The Gunn rat: an experimental model for central deafness

Bilirubin-Induced Transcriptomic Imprinting in Neonatal Hyperbilirubinemia

Recent findings indicated aberrant epigenetic control of the central nervous system (CNS) development in hyperbilirubinemic Gunn rats as an additional cause of cerebellar hypoplasia, the landmark of bilirubin neurotoxicity in rodents. Because the symptoms in severely hyperbilirubinemic human neonates suggest other regions as privileged targets of bilirubin neurotoxicity, we expanded the study of the potential impact of bilirubin on the control of postnatal brain development to regions correlating with human symptoms. Histology, transcriptomic, gene correlation, and behavioral studies were performed. The histology revealed widespread perturbation 9 days after birth, restoring in adulthood. At the genetic level, regional differences were noticed. Bilirubin affected synaptogenesis, repair, differentiation, energy, extracellular matrix development, etc., with transient alterations in the hippocampus (memory, learning, and cognition) and inferior colliculi (auditory functions) but permanent changes in the parietal cortex. Behavioral tests confirmed the presence of a permanent motor disability. The data correlate well both with the clinic description of neonatal bilirubin-induced neurotoxicity, as well as with the neurologic syndromes reported in adults that suffered neonatal hyperbilirubinemia. The results pave the way for better deciphering the neurotoxic features of bilirubin and evaluating deeply the efficacy of new therapeutic approaches against the acute and long-lasting sequels of bilirubin neurotoxicity.

Morphological and Morphometrical Aspects of the Auditory Ossicles in the European Badger (Meles Meles)

Simple Summary

The little-described morphology of the ear ossicles in the badger provides some interesting morphological features alongside some metrical data. For the malleus, we notice the standard framing into the known shape, with the mentioned presence at the level of the column of all three processes (lateral, rostral and medial), from which the rostral one is the most developed. The malleal manubrium is long and triangularly shaped on a cross-section. For the incus we notice the overall shape of a biradicular molar with the existence of the two crura in acute angulation, while the long crus is continuing with the lenticular process. The presence of a bony blade that links to the lenticular process is also noted. For the stapes, the almost equal two crura and the quite round intercrural foramen is described.

Abstract

Given the scarce morphological data regarding the middle ear anatomy of this species, the paper attempts to describe the morphological and morphometrical data of the auditory ossicles in the badger. The study was performed using the standard morphological investigations and provides a complete morphological description of the ossicular assembly (malleus, incus and stapes) with some comparative features and attempts to provide a complete set of standardized metrical data for each ossicle. A more-detailed attempt to compare some functional aspects in the light of combined metrical ratios is also implied.

Diffusion Tensor Imaging of Auditory Pathway in Patients With Crigler-Najjar Syndrome Type I: Correlation With Auditory Brainstem Response

AIM

To evaluate the role of diffusion tensor imaging of the auditory pathway in patients with Crigler Najjar syndrome type I and its relation to auditory brainstem response.

METHODS

Prospective study was done including 12 patients with Crigler Najjar syndrome type I and 10 age- and sex-matched controls that underwent diffusion tensor imaging of brain. Mean diffusivity and fractional anisotropy at 4 regions of the brain and brainstem on each side were measured and correlated with the results of auditory brainstem response for patients.

RESULTS

There was significantly higher mean diffusivity of cochlear nucleus, superior olivary nucleus, inferior colliculus, and auditory cortex of patients versus controls on both sides for all regions (P = .001). The fractional anisotropy of cochlear nucleus, superior olivary nucleus, inferior colliculus, and auditory cortex of patients versus controls was significantly lower, with P values of, respectively, .001, .001, .003, and .001 on the right side and .001, .001, .003, and .001 on left side, respectively. Also, a negative correlation was found between the maximum bilirubin level and fractional anisotropy of the left superior olivary nucleus and inferior colliculus of both sides. A positive correlation was found between the mean diffusivity and auditory brainstem response wave latency of the right inferior colliculus and left cochlear nucleus. The fractional anisotropy and auditory brainstem response wave latency of the right superior olivary nucleus, left cochlear nucleus, and inferior colliculus of both sides were negatively correlated.

CONCLUSION

Diffusion tensor imaging can detect microstructural changes in the auditory pathway in Crigler Najjar syndrome type I that can be correlated with auditory brainstem response.

