The organization of the horizontal semicircular duct, ampulla and utricle in the rat and guinea pig

Irregular primary otolith afferents from the guinea pig utricular and saccular maculae respond to both bone conducted vibration and to air conducted sound

This study sought to identify in guinea pig the peripheral sense organ of origin of otolith irregular primary vestibular afferent neurons having a very sensitive response to both air-conducted sound (ACS) and bone-conducted vibration (BCV). Neurons responding to both types of stimuli were labelled by juxtacellular labelling by neurobiotin. Whole mounts of the maculae showed that some vestibular afferents activated by both ACS and BCV originate from the utricular macula and some from the saccular macula - there is no "afferent specificity" by one sense organ for ACS and the other for BCV - instead some afferents from both sense organs have sensitive responses to both stimuli. The clinical implication of this result is that differential evaluation of the functional status of the utricular and saccular maculae cannot rely on stimulus type (ACS vs BCV), however the differential motor projections of the utricular and saccular maculae allow for differential evaluation of each sense organ.

Diminution of voltage threshold plays a key role in determining recruitment of oculomotor nucleus motoneurons during postnatal development

The size principle dictates the orderly recruitment of motoneurons (Mns). This principle assumes that Mns of different sizes have a similar voltage threshold, cell size being the crucial property in determining neuronal recruitment. Thus, smaller neurons have higher membrane resistance and require a lower depolarizing current to reach spike threshold. However, the cell size contribution to recruitment in Mns during postnatal development remains unknown. To investigate this subject, rat oculomotor nucleus Mns were intracellularly labeled and their electrophysiological properties recorded in a brain slice preparation. Mns were divided into 2 age groups: neonatal (1-7 postnatal days, n = 14) and adult (20-30 postnatal days, n = 10). The increase in size of Mns led to a decrease in input resistance with a strong linear relationship in both age groups. A well-fitted inverse correlation was also found between input resistance and rheobase in both age groups. However, input resistance versus rheobase did not correlate when data from neonatal and adult Mns were combined in a single group. This lack of correlation is due to the fact that decrease in input resistance of developing Mns did not lead to an increase in rheobase. Indeed, a diminution in rheobase was found, and it was accompanied by an unexpected decrease in voltage threshold. Additionally, the decrease in rheobase co-varied with decrease in voltage threshold in developing Mns. These data support that the size principle governs the recruitment order in neonatal Mns and is maintained in adult Mns of the oculomotor nucleus; but during postnatal development the crucial property in determining recruitment order in these Mns was not the modifications of cell size-input resistance but of voltage threshold.

Postnatal development enhances the effects of cholinergic inputs on recruitment threshold and firing rate of rat oculomotor nucleus motoneurons

Changes in the electrophysiological and morphological characteristics of motoneurons (Mns) of the oculomotor nucleus during postnatal development have been reported, however synaptic modifications that take place concurrently with postnatal development in these Mns are yet to be elucidated. We investigated whether cholinergic inputs exert different effects on the recruitment threshold and firing rate of Mns during postnatal development. Rat oculomotor nucleus Mns were intracellularly recorded in brain slice preparations and separated in neonatal (4-7 postnatal days) and adult (20-30 postnatal days) age groups. Stimulation of the medial longitudinal fasciculus evoked a monosynaptic excitatory potential in Mns that was attenuated with atropine (1.5 μM, a muscarinic antagonist). Mns were silent at their resting membrane potential, and bath application of carbachol (10 μM, a cholinergic agonist) induced depolarization of the membrane potential and a sustained firing rate that were more pronounced in adult Mns. Pharmacological and immunohistochemical assays showed that these responses were attributable to muscarinic receptors located in the membrane of Mns. In addition, compared to control Mns, carbachol-exposed Mns exhibited a higher firing rate in response to the injection of the same amount of current, and a decrease in the current threshold required to achieve sustained firing. These latter effects were more pronounced in adult than in neonatal Mns. In conclusion, our findings suggest that cholinergic synaptic inputs are already present in neonatal Mns, and that the electrophysiological effects of such inputs on recruitment threshold and firing rate are enhanced with the postnatal development in oculomotor nucleus Mns. We propose that cholinergic input maturation could provide a greater dynamic range in adult Mns to encode the output necessary for graded muscle contraction.

