Recovery of semicircular canal primary afferent activity in the pigeon after streptomycin ototoxicity

Repair of surviving hair cells in the damaged mouse utricle

Sensory hair cells (HCs) in the utricle are mechanoreceptors required to detect linear acceleration. After damage, the mammalian utricle partially restores the HC population and organ function, although regenerated HCs are primarily type II and immature. Whether native, surviving HCs can repair and contribute to this recovery is unclear. Here, we generated the Pou4f3DTR/+; Atoh1CreERTM/+; Rosa26RtdTomato/+ mouse to fate map HCs prior to ablation. After HC ablation, vestibular evoked potentials were abolished in all animals, with ∼57% later recovering responses. Relative to nonrecovery mice, recovery animals harbored more Atoh1-tdTomato+ surviving HCs. In both groups, surviving HCs displayed markers of both type I and type II subtypes and afferent synapses, despite distorted lamination and morphology. Surviving type II HCs remained innervated in both groups, whereas surviving type I HCs first lacked and later regained calyces in the recovery, but not the nonrecovery, group. Finally, surviving HCs initially displayed immature and subsequently mature-appearing bundles in the recovery group. These results demonstrate that surviving HCs are capable of self-repair and may contribute to the recovery of vestibular function.

Bayesian optimal adaptation explains age-related human sensorimotor changes

The brain uses information from different sensory systems to guide motor behavior, and aging is associated with simultaneous decline in the quality of sensory information provided to the brain and deterioration in motor control. Correlations between age-dependent decline in sensory anatomical structures and behavior have been demonstrated in many sensorimotor systems, and it has recently been suggested that a Bayesian framework could explain these relationships. Here we show that age-dependent changes in a human sensorimotor reflex, the vestibuloocular reflex, are explained by a Bayesian optimal adaptation in the brain occurring in response to death of motion-sensing hair cells. Specifically, we found that the temporal dynamics of the reflex as a function of age emerge from ( r = 0.93, P < 0.001) a Kalman filter model that determines the optimal behavioral output when the sensory signal-to-noise characteristics are degraded by death of the transducers. These findings demonstrate that the aging brain is capable of generating the ideal and statistically optimal behavioral response when provided with deteriorating sensory information. While the Bayesian framework has been shown to be a general neural principle for multimodal sensory integration and dynamic sensory estimation, these findings provide evidence of longitudinal Bayesian processing over the human life span. These results illuminate how the aging brain strives to optimize motor behavior when faced with deterioration in the peripheral and central nervous systems and have implications in the field of vestibular and balance disorders, as they will likely provide guidance for physical therapy and for prosthetic aids that aim to reduce falls in the elderly. NEW & NOTEWORTHY We showed that age-dependent changes in the vestibuloocular reflex are explained by a Bayesian optimal adaptation in the brain that occurs in response to age-dependent sensory anatomical changes. This demonstrates that the brain can longitudinally respond to age-related sensory loss in an ideal and statistically optimal way. This has implications for understanding and treating vestibular disorders caused by aging and provides insight into the structure-function relationship during aging.

Variation in response dynamics of regular and irregular vestibular-nerve afferents during sinusoidal head rotations and currents in the chinchilla

In mammals, vestibular-nerve afferents that innervate only type I hair cells (calyx-only afferents) respond nearly in phase with head acceleration for high-frequency motion, whereas afferents that innervate both type I and type II (dimorphic) or only type II (bouton-only) hair cells respond more in phase with head velocity. Afferents that exhibit irregular background discharge rates have a larger phase lead re-head velocity than those that fire more regularly. The goal of this study was to investigate the cause of the variation in phase lead between regular and irregular afferents at high-frequency head rotations. Under the assumption that externally applied galvanic currents act directly on the nerve, we derived a transfer function describing the dynamics of a semicircular canal and its hair cells through comparison of responses to sinusoidally modulated head velocity and currents. Responses of all afferents were fit well with a transfer function with one zero (lead term). Best-fit lead terms describing responses to current for each group of afferents were similar to the lead term describing responses to head velocity for regular afferents (0.006 s + 1). This finding indicated that the pre-synaptic and synaptic inputs to regular afferents were likely to be pure velocity transducers. However, the variation in phase lead between regular and irregular afferents could not be explained solely by the ratio of type I to II hair cells (Baird et al 1988), suggesting that the variation was caused by a combination of pre- (type of hair cell) and post-synaptic properties.