Is auditory neuropathy an appropriate term? A systematic literature review on its aetiology and pathogenesis

To clarify the aetio-pathogenesis of Auditory Neuropathy Spectrum Disorder (ANSD), a total of 845 papers were divided into four categories: Review, Audiology, Treatment and Aetiology. Aetiology was the topic analysed categorising papers as: Genetics, Histopathology, Imaging and Medical diseases. Isolated ANs were in relation to Otoferlin, Pejvakin and DIAPH3 deficiency, and the syndromes were mainly Charcot Marie Tooth, Friedreich Ataxia, mitochondrial disorders and those associated with optic neuropathies. In histopathology papers, important information was available from analyses on human premature newborns and on some syndromic neuropathies. From cochlear dysmorphism to cerebral tumours associated with ANs, these are described in what is identified as the Imaging area. Finally, the prevalent clinical pathology was bilirubinopathy, followed by diabetes. In conclusion, AN/ANSDs do not refer to a clear pathological condition, but to an instrumental pattern without any evidence of auditory nerve involvement, except in a few conditions. The terms AN/ANSD are misleading and should be avoided, including terms such as “synaptopathy” or “dis-synchrony”.

Auditory brainstem response in preterm infants with bilirubin encephalopathy

AIM

To clarify auditory brainstem response (ABR) in preterm infants with bilirubin encephalopathy and the relationships between ABR and clinical variables.

METHOD

We retrospectively reviewed the ABR waveforms of 56 preterm infants with BE and graded them as "no response", "abnormal interwave separation", or "normal". Patient backgrounds, the peak total bilirubin level, the bilirubin/albumin ratio, verbal communication ability, and newborn hearing screening test results from an automated ABR evaluation had been collected during an earlier nationwide survey.

RESULTS

The frequency of abnormal ABR findings decreased with age. Verbal communication tended to be poorer in patients with more severe ABR abnormalities. ABR findings improved in 7 of 29 infants with available serial ABR data. Both gestational age and the peak total bilirubin level were relatively lower in patients with than in those without improved ABR findings. Newborn hearing screening using automated ABR evaluation yielded data consistent with manual ABR findings in 16 of 20 patients who underwent both examinations.

CONCLUSIONS

ABR abnormalities in preterm infants with bilirubin encephalopathy may improve over time, especially in those with a lower gestational age and peak total bilirubin level. Newborn hearing screening using automated ABR may fail to detect abnormalities in some infants.

Analysis on the MRI and BAEP Results of Neonatal Brain With Different Levels of Bilirubin

BACKGROUND

To explore whether there is abnormality of neonatal brains' MRI and BAEP with different bilirubin levels, and to provide an objective basis for early diagnosis on the bilirubin induced subclinical damage on brains.

METHODS

To retrospectively analyze the clinical data of 103 neonatal patients, to conduct routine brain MRI examination and BAEP testing, and to analyze BAEP and MRI image results of the neonatal patients, who were divided into three groups based on the levels of total serum bilirubin concentration (TSB): 16 cases in mild group (TSB: 0.0-229.0 ěmol/L), 49 cases in moderate group (TSB: 229.0-342.0 ěmol/L), and 38 cases in severe group (TSB ≥ 342.0 ěmol/L).

RESULTS

We found the following: A. Comparison of the bilirubin value of the different group: The bilirubin value of the mild group is 171.99 ± 33.50 ěmol/L, the moderate group is 293.98 ± 32.09 ěmol/L, and the severe group is 375.59 ± 34.25 ěmol/L. The comparison of bilirubin values of the three groups of neonates (p < 0.01) indicates the difference is statistically significant (p < 0.01). B. The weight value of the <2,500 g group is 2.04 ± 0.21 and the ≥2,500 g group is 3.39 ± 0.46; the weight comparison of the two groups indicates that the difference is statistically significant (p < 0.01). C. Comparison of the abnormal MRI of the different groups: The brain MRI result's abnormal ratio of the mild group is 31.25%, the moderate group is 16.33%, and the severe group is 21.05%, but the comparison of brain MRI results of the three neonates groups indicates that the difference is not statistically significant (p > 0.05). D. Comparison of abnormal MRI signal values of globus pallidus on T1WI in different groups: 1. The comparison of normal group signal values with that of mild group (p < 0.05), with that of moderate group, and with that of severe group (p < 0.01) indicates that the difference is statistically significant.

CONCLUSION

At low level of bilirubin, central nervous system damage may also occur and can be detected as abnormality by MRI and BAEP. Meanwhile, MRI and BAEP can also provide early abnormal information for the judgment of central nervous system damage of the children with NHB who have no acute bilirubin encephalopathy (ABE) clinical features, and provide clues for early treatment and early intervention.

Clinicopathological Spectrum of Bilirubin Encephalopathy/Kernicterus

Bilirubin encephalopathy/kernicterus is relatively rare, but continues to occur despite universal newborn screening. What is more interesting is the spectrum of clinical and even neuropathological findings that have been reported in the literature to be associated with bilirubin encephalopathy and kernicterus. In this review, the authors discuss the array of clinicopathological findings reported in the context of bilirubin encephalopathy and kernicterus, as well as the types of diagnostic testing used in patients suspected of having bilirubin encephalopathy or kernicterus. The authors aim to raise the awareness of these features among both pediatric neurologists and neuropathologists.

