Identification of vestibular efferent neurons in the gerbil: histochemical and retrograde labelling

Aged mice are less susceptible to motion sickness and show decreased efferent vestibular activity compared to young adults

INTRODUCTION

The efferent vestibular system (EVS) is a feedback circuit thought to modulate vestibular afferent activity by inhibiting type II hair cells and exciting calyx-bearing afferents in the peripheral vestibular organs. In a previous study, we suggested EVS activity may contribute to the effects of motion sickness. To determine an association between motion sickness and EVS activity, we examined the effects of provocative motion (PM) on c-Fos expression in brainstem efferent vestibular nucleus (EVN) neurons that are the source of efferent innervation in the peripheral vestibular organs.

METHODS

c-Fos is an immediate early gene product expressed in stimulated neurons and is a well-established marker of neuronal activation. To study the effects of PM, young adult C57/BL6 wild-type (WT), aged WT, and young adult transgenic Chat-gCaMP6f mice were exposed to PM, and tail temperature (Ttail ) was monitored using infrared imaging. After PM, we used immunohistochemistry to label EVN neurons to determine any changes in c-Fos expression. All tissue was imaged using laser scanning confocal microscopy.

RESULTS

Infrared recording of Ttail during PM indicated that young adult WT and transgenic mice displayed a typical motion sickness response (tail warming), but not in aged WT mice. Similarly, brainstem EVN neurons showed increased expression of c-Fos protein after PM in young adult WT and transgenic mice but not in aged cohorts.

CONCLUSION

We present evidence that motion sickness symptoms and increased activation of EVN neurons occur in young adult WT and transgenic mice in response to PM. In contrast, aged WT mice showed no signs of motion sickness and no change in c-Fos expression when exposed to the same provocative stimulus.

The Long and Winding Road-Vestibular Efferent Anatomy in Mice

The precise functional role of the Efferent Vestibular System (EVS) is still unclear, but the auditory olivocochlear efferent system has served as a reasonable model on the effects of a cholinergic and peptidergic input on inner ear organs. However, it is important to appreciate the similarities and differences in the structure of the two efferent systems, especially within the same animal model. Here, we examine the anatomy of the mouse EVS, from its central origin in the Efferent Vestibular Nucleus (EVN) of the brainstem, to its peripheral terminations in the vestibular organs, and we compare these findings to known mouse olivocochlear anatomy. Using transgenic mouse lines and two different tracing strategies, we examine central and peripheral anatomical patterning, as well as the anatomical pathway of EVS axons as they leave the mouse brainstem. We separately tag the left and right efferent vestibular nuclei (EVN) using Cre-dependent, adeno-associated virus (AAV)-mediated expression of fluorescent reporters to map their central trajectory and their peripheral terminal fields. We couple this with Fluro-Gold retrograde labeling to quantify the proportion of ipsi- and contralaterally projecting cholinergic efferent neurons. As in some other mammals, the mouse EVN comprises one group of neurons located dorsal to the facial genu, close to the vestibular nuclei complex (VNC). There is an average of just 53 EVN neurons with rich dendritic arborizations towards the VNC. The majority of EVN neurons, 55%, project to the contralateral eighth nerve, crossing the midline rostral to the EVN, and 32% project to the ipsilateral eighth nerve. The vestibular organs, therefore, receive bilateral EVN innervation, but without the distinctive zonal innervation patterns suggested in gerbil. Similar to gerbil, however, our data also suggest that individual EVN neurons do not project bilaterally in mice. Taken together, these data provide a detailed map of EVN neurons from the brainstem to the periphery and strong anatomical support for a dominant contralateral efferent innervation in mammals.

Dopaminergic Inhibition of Na+ Currents in Vestibular Inner Ear Afferents

Inner ear hair cells form synapses with afferent terminals and afferent neurons carry signals as action potentials to the central nervous system. Efferent neurons have their origins in the brainstem and some make synaptic contact with afferent dendrites beneath hair cells. Several neurotransmitters have been identified that may be released from efferent terminals to modulate afferent activity. Dopamine is a candidate efferent neurotransmitter in both the vestibular and auditory systems. Within the cochlea, activation of dopamine receptors may reduce excitotoxicity at the inner hair cell synapse via a direct effect of dopamine on afferent terminals. Here we investigated the effect of dopamine on sodium currents in acutely dissociated vestibular afferent calyces to determine if dopaminergic signaling could also modulate vestibular responses. Calyx terminals were isolated along with their accompanying type I hair cells from the cristae of gerbils (P15-33) and whole cell patch clamp recordings performed. Large transient sodium currents were present in all isolated calyces; compared to data from crista slices, resurgent Na⁺ currents were rare. Perfusion of dopamine (100 μM) in the extracellular solution significantly reduced peak transient Na⁺ currents by approximately 20% of control. A decrease in Na⁺ current amplitude was also seen with extracellular application of the D2 dopamine receptor agonist quinpirole, whereas the D2 receptor antagonist eticlopride largely abolished the response to dopamine. Inclusion of the phosphatase inhibitor okadaic acid in the patch electrode solution occluded the response to dopamine. The reduction in calyx sodium current in response to dopamine suggests efferent signaling through D2 dopaminergic receptors may occur via common mechanisms to decrease excitability in inner ear afferents.

The mammalian efferent vestibular system utilizes cholinergic mechanisms to excite primary vestibular afferents

Electrical stimulation of the mammalian efferent vestibular system (EVS) predominantly excites primary vestibular afferents along two distinct time scales. Although roles for acetylcholine (ACh) have been demonstrated in other vertebrates, synaptic mechanisms underlying mammalian EVS actions are not well-characterized. To determine if activation of ACh receptors account for efferent-mediated afferent excitation in mammals, we recorded afferent activity from the superior vestibular nerve of anesthetized C57BL/6 mice while stimulating EVS neurons in the brainstem, before and after administration of cholinergic antagonists. Using a normalized coefficient of variation (CV*), we broadly classified vestibular afferents as regularly- (CV* < 0.1) or irregularly-discharging (CV* > 0.1) and characterized their responses to midline or ipsilateral EVS stimulation. Afferent responses to efferent stimulation were predominantly excitatory, grew in amplitude with increasing CV*, and consisted of fast and slow components that could be identified by differences in rise time and post-stimulus duration. Both efferent-mediated excitatory components were larger in irregular afferents with ipsilateral EVS stimulation. Our pharmacological data show, for the first time in mammals, that muscarinic AChR antagonists block efferent-mediated slow excitation whereas the nicotinic AChR antagonist DHβE selectively blocks efferent-mediated fast excitation, while leaving the efferent-mediated slow component intact. These data confirm that mammalian EVS actions are predominantly cholinergic.

Activation of GABAB receptors results in excitatory modulation of calyx terminals in rat semicircular canal cristae

Previous studies have found GABA in vestibular end organs. However, existence of GABA receptors or possible GABAergic effects on vestibular nerve afferents has not been investigated. The current study was conducted to determine whether activation of GABAB receptors affects calyx afferent terminals in the central region of the cristae of semicircular canals. We used patch-clamp recording in postnatal day 13-18 (P13-P18) Sprague-Dawley rats of either sex. Application of GABAB receptor agonist baclofen inhibited voltage-sensitive potassium currents. This effect was blocked by selective GABAB receptor antagonist CGP 35348. Application of antagonists of small (SK)- and large-conductance potassium (BK) channels almost completely blocked the effects of baclofen. The remaining baclofen effect was blocked by cadmium chloride, suggesting that it could be due to inhibition of voltage-gated calcium channels. Furthermore, baclofen had no effect in the absence of calcium in the extracellular fluid. Inhibition of potassium currents by GABAB activation resulted in an excitatory effect on calyx terminal action potential firing. While in the control condition calyces could only fire a single action potential during step depolarizations, in the presence of baclofen they fired continuously during steps and a few even showed repetitive discharge. We also found a decrease in threshold for action potential generation and a decrease in first-spike latency during step depolarization. These results provide the first evidence for the presence of GABAB receptors on calyx terminals, showing that their activation results in an excitatory effect and that GABA inputs could be used to modulate calyx response properties.NEW & NOTEWORTHY Using in vitro whole cell patch-clamp recordings from calyx terminals in the vestibular end organs, we show that activation of GABAB receptors result in an excitatory effect, with decreased spike-frequency adaptation and shortened first-spike latencies. Our results suggest that these effects are mediated through inhibition of calcium-sensitive potassium channels.