Regeneration of vestibular horizontal semicircular canal afferents in pigeons

Spontaneous regeneration of vestibular and auditory receptors and their innervating afferents in birds, reptiles, and amphibians are well known. Here, we produced a complete vestibular receptor loss and epithelial denervation using an ototoxic agent (streptomycin), after which we quantitatively characterized the afferent innervation of the horizontal semicircular canals following completed regeneration. We found that calyx, dimorph, and bouton afferents all regenerate in a manner the recapitulates the epithelial topography of normal birds, but over a slow time course. Similar to previous findings in the vestibular otolith maculae, regeneration occurs according to a three-stage temporal sequence. Bouton afferents regenerate during the first month of regeneration, followed by calyceal-bearing afferents in the second and third months. Calyx afferents were the last to regenerate in the final stage of recovery after 3 mo. We also found that regenerated afferents exhibited terminal morphologies that are significantly smaller, less complex, and innervate fewer receptor cells over smaller epithelial areas than those that develop through normative morphogenesis. These structural fiber changes in afferent innervation correlate to alterations in gaze responses during regeneration, although the exact underlying mechanisms responsible for behavioral changes remain unknown. Plasticity in central vestibular neurons processing motion information seem to be required to explain the observed morphologic and response adaptations observed in regenerating vestibular systems.

Resting discharge patterns of macular primary afferents in otoconia-deficient mice

Vestibular primary afferents in the normal mammal are spontaneously active. The consensus hypothesis states that such discharge patterns are independent of stimulation and depend instead on excitation by vestibular hair cells due to background release of synaptic neurotransmitter. In the case of otoconial sensory receptors, it is difficult to test the independence of resting discharge from natural tonic stimulation by gravity. We examined this question by studying discharge patterns of single vestibular primary afferent neurons in the absence of gravity stimulation using two mutant strains of mice that lack otoconia (OTO-; head tilt, het-Nox3, and tilted, tlt-Otop1). Our findings demonstrated that macular primary afferent neurons exhibit robust resting discharge activity in OTO- mice. Spike interval coefficient of variation (CV = SD/mean spike interval) values reflected both regular and irregular discharge patterns in OTO- mice, and the range of values for rate-normalized CV was similar to mice and other mammals with intact otoconia although there were proportionately fewer irregular fibers. Mean discharge rates were slightly higher in otoconia-deficient strains even after accounting for proportionately fewer irregular fibers [OTO- = 75.4 +/- 31.1(113) vs OTO+ = 68.1 +/- 28.5(143) in sp/s]. These results confirm the hypothesis that resting activity in macular primary afferents occurs in the absence of ambient stimulation. The robust discharge rates are interesting in that they may reflect the presence of a functionally 'up-regulated' tonic excitatory process in the absence of natural sensory stimulation.

Recovery of gaze stability during vestibular regeneration

Many motion related behaviors, such as gaze stabilization, balance, orientation, and navigation largely depend on a properly functioning vestibular system. After vestibular insult, many of these responses are compromised but can return during the regeneration of vestibular receptors and afferents as is known to occur in birds, reptiles, and amphibians. Here we characterize gaze stability in pigeons to rotational motion during regeneration after complete bilateral vestibular loss via an ototoxic antibiotic. Immediate postlesion effects included severe head oscillations, postural ataxia, and total lack of gaze control. We found that these abnormal behaviors gradually subsided, and gaze stability slowly returned to normal function according to a temporal sequence that lasted several months. We also found that the dynamic recovery of gaze function during regeneration was not homogeneous for all types of motion. Instead high-frequency motion stability was first achieved, followed much later by slow movement stability. In addition, we found that initial gaze stability was established using almost exclusive head-response components with little eye-movement contribution. However, that trend reversed as recovery progressed so that when gaze stability was complete, the eye component had increased and the head response had decreased to levels significantly different from that observed in normal birds. This was true even though the head-fixed VOR response recovered normally. Recovery of gaze stability coincided well with the three stage temporal sequence of morphologic regeneration previously described by our laboratory.