Bilirubin-Induced Audiologic Injury in Preterm Infants

Although hyperbilirubinemia is extremely common among neonates and is usually mild and transient, it sometimes leads to bilirubin-induced neurologic damage (BIND). The auditory pathway is highly sensitive to the effects of elevated total serum/plasma bilirubin (TB) levels, with damage manifesting clinically as auditory neuropathy spectrum disorder. Compared to full-term neonates, preterm neonates are more susceptible to BIND and suffer adverse effects at lower TB levels with worse long-term outcomes. Furthermore, although standardized guidelines for management of hyperbilirubinemia exist for term and late preterm neonates, similar guidelines for neonates less than 35 weeks gestational age are limited.

Auditory neuropathy--neural and synaptic mechanisms

Sensorineural hearing impairment is the most common form of hearing loss, and encompasses pathologies of the cochlea and the auditory nerve. Hearing impairment caused by abnormal neural encoding of sound stimuli despite preservation of sensory transduction and amplification by outer hair cells is known as 'auditory neuropathy'. This term was originally coined for a specific type of hearing impairment affecting speech comprehension beyond changes in audibility: patients with this condition report that they "can hear but cannot understand". This type of hearing impairment can be caused by damage to the sensory inner hair cells (IHCs), IHC ribbon synapses or spiral ganglion neurons. Human genetic and physiological studies, as well as research on animal models, have recently shown that disrupted IHC ribbon synapse function--resulting from genetic alterations that affect presynaptic glutamate loading of synaptic vesicles, Ca(2+) influx, or synaptic vesicle exocytosis--leads to hearing impairment termed 'auditory synaptopathy'. Moreover, animal studies have demonstrated that sound overexposure causes excitotoxic loss of IHC ribbon synapses. This mechanism probably contributes to hearing disorders caused by noise exposure or age-related hearing loss. This Review provides an update on recently elucidated sensory, synaptic and neural mechanisms of hearing impairment, their corresponding clinical findings, and discusses current rehabilitation strategies as well as future therapies.

Audiologic impairment associated with bilirubin-induced neurologic damage

Hyperbilirubinemia occurs commonly in neonates and is usually mild and transient, with no long-lasting sequelae. However, bilirubin-induced neurologic damage may occur in some infants. The auditory pathway is the most sensitive part of the central nervous system to bilirubin-induced toxicity, and permanent sequelae may result from only moderately elevated total serum/plasma bilirubin levels. The damage to the auditory system occurs primarily within the brainstem and cranial nerve VIII, and manifests clinically as auditory neuropathy spectrum disorder.

Auditory impairment in infants at risk for bilirubin-induced neurologic dysfunction

Classical and subtypes of kernicterus associated with bilirubin toxicity can be differentiated in part with physiological auditory measures that include auditory-evoked potentials and measures of cochlear integrity. The combination of these auditory measures suggests that bilirubin exposure results in auditory system damage initially at the level of the brainstem, progressing to the level of the VIII cranial nerve and then to greater neural centers. There is no evidence of neural damage at the level of the cochlea. Auditory neural damage from bilirubin toxicity ranges from neural timing deficits, including neural firing delays and dyssynchrony, to neural response reduction and even elimination of auditory neural responses. This condition is comprehensively described as auditory neuropathy spectrum disorder. Independent measures of cochlear function and auditory neural function up to the level of the brainstem can effectively diagnose auditory neural damage resulting from bilirubin neurotoxicity. Intervention, including cochlear implants can be effective.

Acute hyperbilirubinaemia induces presynaptic neurodegeneration at a central glutamatergic synapse

There is a well-established link between hyperbilirubinaemia and hearing loss in paediatrics, but the cellular mechanisms have not been elucidated. Here we used the Gunn rat model of hyperbilirubinaemia to investigate bilirubin-induced hearing loss. In vivo auditory brainstem responses revealed that Gunn rats have severe auditory deficits within 18 h of exposure to high bilirubin levels. Using an in vitro preparation of the auditory brainstem from these rats, extracellular multi-electrode array recording from the medial nucleus of the trapezoid body (MNTB) showed longer latency and decreased amplitude of evoked field potentials following bilirubin exposure, suggestive of transmission failure at this synaptic relay. Whole-cell patch-clamp recordings confirmed that the electrophysiological properties of the postsynaptic MNTB neurons were unaffected by bilirubin, with no change in action potential waveforms or current-voltage relationships. However, stimulation of the trapezoid body was unable to elicit large calyceal EPSCs in MNTB neurons of hyperbilirubinaemic rats, indicative of damage at a presynaptic site. Multi-photon imaging of anterograde-labelled calyceal projections revealed axonal staining and presynaptic profiles around MNTB principal neuron somata. Following induction of hyperbilirubinaemia the giant synapses were largely destroyed. Electron microscopy confirmed loss of presynaptic calyceal terminals and supported the electrophysiological evidence for healthy postsynaptic neurons. MNTB neurons express high levels of neuronal nitric oxide synthase (nNOS). Nitric oxide has been implicated in mechanisms of bilirubin toxicity elsewhere in the brain, and antagonism of nNOS by 7-nitroindazole protected hearing during bilirubin exposure. We conclude that bilirubin-induced deafness is caused by degeneration of excitatory synaptic terminals in the auditory brainstem.