A review of efferent cholinergic synaptic transmission in the vestibular periphery and its functional implications

It has been over 60 years since peripheral efferent vestibular terminals were first identified in mammals, and yet the function of the efferent vestibular system remains obscure. One reason for the lack of progress may be due to our deficient understanding of the peripheral efferent synapse. Although vestibular efferent terminals were identified as cholinergic less than a decade after their anatomical characterization, the cellular mechanisms that underlie the properties of these synapses have had to be inferred. In this review we examine how recent mammalian studies have begun to reveal both nicotinic and muscarinic effects at these terminals and therefore provide a context for fast and slow responses observed in classic electrophysiological studies of the mammalian efferent vestibular system, nearly 40 years ago. Although incomplete, these new results together with those of recent behavioral studies are helping to unravel the mysterious and perplexing action of the efferent vestibular system. Armed with this information, we may finally appreciate the behavioral framework in which the efferent vestibular system operates.

Efferent Inputs Are Required for Normal Function of Vestibular Nerve Afferents

A group of vestibular afferent nerve fibers with irregular-firing resting discharges are thought to play a prominent role in responses to fast head movements and vestibular plasticity. We show that, in C57BL/6 mice (either sex, 4-5 weeks old), normal activity in the efferent vestibular pathway is required for function of these irregular afferents. Thermal inhibition of efferent fibers results in a profound inhibition of irregular afferents' resting discharges, rendering them inadequate for signaling head movements. In this way, efferent inputs adjust the contribution of the peripheral irregular afferent pathway that plays a critical role in peripheral vestibular signaling and plasticity.SIGNIFICANCE STATEMENT Vestibular end organs in the inner ear receive efferent inputs from the brainstem. Previously, electrical stimulation of efferents was linked to an increase in resting discharges of afferents and a decrease in their sensitivities. Here, we show that localized thermal inhibition of unmyelinated efferents results in a significant decrease in the activity of afferent nerve fibers, particularly those with irregular resting discharges implicated in responses to fast head movements and vestibular compensation. Thus, by upregulating and downregulating of afferent firing, particularly irregular afferents, efferents adjust neural activity sensitive to rapid head movements. These findings support the notion that peripheral vestibular end organs are not passive transducers of head movements and their sensory signal transmission is modulated by efferent inputs.

The Degeneration of the Vestibular Efferent Neurons After Intratympanic Gentamicin Administration

Intratympanic gentamicin (ITG) has been used to treat refractory Ménière's disease. Disequilibrium after ITG was still a challenge for some patients, and the underlying mechanism is poorly understood. Our previous study demonstrated that gentamicin distributed in the bilateral vestibular efferent neurons (VEN) after ITG; however, does it lead to VEN damage and cause further disequilibrium in patients following ITG? In this study, we observed severe damaged gentamicin-positive neurons of VEN and severe fractured myelin layer plates around neural fibers when viewed under transmission electron microscopy at day 3 after ITG. At day 30, neurons of VEN presented with relatively normal structures. Compared with the control group, the total number of choline acetyltransferase (CHAT) immunolabeling neurons in bilateral VEN showed a significant decrease both at day 3 and day 30. However, there was no significant difference in the total number of CHAT immunolabeling neurons between day 3 and day 30. It indicates that gentamicin is not only retrogradely transported into bilateral VEN, but also results in the degeneration of VEN after ITG. These findings may be related to patients' disequilibrium symptom after ITG. Furthermore, we speculate that VEN may play a role in vestibular compensation.

Reviewing the Role of the Efferent Vestibular System in Motor and Vestibular Circuits

Efferent circuits within the nervous system carry nerve impulses from the central nervous system to sensory end organs. Vestibular efferents originate in the brainstem and terminate on hair cells and primary afferent fibers in the semicircular canals and otolith organs within the inner ear. The function of this efferent vestibular system (EVS) in vestibular and motor coordination though, has proven difficult to determine, and remains under debate. We consider current literature that implicate corollary discharge from the spinal cord through the efferent vestibular nucleus (EVN), and hint at a potential role in overall vestibular plasticity and compensation. Hypotheses range from differentiating between passive and active movements at the level of vestibular afferents, to EVS activation under specific behavioral and environmental contexts such as arousal, predation, and locomotion. In this review, we summarize current knowledge of EVS circuitry, its effects on vestibular hair cell and primary afferent activity, and discuss its potential functional roles.

Muscarinic Acetylcholine Receptors and M-Currents Underlie Efferent-Mediated Slow Excitation in Calyx-Bearing Vestibular Afferents

Stimulation of vestibular efferent neurons excites calyx and dimorphic (CD) afferents. This excitation consists of fast and slow components that differ >100-fold in activation kinetics and response duration. In the turtle, efferent-mediated fast excitation arises in CD afferents when the predominant efferent neurotransmitter acetylcholine (ACh) activates calyceal nicotinic ACh receptors (nAChRs); however, it is unclear whether the accompanying efferent-mediated slow excitation is also attributed to cholinergic mechanisms. To identify synaptic processes underlying efferent-mediated slow excitation, we recorded from CD afferents innervating the turtle posterior crista during electrical stimulation of efferent neurons, in combination with pharmacological probes and mechanical stimulation. Efferent-mediated slow excitation was unaffected by nAChR compounds that block efferent-mediated fast excitation, but were mimicked by muscarine and antagonized by atropine, indicating that it requires ACh and muscarinic ACh receptor (mAChR) activation. Efferent-mediated slow excitation or muscarine application enhanced the sensitivity of CD afferents to mechanical stimulation, suggesting that mAChR activation increases afferent input impedance by closing calyceal potassium channels. These observations were consistent with suppression of a muscarinic-sensitive K⁺-current, or M-current. Immunohistochemistry for putative M-current candidates suggested that turtle CD afferents express KCNQ3, KCNQ4, and ERG1-3 potassium channel subunits. KCNQ channels were favored as application of the selective antagonist XE991 mimicked and occluded efferent-mediated slow excitation in CD afferents. These data highlight an efferent-mediated mechanism for enhancing afferent sensitivity. They further suggest that the clinical effectiveness of mAChR antagonists in treating balance disorders may also target synaptic mechanisms in the vestibular periphery, and that KCNQ channel modulators might offer similar therapeutic value.SIGNIFICANCE STATEMENT Targeting the efferent vestibular system (EVS) pharmacologically might prove useful in ameliorating some forms of vestibular dysfunction by modifying ongoing primary vestibular input. EVS activation engages several kinetically distinct synaptic processes that profoundly alter the discharge rate and sensitivity of first-order vestibular neurons. Efferent-mediated slow excitation of vestibular afferents is of considerable interest given its ability to elevate afferent activity over an extended time course. We demonstrate for the first time that efferent-mediated slow excitation of vestibular afferents is mediated by muscarinic acetylcholine receptor (mAChR) activation and the subsequent closure of KCNQ potassium channels. The clinical effectiveness of some anti-mAChR drugs in treating motion sickness suggest that we may, in fact, already be targeting the peripheral EVS.

