist, CGP 56433A, given systemically during the nerve blockade, aggravated the vestibular asymmetry. The same effect has previously been demonstrated in both short- (days) and long-term (months) compensated rats, by antagonising the GABA(B) receptor. In summary, this study provides the first observations of vestibulo-oculomotor disturbances during the first hour after a rapid and transient unilateral vestibular loss in the rat. By using this method, it is possible to study immediate behavioural consequences and possible neural changes that might outlast the nerve blockade.

Labyrinthectomy changes T-type calcium channels in vestibular neurones of the guinea pig

In the vestibular nuclei of the awake guinea pig, all neurones are spontaneously active. After unilateral labyrinthectomy, this activity virtually disappears on the ipsilateral side, but is completely restored one week later. In a recent study, we observed that the restoration of spontaneous activity was correlated with an increase in pacemaker activity. In the current study, we found that the ratio of medial vestibular nucleus (MVN) neurones endowed with one of the currents known to play a role in pacemaker activity (i.e. low-threshold calcium current; LTCC) increased from 29% in control guinea pigs to 65% in animals labyrinthectomised on the ipsilateral side one week earlier. Yet this change was not correlated with a modification of the ratio of neurones expressing any of the three related protein-channels (alpha1G, alpha1H and alpha1I).

Spontaneous synaptic activity is primarily GABAergic in vestibular nucleus neurons of the chick embryo

The principal cells of the chick tangential nucleus are vestibular nucleus neurons participating in the vestibular reflexes. In 16-day embryos, the application of glutamate receptor antagonists abolished the postsynaptic responses generated on vestibular-nerve stimulation, but spontaneous synaptic activity was largely unaffected. Here, spontaneous synaptic activity was characterized in principal cells from brain slices at E16 using whole cell voltage-clamp recordings. With KCl electrodes, the frequency of spontaneous inward currents was 3.1 Hz at -60 mV, and the reversal potential was +4 mV. Cs-gluconate pipette solution allowed the discrimination of glycine/GABA(A) versus glutamate receptor-mediated events according to their different reversal potentials. The ratio for spontaneous excitatory to inhibitory events was about 1:4. Seventy-four percent of the outward events were GABA(A), whereas 26% were glycine receptor-mediated events. Both pre- and postsynaptic GABA(B) receptor effects were shown, with presynaptic GABA(B) receptors inhibiting 40% of spontaneous excitatory postsynaptic currents (sEPSCs) and 53% of spontaneous inhibitory postsynaptic currents (sIPSCs). With TTX, the frequency decreased approximately 50% for EPSCs and 23% for IPSCs. These data indicate that the spontaneous synaptic activity recorded in the principal cells at E16 is primarily inhibitory, action potential-independent, and based on the activation of GABA(A) receptors that can be modulated by presynaptic GABA(B) receptors.

Role of the flocculus in mediating vestibular nucleus neuron plasticity during vestibular compensation in the rat

We investigated the role of the cerebellar flocculus in mediating the adaptive changes that occur in the intrinsic properties of brainstem medial vestibular nucleus (MVN) neurons during vestibular compensation. Ipsi-lesional, but not contra-lesional, flocculectomy prevented the compensatory increase in intrinsic excitability (CIE) that normally occurs in the de-afferented MVN neurons within 4 h after unilateral labyrinthectomy (UL). Flocculectomy did not, however, prevent the down-regulation of efficacy of GABA receptors that also occurs in these neurons after UL, indicating that these responses of the MVN neurons to deafferentation are discrete, parallel processes. CIE was also abolished by intra-floccular microinjection of the metabotropic glutamate receptor (mGluR) antagonist AIDA, and the protein kinase C inhibitor bisindolymaleimide I (BIS-I). The serene-threonine kinase inhibitor H-7 had no effect when microinjected at the time of de-afferentation, but abolished CIE if microinjected 2 h later. These cellular effects are in line with the recently reported retardatory effects of BIS-I and H-7 on behavioural recovery after UL. They demonstrate that the increase in intrinsic excitability in MVN neurons during vestibular compensation is cerebellum dependent, and requires mGluR activation and protein phosphorylation in cerebellar cortex. Furthermore, microinjection of the glucocorticoid receptor (GR) antagonist RU38486 into the ipsi-lesional flocculus also abolished CIE in MVN neurons. Thus an important site for glucocorticoids in facilitating vestibular compensation is within the cerebellar cortex. These observations ascribe functional significance to the high levels of GR and 11-beta-HSD Type 1 expression in cerebellum.