Hyperbilirubinemia and kernicterus

This article describes new findings concerning the basic science of bilirubin neurotoxicity, new considerations of the definition of clinical kernicterus, and new and useful tools to diagnose kernicterus in older children, and discusses treatments for kernicterus beyond the newborn period and why proper diagnosis is important.

Bilirubin toxicity in the developing nervous system

Bilirubin toxicity remains a significant problem despite recent advances in the care of jaundiced (hyperbilirubinemic) neonates. A recent surge in reported cases of classical kernicterus, due in part to earlier hospital discharge and relaxation of treatment criteria for hyperbilirubinemia, and new reports of hyperbilirubinemia-induced auditory dysfunction using evoked potential based infant testing and hearing screening, underscore the need to better understand how hyperbilirubinemia causes brain damage in some infants, especially because the damage is preventable. Recent progress in understanding bilirubin binding and neurotoxicity resulting from unbound or "free" unconjugated bilirubin, how bilirubin affects the central nervous system in vivo and in vitro, and the use of new clinical tools in neonates, for example magnetic resonance imaging revealing bilateral lesions in globus pallidus and subthalamus, and abnormal brainstem auditory evoked potentials with normal inner ear function, may lead to improved detection and prevention of neurologic dysfunction and damage from bilirubin. Finally, the concern is raised that partial or isolated neurologic sequelae, for example auditory neuropathy and other central auditory processing disorders, may result from excessive amount and duration of exposure to free, unconjugated bilirubin at different stages of neurodevelopment.

Somatosensory and brainstem auditory evoked potentials in the Gunn rat model of acute bilirubin neurotoxicity

Brainstem auditory evoked potentials (BAEPs) are a sensitive indicator of bilirubin neurotoxicity. Somatosensory evoked potentials (SEPs) have been proposed as another measure of toxicity, though the lemniscal pathways that generate the SEP are not involved in kernicterus. In 16 to 17-d-old jaundiced (jj) Gunn rats, serial BAEPs and SEPs were obtained up to 8 h after acute bilirubin toxicity. jjs were injected with 150 mg/kg sulfadimethoxine to displace bilirubin into brain tissue (n = 8); littermate controls were jjs given saline (n = 4) and nonjaundiced given sulfadimethoxine or saline (n = 7). After anesthesia, BAEP and SEP recordings were obtained at baseline and serially after injection. SEPs to median nerve stimulation were recorded from surface electrodes over the brachial plexus (Erb's) and contralateral parietal cortex, and subtracted to obtain central conduction time (CCT). There were no statistically significant different baseline values between groups. Baseline BAEP I, I-II, and I-III were 1.22 +/- 0.13, 1.11 +/- 0.12, and 2.10 +/- 0.15 ms, and SEP Erb's and CCT were 1.48 +/- 0.20 and 5.59 +/- 0.50 ms, respectively (n = 19). At 6.8 +/- 1.5 h after injection BAEP I, I-II, and I-III increased 0.10 +/- 0.08, 0.18 +/- 0.17, and 0.56 +/- 0.33 ms over baseline, respectively (p = 0.005, 0.01, and 0.001, respectively, paired, 1-tailed t-tests), in experimental but not control groups. SEP Erb's decreased slightly, -0.06 +/- 0.04 ms in experimental and -0.08 +/- 0.08 ms in control groups, while CCT did not change significantly. BAEPs were completely abolished in two jjs with no SEP changes. When injection of sulfonamide induced significant peripheral and central BAEP abnormalities in jaundiced rats, no SEP abnormalities occurred. SEPs assess proprioception but not other somatosensory function or sensory integration. The results demonstrate the selectivity of acute bilirubin toxicity for the auditory nervous system.