Restricted loss of olivocochlear but not vestibular efferent neurons in the senescent gerbil (Meriones unguiculatus)

Degeneration of hearing and vertigo are symptoms of age-related auditory and vestibular disorders reflecting multifactorial changes in the peripheral and central nervous system whose interplay remains largely unknown. Originating bilaterally in the brain stem, vestibular and auditory efferent cholinergic projections exert feedback control on the peripheral sensory organs, and modulate sensory processing. We studied age-related changes in the auditory and vestibular efferent systems by evaluating number of cholinergic efferent neurons in young adult and aged gerbils, and in cholinergic trigeminal neurons serving as a control for efferents not related to the inner ear. We observed a significant loss of olivocochlear (OC) neurons in aged compared to young adult animals, whereas the overall number of lateral superior olive (LSO) cells was not reduced in aging. Although the loss of lateral and medial olivocochlear (MOC) neurons was uniform and equal on both sides of the brain, there were frequency-related differences within the lateral olivocochlear (LOC) neurons, where the decline was larger in the medial limb of the superior olivary nucleus (high frequency representation) than in the lateral limb (middle-to-low frequency representation). In contrast, neither the number of vestibular efferent neurons, nor the population of motor trigeminal neurons were significantly reduced in the aged animals. These observations suggest differential effects of aging on the respective cholinergic efferent brainstem systems.

Activation of μ-opioid receptors inhibits calcium-currents in the vestibular afferent neurons of the rat through a cAMP dependent mechanism

Opioid receptors are expressed in the vestibular endorgans (afferent neurons and hair cells) and are activated by the efferent system, which modulates the discharge of action potentials in vestibular afferent neurons (VANs). In mammals, VANs mainly express the μ opioid-receptor, but the function of this receptors activation and the cellular mechanisms by which they exert their actions in these neurons are poorly studied. To determine the actions of μ opioid receptor (MOR) and cell signaling mechanisms in VANs, we made perforated patch-clamp recordings of VANs that were obtained from postnatal days 7 to 10 (P7–10) rats and then maintained in primary culture. The MOR agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO) inhibited the total voltage-gated outward current; this effect was prevented by the perfusion of a Ca²⁺-free extracellular solution. We then studied the voltage-gated calcium current (I_ca) and found that DAMGO Met-enkephalin or endomorphin-1 inhibited the I_Ca in a dose-response fashion. The effects of DAMGO were prevented by the MOR antagonist (CTAP) or by pertussis toxin (PTX). The use of specific calcium channel blockers showed that MOR activation inhibited T-, L- and N-type I_Ca. The use of various enzyme activators and inhibitors and of cAMP analogs allowed us to demonstrate that the MOR acts through a cAMP dependent signaling mechanism. In current clamp experiments, MOR activation increased the duration and decreased the amplitude of the action potentials and modulated the discharge produced by current injection. Pre-incubation with PTX occluded MOR activation effect. We conclude that MOR activation inhibits the T-, L- and N-type I_Ca through activation of a Gα_i/o protein that involves a decrease in AC-cAMP-PKA activity. The modulation of I_Ca may have an impact on the synaptic integration, excitability, and neurotransmitter release from VANs.

The distribution of vestibular efferent neurons receiving innervation of secondary vestibular afferent nerves in rats

OBJECTIVES/HYPOTHESIS

To explore the innervation areas of the medial vestibular nucleus (MVN) afferent neurons onto vestibular efferent neurons in the brain stem of rats.

STUDY DESIGN

A morphology study in the central vestibular system.

METHODS

Two neuronal tracers were used. Lectin PHA-L Conjugates (PHA-L, Invitrogen L - 11270,) was injected into the MVN as an anterograde tracer, and 5% FluoSpheres carboxylate-modified microspheres (MFS, Molecular Probe F-8793) was injected into the contralateral peripheral vestibule using as a retrograde tracer. All animals were allowed to recover for 12 days to facilitate sufficient transportation of the tracers. Then brain stems were sliced coronally on a freezing microtome and observed under a fluorescence microscope and laser confocal microscopy.

RESULTS

Neurons in the MVN labeled with PHA-L exhibited green fluorescence, and their axons were distributed near the genu of the facial nerve (g7) and in the reticulation structure, as well as in the cerebellum or oculomotor-related nuclei. Neurons labeled with red fluorescence of MFS were mainly located dorsomedial and dorsolateral to g7 and in the caudal pontine reticular nucleus (PnC) bilaterally and presented different morphologies at different locations. The synaptic junctions would display color overlap (fluoresced yellow). Under three-dimensional reconstruction of the confocal laser microscopy, the synaptic junctions were visualized dorsomedial and dorsolateral to g7 bilaterally, predominantly ipsilateral to the MVN injection site.

CONCLUSIONS

Morphologic evidence of the distribution of vestibular efferent neurons synapsed by afferent nerves from MVN was demonstrated. These efferent neurons constitute short closed-loop circuits with neurons in the MVN.

Internal models of self-motion: computations that suppress vestibular reafference in early vestibular processing

In everyday life, vestibular sensors are activated by both self-generated and externally applied head movements. The ability to distinguish inputs that are a consequence of our own actions (i.e., active motion) from those that result from changes in the external world (i.e., passive or unexpected motion) is essential for perceptual stability and accurate motor control. Recent work has made progress toward understanding how the brain distinguishes between these two kinds of sensory inputs. We have performed a series of experiments in which single-unit recordings were made from vestibular afferents and central neurons in alert macaque monkeys during rotation and translation. Vestibular afferents showed no differences in firing variability or sensitivity during active movements when compared to passive movements. In contrast, the analyses of neuronal firing rates revealed that neurons at the first central stage of vestibular processing (i.e., in the vestibular nuclei) were effectively less sensitive to active motion. Notably, however, this ability to distinguish between active and passive motion was not a general feature of early central processing, but rather was a characteristic of a distinct group of neurons known to contribute to postural control and spatial orientation. Our most recent studies have addressed how vestibular and proprioceptive inputs are integrated in the vestibular cerebellum, a region likely to be involved in generating an internal model of self-motion. We propose that this multimodal integration within the vestibular cerebellum is required for eliminating self-generated vestibular information from the subsequent computation of orientation and posture control at the first central stage of processing.

Ion channels set spike timing regularity of mammalian vestibular afferent neurons

In the mammalian vestibular nerve, some afferents have highly irregular interspike intervals and others have highly regular intervals. To investigate whether spike timing is determined by the afferents' ion channels, we studied spiking activity in their cell bodies, isolated from the vestibular ganglia of young rats. Whole cell recordings were made with the perforated-patch method. As previously reported, depolarizing current steps revealed distinct firing patterns. Transient neurons fired one or two onset spikes, independent of current level. Sustained neurons were more heterogeneous, firing either trains of spikes or a spike followed by large voltage oscillations. We show that the firing pattern categories are robust, occurring at different temperatures and ages, both in mice and in rats. A difference in average resting potential did not cause the difference in firing patterns, but contributed to differences in afterhyperpolarizations. A low-voltage-activated potassium current (I(LV)) was previously implicated in the transient firing pattern. We show that I(LV) grew from the first to second postnatal week and by the second week comprised Kv1 and Kv7 (KCNQ) components. Blocking I(LV) converted step-evoked firing patterns from transient to sustained. Separated from their normal synaptic inputs, the neurons did not spike spontaneously. To test whether the firing-pattern categories might correspond to afferent populations of different regularity, we injected simulated excitatory postsynaptic currents at pseudorandom intervals. Sustained neurons responded to a given pattern of input with more regular firing than did transient neurons. Pharmacological block of I(LV) made firing more regular. Thus ion channel differences that produce transient and sustained firing patterns in response to depolarizing current steps can also produce irregular and regular spike timing.

Neuropharmacology of vestibular system disorders

This work reviews the neuropharmacology of the vestibular system, with an emphasis on the mechanism of action of drugs used in the treatment of vestibular disorders. Otolaryngologists are confronted with a rapidly changing field in which advances in the knowledge of ionic channel function and synaptic transmission mechanisms have led to the development of new scientific models for the understanding of vestibular dysfunction and its management. In particular, there have been recent advances in our knowledge of the fundamental mechanisms of vestibular system function and drug mechanisms of action. In this work, drugs acting on vestibular system have been grouped into two main categories according to their primary mechanisms of action: those with effects on neurotransmitters and neuromodulator receptors and those that act on voltage-gated ion channels. Particular attention is given in this review to drugs that may provide additional insight into the pathophysiology of vestibular diseases. A critical review of the pharmacology and highlights of the major advances are discussed in each case.

