Distribution of calcitonin gene-related peptide mRNA and immunoreactivity in the rat central and peripheral vestibular system

Opioid receptor activation modulates the calcium current in the cochlear outer hair cells of the rat

Previous works showed that opioid peptides are produced by olivocochlear efferent neurons, while cochlear hair cells express opioid receptors. It has been proposed that opioids protect the auditory system from damage by intense stimulation, although their use for therapeutic or illicit purposes links to hearing impairment. Therefore, it is relevant to study the effect of opioids in the auditory system to define their functional expression and mechanism of action. This study investigated the modulation of the Ca²⁺ currents by opioid peptides in the rat outer hair cells (OHC) using the whole-cell patch-clamp technique. The influence of agonists of the three opioid receptor subtypes (μ, δ, and κ) was studied. The κ opioid receptor agonist U-50488 inhibits the Ca²⁺ currents in a partially reversible form. Coincidently, norbinaltorphimine (a κ receptor antagonist) blocked the U-50488 inhibitory effect on the Ca²⁺ current. The δ- and the μ opioid receptor agonists did not significantly affect the Ca²⁺ currents. These results indicate that the κ opioid receptor activation inhibits the Ca²⁺ current in OHC, modulating the intracellular Ca²⁺ concentration when OHCs depolarize. The modulation of the auditory function by opioids constitutes a relevant mechanism with a potential role in the physiopathology of auditory disturbances.

Differences in the Structure and Function of the Vestibular Efferent System Among Vertebrates

The role of the mammalian vestibular efferent system in everyday life has been a long-standing mystery. In contrast to what has been reported in lower vertebrate classes, the mammalian vestibular efferent system does not appear to relay inputs from other sensory modalities to the vestibular periphery. Furthermore, to date, the available evidence indicates that the mammalian vestibular efferent system does not relay motor-related signals to the vestibular periphery to modulate sensory coding of the voluntary self-motion generated during natural behaviors. Indeed, our recent neurophysiological studies have provided insight into how the peripheral vestibular system transmits head movement-related information to the brain in a context independent manner. The integration of vestibular and extra-vestibular information instead only occurs at next stage of the mammalian vestibular system, at the level of the vestibular nuclei. The question thus arises: what is the physiological role of the vestibular efferent system in mammals? We suggest that the mammalian vestibular efferent system does not play a significant role in short-term modulation of afferent coding, but instead plays a vital role over a longer time course, for example in calibrating and protecting the functional efficacy of vestibular circuits during development and aging in a role analogous the auditory efferent system.

Neuropeptides in sensory signal processing

Peptides released from trigeminal fibers fulfill well-understood functions in neuroinflammatory processes and in the modulation of nociceptive signal processing. In particular, calcitonin gene-related peptide (CGRP) and substance P (SP), released from afferent nerve terminals, exert paracrine effects on the surrounding tissue and this has been recently highlighted by the prominent parcrine role of CGRP in the development of headache and migraine. Some recent communications suggest that these sensory neuropeptides may also modulate the workings of sensory organs and influence afferent signals from nose, tongue, eyes and ears. Here, we briefly review the evidence for modulatory effects of CGRP and SP in the sensory periphery.

Loss of α-Calcitonin Gene-Related Peptide (αCGRP) Reduces Otolith Activation Timing Dynamics and Impairs Balance

Calcitonin gene-related peptide (CGRP) is a neuroactive peptide that is thought to play a role at efferent synapses in hair cell organs including the cochlea, lateral line, and semicircular canal. The deletion of CGRP in transgenic mice is associated with a significant reduction in suprathreshold cochlear nerve activity and vestibulo–ocular reflex (VOR) gain efficacy when compared to littermate controls. Here we asked whether the loss of CGRP also influences otolithic end organ function and contributes to balance impairments. Immunostaining for CGRP was absent in the otolithic end organs of αCGRP null (-/-) mice while choline acetyltransferase (ChAT) immunolabeling appeared unchanged suggesting the overall gross development of efferent innervation in otolithic organs was unaltered. Otolithic function was assessed by quantifying the thresholds, suprathreshold amplitudes, and latencies of vestibular sensory-evoked potentials (VsEPs) while general balance function was assessed using a modified rotarod assay. The loss of αCGRP in null (-/-) mice was associated with: (1) shorter VsEP latencies without a concomitant change in amplitude or thresholds, and (2) deficits in the rotarod balance assay. Our findings show that CGRP loss results in faster otolith afferent activation timing, suggesting that the CGRP component of the efferent vestibular system (EVS) also plays a role in otolithic organ dynamics, which when coupled with reduced VOR gain efficacy, impairs balance.