Immunohistochemical localization of calcium-binding proteins in the brainstem vestibular nuclei of the jaundiced Gunn rat

Vestibular gaze and postural abnormalities are major sequelae of neonatal hyperbilirubinemia. The sites and cellular effects of bilirubin toxicity in the brainstem vestibular pathway are not easily detected. Since altered intracellular calcium homeostasis may play a role in neuronal cell death, we hypothesized that altered expression of calcium-binding proteins may occur in brainstem vestibular nuclei of the classic animal model of bilirubin neurotoxicity. The expression of the calcium-binding proteins calbindin-D28k and parvalbumin in the brainstem vestibular pathways and cerebellum of homozygous recessive jaundiced (jj) Gunn rats was examined by light microscopy and immunohistochemistry at 18 days postnatally and compared to the findings obtained from age-matched non-jaundiced heterozygous (Nj) littermate controls. Jaundiced animals exhibited decreased parvalbumin immunoreactivity specifically in synaptic inputs to superior, medial, and inferior vestibular nuclei, and to oculomotor and trochlear nuclei, whereas the neurons retained their normal immunoreactivity. Jaundiced animals also demonstrated a decrease in calbindin expression in the lateral vestibular nuclei and a paucity of calbindin-immunoreactive synaptic endings on the somata of Deiters' neurons. The involved regions are related to the control of the vestibulo-ocular and vestibulospinal reflexes. Decreased expression of calcium-binding proteins in brainstem vestibular neurons may relate to the vestibulo-ocular and vestibulospinal dysfunction seen with clinical kernicterus, and may provide a sensitive new way to assess bilirubin toxicity in the vestibular system.

Changes in calcium-binding protein expression in the auditory brainstem nuclei of the jaundiced Gunn rat

Sensorineural hearing loss and auditory dysfunction are major sequelae of neonatal hyperbilirubinemia. The sites and cellular effects of bilirubin toxicity in the auditory brainstem pathway are not easily detected. Since altered intracellular calcium homeostasis may play a role in neuronal cell death, we hypothesized that the expression of calcium-binding proteins may be altered in the classic animal model of bilirubin neurotoxicity. The expression of the calcium-binding proteins, calbindin-D28k and parvalbumin, in the brainstem auditory pathway of homozygous recessive jaundiced (jj) Gunn rats was examined by light and electron microscopic immunohistochemistry at 18 days postnatally and compared to the findings obtained from age-matched non-jaundiced heterozygous (Nj) littermate control rats. Immunoreactive staining for both calbindin and parvalbumin was reduced in the cochlear nuclei and the superior olivary complex in jj rats. The extent of the reduction in immunoreactivity was related to the severity of the clinical symptoms. By contrast, immunoreactive staining in other brainstem areas (e.g., dorsal and ventral nuclei of the lateral lemniscus, inferior colliculus), thalamic (medial geniculate body) auditory areas, and neighboring non-auditory structures was similar in jaundiced and control rats. Calbindin-immunoreactive staining in the superior paraolivary and medial superior olivary nuclei in Nj rats was associated with myelinated axons, whereas parvalbumin-immunoreactive staining was localized postsynaptically in neuronal somata and dendrites. Immunoreactive staining for the calcium-binding proteins calbindin and parvalbumin in lower brainstem auditory nuclei shows abnormalities in areas susceptible to the effects of hyperbilirubinemia and provides a sensitive new way to assess bilirubin toxicity in the auditory system.

Bilirubin and the auditory system

The auditory system is highly sensitive to bilirubin toxicity. Damage to the auditory nervous system includes auditory neuropathy or auditory dyssynchrony and auditory processing problems which may occur with or without deafness, hearing loss. Auditory dysfunction may occur in children with or without other signs of classical kernicterus. Bilirubin selectively damages the brainstem auditory nuclei, and may also damage the auditory nerve and spiral ganglion containing cell bodies of primary auditory neurons. The inner ear, thalamic and cortical auditory pathways appear to be spared. Noninvasive auditory neurophysiological tests such as the auditory brainstem response (ABR) or brainstem auditory response (BAER) play an important role in the early detection of bilirubin-induced auditory and central nervous system dysfunction in the neonate.

Glucose 6-phosphate dehydrogenase deficiency with kernicterus: progressive late recovery from profound deafness

In this case report a near-term infant with Glucose 6-Phosphate Dehydrogenase (G6-PD) deficiency had an unconjugated bilirubin level of 703 on the 11th day of life but maintained his haemoglobin levels above 11 gm/dl. At 4 months of age he demonstrated the clinical picture of Kernicterus: profound sensorineural deafness and evidence of encephalopathy. However, by 15 months of age his abnormal cerebral and motor signs had regressed to a near-normal level in parallel with a gradual improvement in hearing, which also reached normal levels, first in the right ear, then in the left. At this age residual mental retardation has not been excluded but his communication skills, though delayed by 4-6 months, were moving towards the level when they would be appropriate for his age.