Retrograde neuron tracing with microspheres reveals projection of CGRP-immunolabeled vestibular afferent neurons to the vestibular efferent nucleus in the brainstem of rats

OBJECTIVE

A new retrograde neuron-tracing technique with microspheres was used to explore the possible innervation of calcitonin gene-related peptide (CGRP)-immunolabeled vestibular afferent neurons in the vestibular efferent immunolabeled nucleus in the brainstem.

METHODS

0.1 microl of 5% microfluorospheres was injected into the area of the vestibular efferent nucleus, which is located lateral to the genu of the facial nerve. CGRP immunohistochemistry was processed in serial sections of the brainstem at the facial nerve genu level. Double-labeled neurons with both CGRP immunoreactivity and microfluorospheres were examined with fluorescence and confocal laser microscopy.

RESULTS

Three types of labeled neurons were observed: (1) neurons only retrogradely microfluorosphere-labeled that were mainly located in the medial vestibular nucleus, lateral vestibular nucleus, superior vestibular nucleus and parvicellular reticular nucleus on the ipsilateral side of the injection; (2) neurons that were both immunolabeled with CGRP and also retrogradedly labeled with microfluorospheres, indicating that they are CGRP cells projecting to the area of vestibular efferent nucleus, these cells were mainly distributed in the superior vestibular nucleus and dorsal vestibular nucleus, and (3) cells only immunolabeled for CGRP that were scattered extensively in the brainstem.

CONCLUSION

The presented methodical contribution demonstrates the suitability of fluorescein-labeled microspheres for retrograde neuronal tracing. The vestibular nuclei contain numerous afferent neurons that send projections to the vestibular efferent nucleus, some of which are CGRP cells. This afferent innervation provides morphological evidence that the vestibular efferent neurons receive input from the vestibular afferent neurons including CGRP cells. These vestibular primary CGRP afferent neurons may have an influence on vestibular efferent neurons. CGRP acts as an important co-transmitter or modulator in the afferent-mediated activity of vestibular efferent neurons, which in turn affect afferents in the vestibular end organs.

Vestibular efferents contain peripherin

Vestibular efferents have a common origin with the motoneurons of the facial nerve. In adults they share a number of common features, such as the same transmitter. Here we show using retrograde transport and immunohistochemistry, that the vestibular efferents, like facial motoneurons, contain peripherin. This supports the suggestion that peripherin-positive fibers at the apex of the cristae ampullaris are efferents.

Polysynaptic inputs to vestibular efferent neurons as revealed by viral transneuronal tracing

The Bartha strain of the alpha-herpes pseudorabies virus (PrV) was used as a retrograde transneuronal tracer to map synaptic inputs to the vestibular efferent neurons of the Mongolian gerbil, Meriones unguiculatus. Although previous experiments have shown that vestibular efferent neurons respond to visual motion and somatosensory stimuli, the anatomic connections mediating those responses are unknown. PrV was injected unilaterally into the horizontal semicircular canal neuroepithelium of gerbils, where it was taken up by efferent axon terminals. The virus was then retrogradely transported to efferent cell bodies, replicated, and transported into synaptic endings projecting onto the efferent cells. Thirty animals were sacrificed at approximately 5-h increments between 75 and 105 h post-infection after determining that shorter time points had no central infection. Infected cells were visualized immunohistochemically. Temporal progression of neuronal infection was used to determine the nature of primary and higher order projections to the vestibular efferent neurons. Animals sacrificed at 80-94 h post-inoculation exhibited immunostaining in the dorsal and ventral group of vestibular efferent neurons, predominately on the contralateral side. Neurons within the medial, gigantocellular, and lateral reticular formations were among the first cells infected thereafter. At 95 h, additional virus-labeled cell groups included the solitary, area postrema, pontine reticular, prepositus, dorsal raphe, tegmental, the subcoeruleus nuclei, the nucleus of Darkschewitsch, and the inferior olivary beta and ventrolateral subnuclei. Analysis beyond 95 h revealed virus-infected neurons located in the vestibulo-cerebellar and motor cortices. Paraventricular, lateral, and posterior hypothalamic cells, as well as central amygdala cells, were also labeled. Spinal cord tissue exhibited no labeling in the intermediolateral cell column, but scattered cells were found in the central cervical nucleus. The results suggest functional associations among efferent feedback regulation of labyrinthine sensory input and both behavioral and autonomic systems, and support a closed-looped vestibular feedback model with additional open-loop polysynaptic inputs.

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.

Alpha-9 nicotinic acetylcholine receptor immunoreactivity in the rodent vestibular labyrinth

Vestibular tissues (cristae ampullares, macular otolithic organs, and Scarpa's ganglia) in chinchilla, rat, and guinea pig were examined for immunoreactivity to the alpha9 nicotinic acetylcholine receptor (nAChR) subunit. The alpha9 antibody was generated against a conserved peptide present in the intracellular loop of the predicted protein sequence of the guinea pig alpha9 nAChR subunit. In the vestibular periphery, staining was observed in calyces around type I hair cells, at the synaptic pole of type II hair cells, and in varying levels in Scarpa's ganglion cells. Ganglion cells were also triply labeled to detect alpha9, calretinin, and peripherin. Calretinin labels calyx-only afferents. Peripherin labels bouton-only afferents. Dimorphic afferents, which have both calyx and bouton endings, are not labeled by calretinin or peripherin. In these experiments, alpha9 was expressed in both calyx and dimorphic afferents. A subpopulation of small ganglion cells did not contain the alpha9 nAChR but did stain for peripherin. We surmise that these are bouton-only afferents. Bouton (regularly discharging) afferents also show efferent responses, although they are qualitatively different from those in irregularly discharging (calyx and dimorphic) afferents, much slower and longer lasting. Thus, regular afferents are probably more affected via a muscarinic cholinergic or a peptidergic mechanism, with a much smaller superimposed fast nicotinic-type response. This latter response could be due to one of the other nicotinic receptors that have been described in studies from other laboratories.

Vestibular afferent innervation in the vestibular efferent nucleus of rats

To delineate the vestibular afferent innervation in the vestibular efferent nucleus in the brainstem, neurobiotin or biotinylated dextran amine was injected into the superior Scarpa's ganglion of Sprague-Dawley rats. The locations of vestibular efferent neurons in the brainstem were identified by neutral red or choline acetyltransferase staining. Of the three pairs of vestibular efferent nuclei, labeled fibers and bouton-like endings were found only within the dorsolateral vestibular efferent nucleus on the ipsilateral side. Labeled afferent terminals with bouton-like varicosities were observed in the vicinity of cell bodies or dendrites of these efferent neurons. Our findings suggest that vestibular primary afferents may exert direct influence on vestibular efferent neurons, constituting an ipsilateral close-loop arrangement in the central vestibular system.

Efferent actions in the chinchilla vestibular labyrinth

Efferent fibers were electrically stimulated in the brain stem, while afferent activity was recorded from the superior vestibular nerve in barbiturate-anesthetized chinchillas. We concentrated on canal afferents, but otolith afferents were also studied. Among canal fibers, calyx afferents were recognized by their irregular discharge and low rotational gains. In separate experiments, stimulating electrodes were placed in the efferent cell groups ipsilateral or contralateral to the recording electrode or in the midline. While single shocks were ineffective, repetitive shock trains invariably led to increases in afferent discharge rate. Such excitatory responses consisted of fast and slow components. Fast components were large only at high shock frequencies (200-333/s), built up with exponential time constants <0.1 s, and showed response declines or adaptation during shock trains >1 s in duration. Slow responses were obtained even at shock rates of 50/s, built up and decayed with time constants of 15-30 s, and could show little adaptation. The more regular the discharge, the larger was the efferent response of an afferent fiber. Response magnitude was proportional to cv*b, a normalized coefficient of interspike-interval variation (cv*) raised to the power b = 0.7. The value of the exponent b did not depend on unit type (calyx vs. bouton plus dimorphic, canal vs. otolith) or on stimulation site (ipsilateral, contralateral, or midline). Responses were slightly smaller with contralateral or midline stimulation than with ipsilateral stimulation, and they were smaller for otolith, as compared to canal, fibers. An anatomical study had suggested that responses to contralateral afferent stimulation should be small or nonexistent in irregular canal fibers. The suggestion was not confirmed in this study. Contralateral responses, including the large responses typically seen in irregular fibers, were abolished by shallow midline incisions that should have severed crossing efferent axons.