Changes in somatodendritic morphometry of rat oculomotor nucleus motoneurons during postnatal development

This work investigates the somatodendritic shaping of rat oculomotor nucleus motoneurons (Mns) during postnatal development. The Mns were functionally identified in slice preparation, intracellularly injected with neurobiotin, and three-dimensionally reconstructed. Most of the Mns (approximately 85%) were multipolar and the rest (approximately 15%) bipolar. Forty multipolar Mns were studied and grouped as follows: 1-5, 6-10, 11-15, and 21-30 postnatal days. Two phases were distinguished during postnatal development (P1-P10 and P11-P30). During the first phase, there was a progressive increase in the dendritic complexity; e.g., the number of terminals per neuron increased from 26.3 (P1-P5) to 47.7 (P6-P10) and membrane somatodendritic area from 11,289.9 microm(2) (P1-P5) to 19,235.8 microm(2) (P6-P10). In addition, a few cases of tracer coupling were observed. During the second phase, dendritic elongation took place; e.g., the maximum dendritic length increased from 486.7 microm (P6-P10) to 729.5 microm in adult Mns, with a simplification of dendritic complexity to values near those for the newborn, and a slow, progressive increase in membrane area from 19,235.8 microm(2) (P6-P10) to 24,700.3 microm(2) (P21-P30), while the somatic area remained constant. In conclusion, the electrophysiological changes reported in these Mns with maturation (Carrascal et al. [2006] Neuroscience 140:1223-1237) cannot be fully explained by morphometric variations; the dendritic elongation and increase in dendritic area are features shared with other pools of Mns, whereas changes in dendritic complexity depend on each population; the first phase paralleled the establishment of vestibular circuitry and the second paralleled eyelid opening.

A new approach to visualizing the membranous structures of the inner ear - high resolution X-ray micro-tomography

CONCLUSION

Through the application of high resolution X-ray micro-tomography and a method of contrast enhancement based on en bloc staining in osmium tetroxide (OsO4), we report an approach that facilitates accurate three-dimensional (3D) reconstruction to reveal the fine structure of the inner ear.

OBJECTIVES

To overcome the problems of artefacts, including tissue distortion and loss of 3D context that are inherent in existing methods that rely on manual dissection and/or histological sectioning.

MATERIALS AND METHODS

A staining protocol was developed that involved the en bloc application of the OsO4 solution (2% w/v) for an extended period of time. The samples were then scanned using an X-ray micro-tomography platform and subsequent 3D visualizations were constructed.

RESULTS

The digital nature of the data allowed a complete 3D contextual visualization to be constructed whereby the individual sensory structures could be seen in relation to other inner ear structures. This included a detailed anatomy of the membranous labyrinth and nerve supply including the spatial configuration of the utricular and saccular maculae. This is a new way of undertaking temporal bone histology.

Bone conducted vibration selectively activates irregular primary otolithic vestibular neurons in the guinea pig

The main objective of this study was to determine whether bone-conducted vibration (BCV) is equally effective in activating both semicircular canal and otolith afferents in the guinea pig or whether there is preferential activation of one of these classes of vestibular afferents. To answer this question a large number (346) of single primary vestibular neurons were recorded extracellularly in anesthetized guinea pigs and were identified by their location in the vestibular nerve and classed as regular or irregular on the basis of the variability of their spontaneous discharge. If a neuron responded to angular acceleration it was classed as a semicircular canal neuron, if it responded to maintained roll or pitch tilts it was classified as an otolith neuron. Each neuron was then tested by BCV stimuli-either clicks, continuous pure tones (200-1,500 Hz) or short tone bursts (500 Hz lasting 7 ms)-delivered by a B-71 clinical bone-conduction oscillator cemented to the guinea pig's skull. All stimulus intensities were referred to that animal's own auditory brainstem response (ABR) threshold to BCV clicks, and the maximum intensity used was within the animal's physiological range and was usually around 70 dB above BCV threshold. In addition two sensitive single axis linear accelerometers cemented to the skull gave absolute values of the stimulus acceleration in the rostro-caudal direction. The criterion for a neuron being classed as activated was an audible, stimulus-locked increase in firing rate (a 10% change was easily detectable) in response to the BCV stimulus. At the stimulus levels used in this study, semicircular canal neurons, both regular and irregular, were insensitive to BCV stimuli and very few responded: only nine of 189 semicircular canal neurons tested (4.7%) showed a detectable increase in firing in response to BCV stimuli up to the maximum 2 V peak-to-peak level we delivered to the B-71 oscillator (which produced a peak-to-peak skull acceleration of around 6-8 g and was usually around 60-70 dB above the animal's own ABR threshold for BCV clicks). Regular otolithic afferents likewise had a poor response; only 14 of 99 tested (14.1%) showed any increase in firing rate up to the maximum BCV stimulus level. However, most irregular otolithic afferents (82.8%) showed a clear increase in firing rate in response to BCV stimuli: of the 58 irregular otolith neurons tested, 48 were activated, with some being activated at very low intensities (only about 10 dB above the animal's ABR threshold to BCV clicks). Most of the activated otolith afferents were in the superior division of the vestibular nerve and were probably utricular afferents. That was confirmed by evidence using juxtacellular injection of neurobiotin near BCV activated neurons to trace their site of origin to the utricular macula. We conclude there is a very clear preference for irregular otolith afferents to be activated selectively by BCV stimuli at low stimulus levels and that BCV stimuli activate some utricular irregular afferent neurons. The BCV generates compressional and shear waves, which travel through the skull and constitute head accelerations, which are sufficient to stimulate the most sensitive otolithic receptor cells.