Excitatory effects of histamine on cerebellar interpositus nuclear cells of rats through H(2) receptors in vitro

Neuroanatomical studies have revealed a direct hypothalamocerebellar histaminergic pathway, and our previous studies have demonstrated an excitatory effect of histamine on granule and Purkinje cells of the cerebellar cortex. In this study, we further investigated the effect of histamine on the neuronal firing of cerebellar interpositus nucleus (IN) by using cerebellar slice preparations. Eighty-seven IN cells were recorded from 38 slices. The vast majority of the cells responded to histamine stimulation with an excitatory response (79/87, 90.8%), and the rest of them showed no reaction (8/87, 9.2%). The histamine-induced excitation was not blocked by application of low-Ca(2+)/high-Mg(2+) medium (n=8), supporting a direct postsynaptic action of histamine. The histamine H(2) receptor antagonist ranitidine effectively blocked the excitatory response of IN cells to histamine (n=23), but the histamine H(1) receptor antagonist triprolidine could not significantly block the histamine-induced excitation, or only very slightly decreased the excitatory effect of histamine on the cells (n=21). On the other hand, the highly selective histamine H(2) receptor agonist dimaprit mimicked the excitatory effect of histamine on IN cells and the dimaprit-induced excitation was also blocked by ranitidine (n=14). Successively perfusing slices with the medium containing ranitidine and triprolidine, respectively, we found that ranitidine exhibited the same blocking effect on the dimaprit-induced excitation, but triprolidine had no such effect (n=8). Moreover, the histamine H(1) receptor agonist 2-pyridylethylamine did not show any effect on the IN cells (n=9). These results demonstrate that histamine excites cerebellar IN cells via the histamine H(2) receptor mechanism. Together with our previous results, we suggest that the hypothalamocerebellar histaminergic fibers may modulate neuronal activities of the cerebellar cortex and deep nuclei in parallel. The significance of the excitatory effect of histamine on the cerebellar nuclear cells is discussed.

The contribution of the intrinsic excitability of vestibular nucleus neurons to recovery from vestibular damage

Damage to the peripheral vestibular system results in a syndrome of ocular motor and postural abnormalities that partially and gradually abate over time in a process known as 'vestibular compensation'. The first, rapid, phase of compensation has been associated with a recovery of spontaneous resting activity in the ipsilateral vestibular nucleus complex (VNC), as a consequence of neuronal and synaptic plasticity. Increasing evidence suggests that normal VNC neurons in labyrinthine-intact animals, as well as ipsilateral VNC neurons following unilateral vestibular deafferentation (UVD), rely to some extent on intrinsic pacemaker activity provided by voltage-dependent conductances for their resting activity. Modification of this intrinsic pacemaker activity may underlie the recovery of resting activity that occurs in ipsilateral VNC neurons following UVD. This review summarizes and critically evaluates the 'intrinsic mechanism hypothesis', identifying discrepancies amongst the current evidence and suggesting experiments that may test it further.

Tonic activity and GABA responsiveness of medial vestibular nucleus neurons in aged rats

The tonic discharge of rat medial vestibular nucleus (MVN) neurons, and their responsiveness to GABA receptor agonists were investigated in slices prepared from aged rats (24 months old). Aged MVN neurons showed regular spontaneous activity similar to that seen in slices from young adults. However the inhibitory effects of the GABA(A) agonist muscimol on the spontaneous activity of aged MVN neurons were significantly greater than in young MVN neurons. Inhibitory responses to the GABA(B) agonist baclofen also tended to be greater in slices from aged animals, but this difference was not statistically significant. The regular discharge of aged MVN neurons at firing rates similar to those in young animals suggests that the intrinsic excitability of MVN cells is maintained with age. The up-regulation of GABA(A) receptor efficacy in aged MVN neurons may compensate for changes in inhibitory inputs from vestibular commissures and cerebellum that may occur with neuronal loss in the aged brain.

Synaptic plasticity in the medial vestibular nuclei: role of glutamate receptors and retrograde messengers in rat brainstem slices