Central Integration of Canal and Otolith Signals is Abnormal in Vestibular Migraine

Vestibular migraine (VM), a common cause of vestibular symptoms within the general population, is a disabling and poorly understood form of dizziness. We sought to examine the underlying pathophysiology of VM with three studies, which involved the central synthesis of canal and otolith cues, and present preliminary results from each of these studies: (1) VM patients appear to have reduced motion perception thresholds when canal and otolith signals are modulated in a co-planar manner during roll tilt; (2) percepts of roll tilt appear to develop more slowly in VM patients than in control groups during a centrifugation paradigm that presents conflicting, orthogonal canal and otolith cues; and (3) eye movement responses appear to be different in VM patients when studied with a post-rotational tilt paradigm, which also presents a canal–otolith conflict, as the shift of the eye’s rotational axis was larger in VM and the relationship between the axis shift and tilt suppression of the vestibulo-ocular reflex differed in VM patients relative to control groups. Based on these preliminary perceptual and eye movement results obtained with three different motion paradigms, we present a hypothesis that the integration of canal and otolith signals by the brain is abnormal in VM and that this abnormality could be cerebellar in origin. We provide potential mechanisms that could underlie these observations, and speculate that one of more of these mechanisms contributes to the vestibular symptoms and motion intolerance that are characteristic of the VM syndrome.

Loss of α-calcitonin gene-related peptide (αCGRP) reduces the efficacy of the Vestibulo-ocular Reflex (VOR)

The neuroactive peptide calcitonin-gene related peptide (CGRP) is known to act at efferent synapses and their targets in hair cell organs, including the cochlea and lateral line. CGRP is also expressed in vestibular efferent neurons as well as a number of central vestibular neurons. Although CGRP-null (-/-) mice demonstrate a significant reduction in cochlear nerve sound-evoked activity compared with wild-type mice, it is unknown whether and how the loss of CGRP influence vestibular system function. Vestibular function was assessed by quantifying the vestibulo-ocular reflex (VOR) in alert mice. The loss of CGRP in (-/-) mice was associated with a reduction of the VOR gain of ≈50% without a concomitant change in phase. Using immunohistochemistry, we confirmed that, although CGRP staining was absent in the vestibular end-organs of null (-/-) mice, cholinergic staining appeared normal, suggesting that the overall gross development of vestibular efferent innervation was unaltered. We further confirmed that the observed deficit in vestibular function of null (-/-) mice was not the result of nontargeted effects at the level of the extraocular motor neurons and/or their innervation of extraocular muscles. Analysis of the relationship between vestibular quick phase amplitude and peak velocity revealed that extraocular motor function was unchanged, and immunohistochemistry revealed no abnormalities in motor endplates. Together, our findings show that the neurotransmitter CGRP plays a key role in ensuring VOR efficacy.

Expression of calcitonin gene-related peptide in efferent vestibular system and vestibular nucleus in rats with motion sickness

Motion sickness presents a challenge due to its high incidence and unknown pathogenesis although it is a known fact that a functioning vestibular system is essential for the perception of motion sickness. Recent studies show that the efferent vestibular neurons contain calcitonin gene-related peptide (CGRP). It is a possibility that the CGRP immunoreactivity (CGRPi) fibers of the efferent vestibular system modulate primary afferent input into the central nervous system; thus, making it likely that CGRP plays a key role in motion sickness. To elucidate the relationship between motion sickness and CGRP, the effects of CGRP on the vestibular efferent nucleus and the vestibular nucleus were investigated in rats with motion sickness.

Colocalization of 5-HT1F receptor and calcitonin gene-related peptide in rat vestibular nuclei

The aim of this study was to determine whether calcitonin gene-related peptide (CGRP) colocalizes with 5-HT(1F) receptor in rat vestibular nuclei using a double immunohistochemical staining procedure. The frequent co-occurrence of migraine and balance disorders suggests a pathophysiologic link between the two. However, the mechanism of migrainous vertigo has not been elucidated, though serotonin (5-HT) and its receptors are believed to involve in the pathogenesis of migrainous vertigo. Furthermore, 5-HT(1F) receptor agonists and CGRP receptor antagonists have recently attracted attention as potential treatments for migraine, and CGRP release from trigeminal neurons has been associated with migraine. This study demonstrates the colocalization of 5-HT(1F) receptor and CGRP in the rat vestibular nuclei, which suggests that 5-HT(1F) receptor regulates the release of CGRP from vestibular nuclei. This finding indicates that 5-HT(1F) receptor agonists may ameliorate migrainous vertigo by attenuating elevated levels of CGRP release from vestibular nuclei.