Temporal sequence of changes in electrophysiological properties of oculomotor motoneurons during postnatal development

The temporal sequence of changes in electrophysiological properties during postnatal development in different neuronal populations has been the subject of previous studies. Those studies demonstrated major physiological modifications with age, and postnatal periods in which such changes are more pronounced. Until now, no similar systematic study has been performed in motoneurons of the oculomotor nucleus. This work has two main aims: first, to determine whether the physiological changes in oculomotor nucleus motoneurons follow a similar time course for different parameters; and second, to compare the temporal sequence with that in other neuronal populations. We recorded the electrophysiological properties of 134 identified oculomotor nucleus motoneurons from 1 to 40 days postnatal in brain slices of rats. The resting membrane potential did not significantly change with postnatal development, and it had a mean value of -61.8 mV. The input resistance and time constant diminished from 82.9-53.1 M omega and from 9.4-4.9 ms respectively with age. These decrements occurred drastically in a short time after birth (1-5 days postnatally). The motoneurons' rheobase gradually decayed from 0.29-0.11 nA along postnatal development. From birth until postnatal day 15 and postnatal day 20 respectively, the action potential shortened from 2.3-1.2 ms, and the medium afterhyperpolarization from 184.8-94.4 ms. The firing gain and the maximum discharge increased with age. The former rose continuously, while the increase in maximum discharge was most pronounced between postnatal day 16 and postnatal day 20. We conclude that the developmental sequence was not similar for all electrophysiological properties, and was unique for each neuronal population.

Transfection of young guinea pig vestibular cells in vitro with an adenovirus vector

This study shows distributions of lacZ-positive cells in the vestibular labyrinthine explants of young guinea pigs with mature ears. When adenovirus lacZ vectors were administered to the vestibular labyrinth following removal of the otoconial membrane, lacZ-positive cells were observed in the mesothelial cells in the perilymphatic space, in the sensory and supporting cells in the utricle and ampulla, and in the transitional and dark cells in the ampulla. When the otoconial membrane was preserved, lacZ-positive cells were not distributed in the utricular sensory epithelium. These findings suggest that adenovirus vectors can transform a variety of vestibular epithelial cells, but that it is difficult for adenovirus vectors to pass through the otoconial membrane.

Morphological, morphometric, and functional differences in the vestibular organ of different breeds of the rat (Rattus norvegicus)

In the laboratory rat, differences in shape, dimension and function of the cochlea have been reported for various breeds. In contrast, no comparable investigations to date have been undertaken for the vestibular organ in different breeds of the rat. Vestibular organs of two breeds of rat (Wistar, Sprague-Dawley) were analyzed morphologically and morphometrically by means of microdissection techniques in order to determine the mechanical sensitivity of the cupula according to Oman et al; (Acta Otolaryngol., 1987;103:1-13, 1987). Differences in shape of the lateral semicircular duct exist between the two breeds and the cupular mechanical sensitivity is significantly higher in Wistar than in Sprague-Dawley rats. With respect to the other semicircular ducts, no differences in shape were found between the two strains. The cupular mechanical sensitivity of the anterior semicircular duct, however, is higher in Wistar than in Sprague-Dawley rats. The breeds also differ in the shape of their utriculus; obviously a correlation exists between the latter and the cupular mechanical sensitivity of the semicircular ducts. There are differences in the vestibular organs between the two breeds of the laboratory rat investigated. The cupular mechanical sensitivity of the semicircular duct does not seem to be correlated to body mass. The size and morphology of the utriculus influence the mechanical sensitivity of a single duct, but differences only become significant if other parameters also differ.