The analysis of cellular-molecular events mediating synaptic plasticity within vestibular nuclei is an attempt to explain the mechanisms underlying vestibular plasticity phenomena. The present review is meant to illustrate the main results, obtained in vitro, on the mechanisms underlying long-term changes in synaptic strength within the medial vestibular nuclei. The synaptic plasticity phenomena taking place at the level of vestibular nuclei could be useful for adapting and consolidating the efficacy of vestibular neuron responsiveness to environmental requirements, as during visuo-vestibular recalibration and vestibular compensation. Following a general introduction on the most salient features of vestibular compensation and visuo-vestibular adaptation, which are two plastic events involving neuronal circuitry within the medial vestibular nuclei, the second and third sections describe the results from rat brainstem slice studies, demonstrating the possibility to induce long-term potentiation and depression in the medial vestibular nuclei, following high frequency stimulation of the primary vestibular afferents. In particular the mechanisms sustaining the induction and expression of vestibular long-term potentiation and depression, such as the role of various glutamate receptors and retrograde messengers have been described. The relevant role of the interaction between the platelet-activating factor, acting as a retrograde messenger, and the presynaptic metabotropic glutamate receptors, in determining the full expression of vestibular long-term potentiation is also underlined. In addition, the mechanisms involved in vestibular long-term potentiation have been compared with those leading to long-term potentiation in the hippocampus to emphasize the most significant differences emerging from vestibular studies. The fourth part, describes recent results demonstrating the essential role of nitric oxide, another retrograde messenger, in the induction of vestibular potentiation. Finally the fifth part suggests the possible functional significance of different action times of the two retrograde messengers and metabotropic glutamate receptors, which are involved in mediating the presynaptic mechanism sustaining vestibular long-term potentiation.

Intrinsic excitability changes in vestibular nucleus neurons after unilateral deafferentation

Two synergistic plastic mechanisms have recently been identified in rat medial vestibular nucleus (MVN) neurons during 'vestibular compensation', the behavioral recovery that follows damage to the vestibular receptors or nerve of one inner ear. Ipsi-lesional MVN neurons develop a significant increase in their intrinsic excitability, and a marked decrease in the functional efficacy of GABA(A) and GABA(B) receptors, within 4 h of unilateral vestibular deafferentation. These mechanisms presumably counteract the disfacilitation and excessive commissural inhibition of the ipsi-lesional cells after deafferentation, and thus promote the recovery of resting activity. In this study, we investigated the intrinsic membrane properties and spike firing characteristics of rostral ipsi-lesional MVN neurons in slices from animals that underwent vestibular compensation for either 24-72 h or 7-10 days. Significant changes were observed in the spontaneous in vitro discharge rate, resting membrane potentials and voltage-activated membrane conductances of type B cells, but not type A cells. There was a significant increase in the number of type B(LTS) cells compared to normal. These findings indicate that during vestibular compensation marked changes occur in ion channel expression and function selectively in type B MVN neurons. These changes are appropriate to increase the responsiveness of type B cells both to their own intrinsic pacemaker-like membrane conductances and excitatory synaptic inputs. Together with the downregulation of inhibitory receptor efficacy, this increased intrinsic excitability may be sufficient to restore the resting discharge of the deafferented neurons in vivo. These results therefore provide further evidence for synaptic and neuronal plasticity in ipsi-lesional MVN neurons during vestibular compensation.

Modification of the pacemaker activity of vestibular neurons in brainstem slices during vestibular compensation in the guinea pig

In the guinea pig, unilateral labyrinthectomy causes an immediate and severe depression of the spontaneous activity of the ipsilateral central vestibular neurons, which subsequently recovers completely within one week. A possible underlying mechanism could be an increase in the endogenous activity of the neurons deprived of their labyrinthine input. Here, we addressed this hypothesis. The endogenous activity of the neurons was assessed by their spontaneous activity recorded extracellularly in brainstem slices in the presence of a cocktail of neurotransmitter blockers (CNQX, D-APV, bicuculline and strychnine) which freed them from their main synaptic influences. The left medial vestibular nucleus (MVN) was explored in a very systematic way and strict methodological precautions were taken in order to validate comparisons between the numbers of spontaneously active neurons recorded in the MVN of distinct slices. In the presence of neurotransmitter antagonists, the mean number of spontaneously active neurons detected in a single MVN increased dramatically from 9.5 in slices from control guinea pigs to 26.3 in slices from animals labyrinthectomized on the left side one week beforehand. The mean firing rate of the recorded neurons also increased from 7.5 +/- 5.6 spikes/s in slices from control animals to 12.3 +/- 7.6 spikes/s in slices from guinea pigs labyrinthectomized one week beforehand. These results show that deprivation of the vestibular neurons of their labyrinthine input caused a change in the deprived neurons themselves. They suggest that an increase in pacemaker activity might be a factor responsible for the restoration of spontaneous activity in the vestibular neurons after labyrinthectomy.