Urocortin expression in mouse cochlear nucleus and Scarpa's ganglion

OBJECTIVES/HYPOTHESIS

Clinical and basic studies have correlated tinnitus with stress. Although the etiology of tinnitus is unknown, the cochlear nucleus (CN) appears to play a role. To better understand the potential impact of stress on tinnitus and modulation of the central auditory system in general, the goal of the current study was to examine the presence and distribution of axon terminals containing urocortin in the CN of the mouse.

STUDY DESIGN

Prospective description of histological findings.

METHODS

Three different forms of urocortin were labeled in brainstem sections collected from 10 wild-type mice by immunohistochemistry. Immunoreactive terminal fibers in the CN were digitally photographed, as well as reconstructed in the CN under a drawing tube attached to a light microscope.

RESULTS

Specific staining was found in en passant type fibers scattered throughout the CN but situated mostly within the granule cell domains. Clusters of labeled fibers were observed in the nerve root. Labeled axons were observed in the three tracts known to carry olivocochlear fibers to the CN, as well as in the olivocochlear bundle itself. As the axons within the olivocochlear bundle departed the brainstem in the vestibular nerve, numerous labeled en passant fibers were observed among somata of the vestibular ganglion (Scarpa's). Centrally, labeled axons were followed from the CN to the lateral superior olive, an established source of urocortin-positive efferents to the cochlea. These findings indicate that lateral olivocochlear efferents innervate the CN and Scarpa's ganglion, and also that urocortin is likely a neuromodulator in particular CN circuits.

CONCLUSIONS

The current study supports innervation of specific regions of the mouse CN and Scarpa's ganglion by neurons expressing urocortin. The innervation may be one substrate by which stress modulates particular CN processes. Further studies are necessary to establish the role of urocortin in CN models of tinnitus.

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.

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.

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.

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.

Fiber diameter distributions in the chinchilla's ampullary nerves

A morphometric study of the chinchilla's ampullary nerves was conducted to produce an unbiased accounting of the diameter distribution of their constituent fibers. Diameter analyses were determined from 1 microm plastic-embedded nerve sections taken at a plane immediately proximal to the sensory epithelium. We found these nerves to be composed of 2094+/-573 fibers, having diameters that ranged from 0.5 to 8 microm. The distributions of diameters were positively skewed, where approximately 75% of the fibers were found to have diameters less than 3.5 microm. An analysis of the spatial distribution of diameters within the nerve section revealed that the lateralmost areas of the nerve contained larger fractions of fibers within the smallest diameter quintiles, and the central area harbored greater proportions of the larger diameter quintiles. However, significant fractions of all quintiles were found in all areas. These data were integrated with available data of Fernandez et al. (1998) to produce diameter estimates of calyx, dimorphic, and bouton morphology subpopulations. In view of a general relationship between diameter, innervation locus, and an afferent's physiologic characteristics, these data provide the basis for developing a perspective for the in situ distribution of afferent response dynamics.

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.

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.

Immunohistochemical investigation of enkephalins in the human inner ear

Enkephalins are generally considered as neuropeptides in the central and peripheral nervous system of mammals bound to three large precursor molecules. Several animal studies demonstrated the distribution of met- and leu-enkephalin-like immunoreactivities in neurons and terminals of the lateral olivocochlear system. The immunostainings in the medial system are more controversial. No data about the presence of different enkephalin sequences in the vestibular efferent terminals are known. In the present study, the ultrastructural localization and distribution of immunoreactivities for six different antibodies against met- and leu-enkephalins in the human cochlear and vestibular periphery were investigated. A modified method of pre-embedding immunoelectronmicroscopy was applied. Met- and leu-enkephalin-like immunoreactivities were observed in the efferent terminals of the human outer and inner hair cell region. Using different met- and leu-enkephalin antibodies, the distribution of immunoreactivities remained similar. In the five human vestibular endorgans, enkephalin-like immunostaining was absent.