Expression of GATA3 and tenascin in the avian vestibular maculae: normative patterns and changes during sensory regeneration

Sensory receptors in the vestibular organs of birds can regenerate after ototoxic injury. Notably, this regenerative process leads to the restoration of the correct patterning of hair cell phenotype and afferent innervation within the repaired sensory epithelium. The molecular signals that specify cell phenotype and regulate neuronal guidance during sensory regeneration are not known, but they are likely to be similar to the signals that direct these processes during embryonic development. The present study examined the recovery of hair cell phenotype during regeneration in the avian utricle, a vestibular organ that detects linear acceleration and head orientation. First, we show that Type I hair cells in the avian vestibular maculae are immunoreactive for the extracellular matrix molecule tenascin and that treatment with the ototoxic antibiotic streptomycin results in a nearly complete elimination of tenascin immunoreactivity. Cells that express tenascin begin to recover after about 2 weeks and are then contacted by calyx terminals of vestibular neurons. In addition, our previous work had shown that the zinc finger transcription factor GATA3 is uniquely expressed within the striolar reversal zone of the utricle (Hawkins et al. [2003] Hum Mol Genet 12:1261-1272), and we show here that this regionalized expression of GATA3 is maintained after severe hair cell lesions and after transplantation of the sensory epithelium onto a chemically defined substrate. In contrast, the expression of three other supporting cell markers--alpha- and beta-tectorin and SCA--is reduced following ototoxic injury. These observations suggest that GATA3 expression may maintain positional information in the maculae during sensory regeneration.

Emergence of Action Potential Generation and Synaptic Transmission in Vestibular Nucleus Neurons

Principal cells of the chick tangential nucleus are vestibular nucleus neurons in the hindbrain. Although detailed information is available on the morphogenesis of principal cells and synaptogenesis of primary vestibular fibers, this is the first study of their early functional development, when vestibular terminals emerge at embryonic days 10 and 13 (E10 and E13). At E10, 60% of principal cells generated spikes on depolarization, whereas 50% exhibited excitatory postsynaptic currents (EPSCs) on vestibular-nerve stimulation. The frequency was 0.2 Hz for glutamatergic spontaneous EPSCs (sEPSCs) at −60 mV, and 0.6 Hz for spontaneous inhibitory postsynaptic current (sIPSC) at +10 mV and completely GABAergic. All of these synaptic events were TTX-insensitive, miniature events. At E13, 50% of principal cells generated spikes on depolarization and 82% exhibited EPSCs on vestibular-nerve stimulation. The frequency was 0.7 Hz for sEPSCs at −60 mV, and 0.8 Hz for sIPSCs at +10 mV. Most principal cells had sIPSCs composed of both GABAergic (75%) and glycinergic (25%) events, but a few cells had only GABAergic sIPSCs. TTX decreased the frequency of EPSCs by 12%, and the IPSCs by 17%. In summary, at E10, some principal cells generated immature spikes on depolarization and EPSCs on vestibular-nerve stimulation. At E10, GABAergic events predominated, AMPA events had low frequencies, and glycinergic activity was absent. By E13, glycinergic events first appeared. This data were compared systematically to that obtained from the late-term embryo and hatchling to reveal the long-term sequence of changes in synaptic events and excitability and offer a broader understanding of how the vestibular system is assembled during development.

Regeneration of vestibular otolith afferents after ototoxic damage

Regeneration of receptor cells and subsequent functional recovery after damage in the auditory and vestibular systems of many vertebrates is well known. Spontaneous regeneration of mammalian hair cells does not occur. However, recent approaches provide hope for similar restoration of hearing and balance in humans after loss. Newly regenerated hair cells receive afferent terminal contacts, yet nothing is known about how reinnervation progresses or whether regenerated afferents finally develop normal termination fields. We hypothesized that neural regeneration in the vestibular otolith system would recapitulate the topographic phenotype of afferent innervation so characteristic of normal development. We used an ototoxic agent to produce complete vestibular receptor cell loss and epithelial denervation, and then quantitatively examined afferent regeneration at discrete periods up to 1 year in otolith maculas. Here, we report that bouton, dimorph, and calyx afferents all regenerate slowly at different time epochs, through a progressive temporal sequence. Furthermore, our data suggest that both the hair cells and their innervating afferents transdifferentiate from an early form into more advanced forms during regeneration. Finally, we show that regeneration remarkably recapitulates the topographic organization of afferent macular innervation, comparable with that developed through normative morphogenesis. However, we also show that regenerated terminal morphologies were significantly less complex than normal fibers. Whether these structural fiber changes lead to alterations in afferent responsiveness is unknown. If true, adaptive plasticity in the central neural processing of motion information would be necessitated, because it is known that many vestibular-related behaviors fully recover during regeneration.