Gamma amino butyric acid (GABA) immunoreactivity in the vestibular nuclei of normal and unilateral vestibular neurectomized cats

Recent neurochemical investigations of the central vestibular pathways have demonstrated that several neurotransmitters are involved in various operations required for stabilizing posture and gaze. Neurons of the vestibular nuclei (VN) receive GABAergic inhibitory afferents, and GABAergic neurons distributed throughout the vestibular complex are implicated in inhibitory vestibulo-ocular and vestibulo-spinal pathways. The aim of this study was to analyse the modifications of GABA immunoreactivity (GABA-ir) in the cat VN after unilateral vestibular neurectomy (UVN). Indeed, compensation of vestibular deficits is a good model for studying adult central nervous system (CNS) plasticity and the GABAergic system is involved in CNS plasticity. We studied GABA-ir by using a purified polyclonal antibody raised against GABA. Light microscopic preparations of thin (20 microm) sections of cat VN were used to quantify GABA-ir by an image analysing system measuring GABA-positive punctate structures and the number of GABA-positive neurons. Both the lesioned and intact sides were analysed in three populations of UVN cats killed at different times after injury (1 week, 3 weeks and 1 year). These data were compared to those collected in normal unlesioned and sham-operated cats. Results showed a spatial distribution of GABA-ir in the control cats that confirmed previous studies. GABA-ir neurons, fibres and nerve terminals were scattered in all parts of the VN. A higher concentration of GABA-positive neurons (small cells) was detected in the medial and inferior VN (MVN and IVN) and in the dorsal part of the lateral VN (LVNd). A higher level of GABA-positive punctate structures was observed in the MVN and in the prepositus hypoglossi (PH) nucleus. Lesion-induced changes were found at each survival time. One week after injury the number of GABA-positive neurons was significantly increased in the MVN, the IVN and the dorsal part of the LVN on the lesioned side and in the ventral part of the LVN on the intact side. One year later a bilateral increase in GABA-positive neurons was detected in the MVN whilst a bilateral decrease was observed in both the SVN and the ventral part of the LVN. Changes in the GABA-staining varicosities did not strictly coincide with the distribution of GABA-ir cells, suggesting that GABA-ir fibres and nerve terminals were also modified. One week and later after injury, higher GABA-staining varicosities were seen unilaterally in the ipsilateral MVN. In contrast, bilateral increases (in PH) and bilateral decreases (in SVN and the ventral part of the LVN) were recorded in the nearly (3 weeks) or fully (1 year) compensated cats. At this stage GABA-staining varicosities were significantly increased in the lesioned side of the MVN. These findings demonstrate the reorganization of the GABAergic system in the VN and its possible role in recovery process after UVN in the cat. The changes seen during the acute stage could be causally related to the VN neuron deafferentation, contributing to the static vestibular deficits. Those found in the compensated cats would be more functionally implicated in the dynamic aspects of vestibular compensation.

Differential regulation of GABA(A) and GABA(B) receptors during vestibular compensation

We investigated changes in intrinsic excitability and GABA receptor efficacy in rat medial vestibular nucleus (MVN) neurons following 48 h and 7-10 days of behavioral recovery after unilateral labyrinthectomy (UL) in the rat. The mean in vitro discharge rate of rostral ipsilesional MVN cells at both time points was significantly higher than normal, indicating that the intrinsic excitability of the deafferented cells undergoes a sustained up-regulation during vestibular compensation. In slices from animals that had compensated for 7-10 days after UL, the responsiveness of rostral ipsilesional MVN cells to the GABA(A) agonist muscimol was not different from normal, while the responsiveness to the GABA(B) agonist baclofen was significantly down-regulated. This is in contrast to the situation soon after UL, where the efficacy of both GABA(A) and GABA(B) receptors is markedly down-regulated. The recovery of fast GABA(A) mediated neurotransmission by 7-10 days post-UL presumably enables ipsilesional cells to again respond to vestibular stimulation, through commissural inhibitory modulation from the intact side. The permanent loss of excitatory input from the lesioned side may be, in effect, counteracted by the long-term down-regulation of slow GABA(B) receptors in the de-afferented neurons.

Modulation of rat medial vestibular nucleus neurone activity by vasopressin and noradrenaline in vitro

In the present study, we examined the effects of bath application of vasopressin and noradrenaline on the spontaneous tonic discharge of medial vestibular nucleus (MVN) neurones and investigated if there is an interaction between the two drugs in an in vitro slice preparation of the rat brainstem containing the MVN. The results showed that vasopressin did not affect the spontaneous discharge rate of MVN neurones when applied either as a 60 s pulse or when the drug continuously perfused the slice for a period of 10 min. In contrast, noradrenaline affected the spontaneous discharge rate of the majority of cells tested (53/60, 88%). Noradrenaline excited the majority (46/53, 87%) of MVN neurones through both alpha1 and beta noradrenergic receptor-linked mechanisms. The remaining cells (7/53, 13%) were inhibited by noradrenaline through an alpha2 noradrenergic receptor-linked mechanism. Neither the excitatory nor inhibitory effects of noradrenaline were modified by vasopressin when the two drugs were applied together.