Ultrastructural observations of efferent terminals in the crista ampullaris of the toadfish, Opsanus tau

The present study was conducted to visualize the ultrastructural features of vestibular efferent boutons in the oyster toadfish, Opsanus tau. The crista ampullaris of the horizontal semicircular canal was processed for and examined by routine transmission electron microscopy. The results demonstrate that such boutons vary in size and shape, and contain a heterogeneous population of lucent vesicles with scattered dense core vesicles. Efferent contacts with hair cells are characterized by local vesicle accumulations in the presynaptic terminal and a subsynaptic cistern in the postsynaptic region of the hair cell. Serial efferent to hair cell to afferent synaptic arrangements are common, particularly in the central portion of the crista. However, direct contacts between efferent terminals and afferent neurites were not observed in our specimens. The existence of serial synaptic contacts, often with a row of vesicles in the efferent boutons lining the efferent-afferent membrane apposition, suggests that the efferent influence on the crista may involve both synaptic and nonsynaptic, secretory mechanisms. Further, it is suggested that differences in more subtle aspects of synaptic architecture and/or transmitter and receptor localization and interaction may render the efferent innervation of the peripheral crista less effective in influencing sensory processing.

Expression and distribution of μ opioid receptors in the inner ear of the rat

Opioid peptides have demonstrated modulatory effects on the vestibular afferent discharge and are putative vestibular efferent neuromodulators. The distribution of their receptors in the mammalian vestibular epithelia is not known. We used reverse transcriptase-polymerase chain reaction (RT-PCR), in situ hybridization, Western blots and immunohistochemistry to study the expression of mu opioid receptor (MOR) in the Scarpa's ganglia and cristae ampullares of rats. MOR transcript was only detected in the somata of the vestibular afferent neurons. MOR-like immunoreactivity was observed in the somata of vestibular afferents and in nerve terminals in the cristae ampullares epithelia both in the center and peripheral regions. Double labeling of cristae sections with the MOR1 antibody in combination with antibodies against calretinin (a marker for vestibular afferents terminating in calices) and peripherin (a marker for afferents terminating in boutons), respectively showed that MOR1 immunoreactivity was in calyx, dimorphic and bouton vestibular afferents. MOR immunoreactivity was not detected in vestibular efferent fibers identified with choline acetyltransferase immunohistochemistry. These results indicate that MOR may mediate effects of vestibular efferents on afferents.

Ultrastructural observations of efferent terminals in the crista Ampullaris of the toadfish, opsanus tau

The present study was conducted to visualize the ultrastructural features of vestibular efferent boutons in the oyster toadfish, Opsanus tau. The crista ampullaris of the horizontal semicircular canal was processed for and examined by routine transmission electron microscopy. The results demonstrate that such boutons vary in size and shape, and contain a heterogeneous population of lucent vesicles with scattered dense core vesicles. Efferent contacts with hair cells are characterized by local vesicle accumulations in the presynaptic terminal and a subsynaptic cistern in the postsynaptic region of the hair cell. Serial efferent to hair cell to afferent synaptic arrangements are common, particularly in the central portion of the crista. However, direct contacts between efferent terminals and afferent neurites were not observed in our specimens. The existence of serial synaptic contacts, often with a row of vesicles in the efferent boutons lining the efferent-afferent membrane apposition, suggests that the efferent influence on the crista may involve both synaptic and nonsynaptic, secretory mechanisms. Further, it is suggested that differences in more subtle aspects of synaptic architecture and/or transmitter and receptor localization and interaction may render the efferent innervation of the peripheral crista less effective in influencing sensory processing.

Opioid receptors mediate a postsynaptic facilitation and a presynaptic inhibition at the afferent synapse of axolotl vestibular hair cells

This study was designed to determine the effects of opiate drugs on the electrical activity of afferent neurons and on the ionic currents of hair cells from semicircular canals. Experiments were done on larval axolotls (Ambystoma tigrinum). The multiunit spike activity of afferent neurons was recorded in the isolated inner ear under both resting conditions and mechanical stimulation. Ionic currents were recorded using voltage clamp of hair cells isolated from the semicircular canal. In the isolated inner-ear preparation, microperfusion of either non-specific opioid receptor antagonist naloxone (10 nM to 1 mM), mu receptor agonist [D-Ala(2), N-Me-Phe(4),Gly(5)-ol]-enkephalin (1 pM to 10 microM), or kappa receptor antagonist nor-binaltorphimine (10 nM to 100 microM) elicited a dose-dependent long-lasting (>5 min) increase of the electrical discharge of afferent neurons. The mu receptor agonist funaltrexamine (1 nM to 100 microM) and the kappa receptor agonist U-50488 (1 nM to 10 microM) diminished the basal spike discharge of vestibular afferents. The delta receptor agonist D-Pen(2)-D-Pen(5)-enkephalin (1 nM to 10 mM) and the antagonist naltrindole (1 nM to 10 mM) were without a significant effect. The only drug that displayed a significant action on hair-cell ionic currents was trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl) benzeneacetamide methanesulfonate (U-50488) that reduced the Ca(2+) current in a dose-dependent fashion. On its own, mu receptor agonist [D-Ala(2), N-Me-Phe(4),Gly(5)-ol]-enkephalin (0.01 and 10 microM) significantly potentiated the response of afferent neurons to the excitatory amino acid agonist (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (0.1 microM), while synaptic transmission was blocked by the use of high-Mg(2+), low-Ca(2+) solutions. Our data indicate that the activity of vestibular afferent neurons may be regulated in a complex fashion by opioid receptors: mu opioid receptors mediating an excitatory, postsynaptic modulatory input to afferent neurons, and kappa receptors mediating an inhibitory, presynaptic input to hair cells.

Central projections of the utricular nerve in the gerbil

The central projections of primary afferent fibers in the utricular nerve, which convey linear head acceleration signals to neurons in the brainstem and cerebellum, are not completely defined. The purpose of this investigation was twofold: 1) to define the central projections of the gerbil utricular afferents by injecting horseradish peroxidase (HRP) and biotinylated dextran amine (BDA) into the utricular macula; and 2) to investigate the projections of individual utricular afferents by injecting HRP intracellularly into functionally identified utricular neurons. We found that utricular afferents in the gerbil projected to all divisions of the vestibular nuclear complex, except the dorsal lateral vestibular nucleus. In addition, terminals were observed in the interstitial nucleus of the eighth nerve, nucleus Y, external cuneate nucleus, and lobules I, IV, V, IX, and X of the cerebellar vermis. No projections appeared in the flocculus or paraflocculus. Fibers traversed the medial and intermediate cerebellar nuclei, but terminals appeared only occasionally. Individual utricular afferents collateralize extensively, projecting to much of the brainstem area innervated by the whole of the utricular nerve. This study did not produce complete filling of individual afferent collateral projections into the cerebellar cortex.

