Hair cell numbers do not decrease in the crista ampullaris of geriatric gerbils

Molecular and Functional Changes to Postsynaptic Cholinergic Signaling in the Vestibular Sensory Organs of Aging C57BL/6 Mice

Cholinergic circuits in the central nervous system are vulnerable to age-related functional decline, but it is not known if aging impacts cholinergic signaling in the vestibular sensory organs, which are critically important to balance maintenance and visual gaze stability. We have previously shown cholinergic neurotransmission between vestibular efferent terminals and type II mechanosensory hair cells requires the alpha9 (Chrna9) nicotinic receptor subunit. Homozygous knockout of the alpha9 subunit causes vestibulo-ocular reflex adaptation deficits that mirror those observed in aged mice. This prompted examination of cholinergic signaling in the vestibular sensory organs of aged mice. We confirmed older (>24 months) mice had impaired performance in a balance beam task compared to young (3-4 months) adult mice. While there was no qualitative loss of cholinergic axon varicosities in the crista ampullaris of old mice, qPCR analysis revealed reduced expression of nicotinic receptor subunit genes Chrna1, Chrna9, and Chrna10 in the cristae of old relative to young mice. Functionally, single-cell patch clamp recordings taken from type II vestibular hair cells exposed to acetylcholine show reduced conductance through alpha9/10 subunit-containing nicotinic receptors in older mice, despite preserved passive membrane properties and voltage-activated conductances. These findings suggest that cholinergic signaling in the peripheral vestibular sensory organs is vulnerable to aging processes, manifesting in dynamic molecular and functional age-related changes. Given the importance of these organs to our everyday activities, and the dramatic increase in fall incidence in the older, further investigation into the mechanisms of altered peripheral vestibular function in older humans is warranted.

Further studies of the effects of aging on arginine metabolites in the rat vestibular nucleus and cerebellum

Some studies have demonstrated that aging is associated with impaired vestibular reflexes, especially otolithic reflexes, resulting in postural instability. However, the neurochemical basis of these age-related changes is still poorly understood. The l-arginine metabolic system has been implicated in changes in the brain associated with aging. In the current study, we examined the levels of l-arginine and its metabolizing enzymes and downstream metabolites in the vestibular nucleus complex (VNC) and cerebellum (CE) of rats with and without behavioral testing which were young (4months old), middle-aged (12months old) or aged (24months old). We found that aging was associated with lower nitric oxide synthase activity in the CE of animals with testing and increased arginase in the VNC and CE of animals with testing. l-citrulline and l-ornithine were lower in the VNC of aged animals irrespective of testing, while l-arginine and l-citrulline were lower in the CE with and without testing, respectively. In the VNC and CE, aging was associated with lower levels of glutamate in the VNC, irrespective of testing. In the VNC it was associated with higher levels of agmatine and putrescine, irrespective of testing. In the CE, aging was associated with higher levels of putrescine in animals without testing and with higher levels of spermine in animals with testing, and spermidine, irrespective of testing. Multivariate analyses indicated significant predictive relationships between the different variables, and there were correlations between some of the neurochemical variables and behavioral measurements. Cluster analyses revealed that aging altered the relationships between l-arginine and its metabolites. The results of this study demonstrate that there are major changes occurring in l-arginine metabolism in the VNC and CE as a result of age, as well as behavioral activity.

Age-Related Neurochemical Changes in the Vestibular Nuclei

There is evidence that the normal aging process is associated with impaired vestibulo-ocular reflexes (VOR) and vestibulo-spinal reflexes, causing reduced visual acuity and postural instability. Nonetheless, the available evidence is not entirely consistent, especially with respect to the VOR. Some recent studies have reported that VOR gain can be intact even above 80 years of age. Similarly, although there is evidence for age-related hair cell loss and neuronal loss in Scarpa's ganglion and the vestibular nucleus complex (VNC), it is not entirely consistent. Whatever structural and functional changes occur in the VNC as a result of aging, either to cause vestibular impairment or to compensate for it, neurochemical changes must underlie them. However, the neurochemical changes that occur in the VNC with aging are poorly understood because the available literature is very limited. This review summarizes and critically evaluates the available evidence relating to the noradrenaline, serotonin, dopamine, glutamate, GABA, glycine, and nitric oxide neurotransmitter systems in the aging VNC. It is concluded that, at present, it is difficult, if not impossible, to relate the neurochemical changes observed to the function of specific VNC neurons and whether the observed changes are the cause of a functional deficit in the VNC or an effect of it. A better understanding of the neurochemical changes that occur during aging may be important for the development of potential drug treatments for age-related vestibular disorders. However, this will require the use of more sophisticated methodology such as in vivo microdialysis with single neuron recording and perhaps new technologies such as optogenetics.