Cochlear microphonics in the jaundiced Gunn rat

INTRODUCTION

Bilirubin toxicity causes brain damage and deafness. Brainstem auditory areas are damaged, but the effects of bilirubin toxicity on the peripheral auditory system are less well defined. As a first step in the study of cochlear physiology, we performed studies of scalp-derived cochlear microphonic (CM) responses in the jaundiced (jj) Gunn rat model of acute bilirubin toxicity.

MATERIALS AND METHODS

CMs were obtained from scalp needle electrodes in response to acoustically delayed 500-, 1,000-, 2,000-, 4,000-, 6,000-, and 8,000-Hz tonebursts delivered by overhead speakers, and brainstem auditory-evoked potentials (BAEPs) were obtained to clicks. At 18 days of age, one nonjaundiced (Nj) and two jj Gunn rats in each of four litters were anesthetized. CMs and BAEPs were performed before and 4 hours after either (1) sulfonamide was injected into jj rats to produce acute bilirubin toxicity, (2) jj controls were injected with saline, or (3) Nj controls were given sulfonamide. In a second experiment, 16-day-old jj-sulfa and Nj-saline littermates were studied with insert speakers at 60 and 75 dB hearing level (HL) at baseline and 6 hours later, and CM amplitude was analyzed with a fast fourier transformation.

RESULTS

No statistically significant differences were found by repeated measures analysis of variance in the CMs in either experiment between groups or after sulfonamide, despite BAEP changes of decreased amplitude of waves II and III and increased latency of I-II and I-III interwave intervals in jjs given sulfa.

CONCLUSION

Alterations of CM after acute bilirubin toxicity did not occur at a time when there was electrophysiologic evidence of brainstem dysfunction. Although more subtle effects might be detected with a larger sample studied at longer intervals after the insult, CM seems insensitive or less sensitive to acute bilirubin toxicity than brainstem auditory function. These results suggest that retrocochlear tests of central auditory function may be more sensitive to the effects of hyperbilirubinemia than tests of peripheral auditory function in humans.

Morphological changes in the cochlear nucleus and nucleus of the trapezoid body in Gunn rat pups

Mechanisms underlying bilirubin encephalopathy and hearing loss remain poorly understood, including the way bilirubin enters the nervous system and how bilirubin accumulates in circumscribed regions of the brain. The present experiments examined the auditory brainstem in heterozygous (Nj) and homozygous (jj) Gunn rats at an age when serum bilirubin levels were highest, and after brain bilirubin concentration was artificially raised by sulfadimethoxine administration. In four litters of 11-12 days old Gunn rats, Nj and jj littermates received a single intraperitoneal injection of sulfadimethoxine (100 mg/kg) or a comparable volume of saline. At 16-17 days of age, brainstem auditory evoked potentials were recorded to assess the severity of bilirubin toxicity in the Nj and jj animals. Following the recordings, each animal was perfusion-fixed and frozen sections of the brainstem were cut in the transverse plane from medullary through mesencephalic levels. Sections were mounted on slides, stained with thionin and coded to avoid observer bias. Quantitative analysis revealed no differences between saline and sulfa-treated Nj rats for cochlear nucleus volume, or for cell size in the cochlear nucleus or superior olive. In the sulfa-treated jj rats, cochlear nucleus volume, and cross-sectional areas of spherical cells in the anteroventral cochlear nucleus and principal cells in the nucleus of the trapezoid body, were all significantly smaller than in the combined groups of Nj animals. The affected areas in the cochlear nucleus and superior olive are innervated by large axosomatic end-bulbs of Held or calyceal endings, and were associated with bilirubin staining of glia in the most severely jaundiced jj sulfa-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Brainstem auditory evoked potentials correlate with morphological changes in Gunn rat pups

The relationship of brainstem structure and function in bilirubin encephalopathy is incompletely understood. The present experiments compare quantitative measures of brainstem structures with brainstem auditory evoked potentials (BAEPs) in infant jaundiced (jj) and nonjaundiced (Nj) Gunn rats. Ten jj's from 4 litters were injected with sulfadimethoxine at 11-12 days of age to raise their brain bilirubin concentration. Littermate controls were jj's given saline, and Nj's given sulfadimethoxine or saline. At 15-17 days of age BAEPs were recorded, and rats were prepared for histological examination, as was reported in the previous paper (Conlee and Shapiro, 1991). Significant differences between groups were seen for BAEP wave I latency (P = 0.002). I-II interwave interval (P = 0.001), and amplitudes of waves I, II, III, and IV (each P less than 0.0005) due to increased latencies and decreased amplitudes in the jj-sulfa group. Animals with the most severe BAEP abnormalities had the most severe histological abnormalities. Cochlear nucleus volume had a positive linear correlation with the amplitude of BAEP waves I, II, and IV, and an inverse correlation with wave I latency and I-II interwave interval (P less than or equal to 0.001). The highest correlations were BAEP I-II interwave interval and amplitude of waves I and II with cochlear nucleus volume (r = -0.78, 0.71 and 0.70, respectively, P less than 0.0005).(ABSTRACT TRUNCATED AT 250 WORDS)