Reflections of efferent activity in rotational responses of chinchilla vestibular afferents

To study presumed efferent-mediated responses, we determined if afferents responded to head rotations that stimulated semicircular canals other than the organ being innervated. To minimize stimulation of an afferent's own canal, its plane was placed nearly orthogonal to the rotation plane. Otolith units were tested in a horizontal head position with the ear placed near the rotation axis to minimize linear forces. Under these circumstances, angular-velocity trapezoids (2-s ramps, 2-s plateau) evoked excitatory responses for both rotation directions. These type III responses were considerably larger in decerebrate than in anesthetized preparations. In addition to their being exclusively excitatory, the responses resembled those obtained with electrical stimulation of efferent pathways in including per-stimulus and more prolonged post-stimulus components and in being larger in irregularly discharging than in regularly discharging units. Responses, which were not seen for rotations <80 degrees/s, grew as velocity increased between 80 and 500 degrees/s but were seldom larger than 20 spikes/s. Complete section of the VIIIth nerve abolished type III responses, leaving conventional afferent responses intact. To study the separate contributions of canals on the two sides, responses were compared when the labyrinths were intact and when the ipsilateral or contralateral horizontal canal was mechanically inactivated. Both sides contributed to the efferent-mediated responses. That afferents could be influenced from the contralateral labyrinth was confirmed with the use of unilateral galvanic currents. Following inactivation, excitatory responses were produced by rotations exciting or inhibiting the intact horizontal canal with the responses resulting from excitatory rotations being much larger. Such a response asymmetry is consistent with a semicircular-canal origin for the type III responses. A similar asymmetry was seen in the post-stimulus responses to contralateral cathodal (excitatory) and anodal (inhibitory) galvanic currents. We conclude that the efferent system receives a sufficiently powerful vestibular input from both the ipsilateral and contralateral labyrinths to affect afferent discharge.

Development of the rat efferent vestibular system on the ground and in microgravity

We investigated whether plastic changes occurred in the organization of the vestibular efferent network in the rat utricle during a 17-day episode of microgravity, from postnatal (PN) day 8 to PN23, and on return to earth on PN25. We also determined the normal pattern of efferent development from birth to PN25. Immunofluorescence experiments were performed with a specific biochemical marker of the efferent system, the calcitonin gene-related peptide (CGRP), and vibratome sections of the utricles were analyzed by laser scanning confocal microscopy. At birth, a few efferent fibers were detected beneath the sensory epithelium. These then massively invaded the epithelium between PN2 and PN4. At the time of launch, PN8, most fiber paths in the utricular epithelium, after following transient courses (towards the epithelial surface for example) returned to the base and were stabilized in the lower part of the epithelium, in which they established synaptic contacts with sensory cells, except at a few immature locations. The main difference between this stage (on PN8) and subsequent more mature stages was the lower density of fibers and synapses in the utricle. The maturation of the vestibular efferent system was similar in microgravity and on the ground. Thus, maturation of the efferent system between PN8 and PN23 was not sensitive to a change in gravitational environment. These results suggest that periods of microgravity at earlier stages are required to identify critical periods in peripheral vestibular system development.

Vestibular efferent neurons project to the flocculus

A bilateral projection from the vestibular efferent neurons, located dorsal to the genu of the facial nerve, to the cerebellar flocculus and ventral paraflocculus was demonstrated. Efferent neurons were double-labeled by the unilateral injections of separate retrograde tracers into the labyrinth and into the floccular and ventral parafloccular lobules. Efferent neurons were found with double retrograde tracer labeling both ipsilateral and contralateral to the sites of injection. No double labeling was found when using a fluorescent tracer with non-fluorescent tracers such as horseradish peroxidase (HRP) or biotinylated dextran amine (BDA), but large percentages of efferent neurons were found to be double labeled when using two fluorescent substances including: fluorogold, microruby dextran amine, or rhodamine labeled latex beads. These data suggest a potential role for vestibular efferent neurons in modulating the dynamics of the vestibulo-ocular reflex (VOR) during normal and adaptive conditions.

Morphine inhibits an alpha9-acetylcholine nicotinic receptor-mediated response by a mechanism which does not involve opioid receptors

Nicotinic acetylcholine (nACh) receptors are known to be targets for modulation by a number of substances, including the opiates. It is known that acetylcholine (ACh) coexists with opioid peptides in cochlear efferent neurons, and such a colocalization has been proposed for the vestibular system. In the present study we test the hypothesis that morphine, an opioid receptor agonist with a broad spectrum of selectivity, modulates alpha9nACh receptor-mediated responses in frog vestibular hair cells. Morphine dose-dependently and reversibly inhibited ACh-induced currents as recorded by the perforated patch-clamp method. In the presence of morphine the ACh dose-response curve was shifted to the right in a parallel fashion, suggesting a competitive interaction. However, naloxone did not antagonize the inhibition produced by morphine. To test the hypothesis that morphine could interact with the alpha9nACh receptor without the involvement of opioid receptors, experiments were performed using Xenopus laevis oocytes injected with the alpha9nACh receptor cRNA. The currents activated by ACh in Xenopus oocytes, a system that lacks opioid receptors, were also dose-dependently inhibited by morphine. We conclude that morphine inhibits the alpha9nACh receptor-mediated response in hair cells and Xenopus oocytes through a mechanism which does not involve opioid receptors but may be a direct block of the alpha9nACh receptor.

Calcitonin gene-related peptide immunoreactivity in the rat efferent vestibular system during development

The organization of the efferent fiber network during postnatal development was investigated by immunocytochemical detection of the calcitonin gene-related peptide (CGRP) in rat vestibular receptors from postnatal day 0 (PD 0) to adulthood. CGRP was detected at birth in a few efferent fibers below the sensory epithelia of cristae and maculae. Thereafter, the nerve fibers in the cristae progressively invaded the epithelia with an apex to base gradient from PD 2 to PD 4. There was also a rearrangement of the fibers during maturation of the efferent innervation, such that after reaching the surface of the epithelium, they turned back and moved towards the base of the sensory cells, producing numerous synaptic contacts. Analysis of surface preparations of utricules showed the irregular and asymmetric topographic organization of the efferent fiber network and the extensive, complex distribution of this innervation. The presence and broad distribution of CGRP in the epithelium at critical stages of development and synaptogenesis suggests that it is involved in the maturation of vestibular receptors.

Three-dimensional analysis of vestibular efferent neurons innervating semicircular canals of the gerbil

Anterograde labeling techniques were used to examine peripheral innervation patterns of vestibular efferent neurons in the crista ampullares of the gerbil. Vestibular efferent neurons were labeled by extracellular injections of biocytin or biotinylated dextran amine into the contralateral or ipsilateral dorsal subgroup of efferent cell bodies (group e) located dorsolateral to the facial nerve genu. Anterogradely labeled efferent terminal field varicosities consist mainly of boutons en passant with fewer of the terminal type. The bouton swellings are located predominately in apposition to the basolateral borders of the afferent calyces and type II hair cells, but several boutons were identified close to the hair cell apical border on both types. Three-dimensional reconstruction and morphological analysis of the terminal fields from these cells located in the sensory neuroepithelium of the anterior, horizontal, and posterior cristae were performed. We show that efferent neurons densely innervate each end organ in widespread terminal fields. Subepithelial bifurcations of parent axons were minimal, with extensive collateralization occurring after the axons penetrated the basement membrane of the neuroepithelium. Axonal branching ranged between the 6th and 27th orders and terminal field collecting area far exceeds that of the peripheral terminals of primary afferent neurons. The terminal fields of the efferent neurons display three morphologically heterogeneous types: central, peripheral, and planum. All cell types possess terminal fields displaying a high degree of anisotropy with orientations typically parallel to or within +/-45 degrees of the longitudinal axis if the crista. Terminal fields of the central and planum zones predominately project medially toward the transverse axis from the more laterally located penetration of the basement membrane by the parent axon. Peripheral zone terminal fields extend predominately toward the planum semilunatum. The innervation areas of efferent terminal fields display a trend from smallest to largest for the central, peripheral, and planum types, respectively. Neurons that innervate the central zone of the crista do not extend into the peripheral or planum regions. Conversely, those neurons with terminal fields in the peripheral or planum regions do not innervate the central zone of the sensory neuroepithelium. The central zone of the crista is innervated preferentially by efferent neurons with cell bodies located in the ipsilateral group e. The peripheral and planum zones of the crista are innervated preferentially by efferent neurons with cell bodies located in the contralateral group e. A model incorporating our anatomic observations is presented describing an ipsilateral closed-loop feedback between ipsilateral efferent neurons and the periphery and an open-loop feed-forward innervation from contralateral efferent neurons. A possible role for the vestibular efferent neurons in the modulation of semicircular canal afferent response dynamics is proposed.