Voltage-gated calcium channels contribute to the pattern of the resting discharge in guinea pig medial vestibular nucleus neurons

In brainstem slices of guinea pigs perfused with artificial cerebro-spinal fluid (ACSF), the discharge of all the spontaneously active neurons of the medial vestibular nucleus (MVN) is regular. It has been reported that prolonged exposure to a low Ca(2+) medium could induce these neurons to fire bursts of spikes. In this study, we performed a systematic exploration of the spontaneous activity of the guinea pig MVN neurons by extracellular recordings in slices perfused either with a low Ca(2+)-high Mg(2+) medium, or with ACSF added with omega-agatoxin-IVA and with omega-conotoxin-GVIA. The percentage of recorded neurons which fired bursts, was 67% in low Ca(2+)-high Mg(2+) medium and 34% under the action of Ca(2+) channel blockers. These results show that the sensitivity of the firing properties to divalent cations is not shared by all of the MVN neurons and that the regularity of firing of a class of MVN neurons depends on the Ca(2+) channels they express in their membranes.

Multiple-site optical recording of mouse brainstem evoked by vestibulocochlear nerve stimulation

We used optical imaging to investigate the mouse cochlear and vestibular nucleus in brainstem slices using a voltage-sensitive dye, RH 155. As a result, the spatiotemporal patterns of excitatory propagation were shown. These optical signals consisted of two components consisting of a spike-like fast signal and a long-lasting slow signal. All responses were abolished by tetrodotoxin. The slow signals were eliminated under a Ca(2+)-free solution. In addition, synaptic fatigue was also observed. The present study indicated the feasibility of optical recording for visually revealing the synaptic transmission in both the vestibular and cochlear nucleus.

Rapid compensatory changes in GABA receptor efficacy in rat vestibular neurones after unilateral labyrinthectomy

1. The inhibitory effects of the GABAA agonist muscimol and the GABAB agonist baclofen on tonically active medial vestibular nucleus (MVN) neurones were recorded in slices of the rat dorsal brainstem in vitro, to determine whether any changes occurred in the functional efficacy of GABAergic inhibition in these cells during the initial rapid stage of 'vestibular compensation', the behavioural recovery that takes place after unilateral labyrinthectomy (UL). These experiments were carried out in preparations where the midline was cut, severing all commissural connections between the two vestibular nuclei. 2. Slices of the MVN were prepared from normal animals and animals that had been unilaterally labyrinthectomised 4 h earlier. The mean in vitro discharge rate of MVN neurones in the rostral region of the ipsi-lesional nucleus after UL was significantly higher than that in control slices, confirming our earlier reports of an increase in intrinsic excitability of these cells in the early stage of vestibular compensation. The in vitro discharge rates of caudal ipsi-lesional MVN cells, and rostral and caudal contra-lesional MVN cells, were not different from controls. 3. Muscimol and baclofen caused reversible, dose-related inhibition of the tonic discharge rate of MVN cells in control slices. In slices prepared from UL animals, MVN cells in the rostral region of the ipsi-lesional nucleus showed a marked downregulation of their response to both muscimol and baclofen, seen as a rightward shift and a decrease in slope of the dose-response relationships for the two agonists. In the contra-lesional nucleus, there was a small but significant upregulation of the responsiveness of both rostral and caudal MVN cells to baclofen, and a marked upregulation of the responsiveness of caudal MVN cells to muscimol. 4. In slices from animals that had undergone bilateral labyrinthectomy 4 h earlier, the downregulation of the functional efficacy of GABA receptors in the rostral MVN cells did not occur. The changes in GABA receptor efficacy after UL are therefore not due to the vestibular de-afferentation itself, but are instead due to the imbalance in excitability of the vestibular nuclei of the lesioned and intact sides, and the enhanced commissural inhibition of the ipsi-lesional MVN cells that follows UL. 5. The downregulation of GABA receptor efficacy in the ipsi-lesional MVN neurones is functionally compensatory, in that their response to commissural and cerebellar inhibitory drive will be significantly reduced after UL. Their intrinsic membrane conductances, and their remaining excitatory synaptic inputs, will consequently be more effective in causing depolarisation and the restoration of resting activity. Simultaneously the upregulation of GABAergic efficacy in the contra-lesional MVN will tend to reduce the hyperactivity on the contralateral side. These adaptive changes therefore represent a plausible cellular mechanism for the recovery of resting discharge in the ipsi-lesional MVN neurones, and the 're-balancing' of the excitability of the vestibular neurones of the lesioned and intact sides, as occurs after UL in vivo. 6. We propose that the adaptive regulation of the functional efficacy of GABA receptors in the MVN neurones may be an important cellular mechanism for the 'homeostasis of bilateral excitability' of the vestibular nuclei of the two sides.