The influence of acoustic and static stimuli on development of inner ear sensory epithelia

Mechanical stimuli affect differentiation of specific cell types in several organs of mouse fetuses that develop without any skeletal musculature. To that end, we employed Myf5(-/-):MyoD(-/-) mouse embryos that completely lack skeletal musculature, and analyzed the development of sensory fields in the inner ear. Amyogenic fetuses lack skeletal muscles that move the chain of three middle ear ossicles which normally transfers sound vibrations. They also cannot tilt their head, which prevents the perception of angular acceleration. While our findings in the spiral organ of Corti are surprisingly normal, our results show that the development of cristae ampullaris, vestibular sensory fields sensitive to the angular acceleration, was the most affected. In cristae, hair cells and supporting cells were significantly smaller in the mutant embryos, but hair cells completely lacked tenascin, while supporting cells were more numerous. In maculae, supporting cells were significantly smaller but more numerous in the mutants. Here, we propose that our finding of a specific type I hair cell absence in the mutant's crista may now be employed in the identification of a profile of genes specific for the lacking cell type.

Spontaneous hair cell regeneration in the mouse utricle following gentamicin ototoxicity

Whereas most epithelial tissues turn-over and regenerate after a traumatic lesion, this restorative ability is diminished in the sensory epithelia of the inner ear; it is absent in the cochlea and exists only in a limited capacity in the vestibular epithelium. The extent of regeneration in vestibular hair cells has been characterized for several mammalian species including guinea pig, rat, and chinchilla, but not yet in mouse. As the fundamental model species for investigating hereditary disease, the mouse can be studied using a wide variety of genetic and molecular tools. To design a mouse model for vestibular hair cell regeneration research, an aminoglycoside-induced method of complete hair cell elimination was developed in our lab and applied to the murine utricle. Loss of utricular hair cells was observed using scanning electron microscopy, and corroborated by a loss of fluorescent signal in utricles from transgenic mice with GFP-positive hair cells. Regenerative capability was characterized at several time points up to six months following insult. Using scanning electron microscopy, we observed that as early as two weeks after insult, a few immature hair cells, demonstrating the characteristic immature morphology indicative of regeneration, could be seen in the utricle. As time progressed, larger numbers of immature hair cells could be seen along with some mature cells resembling surface morphology of type II hair cells. By six months post-lesion, numerous regenerated hair cells were present in the utricle, however, neither their number nor their appearance was normal. A BrdU assay suggested that at least some of the regeneration of mouse vestibular hair cells involved mitosis. Our results demonstrate that the vestibular sensory epithelium in mice can spontaneously regenerate, elucidate the time course of this process, and identify involvement of mitosis in some cases. These data establish a road map of the murine vestibular regenerative process, which can be used for elucidating the molecular events that govern this process.

Structural and functional changes in the cristae ampullares of aged gerbils

We have reported that calretinin and calbindin staining of calyxes in the apical region of the cristae is reduced or absent in old gerbils (>or=35 months) that had normal numbers of hair cells [Kevetter GA, Leonard RB (2002) Decreased expression of calretinin and calbindin in the labyrinth of old gerbils. Brain Res 957:362-365]. Here we examine the ability of primary afferents in aged gerbils to carry a tracer injected into the vestibular nuclear complex to their terminals in the cristae. Calyxes throughout the cristae were well labeled in a young animal with such an injection. In the aged animals, many calyxes were only partially filled or not filled at all. In some cases labeled axons were also missing from the stroma underlying the missing calyxes. There is a strong correspondence between the region where the calyxes were not filled and the absence of calretinin immunostaining. To determine if afferents from the cristae are functionally abnormal, we recorded from their axons and attempted to activate them with natural stimulation. Among afferents that could be activated, we encountered many afferents that had spontaneous activity but could not be modulated with natural stimulation. When tested, the firing rate of these afferents could be modulated with galvanic stimulation, and/or they could be activated by pulsed electrical stimulation. We also encountered afferents that had no spontaneous activity. The presence of these axons was revealed by an injury discharge that could not be modulated with natural stimulation. When tested, these axons could be activated with pulsed electrical stimulation. In some instances we encountered two or more such afferents in a row, an event we have not seen in young animals. We suggest that the simplest explanation for these observations is that calyxes are being lost in old animals.

Regional estimates of hair cells and supporting cells in the human crista ampullaris

Regional estimates of type I and type II vestibular hair cells (HC) and supporting cell (SC) numbers were obtained from the horizontal crista ampullaris by using design-based stereology in human. Cristae were microdissected from temporal bones obtained post-mortem (N=16, age range 26-98 years). Three groups were made according to age: group 1, n=5, ages between 26 and 67 years, average age 51 years; group 2, n=4, average age 84 years; and group 3, n=7, average age 94 years. For group 1, the average total HC number was 8,005+/-214, corresponding to 4,119+/-107 type I HC, 3,886+/-117 type II HC, and 10,274+/-224 SC. The type I:type II HC ratio was 1.06+/-0.01, and HC density was 0.80 cells/100 microm2. For group 2, the average total HC number was 7,074+/-489, corresponding to 3,733+/-212 type I HC, 3,341+/-314 type II HC, and 9,321+/-858 SC. The type I:II HC ratio was 1.12+/-0.06, and HC density was 0.75 cells/100 microm2. For group 3, the average HC number was 6,009+/-327, corresponding to 3,380+/-223 type I HC, 2,628+/-235 type II HC, and 10,185+/-182 SC. The type I:II HC ratio was 1.34+/-0.10, and HC density was 0.63 cells/100 microm2. A significant decline in type I, type II, and total HC number and density was found in groups 2 and 3, with individuals exceeding the average human life span.