Saccule contribution to immediate early gene induction in the gerbil brainstem with posterior canal galvanic or hypergravity stimulation

Immunolabeling patterns of the immediate early gene-related protein Fos in the gerbil brainstem were studied following stimulation of the sacculus by both hypergravity and galvanic stimulation. Head-restrained, alert animals were exposed to a prolonged (1 h) inertial vector of 2 G (19.6 m/s2) head acceleration directed in a dorso-ventral head axis to maximally stimulate the sacculus. Fos-defined immunoreactivity was quantified, and the results compared to a control group. The hypergravity stimulus produced Fos immunolabeling in the dorsomedial cell column (dmcc) of the inferior olive independently of other subnuclei. Similar dmcc labeling was induced by a 30 min galvanic stimulus of up to -100 microA applied through a stimulating electrode placed unilaterally on the bony labyrinth overlying the posterior canal (PC). The pattern of vestibular afferent firing activity induced by this galvanic stimulus was quantified in anesthetized gerbils by simultaneously recording from Scarpa's ganglion. Only saccular and PC afferent neurons exhibited increases in average firing rates of 200-300%, suggesting a pattern of current spread involving only PC and saccular afferent neurons at this level of stimulation. These results suggest that alteration in saccular afferent firing rates are sufficient to induce Fos-defined genomic activation of the dmcc, and lend further evidence to the existence of a functional vestibulo-olivary-cerebellar pathway of adaptation to novel gravito-inertial environments.

Efferent neurons and vestibular cross talk in the frog

A galvanic stimulus (30- to 120-s, 0.3-mA constant current pulse) was used to depolarize the spike-generating region of horizontal and anterior canal afferent neurons. The galvanically induced spike activity from these neurons served as a driving input to the efferent vestibular system in the bullfrog. Efferent-mediated effects were assessed by intracellular recordings of posterior canal afferent spike activity, either ipsilateral or contralateral to the driving stimulus. Ipsilateral to the driving stimulus, efferent-mediated spike rate changes occurred in 62 (39%) of 158 posterior canal afferent neurons. Ipsilateral efferent-mediated effects were overwhelmingly excitatory (92%). Of responding units, 3% were inhibited during stimulus application and 5% showed mixed responses involving 3-20 s of inhibition followed by facilitation. Contralateral to the driving stimulus, efferent-mediated spike rate changes occurred in 18 (23%) of 77 posterior canal afferent neurons. Contralateral efferent-mediated effects were overwhelmingly inhibitory (95%). Only one unit was facilitated during stimulation and no mixed responses to contralateral stimulation were observed. Analysis of the coefficient of variation in interspike intervals (CV) before and during stimulation showed no significant efferent-mediated effects on spike train noise. Comparisons of resting spike rates between units showing efferent-mediated effects and those that did not were in general agreement with previous studies. Responding units had a lower mean spike rate (6.8 +/- 0.70 spikes/s, mean +/- SE) than did nonresponding units (10.7 +/- 0.42 spikes/s, mean +/- SE; P < 0.001; 2-tailed t-test of log-normalized data). Comparison between groups in the regularity of their resting spike rates, as quantified by CV, showed considerable overlap. When responding and nonresponding units with similar resting spike rates were compared, responding units had more irregular resting spike rates than did nonresponding units (P < 0.004; 2-tailed, paired t-test). In most cases (77%) the temporal pattern and general shapes of efferent-mediated responses mirrored the driving input of the galvanically activated afferent neurons. The other 23% of efferent-mediated responses exhibited a marked adaptation of the response. Adapting and nonadapting units were not significantly different in their mean resting spike rates or in the regularity of their resting spike rates.

Distribution of calcitonin gene-related peptide immunoreactivity in vestibular efferent neurons of the chinchilla

The distribution of calcitonin gene-related peptide immunoreactivity (CGRPi) within efferent vestibular neurons in the chinchilla was investigated using fluorescent retrograde labeling combined with immunohistochemistry. Efferent vestibular neurons were found bilaterally in clusters: dorsolateral (group E1) and medial (group E2) to the genu of CN VII, as well as ventromedial to the descending CN VII fibers in the parvicellular reticular formation (PCR). The percentage of retrogradely labeled cells containing CGRPi was 77.1 +/- 5.7 for group E1 neurons, 90.3 +/- 3.8 in the E2 region. Among the PCR efferents more then half of the neurons (61.4 +/- 19.9%) expressed CGRP peptide or message. The wide distribution of CGRP among vestibular efferent neurons suggests that CGRP plays an important role in vestibular efferent function. In addition, the differential distribution among the groups of vestibular efferent neurons suggests that efferent modulation of vestibular function is different between the E cell group efferent neurons and the PCR efferent neurons.

Transneuronal pathways to the vestibulocerebellum

The alpha-herpes virus (pseudorabies, PRV) was used to observe central nervous system (CNS) pathways associated with the vestibulocerebellar system. Retrograde transneuronal migration of alpha-herpes virions from specific lobules of the gerbil and rat vestibulo-cerebellar cortex was detected immunohistochemically. Using a time series analysis, progression of infection along polyneuronal cerebellar afferent pathways was examined. Pressure injections of > 20 nanoliters of a 10(8) plaque forming units (pfu) per ml solution of virus were sufficient to initiate an infectious locus which resulted in labeled neurons in the inferior olivary subnuclei, vestibular nuclei, and their afferent cell groups in a progressive temporal fashion and in growing complexity with increasing incubation time. We show that climbing fibers and some other cerebellar afferent fibers transported the virus retrogradely from the cerebellum within 24 hours. One to three days after cerebellar infection discrete cell groups were labeled and appropriate laterality within crossed projections was preserved. Subsequent nuclei labeled with PRV after infection of the flocculus/paraflocculus, or nodulus/uvula, included the following: vestibular (e.g., z) and inferior olivary nuclei (e.g., dorsal cap), accessory oculomotor (e.g., Darkschewitsch n.) and accessory optic related nuclei, (e.g., the nucleus of the optic tract, and the medial terminal nucleus); noradrenergic, raphe, and reticular cell groups (e.g., locus coeruleus, dorsal raphe, raphe pontis, and the lateral reticular tract); other vestibulocerebellum sites, the periaqueductal gray, substantia nigra, hippocampus, thalamus and hypothalamus, amygdala, septal nuclei, and the frontal, cingulate, entorhinal, perirhinal, and insular cortices. However, there were differences in the resulting labeling between infection in either region. Double-labeling experiments revealed that vestibular efferent neurons are located adjacent to, but are not included among, flocculus-projecting supragenual neurons. PRV transport from the vestibular labyrinth and cervical muscles also resulted in CNS infections. Virus propagation in situ provides specific connectivity information based on the functional transport across synapses. The findings support and extend anatomical data regarding vestibulo-olivo-cerebellar pathways.

Development of the labyrinthine efferent system

The data presented here show that labyrinthine and facial branchiomotor efferent cells in the chicken and the mouse become postmitotic overlappingly, both spatially and temporally. Differential migration of labyrinthine efferents and facial motoneurons leads to the already described distinct distribution of labyrinthine efferents and facial motoneurons in adult brains. Differences exist between the chicken and the mouse with respect to the origin of labyrinthine efferents (rhombomere 4 and 5 for the chicken; rhombomere 4 alone for the mouse) and the way contralateral labyrinthine efferents form (migration across the floor plate in the chicken; extension of an axon across the floor plate in the mouse). The different routes taken by migrating motoneurons may all be mediated by substances released from the floor plate, some of which were recently characterized. Labyrinthine efferent axons and facial motoneuron axons segregate at distinctly different areas in the chicken and mouse: outside the brain in the former and inside the brain in the latter. Examination of the possible basis for pathway selection tends to support the idea that efferents use intact afferent fibers as highways for their navigation to distinct sensory epithelia.