Dynamics and the single spike

Responses of vestibular primary afferent neurons to head rotation exhibit fractional-order dynamics. As a consequence, the head tends to be in a localized region of its state-space at spike times of a particular neuron during arbitrary head movements, and single spikes can be interpreted as state measurements. We are developing a model of neural computations underlying trajectory prediction and control tasks, based on this experimental observation. This is a step toward a formal neural calculus in which single spikes are modeled realistically as the operands of neural computation.

Posture, head stability, and orientation recovery during vestibular regeneration in pigeons

Compensatory behavior such as oculomotor, gaze, and postural responses that occur during movement largely depend upon a functioning vestibular system. In the present study, the initial loss and subsequent recovery of postural and head stability in pigeons undergoing vestibular regeneration were examined. Adult pigeons were trained to manipulate a straight run chamber to peck an illuminated key for fluid reward. Six behavioral measures assessing performance, posture, and head stability were quantified. These included run latency, steps (walking), path negotiation (lane changes), gaze saccades, head bobs, and head shakes. Once normative values were obtained for four birds, complete lesion of all receptor cells and denervation of the epithelia in the vestibular endorgans were produced using a single intralabyrinthine application of streptomycin sulfate. Each bird was then tested at specific times during regeneration and the same behavioral measures examined. At 7 days post-streptomycin treatment (PST), all birds exhibited severe postural and head instability, with tremors, head shakes, staggering, and circling predominating. No normal trial runs, walking, gaze saccades, or head bobs were present. Many of these dysfunctions persisted through 3-4 weeks PST. Gradually, tremor and head shakes diminished and were replaced with an increasing number of normal head bobs during steps and gaze saccades. Beginning at 4 weeks PST, but largely inaccurate, was the observed initiation of directed steps, less staggering, and some successful path negotiation. As regeneration progressed, spatial orientation and navigation ability increased and, by 49 days PST, most trials were successful. By 70 days PST, all birds had recovered to pretreatment levels. Thus, it was observed that ataxia must subside, coincident with normalized head and postural stability prior to the recovery of spatial orientation and path navigation recovery. Parallels in recovery were drawn to hair cell regeneration and afferent responsiveness, as inferred from present results and those in other investigations.

Return of potassium ion channels in regenerated hair cells: possible pathways and the role of intracellular calcium signaling

Recent electrophysiological studies in pigeon have demonstrated that potassium channels are completely functional in regenerated type II hair cells at 21 days post-treatment (PT) with ototoxic doses of streptomycin. The currents return in the order they appear during development. The mixture of ionic currents in a regenerated type II hair cell in a particular region of the neuroepithelium is the same as in its ancestor in that region. The return of currents in regenerated type I hair cells is more complicated. The dominant conductance gKI is not present until after 70 days PT. Before 70 days, the ionic currents in type I hair cells resemble those of regenerated type II hair cells, suggesting that the ionic currents in type II hair cells might be precursors of the ionic currents in regenerated type I hair cells. New data show that at one year PT, the kinetics and drug sensitivity of the dominant K+ conductance in type I hair cells are identical to gKI. Supporting cells, believed to be the precursors of regenerated type II hair cells, have effectively no voltage-gated outward potassium channels, suggesting that regenerated type II hair cells must develop these channels de novo. The next step is to understand the mechanisms by which the potassium channel protein is synthesized, migrates through the cytosol, and is inserted into the plasmalemma of regenerating hair cells. These mechanisms are unknown. We propose that intracellular calcium is involved in this process, as well as in the differentiation, proliferation, and gene regulation of precursor cells fated to become hair cells.