Auditory nerve of the normal and jaundiced rat. I. Spontaneous discharge rate and cochlear nerve histology

Hyperbilirubinemia is a major problem in neonatal intensive care. Hearing impairment is one of its sequelae. Although lesions of the central auditory pathways are known to be associated with this disorder in both humans and homozygous Gunn rats, the presence of cochler pathology is still controversial. The purpose of this study was to examine the functional integrity of the peripheral auditory system in the Gunn rat. The Gunn rat is a mutant of the Wistar strain with congenital deficiency of the liver enzyme uridine diphosphoglucuronyl transferase which is essential for bilirubin conjugation. This deficiency is inherited as an autosomal recessive trait, with the homozygous animals (jj) showing evidence of bilirubin encephalopathy. The heterozygotes (Jj) have 50% enzyme deficiency and are not jaundiced. The Long-Evans rat served as a control. The approach was to study the discharge characteristics fo single auditory nerve fibers using standard procedures in a closed and calibrated sound system. Various response measurements which would reveal pathological processes in the cochlea were analyzed. In this study, spontaneous discharge rate distribution and interspike interval statistics derived from Gunn rat auditory nerve recordings were found to be within the normal range, and cochlear nerve histology showed no evidence of neuropathy.

Brain stem auditory evoked potentials in jaundiced Gunn rats

Bilirubin encephalopathy causes potentially preventable brain damage and hearing loss. The site of auditory dysfunction is controversial, despite pathologic studies showing damage to brain stem auditory nuclei in humans and experimental animals. We studied the effects of bilirubin toxicity on the auditory system of homozygous jaundiced Gunn rats by use of brain stem auditory evoked potentials. Small but statistically significant abnormalities were found for wave latencies, interwave intervals, and amplitudes.

Development of brainstem auditory evoked potentials in heterozygous and homozygous jaundiced Gunn rats

Bilirubin toxicity is a significant clinical problem causing neurologic and audiologic sequelae. To better understand the pathogenesis of bilirubin toxicity in the immature nervous system we studied the development of brainstem auditory evoked potentials (BAEPs) in jaundiced (jj) Gunn rats and their non-jaundiced (Jj) littermates. Littermate pairs of Jj and jj rats were studied serially from early infancy to adulthood. Replicated BAEPs to click stimuli at two different intensities (45 and 75 dB SPL) and rates (33 and 89/s) were obtained from animals anesthetized with ketamine and acepromazine and maintained at a constant rectal temperature. Jaundiced (jj) rats had increased latencies of waves II and III and the I-II and I-III intervals, and decreased amplitudes of waves II and III from 17 days of age through adulthood. For both groups, all latencies and interwave intervals decreased with age (P less than 0.0001 for each wave and interwave interval by repeated measures ANOVA), and the amplitude of II increased with age (P less than 0.0001). No group differences were found in wave I latency or amplitude, or in the latency change of waves I, II or III as a function of intensity (about 11 microseconds/dB at all ages), suggesting that peripheral auditory function is normal in jj rats. Finally, there were no different effects of stimulation rate on BAEP wave latencies between groups. The findings suggest dysfunction of the central (brainstem) auditory pathways at and rostral to the cochlear nuclei, and are consistent with studies showing destruction of the cochlear nuclei in this animal model and in humans with bilirubin toxicity. The central abnormalities previously found in adult, jaundiced rats are now demonstrated in animals as early as 17 days of age, when serum bilirubin concentration is maximum. The BAEP findings are similar to changes found in hyperbilirubinemic human neonates, and support the use of the Gunn rat animal model for the study of bilirubin encephalopathy.

The effects of brain bilirubin deposition on auditory brain stem evoked responses in rats

Auditory brain stem evoked responses (ABR) in newborn infants are altered by elevated serum bilirubin levels and may be used as an indicator of bilirubin toxicity. The hypothesis of our study was that hyperbilirubinemia may affect ABR's because of bilirubin deposition in the brain stem. We examined this hypothesis by studying 21 male adult rats divided into three groups: group A, control (n = 8); group B, low bilirubin (n = 6); and group C, high bilirubin (n = 7). Each experimental group was studied over 150 min. The control group received a buffer solution. The low bilirubin group received a low dose bilirubin bolus of 50 mg/kg followed by continuous infusion of 20 mg/kg/h. The high bilirubin group received a bolus of 100 mg/kg bilirubin followed by continuous infusion of 40 mg/kg/h. All groups received sulfisoxazole (50 mg/kg X 3) during the second hour of the study. Auditory evoked potentials were recorded at 0 and 150 min. At the end of the study, the brains were analyzed for bilirubin content. Bilirubin deposition in the 3 groups was 0.6 +/- 0.28 micrograms/g, 0.93 +/- 0.07 micrograms/g and 3.2 +/- 2.2 micrograms/g for the control, low bilirubin and high bilirubin groups, respectively. Bilirubin deposition in the brain was associated with a significant amplitude reduction of Wave I and III, but had no effect on absolute latencies and interpeak latencies. Very high brain bilirubin concentrations were associated with absence of Waves I and IV. Wave I was also significantly reduced in the low bilirubin group where there was slightly increased bilirubin deposition in the brain. We conclude that ABR changes in the form of wave amplitude reduction were associated with brain stem and cerebellum bilirubin deposition. We speculate that previously reported observations of abnormal ABR changes in hyperbilirubinemic newborn infants are associated with bilirubin deposition.