Development of blood-labyrinth barrier in the semicircular canal ampulla of the rat

Cationic polyethyleneimine (PEI) administered intravenously was transported to anionic sites on the capillary and subepithelial basal laminae (BL) in the vestibular labyrinth. Therefore, changes in the PEI distribution on the BL reflect changes in the transport system in the vestibular labyrinth. A 0.1% PEI solution was administered intravenously (7.5 ml/kg) to developing (1, 4, 7, 14 days after birth) and adult rats in order to investigate the development of the macromolecular transport in the ampulla of the semicircular canal as a function of age. After 1 h, the bony labyrinth was removed and embedded in Epoxy resin. Ultrathin sections of the ampulla were then examined with a transmission electron microscope. In the subepithelial BL in the dark cell area and capillary BL in the crista ampullaris, the PEI distribution in both 1- and 4-day-old rats was markedly increased compared to that in either 7-, 14-day or adult rats. In the sensory cells in 1-, 4-day or 7-day-old rats, PEI density and area was significantly greater than in the adult rats. These findings suggest that the macromolecular transport system in the developing rat ampulla becomes mature by 14 days after birth and that the maturation of its transport system in the ampulla is strongly associated with that in the stria vascularis.

Short latency vestibular evoked potentials (VsEPs) to linear acceleration impulses in rats

In this study, short latency (t < 12.7 ms) vestibular evoked potentials (VsEPs) in response to linear acceleration impulses were recorded in 37 rats. A new technique (based on a solenoid) was used for generating linear force impulses that were delivered to the animal's head. The impulse had a maximal peak acceleration of 12 g. During the impulse, the displacement was 50 microns (at 4 g) and the rise time was 1.0 ms. A stimulation rate of 2/s was usually used. The VsEPs (averaged responses to 128 stimulations, digital filter: 300-1500 Hz) were recorded with electrodes on pinna and vertex, and were composed of 4-6 clear waves with mean amplitudes (for a 4 g stimulus) of 1-5 microV. The VsEPs were resistant to white noise masking, and were significantly suppressed (P < 0.05) following bilateral application of a saturated KCl solution to the inner ear, showing that contributions of the auditory and somatosensory systems are negligible. The latency of the response decreased as a power law function of stimulus magnitude, and the amplitude of the first wave increased as a sigmoid function of stimulus magnitude. VsEP responses were still present at the lowest intensities attainable (0.06-0.4 g) and reached saturation at 9 g. The amplitude of the later components was reduced when stimulus rate was elevated to 20/s. These results suggest that VsEPs in response to linear accelerations are similar in their nature to VsEPs in response to angular acceleration impulses that were previously recorded. These VsEPs to linear accelerations are most likely initiated in the otolith organs.

Dimensions of the horizontal semicircular duct, ampulla and utricle in rat and guinea pig

The dimensions of the membranous labyrinth partly determine the mechanical operation of the semicircular canal system. This study provides, for the first time, extensive measures in individual specimens of the sizes, cross-sectional shapes and areas of the horizontal semicircular duct, ampulla and utricle in the rat and the guinea pig. The membranous labyrinths were fixed in Karnovsky's fixative, exposed, photographed, sectioned, oriented perpendicular to the line of sight and then measured using a calibrated graticule in the eye piece of an operating microscope. As well as the expected size differences between these species, there are major differences in the shape of the utricle.

The response of primary horizontal semicircular canal neurons in the rat and guinea pig to angular acceleration

In rats and guinea pigs, primary afferent neurons from the horizontal semicircular canal were divided into two categories, regular and irregular, on the basis of the regularity of their resting activity. Regular neurons tend to have higher average resting rates than irregular neurons and in response to a constant angular acceleration stimulus of 16.7 deg/s2 regular neurons tended to have lower sensitivity and longer time constants than irregular cells. Some irregular neurons are more sensitive to incremental accelerations than to decremental accelerations of the same magnitude, whereas regular neurons tend to show symmetrical sensitivity. In response to sinusoidal angular acceleration stimuli (fixed frequencies) in the range 0.01-1.5 Hz, cells which fired regularly at rest tended to have smaller gain and longer phase lag re acceleration at most frequencies than irregular cells. Transfer functions were obtained for averaged data for regular and irregular neurons separately in both species. In both species there is evidence of systematic variation between neurons within each category, and this systematic variation is obscured by averaging across neurons.