Hair cell and supporting cell density and distribution in the normal and regenerating posterior crista ampullaris of the pigeon

The numbers of supporting cells and the numbers and types of hair cells in three distinct longitudinal regions through the posterior canal cristae of control and streptomycin-treated pigeons were determined using stereological techniques. For control cristae, type I (3758) and type II (3517) hair cells occurred in approximately equal numbers. However, the proportions varied in different longitudinal zones: Zone I (peripheral region) had four times more type II hair cells (2083) than type I (483), while Zone II (intermediate region) had almost seven times more type I (2517) than type II (367) hair cells and Zone III (central region) had relatively equal numbers of type I (758) and type II (1067) hair cells. Novel findings included the following: (1) immediately after the post-injection sequence (PIS) of streptomycin, there was a significant reduction in both hair cells (-93%) and supporting cells (-45%); (2) by 70 days after the PIS, the population of type I hair cells returned to control values (however, the normal complement of complex calyces took 1 year to recover); (3) during the first 143 days after the PIS, the number of type I and type II hair cells across all zones returned linearly with about the same slope (46 and 43 cells per day, respectively), although the rate of return differed significantly in different zones; (4) there was a massive overproduction of hair cells (+150%) and supporting cells (+120%) during the first 5 months of recovery; and (5) during the first year after the PIS, both hair cells and supporting cells increased and their increases in numbers were correlated (r = 0.88, P < 0.01). Knowledge of the sequence and numbers of regenerating hair cells may help elucidate common modes of cell survival, recovery, and compensation from neural insult.

Spontaneous activity in the statoacoustic ganglion of the chicken embryo

Statoacoustic ganglion cells in the mature bird include neurons that are responsive to sound (auditory) and those that are not (nonauditory). Those that are nonauditory have been shown to innervate an otolith organ, the macula lagena, whereas auditory neurons innervate the basilar papilla. In the present study, single-unit recordings of statoacoustic ganglion cells were made in embryonic (E19, mean = 19.2 days of incubation) and hatchling (P6-P14, mean = 8.6 days posthatch) chickens. Spontaneous activity from the two age groups was compared with developmental changes. Activity was evaluated for 47 auditory, 11 nonauditory, and 6 undefined eighth nerve neurons in embryos and 29 auditory, 26 nonauditory, and 1 undefined neurons in hatchlings. For auditory neurons, spontaneous activity displayed an irregular pattern [discharge interval coefficient of variation (CV) was >0.5, mean CV for embryos was 1.46 +/- 0.58 and for hatchlings was 1.02 +/- 0.25; means +/- SD]. Embryonic discharge rates ranged from 0.05 to 97.6 spikes per second (sp/s) for all neurons (mean 18.6 +/- 16.9 sp/s). Hatchling spontaneous rates ranged from 1.2 to 185.2 sp/s (mean 66.5 +/- 39.6 sp/s). Discharge rates were significantly higher for hatchlings (P < 0.001). Many embryonic auditory neurons displayed long silent periods between irregular bursts of neural activity, a feature not seen posthatch. All regular bursting discharge patterns were correlated with heart rate in both embryos and hatchlings. Preferred intervals were visible in the time interval histograms (TIHs) of only one embryonic neuron in contrast to 55% of the neurons in posthatch animals. Generally, the embryonic auditory TIH displayed a modified quasi-Poisson distribution. Nonauditory units generally displayed regular (CV <0.5) or irregular (CV >0.5) activity and Gaussian and modified-Gaussian TIHs. Long silent periods or bursting patterns were not a characteristic of embryonic nonauditory neurons. CV varied systematically as a function of discharge rate in nonauditory but not auditory primary afferents. Minimum spike intervals (dead time) and interval modes for auditory neurons were longer in embryos (dead time: embryos 2.88 +/- 6.85 ms; hatchlings 1.50 +/- 1.76 ms; modal intervals: embryo 10.09 +/- 22.50 ms, hatchling 3.54 +/- 3.29 ms). The results show that significant developmental changes occur in spontaneous activity between E19 and posthatch. It is likely that both presynaptic and postsynaptic changes in the neuroepithelium contribute to maturational refinements during this period of development.