Lesion-induced plasticity in rat vestibular nucleus neurones dependent on glucocorticoid receptor activation

1. We have recently shown that neurones in the rostral region of the medial vestibular nucleus (MVN) develop a sustained increase in their intrinsic excitability within 4 h of a lesion of the vestibular receptors of the ipsilateral inner ear. This increased excitability may be important in the rapid recovery of resting activity in these neurones during 'vestibular compensation', the behavioural recovery that follows unilateral vestibular deafferentation. In this study we investigated the role of the acute stress that normally accompanies the symptoms of unilateral labyrinthectomy (UL), and in particular the role of glucocorticoid receptors (GRs), in the development of the increase in excitability in the rostral MVN cells after UL in the rat. 2. The compensatory increase in intrinsic excitability (CIE) of MVN neurones failed to occur in animals that were labyrinthectomized under urethane anaesthesia and kept at a stable level of anaesthesia for either 4 or 6 h after UL, so that they did not experience the stress normally associated with the vestibular deafferentation syndrome. In these animals, 'mimicking' the stress response by administration of the synthetic GR agonist dexamethasone at the time of UL, restored and somewhat potentiated CIE in the MVN cells. Administration of dexamethasone in itself had no effect on the intrinsic excitability of MVN cells in sham-operated animals. 3. In animals that awoke after labyrinthectomy, and which therefore experienced the full range of oculomotor and postural symptoms of UL, there was a high level of Fos-like immunoreactivity in the paraventricular nucleus of the hypothalamus over 1.5-3 h post-UL, indicating a strong activation of the stress axis. 4. The GR antagonist RU38486 administered at the time of UL abolished CIE in the rostral MVN cells, and significantly delayed behavioural recovery as indicated by the persistence of circular walking. The mineralocorticoid receptor (MR) antagonist spironolactone administered at the time of UL had no effect. 5. Vestibular compensation thus involves a novel form of 'metaplasticity' in the adult brain, in which the increase in intrinsic excitability of rostral MVN cells and the initial behavioural recovery are dependent both on the vestibular deafferentation and on the activation of glucocorticoid receptors, during the acute behavioural stress response that follows UL. These findings help elucidate the beneficial effects of neuroactive steroids on vestibular plasticity in various species including man, while the lack of such an effect in the guinea-pig may be due to the significant differences in the physiology of the stress axis in that species.

Modulation of vestibular function by nociceptin/orphanin FQ: an in vivo and in vitro study

The effects of nociceptin (orphanin FQ) on medial vestibular nucleus (MVN) neurons in vitro, and on vestibulo-ocular reflex (VOR) function in vivo, were investigated in order to determine the role of 'opioid-like orphan' (ORL1) receptors in modulating vestibular reflex function in the rat. Nociceptin (100 nM-1 microM) potently inhibited the spontaneous discharge of the majority (86%) of MVN neurons tested in the rat dorsal brainstem slice preparation in vitro. This inhibition was dose-dependent and persisted after blockade of synaptic transmission in low Ca2+/Co2+ medium. The inhibitory effects were insensitive to the opioid antagonist naloxone, but were effectively antagonised by the selective ORL1 receptor antagonist, [Phe1Psi(CH2-NH)Gly2]Nociceptin(1-13)NH2. The majority of MVN neurons ( approximately 70%) were inhibited by both nociceptin and the delta-opioid receptor agonist, [D-ala2, D-leu5]-enkephalin (DADLE), while a minority of cells (approximately 30%) were selectively responsive either to DADLE or to nociceptin, but not both. Co-application of nociceptin and DADLE to neurons that were responsive to both agonists, resulted in an inhibitory response that was the same as or less than the inhibition evoked by either agonist alone. Intracellular whole-cell patch clamp recordings from identified Type A and Type B MVN cells showed that both these cell types are responsive to nociceptin, which induced membrane hyperpolarisation and decrease in input resistance consistent with its known effects on membrane K currents in other cell types. In alert rats, i.c.v. injection of nociceptin caused a significant decrease in the gain of the hVOR and resulted in a prolongation of post-rotatory nystagmus in darkness. The decrease in VOR gain and the increase in the VOR time-constant was significant even at low doses of nociceptin which did not cause other observable behavioural effects. These findings demonstrate that endogenously released nociceptin may have a hitherto unexplored role in the functional modulation of the neural pathways that mediate vestibular reflexes in vivo.