Combined electrophysiological and autoradiographic delimitation of retrocochlear dysfunction in a mouse mutant

The hereditary retrocochlear dysfunction in the quivering (qv) mouse was investigated with autoradiography and single unit recordings. Whilst the cochlea appears to function normally, earlier studies had indicated some single unit dysfunction detectable at the level of the cochlear nucleus (CN) and abnormality of auditory evoked potentials recorded at the inferior colliculus (IC). The present study investigated the possibility of progressive deterioration of function at successive higher levels in the auditory system. The 2-deoxyglucose technique illustrated auditory activity in the CN of quivering mice similar to that seen in normally-hearing control animals. There was only a slight increment in metabolic activity detectable at the level of the IC. Electrophysiology demonstrated that this minimal IC activity was the result of abnormally raised thresholds associated with all single units recorded, rather than of activity in a few normally responding cells. There was no evidence from autoradiography for any enhanced auditorily evoked metabolic activity in either the superior olivary complex or the lateral lemniscus. This study suggests that the retrocochlear dysfunction in quivering mice is due to a specific abnormality at a low stage in the auditory pathway rather than being non-specific and cumulative over stages.

Genetic factors affecting hearing development

Both genetic background and single gene mutations may affect the development of the auditory system. A classification system is presented for those single gene mutations causing hearing impairment. The new feature of this classification is the inclusion of a category for hereditary deafness of central origin. The other categories involve peripheral abnormalities and are: morphogenetic defects, in which the overall structure of the labyrinth is deformed; neuroepithelial degeneration, in which the primary defect appears to occur in the organ of Corti; and cochleo-saccular degeneration, where the stria vascularis is abnormal and Reissner's membrane collapses, leading to further degeneration.

Brain-stem auditory evoked potentials in rats with high-dose pentobarbital

Brain-stem auditory evoked potentials (BAEPs) are relatively resistant to alteration by barbiturate drugs, but the effects of the high doses that are used clinically to produce deep barbiturate coma for the treatment of intracranial hypertension and postischemic anoxic encephalopathy on BAEPs are unknown. We gave high-dose pentobarbital infusions to mechanically ventilated rats while recording serial BAEPs from the scalp. Pentobarbital progressively suppressed and then abolished all peaks. First the later waves, then all but the first wave, and finally all waves were abolished at intravenous doses of 120, 220, and 260 mg/kg, respectively, in addition to the initial anesthetic dose of 60 mg/kg i.p. The changes were at least partially reversible; peaks returned in reverse order of their disappearance. Peak latencies increased with dose. The results show a significant effect of pentobarbital on BAEPs in the rat, suggesting that BAEPs may be useful in assessing the depth of and recovery from barbiturate coma.

Single unit responses in the cochlear nucleus of the deaf quivering mouse

Mice homozygous for the autosomal recessive gene quivering do not have a classical Preyer reflex and appear to be deaf. Round window recordings including both cochlear microphonics and compound action potentials failed to reveal any abnormality. However, auditory-evoked potentials recorded from the inferior colliculus (IC) are small with long latencies, and the thresholds are at least 50 dB higher than those recorded in controls. This suggests that the auditory deficit arises in the auditory pathway between the cochlear nerve and IC and underlines the need for a description of the functioning of the cochlear nucleus (CN). Single units were recorded extracellularly from the CN in 9 mutants (qv/qv) and 11 control animals (+/qv, or +/+) in the age range 60-120 days. The spike response pattern in mutant animals was broadly similar to that in the controls: a sustained response with monotonic rate-intensity functions. In addition the mean Q10dB for units in the mutants was similar to that of the controls. However, in mutants the group mean threshold at the characteristic frequency was higher and the latency to the first evoked spike at 20 dB above threshold was longer than in controls. Some unit responses in the mutants were similar to those of the controls. Nevertheless, in the quivering mouse, evidence now exists of single unit dysfunction in the cochlear nucleus.