A simple technique for introducing anterograde and retrograde tracers into the vestibular and cochlear sensory organs

The standard method for labeling the afferent and efferent innervation of the cochlear and vestibular sensory organs is by microinjection of tracer substances into the labyrinth. Injection of small amounts of tracer often result in incomplete and inconsistent labeling, but large injections can cause spurious labeling of brainstem structures due to diffusion from perilymph to cerebrospinal fluid. Effective labeling with minimal artifact can, however, be achieved by a relatively simple method involving placement of a tracer-saturated pledglet of gelatin sponge in the round window after rupture of its membrane. The gelatin sponge simultaneously acts as a continuous-release vehicle for the tracer and prevents reflux of perilymph and tracer into the middle ear cavity. Use of this technique produces labeling with a degree of intensity and anatomic detail that rivals that seen with more complicated methods of tracer placement.

The effect of somatosensory stimulation on second-order and efferent vestibular neurons in the decerebrate decerebellate guinea-pig

The extracellularly recorded activity of medial vestibular nucleus neurons and efferent vestibular neurons was analysed in the decerebrate decerebellate guinea-pig. Neurons were identified by means of electrical stimulation of the anterior semicircular canal. Thirty-six neurons were monosynaptically activated during semicircular canal stimulation. These cells were regarded as second-order vestibular neurons. Thirty neurons were antidromically activated and therefore identified as efferent vestibular neurons. Both types of neurons investigated had spontaneous impulse activity. All neurons responded to sinusoidal roll tilt. All the second-order vestibular neurons were excited during ipsilateral tilt and inhibited by contralateral tilt. Eighteen efferent vestibular neurons also showed this pattern, while the remaining 12 were excited by contralateral tilt and inhibited by ipsilateral tilt. Some neurons responded to passive forelimb extension or pressure of the forelimb plantar surface; none of the neurons responded to passive forelimb flexion or light plantar touch. Eleven second-order neurons (30%) were excited by somatosensory stimuli, seven (20%) were inhibited and 18 (50%) showed no response. Twenty efferent neurons (67%) were excited by somatosensory stimuli, none were inhibited and 10 (33%) showed no response. The responses of vestibular neurons to somatosensory stimulation are discussed with respect to their importance in vestibulospinal control during locomotion.

Distribution of acidic fibroblast growth factor mRNA-expressing neurons in the adult mouse central nervous system

The distribution of acidic fibroblast growth factor (aFGF) mRNA-expressing neurons was studied throughout the adult mouse central nervous system (CNS) with in situ hybridization histochemistry using a radiolabelled synthetic oligodeoxynucleotide probe complementary to the mRNA of human aFGF. We report here a widespread distribution of aFGF mRNA in several defined functional systems of the adult mouse brain, whereby the highest levels of aFGF mRNA were found in large somatomotor neurons in the nuclei of the oculomotor, trochlear, abducens, and hypoglossal nerves; in the motoneurons of the ventral spinal cord and the special visceromotor neurons in the motor nucleus of the trigeminal nerve; and in the facial and ambiguus nuclei. Labelled perikarya were also detected in all central structures of the auditory pathway including the level of the inferior colliculus, i.e., the lateral and medial superior nuclei; the trapezoid, cochlear, and lateral lemniscal nuclei; and parts of the anterior colliculus. Furthermore, many aFGF-positive cell bodies were found in the vestibular system and other structures projecting to the cerebellum, in the deep cerebellar nuclei, in somatosensory structures of the medulla (i.e., in the gracile, cuneate, and external cuneate nuclei), as well as in the spinal nucleus of the trigeminal nerve. The findings that aFGF mRNA is expressed in all components of several well-defined systems (i.e., in sensory structures) as well as in central neurons that process sensory information and, finally, in some efferent projections point towards a concept of aFGF expression primarily within certain neuronal circuitries.

Subcellular innervation patterns of the calcitonin gene-related peptidergic efferent terminals in the chinchilla vestibular periphery

We examined the ultrastructural distribution of calcitonin gene-related peptide immunoreactivity in the peripheral vestibular system of the chinchilla to study the innervation patterns of this efferent neuropeptide. Immunoelectron microscopic localization of calcitonin gene-related peptide immunoreactive terminals in the maculae and cristae revealed an extensive innervation pattern on the afferent vestibular pathway. Calcitonin gene-related peptide immuno-reactive terminals made synaptic contacts with the unmyelinated portions of the primary afferent vestibular dendrites innervating both type I and type II hair cells. Abundant synaptic contact between calcitonin gene-related peptide immunoreactive terminals and the chalices surrounding type I hair cells was observed. Direct contact between calcitonin gene-related peptide immunoreactive terminals and type II hair cells was observed. In addition, vesiculated efferent terminals without calcitonin gene-related peptide immunoreactivity were seen synapsing on the chalices of type II hair cells and on the surrounding type I hair cells. The primary afferent somata in the vestibular ganglion of Scarpa did not contain calcitonin gene-related peptide immunoreactivity. Unmyelinated calcitonin gene-related peptide immunoreactive axons passed among the primary afferent fibers in Scarpa's ganglion, and these fibers continued through the subepithelial regions of the vestibular end-organs. The calcitonin gene-related peptide immunoreactive axons ramified to produce numerous calcitonin gene-related peptide immunoreactive terminals throughout the neurosensory epithelium of the maculae and cristae. These data suggest that calcitonin gene-related peptide-mediated modulation of the afferent vestibular system is functionally important.

Coexistence of calcitonin gene-related peptide and choline acetyltransferase in eel efferent neurons

We applied choline acetyltransferase, (ChAT) and calcitonin gene-related peptide (CGRP) immunocytochemistry to the efferent neurons that innervate the lateral line and the ear of the eel. Strong immunoreactivity to the ChAT antiserum was observed in neurons located within the octavolateralis efferent nucleus that could be distinguished, on the basis of their form, location and dendritic organization, from the ChAT-immunopositive motoneurons of the adjacent facial motor nucleus. Both facial motoneurons and efferent neurons were found to be immunopositive for CGRP, although the reaction was always stronger in the motoneurons. Double labelling experiments established the presence of both ChAT and CGRP in many efferent neurons. The results are evidence that cholinergic efferent neurons supplying end organs of different modalities may also produce calcitonin gene-related peptide.

Bilateral communication between vestibular labyrinths in pigeons

Extracellular action potentials from single horizontal semicircular canal primary afferent fibers were recorded in paralysed decerebrate pigeons during pulse mechanical stimulation of the contralateral horizontal semicircular canal. Clear responses to the contralateral membranous duct displacement stimuli were observed in 51% of the tested 158 horizontal semicircular canal afferents. Generally, three different types of responses were obtained in the primary afferent fibers including excitation, inhibition, and a few complex type neural activity profiles. Inhibitory responses were of larger amplitude and had longer time constants than did excitatory responses. The few complex type responses observed were characterized by an initial excitatory discharge followed by a longer duration decrease in the fiber's firing rate. The sensitivity to stimulation and type of response obtained for each afferent was significantly correlated with the fiber's coefficient of variation value. Regular firing afferents were less sensitive and exhibited primarily excitatory responses (71%) to contralateral canal stimulation. Irregular firing afferents were more sensitive and exhibited mostly inhibitory responses (84%). The present results demonstrate that a communication network for information exchange between the bilateral labyrinths exists in pigeons. The observed responses in primary afferent fibers to contralateral horizontal semicircular canal stimulation are proposed to be mediated by the vestibular efferent system, which could provide an anatomical pathway for information exchange from vestibular receptors on opposite sides of the head.