Spike discharge regularity of vestibular neurons in labyrinthectomized guinea pigs

Single unit activity of second-order vestibular neurons was recorded in alert guinea pigs. Here, we compared the spike discharge regularity (measured by calculating the coefficient of variation (CV)) of neurons from control animals with those from animals labyrinthectomized 1 week before. The mean CV (+/-SD) were the same in both groups (0.72+/-0.43 vs. 0.70+/-0.39). Furthermore, in both groups, the CV was related to the resting rate (RR) according to the same law (CV = 4/square root of RR). Because the discharge of a neuron is more regular when it is due to a pacemaker activity than when it is due to the synaptic drive, we conclude that restoration in the firing rate after labyrinthectomy is due to increase in the synaptic drive rather than to increase in the (intrinsic) pacemaker activity.

Effects of the GABA agonists baclofen and THIP on long-term compensation in hemilabyrinthectomised rats

Horizontal eye movements, elicited by sinusoidal rotation in darkness, were recorded with a magnetic search coil technique in pigmented rats, hemilabyrinthectomised 8-12 weeks before the investigation. Separate gains during rotation towards the lesioned side (LS) and the intact side (IS) were calculated by a computer program, demonstrating an asymmetry. Systemic single administration of the GABAB agonist baclofen caused a dose-related temporary rebalancing of the compensatory eye movements to the LS and the IS. At an optimal dose of 14 micromol/kg b.wt symmetry was achieved by excitation of eye movements during rotation to the LS and depression during rotation to the IS. Administration of the GABAA agonist THIP did not obviously reduce the asymmetry. It is suggested that stimulation of GABAB receptors modifies the tonic imbalance between the bilateral vestibular nuclei and/or the central processing of the input from the peripheral sensory organs.

Effects of toluene on tonic firing and membrane properties of rat medial vestibular nucleus neurones in vitro

The effects of toluene on discharge rate and membrane properties of tonically active medial vestibular nucleus (MVN) neurones were investigated in an in vitro slice preparation of the dorsal brainstem of the rat. The majority of the cells (50/56) were inhibited in a dose-dependent manner by toluene. The inhibitory effects of toluene persisted after blockade of synaptic transmission. Complementary patch-clamp recordings showed that toluene caused a hyperpolarisation of 2-5 mV associated with an increase in membrane conductance. These findings indicate that toluene might interfere with specific ion channels or the receptors regulating them along the cell membrane. The effective toluene concentrations used in this experiment are comparable to the concentrations producing vestibulo-ocular disturbances in vivo.

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.

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.

Postnatal development of spontaneous tonic activity in mouse medial vestibular nucleus neurones

The timecourse of the appearance and maturation of intrinsic tonic activity in medial vestibular nucleus (MVN) neurones was examined using extracellular single unit recording techniques in slices of the dorsal brainstem prepared from balb/c mice at specific stages during the first postnatal month. In slices from animals at postnatal day 5 (P5), the intrinsic spontaneous discharge rate was low (< 5 impulses/s on average). Over the period P10 to P30 this gradually increased to levels comparable to those of adult MVN cells in vitro. There was a rostro-caudal gradient in the time-course of development of tonic activity, such that cells located rostrally within the MVN developed higher frequencies of tonic discharge earlier than caudally located cells. The opening of the eyes around P14 was associated with a significant increase in the mean discharge rate of caudally located, but not rostrally located, MVN cells.

Optokinetic and vestibulo-ocular reflex adjustment by GABA antagonists

We determined if high and low doses of anti-GABAergic drugs have opposite effects on the visuo-vestibular activity in pigmented rats and examined a possible correlation with the level of GABA in the related structures. First, the horizontal optokinetic and vestibulo-ocular reflexes of most animals were depressed by high doses of anti-GABAergic drugs (10(-3) M purified picrotoxin or 10(-6) M picrotoxin in unpurified vegetal extract). Simultaneously, a drop in GABA level in the cerebellum and posterior brainstem was detected. Second, after a subsequent injection (1 ml) of the diluted extract (10(-13) M picrotoxin), the reflexes returned to normal despite the fact that no correlation with the GABA level was found. These results demonstrate that small doses of anti-GABAergic drugs reverse the depressive effect created by large doses of these drugs on the oculomotor system, and even adjust the reflexes to the stimulation. This adjustment, without correlation with the GABA level, suggest a powerful effect of very low dose of the drug to modulate either the activity of the cerebellar inhibiting input or of the vestibular nuclei neurons or to trigger the adaptation by other neurotransmitter systems involved in the performances of the reflexes.