Molecules, Motion, and Man

The application of cell and molecular biology techniques to vestibular research is resulting in rapid changes in our understanding of the fundamental mechanisms of vestibular function. The clinical problems encountered in space travel together with the acute and chronic vestibular dysfunction affecting many of the patients otolaryngologists care for have driven this research at a rapid pace. A review of these methods and highlights of the major advances are discussed. (Otolaryngol Head Neck Surg 1998;118:S16-S24.)

ARIA is heavily expressed in rat peripheral auditory and vestibular ganglia

The ARIA (acetylcholine receptor inducing activity) polypeptide is a member of the neuregulin gene family. It was originally purified on the basis of its ability to induce skeletal muscle nicotinic acetylcholine receptors (nAChRs). ARIA mRNA is expressed in ventral horn motor neurons and brain cholinergic neurons. We report here that ARIA mRNA is heavily expressed in the embryonic, developing, and adult peripheral auditory and vestibular ganglia, the spiral ganglion and Scarpa's ganglion. Neither ganglion is cholinergic, but both express mRNAs for nicotinic and muscarinic receptors. The expression of ARIA in these ganglia may be related to the regulation of cholinergic receptors or a more general role for ARIA in growth and development.

Molecular temporal bone pathology: I. Historical foundation

The application of molecular biology techniques to temporal bone research is resulting in rapid changes in our understanding of the fundamental mechanisms of auditory, facial nerve, and vestibular function. The use of the polymerase chain reaction, cDNA libraries, and in situ hybridization histochemistry, the determination of genetic defects, and the manipulation of transgenic animals are the molecular biology tools that are available to approach these research problems. Knowledge of the molecular pathology that results in the otologic and neuro-otologic dysfunction many of our patients experience is currently in its infancy. A review of the historical foundation of temporal bone pathology and the evolution of the application of cell and molecular biology methods to archival celloidin-embedded human temporal bone material is presented.

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.

Distribution of efferent cholinergic terminals and alpha-bungarotoxin binding to putative nicotinic acetylcholine receptors in the human vestibular end-organs

Although acetylcholine (ACh) has been identified as the primary neurotransmitter of the efferent vestibular system in most animals studied, no direct evidence exists that ACh is the efferent neurotransmitter of the human vestibular system. Choline acetyltransferase immunohistochemistry (ChATi), acetylcholinesterase (AChE) histochemistry, and alpha-bungarotoxin binding were used in human vestibular end-organs to address this question. ChATi and AChE activity was found in numerous bouton-type terminals contacting the basal area of type II vestibular hair cells and the afferent chalices surrounding type I hair cells; alpha-bungarotoxin binding suggested the presence of nicotinic acetylcholine receptors on type II vestibular hair cells and on the afferent chalices surrounding type I hair cells. This study provides evidence that the human efferent vestibular axons and terminals are cholinergic and that the receptors receiving this innervation may be nicotinic.

Neuroactive substances in the human vestibular end organs

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

Cholinergic agonists increase intracellular calcium concentration in frog vestibular hair cells

Acetylcholine (ACh) is usually considered to be the neurotransmitter of the efferent vestibular system. The nature and the localization of cholinergic receptors have been investigated on frog isolated vestibular hair cells (VHCs), by measuring variations of intracellular calcium concentration ([Ca2+]i), using calcium sensitive dye fura-2. Focal iontophoretic ACh (1 M, 300 nA.40 ms) application induced a rapid increase in [Ca2+]i, reaching a peak in 20 s and representing about 5-fold the resting level (from 61 +/- 6 to 320 +/- 26 nM). Applications of muscarinic agonists as methacholine and carbachol induced weaker calcium responses (from 78 +/- 25 to 238 +/- 53 nM) than the one obtained with ACh applications. These muscarinic agonists were efficient only in precise zones. Desensitization of muscarinic receptors to successive stimulations was significant. Perfusion of nicotine or 1,1-dimethyl-4-phenyl-piperazinium (DMPP), a nicotinic agonist, induced an increase in [Ca2+]i only in some cells (4/28 with DMPP). These results indicated the presence of cholinergic receptors on frog VHCs: muscarinic receptors were more responsive than nicotinic receptors. Presence of muscarinic and nicotinic receptors in the membrane of VHCs could indicate different modulations of VHCs activity mediated by [Ca2+]i and involving an efferent control which represents a central regulation of the vestibular afferent message.

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.