The role of GABA in NMDA-dependent long term depression (LTD) of rat medial vestibular nuclei

The role of GABA in NMDA-dependent long term depression (LTD) in the medial vestibular nuclei (MVN) was studied on rat brainstem slices. High frequency stimulation (HFS) of the primary vestibular afferents induces a long lasting reduction of the polysynaptic (N2) component of the field potentials recorded in the dorsal portion of the MVN. The induction but not the maintenance of this depression was abolished by AP5, a specific blocking agent for glutamate NMDA receptors. The involvement of GABA in mediating the depression was checked by applying the GABAA and GABAB receptor antagonists, bicuculline and saclofen, before and after HFS. Under bicuculline and saclofen perfusion, HFS provoked a slight potentiation of the N2 wave, while the N2 depression clearly emerged after drug wash-out. This indicates that GABA is not involved in inducing the long term effect, but it is necessary for its expression. Similarly, the LTD reversed and a slight potentiation appeared when both drugs were administered after its induction. Most of these effects were due to the bicuculline, suggesting that GABAA receptors contribute to LTD more than GABAB do. According to our results, it is unlikely that the long lasting vestibular depression is the result of a homosynaptic LTD. On the contrary, our findings suggest that the depression is due to an enhancement of the GABA inhibitory effect, caused by an HFS dependent increase in gabaergic interneuron activity, which resets vestibular neuron excitability at a lower level.

Effects of histamine and betahistine on rat medial vestibular nucleus neurones: possible mechanism of action of anti-histaminergic drugs in vertigo and motion sickness

The tonic discharge of 71 medial vestibular nucleus (MVN) neurones was recorded in slices of the dorsal brainstem of young adult rats. Bath application of histamine caused a dose-related excitation in 59 of the 71 cells (83%), the remaining 12 (17%) being unresponsive. Dimaprit, a selective H2 agonist, also caused excitation in all 20 cells tested. The histamine-induced excitation and the response to dimaprit were antagonised by the selective H2 antagonist ranitidine, confirming that the H2 subtype of histamine receptor is involved in mediating the effects of histamine on these cells. Triprolidine, a selective H1 antagonist, also antagonised the excitation caused by histamine, at a concentration (0.3 microM) which left the H2 receptor-mediated response to dimaprit unchanged. Thus the excitatory effects of histamine on MVN cells in the rat involve two components mediated through H1 and H2 receptor-linked mechanisms, respectively. Betahistine, a weak H1 agonist and H3 antagonist, had little excitatory action when applied on its own, but significantly reduced the excitation caused by histamine when the two drugs were applied together. The effects of betahistine were consistent with a partial-agonist action at H1 receptors on MVN cells, reducing the excitatory responses to histamine presumably by occupying these receptor sites in competition with the exogenously applied neurotransmitter. This partial-agonist action of betahistine may be an important part of its mechanism of action in the symptomatic treatment of vertigo and motion sickness, since it is likely to occur not only in the MVN but also in many brain regions, including the thalamus and cortex, which express H1 receptors and which are innervated by the hypothalamic histaminergic system. Thus the effectiveness of betahistine and other anti-H1 drugs against motion sickness may be explained by their action in reducing the effects of the excess histamine release induced in such conditions in various brain areas, including the MVN.

Direct and indirect effects of muscimol on medial vestibular nucleus neurones in guinea-pig brainstem slices

Inhibitory amino acids are considered as major transmitters in the vestibular system. Using intracellular recordings in slices, we applied gamma-aminobutyric acid (GABA) and muscimol (a specific agonist of the GABAA receptor) to the two main types of medial vestibular nucleus neurones (A and B MVNn). In either a high Mg2+/low Ca2+ solution, or a solution containing tetrodotoxin, all MVNn were hyperpolarized by GABA and muscimol. This indicates that both types of MVNn are endowed with postsynaptic, hyperpolarising GABAA receptors. In a normal medium, about half of A and B MVNn were, in contrast, depolarised by GABA and muscimol, whereas the remaining cells were hyperpolarised. These results could be due to a modulation by GABA and muscimol of a tonic GABA release in the slice. Such a release was, indeed, suggested by results showing the depolarising effect of either tetrodotoxin (TTX) or bicuculline, when applied alone. The cells that were depolarised by GABA or muscimol in control conditions were always hyperpolarised in the presence of TTX. Our data therefore suggest that GABA acting at GABAA receptors in the medial vestibular nucleus can play a role either through a postsynaptic hyperpolarising action or indirectly by inhibiting a tonic GABA release, probably resulting from the spontaneous activity of local inhibitory interneurones. A GABAergic regulation of these interneurones could be important in processes of vestibular habituation and/or adaptation.